Formation Conditions and Sedimentary Characteristics of a Triassic Shallow Water Braided Delta in the Yanchang Formation, Southwest Ordos Basin, China
Formation Conditions and Sedimentary Characteristics of a Triassic Shallow Water Braided Delta in the Yanchang Formation, Southwest Ordos Basin, China
Ziliang Liu 0 1
Fang Shen 0 1
Xiaomin Zhu 0 1
Fengjie Li 0 1
Mengqi Tan 0 1
0 1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology , Chengdu , China , 2 School of Energy, Chengdu University of Technology , Chengdu , China , 3 State Key Laboratory of Geological Processes and Mineral Resources , China , University of Petroleum , Beijing , China , 4 Institute of Sedimentary Geology, Chengdu University Technology , Chengdu , China
1 Academic Editor: Vanesa Magar, Centro de Investigacion Cientifica y Educacion Superior de Ensenada , MEXICO
A large, shallow braided river delta sedimentary system developed in the Yanchang Formation during the Triassic in the southwest of the Ordos basin. In this braided delta system, abundant oil and gas resources have been observed, and the area is a hotspot for oil and gas resource exploration. Through extensive field work on outcrops and cores and analyses of geophysical data, it was determined that developments in the Late Triassic produced favorable geological conditions for the development of shallow water braided river deltas. Such conditions included a large basin, flat terrain, and wide and shallow water areas; wet and dry cyclical climate changes; ancient water turbulence; dramatic depth cycle changes; ancient uplift development; strong weathering of parent rock; and abundant supply. The shallow water braided river delta showed grain sediment granularity, plastic debris, and sediment with mature composition and structure that reflected the strong hydrodynamic environment of large tabular cross-bedding, wedge cross-bedding, and multiple positive rhythms superimposed to form a thick sand body layer. The branch river bifurcation developed underwater, and the thickness of the sand body increased further, indicating that the slope was slow and located in shallow water. The seismic responses of the braided river delta reflected strong shallow water performance, indicated by a progradation seismic reflection phase axis that was relatively flat; in addition, the seismic reflection amplitude was strong and continuous with a low angle and extended over considerable distances (up to 50 km). The sedimentary center was close to the provenance, the width of the river was large, and a shallow sedimentary structure and a sedimentary rhythm were developed. The development of the delta was primarily controlled by tectonic activity and changes in the lake level; as a result, the river delta sedimentary system eventually presented a “small plain, big front” character.
Funding: This research was financially supported
jointly by the National Natural Science Foundation of
China (grant no. 41272133) and the National
Demonstration Project of China (grant no.
2008ZX05044-1-3-2). The funders had no role in
study design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing Interests: The authors have declared
that no competing interests exist.
The concept of a shallow water delta was first introduced by Fisk. In Fisk’s research on the
Mississippi River Delta, he divided the fluvial-dominated delta into two types: deep and shallow
water. Donaldson further summarized the concept of a shallow water delta and discussed
the effects of water depth on delta development. Postma divided the low-position
fluvialdominated deltas of basins into shallow and deep water types and further distinguished 8 types
of shallow-water deltas. Based on the classification of Postma, the study of shallow water
deltas has gradually progressed in recent years, mainly with respect to delta formation
dynamics, sedimentation processes, microfacies recognition and sand body configuration[4,5,6,7,8,9].
Such studies have proposed that distributary channel sand form the skeleton of shallow water
deltas, with the mouth bar undeveloped at the front of shallow water deltas. The sand body
distribution is significantly affected, and it is not only distributed in a strip but also concentrated
in certain areas.
In the 1980s, Chinese scholars began to study shallow water deltas. Gong Shaoli discussed a
mainly fluvial shallow water delta that had developed in Henan Yu County in the late Early
Permian. Su Chunqian observed distinctly different sediment types between the southern
margin of the Ordos basin carboniferous delta and the traditional Gilbert delta. These deltas’
sedimentary sequence was given priority and thinned going upwardin the presence of the
delta’s front forward coarsening upward sequence, which has the distinct characteristics of a
shallow water delta. Li et al. studied structures of the Carboniferous and Permian periods and
discovered an enormous complex shallow water delta sedimentary system in which a
distributary channel was developed along with frequent branches with a small shunt that were
accompanied with limited front delta deposition. Lou et al. indicated that a shallow water delta
had developed in the Putaohua oil layer in the northern Songliao basin; the delta had the
unique characteristic of being an underwater distributary channel and had developed by the
spread of delta-front deposits. A delta plain and delta front developed, the former developing
into a flat structure and the latter spreading over the plain. There was no Gilbert-type mode of
delta formation with a top set, foreset, and bottom set.
In the 21st century, the study of shallow water deltas by Chinese scholars has mainly focused
on the Songliao basin[14,15,16,17,18,19,20,21,22,23,24], Ordos basin[25,26,27,28,29], and
Sichuan basin. The study of shallow water deltas has revealed their outstanding
characteristics, with underwater distributary channels as the main body and not debouch bar sediments,
unlike conventional Gilbert deltas, which show small plain, big front characteristics and
present a broad distribution over plains. The Ordos basin shows the characteristics of a basin full of
sand. Furthermore, fluvial and paleotopographic lake-level changes affected the growth of
shallow water deltas. Shallow water deltas developed on a flat terrain due to the structural stability
of the platform and the presence of shallow water, under the formation conditions of a
subsidence over an overall slow basin depression.
The delta located at the margins of the southwest Ordos basin of the Yanchang Formation
has always been a hotspot for geologists in China and abroad, especially in recent years. An
oilfield with reserves of 100 million tons was found close to the Xifeng, Huanxian, Zhenbei,
HuaQing areas, promoting additional research on depositional systems, source directions,
sequence filling style and lithofacies paleogeographic distribution. Certain researchers believe
that the research areas were mainly underwater alluvial fans of deposition, whereas others
have advanced the viewpoint that the region was an ordinary river delta as well as a theory
involving fan delta deposits. More recently, the formation of braided river deltas and sandy
debris flows has been proposed. Therefore, to strengthen the study of sedimentary system
types and sedimentary characteristics, it is more advantageous to conduct precise research on
oil and gas exploration and development in the future. Based on field work conducted in the
Yan River and five additional outcrop sections and a detailed description of the cores of more
than 70 wells, it was proposed that the southwest margin of the Ordos basin was full of large
areas of shallow water that persisted across the gentle slope topography. In addition, there were
frequent elevations of lake levels that resulted in a distinctive shallow braided river delta
sedimentary system during the Triassic. In this paper, the sedimentary characteristics of these
areas are discussed.
Regional Geological Background
The Ordos basin belonged to a type of platform structure sedimentary basin. After the
Hercynian movement, the Ordos basin became an inland depression basin during a depositional
stage following the Late Triassic. During this stage, the feature of inconformity was that of a
depocenter and subsidence center, and the center of the sedimentary basin was the same as the
center of the depocenter, which had thin and stable deposits. The subsidence center in the
western basin was accompanied by thick and varying layers of deposits. The study area was
located in the northern middle of the Shanxi slope in the Qingyang nose fold belt, which is a flat
structure, and a small nose-like uplift developed along the western monocline (Fig 1). The area
is characterized by low elevation in the west and high elevation in the east, with sediments
originating from the west and southwest directions. Its system characteristics and the
characteristics of the sediment changed with tectonic subsidence and the lake level during the Yangchang
Fig 2. Tectonic-sedimentary evolution of the Ordos basin (red/yellow represent the oil/gas deposits).
Development Conditions of a Shallow Water Delta
3.1 The area remained in a subsidence stage, with wide areas of basin and a slow slope
The Ordos basin was a craton succession basin that was filled with marine deposits from
the early Paleozoic period that turned into sea-land transition deposits in the late Paleozoic
(green part), as shown in Fig 2. The ocean terminated by the end of the late Paleozoic and
entered the stage of inland lake basin development, and the Yangchang period of the Late Triassic
epoch was a prime period in the development of inland lake basins (Fig 2, blue-gray section),
as indicated by the deposition of thick, high-quality source rocks.
Affected by Indosinian movement, a large and wide area of the water’s surface developed in
the Yangchang period of the Late Triassic in the Ordos basin, which was accompanied by the
formation of a flat terrain and shallow water caused by wet and dry cyclical climate changes. In
addition, the Yimeng uplift, Jinxi flexure belt, Weibei uplift, and thrust belts developed in the
western margin and adjacent ancient land. A system of source reservoir cap rocks developed
from an adequate supply of terrigenous clastic sediments, such as those of lakes and rivers. The
basin formed by enclosing the area until the early Cretaceous, which led to significantly
narrowed areas of sedimentary deposits.
There was only one set of shallow lacustrine facies during the process of steady subsidence
accumulation of the Yanchang Formation in the Late Triassic for the Ordos basin. The age is
equivalent to that of the Carnian to the Rhaetic according to the standard geological age
(228.7–199.6 Ma), and the duration was approximately 28 Ma. The entire Yanchang Formation
was divided into 10 reservoir groups from bottom to top (from top to bottom, the Chang 1,
Chang 2. . . Chang 10 reservoir group, respectively). The main lithology of the Yanchang
Formation was thick massive fine to coarse grain feldspar sandstone. However, the Chang 4 + 5
and Chang 7 formations were mainly composed of mud shale because mud shale was the main
hydrocarbon source rock. The mudstone of the Chang 8-Chang 6 formations were rich in
plant branch fossil fragments, and leaf segments and flap gill class fossils were also found in the
mudstone layer of the Chang 8 formation. Ostracoda as well as scales and fish fossils were
found in the mudstone of the Chang 7 and Chang 6 formations. These fossil assemblages reflect
the relatively shallow lake sedimentary environment.
3.2 Ancient uplift developed in the western and southern marine, with an adequate provenance provided by the humid ancient climate and the strong weathering of parent rock
There was a nearly NS-trending sinistral shear stress field in eastern China that was caused by
the subduction of the Eurasian continent by the Kula Plate in the Mesozoic period, which
resulted in an uplift of northern China in a region currently occupied by the Bohai Bay basin.
The thrust uplift at the western margin of the Ordos thrust belt began in the Early Triassic and
peaked in the Yanshanian period, during which it formed a series of ancient uplifts, including
the southern Qinling ancient land, the western margin of the west Gansu ancient land, and the
Ningxia Liu Panshan ancient land (Fig 3).
The plants of the Longdong region mainly reflected the tropical and subtropical climatic
conditions of evergreen forests in the Middle-Late Triassic. Wet raw ferns flourished with
clubmosses and true ferns, such as Cyatheaceae, Dipteridaceae and Selaginellaceae. The flora were
mainly distributed in tropical and subtropical moist areas and reflected the wet raw
environment. This type of forest vegetation reflected the subtropical climate characteristics of the area.
However, the lack of Classopollis of Cheirolepidiaceae and Ephaedripites of Ephedraceae
pollen during the Middle-Late Triassic indicated a drought-prone environment. Therefore, the
climate of the Longdong areas belonged to a warm-temperate humid subtropical moist climate or
hot and humid climate during the Middle-Late Triassic period, which was warm and
humid and had abundant rainfall. The humid paleoclimate increased the physical weathering
of the peripheral ancient land in the Late Triassic period, and it also provided water for the
formation of large rivers. Under the alluvial action of large rivers, a significant amount of debris
could have provided an adequate provenance for the formation of the shallow water delta.
3.3 The lake level cycle changed dramatically due to ancient water turbulence
There was imperceptible horizontal bedding and a continuous rhythm that developed in the
deep water areas of the basin. The sedimentary marks of a flysch structure, slot mold and ditch
mold indicated deep lake turbidite. The shallow water areas were marked by various types of
bedding, including parallel bedding, bedding and sand grain bedding, such as ripple, mix
Fig 3. Ancient land in the southwestern margin of the Ordos basin.
structure and erosion scour. The sediment produced by water was represented by a
precipitation-induced impression and featured rill mark level structures.
The Longdong region of the Ordos basin showed distinct cycle variation and frequent
changes in lake water level characteristics, based on research related to lithological-electrical
analyses, characteristics of the biomarkers, sedimentary structure and lithology marks, such
as trace fossils (burrow, footprint, and mark up), and other biological disturbance structure
The Chang 8 and Chang 6 formations were composed of a complex sandstone, as indicated by
an outcropped section of the Yanchang Formation in the Ordos basin. This outcrop revealed
lithic arkose, feldspathic litharenite and arkose, with a small amount of feldspar lithic quartz
sandstone (Fig 4). Each component was contained in the sandstone, including quartz (average
32.03%), feldspar (average 31.05%), and a high lithic content (average 36.30%). However, the
Fig 4. Ternary diagram of the sandstone types of the Yanchang group in the southwestern margin of the Ordos basin.
type of rock debris varied. The Kongdong Mountain section mainly contains magmatic rock
debris (average 10.60%) and sedimentary rock debris (average 22.00%), and the Cedipo and
Ruishui River profile section mainly features magmatic rock and metamorphic rock debris.
Compared to the southwest margin of the basin outcrop section, the Yanhe profile section in
the eastern part of the basin showed a single type of sandstone and mineral composition,
mainly feldspar sandstone and lithic feldspar sandstone. Each component was contained in the
sandstone, including quartz and feldspar, with higher quartz (average 31.50%) and feldspar
(41.65%) contents and a lower lithic content (average 6.8%). A large amount of debris consisted
of metamorphic rock and sedimentary rock, and a small portion of magmatic rock
Analysis of rigid and plastic mica and lithic samples from the Yanchang Formation revealed
that debris from the reservoirs was mainly composed of plastic debris, accounting for
approximately 64% of the total debris and mica. Among the debris, mica, eruptive rock debris and
phyllite rock debris were relatively well developed, and each accounted for approximately 24%,
Fig 5. Debris composition of the Yanchang Fm. in the southwestern margin of the Ordos basin.
16% and 24% of the total composition, respectively. The rigid lithic content was observed to be
low, constituting approximately 36% of the total debris and mica content. Dolomite debris and
quartzite debris were relatively well developed and accounted for approximately 10% of the
total composition. The composition maturity was low, with a mean of 0.65 (Fig 5). The
composition maturity of the stratum below the Chang 7 group in the Yanchang Formation was less
than 0.5, whereas that of the upper stratum was relatively high and even reached values greater
Based on the aforementioned data, the Yanchang Formation of the Longdong region
showed a sedimentary environment exhibiting strong water performance and was close to the
provenance. This result is reflected by the relatively low quartz content, high feldspar and lithic
content and lower compositional maturity of the sandstone. The plastic components in the
debris content were relatively high, and the rigid component content was low; in addition, the
content of the matrix was low. The primary rock types were coarse- to fine-grained lithic
arkose and feldspathic lithic sandstone with medium separation, and the separation of the Chang
6 and 8 groups in the Yanchang Formation was relatively poor, with most of the particles
having a subangular shape.
4.2 Sedimentary structure characteristics
The southwest margin of the Ordos basin possesses typical strong hydrodynamic depositional
features, as evidenced by the distinct size of the erosion interface, the directional arrangement
of gravel, trough cross-bedding, wedge cross-bedding, parallel bedding, etc.
The Ruishui River outcrop section is a representative section of the “slender Yanchang
Formation" in the southwest margin of the Ordos basin. The Chang 10 group of the Yanchang
Formation in this section developed the following characteristics: a thick layer of gray, celadon
or massive fine sandstone (40 ~ 100 m), large tabular cross-bedding, and wedge cross-bedding.
The lower formation developed amaranth mudstone unconformably in contact with the
Zhifang Fm., and a wormhole was also visible. A distinct fault developed between the interface
layer, but there was no obvious dislocation. The Chang 9 and Chang 8 reservoirs occurred
mainly on dark gray to gray thick massive sandstone that was clipped with a thin carbonaceous
mudstone layer (20 ~ 30 cm); in addition, large tabular cross-bedding and scour mud gravel
could be observed (Fig 6A). A directional arrangement of gravel, which reached a thickness of
10 cm among the sandstone gravel, had an average particle size of 3~5 cm. Another
carbonaceous mudstone thin layer (20 to 30 cm) was associated with the delta channel sandstone thick
layer. All of these characteristics reflect a shallow sedimentary environment.
All types of features associated with the ancient environment can be commonly found in
cores of the Yanchang Formation in the Longdong area, including a wash surface (Fig 6B, 6C
and 6D), tabular cross-bedding (Fig 6E), wedge cross-bedding (Fig 6F), trough cross-bedding
and parallel bedding. These features mainly appear in the thick sandstone of the distributary
channel. Multiple cross-beddings with laminated flat, tilt progradations, and an oblique
orientation with formation interfaces and formation thicknesses larger than 3 cm all reflect strong
current and shallow sedimentary characteristics.
4.3 Sand body vertical superimposed development features
The branch channel was well developed in the shallow water delta plain and front portion of
the Yanchang Formation. Because of the shallow water, the flat bottom of the river, the
sufficient source supply, and frequent diversions, a continuous positive rhythm occurred from the
direction of the lake basin edge to the center of the lake in the delta plain and front sedimentary
environments, which was accompanied by an increased thickening intercalation. A "without
mud" intermittent positive rhythm developed in the delta plain, and a “within mud"
intermittent positive rhythm mainly developed in the delta front.
The "without mud" intermittent positive rhythm refers to no mudstone (clip) existing
between the two positive rhythm insulation layers, constitutes the discontinuity of the
sedimentary lithology of the scour contact between sand and reflects frequent and strong hydrodynamic
activities. A washing surface existed in the thick sand body, and a gravel directional
arrangement and wedge cross-bedding was developed in the thick sandstone. The "without mud"
intermittent positive rhythm of the sedimentary sequence was mainly developed in the delta plains
and underwater branch channels in the delta front, corresponding to a lithology composed of
gravelly coarse sandstones. Large cross-bedding developed, and a sandstone thickness in a
single layer measuring approximately 0.5–3 m could be found in the Chang 3 group of the Yanhe
profile section (Fig 7A) and the Well Yan 40 (Fig 8).
The “within mud" intermittent positive rhythm indicates the existence of mudstone (clip)
between the two positive rhythm insulation layers, and it reflects the strong hydrodynamic
effect of the process of subaqueous distributary channel sedimentation. The rhythm mainly
developed in the subaqueous distributary channel in the delta front. Medium-sized cross-bedding
was developed, and the main lithology was fine sandstone; the single-layer thickness of
sandstone was 1.5–3.1 m. A series of environmental indicators was observed, including a wash
surface, directional arrangement of sandstone gravel, wedge cross-bedding in fine sandstone or
wavy parallel bedding, cross-bedding among fine sandstone, silty mudstone or argillaceous
siltstone with horizontal bedding, bottom rock interface contact with underlying sandstone at the
Fig 6. Typical sedimentary structure of the shallow water delta of the Yanchang Fm. in the southwestern margin of the Ordos Basin. A: Ruihe profile
section, including the thick sandstone located at the bottom of the Chang 8 formation and gravel directional arrangement; B: Well Yan 22, TVD2577.5 m,
Chang 6 formation, directional distribution of mud stone and gravel layers; C: Well Huan 73, TVD2392 m, Chang 8–1 formation, wash surface structure and
sandstone gravel at the bottom in a directional arrangement; D: Well Huan 27, TVD269 9.96 m, Chang 8–1 formation, scour surface at the bottom of the
sandstone; E: Well Zhen 9, TVD 2548.3 m, Chang 8–1 formation, large wedge cross-bedding; and F: Well Li 92, TVD 1819.3 m, Chang 3–3 formation, wedge
cross-bedding and parallel bedding.
wash surface, and upper sandstone contact with the overlying sandstone or mudstone. These
features could be found in Well Li 56 (Fig 7B) and the Chang 8–1 group of the Yanhe profile
section (Fig 7B).
Fig 7. Detected outcrop profiles illustrating the composition of the submarine distributary sand bodies in the Yanhe section.
4.4 Plane distribution characteristics of a sand body
The provenance of the Yanchang Formation in the Longdong region is derived from the
southwest shallow braided delta of the Ordos basin. The delta front is a composite of 3–7 underwater
branch channels, each of which could be split into 2–4 secondary branch rivers. Underwater
branch channels presented branching and were mixed in the distribution. The single channel
sandstone had a thickness of 0.8–6.6 m, the cumulative thickness of the channel sandstone was
approximately 4–65 m, and the sand stratum ratio was 5–90%.
Single branches could extend over long distances in the direction of the central of basin.
During the basin’s peak stage of evolution, the lake level increased and the lake surface area
was large. The distance from the delta front to the center of the basin was relatively short at
Fig 8. Sedimentary cycles of the shallow braided delta of the Yanchang Fm. in the Longdong area of the Ordos basin (Wll Yan40).
9–46 km. During the evolution of the basin, a no-peak stage occurred and the lake level
declined. The area of the lake was relatively small and had a shallow water level, and the distance
from the delta front to the center of the basin was relatively long at 18–100 km (Fig 9). In short,
the Yanchang Formation had the underwater distributary channel sedimentary characteristics
of a delta front in a shallow water environment, which was revealed by an underwater
distributary channel with a thick sand body that extended over long distances; this finding suggests the
presence of adequate supplies, shallow water, and strong hydrodynamic forces.
4.5 The seismic response characteristics
During the Yanchang Formation sedimentary period, the Ordos basin was in the depression
stage, accompanied by a shallow water level, a large amount of water space and adequate
supplies. In addition, there were frequent fluctuations in the lake level; therefore, a braided delta
with coarse grains developed under strong hydrodynamic effects in the Longdong region.
Several types of seismic progradation reflections indicated the delta sediment transfer direction,
which pointed to the center of the basin. The foreset form represented the delta sedimentary
types, and the foreset shape indicated the sedimentary environment with hydrodynamic effects
as well as a terrain slope that was steep or slow. The study area’s progradation of seismic
reflection appeared relatively flat and typically ranged from 2 to 4 in the phase axis, with a strong
seismic reflection amplitude. In addition, the reflection showed better continuity with a low
angle and extended over long distances (up to 50 km). The slope progradation seismic
Fig 10. Seismic reflection of the shallow braided delta of the Yanchang Fm. in the Longdong area (line:glq_04glq03b_stk).
reflection indicated a braided delta sedimentary environment with a shallow water level and
strong hydrodynamic forces. It could be concluded from the seismic profile (Fig 10) that the
seismic response of the breadth of the braided river delta could reach 3–5 km, which was composed
of one lenticular compound and domal structures that were reflected in the seismic response.
All of the seismic reflections could be found in Well 76 (TVD2007.1–2020.0 m) of the Chang 6
group of the Yanchang Formation by drilling. A typical delta front underwater branch river with
intermittent positive rhythm was observed in the sedimentary sequence; from bottom to top, the
sequence transitioned from sandstone or mudstone with a boulder wash surface, to
mediumsized fine sandstone with wedge cross-bedding, to sandstone with visible parallel bedding. The
thickness of the intermittent positive rhythm was approximately 0.5–5.6 m.
Sedimentary Model of the Shallow Braided Delta
The braided delta had a "small plain, big front” distribution and the vertical evolution
characteristics of the Yanchang Formation in the Longdong region of the Ordos basin. The delta’s
sedimentary evolution was clearly controlled by tectonic movement and lake level changes.
Due to the distinct lake level changes during the period of lake water levels, branch channels
extended to the central basin; however, the branch channels were reformed by the increased
lake levels, which resulted in a braided delta with "small plain, big front” sedimentary
The Yanchang Formation of the Ordos basin formed in a complete cycle of the inland
depression lake basin, which experienced 4 complete return stages from the initial depression
Fig 11. Schematic sedimentary model showing the evolution of the Yanchang Formation in the southwestern margin of the Ordos basin.
stage as well as a severe depression stage to an uplift shrinking vanished stage. The
sedimentary system scale, sedimentary characteristics and evolution process of the shallow water
braided delta were controlled by tectonic evolution (Fig 11).
During the Chang 10-Chang 9 sedimentary period, the Ordos basin was under continuous
stress, which was inherited from the Early Triassic to Middle Triassic. The Ordos basin
experienced an initial stage characterized by a lake basin depression; then, the basin began to develop,
and the initial shape of lakes formed within shallow water levels. The study area is close to a
provenance. Plenty of detrital material was deposited in the local area[33,39], and a shallow
lake sedimentary system developed in the study area. Four braided river delta fronts were
distributed from the northwest to southeast on the plain. The width of the braided river delta
front was approximately 40 km in the southwestern part. The formation thickness was 31–90
m, the sand body thickness was 8–28 m, and the sand formation ratio was approximately 20–
50%; however, the local area was composed almost entirely of sandstone (Yan 47, the ratio
could reach up to 100%).
During the Chang 8 sedimentary period, the lake basin had a widened scope and large water
depth, and it was characterized by a large area of lakeside swamping that was associated with
the braided river delta channel sand body and a thin coal layer. The braided delta plain was
mainly composed of coarse sandstone gravel, sandstone gravel and peat swamp deposits. The
plain distributary channel deposit was mostly typical, with multiple superpositions at the
bottom of the scour surface, developmental groove cross-bedding, wedge cross-bedding and
parallel bedding. Overlying the thin layer (less than 1 m) of a coal seam was the sedimentation of
the vertical structure of the discontinuous depositional sequence (Fig 12).
The braided delta front mainly developed in alternating layers of fine sandstone and
argillaceous siltstone and was characterized by a common scour surface, parallel bedding, trough
cross-bedding, wavy cross-bedding, slump deformation structure, structure and plant debris
bones wormholes. In the vertical direction, sedimentation particles of up to a fine size were in
contact with the underlying dark mudstone mutation of the sedimentary sequence, especially
in the long 8 reservoir group of the common coal core wire (Fig 12).
Fig 12. Sedimentary cycles of the shallow braided delta of the Yanchang Fm. in the Longdong area of the Ordos basin (Wll Li56).
During the Chang 7 sedimentary period, the basin water level sharply increased and the
basin area expanded significantly when a set of large thickness dark mudstones with high
organic and shale contents was developed (Zhang Jiatan shale); in addition, the delta receded to
the southwest provenance, and the delta area receded less than 40 km in the study area.
During the Chang 6 to Chang 4+5 sedimentary period, the lake basin returned to the uplift
stage and began to shrink. This process was associated with a gradually increasing supply, and
a series of constructive river-lake delta sedimentary sand was developed in the basin. The Wu
Qi delta extended from the northeast to the Baibao region, and a set of thick sand layers was
deposited in the meandering river delta front.
During the Chang 3 to Chang 1 sedimentary period, the lake basin entered the shrinking to
dying stage. Tectonic activity decreased significantly, and the lake basin filled with ooze and
then atrophied and died gradually. In addition, during the Chang 3 sedimentary period, a
significant deltaic progradation occurred on the margin of the basin, and the delta immigrated
from the southwest to the lake center. During the Chang 2–1 sedimentary period, due to the
strong denudation followed by the huge uplift of the basin, the southwest margin of the basin
was nearly denuded, and the northwest section of the reservoir remained incomplete. A
significant portion of the basin was swamped, and a coal seam and wide coal line developed. The
entire lake basin was submerged and buried.
During the Yanchang Formation period in the late Triassic, a shallow braided river delta
developed under favorable geological conditions, i.e., shallow water and a large-area basin with a flat
terrain and wide and shallow water areas. In addition, there was a wet and dry cyclical climate,
ancient water turbulence, dramatic changes in the depth cycle, ancient uplift development,
strong weathering on the parent rock, and abundant supply.
An underwater distributary channel and mouth bar developed in the shallow braided river
delta but were not well developed in the Yanchang Formation. The characteristics of these
structures were indicative of a thin coal seam line (coal). All of the abovementioned features
indicated a shallow water braided river delta with grain sediment granularity, plastic debris, and
sediment of mature composition and structure that reflected the strong hydrodynamic
environment of large tabular cross-bedding, wedge cross-bedding, multiple positive rhythms
superimposed to form a thick layer of sand, and procreation of seismic reflections that appeared to
be relatively flat and low-angle.
During the development of the shallow braided river delta in the Yanchang Formation
sedimentary period, the sedimentary center was close to a provenance, the width of the river was
even greater, a shallow sedimentary structure and a sedimentary rhythm developed, and delta
development was mainly controlled by tectonic activity and changes in lake levels. As a result,
a "small plain, big front" characteristic was eventually developed in the shallow braided river
delta sedimentary system.
Conceived and designed the experiments: ZL XZ. Performed the experiments: ZL. Analyzed
the data: ZL XZ FS. Wrote the paper: ZL FS. Map digitizing: FL MT.
1. Fisk HN ( 1954 ) Sedimentary framework of the modern Mississippi Delta . Journal of Sedimentary petrology 24 ( 2 ): 76 - 99 .
2. Donaldson AC ( 1974 ) Pennsylvanian sedimentation of central Appalachians . Thegeological Society of America 148(special paper) : 47 - 48 .
3. Postma G ( 1990 ) An analysis of the variation in delta architecture . Terranova 2 ( 2 ): 124 - 130 .
4. Lemons DR , Chan MA ( 1999 ) Facies architecture and sequence stratigraphy of fine-grained lacustrine deltas along the eastern margin of late Pleistocene Lake Bonneville, northern Utah and southern Idaho . AAPG Bulletin 83 ( 4 ): 635 - 665 .
5. Boris K , Andreas B , Thomas A ( 2005 ) 3-D sedimentary architecture of a Quaternary gravel delta (SWGermany): Implications for hydrostratigraphy . Sedimentary Geology 181 ( 3-4 ): 147 - 171 .
6. García-García F , Fernández J , Viseras C , Soria JM ( 2006 ) Architecture and sedimentary facies evolution in a delta stack controlled by fault growth (Betic Cordillera, southern Spain, late Tortonian) . Sedimentary Geology 185 ( 1-2 ): 79 - 92 .
7. Cornel O , Janok PB ( 2006 ) Terminal distributary channels and delta front architecture of river-dominated delta systems . Journal of sedimentary research 76 : 212 - 233 .
8. Lee K , Mcmechan GA , Gani MR , Battacharya JP , Zeng X , et al. ( 2007 ) 3-D architecture and sequence stratigraphic evolution of a forced regressive toptruncated mixed-influenced delta, Cretaceous Wall Creek sandstone , Wyoming, USA. Journal of Sedimentary Research 77 ( 4 ): 284 - 302 .
9. Longhitano SG ( 2008 ) Sedimentary facies and sequence stratigraphy of coarse-grained Gilbert-type deltas within the Pliocene thrust-top Potenza Basin (Southern Apennines, Italy) . Sedimentary Geology 210 ( 3-4 ): 87 - 110 .
10. Gong SL ( 1986 ) Shallow delta and coal environment of upper Permian in Yuxian,Henan . Coal Geology & Exploration (06: ) : 2 - 9 .
11. Su CQ , Liu JQ ( 1993 ) Shallow Delta system of the carboniferous system in hulusitai . Journal of Changan university 15 (S1): 23 - 28 .
12. Li ZX , Wei JC , Li SC ( 1995 ) The depositional system of fluvial-controlled shallow water delta and coalaccumulation analysis in western Shandong . Coal Geology & Exploration 15 ( 02 ): 7 - 13 .
13. Lou ZH , Lan X , Lu QM , Cai X ( 1999 ) Controls of the Topography, Climate and lake level fluctuation on the depositional environment of a shallow-water delta-a case study of the Cretaceous Putaohua reservoir in the northern part of Songliao basin . ACTA Geologica Sinica 73 ( 01 ): 83 - 92 .
14. Wang GH , Chen HH , Jiang T , Zhao BF , Xu DH , et al. ( 2012 ) Sandbodies Frameworks of Subaqueous Distributary Channel in Shallow-Water Delta, Xinli Area of Songliao Basin . Earth Science-Journal of China University of Gensciences 37 ( 03 ): 556 - 564 .
15. Zhao BF , Wang JH , Xu DH , et al. ( 2012 ) Semi-quantitative Research on Subaqueous Distributary Channel Sandbodies of the 3th Member of Nenjiang Formation in Xinli-XinbeiArea, Songliao Basin . ACTA Sedimentologica Sinica 30 ( 03 ): 511 - 521 .
16. Liu Y , Zhu XM , Yuan HQ , et al. ( 2010 ) New explanation for shallow delta high-resolution sequence stratigraphy of Putaohua oil-bearing layer of member 1 of Yaojia formation of Cretaceous in Sanzhao sag . Journal of China University of Petroleum (Edition of Natural Science) 34 ( 04 ): 12 - 18 .
17. Ya DJ , Zhang JL , Zhao HJ ( 2011 ) Shallow-water delta sedimentary characteristic and evolution of the 1st member of Yaojia formation, Heidimiao area . Periodical of Ocean University of China (Science &Technology Edition) (S1: ): 299 - 304 .
18. Xiao DS , Lu SF , Chen HF , et al. ( 2011 ) Sedimentary Characteristics and Model of Shallow Delta of Quan-4 Member in the West of Daqing Placanticline . Science Technology and Engineering 11 ( 21 ): 4986 - 4992 .
19. Sun Y , Ma SZ , Jiang HF , et al. ( 2010 ) Sedimentary Mode of Shallow Lacustrine Fluvial-Dominated Delta of Putaohua Reservoirs in the Sanzhao Depression, Songliao Basin . ACTA Geologica Sinica 84 ( 10 ): 1502 - 1509 .
20. Li Y , Zhu XM , Song YQ , et al. ( 2013 ) Sendimentary Characteristics and Evolution of Shallow-water Delta of the Lower Cretaceous Fuyu Reservoir in the Yushulin Oilfield , Songliao Basin . Geological Journal of China Universities ( 01 : ): 23 - 31 .
21. Wang JG , Wang TQ , Wei PS , et al. ( 2007 ) Sedimentary mode of shallow lacustrine delta of large continental basin-An example from Putaohua Formation in northern Songliao Basin . Northwest Oil & Gas Exploration ( 02 : ): 28 - 34 .
22. Wang LW ( 2012 ) Forming Conditions and Depositional Characteristics of Shallow-water Deltas in Depression Basins: A case study of K2yl in the south of Songliao Basin . ACTA Sedimentologica Sinica 30 ( 06 ): 1053 - 1060 .
23. Zhu XM , Liu Y , Fang Q , et al. ( 2012 ) Formation and sedimentary model of shallow delta in large-scale lake.example from Cretaceous Quantou Formation in SanzhaoSag, Songliao Basin . Earth Science Frontiers 19 ( 01 ): 89 - 99 .
24. Lu J , Lu YF , Liu ZB ( 2010 ) Sedimentary Features and Oil Accumulaltion Mode of Shallow-water Delta of Putaohua Oillayer in Sanzhao Depression . Science Technology and Engineering 10 ( 34 ): 8407 - 8412 .
25. Zhu XM , Deng XQ , Liu ZL , et al. ( 2013 ) Sedimentary characteristics and model of shallow braided delta in large-scale lacustrine: An example from Triassic Yanchang Formation in Ordos Basin . Earth Science Frontiers 20 ( 02 ): 19 - 28 .
26. Zhang XM , Yin GR , Hu R , et al. ( 2011 ) Sedimentary Characteristics and Well Spacing Methods of Shallow Water Delta-An Example from TiebianchengArea . Journal of Chongqing University of Science and Technology (Natural Science Edition) 13 ( 04 ): 47 - 51 .
27. Li J , Chen HD , Lin LB , et al. ( 2011 ) Ggenesis and distribution pattern of shallow water delta sandbodies in Member 8 of lower Shihezi Formation in the northwest Ordos basin . Journal of Chengdu University of Technology (Science & Technology Edition) 38 ( 02 ): 132 - 139 .
28. Liu ZL , Zhu XM , Liao JJ , et al. ( 2013 ) Sequence stratigraphy and genesis of sand bodies of the Upper Triassic Yanchang Formation in the southwestern margin of Ordos Basin . Earth Science Frontiers 20 ( 02 ): 1 - 9 .
29. Wang CY , Zheng RC , Tian YQ , et al. ( 2011 ) Sedimentary characteristics and hydrocarbon accumulation law of shallow-water delta of chang9 reservoir in longdong area . Journal of Oil and Gas Technology 33 ( 08 ): 11 - 15 .
30. Liu LH , Zhu RK , Luo P , et al. ( 2009 ) Characteristics and Depositional Models for the Shallow-water Deltas of the 5th-6th Interval , Xujiahe Formation , Upper Triassic in Central Sichuan Basin,China. Geoscience 23 ( 04 ): 667 - 675 .
31. Guo ZM ( 1989 ) Tectonic characteristics and oil-gas exploration of Shanganning Basin . Beijing: Petroleum Industry Press.
32. Zhang SN , Hu JN , et al. ( 2000 ) The sedimentary characteristics of Yanchang Formation in Zhenyuan and Jingchuan districts, Southern part of Ordos Basin . Journal of Mineralogy and Petrology 20 ( 4 ): 25 - 30 .
33. Luo JL , Li ZX , Shi CG , et al. ( 2008 ) Depositional systems and provenance directions for the Chang 6 and Chang 8 reservoir groups of the Upper Triassic Yanchang Formation in the southwestern Ordos basin , China. Geologcal Bulletin of China 27 ( 1 ): 101 - 111 .
34. Li XB , Fu JH , Chen QL , et al. ( 2011 ) The Concept of Sandy Debris Flow and Its Application in the Yanchang Formation Deep Water Sedimentation of the Ordos Basin . Advances in Earth Science ( 03 : ): 286 - 294 .
35. Hu JY ( 1992 ) Theoretical basis of Nonmarine petroleum geology of China . Beijing: Petroleum Industry Press.
36. Zhai GM ( 1992 ) Petroleum Geology of China (volume12) . Beijing: Petroleum Industry Press.
37. Ji LM , Meng FW , Wu T , et al. ( 2006 ) Paleoclimatic Characteristics During Sedimentary Period of Main Source Rocks of Yanchang Formation (Triassic) in Eastern Gansu . ACTA Sedimentologica Sinica 24 ( 03 ): 426 - 431 .
38. Yang H , Liu ZL , Zhu XM , et al. ( 2013 ) Perovenance and depositional systems of the Upper Triassic Yanchang Formation in the southwestern Ordos Basin . China. Earth Science Frontiers 20 ( 2 ): 010 - 018 .
39. Liu ZL , Wang DY , Wang F , et al. ( 2005 ) The microfacies combination and characteristic of main sandbodies in Xifeng oilfield, Shan-gan-ning basin . Acta Sedimentologica Sinica 24 ( 02 ): 248 - 254 .