Delta Lithologic Facies Classification

1. 2. Fan delta front
The facies belt of a fan delta front mainly comprises conglomeratic and sandstone mixed with grayish-green mud sand and a little inferior oil shale.
The lithology of this facies belt varies considerably, and this facies belt is the best developed part in a fan delta sand body, which can be further divided into a subaqueous distributary channel, subaqueous distributary mouth bar, and sheet sand.
The subaqueous distributary channel is a combination of conglomeratic and sandstone, mixed with thin layers of mudstone, and it develops megascopic trough cross bedding, parallel bedding, and cross bedding.
The thickness of a single sequence is 0.3–2 m, and positive rhythmic layer fining upward is presented.
The superimposed sand body thickness of a multilayer river can reach tens of meters, and its SP curve mostly shows a cylinder shape.
The river mouth bar is formed by the inter bedding of well sorted pebbly sandstone and sandstone with gray mudstone.
Bedding, dominated by low-angle planar cross bedding and parallel bedding, is developed.
The SP curve shows a funnel shape-bell shape or fore set finger shape.
In certain fan-delta front facies, the river mouth bar is poorly developed or not developed at all.
Sheet sand, which is a sand body, can be seen as a thin layer distributed at the outer edge of a river mouth bar, with lithology fining and dominated by sedimentary sandstone.
The SP curve shows finger and serrated shapes.
Gilbert-type fan deltas are also common in ancient lakes located at the crack edge of a lacustrine basin and steep slope zone
3. Front (fan) delta
Front (fan) deltas enter semi-deep lake areas, and their lithology is light and dark gray mudstone mixed with a little sandstone, siltstone, calcareous shale, and oil shale.
Many ostracoda and pyrites are contained in the mudstone.
The sandstone mainly develops miniature wave ripple cross bedding and ripple crossing bedding.
The SP curve shows a serrated or the low-amplitude flat shape.
The underlying layer of a regressive fan delta is mainly deep lake sub facies.
Front contemporaneous deposit is mostly deep lake sub facies also.
Its main sequence is coarsening upward, the sand body plane is shown to be fan shaped, and the section form is lenticular.
1. Topset (fan delta plain)
This is mainly the braided river sedimentation of a fan delta plain, which mainly contains coarse-grained sediments including gravelstone, conglomeratic, and sandstone supported by matrix or particles.
Channel sand with a thickness of up to tens of meters is deposited by the longitudinal bar and transverse bar in the channel.
The inter channel sediments are mostly amaranth and variegated mudstone; nodules may be contained, and mud cracks can be seen.
In addition, high-angle planar cross bedding can be seen in the channel.
2. Foreset (fan delta front)
The foreset is the most developed part of a fan delta sand body, which is composed mainly of high-angle conglomeratic and foreset sandstone, and it can be divided further into sand microfacies such as subaqueous distributary channels and front bars.
(1) Subaqueous distributary channel
This is mainly composed of conglomeratic and sandstone, and high-angle planar and trough cross beddings are developed.
Local conglomerates are shown as directional alignment, and many intrastratal scour surfaces, lag gravels, and boulder clays are present.
The interchannel sediment is mainly sand shale interbed, formed because of an overflowing channel or crevasse in the flood period.
The SP curve of distributary channel deposit is dominated by cylinder and serrated cylinder shapes.
(2) Front bar
In the depositional sequence of Gilbert-type fan deltas, the high-angle foreset gravel bar is well developed.
The thickness of a single layer of conglomeratic is usually more than 1.5 m.
It is in abrupt contact between the bottom and the underlying layer, and conglomerate is mostly shown as having imbricate or directional alignment with a foreset angle of 25°–45°, coarsening upward or with alternating intrastratal grain size change.
Toward the basin, gravel foreset gradually changes into sandy foreset.
The top and bottom of the sand layer mostly show gradual change with the fine-coarse-fine sequence characteristic.
Outside the river mouth bar, thin frontal sheet sand that transits into the bottom set can be developed.
The SP curve of the front bar shows a serrated funnel shape-cylinder shape combination and a progradational finger shape.
(3) Bottom set (prodelta)
The bottom set comprises fine-grained sediments of the front (fan) delta, and the lithology is gray to dark gray mudstone, shale, and oil shale, possibly mixed with a few thin siltstone layers.
The SP curve mostly shows a flat straight line.
11.7.3.3 Gilbert-Type Model
Gilbert-type fan deltas are also common in
ancient lakes located at the crack edge of a
lacustrine basin and steep slope zone.
This type of fan delta has an
obvious three-tiered structure, comprising topset,
foreset, and bottom set (Table 11.16).

4. Fig. 11.55 The face zones of fan delta occur generally completely, with well
developed upward coarsening sequence characteristics
1. Fan delta plain
Fan delta plain subfacies are mainly composed of a mixture of conglomerate and conglomeratic, mingled with red, yellow, grayish-green, and variegated
mudstone and dominated by braided river sedimentation.
The single sequence is fining upward.
Relatively megascopic planar cross bedding and parallel bedding can be seen in conglomeratic.
Mixed massive argillaceous conglomerates or conglomerates supported by matrix can also be seen in the coarse clastic profile, belonging to onshore
detrital flow deposit. The SP curve of the fan delta plain facies belt mainly shows a serrated low-amplitude cylinder shape.

5. 11.8 Braid Delta
A braid delta is a type of coarse-grained delta commonly formed in the short-axis direction of a lacustrine basin.
It can also be developed when the slope of a basin is narrow in the long-axis direction and the provenance is near.
Nemec and Steel (1988) divided braided rivers into single braided river deltas and braided plain deltas according to the quantity, in which the former takes the single braided river as its source and the latter takes
the braided plain as its source.
There is a large gradient on the shore side and the subaqueous slope of this type of delta, the lakeshore is close to the piedmont, and current is short, which means it enters lake water only when developed in the braided river stage.
Hence, a braid delta is formed, which shortens the flow path of the river before it enters the lake.
Accordingly, it is called a short flow path delta.
In sag ponds, the braid delta is commonly developed and is mainly distributed on the gentle slope side of the short axis (or narrow steep slope in the long axis direction).
On the short-axis steep side of a lacustrine basin, where the slope is steep and close to the mountains, the alluvial fan directly enters into the lake to form a fan delta, but in the continuous fore set of the fan delta, the slope increases and becomes
gentle, resulting in a gradual transformation to a braid delta (Fig. 11.57).
Hence, braid deltas fall under the scope of fan deltas.
A single braided river delta refers to a coarse-grained delta rich in sand and gravel, and is formed due to the braided river plain by the fore set of a single bed load river entering a stable water body (Yu et al. 1995; Fig. 11.56).

6. 11.8.1 Differences Among Braid Deltas, Normal Fan Deltas, and Fan Deltas
The sedimentary characteristics of a braid delta lie between those of a normal delta and fan delta.
11.8.1.1 Difference from Normal Delta
The sedimentary characteristics of braided river deltas are similar to those of normal deltas, however the biggest differences between them are their source and particles. In general, a braided river delta or short flow path delta is supplied by a braided river, while a normal delta or
long
flow path delta is mainly supplied by a meandering river.
 In addition, the granularity of a braid delta is usually coarse; hence, it is called a coarse-grained delta.
 However, the granularity of a normal delta is finer than that of a braid delta; hence, it is called a fine-grained delta.
Braid delta sub facies can also be divided into three sections, namely, delta plain, delta front, and prodelta sedimentation.
However, it is commonly divided by quartering, which means
dividing the delta plain into (upper and lower parts), and each sub facies and microfacies are different from those of a normal delta.
(1) The distributary channel of a braided river plain has the characteristics of a braided river, which means channel deposit is tabular and has high width/thickness ratio; clastic particles are relatively coarse; the contents of sand and gravel are high
(normal delta is dominated by sand and silt); channels have no typical “dual structure” feature, which means few topset sub facies or overbank sediments; and channels are not stable and easy to migrate, thus coarse clastic sand bodies are usually distributed in pieces on the plane.
(2) A braided river is developed in a subaqueous distributary channel.
Owing to the large flow magnitude of a braided river and the abundance of fragmentary material, the bed load/suspended matter ratio is high.
Therefore, after entering into the water body, the channel depositional facies is relatively developed, followed by the distributary mouth bar, which is very different from the pattern observed in a normal delta.
When the source supply is adequate, the terrain slope is medium, distributary mouth bar is relatively developed, and its plane modality is mainly shown as a rhombus sand bar.
(3) The size of a braided delta is smaller than that of a normal fine-grained delta, however braided deltas are often distributed in groups, especially in steep terrain.
11.8.1.2 Difference from Fan Delta
Braided deltas and fan deltas are coarse grain deltas. Some scholars hold the opinion that they can be merged into fan deltas, but their sedimentary characteristics are obviously different.
The main difference lies in their development states of source supply and gravity flow.
The supply source of a braided delta is a braided river, while that of a fan delta is an alluvial fan (including dry fan and wet fan), and no debris flow is developed on a braided delta plain.
However, debris flow is commonly seen on a fan delta plain, and dry arid fan delta debris flow is especially developed.
Specifically, the following points should be noted (Table 11.17):
①Gravity flow sedimentation of a fan delta is usually better developed than that of a braid delta, and debris flow is particularly common in a fan delta plain.
② The granularity of a fan delta is much coarser than that of a braided (river) delta.
③ Fan deltas mainly take Gm, Gp, Gi, and Gt as their main Litho facies, while Litho facies Sm, Sh, and Fh are less developed compared to those of braid deltas.
④ The vertical sedimentary sequence of a fan delta is dominated by conglomerates with rapid granularity change, while the sequence granularity change of a braid delta is relatively slow; a fan delta shows a relatively wider range of granularity
variation than a braid delta.
⑤The distributary channel of a braid delta is a fine-grained straight river or a meandering river with low sinuosity.
⑥ Both deltas do not develop the coarsening-upward sequence well, however conglomerate facies may occur on certain sequences of a fan delta, while the coarsest particle in a braid delta is medium sand, because a braid delta mostly
comprises fine sand-silt.

7. Fig. 11.56 Sedimentary sketch map of all types of delta (after Einsele 2000).
a) Marine fan delta formed due to alluvial fan or braided river plain prograding seaward.
Notably, a coarse-grained beach ridge (rock river gravel) can form a lagoon or pond free from waves and air flow, and the silt and mudstone of an ancient
beach ridge and lagoon facies may be covered by fluvial deposits.
b)It is not only affected by sedimentary supply (HI high supply; MI medium supply) but also divided into different forms, namely, wave-controlled and tide-
dominated (LE low-energy; ME medium-energy; HE high-energy) megascopic marine delta.
C) Different sub-environments of megascopic lobate wave-controlled to tide-dominated delta system (similar to the modern Niger Delta)
Fig. 11.57 Sedimentary model map of braid delta in Buliang river, Daihai

8. Fig. 11.68 Physical equivalence map drawn by interpolation method reflects the gradual change of plane physical properties of the sand body in estuary dam

9. 11.10.2.1 Bounding Surface Hierarchy
Through research on continental delta sedimentation, the bounding surface hierarchy of continental deltas can be divided into 6 levels (Fig. 11.69):
the boundaries of levels ④, ⑤ and ⑥ should be considered when the genetic unit is divided because the former three levels of bounding surface mainly control second or third developments but have no significant influence
on the evaluation, prediction, and primary recovery of reservoirs.
All existing reservoir geological models established in China employ levels ④ and ⑤ or the latter three levels for demarcation, especially in interstratified and intra stratal heterogeneity research.
The last level of surface is usually the standard for the division of whole oilfields or depositional system surfaces.
11.10.2.2 Delta Lithologic Facies Classification
The division of interfaces causes some difficulties in coring, however research on lithofacies association types compensates for the aforementioned difficulties.
It not only reflects the differences of genetic sand bodies due to different sedimentations but also clarifies their vertical rhythms, physical properties, and heterogeneity.
Continental lacustrine basin deltas can be divided into 14 types of basic lithofacies:
① massive conglomerate facies (Gm);
② imbricate conglomerate facies (Gi);
③ flood bedding conglomerate facies (Gf);
④ planar cross bedding conglomerate facies (Sp);
⑤ parallel bedding conglomerate facies (Sh);
⑥ trough cross bedding conglomerate facies (St);
⑦ massive conglomerate facies (Sm);
⑧ swash bedding conglomerate facies (Ss);
⑨ wave cross bedding conglomerate facies (Sw);
⑩ wavy-interrupted wavy cross bedding, fine-sand facies (Fr);
⑪ parallel bedding silt lithofacies (Fh);
⑫ massive bedding silt lithofacies (Fm);
⑬ silt and muddy thin interbed complex beddings facies (Fc); and
⑭ mudstone facies (M). Mudstone facies can be subdivided into two types based on genesis and color.
The first is dark gray mudstone facies (M1), and it is usually the product of lacustrine mud;
the second is purplish gray, brownish red massive silty mudstone facies (M2), and it is the product of overbank deposits on a delta plain.
Fig. 11.69 Based on the study of continental deltaic deposits, different deposition scale interface can be divided into six grades.
A) Depositional system scale; B) sand body scale; C) core scale; and ①–⑥ represent the six surface levels (see text for explanation)

10. 11.11 Relations Between Delta Deposit and Petroleum
There is a close relationship between delta deposition, and petroleum generation and accumulation.
As is well known, a prospective oil and gas field must provide the following basic geological conditions: “source, reservoir, seal, trap, migration, and preservation.
” The existence, quality, and cooperative relationship of these conditions directly affect the formation and scale of petroleum reservoirs.
Delta depositional systems usually have general conditions such as source-reservoir-cap, and it is shown in petroleum exploration that delta deposits lead to petroleum accumulation.
According to domestic and overseas research on petroleum exploration data, petroleum is mainly accumulated in the sea-land transitional zone developed on delta deposits, that is, in the area near the coastline.
In addition, there are many megascopic and super-huge oil and gas fields.
Typical examples include the Kuwait Burgan oilfield and the Venezuela Maracaibo Basin Bolivar oilfield.
Very thick delta sedimentation can be formed due to the gradual slow subsidence of the basin and repeated advance and retreat of this belt in geological development history.
This sedimentation creates favorable conditions for petroleum formation and accumulation, and oil is mainly accumulated in the transitional zone and in the indented stratum of neritic deposit.
11.11.1 Source Bed
Mudstone with a dark color and pure quality in the prodelta is a good unboiled oil facies belt.
The river brings large amounts of mud sand to the delta and also organic substances.
These organic substances deposited in a prodelta with floating mud, bring rich nutrients for organisms in a lacustrine basin or a sea basin, thus driving their reproduction and growth.
Therefore, the mudstone in prodelta contains rich terrestrial sources and organic matters in situ.
In addition, a prodelta is composed mainly of argillaceous sediment characterized by very thick, wide distribution, and the prodelta environment is generally a reduction or weak reduction environment, which is favorable for the preservation
and transformation of organic matter, as well as rapid deposition and burial of deltas, because waves cannot reach this environment.
Therefore, predelta mudstone and silty mudstone can be considered good source rocks.
In addition, lagoon depositions associated with deltaic depositional systems are important source rocks.
This geological analysis has been proven using organic geochemical analysis indicators.
11.11.2 Reservoir
In delta deposits, good reservoir sand bodies are usually well developed, such as the distributary mouth bar of fluvial-dominated deltas, front sheet sand, distributary channel sand, beach sand and barrier bar of wave-dominated deltas, all of
which have good reservoir properties.
Distributary channel sandstone is generally not favorable compared to other sand bodies owing to its distance from the oil source area.
Therefore, in ancient delta sediments, the main reservoir is constituted of delta front sand and coastal sand, which are closely associated with delta destruction.
There, the predelta adjacent to the delta front sand is crossed as finger shape to form a compound reservoir, leading to favorable petroleum accumulation conditions.
This is a typical feature of many delta fronts in major oil and gas fields worldwide. However, in the delta system, many large structures are not petroliferous.
This is because they are located in the bay argillaceous sediment of poor sandstone between deltas.
In addition, petroleum reservoirs are not distributed in the development zone of coastal plain and barrier sand bar.
11.11.3 Seal Bed
In delta deposits, large-scale seal beds, such as swamp deposits, inter distributary bays, continental shelves, and predelta mud, can be considered seal beds.
In the process of transgression, mudstone overlaps on the reservoir to form a regional seal bed.
Moreover, in the process of advance and retreat during delta formation, the seal bed, source bed, and petroleum reservoir together compose a good source-reservoir-cap.
11.11.4 Trap
In delta deposits, in terms of petroleum reservoir sandstone, most sands are yielded in the shape of lens, except sheet sand and distributary channel sand.
This makes it easy to form stratigraphic-lithologic traps, and of course, structural traps can be formed as well.
In fluvial-dominated delta deposits, for instance, they are often associated with the contemporaneous fault and traction structure formed therefrom, diapir structure, and salt-dome structure.
Hence, many types of traps can be formed.
Most traps are formed in the process of deposition in earlier times, which is conducive to petroleum accumulation and formation.
For example, the Meso-Cenozoic oil and gas fields in Mexico Bay in the United States were formed thusly.
To summarize, delta deposits not only have very thick source beds, but also have high-quality reservoirs with good sorting.
In addition, owing to frequent local marine transgression and regression and large amplitude in the delta sedimentation process, many good source-reservoir-cap associations can be formed, leading to the formation of a rich petroleum
accumulation belt. Therefore, it is very important to study the sedimentary characteristics of deltas when searching for petroleum.

11. 11.11.5 Comparison Between Source-Reservoir and Delta Trap:-
The geometric shapes and horizontal superimposition relationships of different delta systems have their own characteristics (Fig. 11.70), which leads to differences in reservoirs.
Moreover, the characteristics of deposition determine whether source beds are different from trap types (Table 11.28).
This is because the formation diversity of deltas leads to huge differences in the productive capacities of various reservoir types.
11.11.5.1 Fluvial-Dominated Delta System:-
Given the popularity of the Mississippi Delta mode, many descriptions and explanations of fluvial-dominated delta reservoirs have been reported and published.
In the pay bed from the Pleistocene to the early Paleozoic, distributary channels, distributary mouth bars, and delta-front sheet sand are recognized reservoirs (Table 11.29).
Reservoir sand bodies in the lower delta plain are usually multilateral, branched, isolated, irregular, and lens-shaped on the cross section (Fig. 11.70a).
Inter distributary crevasse splays are secondary in volume, but they may be partially or completely isolated from other sand bodies to form potentially important reservoirs.
In large deltas, a separate crevasse fan (lobate) can accommodate up to millions of barrels of oil in isolated sand body traps.
In addition, thin locally destructive sand bars can also form small but high-yield isolated sandstone reservoirs.
Toward the direction of land, the upper delta plain facies association is generally a channel-filling facies reservoir of the suspended load type.
Carbonaceous sediments of delta plain mud and prodelta rich in organic matter surround potential reservoirs and form finger-like intersections with them.
The inherent strong river superimposition of a delta system is conducive to the formation of terrigenous and herbaceous organic matter, which generate gas easily.
Local or regional destructive transgressive mudstone, which offers good sealing, can be the best seal bed.
Reservoirs in a delta may be composed of a channel-filling sand body, distributary mouth bar, distal bar, and delta-front sheet sand.
Each sand body has distinct characteristics in terms of reservoir, permeability, and geometry.
Later burial and diagenesis may enhance these differences.
In older delta systems, perhaps only coarse channel-filling sand has the permeability required to produce oil and gas.
On the contrary, in a different diagenetic system, a well-sorted distributary mouth bar is probably the only reservoir, while symbiotic channel fillings are
dense and oil-free sand bodies.
On vast and flat delta plains, a variety of isolated or partially isolated sand bodies show very different trends, which provide a variety of forms of potential stratigraphic traps in fluvial-dominated deltas.
11.11.5.2 Wave-dominated Delta System:-
When the transformation effect of waves is enhanced, isolated mouth bars are connected, forming transversely vast, interconnected beach ridges and coastal barrier sand bodies (Fig. 11.70b).
Owing to multiple high-permeability, well-sorted, directionally arranged delta fronts and river channel sand bodies, wave-dominated deltas are the best places for the formation of high-quality reservoirs.
However, the high degree of connectivity of sand bodies, funnel-shaped distribution of sand, and connection with the river system in the updip direction decrease the possibility of forming stratigraphic traps.

12. Structural traps, including growth faults and diapir structures, are the main trap types in this type of delta.
The dominant effect of oceans leads to a low deposition rate in the pro delta, adjacent shelf, and slope environment.
Therefore, most marine organic matter can mix in the region, and the opportunities for bacterial decomposition of herb detritus materials from the river increase.
All these factors are conducive to the formation of relatively good source rocks. However, in proportion, the volume of potential oil source rock facies is usually smaller, and owing to slow
accumulation, and some organic substances may be oxidized completely.
As in the case of fluvial-dominated deltas, it is possible to produce oil-rich sources by providing petroleum source materials from a nearby depositional system.
11.11.5.3 Tide-dominated Delta System
Only a small amount of data has been proven for the development geological characteristics of Tide-Dominated Delta Systems, however they can be deduced reasonably as characteristics
of petroleum reservoirs.
The Potential Sources Include Pre delta Mud (Generating Gas and Some Oil) and Organic Matter in a Delta Plain Marsh (Generating Gas).
Although the reservoirs are products of transformation by the ocean, they might be complex and discontinuous, except those in the most sand-rich systems.
Potential reservoir sandstones include
① Large-Scale Distributary Channel Filling and the Associated Delta Plain Crevasse Splay Opened Toward the Basin Direction;
② Those Found in the Area Between the Delta Front and the Isolated Tidal Current Ridge Sand Body (Fig. 11.70c).
These two types of reservoirs can show the external shape and internal separation of dip orientation.
The potential for stratigraphic traps ranges from good to poor, depending on the sand-carrying capacity of the river system and the range and intensity of delta edge transformation.
 In Tidal Channel Sands and Bar Sands Formed due to Tides, a Large Amount of Mud Sediments and Good Preservation
may Result in Complex Permeability Levels and Heterogeneity Features.
(1) For a Long Time, deltas have been deemed as the biggest “machines” for generating petroleum.
Although there are a few simple ideas about the mechanism of petroleum formation in delta deposits, the fact that deltaic depositional systems have the potential for generation, and reservoir
and cap formation holds.
(2) In Each Delta System, there are two large reservoirs. The distributary channel forms a discontinuous reservoir network with irregular dip orientation, while the delta front sand
provides a wide sand collection with fine granularity and good sorting.
They are concentrated along the front zone of the delta sedimentary body. Crevasse splays, prodelta slip fault structures, and partially destructive sand bodies constitute quantitatively
unimportant but locally important oil and gas producing reservoirs.
(3) Source Bed Quality generally ranges from low to medium, and it contains a large number of type III kerogens.
Therefore, a delta system with an internal oil source is often dominated by gas yield. However, prodelta sediments and continental shelf and slope sediments are crossed in a finger shape,
causing the prodelta produce a greater amount of source rock.
The abundance of source rock is basically affected by the deposition rate and the abundance control of offsite herbal organic matter or shelf plankton in prodelta environments.
(4) In Each Delta System, the structure in same sedimentary period is the most common trap type.
Their economic importance is determined by the progradation scale of the delta.
All Neogene delta basins are characterized by progradational sediments with growth fault or diapir trap measuring hundreds of meters in thickness. Although similar features appear in the delta
systems of simple or complex craton inner basins, the scale is generally too small to form petroleum traps of economic value.

13. Fig. 11.70
Geometric morphology and lateral overlap relation formed by Sand Bodies in Different Delta Regime has its own
characteristics.
A Fluvial-dominated (Trap is Abundance of Stratum – Lithology traps)
B Wave-dominated, (Trap is Abundance of Structural Traps)
C Tide-dominated (Trap is Dominated by Stratigraphic Traps)
a) Fluvial-dominated
b) Wave-dominated
c) Tide-dominated

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