Origin and depositional model of deep-water lacustrine sandstone deposits in the 7th and 6th members of the Yanchang Formation (Late Triassic), Binchang area, Ordos Basin, China
Origin and depositional model of deep-water lacustrine sandstone deposits in the 7th and 6th members of the Yanchang Formation (Late Triassic), Binchang area, Ordos Basin, China
Xi-Xiang Liu 0 1 2 3
Xiao-Qi Ding 0 1 2 3
Shao-Nan Zhang 0 1 2 3
Hao He 0 1 2 3
0 College of Energy, Chengdu University of Technology , Chengdu 610059, Sichuan , China
1 School of Geoscience and Technology, Southwest Petroleum University , Chengdu 610500, Sichuan , China
2 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University , Chengdu 610500, Sichuan , China
3 Second Production Plant, PetroChina Changqing Oilfield Company , Qinyang 745000, Gansu , China
Sandstones attributed to different lacustrine sediment gravity flows are present in the 7th and 6th members of the Yanchang Formation in the Ordos Basin, China. These differences in their origins led to different sandstone distributions which control the scale and connectivity of oil and gas reservoirs. Numerous cores and outcrops were analysed to understand the origins of these sandstones. The main origin of these sandstones was analysed by statistical methods, and well logging data were used to study their vertical and horizontal distributions. Results show that the sandstones in the study area accumulated via sandy debris flows, turbidity currents and slumping, and sandy debris flows predominate. The sandstone associated with a single event is characteristically small in scale and exhibits poor lateral continuity. However, as a result of multiple events that stacked gravity flow-related sandstones atop one another, sandstones are extensive overall, as illustrated in the cross section and isopach maps. Finally, a depositional model was developed in which sandy debris flows predominated and various other types of small-scale gravity flows occurred frequently, resulting in extensive deposition of sand bodies across a large area.
Sediment gravity flows; Sandy debris flows; Binchang area; Yanchang Formation; Ordos Basin
& Xiao-Qi Ding
Discussion of the origin of deep-water sandstones began
with ‘‘Turbidity currents as a cause of graded bedding’’
(Kuenen and Migliorini 1950). Then, Bouma (1962)
proposed the ‘‘Bouma sequence’’, which summarized the
vertical sequence of sedimentary structures in turbidites
and became the criterion for recognizing turbidites. Based
on these criteria, other scholars contributed valuable
insights related to this type of deposit and improved
turbidity current theory (Yang et al. 2015a). In the same time
frame, many depositional models were developed, such as
the ancient fan model, the modern submarine fan model
and the general fan model (Shanmugam 2000). All these
models suggest that turbidity-related sand bodies feature
fan-shaped deposits. This consensus has been widely
applied in the exploration of deep-water sandstone
reservoirs. However, as more deep-water sandstone reservoirs
have been discovered, the turbidity current theory has
begun to fall short. Shanmugam (1996) noted that high
particle concentrations ([30% by volume) lead the support
mechanism in the flow change to disperse pressure and
viscosity of clay minerals which essentially represent the
physical properties of the sandy debris flow. Other scholars
noted that sandy debris flows tend to be deposited in the
form of lumps, which modelled as interrupted sand bodies
(Shanmugam and Moiola 1997) and tongue-like sand
bodies (Zou et al. 2012). Distribution patterns of these
deposits are entirely different from those of turbidites.
Recently, scholars described a special type of turbidity
current called hyperpycnal flow (Mulder et al. 2003; Zavala
et al. 2011). Unlike turbidity currents, hyperpycnal flow is
of significantly long duration because it is caused by
seasonal floods, snow-melt floods and melting glaciers
(Huneke and Mulder 2011). Therefore, the resulting
sandstones are interrupted less than those related to waning
turbidity currents or sandy debris flows and show
gradational changes in the vertical and the horizontal direction
(Zavala and Arcuri 2016).
The above analyses suggest that reservoir characteristics
such as the sand distribution pattern, scale and continuity
of these various deep-water sandstones differ significantly
because of variations in transport and depositional models.
The 7th and 6th members of the Yanchang Formation in
the Binchang area of the Ordos Basin were deposited in a
deep-water environment (Lei et al. 2015) and are the main
production formation of tight oil in China (Yang et al.
2012; Ding et al. 2013; Yuan et al. 2015). Understanding
the origin, distribution pattern, scale and continuity of this
sandstone is important, and these factors must be
considered during exploration and decision-making in the
development of the oil field.
This paper aims to discuss the main origin of sand
bodies by summarizing the sedimentary characteristics
observed in cores and outcrops and then to determine the
scale and distribution pattern based on well logging
information. Finally, it aims to construct a depositional
model for these deep-water lacustrine sandstones and
provide a reference for further exploration and
development of this area.
2 Geological background
The Ordos Basin is a multi-episodic cratonic basin in
central China that covers an area of 370,000 km2. The
basin is bordered by the Yin Mountain on the north, the
Liupan Mountain on the west, the Lu¨liang Mountain to the
east and the Qin Mountain on the south. The study area is
located around Xunyi and Binxian, near the Weibei uplift,
on the southern Yi-Shan slope in the Ordos Basin (Fig. 1a–
c). The basement of the Ordos Basin is Archaeozoic and
lower Proterozoic metamorphic rocks. The basin has
accumulated sediments since the Mesoproterozoic,
resulting in a sediment column that is 5,000–10,000 m thick. The
sedimentary rocks in the basin that predate the Permian
were deposited under marine conditions (He 2003). By the
middle Permian to Late Triassic, the northern China Plate
and Yangtze Plate began to collide and merge (Deng et al.
2013; Yang and Deng 2013). This collision caused the Qin
Mountains to be uplifted and the residual marine basin to
close. The collision also resulted in southward migration of
the basin centre and steepened the bottom topography in
the southern portion of the basin. By the Late Triassic,
when deposition of the Yanchang Formation began, the
basin was a deltaic–lacustrine depositional system
(Fig. 1b) with a 900–1600-m-thick set of clastic rocks that
recorded a complete cycle of lacustrine basin initiation,
development and cessation (Fu et al. 2013; Liu et al.
2015a). The formation can be divided into 10 members
(Fig. 2). Member 10 was deposited during the initial
creation of the lake. Members 9 and 8 were deposited during
an episode of major transgression. Member 7 was
deposited during an episode of accelerated subsidence, when the
water depth increased from 50 to 120 m, causing
significant lake expansion. Laterally extensive black shale was
deposited during this time (Lei et al. 2015). Deep-water
sandstones of Member 7 were deposited on top of the black
shale. The lake began to shrink during the deposition of
Member 6. Members 5–1 were deposited during a period of
major lake contraction, after which the lake gradually
disappeared. Adequate coring data from the Yanchang
Formation and outcrops around the study area make it an
ideal place to study the origin of deep-water lacustrine
3 Origin analysis of deep-water sandstones
3.1 Facies and interpretation
Based on the observation of 42 cored wells and outcrops
near the study area, 7 typical facies were identified
according to their lithologies and sedimentary
characteristics (Table 1).
3.1.1 Facies A
Facies A is composed of black-dark grey mudstone and
shale. Silty laminaes are ubiquitous in the mudstone
(Fig. 3a). The thickness of the mudstone (or shale) layer
reaches 40 m in the study area.
The thick black mudstone reflects a reducing and weak
hydrodynamic condition in the deposition area. This facies,
therefore, represents a deep-water environment.
3.1.2 Facies B
Facies B is composed of light to dark grey fine-grained
sandstone with an individual bed thickness ranging from 2
to 3 m. No significant grading is observed. Abrupt contacts
with mudstones are present at both the bottom and top
surface of sandstones. Most of the top contact surfaces are
irregular (Fig. 3b). Dark muddy clasts are ubiquitous and
Jh37 Jh67 Zhengning
The study area
109°40′ 111°40′ Floodplain
usually exhibit a planar fabric (Fig. 3c). Yellow muddy
clasts are also present, some of which are coated with black
mudstone (Fig. 3d). Unlike clasts commonly found at the
bottom of a layer in a fluvial environment, these clasts are
primarily found in the middle or upper parts of sandstones.
The presence of clasts in the middle or upper parts of
sandstones indicates the viscosity and density of the
transporting fluid, which was able to support mudstone
clasts and allow grains to deposit via freezing en masse and
without grading. This is typical of sediments transported by
plastic sandy debris flows (Zou et al. 2012; Shanmugam
2013; Xian et al. 2013). The planar fabric of the clasts
further supports the hypothesis that the sediment was
transported by plastic flows (Talling et al. 2012a; Yang
et al. 2014). The presence of mudstone coats can be
attributed to muddy slurry adhering to the clasts as they
were rolled or spun by the shear stresses along their upper
and lower surfaces (Li et al. 2014, 2016). These features
also indicate that the deposits did not result from a turbidity
current (with water between grains). Therefore, it can be
concluded that this facies is the product of sandy debris
3.1.3 Facies C
Facies C is composed of light grey sandstone lacking
grading, sedimentary structures or muddy clasts. The
bottom and top of the sandstone are typically abrupt contacts
with mudstones. However, the bottom contact surfaces are
very smooth and the top contact surfaces are rough and
irregular (Fig. 4). In outcrops, this facies displays abrupt
lateral terminations with tongue-like shapes (Fig. 5a).
Massive sandstones such as these could have been
deposited by grain flow, sandy debris flow or
hyperpycnal flow (Bagnold 1956; Zavala 2006; Li et al. 2011a;
Gao et al. 2012). Hampton (1975) concluded, based on
experimentation, that a clay content of less than 2% is
required for the development of grain flow. However, the
thin section observation and X-ray diffraction analysis
show that the average clay content in these sandstones is
as high as 8.6%, which suggests that these sandstones did
not form via grain flow. Hyperpycnal flow, however, is
quasi-steady flow that produces sand bodies with good
lateral continuity from the source to the depositional
district (Tan et al. 2015; Yang et al. 2015b). However,
Eolian and alluvial
Fluvial and delta
Fig. 2 Stratigraphic column showing the Mesozoic stratigraphy, lithology and depositional environments in the study area, southern part of the
Ordos Basin, central China
the sand bodies in the study area are generally
characterized by small scales and poor continuity (see
chapter 4.2). Hyperpycnal flows significantly erode the
underlying layer, but sandstones observed in cores and
outcrops in the study area display typically smooth
contacts with underlying layers. Smooth bottom contacts
are more likely produced by hydroplaning associated
with sandy debris flows (Marr et al. 2001). The abrupt
termination of sandstones and the irregular contact
surfaces with overlying layers also indicate that the
sediment was deposited by the freezing en masse of a plastic
sandy debris flow (Talling et al. 2012b; Shanmugam
2016). Therefore, these massive sandstones are more
likely the result of sandy debris flow deposition.
Table 1 Characteristics of different facies in the study area
Facies Lithology and sedimentary structure
Black-dark grey mudstone or shale. Silty laminae are ubiquitous
Light grey massive sandstones. Their bottom and top often are in abrupt contact with mudstones. The bottom contact surface usually
very smooth and the top contact surface rough and irregular
Light grey fine-to-silt sandstones or mudstones with lenticular, flaser or wavy bedding and normal grading can be observed. Their
bottom often has abrupt contact with mudstones
Grey sandstones with deformation structures, such as glide planes, steep fabric and slump folds
Fig. 4 Photographs of facies C showing massive sandstone with an irregular top surface and a smooth bottom surface, well Jh25,
Mud clasts with planar fabric
3.1.4 Facies D
Facies D is composed of grey fine to silty sandstone with
parallel bedding and small-scale cross-bedding. Weak
normal grading can be observed. The thickness of facies D
is no more than 10 cm. It typically overlies facies B or
facies C (Fig. 5b–d).
The parallel bedding or cross-bedding is related to the
traction process. However, the scale of the traction
structure is small which reflects the weak hydrodynamic
condition and a small impact area. Additionally, normal
grading usually indicates that this facies is associated with
turbidity currents (Mulder and Alexander 2001). Therefore,
we conclude that as a debris flow advanced through the
water, shear stress generated along the upper boundary of
the water–slurry interface. Then, the shear stress leads to
erosion and entrainment of sediment from the surface of
the debris flow into the clear water above (Marr et al.
2001). The entrainment of sediment into the overriding
clear water resulted in the formation of a dilute subsidiary
turbidity current and a traction ability in the surrounding
water. Thus, this facies with weak normal grading and
small traction structure is generally observed above facies
B and facies C and could be a criterion for identify sandy
debris flow of different stages. This phenomenon has been
observed by other scholars (Yang et al. 2014; Liu et al.
3.1.5 Facies E
Facies E is composed of light grey fine to silty sandstone or
mudstone with lenticular (Fig. 6a), flaser (Fig. 6b) or wavy
bedding (Fig. 6c). Most of the sand beds are no more than
1 m thick. A transition from flaser bedding at the bottom to
lenticular bedding at the top can be observed where the
sand body is thick enough. Normal grading can also be
observed, and the bottom of the sandstone has abrupt
contacts with mudstone (Fig. 6).
The wavy, flaser and lenticular bedding are all related to
traction processes (Mulder and Alexander 2001), and the
Sandy debris flow
Jh58 Cored well
normal grading suggests that the sediments were deposited
by waning turbidity currents that deposited coarse-grained
material first followed by fine-grained material (Li et al.
2011b, c; Pu et al. 2014). However, as in facies D, these
beds are thin, which indicates that the energy of the
currents was low. Therefore, we conclude that the deposits
may have been formed by subsidiary turbidity currents
transformed from sandy debris flows.
3.1.6 Facies F
Facies F is composed of grey sandstone with deformation
structures such as glide planes (Fig. 7a), steep fabric
(Fig. 7b) and slump folds (Fig. 7c). The bottoms and tops
are typically abrupt contacts with mudstones, which also
display deformation structures (Fig. 7). The deformed
layers could reach 4 m in thickness.
Table 2 Characteristics of sandstones of various origins in the Binchang area, Ordos Basin, China
Sandy debris flow
Fig. 9 Thicknesses of sandstone bodies of various origins
The slump folding suggests that the sediment
experienced soft-sediment deformation. The glide plane is a
dislocation in the sedimentary structure or bedding, which
suggest internal soft-sediment deformation. The steep
fabric reflects that the sandstone experienced slumping
(Herrington et al. 1991). Thus, this facies is attributed to
synsedimentary sand slumping.
3.1.7 Facies G
Facies G is composed of black mudstone and light grey
silty mudstone that is characterized primarily by steeply
dipping mud layers (steep fabric) and sandstone injection
(Fig. 7d). This facies is usually adjacent to facies F
The black mudstone indicates deposition in a deep-water
environment. In this environment, the steeply dipping mud
layers can only be attributed to mud slumping. The injected
sand is also related to loading induced by slumping
(Shanmugam 2012). This facies is therefore the product of
3.2 Spatial distribution of sandstones
The sedimentary facies control the distribution of the sand
bodies, and the origin of the sandstones can be deduced
from the geometry of the sandstone beds. Based on the
analysis of logging data in cored wells, a logging
identification standard has been established. With the
identification standard, sand bodies with a thickness of at least
1 m were distinguished. Then, sand-thickness contour
maps of the various submembers were prepared.
The isopach map illustrates the localized thickening and
thinning that is common in these beds. Sand bodies thicker
than 5 m exist in the form of lumps and display rapid
lateral thickness change (Fig. 8). Amy et al. (2005) noted
that this horizontal distribution of sand bodies is a typical
characteristic of sandy debris flows. Moreover, the thick
sand bodies distributed primarily in the north-eastern part
of the study area, far from the sedimentary sources. This
suggests that the sandstone may not have been transported
by waning traction flows whose sediment thickness
decreases with the transporting distance (Shanmugam
3.3 Origins of deep-water sandstones
From the above analyses, black mudstones (shales) are
widely distributed, and the coarsest grains in the 7th and
6th members of the Yanchang Formation are fine sand,
which indicate weak hydrodynamic conditions. The rocks
lack large-scale bedding, indicating weak traction
processes. The lateral continuity of the sandstone bodies is
poor, which suggests that the sand was not transported by
flows of long duration. The thick sandstone beds display
abundant characteristics of plastic flow transport such as
massive sandstones (facies B), mud clasts with planar
fabrics, irregular upper contacts and mud coatings.
Additionally, previous investigators noted that during the
4.1 The main origin of deep-water sandstone
deposition of the Yanchang Formation, strong tectonic
movements such as earthquakes and volcanic eruptions
occurred frequently; these events could trigger gravity
flows (Deng et al. 2013; Zhang et al. 2014). Thus, we
conclude that the deep-water sandstones of the 7th and 6th
members of the Yanchang Formation are the product of
sediment gravity flows including sandy debris flows,
subsidiary turbidity currents and sandy slumping (Table 2).
Based on the above-mentioned standard, the facies of the
deposits in the study area were identified by observing
715.3 m of cores collected from 42 cored wells. In total,
the black mudstones are 273.1 m, sandstones attributed to
sandy debris flows are 354.5 m, turbidite sandstones are
65 m, and the sandstones attributed to sand slumping are
22.7 m. In terms of thickness, the 6th and 7th members of
the Yanchang Formation are dominated by sandy debris
flow sandstones followed by turbidite sandstones. The
slump sand bodies are present in only a few wells.
A statistical analysis of the thicknesses of single-event
sand bodies attributed to sandy debris flows shows that the
layers are generally less than 2 m thick (Fig. 9, blue bars).
However, some thick sandstone bodies of this type are
developed in local areas.
The total thickness of the turbidite sandstones is small,
and most of the single-event sand bodies of this type are
less than 1 m thick (Fig. 9, brown bars), which suggests
that the scale of turbidity currents in the study area was
small. Thus, turbidity currents were not the main origin of
the deep-water sandstone in the study area.
Some thick sand bodies can be attributed to slumping
(Fig. 9, green bars). However, the total thickness of this
type of sandstone is small, and this type is observed in only
a few wells. Overall, this type of sand body is not abundant
in the study area.
Therefore, the thick sand bodies in the study area were
deposited primarily by sandy debris flows. The sand bodies
attributed to turbidity currents come second, and the
slumping-related sandstone bodies are present only in local
A cross section through 6 wells, with an average well
spacing of approximately 1 km, was made (Fig. 10) to
further investigate the scale of the sand bodies in the study
area. Based on the lateral distribution characteristics of the
sand bodies in the cross section, the sand bodies in study
area are suggested to have the following features:
4.2 Scale of deep-water sandstones
Fig. 10 Cross section of Chang 7 and Chang 6 members. See Fig. 1c for the location of the section
Sand in the delta
front is the source
of gravity flow
Sand bodies in the study
area are characterized
by the small scale and
The scale of single-event sand bodies is small, and
individual sand bodies are usually less than 2 km in
The single-event sandstone bodies are thin, although
the frequency of gravity flow deposition resulted in a
large cumulative thickness.
The frequency of gravity flow events varied
significantly from one neighbouring well to the next, and
thick sandstones are developed only in small area.
The above features can be observed directly in the
outcrops. One particular outcrop in the study area exhibits
two sets of sandy debris flow sandstones at the top and
bottom (Fig. 5). The sand bodies at the top are
approximately 6 m thick but pinch out laterally within 15 m
(Fig. 5a). The sand bodies at the bottom are only 2 m thick
but display parallel bedding and planar fabric (Fig. 5b–d),
which indicates that the sand bodies are the result of two
stages of sandy debris flows. All the above evidence
adequately supports the conclusion that the sand bodies in the
study area are characterized by small individual events that
rapidly thin out laterally, which leads to poor continuity.
However, a large cumulative thickness resulted from the
stacking of multiple gravity flow deposits.
4.3 Sedimentary models
Based on the above analysis, a sedimentary model of the
deep-water sandstones in the Binchang area was developed
(Fig. 11). A significant number of deposits in the study area
were re-mobilized by sediment gravity flow and
re-transported from a gently sloping front delta area to the adjacent
lake plain due to an earthquake disturbance or the naturally
unstable nature of the delta front area (i.e. sand deposited
on water-bearing muddy deposits) (Zhang et al. 2014).
Thus, the sand in the delta front area is the source of the
deep-water sandstones. Re-transportation occurred mainly
in the form of sandy debris flows and turbidity currents.
Due to the short travel distance of the slumping-related
sand (Shanmugam 2012), these deposits are believed to be
present near the delta front zone, outside the study area.
The thick sand bodies in the study area are generally
present in the form of lumps. These thick sand bodies, in
most cases, resulted from the stacking of sediment gravity
flows, in the form of stacking and interfingering of multiple
sandy debris flows and turbidity currents (Fig. 11a–c). The
individual subsidiary turbidity current deposits are thin but
extensively distributed around the thick sand bodies
(Fig. 11d). According to the drilling situation and previous
research, the thick layers of sand are generally located far
from the material source areas where palaeo-water depths
were greater (Fu et al. 2013). This distribution is due to the
nature of sandy debris flows, which, under the influence of
gravity, are likely to move into deeper water where the
gravitational potential energy is lower.
Three different types of sandstone were identified in
the study area. The massive sandstone (facies C) and
the sandstone containing muddy clasts and ‘‘mud
coatings’’ (facies B) are the products of sandy debris
flows. Facies D and facies E, which display traction
structures and normal grading, represent deposition by
turbidity currents. These deposits are thin, which
indicates that the energy of the turbidity currents was
low. Therefore, the deposits are interpreted as being
the products of subsidiary turbidity currents generated
by sandy debris flows. Facies F and G, which are
characterized by soft-sediment deformation, are the
product of sand slumping.
The sandstones in the study area were deposited
mainly by sandy debris flows. This type of sandstone is
characterized by small-scale single events, rapid lateral
pinch outs and multi-stage development. The turbidite
sandstones are widespread, but the scale of beds
represented by single events was too small to form
A depositional model was developed in which sandy
debris flows predominated, turbidity currents were
secondary and sandy slumping was localized. In this
model, the sandstones developed far from their source
at substantial palaeo-water depths. The thick
sandstones resulted from the accumulation of multiple
gravity flow deposits, which led to lump plan-form
geometry. The turbidites generally overlie the sandy
debris flow deposits or are interbedded with mudstone
near the sandy debris flow deposits. And
lumpingrelated sandstone bodies are present only in local areas.
Acknowledgements This work was supported by the Science
Foundation Programs (41302115). We are grateful to two reviewers
who provided useful comments to help improve the manuscript and
the senior engineer He Pumin, North China Company of SINOPEC,
who assisted in the observation of cores.
Open Access This article is distributed under the terms of the
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