The Heshui area is an important oil-generating area located in the southwest of the Ordos Basin, which is located in Qingcheng and Heshui County, Gansu Province, adjacent to Qingyang in the west, Ta’erwan in the east, Chenghao in the north, and Ning County in the south, with an exploration area of about 750 km². It is tectonically located in a tectonic depression in the middle part of the Yi-Shaan Slope in Ordos Basin, which is at the intersection center of multiple depositional systems of Yanchang Formation in Ordos Basin, and local small nose-shaped uplift is developed (Figure 1). The Chang 6 oil reservoir group is gravity-flow deposits of various types developed in the context of the semi-deep and deep lake (Liu et al., 2015; Xu et al., 2016; Li X. et al., 2018; Ran and Zhou., 2019) during a special period of return uplift after the maximum expansion period of Chang 7 Lake Basin (Figure 2). It has been an important exploration section for unconventional petroleum such as deep-water tight sandstone reservoirs and mud shale reservoirs in Ordos Basin in recent years.

As the subsidence center of the Ordos Basin during the Yanchang period, the Heshui area inherited the same sedimentary and stratum characteristics as the Ordos Basin. The Ordos Basin, a craton foreland basin located on the North China Plain, is characterized by a stable basement, multi-phase fault depression, obvious migration oscillation, and multi-phase cycle superposition (Zhang, 1989; He, 2003; Yu et al., 2010). During the Triassic period, the thrust and nappe of the North Qinling led to the rapid expansion of the base plane and the deepening of water. Due to the influence of the Indochina movement in the Late Triassic period, the Ordos Basin gradually transformed from marine to lacustrine basin deposits and developed a set of lacustrine terrigenous clastic rocks with a thickness of about 1,300 m, that is, the Yanchang Formation. The bottom of the Yanchang Formation is relatively flat, and the denudation at the top varies greatly. The Yanchang Formation is in pseudo-integrated contact with the overlying Yan’an Formation or Fuxian Formation (He, 2003; Li Z. et al., 2018; Xian et al., 2018). From top to bottom, the Yanchang Formation can be divided into ten oil reservoir sections covering Chang 1 to Chang 10. The features of depositional evolution reflect the entire formation and extinction processes of lake basins, during which a variety of depositional systems such as alluvial fans, rivers, deltas, and lakes were developed (Figure 2). The deep-water gravity-flow deposits of Chang 6 in the Heshui area can be regarded as an underwater extension system deposited in the semi-deep and deep lake. It is formed by front deposits of the large-scale prograded delta, which migrated to the center of the lake basin and collapsed under the action of flood, earthquake, and volcano (Li et al., 2009; Zou et al., 2012; Li X. et al., 2018).

The samples of 1859.3 m from Zhuang 42 well, 1,695.7 m from Zhuang 35 well, and 1,654.4 m from Ban 29 well were taken. The cumulative curves of particle size probability show large suspended component with low gradient (Figure 3A); suspended component accounts for most of the content with a slight amount of saltation population content with the size less than 1ɸ, and low gradient (Figure 3C); higher saltation population content and the cut-off point between this content and the suspended component is at about 2ɸ, and the gradient is low overall (Figure 3E). The Im values of slide-slump rocks, sandy debris flow, and turbidity current are 3.13, 2.28, and 2.32, respectively (Figure 3B, Figures 3D,F).

Thin section identification is adopted for the zonal statistics of the heavy minerals in the study area, and the results obtained are as follows: the northeast and southwest of the study area are dominated by zircon. However, the former is observed to develop a small amount of tourmaline and garnet, while the latter a small amount of leucite and garnet. As for the central part, zircon still dominates, followed by tourmaline and garnet. A small amount of albite is also developed, showing its inheritance of the characteristics of the northeastern maternal provenance (Table 5). The ZTR index refers to the proportion of zircon, rutile, and tourmaline, three super-stable heavy minerals, in transparent heavy minerals.

The determination results of rare earth elements of nine rock samples from wells Le 36, Ban 33, Zhuang 37, and Zhuang 42 reveal that the REE distribution features right gentle inclination of rich in light rare earth (LREE) and depletion in heavy rare earth (HREE), with obvious negative Eu anomaly (Figure 4A). La/Yb-∑REE source rock identification chart is adopted to analyze 20 sandstone samples in the study area, showing that most data points are distributed in the basaltic region and depositional rock region, and a few in the granitic region and alkaline basaltic region (Figure 4B).

Based on the results obtained from the core observations of several coring wells in the study area, the fluid rheological characteristics, and the methods employed to classify deep-water gravity-flow inside and outside China, we divided the gravity-flow in the study area into three main types: slide-slump featuring mass transport, sandy debris flow with plastic rheological properties, and turbidity current with Newtonian rheological characteristics.

The samples of 1859.3 m from Zhuang 42 well, 1,695.7 m from Zhuang 35 well, and 1,654.4 m from Ban 29 well were taken from three typical gravity-flow types: slide-slump rocks, sandy debris flow, and turbidity current. According to particle size analysis, both slide-slump rocks and sandy debris flow have high matrix strength and rapid depositional process with the deposits mainly being transported by the suspension. However, turbidity current, which is transported by both suspension and jumping, features a tractive current. The sortability is closely related to their development mechanisms and flow transformation, in terms of which, the performance of sandy debris flow ranks first, turbidity current comes second, while slide-slump the last.

Gravity drives the gravity-flow to be transported along the slope. Flow transformation, which occurs continuously with the participation of surrounding water bodies, can lead to the spatial superposition of gravity-flow of diverse types, forming interdeposition between multiple rock types and facilitating hydrocarbon generation and storage. At the same time, the flow transformation mechanism of gravity-flow also explains the differences in particle sorting of gravity-flow of various types.

1) Transformation of slide-slump to sandy debris flow

Semi-consolidated or unconsolidated deposits that stake high up the slope contain large clasts of mud and gravel, which slide down the slope in blocks due to geological events. The high matrix strength within the blocks causes large mud clasts and gravel to collide and crush under shear stress, and then deform to form gravels of varying sizes and shapes, which are distributed randomly in the sandy matrix. The participation of surrounding water prompts liquefaction of deposits to a certain extent, with the coarser gravels preferentially unloaded and deposited at the bottom. At this point, the remaining fluid is diluted, and the overall flow regime is transformed to develop a sandy debris flow with a granular support structure.

2) Transformation from sandy debris flow to turbidity current

The concentration of sandy debris flows is higher, and the supporting matrix is formed by the dispersion pressure of inter-particle collisions, heterogeneous base support, and buoyancy. Due to the lubrication of the slope and the soft mud at the bottom of the lake basin, the coarse stream of the sandy debris carrying a small amount of coarse and gravel continues to flow and undergo transformation by the liquefaction of surrounding water. The liquefaction bumped the coarser fractions and then deposited them to the bottom of the fluid, reducing the concentration of the remaining fluid. The upper part of the stagnation point at the front end of the fluid mixes with the environmental water first, creating water circulation in a small range at the head of the sandy debris flow. During the process, the head of the sandy debris flow is the first to convert to less dense turbidity deposits, which erodes the underlying strata as it flows along the slope. Such turbidity current only stops until the new sediment involved in erosion is less than the sediment deposited. Thus, turbidity currents, though being indicative of distal deposits, are non-homogeneous within layers, and the sortability of the debris fraction is poorer than that of the sandy debris flow. The single-phase turbidity current deposits are thin and interbedded frequently with sandy clastic flows longitudinally.

The above analysis reveals that the study area mainly develops three flow types of gravity flow, namely, slide-slump, sandy debris flow, and turbidity flow. The sedimentological evidence indicates geological events such as volcanoes and earthquakes. The lake basin bottom shape, provenance supply, and triggering mechanisms obviously affect the distribution and formation of gravity flow.

The southwest Qinling has relatively insufficient supply, which is mostly driven by geological events such as volcanoes and earthquakes. Deposits bypass only because of the slope-fold zone at the bottom of the lake basin where slump turbidite develops. The well Ban29 in the southwest is selected as a typical well. The histogram shows the inter-deposition of thin siltstone or muddy siltstone and mudstone, as well as sedimentary structures such as wavy bedding, convolute bedding, deformation slump, and flame structures. All of them are considered to be triggered by geological events (Figure 9A, Figure 2 for the location of well). The deposits remain temporarily at high places collapsed, sliding down along the slope-break zone. In addition, they undergo four evolutionary stages of sliding, slumping, debris flow, and turbidity current during the transition from proximal to distal sediments. Single-phase slump turbidite can be subdivided into three microfacies, including proximal slide-slump, debris flow lobe, and distal turbidite lobe based on flow transformation and spatial distribution (Figure 10B; Figure 2 for the location of well).

Interwell correlation—well Xi160-well Ban21-well Xi140-well Ban10-well Zhuang215-well Ban15-well Zhuang76 is selected to compare the lateral distribution of sedimentary facies (Figure 10; see Figure 2 for the location of the line of interwell correlation). The section is distributed in a northeast to southwest direction, parallel to the input direction of the outcrop. It is manifested by the development of sublacustrine fans in the northeast, with large thickness and good spatial continuity, and the long extension of sand bodies. The sedimentary slump-turbidite in the southwest is isolated and discontinuous, with shorter extensions.

The northeastern part that is well supplied with material resources has a wide and gentle bottom shape. The well Zhuang42 in northeast is selected as a typical well. From the histogram, it can be seen that multi-stage fine sandstone and siltstone are superimposed to form a huge, massive sand body. Muddy tearing chips or plant fragment fossil can be seen inside, and the load cast can be seen at the bottom (Figure 9B). Deposits accumulated on the front of the delta are transported by flood into the deep lake, forming a sublacustrine fan: a gravity-flow fan featuring watercourse (Figure 11A). During the rainy season, flood carries shallow-water deposits to the basin through the recharge channel. At the same time, high-density gravity-flow consisting of sand, mud, and gravel enters the semi-deep and deep lake, eroding the underlying unconsolidated soft deposits to form a lacustrine subaqueous channel (Figure 11C). As the slope of the basin decreases, the kinetic energy of the channel decreases, forming a longitudinally and horizontally interconnected braided channel (Figure 11C). In this channel, massive, homogeneous fine-siltstone is distributed inside and mud-rich tear debris at the top (Figures 4F,G). Evident scouring marks observed at the lithologic interface (Figure 4H) are typical sandy clastic flow deposits. The multi-phase braided channels are prone to superposition or interbedded with thin mudstone. On the flanks of the channels, typical turbidites with Bouma sequences of B–E and C–E sections (Figure 11C) are formed, featuring the positive rhythmical progression of siltstone, muddy siltstone, and argillaceous siltstone with a thickness of less than 1 m. The microfacies are formed by the overflow of deposits in the braided channels. On entering the lake bottom plain, the erosion of gravity-flow on the overlying deposits weakens and the channels disappear, forming a wide area of sheet sand in outer fan architecture, which interbeds with semi-deep lacustrine-deep lacustrine mudstone. In these strata, sandy lens (Figure 4M) and sections C, D, and E of the Bouma sequences (Figures 4K,L) are visible in the core so they are typical turbidite deposits. According to lithological signatures and sand thickness, the gravity-flow in the northeastern part is controlled by the middle and outer sides of the sublacustrine fan, which can be further subdivided into four microfacies covering braided channel, channel flank wing, interchannel, and sheet sand.

The southwestern part of the study area is dominated by slump turbidites sedimented by event collapse, forming disconnected sand bodies in the shape of an isolated island (Figure 12A). Statistically, the single-phase slump turbidites in the study area are no more than 2 m in thickness, and are extended within 15 km. They are also independent of each other (Figure 12B). As for the bottom shape with a certain slope drop angle, gravity-flow undergoes multiple flow transformations along the slope. The superstition and development of multi-stage slump lobes lead to the formation of a mixed event layer of sandy debris flow and turbidity current, i.e., fine-grained depositional rock interbedded with siltstone and mudstone. The sudden and rapid deposits of the event gravity-flow enable it to bury and preserve organic matter in a relatively short period, indirectly promoting hydrocarbon generation and expulsion. As a result, the fine-grained depositional rock developed from the fluid transformation in the slump turbidites is in a favorable position for hydrocarbon generation and enrichment. Besides, the brittle minerals of terrestrial origin are abundant in this area, including quartz, feldspar, and calcite, determining the fracturing parameters in the development of unconventional petroleum, and serving as a potential “sweet spot” for deep-water tight oil and gas.

Flood-forming sublacustrine fans are widely developed in the northeastern part of the study area. Multiple sublacustrine fans may be stimulated simultaneously in the temporary sediment retention areas at the gentle slope of the delta front by multi-stage floods, and the superposition of multi-phase fans controls the distribution range of contiguous sand bodies in the study area (Figures 12A,B). The thickness ranged from 1.8 to 13.7 m for the sandy debris flow with better-sorting properties in the middle fan braided channel microfacies, with the average being 7.6 m. The sheet sand in the outer fan is turbidite sediment transformed by the liquefaction and dilution of sandy debris flow. The physical property of argillaceous siltstone and muddy siltstone between multi-phase fans becomes poor in the burial evolution given the strong compaction and recrystallization of the heterogeneous base, forming favorable caprock and plugging zone. The thick sand body of sandy debris flows genesis in the middle fan, and the sheet sand of current turbidity genesis in the outer fan can be employed as transport and reservoir layers. Under the microscope, the intergranular pores remaining between terrestrial clastic particles such as quartz and feldspar can be seen (Figures 13A–C). During the diagenesis stage, the feldspar is easily dissolved to form solution expansion pores (Figure 13D) or intragranular dissolved pores (Figure 13E). In addition, due to tectonic action, structural fractures are easily formed in brittle mineral-rich rocks (Figure 13F), providing effective space for oil and gas storage. The periodicity of sedimentation explains the variability of the rock-forming evolution process, which controls the formation of deep-water lithological traps and hydrocarbon enrichment under deep burial conditions. On the other hand, floods transport a large amount of organic matter from terrestrial sources and shallow waters, and the enrichment of organic matter changes the original ecological environment of the basin and promotes the survival and reproduction of microorganisms, which directly or indirectly increases the content of organic carbon (Khripounoff et al., 2009; Yuan et al., 2015) and improves the hydrocarbon generation potential.