Shallow water deltas commonly develop in basins with stable structure, gentle terrain, shallow water body and abundant source material^[1,2,3,4,5]. These deltas have distributary channels as main sandbody framework and no Gilbert type three-layer structure. Fisk et al.^[6] divided fluvial deltas into deep water type and shallow water type when studying the Mississippi River, and put forward the concept of shallow water delta for the first time. Postma^[7] classified the deltas in low-energy basins into two categories: shallow water delta and deep water delta, and pointed out that the sedimentary characteristics of them were controlled by the depositional process and tectonic setting jointly.

Large scale oil and gas reservoirs have been found in many shallow water deltas of large lacustrine basins in China. The distribution and genesis of sand bodies have always been research focuses in oil and gas exploration. Chinese researchers have put forward many different viewpoints, such as "base level cycle controls sand distribution^[8], multiple factors control sand distribution in fault depression lacustrine basin^[9], lake shoreline controls sand distribution^[10,11], and river erosion controls distribution of thick-layer sand and shoreline migration controls thin-layer sand"^[12].

At the beginning of the 20th century, the Changqing Oilfield successively discovered large oil fields of 100 million tons and over 100 million tons in Chang 8 Member of Yanchang Formation at Xifeng area and Jiyuan area of Longdong, Ordos Basin. Since 2015, with the exploration extending to the center of the lacustrine basin, six oil and gas-rich areas have been confirmed in Chang 8₂ sub- member of Longdong area, including L9 well block, Z91 well block and H82 well block. Among them, Well l152 obtained 200 t/d high-yield industrial oil flow, showing that this area has bright prospects of development and exploration and huge resource potential. So far, the researches on the sedimentary characteristics of Chang 8 Member in Longdong area have mainly focused on Chang 8₁ sub-member, but shallow water deltas of Chang 8₂ sub-member, especially the genesis of thick sand bodies in Chang 8₂ sub-member, have hardly been examined. Based on the description and evaluation of cores taken from 47 wells (including rock type, granularity analysis, sedimentary structures, ancient organisms, etc.) and log data of 130 wells, together with the previous research results on the paleogeographic setting of the lacustrine basin during Chang 8 Member period^{[13,14,15,16]}, the characteristics of sand bodies in shallow water delta are figured out, and the genetic types of thick layer sand bodies are classified according to the principle of sediment volume partition and the law of accommodation space variation.

The Longdong area is located in the southwest of the Ordos Basin. This region covers four structural units, namely, the western margin thrust belt, the Tianhuan depression, the Yishaan slope and the Weibei uplift^[17] (Fig. 1a). The regional structure is a monocline low in the west and high in the east, with a dip angle of less than 1°. The area has small nose shaped uplifts and no faults and folds, indicating the area has been stable in structure. The Triassic Yanchang Formation is a set of fluvial lacustrine terrigenous clastic rock depositional system accumulated in the process of continuous depression and stable subsidence of the basin^[18]. It is divided into 10 members of Chang 1 to Chang 10 from top to bottom^[19] (Fig. 1b). The Chang 6 to Chang 8 members are well preserved and higher in drilling degree.

The Yanchang Formation is divided into 1 super long-term cycle, 5 long-term cycles (SQ1—SQ5) and 17 medium-term cycles^[20]. Chang 8 Member is located between the first flooding surface (top of Chang 9₁ sub-member) and the maximum flooding surface (lower part of Chang 7₃ sub-member) of the super long-term cycle, and spans two long-term cycles SQ2 and SQ3. During this period, the lacustrine basin experienced multiple times of rises and falls, consequently, delta progradation and retrogradation occurred in a short period. The Chang 8₂ sub-member was formed in the process of the base level fall after the flooding period when the top of the Chang 9 Member deposited. The Chang 8₂ sub-member, 35-45 m thick, is located at the top of the long-term cycle SQ2, and corresponds to a complete mid-term cycle with nearly equal thickness of ascending and descending half cycles^[21] (Fig. 1c), representing the change process of water body from shallow to deep and then to shallow again.

Previous studies have shown that shallow water deltas developed in Chang 8₂ sub-member of Longdong area, Ordos Basin. Based on the core data of 47 coring wells and logging data of 130 wells, the authors further analyze the rock type, grain size characteristics, sedimentary structures and paleontological markers of Chang 8₂ sub-member in the study area.

The Chang 8₂ sub-member in Longdong area is a set of fine-grained gray mudstone and grayish green siltstone interbeds, showing dark color characteristic under reduction conditions. The rocks are mainly feldspathic lithic sandstone, lithic arkose, and a small amount of lithic sandstone, and characterized by high content of metamorphic rock debris. The sandstone is relatively fine in grain size, mainly fine sandstone, and medium to poor in sorting. The clastic particles in the sandstone are sub-round to sub-angular, and medium to poor in roundness. The granularity probability cumulative curves of 20 core samples can be divided into two-segment type (Fig. 2a) and three-segment type (Fig. 2b).

The two-segment probability cumulative curves have a total content of bouncing population of 60%, and the inclination of straight section of 63 °, indicating bouncing is the main transport mode, whereas the suspension population is limited, and the particle size at the intersection point of jump population and suspension secondary population is 3.35. The grain size frequency curves of these samples show a single peak, which indicates that the sediments are simple in composition and is affected by relatively stable water flow during sedimentation process. This kind of samples account for 35%, and are mainly from the southwest of the study area far away from the center of the lacustrine basin (Fig. 2c), representing the distributary channel sedimentary environment formed under strong hydrodynamic conditions.

The three-segment probability cumulative curves have a total content of bouncing population of 70%, the inclination of straight section of 71°, and the particle size at the intersection point with suspension population of 3.75. The grain size frequency curves of these samples are bimodal, indicating that the sediments were reformed by river and lake water jointly during deposition. This kind of the samples accounts for 65%, and are mostly from the area near the center of the lacustrine basin (Fig. 2c), which represent the turbulent flow environment and are supposed to be estuary bar deposit.

There are many types of sedimentary structures in the study area. The medium-fine sandstone has trough cross bedding (Fig. 3a), parallel bedding (Fig. 3b), scouring surface and mud gravels (Fig. 3c) which indicate high-energy environment such as distributary channel. The siltstone and fine sandstone have cross bedding and wavy bedding (Fig. 3d) which suggest low-energy sedimentary environment, and are mostly the result of joint effects of river and wave, indicating the water body was turbulent during sedimentation of these sandstones. The paleobiomarkers widely distributed in the study area are worm hole (Fig. 3e), bivalves (Fig. 3f), fish scales (Fig. 3g) and plant fossils (Fig. 3h-3j). Among them, Pecopteris and Taeniopteris belong to polypodiales of leptosporangiatae in filicinae, with thick midvein, flat and intact pinnate leaves on both sides, and are preserved fairly well (Fig. 3h, 3i). This kind of polypodiales plants preferred continuous warm and humid climate, and often grew in marsh areas^[23]. Neocalamites fossils with clear vein traces are widely distributed in the study area (Fig. 3j). Neocalamites was the main coal making herb of Mesozoic era^[24], and mostly grew in shallow water environment (less than 5 m deep) ^[14]. According to observation of field outcrop sections, Chang 8₂ sub-member has 2-3 coal seams (Fig. 3k) of no more than 10 cm generally. Light, soiling hands, the coal is mostly sandwiched between black mudstone, to some extent, indicating a sedimentary environment with shallow water and flourishing hygrophilous plants. Compared with the thick coal seams in other members of the Yanchang Formation, the thin coal seams in the Chang 8₂ sub-member may reflect the frequent fluctuation of lake level and frequent transgression and regression of lake water during this sedimentary period, which made the swamp environment difficult to last.

Many researchers tried to reveal the paleoclimate during the deposition of Chang 8 Member by studying sporopollen assemblage types, trace elements and rock mineral composition changes of mudstone samples^[25,26,27]. They reached the conclusions that the lacustrine basin during the deposition of Chang 8 Member was in a warm and humid tropical subtropical low latitude environment; the lake water was brackish; the climate was relatively dry, with the evaporation greater than the precipitation; and the lake water supply mainly relied on seasonal floods. These climate conditions were not conducive to the formation and development of deep-water lake. To sum up, the sand bodies of Chang 8₂ sub-member are formed in shallow water delta sedimentary environment in the transition area between river and lake.

On the basis of previous studies on sedimentary facies^[10], the main body of shallow water delta in the study area is divided into three sedimentary facies zones, namely delta plain, delta inner front and delta outer front (Fig. 2c) according to the evidences of biomarkers, sedimentary structures, hydrodynamic conditions and different lake levels (Table 1). During the sedimentary period of Chang 8 Member, Longdong area was mainly affected by the southwest provenance, and a set of shallow water braided river delta depositional system in SW-NE direction developed under the gentle slope transportation near provenance. The delta plain was located above the average high water level, and several braided river channels under the control of fluvial process advanced from the edge to the center of the lake basin. In the inner front of the delta, the location of the lake shoreline between the average high water level and the average low water level changed greatly and shifted frequently; the distributary channels branched and then merged under the effect of shoreline shift. In Huanxian-Huachi-Taerwan line, the braided river delta front intersected with the meandering river delta front in the northeast of the basin, and extended from the southeast of Zhengning. Due to the weak hydrodynamic condition of the delta front, the branching degree of distributary channel reduced, and estuary bars developed.

Based on the sedimentary characteristics of shallow water delta of Chang 8₂ sub-member in Longdong area, Ordos Basin, combined with logging curve characteristics of different sand bodies, the sand body characteristics in the sedimentary system were comprehensively analyzed. According to the differences in formation process, three main sand body types have been identified, namely, distributary channel sand body, estuary bar sand body and distributary channel-estuary bar composite sand body.

The distributary channel sand body is formed by clastic materials filling the river channel eroding early stage sediment. Observation of cores shows this kind of sand body is mainly grayish white and grayish green fine sandstone of medium sorting, with occasionally argillaceous intercalation. Plant stem and leaf fragments can be observed in some gray mudstone samples. From the bottom to the top, the sedimentary sequence includes fine siltstone with mud gravel and scour surface, siltstone with large-scale trough cross bedding or parallel bedding, siltstone and fine sandstone with medium to large trough or plate-shaped cross bedding, etc. The sedimentary sequence shows overall positive rhythm or discontinuous positive rhythm fining upward, indicating that the sedimentary environment had hydrodynamic power weakening and material supply reducing gradually. The sand bodies are generally 6-8 m thick each. They appear as weak serrated box shape and strongly serrated box shape with medium high amplitude on natural gamma log curve, and are in gradual contact on top and abrupt contact in the bottom generally. The degree of serration can reflect the turbulence degree of sedimentary environment to a certain extent (Fig. 4).

Under the sedimentary background of frequent fluctuations of lake level, the distributary channels dispersed in water flow and reduced in flow speed after entering into the lake. Subsequently, a large quantity of sandy materials deposited at the estuary to form estuary bar sand body. Observation of cores show the estuary bar sand bodies are mainly grayish white siltstone, with common argillaceous bands, strong heterogeneity, and sand cross bedding, wavy bedding and deformation bedding. Generally, the estuary bars have reverse rhythm of grain size coarsening upward, content of sandstone increasing, and silty mudstone, argillaceous siltstone and medium fine sandstone from bottom to top. These sand bodies are generally 2-6 m thick each. They take on serrated funnel shape with medium high amplitude, and are in abrupt contact at the top and gradual contact at the bottom on log curves (Fig. 5).

With the river controlled shallow water delta advancing to the center of the lacustrine basin, progradation made the distributary channel erode and scour the early sedimentary estuary bar sand body. The residual estuary bar and distributary channel combined into the composite sand body of distributary channel-estuary bar, and the contact surface between them is a small scouring surface. On lithologic section, this kind of sand body appears as positive rhythm distributary channel sand body superimposed with reverse rhythm estuary bar sand body, with composite rhythm on the whole. These sand bodies are 9-13 m thick each, and take on the composite feature of bell shape in the upper part and funnel shape in the lower part on log curves (Fig. 6).

According to the identification characteristics of sand bodies of different genetic types, the distribution pattern of medium-thick sand bodies in shallow water delta sedimentary system of Chang 8₂ sub-member in Longdong area, Ordos Basin has been studied by using core, drilling and logging data comprehensively.

It is found through this study that the distribution of sand bodies more than 20 m thick in the study area has obvious zonality on the plane (Fig. 7). According to the shape and distribution position of the sand bodies, the sand bodies more than 20 m thick can be divided into two types: the first type in wide strip shape occurring in the inner front facies of shallow water delta are located in the southwest of Huanxian County, and in NE-SW strike consistent with the extension direction of the main river channel, with extension distance of about 30 km and planar width of 7-13 km. The second type in irregular lump or branch shapes are distributed on both sides of the average low water level along Shangliyuan-Qingcheng- Heshui line.

These two types of thick bedded sand bodies reflect the cumulative thickness of sand bodies, including single sand bodies and composite superimposed sand bodies. The former refer to the sand bodies which are continuous vertically and horizontally, but separated by mudstone or impermeable interlayer from the sand bodies above or below; the latter refer to the sand body combination composed of several sub-units with spatial genetic relationship in a certain geological period and sedimentary discontinuities inside. It is necessary to compare the thick sand bodies of different shapes controlled by lake shoreline to find out the distribution characteristics of single sand bodies and composite superimposed sand bodies in different sedimentary stages in vertical direction.

The sand bodies of Chang 8₂ sub-member in Ordos Basin had sediment sources of multiple directions such as southwest, northwest and northeast^[29], and the sand bodies in Longdong area were mainly controlled by the southwest provenance^[28]. In order to eliminate the influence of different provenances on sand body deposition, thick sand bodies near each other in different areas under the control of lake shoreline were selected for section comparison, and were named as area I and area II respectively (Fig. 7).

4.2.1. Along the direction of main provenance

Along the direction of main provenance, the two well-tie sections are in NE-SW direction. In the two sections, the sand body of Chang 8₂² sub-layer is in retrogradation superposition pattern; the sand body of sub-layer Chang 8₂¹ is in progradational superposition pattern; and the sand body of sub-layer Chang 8₂² has better continuity than that of sub-layer Chang 8₂¹ (Fig. 8). The sand bodies on section B-B' decrease in cumulative thickness and number and get poorer in continuity towards the center of the lacustrine basin gradually (Fig. 8a). This section has superimposed distributary channel sand bodies in the southwest, distributary channel-estuary bar composite sand bodies in the middle, and smaller distributary channel sand bodies and no estuary bar sand body in the northeast. Section C-C' has mainly distributary channel sandbodies and distributary channel-estuary bar composite sandbodies. The sand bodies have little change in cumulative thickness on the whole and good continuity (Fig. 8b).

4.2.2. Perpendicular to the direction of main provenance

The two well-tie sections perpendicular to the main provenance are in NW-SE direction, and both have sand to ground ratios of higher than 50%. The distributary channel and composite distributary channel-estuary bar sand bodies account for most of the thick sand bodies (Fig. 9). Composite superimposed distributary channel- estuary bar sand bodies contribute as high as 60% of the sand bodies in Well Z126 (Fig. 9a). In Well L200, Well l189 and Well L148, single distributary channel sand bodies take a higher proportion (over 50%) than composite superimposed sand bodies (Fig. 9b).

In Well Z126, L200, L189 and L148, the distributary channel single sand bodies and distributary channel-estuary bar composite sand bodies are similar in vertical distribution position. The distributary channel single sand bodies mainly occur in the upper part of Chang 8₂² sub-layer and the lower part of Chang 8₂¹ sub-layer. These sand bodies are lenticular with flat top and convex bottom and poor in lateral connectivity. Vertically, the sand bodies of different stages, separated by mudstone, have no direct contact and connectivity. The composite superimposed sand bodies of distributary channel-estuary bar mostly exist in the middle of Chang 8₂² sub-layer and Chang 8₂¹ sub-layer. The vertical superposition pattern shows that the distributary channel sandbodies formed in the later stage have obvious cutting, erosion and scouring effects on the estuary bar sand bodies formed early. This kind of composite sand bodies differ widely in lateral contact relationship on the two sections. On the A-A' section, they show no obvious lateral contact, while on the D-D' section, they shows a large lateral migration range. The adjacent estuary bar sand body and distributary channel sand body of the same stage are likely to connect with each other and form a small connected sand bar. The middle part of Chang 8₂² sub-layer in Well L200 and L189 are good examples.

Previous studies have shown that the depositional period of Triassic Yanchang Formation was the heyday of lacustrine basin development in Ordos Basin, and the formation and evolution of the lacustrine basin were closely related to Indosinian tectonic movement^[30]. The sedimentary period of Chang 8 Member in Longdong area corresponds to the early stage of episodeⅠof Indosinian movement. The Indosinian Movement was weak in this period, the Ordos Basin basically inherited the structural stable pattern since the late Hercynian period^[31]. The regional tectonic activity was relatively weak, and the basement of the basin subsided steadily. The lacustrine basin was about (6.4-7.7)×10⁴ km², quite flat in paleotopography, with a slope of less than 0.1°. On the whole, it showed the sedimentary characteristics of shallow water delta with "stable basin structure and flat bottom shape"^[11]. The Chang 8₂ sub-member corresponds to a complete mid-term cycle, which indicates that the sedimentary period of this sub-member witnessed the evolution from shallow to deep and then shallow water body again. In different stages of mid-term base level changes, single distributary channel sand bodies and composite sand bodies of distributary channel-estuary bar, the main contribution types of thick sand bodies, are different in structural characteristics.

There are many geological factors affecting the formation, type, distribution and structural characteristics of sand bodies, such as structure, climate, sediment supply, base level change, etc.^[32], and these factors also interact with each other. But the base level movement does not depend on the specific environment, and is ubiquitous and continuous^[33], the accommodation space often changes correspondingly with this abstract dynamic equilibrium section^[34]. Taking Well Z126, L200, L189 and L148 as examples, the genesis of the thick sand bodies in Chang 8₂ sub-member was analyzed, and the related genetic types have been established according to the base level cycle change of Chang 8₂ sub-member, the accommodation space change law and sediment volume distribution principle.

Type 1 is superimposed distributary channel sand body in large accommodation space (Fig. 10a), which mainly developed in the late rise and early fall stages of the mid-term base level, corresponding to the middle-upper part of Chang 8₂² sub-layer and the lower part of Chang 8₂¹ sub-layer. The sand bodies were formed in the under compensation or weak compensation sedimentary background when the sediment supply (S) was less than the accommodation space growth rate (A). The sand bodies are mostly vertically superimposed, relatively isolated lenticular braided channels. There are often argillaceous interlayers between channel sand bodies of different stages. The two stages of sand bodies are about 8-13 m thick combined. These sand bodies are widespread in the sedimentary facies of inner and outer front of shallow water delta.

Type 2 is superimposed composite sand body in small accommodation space (Fig. 10b, c). These sand bodies mainly developed in the middle stage of the mid-term base level rise or fall, corresponding to the middle-upper part of Chang 8₂² sub-layer and the lower part of Chang 8₂¹ sub-layer. They were formed in the over compensation sedimentary background with S greater than A. With the advance of river controlled shallow water delta toward the lake, progradation resulted in strong erosion and scouring of the late distributary channel to the early deposited estuary bar, thus forming the distributary channel- estuary bar composite superimposed sand body. This kind of sand body is mainly composed of lenticular braided channel and estuary bar sand bodies, and is about 7-10 m thick combined. Among them, the composite sand bodies in the outer front of the shallow water delta (Fig. 10c) often have better lateral connectivity than those in the inner front of the shallow water delta (Fig. 10b). The reason is that in the estuary area near the average low water level line of the outer front of delta, the river erosion is weak, and the supporting effect of the lake water body is stronger. In the area with gentle terrain, the fluvial process is weaker, and the lake hydrodynamic force has stronger effect on the lake shore sediment, so the composite sand bodies formed by distributary channel sand bodies scouring the early estuary bar sand bodies are likely to form contiguous distribution horizontally.

The theory of shallow water delta tends mature, and the sedimentary system of shallow water delta has been constantly improved. As unconventional oil and gas exploration and development goes deeper, unconventional reservoirs formed in shallow water delta environment have become important exploration targets. The Chang 8₂ sub-member in Longdong area has the characteristic of "sand widespread in the whole basin". The reservoir of this sub-member has an average porosity of 8.8% and average permeability of 0.64×10^-3 μm², representing typical low porosity and low permeability reservoir^[35]. The genetic types of thick sand bodies in the Chang 8₂ sub-member of Longdong area are figured out. By comparing the planar distribution of thick sand bodies (Fig. 7) and the planar distribution of permeability (Fig. 11), it is found that the irregular lump or branch shape thick sand bodies on both sides of the average low water level line have higher permeability. These thick sand bodies are mainly distributed in Huanxian-Qingcheng-Heshui area, which is one of the important oil exploration and development zones in Longdong area.

The sedimentary period of Chang 8₂ sub-member in the Longdong area of Ordos Basin has a shallow lake sedimentary background with stable structure and flat terrain (with a slope of less than 0.1°). The sedimentary characteristics of shallow water delta are further analyzed from the aspects of rock type, grain size, sedimentary structure and paleontological markers. The shallow water delta can be divided into delta plain, delta inner front and delta outer front in terms of sedimentary facies zone.

There are three main types of sand bodies in Chang 8₂ sub-member of Longdong area, namely, distributary channel sand body, estuary bar sand body and distributary channel-estuary bar composite superimposed sand body. The thick sand bodies formed by sand bodies of different genesis have obvious zonality in planar distribution, and can be divided into two types: the first type mainly occurs in the inner front facies zone of shallow water delta, is consistent with the extension direction of main river channel, and in "wide strip" shape; the second type is mainly distributed on both sides of the average low water level line along Shangliyuan-Qingcheng-Heshui line, is scattered and in irregular lump or branch shapes.

The genetic types of thick sand bodies in Chang 8₂ sub-member can be divided into superimposed distributary channel sand bodies in high accommodation space and superimposed composite sand bodies in low accommodation space. According to the difference of lateral connectivity of composite sand bodies, the latter can be further divided into two subgroups: the inner front type of shallow water delta and the outer front type of shallow water delta.