ABSTRACT
The Sacha field is a prolific producer of hydro-
carbons from the Cretaceous Hollin and Napo for-
mations in the ...
of oil. To enhance further oil production, it is
important to gain a clear understanding of the
reservoir in terms of its ...
we were able to document the occurrence of some
key sedimentary features, such as double mud lay-
ers (also known as mud c...
Sandstones of the Hollin and Napo formations are
believed to be derived from the east, perhaps from
two intrabasinal highs...
facies contains quartz granules about 3 mm in size.
Rare mudstone clasts are also present. The cross-
bedded sandstone is ...
tidal and fluvial channels is that tidal channels
exhibit cross-beds with mud drapes, whereas cross-
beds in fluvial chann...
Interpretation
Thick-thin alternations of successive sand layers
or bundles reflect (semi-) diurnal tidal inequality
(de B...
appear as a series of small-amplitude crinklets
(Terwindt, 1981; Banerjee, 1989). Crinkled laminae
such as these are confo...
Heterolithic Facies with Flaser-Bedded
Sandstone and Rhythmites (Tidal Sand Flat)
Description
Heterolithic facies with fla...
Mud-draped silty ripples and double mud layers,
composed of clay laminae 2–3 mm thick, are pres-
ent (Figure 16). Thick-th...
relatively level area of mud (silt and clay) accumula-
tion along the margins of an estuary. The marginal
area, which we c...
Interpretation
The glauconite in these cores is interpreted to be
mostly in situ in origin. Glauconitic deposits are
wides...
Tangential lower contacts of cross-beds are evident.
Mud drapes, double mud layers, and rhythmic bed-
ding are present. So...
(2) There is also agreement that the Napo
Formation represents deposition on a transgressive
shelf.
(3) De Souza Cruz (198...
DEPOSITIONAL ENVIRONMENTS AND
MODELS
Evidence for Tidal Processes
Tidal processes and related depositional features
have b...
peak current velocities are represented by alternat-
ing thick and thin sand layers or bundles, respec-
tively. This alter...
three common environments (i.e., fluvial, estuar-
ine, and deltaic).
Earlier workers suggested a fluvial environment
for t...
Characteristics of a river-dominated delta include
the following (modified after Dalrymple et al.,
1992; Boyd et al., 1992...
into two end members (Figure 24): (1) wave-dominated
estuary and (2) tide-dominated estuary. Bay-head
deltas, central basi...
were repeated again during deposition of the Napo “U”
interval. Finally, deposition of the “A” limestone took place
(see F...
Although both the Hollin and Napo formations
exhibit similar depositional facies and episodes of
drowning, there is an imp...
the modern Bristol Channel estuary (United
Kingdom) may also be considered an analog to the
tidal sand bars in the Hollin ...
discernible incision at the base of Napo “T” and
Napo “U” reservoirs (Figure 26). In fact, both the
Napo “T” shale and Nap...
Shanmugam et al. 675
Figure 25—Interpreted
paleogeography of the
Hollin Formation showing
four stages of evolution
from bo...
676 Oriente Basin, Ecuador
Figure26—North-southstratigraphicwell-logcrosssectionacrosstheSachafield.Eachcorrelatableunit(e...
the Sacha cores that would indicate a lowering of sea
level. In contrast, there is ample evidence for deepen-
ing or drown...
678 Oriente Basin, Ecuador
Figure28—East-westseismicprofileacrossSachafieldshowingthebaseCretaceousangularunconformity.Not...
both beneath the incision and within the valley fill
(Figure 27). In describing properties of parase-
quences, Kamola and ...
solely on its wireline log motif, could be misinter-
preted as a fluvial channel facies.
Reservoir Facies III
Reservoir fa...
conventional fluvio-deltaic model would predict
north-south–trending distributary mouth bars with
an easterly sediment sou...
stratigraphic organization of incised valley systems:
implications to hydrocarbon exploration and production:
Canadian Soc...
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  1. 1. ABSTRACT The Sacha field is a prolific producer of hydro- carbons from the Cretaceous Hollin and Napo for- mations in the Oriente basin, Ecuador. To under- stand the depositional origin of these reservoirs, we did a detailed sedimentological study using 516 ft (157 m) of conventional core from seven wells. This study reveals seven lithofacies: (1) cross-bedded sandstone with erosional base (fluvial channels), (2) heterolithic facies with erosive-based, cross- bedded sandstone (tidal channels), (3) heterolithic facies with cross-bedded sandstone showing full- vortex structures, crinkled laminae, sandy rhyth- mites, and double mud layers (tidal sand bars), (4) heterolithic facies with flaser-bedded sandstone (tidal sand flats), (5) muddy rhythmites with silty lenticular beds and double mud layers (subtidal mud flats), (6) bioturbated glauconitic sandstone (sandy shelves), and (7) bioturbated and laminated mudstone (muddy shelves). Based on the presence of mud drapes on bed forms, heterolithic facies, double mud layers, bidi- rectional (i.e., herringbone) cross-bedding, sandy rhythmites, thick-thin alternations of silt and clay lay- ers showing cyclicity (muddy rhythmites), crinkled laminae, and deepening-upward (i.e., transgressive) 652 AAPG Bulletin, V. 84, No. 5 (May 2000), P. 652–682. ©Copyright 2000. The American Association of Petroleum Geologists. All rights reserved. 1Manuscript received September 5, 1997; revised manuscript received August 31, 1998; final acceptance October 30, 1999. 2Department of Geology, The University of Texas at Arlington, Box 19049, Arlington, Texas 76019; e-mail: shamma@fastlane.net 3Mobil New Exploration and Producing Ventures, P.O. Box 650232, Dallas, Texas 75265-0232. 4Petroproducción, Unit of Research and Laboratories, Quito, Ecuador. We thank Manuel Berumen (Mobil) for assistance during core and outcrop examination; Joe Hayden (Mobil) for seismic interpretation; Jorge Montenegro and Carlos Huaman (Petroproducción, Quito, Ecuador) for discussion; R. J. Moiola, J. B. Wagner, M. Berumen, D. W. Kirkland, and P. L. Kirkland for reviewing an earlier version of the manuscript; J. E. Krueger for managerial support; and M. K. Lindsey for drafting. We wish to thank Petroproduccion, Amoco, and Mobil for granting permission to publish this paper. We thank Bulletin Associate Editor J. A. May for his critical comments that considerably improved the manuscript, Bulletin reviewers H. J. White and K. W. Shanley for their helpful reviews, and AAPG Editor N. F. Hurley for his constructive comments. Tide-Dominated Estuarine Facies in the Hollin and Napo (“T” and “U”) Formations (Cretaceous), Sacha Field, Oriente Basin, Ecuador1 G. Shanmugam,2 M. Poffenberger,3 and J. Toro Álava4 successions, we interpret the cored intervals of the Hollin and Napo formations to represent tide- dominated estuarine facies. We propose four stages of deposition for the Hollin Formation (oldest to youngest) following the regional uplift and erosion of the Misahualli volcanics: (1) the first stage (during deposition of the lower Hollin) represents minor flu- vial channels (low-sinuosity streams) and common tide-dominated estuary, (2) the second stage (during deposition of the lower and upper Hollin) repre- sents a well-developed tide-dominated estuary, (3) the third stage (during deposition of the upper Hollin) represents drowning of a tide-dominated estuary, and (4) the final stage (during deposition of the upper Hollin) represents well-developed shelf environments in the Sacha field area. During Napo “T” and “U” deposition, stages two, three, and four were repeated. Previous interpretations that the Hollin and Napo formations represent fluvio-deltaic environments are not supported by this study. A tide-dominated estu- arine setting is proposed instead. An important aspect of our work is that tidal sand bars interpret- ed in the Sacha area are predicted to trend east- west, paralleling the direction of sediment trans- port. In contrast, the conventional fluvio-deltaic model would predict north-south–trending dis- tributary mouth bars with an easterly sediment source. Outcrop, core, seismic, or well data do not corroborate an incised valley-fill model that was applied to the Hollin and Napo formations by other workers. Estuarine facies are quite complex, as this study shows, and may not always fit into a general incised valley-fill model. INTRODUCTION The Sacha oil field of the Oriente basin is located about 180 km east of the capitol city of Quito, Ecuador (Figure 1). Texaco discovered the field in February 1969 and it went on production in July 1972 (Canfield et al., 1982). Through 1995 the Sacha field had produced over 530 million barrels
  2. 2. of oil. To enhance further oil production, it is important to gain a clear understanding of the reservoir in terms of its depositional origin. The primary purpose of this study was to devel- op a viable sedimentological model to predict the distribution of the Cretaceous Hollin and Napo reservoirs in the Sacha field. Our objectives were to (1) describe cores and interpret depositional processes, (2) calibrate depositional facies with wireline logs, (3) establish sand-body geometries using stratigraphic correlations of well logs and seismic data, and (4) develop a depositional model by integrating core, outcrop, log, and seis- mic data. The principal reservoir, the Lower Cretaceous Hollin Formation (Figure 2), traditionally has been considered as braided fluvial deposits with sheet- like geometries (Canfield et al., 1982; White et al. 1995). Macellari (1988) proposed a fluvio-deltaic environment for the Hollin Formation. White et al. (1995) interpreted the overlying Upper Cretaceous Napo Formation as fluvio-deltaic deposits in an incised valley-fill setting. Our study, based on con- ventional cores from the Sacha field area, shows that tidal processes were much more important than fluvio-deltaic processes in depositing sands of both the Hollin and Napo formations. A possible reason for this difference in interpretation is that Shanmugam et al. 653 Figure 1—Location maps (two inset maps) showing structural features and distribution of producing fields (black patches) in the Oriente basin, Ecuador (compiled from Canfield et al., 1982; Dashwood and Abbotts, 1990; White et al., 1995), and the outline of the Sacha field showing line of a north-south well-log cross section (Figure 26), posi- tion of an east-west seismic profile (Figure 28), and cored wells used in this study.
  3. 3. we were able to document the occurrence of some key sedimentary features, such as double mud lay- ers (also known as mud couplets), tidal rhythmites, full-vortex structures, and crinkled laminae. Be- cause some of these features are subtle and difficult to observe at core scale, they might be overlooked or even mistaken for something else (e.g., crinkled laminae might be misidentified as stylolites). In cross-bedded sandstone, failure to recognize these tidal features in cores can result in misinterpreting the sandstone as possible braided fluvial deposits. Kuecher et al. (1990) reported cases in which sand- stones of tidal origin have been misinterpreted as deposits of fluvial origin in the United States. Our observations and interpretations have important implications for developing alternative sedimento- logic models for the Hollin and Napo formations with different orientations of sand bodies (Shanmugam et al., 1998). GEOLOGIC SETTING The Oriente basin, which covers about 100,000 km2, lies between the Andes on the west and the Guyana shield on the east (Figure 1). The basin extends northward into the Putamayo basin in Colombia, and southward into the Maranon basin in Peru (Figure 1). These basins are part of the sub- Andean foreland zone that stretches from Venezuela to southern Chile (Gansser, 1973). The Sacha field is a large, very low relief structure that lies in the axial region of the Oriente basin. The producing structures are north-south–trending anti- clines, usually faulted on one flank (Canfield et al., 1982). Oil accumulations in the Oriente basin are found in the Cretaceous sandstones of the Hollin and Napo formations. Local stratigraphic subdivi- sions of the Hollin and Napo formations are shown in the type log from the Sacha 130 well (Figure 3). 654 Oriente Basin, Ecuador Figure 2—Generalized stratigraphic column, Oriente basin, Ecuador (from Smith, 1989).
  4. 4. Sandstones of the Hollin and Napo formations are believed to be derived from the east, perhaps from two intrabasinal highs (Figure 1), the Aguarico arch to the north and the Cononaco arch to the south (White et al., 1995). Oil accumulations in the Hollin Formation are structurally controlled, whereas oil accumulations in the Napo Formation are both structurally and stratigraphically controlled (Canfield et al., 1982). The source rocks for these reservoirs are considered to be organic-rich shales of the Napo Formation (Dashwood and Abbotts, 1990). Biological marker data of the oils show very good correlation with bio- logical marker data of organic extracts from the Napo Formation (Mello et al., 1995). Geochemical analyses indicate that the oil migrated into these structures from Cretaceous source rocks in the east- ern Cordillera and southernmost Oriente basin (Dashwood and Abbotts, 1990). The oil is trapped in structures of Cretaceous–Oligocene age (Canfield et al., 1982). CORE STUDY We described 516 ft (157m) of conventional core from the Sacha field (Figure 1, Table 1). All cored wells are straight holes. The cored intervals are com- posed of consolidated fine-grained sandstone and mudstone. Cores were examined for (Figure 4) (1) bedding contacts, (2) bed-thickness variations, (3) grain-size variations, (4) lithologic variations, (5) pri- mary physical sedimentary structures, (6) biological sedimentary structures, (7) syndepositional and postdepositional sedimentary structures, and (8) oil staining. Core depths are measured depths in feet. Seven lithofacies are described in the cored inter- vals, and each type is interpreted to represent a spe- cific depositional facies (Tables 2–4). Cross-Bedded Sandstone with Erosional Bases (Fluvial Channel) Description Cross-bedded sandstone with erosional bases is present in the lower Hollin (well SA 133) and in the upper Hollin (well SA 130) intervals but is absent in the Napo “T” and “U” intervals. This facies is com- posed of dark gray (oil-stained), fine- to medium- grained sandstone. Sand grains are moderately well sorted and subangular to subrounded. Depositional matrix is generally low because sand usually com- prises 100% of this facies. The most diagnostic fea- ture is cross-stratification. Planar cross-stratification is common, with dips of cross-beds ranging from 10 to 20°. In the lower Hollin, the basal part of this Figure 3—Type log and lithostratigraphy from the Sacha 130 well showing cored (hachured) intervals.
  5. 5. facies contains quartz granules about 3 mm in size. Rare mudstone clasts are also present. The cross- bedded sandstone is interbedded with very fine grained sandstone with mud layers (i.e., mud drapes) in the lower Hollin (well SA 133). This facies has a thickness of up to 5 m in core and exhibits a “blocky” motif in wireline logs (Figure 3). Interpretation On the basis of cross-stratification and basal lags, this facies is interpreted as high-energy fluvial chan- nels with traction structures. A lack of interbedded fine-grained (levee) facies suggests low-sinuosity streams. The interbedded sandstone units with abundant mud drapes indicate some tidal influ- ence. The vertical gradation of these fluvial chan- nels (e.g., 9930.5 ft, 3028.8 m, well SA 133) into tidal channels likely indicates a transgressive coastal plain setting. Heterolithic Facies with Erosive-Based Cross-Bedded Sandstone (Tidal Channel) Description A heterolithic facies with erosive-based, cross- bedded sandstone is present in the lower Hollin (well SA 133), upper Hollin (well SA 122), and Napo “U” (well SA 119) intervals. This facies is composed of light brown (oil-stained), mud- draped, cross-bedded, fine-grained sandstone. Cross-beds dip up to 18°. Some intervals show bidirectional cross-beds. Sand grains are poorly sorted and subrounded. Depositional matrix is low to moderate because sand comprises 90–100% of this facies. This facies is 1.5 m thick in the SA 122 well and shows a fining-upward trend with a basal erosional surface and a basal lag com- posed of carbonaceous and mudstone clasts. Carbonaceous fragments are common throughout this facies. Interpretation Cross-beds, erosional bases, basal lags, and fining- upward trends provide evidence for channel depo- sition. Bidirectional cross-beds and foresets with mud drapes indicate deposition in tidal channels. Elliott (1986) considered cross-bedded sandstone with an erosional base, basal lags, and fining- upward trends in association with heterolithic facies and flaser bedding to represent estuarine tidal channels. Shanley et al. (1992) interpreted cross- beds with mud drapes as tidally influenced fluvial strata. In this study, the main difference between 656 Oriente Basin, Ecuador Table 1. Cored Wells Used in This Study Interval Thickness Total Thickness Formation Well* (ft) (m) (ft) (m) (ft) (m) Napo “U” SA 119 9450–9510 2882–2900 60 18 Napo “U” SA 126 9425–9455 2874–2883 30 9 Napo “U” SA 129 9577–9608 2920–2930 31 9.4 Napo “U” SA 132 9393–9454 2864–2883 61 18.6 Total Napo “U” Thickness 182 (35.3%) 55 Napo “T” SA 126 9652–9682 2943–2953 30 9 Napo “T” SA 129 9768–9795 2979–2987 27 8.2 Napo “T” SA 130 9650–9675 2943–2950 25 7.6 Napo “T” SA 133 9685–9715 2953–2963 30 9 Total Napo “T” Thickness 112 (21.7%) 34 Upper Hollin SA 122 9831–9886 2998–3015 55 16 Upper Hollin SA 126 9825–9885 2996–3014 60** 18 Upper Hollin SA 129 9935–9953 3030–3035 18 5 Upper Hollin SA 129 9965–9993 3039–3047 28 8.5 Upper Hollin SA 130 9870–9901 3010–3019 31 9.4 Total Upper Hollin Thickness 192 (37.2%) 58 Lower Hollin SA 133 9910–9940 3022–3031 30 9 Total Lower Hollin Thickness 30 (5.8%) 9 Total Thickness All Formations 516 (100%) 157 *SA = Sacha. **2 ft (0.6 m) of core is missing in the core box.
  6. 6. tidal and fluvial channels is that tidal channels exhibit cross-beds with mud drapes, whereas cross- beds in fluvial channels do not typically show mud drapes. Heterolithic Facies with Cross-Bedded Sandstone, Full-Vortex Structures, Rhythmites, and Double Mud Layers (Tidal Sand Bar) Description A heterolithic facies with cross-bedded sand- stone, full-vortex structures, rhythmites, and dou- ble mud layers is common in the upper Hollin (wells SA 122, SA 126, and SA 130), Napo “T” (wells SA 126 and SA 133), and Napo “U” (wells SA 126, SA 129, and SA 132) intervals. This facies is composed of light brown to dark brown (oil- stained), fine- to medium-grained, cross-bedded sandstone with abundant mud drapes (3–4 mm to 8 cm in thickness) (Figure 5). Sand layers com- monly vary in thickness from 3 mm to 3 cm. Sand grains are moderately to poorly sorted and sub- rounded. Depositional matrix is generally low with visual estimates of sand near 100%. Mud drapes are ubiquitous, resulting in a heterolithic facies (Figures 5, 6). Rhythmic alternation of the sandstone and mudstone layers (i.e., sandy rhyth- mites) is a diagnostic feature. Thick-thin alterna- tions of successive sand layers (i.e., bundles) are well developed in some intervals. Double mud lay- ers are common (Figures 5, 6). Mud offshoots (i.e., top-truncated drapes) in ripples also are common. Dips of cross-beds range from 15 to 36°. In rare cases, bidirectional (i.e., herringbone) cross- bedding is present. Some cross-bedded units dip 22° (well SA 126) and show normal grading along foresets. Graded beds are 2–3 cm in thickness. Internal truncation surfaces (i.e., reactivation sur- faces) generally dip at lower angles than dips of associated cross-beds (Figure 7). Some cross-beds show mud-draped tangential toesets and fanning (i.e., thickening) of the foresets (full-vortex struc- tures) (Figure 8A). Crinkled laminae are common, and they are conformable to ripple bed forms (Figure 8B). Small carbonaceous mudstone clasts are present in some intervals (well SA 126, 9859 ft, 3006.9 m). Individual sandstone beds range in thickness from 5 cm to 2 m. Amalgamated units are 5–10 m thick (e.g., Figure 9). Shanmugam et al. 657 Figure 4—Symbols used in sedimentological logs (for Figures 9, 15, 21, and 22).
  7. 7. Interpretation Thick-thin alternations of successive sand layers or bundles reflect (semi-) diurnal tidal inequality (de Boer et al., 1989). Mud drapes along pause planes, reactivation surfaces, and crinkled laminae likely indicate slack-water periods (Terwindt, 1981; Banerjee, 1989). During periods of higher energy, current activity maintained ripples and cross-beds (i.e., avalanching phase), whereas during periods of low energy, mud was deposited from suspen- sion. Mud offshoots in the rippled sands represent low-angle foresets caused by settling of mud from suspension (i.e., nonavalanching phase) on the lee side of ripples. Double mud layers have been ascribed to alternating ebb and flood tidal currents with extreme time-velocity asymmetry in subtidal settings (Visser, 1980). The thick sand units likely reflect deposition during dominant tides, whereas the thin sand units are probably products of subor- dinate tides. The crinkled laminae tend to mimic stylolites; how- ever, they are not stylolites. When a section that is transverse to the megaripple foreset passes through the crest line of the ripple trains, the mud-draped ripples 658 Oriente Basin, Ecuador Table 2. Depositional Facies in the Hollin Formation as a Percentage of Cored Interval* Lower Hollin Upper Hollin Well SA 133 Well SA 122 Well SA 126 Well SA 129 Well SA 130 Facies (30 ft, 9 m) (55 ft, 16 m) (58 ft, 17 m) (46 ft, 14 m) (31 ft, 9.4 m) 1. Fluvial Channel (%) 67 – – – 39 2. Tidal Channel (%) 13 9 – – – 3. Tidal Sand Bar (%) 13 49 21 2 61 4. Tidal Sand Flat (%) 7 29 38** – – 5. Tidal Mud Flat (%) – 13 17 – – 6. Shelf Sand (%) – – 21 87 – 7. Shelf Mud (%) – – 3 11 – * In well names, SA = Sacha. **Contains less than 1% marsh facies. Table 3. Depositional Facies in the Napo “T” as a Percentage of Cored Interval* Well SA 126 Well SA 129 Well SA 130 Well SA 133 Facies (30 ft, 9 m) (27 ft, 8.2 m) (25 ft, 7.6 m) (30 ft, 9 m) 1. Fluvial Channel (%) – – – – 2. Tidal Channel (%) – – – – 3. Tidal Sand Bar (%) 23 – 12 17 4. Tidal Sand Flat (%) 40 – 36 40 5. Tidal Mud Flat (%) 37 – 52 10 6. Shelf Sand (%) – 67 – 33 7. Shelf Mud (%) – 33 – – *In well names, SA = Sacha. Table 4. Depositional Facies in the Napo “U” as a Percentage of Cored Interval* Well SA 119 Well SA 126 Well SA 129 Well SA 132 Facies (60 ft, 18 m) (30 ft, 9 m) (31 ft, 9.4 m) (61 ft, 18.6 m) 1. Fluvial Channel (%) – – – – 2. Tidal Channel (%) 5 – – – 3. Tidal Sand Bar (%) – 33 22 53 4. Tidal Sand Flat (%) – 67 39 23 5. Tidal Mud Flat (%) – Trace 10 16 6. Shelf Sand (%) – – – – 7. Shelf Mud (%) 95 – 29 8 *In well names, SA = Sacha.
  8. 8. appear as a series of small-amplitude crinklets (Terwindt, 1981; Banerjee, 1989). Crinkled laminae such as these are conformable to ripple bed forms in tidal facies, but stylolites are not conformable to bed- form surfaces. The general absence of burrows in this facies suggests that the rate of sedimentation was high and therefore hostile to infaunal burrowers. Rhythmic alternation of sand and mud layers pro- vides evidence for tidal deposition (i.e., sandy tidal rhythmites). Other features of the inclined het- erolithic facies are analogous to those described for tidal sand bars (e.g., Dalrymple et al., 1992). The presence of tangential basal contacts, steeply dip- ping foresets (up to 36°), and fanning of the fore- sets may be equivalent to the full-vortex part of tidal bundles described by Terwindt (1981) for mesotidal deposits of the North Sea (Figure 10). Tidal bundles represent a lateral succession of cross-strata deposited in one event by the dominant tide (Terwindt, 1981). In the upper Hollin, small- scale tidal bundles are recognized, which may be comparable with sigmoidal tidal bundles described by Mutti et al. (1985). We interpret the full-vortex structures to be products of migrating megaripples, which are common in tide-dominated estuaries (Nio and Yang, 1991; Harris, 1988). Reactivation surfaces similar to those in the cross-bedded sand- stones also have been reported in tidal sand bars (Klein, 1970). In the Hollin Formation, the occur- rence of tidal sand bars above fluvial channels sug- gests a transgressive phase of deposition (Figure 9). Shanmugam et al. 659 Figure 5—Core photograph of heterolithic facies show- ing cross-bedded sandstone with double mud layers (arrow). Note rhythmic alternation of thick and thin sand layers. Each mud layer represents a period of slack-water deposition. Tidal cyclicity is poorly devel- oped because of merging of mud layers (black). Tidal sand bar facies. Upper Hollin, 9871.5 ft (3010.8 m), Sacha 130 well. Figure 6—Core photographs of fine-grained sandstone showing horizontal stratification with double mud lay- ers (arrow). Sand layers range in thickness from 3 mm to 1 cm. Upper Hollin, 9846 ft (3003.0 m), Sacha 122 well.
  9. 9. Heterolithic Facies with Flaser-Bedded Sandstone and Rhythmites (Tidal Sand Flat) Description Heterolithic facies with flaser-bedded sandstone and rhythmites is rare in the lower Hollin (well SA 133) but common in the upper Hollin (wells SA 122 and SA 126), Napo “T” (wells SA 126, SA 130, and SA 133), and Napo “U” (wells SA 126, SA 129, and SA 132) intervals. This facies is composed of light gray, very fine grained, ripple-bedded sand- stone with abundant mud drapes (2–3 mm to 1 cm thick). Sands are poorly sorted and subrounded. Depositional matrix varies from moderate to high because sand comprises 50–100% of this facies. Flaser bedding is diagnostic of this facies (Figure 11). Rhythmic bedding (rhythmites) of sand and mud layers is common (Figure 12). Double mud lay- ers (Figure 13), wavy bedding (Figure 13), and lenticular bedding are also common. Reddish brown elongate mudstone clasts (siderite?) are also present (Figure 14A). Dimensions of clasts are up to 7 cm long and 1.5 cm thick. Carbonaceous, glau- conitic, and micaceous fragments are dispersed throughout. In some intervals, there is a concentra- tion of carbonaceous fragments and plant resins (Figure 15). These resin (i.e., amber) particles are golden yellow in color and vary in size from a few millimeters to a centimeter. Crinkled laminae are also associated with this facies and are common in some upper Hollin inter- vals (Figure 14B). Merging of crinkled laminae is present (Figure 14B). Individual sandstone beds range in thickness from 3 to 35 cm (well SA 126, upper Hollin). Amalgamated units show a thickness of up to 5 m (well SA 126, Napo “U”). Bioturbation is common, and some intervals in the Napo “T” contain Rhyzocorallium (well SA 133, 9690 ft, 2955.4 m; well SA 126, 9660 ft, 2946.3 m) and Ophiomorpha trace fossils (well SA 133, 9698 ft, 2957.8 m). Napo “U” cores exhibit Skolithos (well SA 132, 9411 ft, 2870.3 m) and Ophiomorpha trace fossils (well SA 132, 9418 ft, 2872.4 m). In some cases, this lithofacies grades vertically into bioturbat- ed glauconitic sandstone (i.e., sandy shelf). Interpretation The common occurrence of double mud layers indicates a subtidal environment (Visser, 1980). The elongate mudstone clasts in this facies may have orig- inally been emplaced as double mud layers, which were later broken up by tidal currents. Elongate mud- stone clasts have been reported from tidal sand sheets (Banerjee, 1989). Flaser bedding, wavy bed- ding, and lenticular bedding are also evidence of a tidal-flat environment (Reineck and Wunderlich, 1968). We interpret this facies to be a tidal sand flat. Associated intervals of concentrated carbonaceous fragments with resin particles may be interpreted as a marsh environment (Figure 15); however, we give little importance to marsh facies because it compris- es less than 1% of all cored intervals. Also, evidence of rooting is lacking. Conceivably, these carbona- ceous fragments were transported onto the tidal flat. Mudstone with Lenticular Bedding and Rhythmites (Subtidal Mud Flat) Description Mudstone with lenticular bedding and rhythmites is present in the upper Hollin (wells SA 122 and SA 126), Napo “T” (wells SA 126, SA 130, and SA 133), and Napo “U” (wells SA 126, SA 129, and SA 132) intervals. This facies is composed of medium to dark gray, silty mudstone. Lenticular bedding caused by starved ripples of silt are common (Figure 16). 660 Oriente Basin, Ecuador Figure 7—Core photograph showing sandstone with mud-draped reactivation surface (arrow). Note steeply dipping cross-stratification below reactivation surface. Tidal sand bar facies. Upper Hollin, 9887 ft (3015.5 m), Sacha 130 well.
  10. 10. Mud-draped silty ripples and double mud layers, composed of clay laminae 2–3 mm thick, are pres- ent (Figure 16). Thick-thin alternations of silt and clay layers show cyclicity (Figure 17); they are called “rhythmites.” Carbonaceous fragments are common, and siderite layers and pyrite nodules are rare. This facies ranges in thickness from several cm to 3 m and is commonly associated with tidal channel and tidal sand bar facies. Interpretation Lenticular bedding is common in tidal-flat envi- ronments (Reineck and Wunderlich, 1968), as well as other environments. Because of the presence of double mud layers and rhythmites, this facies is interpreted as a subtidal mud flat (Nio and Yang, 1991). Thick-thin alternations of silt and clay layers showing cyclicity have been interpreted to repre- sent tidal influence on inner estuarine sediments (Kuecher et al., 1990). The silt layers represent trac- tion deposition from ebb and flood tides, whereas the clay layers represent deposition from suspension during slack-water periods. The thin layers are inter- preted to be deposits of neap tides and the thick lay- ers to be deposits of spring tides. The absence of burrows in this facies suggests that either the rate of sedimentation was too high or the salinity was too low to support burrowing organisms. We envision a Shanmugam et al. 661 Figure 8—(A) Core photograph showing cross-bedded fine-grained sandstone with tangential lower contacts and mud-draped toesets. Note fanning of the foreset or full-vortex structure (i.e., double-headed arrow). Also note the lower bounding surface with wavy mud drapes. Tidal sand bar facies. Upper Hollin, 9880 ft (3013.4 m), Sacha 130 well. (B) Core photograph showing fine-grained sandstone with multiple crinkled laminae composed of mud layers (horizontal arrow). Note crinkled laminae are conformable to ripple bed forms (vertical arrow). Tidal sand bar facies. Lower Hollin, 9932 ft (3029.2 m), Sacha 133 well.
  11. 11. relatively level area of mud (silt and clay) accumula- tion along the margins of an estuary. The marginal area, which we call a subtidal mud flat, was likely covered by shallow water. Bioturbated Glauconitic Sandstone (Sandy Shelf) Description Bioturbated glauconitic sandstone is present in the upper Hollin (wells SA 126 and SA 129) and Napo “T” (wells SA 129 and SA 133) intervals (Figure 18). This facies is composed of greenish gray to light brown (oil-stained), very fine to fine-grained sandstone. Sand grains are moderately to poorly sorted and subround- ed. Some intervals are argillaceous, with high deposi- tional matrix (>10%). Sand comprises 80–100% of this facies. Bioturbation is ubiquitous (Figure 18). Glauconite content is up to 40% in the Napo “T” (well SA 133, 9688 ft, 2954.8 m). We observed faint planar cross-stratification in the upper Hollin (well SA 129, 9951 ft, 3035 m). Mudstone clasts (4 cm) and mud layers are also present. Calcareous pelecypod frag- ments and Rhyzocorallium and Ophiomorpha trace fossils are evident. Pyrite and siderite nodules are rare. Individual depositional units are difficult to recognize because of bed destruction by bioturbation. This facies reaches thicknesses of 6 m or more due to amalgamation. 662 Oriente Basin, Ecuador Figure 9—Sedimentological log of core from the Sacha 130 well showing tidal sand bar facies overlying fluvial channel facies, indicative of a transgressive phase. Lower to upper Hollin. Note that these cored facies show blocky log motif (see Figure 3). See Figure 4 for explanation of symbols.
  12. 12. Interpretation The glauconite in these cores is interpreted to be mostly in situ in origin. Glauconitic deposits are widespread on present-day continental shelves and slopes at water depths from 50 to 500 m (Odin, 1985). On the basis of the abundance of glauconite and extensive bioturbation, we interpret this facies to be a sandy shelf environment. The term “shelf” is defined here as an open, shallow-marine setting. Many intervals of the Napo Formation contain trans- ported glauconite that has been attributed to accre- tional origin by which inorganic bodies grow larger by the addition of fresh particles to the outside (Lopez and Vera, 1992). There is, however, no evidence of wave processes in this facies, perhaps because of bio- turbation or deposition below wave base. Bioturbated and Laminated Mudstone (Muddy Shelf) Description Bioturbated and laminated mudstone is present in the upper Hollin (wells SA 126 and SA 129), Napo “T” (well SA 129), and Napo “U” (wells SA 119, SA 129, and SA 132) intervals. This facies is composed of dark gray, silty mudstone. Thin inter- vals of skeletal wackestone, composed of pelecy- pods, are present in the Napo “U” (Figure 19). Faint horizontal laminae and lenticular silt layers are also present. We observed rare synaeresis cracks in the upper Hollin (well SA 129, 9988 ft, 3046.3 m). Burrows and Teichichnus are present in this facies. Scattered carbonaceous fragments and pyrite nod- ules are also present. Some intervals show fissility. Approximately 15 m of this facies occurs in the Napo “U” interval. Interpretation We interpret this facies to represent a muddy shelf environment. Pelecypod fragments appear to have undergone minor transport in the shelf environment. OUTCROP STUDY Cross-Bedded Sandstone with Rhythmic Bedding and Double Mud Layers (Fluvial to Tidal Channels) Description The basal Hollin Formation is exposed at Hollin Loreto Coca Road, located nearly 70 km southwest of the Sacha field (Figure 1). At this location, the Hollin Formation is separated from the underlying Misahualli volcanics by a well-developed angular unconformity (Figure 20). Oil seeps are extensive. The basal Hollin is composed of reddish brown, peb- bly sandstone with a matrix ranging in size from coarse to fine grained and in sorting from medium to poor. Quartz pebbles are up to 7 mm in size. Planar cross-stratification, trough cross-stratification, and horizontal stratification are common (Figure 21). Shanmugam et al. 663 Figure 10—A model for tidal bundles. The term “mud couplet” refers to double mud layers. Core photographs (e.g. Figure 8A) in this paper may be compared with the probable view in core outlined by the three boxes. Simplified from Terwindt (1981) and Banerjee (1989).
  13. 13. Tangential lower contacts of cross-beds are evident. Mud drapes, double mud layers, and rhythmic bed- ding are present. Some siltstone intervals contain quartz granules and Planolites. Carbonaceous and resin (i.e., amber) fragments are common. Carbonaceous mudstone clasts are up to 80 cm long and 10 cm thick. There are no basal pebble lags at this locality. Interpretation The measured interval of the Hollin Formation is interpreted to represent fluvial- to tidal-channel facies. This portion of the outcrop is somewhat analogous to the cored interval of the lower Hollin in the SA 130 well (Figure 9). De Souza Cruz (1989) also studied this outcrop. There are both similari- ties and differences in interpretations between De Souza Cruz (1989) and our study (Table 5): (1) De Souza Cruz (1989) and this study agree with the upper Hollin being interpreted as a tidal bar facies. 664 Oriente Basin, Ecuador Figure 11—Core photograph of very fine grained sand- stone showing flaser bedding (arrow). Tidal sand flat facies. Napo “U,” 9408 ft (2869.4 m), Sacha 132 well. Figure 12—Core photograph showing rhythmic alterna- tion of sandstone and mudstone (arrows) units. Note mud-draped ripples (flaser) in sandstone. Tidal sand flat facies. Napo “U,” 9416.5 ft (2872.0 m), Sacha 132 well.
  14. 14. (2) There is also agreement that the Napo Formation represents deposition on a transgressive shelf. (3) De Souza Cruz (1989) identified eolian facies in the lower Hollin; however, we did not recognize eolian facies in the basal part of the outcrop. (4) Unlike the study by De Souza Cruz (1989), we recognize tide-dominated estuarine facies throughout the Hollin and Napo formations. Shanmugam et al. 665 Figure 13—Core photograph of very fine grained sand- stone showing wavy bedding. Note rhythmic alternation of mudstone (horizontal arrows) and sandstone. Also note double mud layers (vertical arrow). Tidal sand flat facies. Napo “T,” 9663.5 ft (2947.3 m), Sacha 126 well. Figure 14—(A) Core photograph showing fine-grained sandstone with elongate mudstone (sideritic?) clasts (arrow). Tidal sand-flat facies. Upper Hollin, 9841.5 ft (3001.6 m), Sacha 126 well. (B) Core photograph showing fine-grained sandstone with crinkled laminae (arrows). Note merging of crinkled laminae in the middle. Tidal sand flat facies, upper Hollin, 9870 ft (3010.3 m), Sacha 126 well.
  15. 15. DEPOSITIONAL ENVIRONMENTS AND MODELS Evidence for Tidal Processes Tidal processes and related depositional features have been discussed by many workers (e.g., Klein, 1970; Visser, 1980; Terwindt, 1981; Banerjee, 1989; Nio and Yang, 1991). Sedimentary features indica- tive of tidal processes in the Hollin and Napo for- mations include the following (see Table 6): (1) heterolithic facies, (2) rhythmic alternation of sand- stone-shale couplets (sandy tidal rhythmites), (3) thick-thin alternations of silt and clay layers showing cyclicity (muddy tidal rhythmites), (4) double mud layers, (5) cross-beds with mud-draped foresets, (6) bidirectional (herringbone) cross-bedding, (7) reacti- vation surfaces, (8) crinkled laminae, (9) elongate mudstone clasts, (10) full-vortex structures, (11) flaser bedding, (12) wavy bedding, and (13) lentic- ular bedding. Diurnal inequality and tidal cyclicity are consid- ered to be diagnostic properties of clastic tidal deposits (de Boer et al., 1989; Kuecher et al., 1990; Nio and Young, 1991). Most areas of the Earth experience semidiurnal (i.e., two tides per day) periodicity (de Boer et al., 1989). One key element of the tidal system is that alternating high and low 666 Oriente Basin, Ecuador Figure 15—Sedimentological log of core from the Sacha 126 well, upper Hollin. See Figure 4 for explanation of symbols.
  16. 16. peak current velocities are represented by alternat- ing thick and thin sand layers or bundles, respec- tively. This alternation of thick and thin sand bun- dles reflects alternating ebb and flood episodes known as diurnal inequality (de Boer et al., 1989). Thick-thin alternations of sand bundles, which are unique to the tidal regime (de Boer et al., 1989), are evident in the upper Hollin (Figure 5). In addition to diurnal inequality, clastic tidal deposits also exhibit cyclicity (de Boer et al., 1989). Tidal units tend to thicken progressively to a maxi- mum (spring tide), then thin to a minimum (neap tide), and then thicken to a next maximum (spring tide), resulting in a complete cycle every 14 days; therefore, sediments deposited over a period of 14 days should ideally be composed of 28 sand bun- dles in a semidiurnal tidal regime (i.e., two tides a day), or 14 sand bundles in a diurnal tidal regime (i.e., one tide a day). In some cases, sand deposi- tion may not occur during neap tides, resulting in a less than ideal number of bundles due to merging of clay layers. In the upper Hollin there is evidence for tidal cyclicity in some of the mudstones (Figure 17). The thick silt-rich and thin clay-rich mud layers are interpreted to be products of spring and neap tides, respectively. Although the exact number of mud layers is difficult to count because of merging, the deposition of mudstone probably took place under a semidiurnal regime. This observation is based on the thick-thin alternations of sand bun- dles, typical of semidiurnal regime, observed in the upper Hollin sandstone (Figure 5). Fluvial vs. Estuarine vs. Deltaic Environments Tidal features that we documented in this study can be interpreted to occur in more than one set- ting (e.g., tide-dominated deltas, bayhead deltas, tide-dominated estuaries). In arriving at a reason- able interpretation of depositional setting for the Hollin and Napo formations, we considered the Shanmugam et al. 667 Figure 16—Core photograph showing mudstone with lenticular bedding (horizontal arrow). Note double mud layers near the top (vertical arrow). Tidal mud flat facies. Napo “T,” 9665 ft (2947.8 m), Sacha 126 well. Figure 17—Core photograph of mudstone showing alternating silt and clay layers exhibiting thick-thin cyclicity. This pattern may indicate tidal rhythmites. The silt layers are interpreted to represent traction deposition from ebb and flood tides, whereas the clay layers are interpreted to represent slack-water deposi- tion. The thin silt layers are interpreted to be deposits of neap tides, and the thick silt layers are interpreted to be deposits of spring tides. Subtidal mud flat facies. Upper Hollin, 9882 ft (3014.0 m), Sacha 122 well.
  17. 17. three common environments (i.e., fluvial, estuar- ine, and deltaic). Earlier workers suggested a fluvial environment for the Hollin Formation and coastal plain environ- ments with bayhead deltas for the Napo Formation in the Oriente basin (e.g., White et al., 1995). Macellari (1988) proposed a river-dominated deltaic environ- ment for the Hollin Formation. Core data from this study, however, provide evidence that both the Hollin and Napo formations in the Sacha field area were deposited in a tide-dominated estuarine envi- ronment (Table 7). A possible reason for this differ- ence in interpretation is that we were able to docu- ment some key sedimentary features, such as double mud layers (also known as mud couplets), crinkled laminae, and full-vortex structures in cross-bedded sandstone units. Even in the lowermost Hollin exposed at the Hollin Loreto Coca Road, there is evi- dence for tidal influence. Thus, tidal environments persisted throughout the deposition of the Hollin and Napo formations in the Sacha area. The distinction between estuarine and fluvio- deltaic environments has important implications for petroleum geology, including the distribution of sand bodies. Estuarine tidal sand bars are aligned parallel to depositional dip, whereas delta-front sands are typically aligned along strike. This differen- tial distribution of sand bodies is critical in mapping subsurface trends. To help distinguish between the estuarine and fluvio-deltaic interpretations, charac- teristics of these two environments are summarized in the following paragraph. Characteristics of an estuary typically include the following (modified after Dalrymple et al., 1992): • Represents the seaward portion of a drowned valley system • Receives sediment from both fluvial and marine sources • May contain tidal, wave, and fluvial facies • May represent bidirectional sediment trans- port (i.e., seaward and landward) • Can exist only during rising sea level (i.e., transgressive) • Commonly exhibits deepening-upward successions • Fills during falling or stable sea level • Can become sites of river-dominated deltas only after the estuaries get filled completely 668 Oriente Basin, Ecuador Figure 19—Core photograph showing wackestone with calcareous shell (pelecypod) fragments. Muddy shelf facies. Napo “U,” 9459 ft (2884.9 m), Sacha 119 well. Figure 18—Core photograph showing bioturbated glau- conitic sandstone with an Ophiomorpha trace fossil. Sandy shelf facies. Upper Hollin, 9984 ft (3045.1 m), Sacha 129 well.
  18. 18. Characteristics of a river-dominated delta include the following (modified after Dalrymple et al., 1992; Boyd et al., 1992): • Represents seaward protrusion of the coast- line of fluvial origin • Receives sediment from both fluvial and marine sources • Contains fluvial (dominant), wave, and tidal facies • Represents unidirectional sediment transport (i.e., seaward) • Can exist only when sediment supply exceeds sea level rise • Exhibits a progradational trend To form river-dominated deltas in our study area, the estuary had to have been filled completely with sediment prior to deltaic progradation. Filling of an estuary is normally indicated by shallowing-upward successions, commonly capped by fluvial or marsh facies; however, there is no evidence for shallowing- upward successions and abandonment in the cored intervals of the Sacha field. In fact, fluvial facies are extremely rare in the cored intervals of the upper Hollin interval and absent in the cored intervals of the Napo “T” and Napo “U” intervals. Except for the fluvial facies recognized in the lower Hollin, there is no evidence of progradation. The vertical distribu- tion of facies in the lower Hollin (well SA 133), upper Hollin (well SA 130), Napo “T” (well SA 133), and Napo “U” (wells SA 129 and SA 132) intervals shows a deepening-upward trend, suggesting transgressive deposition (Figures 22, 23); therefore, the deltaic progradation and fill model is not a viable model. Another possibility is that the Hollin and Napo for- mations may represent tide-dominated deltas. According to Galloway (1975), who introduced the concept, tide-dominated deltas represent mainly estuarine settings. As has been mentioned, estuarine and deltaic environments differ from one another. There is a question as to whether tide-dominated deltas are true deltas (i.e., progradational systems). Walker (1992) even advocated abandoning the con- cept of tide-dominated deltas. In light of these prob- lems, we do not consider tide-dominated deltas as a viable depositional setting. Proposed Tide-Dominated Estuary Model Although the common perception is that all estu- aries are tide dominated, Dalrymple et al. (1992) and Zaitlin et al. (1994a, b), using physical process- es and facies, made a formal distinction of estuaries Shanmugam et al. 669 Figure 20—Outcrop photograph showing an angular unconformity (arrow and dashed line) between the basal con- tact of the Hollin Formation and the underlying Misahualli volcanics. Hollin Loreto Coca Road (see Figure 1 for location). The basal part of the Hollin Formation exhibits features of both fluvial and tidal channel facies (see Fig- ure 21 for a measured section).
  19. 19. into two end members (Figure 24): (1) wave-dominated estuary and (2) tide-dominated estuary. Bay-head deltas, central basins, flood-tidal deltas, washovers, and barrier bars, although they may be influenced by tides, characterize wave-dominated estuaries. In the Hollin and Napo formations of the Sacha area, we do not recognize any of these wave-dominated deltaic facies. We do recognize, however, major facies of a tide- dominated estuary. Evidence for a tide-dominated estu- ary in the Hollin, Napo “T,” and Napo “U” includes (1) an erosional unconformity at the base of the Hollin, (2) tidal channels and associated fluvial chan- nels, (3) tidal sand bars, (4) tidal sand flats, (5) subtidal mud flats, (6) sandy and muddy shelves, and (7) deep- ening-upward (i.e., transgressive) successions. In par- ticular, the preservation of delicate mud drapes indi- cates a protected environment, such as an estuary. The depositional model proposed for the Hollin and Napo formations is a modified version of the general model proposed for a tide-dominated estuary by Dalrymple et al. (1992). We propose the following general stages of deposition for the Hollin Formation (oldest to youngest) following the regional uplift and erosion of the Misahualli volcanics (Figure 25): Stage 1: Minor fluvial channels (low-sinuosity streams) and common tide-dominated estuary dur- ing lower Hollin deposition. Stage 2: Well-developed tide-dominated estuary and shelf environments during lower and upper Hollin deposition. Stage 3: Drowned tide-dominated estuary during upper Hollin deposition. Stage 4: Well-developed shelf environments (i.e., complete drowning) with glauconitic sands and muds during the final phase of upper Hollin deposition. During Napo “T” deposition, stages 2, 3, and 4 were repeated. Following deposition of the “B” limestone and overlying shales, stages 2, 3, and 4 670 Oriente Basin, Ecuador Figure 21—Sedimentological log of the outcrop at Hollin Loreto Coca Road (see Figure 1 for location). Note base Cretaceous unconformity and its angular relationship with underlying volcanics. The measured interval is interpreted to be composed of mixed fluvial and tidal channel facies. See Figure 4 for explanation of symbols.
  20. 20. were repeated again during deposition of the Napo “U” interval. Finally, deposition of the “A” limestone took place (see Figure 3). Thecarbonateintervalssignifyregionaltrans- gressivedeposition.Because we interpret the upper Hollin, Napo “T,” and Napo “U” formations to be tide-dominat- ed estuarine facies, we suggest that the tidal environ- ment persisted throughout the deposition ofthe Hollin and Napo formations (i.e., time transgressive). Shanmugam et al. 671 Table 5. Comparison of Interpretations of Depositional Facies for the Hollin Loreto Coca Road Outcrop by De Souza Cruz (1989) and by This Study Study De Souza Cruz (1989) This Study* Data Outcrop and core descriptions Core and limited outcrop descriptions Napo “U” Transgressive shelf Tide-dominated estuary and shelf Napo “T” Transgressive shelf Tide-dominated estuary and shelf Upper Hollin Estuarine tidal bars Tide-dominated estuary and shelf Middle Hollin Lacustrine delta front (Gilbert type) – Lower Hollin Braided fluvial and eolian Tide-dominated estuary and distal fluvial** *Palynological study of limited shale samples from the basal Hollin exposed at the Hollin Loreto Coca outcrop (Petroproduccion internal report) suggests continental environments. **Lower Hollin interpretation is based on outcrop study; the remaining interpretations are based on core. Table 7. Summary of Features Recognized in the Hollin and Napo Formations* Feature Figure Number (This Paper) Related Reference Heterolithic facies 5 Terwindt (1981) de Boer et al. (1989) Rhythmic alternation of 5 and 6 Nio and Yang (1991) sandstone-shale couplets (sandy rhythmites) Thick-thin alternations of silt and clay layers 17 Kuecher et al. (1990) showing cyclicity (muddy rhythmites) Double mud layers 5 and 6 Visser (1980) Cross beds with mud-draped foresets 5 Terwindt (1981) Bidirectional cross-bedding – Terwindt (1981) Reactivation surfaces 7 Klein (1970) Crinkled laminae 8B and 14B Terwindt (1981) Elongate mudstone clasts 14A Banerjee (1989) Feines and Tastet (1998) Full-vortex structures 8A Terwindt (1981) Flaser bedding 11 Reineck and Wunderlich (1968) Wavy bedding 13 Reineck and Wunderlich (1968) Lenticular bedding 16 Reineck and Wunderlich (1968) *Many of these features are considered to be indicative of deposition from tidal processes. Table 6. Comparison of Interpretations of Depositional Facies for the Hollin and Napo Formations by White et al.* and by This Study White et al. (1995) This Study Data type Core and outcrop descriptions Core and limited outcrop descriptions Napo “U” Incised valley-fill fluvial and deltaic Tide-dominated estuary and shelf estuary parasequences shelf Napo “T” Incised valley-fill fluvial and deltaic Tide-dominated estuary and shelf estuary parasequences shelf Upper Hollin Coastal plain tidal shoreline Tide-dominated estuary and shelf parasequences Main Hollin Alluvial braid plain coastal plain Tide-dominated estuary and fluvial *White et al. (1995).
  21. 21. Although both the Hollin and Napo formations exhibit similar depositional facies and episodes of drowning, there is an important difference. The basal Hollin is marked by a major angular unconformity, indicating erosion prior to deposition. In contrast, the Napo “T” and “U” formations rest on shelf facies with- out any evidence for erosion. Stratigraphic correla- tions show that the shelf facies beneath the tidal facies in the Napo maintains a uniform thickness regionally, indicating a lack of incision prior to Napo deposition (Figure 26). Tide-dominated estuaries commonly occur in mesotidal and macrotidal ranges (Harris, 1988). Davies (1964) defined microtidal, mesotidal, and macrotidal ranges as 0–2 m, 2–4 m, and >4 m, respectively; there- fore, we infer that the tidal range for the Sacha area was likely more than 2 m. Double mud layers in cross-beds of the Hollin and Napo resemble those of the modern Oosterschelde estuary in the Netherlands (Visser, 1980). The mouth of the Oosterschelde estuary is 7.4 km wide, and its tidal range is 3.5 m. In terms of depo- sitional processes, large-scale bed forms observed in 672 Oriente Basin, Ecuador Figure 22—Sedimentological log of core from the Sacha 132 well Napo “U” sand showing a transgressive (deepening-upward) phase. Note that this cored interval, composed of multiple depositional facies, shows a fining-upward log motif (see Figure 23). In the absence of core, this interval could be misinterpreted as a channel-fill facies based on a fining-upward log motif. See Figure 4 for explanation of symbols.
  22. 22. the modern Bristol Channel estuary (United Kingdom) may also be considered an analog to the tidal sand bars in the Hollin and Napo formations. The linear bed forms in the Bristol Channel estuary are 2–10 m in thickness, hundreds of meters in width, and several kilometers in length. Their long axes are aligned parallel to the tidal flow (Harris, 1988). The mouth of the Bristol Channel estuary is 38.8 km wide and its tidal range is 8 m (Harris, 1988). Individual tidal sand bar units in the Hollin reach up to 2 m in thickness. Conceivably, widths of paleoestuaries in the study area may have been in the range of tens of kilometers, exceeding the size of the Sacha field (see Figure 25). An important outcome of the proposed tide- dominated estuarine model is that tidal sand bars in the Sacha area are predicted to align in an east-west direction, paralleling the direction of sediment transport (Figure 25). In contrast to the proposed model, the conventional fluvial-deltaic model would predict north-south–trending distributary mouth bars with an easterly sediment source. Difficulties of an Incised Valley-Fill Model In a sequence stratigraphic framework, the con- cept of incised valley-fill systems is quite popular (Zaitlin et al., 1994b). White et al. (1995) interpreted the Hollin Formation to represent fluvial paleovalley deposits associated with coastal-plain deposits. Such valley-fill successions may be considered to be coastal- plain incised valley systems (Zaitlin et al., 1994b); however, there are some difficulties in advocating an incised valley-fill model of Zaitlin et al. (1994b) for the Hollin and Napo formations in the Sacha area. An incised valley-fill system is characterized by a basal, regional, erosional surface forming a sequence boundary (Zaitlin et al., 1994b) (Figure 27). The presence of an angular unconformity at the base of the Hollin Formation exposed at the Hollin Loreto Coca roadcut indicates a regional surface of erosion; however, an erosional surface does not necessarily mean a deep incision. Incised valley systems may reach lengths in excess of hundreds of kilometers, widths of tens of kilometers, and depths to hundreds of meters (Zaitlin et al., 1994b). If so, evidence for significant incision may be established from seismic data and regional correlations. Seismic data clearly show truncated reflections, suggesting regional erosion at the base of the Hollin Formation (Figure 28). Seismic data, however, do not show clear evidence for laterally confined, deep incised valley systems in the Sacha area. Again, we make a distinction between erosion (shallow) and incision (deep) in terms of depth. The Hollin and Napo formations exhibit overall parallel and continuous reflection patterns (Figure 28). This could be interpreted to mean that deposi- tion of the Hollin and Napo took place on a nearly flat erosional surface without a recognizable incised valley morphology in the Sacha area. Another possi- bility is that the incised valley in the Sacha area, if present, is below the seismic resolution. Detailed stratigraphic correlations are helpful in recognizing erosional and incisional features; how- ever, a stratigraphic cross section does not show Shanmugam et al. 673 Figure 23—Fining-upward wireline log motif of cored interval (hachured) in the Sacha 132 well, Napo “U” sand (see Figure 22).
  23. 23. discernible incision at the base of Napo “T” and Napo “U” reservoirs (Figure 26). In fact, both the Napo “T” shale and Napo “U” shale maintain their thickness through the entire length of the north- south cross section of the Sacha field (Figure 26). These underlying shale units should exhibit a lateral change in thickness, if they had indeed experienced a major downcutting or incision. This is perhaps the most convincing evidence for lack of incision beneath the Napo “T” and Napo “U” sands; howev- er, if an incised valley is much wider than the length of our stratigraphic cross section, then we cannot establish the existence of an incised valley without additional seismic data and a regional study. The regional sequence boundary in an incised valley-fill system is related to relative sea level fall (Zaitlin et al., 1994b). In the Sacha field area, the angular unconformity at the base of the Hollin is considered to be the result of uplift and erosion associated with the early Mesozoic tectonic activity in the Oriente basin (Dashwood and Abbotts, 1990; Balkwill et al., 1995). Although most sequence stratigraphers consider uplift under the all-inclusive term “relative sea-level fall,” we wish to emphasize the tectonic origin of the base Cretaceous uncon- formity. Estimated tectonic subsidence rate during deposition of the Hollin and Napo formations ranges from 3.714 to 9.143 m/m.y. for the middle north of the Oriente basin (Jaillard, 1995), indicat- ing that the basin was tectonically active during the Cretaceous deposition. The current trend in sequence stratigraphy is to explain most coastal erosion and deposition related to incised valley-fill systems by allocyclic processes, such as sea level changes (Zaitlin et al., 1994b); how- ever, there is no evidence for subaerial exposure in 674 Oriente Basin, Ecuador Figure 24—Morphological components in plan view of two end members of estuarine models. (A) Spatial distribution of facies in a tide-dominated estuary. (B) Spatial distribution of facies in a wave-dominated estuary. Vertical dashed lines show facies boundaries. Wave-dominated estuarine facies are considered to be the typical fill of an incised valley (see Figure 27). Simplified from Dalrymple et al. (1992). Reprinted with permis- sion from SEPM.
  24. 24. Shanmugam et al. 675 Figure 25—Interpreted paleogeography of the Hollin Formation showing four stages of evolution from bottom (older) to top (younger). (1) Depositional model for the lower Hollin showing fluvial and tide-dominated estuarine facies. During this time, deposition took place above the unconformity that separates the Hollin from the underlying Misahualli volcanics. (2) Depositional model for the Lower to upper Hollin showing tide-dominated estuarine and shelf facies. Note well-developed sand bars and sand flats in the estuary. During this time, tidal facies were deposited over fluvial and tidal facies. (3) Depositional model for the upper Hollin showing drowned tide-dominated estuarine facies with well-developed open shelf facies. During this time, shelf facies were deposited over tidal facies. (4) Depositional model for the upper Hollin showing complete drowning of the area by transgression and establishment of widespread shelf facies. Approximate position of the Sacha area is shown by a rectangle to illustrate dominant facies encountered in the cores.
  25. 25. 676 Oriente Basin, Ecuador Figure26—North-southstratigraphicwell-logcrosssectionacrosstheSachafield.Eachcorrelatableunit(e.g.,upperHollin)iscomposedof multipledepositionalfacies(e.g.,tidalchannels,tidalbars).NotetheuniformthicknessofNapo“T”and“U”shales,suggestingalackofmajor incision.SeeFigure1forlineofcrosssection.
  26. 26. the Sacha cores that would indicate a lowering of sea level. In contrast, there is ample evidence for deepen- ing or drowning in the lower Hollin, upper Hollin, Napo “T” (Sacha 133), and Napo “U” intervals, indi- cating possible rises in sea level. Autocyclic process- es, such as lateral migration of facies, can also explain the vertical changes in facies. An incised valley-fill system is characterized by a basinward shift in facies (Zaitlin et al., 1994b). This is typically the result of a fluvial valley incising into the exposed shelf during falling sea level. As a result, fluvial deposits overlie shallow-marine parasequences (Figure 27); however, such a relationship is absent in the study area because tidal and fluvial facies of the Hollin unconformably overlie the Misahualli vol- canics, not shallow-marine deposits (Figure 21). More important, there is no evidence in the core for fluvial erosion within the Napo Formation in the Sacha area. In fact, the fluvial facies are completely absent in the Napo Formation in our study area. Coarsening-upward parasequences are one charac- teristic of incised valley-fill systems, often occurring Shanmugam et al. 677 Figure 27—Generalized model for incised valley-fill system. Note that the incised valley is filled with wave-dominated estuarine facies (see Figure 24B). Also note coarsening-upward trends of parasequences. From Zaitlin et al. (1994b). Reprinted with permission from SEPM.
  27. 27. 678 Oriente Basin, Ecuador Figure28—East-westseismicprofileacrossSachafieldshowingthebaseCretaceousangularunconformity.Notetruncatedreflectionssuggesting erosionalongtheunconformity.AlsonotealackofdeepincisionatthebaseoftheHollinandwithintheNapoformations.SeeFigure1forposi- tionofseismicprofile.
  28. 28. both beneath the incision and within the valley fill (Figure 27). In describing properties of parase- quences, Kamola and Van Wagoner (1995, p. 29) stat- ed, “In siliciclastic, shallow-marine settings, parase- quences are composed of beds and bedsets that record continuous, gradual upward shallowing. . . . Most parasequences are marked by an upward-coars- ening change in grain size, an upward decrease in mud, an upward increase in thickness of the beds, and an upward change in the beds and the trace fos- sils reflecting an upward decrease in water depth. The grain-size and bed-thickness trends are reversed in parasequences composed of subtidal-tidal flat deposits, where deposition still records an upward decrease in water depth.” As one might expect, parasequences do not occur in the underlying pre- Hollin Misahualli volcanics. More important, coarsen- ing upward and thickening-upward trends are absent in the Hollin and Napo formations. Although parase- quences in subtidal environments may show fining- upward trends, the Hollin and Napo formations do not show shallowing-upward trends that are charac- teristic of some parasequences. An incised valley-fill system is generally charac- terized by a vertical association of wave-dominated estuarine facies composed of fluvial, bayhead delta, central basin, and barrier beach in an ascending order (Figure 27); however, wave-dominated estu- arine facies are absent in the Sacha core. Outcrop, core, seismic, or well data do not cor- roborate an incised valley-fill model, applied to the Hollin and Napo formations by other workers. Estuarine facies are quite complex, as this study shows, and may not always fit into a general incised valley-fill model. RESERVOIR FACIES In the core and outcrop examined, we recognize four reservoir facies on the basis of lithofacies (i.e., sand percent), inferred sand-body geometry, bed continuity and connectedness, and depositional permeability barriers. All four facies types generally exhibit similar porosity values (15–20%), but their permeability values differ. Reservoir Facies I Reservoir facies I comprises tidal sand bars that are dominantly composed of fine- to medium- grained sand. This facies is most common in the outer estuarine setting (Johnson and Levell, 1995), closely associated with tidal sand flats (reservoir facies III). Sand percentage in this facies is common- ly 100%. The sand is clean and devoid of deposition- al matrix because strong tidal traction processes winnowed the associated fines. Measured perme- ability (horizontal) ranges from 1 to 9300 md. Although mud drapes are ubiquitous in this facies, they are not considered to be major perme- ability barriers because they tend to be too thin (i.e., 3–4 mm thick) and discontinuous. This facies is expected to have good vertical and lateral com- munication. This reservoir facies is characterized by an elon- gate bar geometry, parallel to depositional dip. Vertical amalgamation of sand packages is a diag- nostic feature of this facies (e.g., well SA 130, upper Hollin), where multiple bars are stacked both laterally and vertically, and the deposit is more sheetlike. Individual bars range in thickness from 5 cm to 2 m. Amalgamated units show a thickness of up to 10 m. As indicated by the gamma-ray curve, this facies is characteristically clean and exhibits a blocky character; however, the wireline-log motif alone is not a reliable indicator of depositional facies. For example, the blocky log motif in the Sacha 130 well (Figure 3) represents both fluvial channel and tidal sand bar facies (Figure 9). Reservoir Facies II Reservoir facies II comprises both tidal and flu- vial channels. This facies commonly occurs in the inner estuarine setting. Although both fluvial and tidal channels are cross-bedded, the tidal channels are distinguished by their mud drapes. This facies is composed dominantly of medium- to fine-grained sand. Sand percentage is commonly 90–100%. Sands in this facies are not as clean when compared to tidal sand bars because of moderate matrix con- tent; therefore, reservoir quality is somewhat lower. Measured permeability (horizontal) ranges from 30 to 3400 md. We infer that most of these channel sands are lenticular in geometry. The tidal channel facies is 1–2 m thick. The fluvial channel facies is up to 5 m in core and up to 10 m in outcrop. Fluvial channels tend to exhibit slightly better reservoir quality than tidal channels. Comparing the wireline logs to the core, this facies exhibits fining-upward and blocky motifs. Again, caution must be exercised in using log motifs to interpret depositional facies because both trans- gressive sequences and fluvial sequences can show a fining-upward log motif. For example, the cored interval of the Napo “U” in the Sacha 132 well shows a transgressive phase of deposition in which tidal sand bar facies grades vertically into shelf facies (see Figure 22). This transgressive interval is seen as a fin- ing-upward trend in wireline logs (see Figure 23). In the absence of core, this fining-upward trend, based Shanmugam et al. 679
  29. 29. solely on its wireline log motif, could be misinter- preted as a fluvial channel facies. Reservoir Facies III Reservoir facies III is composed of tidal sand-flat deposits and comprises dominantly fine-grained sand. The most diagnostic feature of this facies is its flaser, wavy, and lenticular bedding. Sand percent- ages of this facies range from 50 to 100%. Sands are fine grained and have a high matrix content. The sands thus are not as clean as sands of tidal sand bars and channels; therefore, the reservoir quality of this facies is lower than the tidal sand bar and channel facies. Measured permeability (horizontal) ranges from 0.1 to 1000 md. Reservoir facies III is one of the most common facies in the Hollin, Napo “T,” and Napo “U” inter- vals (Tables 2–4). Although this facies can occur throughout the estuary, it commonly occurs land- ward of the tidal bars. Sand bodies are likely to be laterally extensive (i.e., sheetlike geometry) and are often connected to tidal sand bars. The sand bars tended to migrate over the sand flats during megaripple formation. As a result, these connect- ed facies may behave as single reservoir flow units. In the tidal-flat facies, neap bundles with higher numbers of crinkled laminae (i.e., mud lay- ers) may be of poorer reservoir quality than spring bundles with fewer crinkled laminae. Individual sandstone units range in thickness from 3 to 35 cm. Vertically amalgamated units show thickness- es of up to 5 m. In wireline logs, this facies exhibits an irregular character in comparison to the other two sand-rich facies (i.e., reservoir facies I and II) because it contains more intercalat- ed finer grained material. Reservoir Facies IV Reservoir facies IV is composed of shallow- marine shelf sands. This reservoir facies is consid- ered to be the least important of the four types because of (1) high glauconite content, which can result in considerable compaction and reduction in primary porosity, (2) high depositional matrix, which can occlude primary porosity, and (3) high bioturbation, which can mix sand and mud result- ing in a poorly sorted texture. Measured permeabil- ity (horizontal) ranges from 0.06 to 150 md. The sandstones are very fine to fine grained. The sand is moderately to poorly sorted and subrounded. Some intervals are argillaceous because of the high depo- sitional matrix. Sand content ranges from 80 to 100%. Glauconite content is up to 40%. Calcareous shell fragments and bioturbation are ubiquitous. This facies varies from bars to sheetlike geometry. Thicknesses of individual units are difficult to deter- mine, but this facies can reach a thickness of 6 m or more due to vertical amalgamation. Commonly, there is a gradation between this facies and the three tidal reservoir facies. CONCLUSIONS The Cretaceous Hollin and Napo formations in the Sacha field are prolific producers of hydrocar- bons in the Oriente basin, Ecuador. To enhance fur- ther oil production, it is important to gain a clear understanding of the reservoir in terms of its depo- sitional origin. A sedimentological analysis using 516 ft (157 m) of conventional core from seven wells established seven depositional facies, namely (1) fluvial channels, (2) tidal channels, (3) tidal sand bars, (4) tidal sand flats, (5) subtidal mud flats, (6) sandy shelves, and (7) muddy shelves. The seven depositional facies can be grouped into four reservoir facies: (1) tidal sand bars with excellent reservoir properties (i.e., 100% sand, low matrix, elongate bar geometry), (2) fluvial and tidal chan- nels with good reservoir properties (i.e., 90–100% sand, moderate matrix, lenticular geometry), (3) tidal sand flats with moderate properties (i.e., 50–100% sand, high matrix, sheet geometry), and (4) shelf sands with relatively poor properties (i.e., 80–100% sand with high matrix and glauconite, bar to sheet geometry). Based on the presence of mud drapes on bed forms, heterolithic facies, double mud layers, bidi- rectional (i.e., herringbone) cross-bedding, sandy tidal rhythmites, muddy tidal rhythmites, crinkled laminae, flaser bedding, wavy bedding, lenticular bedding, and deepening-upward (i.e., transgres- sive) sequences, we interpret the cored intervals of the Hollin and Napo formations to represent tide- dominated estuarine facies. We propose four stages of deposition for the Hollin Formation (oldest to youngest) following the regional uplift and erosion of the Misahualli vol- canics: stage 1 (lower Hollin deposition) represents minor fluvial channels (low-sinuosity streams) and common tide-dominated estuary; stage 2 (lower and upper Hollin deposition) represents a well devel- oped tide-dominated estuary; stage 3 (upper Hollin deposition) represents a drowning of tide-dominated estuary; and stage 4 (upper Hollin deposition) rep- resents well-developed shelf environments in the Sacha field area. Stages 2–4 are repeated during Napo “T” and “U” deposition. An important aspect of the proposed model is that tidal sand bars in the Sacha area are predict- ed to align in an east-west direction paralleling the direction of sediment transport, whereas the 680 Oriente Basin, Ecuador
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  31. 31. stratigraphic organization of incised valley systems: implications to hydrocarbon exploration and production: Canadian Society of Petroleum Geologists, Calgary, 17 p. Zaitlin, B. A., R. W. Dalrymple, and R. Boyd, 1994b, The stratigraphic organization of incised-valley systems associated with relative sea-level change, in R. W. Dalrymple and B. A. Zaitlin, eds., Incised-valley systems: origin and sedimentary sequences: SEPM Special Publication 51, p. 45–60. 682 Oriente Basin, Ecuador G. (Shan) Shanmugam Shan Shanmugam received his Ph.D. in geology from the University of Tennessee in 1978 and joined Mobil the same year. His publications (1 book, 75 papers, and 65 abstracts) include AAPG Bulletin articles on secondary porosity (1984, 1985), oil generation from coal in Australia (1985), unconformity-related porosity in Alaska (1988), submarine-fan lobes (1991), bottom-current reworked sands in the Gulf of Mexico (1993), sandy slump and debris-flow reservoirs in offshore Norway (1994), reinter- pretation of classic turbidites of the Jackfork Group in Arkansas (1995), basin-floor fans in the North Sea (1995), replies to six discussions on turbidite controversy (1997), and tide-dominated estuarine facies in Ecuador (this paper). He has been included in the Millennium Edition (2000–2001) of Marquis Who’s Who in Science and Engineering among 470 geologists chosen from 40 coun- tries. In January 2000, he retired from Mobil and joined The University of Texas at Arlington as an adjunct profes- sor of geology. Mike Poffenberger Mike Poffenberger is currently employed by Mobil New Exploration and Producing Ventures as a senior staff geolo- gist in Dallas, Texas. He received his B.S. (1983) and M.S. (1986) degrees in geology from Louisiana Tech University. He joined Mobil Oil in 1985 and has worked numerous producing and exploration projects domestically and internationally including the U.S. Gulf Coast, Mexico, Ecuador, and circum-Mediterranean. He is currently assigned to exploration studies in Tunisia. Jorge Toro Álava Jorge Toro Álava received a degree in geotechnical engineering (1994) from the Escuela Politecnica Nacional in Quito (Ecuador) and a M.Sc. degree in geology from the Universite Joseph Fourier in Grenoble (France). He worked in soil and rock mechanics, natural hazards, and microseismic volcanology, but mainly in geodynamics characterization and basin analysis of the Cretaceous and Tertiary basins located in the back-arc, Andean arc, and foreland of Ecuador. Since joining Petroproducción (filial of Petroecuador) in 1994, he worked in sedimentology, stratigraphy, regional geology, and reservoir characteriza- tion of the Cretaceous sediments of the Oriente basin. ABOUT THE AUTHORS

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