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-
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.
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).
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
Oriente basin, Ecuador
(from Smith, 1989).
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
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
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.
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).
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)
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
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.
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
Heterolithic Facies with Cross-Bedded
Sandstone, Full-Vortex Structures,
Rhythmites, and Double Mud Layers
(Tidal Sand Bar)
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
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).
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-
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.
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-
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.
Heterolithic Facies with Flaser-Bedded
Sandstone and Rhythmites (Tidal Sand Flat)
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).
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)
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.
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
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.
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.
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
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
662 Oriente Basin, Ecuador
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.
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
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
We interpret this facies to represent a muddy shelf
environment. Pelecypod fragments appear to have
undergone minor transport in the shelf environment.
Cross-Bedded Sandstone with Rhythmic
Bedding and Double Mud Layers (Fluvial to
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
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.
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
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.
(2) There is also agreement that the Napo
Formation represents deposition on a transgressive
(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.
DEPOSITIONAL ENVIRONMENTS AND
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-
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
log of core from the
Sacha 126 well, upper
Hollin. See Figure 4 for
explanation of symbols.
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.
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
• Receives sediment from both fluvial and
• 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.,
• Commonly exhibits deepening-upward
• 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.
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
• Contains fluvial (dominant), wave, and tidal
• Represents unidirectional sediment transport
• 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).
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
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
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
log of the outcrop at Hollin
Loreto Coca Road (see
Figure 1 for location).
Note base Cretaceous
unconformity and its
angular relationship with
The measured interval
is interpreted to be
composed of mixed
fluvial and tidal channel
facies. See Figure 4 for
explanation of symbols.
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
**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
Main Hollin Alluvial braid plain coastal plain Tide-dominated estuary and fluvial
*White et al. (1995).
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
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
log of core from the
Sacha 132 well Napo
“U” sand showing a
phase. Note that this
composed of multiple
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.
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).
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
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
components in plan view
of two end members of
(A) Spatial distribution of
facies in a tide-dominated
estuary. (B) Spatial
distribution of facies in a
Vertical dashed lines
show facies boundaries.
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.
Shanmugam et al. 675
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
facies. During this time,
deposition took place
above the unconformity
that separates the Hollin
from the underlying
(2) Depositional model for
the Lower to upper Hollin
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
estuarine facies with
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
widespread shelf facies.
Approximate position of
the Sacha area is shown
by a rectangle to illustrate
encountered in the cores.
676 Oriente Basin, Ecuador
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.
678 Oriente Basin, Ecuador
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
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-
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
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
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
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
conventional fluvio-deltaic model would predict
north-south–trending distributary mouth bars with
an easterly sediment source.
Outcrop, core, seismic, or well data do not cor-
roborate an incised valley-fill model, as applied to
these deposits by other workers. Estuarine facies
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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 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