The Lithological Features of Sublacustrine Fans and Significance to Hydrocarbon Exploration: A Case Study of the Chang 7 Interval of the Yanchang Formation, Southeastern Ordos Basin, North China

State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an, Shaanxi 710069, China National Engineering Laboratory of Low-Permeability Oil & Gas Exploration and Development, Xi’an, Shaanxi 710018, China Exploration and Development Research Institute, PetroChina Changqing Oilfield Company, Xi’an, Shaanxi 710018, China Shaanxi Key Laboratory of Advanced Stimulation Technology for Oil & Gas Reservoirs, College of Petroleum Engineering, Xi’an Shiyou University, Xi’an 710021, China


Introduction
The continuous development of new technology and advances in knowledge on oil and gas exploration and development have remarkably improved exploration and development in deep-water basins worldwide. Major discoveries of oil and gas in tight sandstones include the Gulf of Mexico, the North Sea, and the Norwegian Sea [1][2][3][4][5][6][7][8][9][10]. These discov-eries prompted deep-water exploration in marine basins such as the Qiongdongnan and the Yinggehai and continental basins such as the Ordos and the Bohai Bay in China. Concurrently, theoretical advancements improved understanding of deep-water sedimentation [11,12]. Deep-water gravity flows sedimentary models were primarily developed between 1950 and 1970, such as the Bouma sequence, the York model, and the ancient submarine fan model [13][14][15][16][17][18].
The study of deep-water sediments in the Ordos Basin began in the 1970s, and divergent views exist on the large scale development of sandstones attributed to deep-water gravity flows in the basin. Chen et al. concluded that the deep-water sandstones of the Chang 6 and Chang 7 intervals are primarily turbidite deposits including slope displacement turbidite fans and slumping turbidite fans [37][38][39][40][41]. Zou et al. reported expansively developed debris flow sand bodies in the Chang 6 and Chang 7 intervals in the center of the basin [42][43][44][45][46]. Recent studies indicate that turbidity flow and debris flow deposits exist in the Ordos Basin [47][48][49][50][51][52]. In addition, Yang et al. found a gravity flow-induced sandstone different from those of debris flow and slumping turbidite sediments of the Chang 6 and Chang 7 intervals in the southern part of the basin, which are considered as hyperpycnal flow sediments [53,54].
The knowledge from deep-water sedimentary research effectively guides the exploration and development of the Chang 7 reservoir formation and remains significant for exploration in the Ordos Basin. The Changqing Oilfield Company made a significant discovery in the Xin'anbian area of the central and western part of the basin in 2014. This represents the first tight field in China with over 100 million tons of proven reserves and highlights the significant exploration potential of the Chang 7 interval [55][56][57]. However, when compared with the central and western parts of the basin, no major discovery exists from exploration for tight oil in the southeast, with only occasional tight oil-enriched areas such as the Zhidan and the Ganquan. Detailed studies on the genetic types and sedimentary models of sand bodies in the Chang 7 interval in the southeast of the basin are scant. This study examines drilling core, logging, and analytical test data for more than 300 wells in the southeastern Ordos Basin and eight outcrop profiles (Shanshui River, Jinsuoguan, Yao Xian, Xuefengchuan, Hongshiya, Shiwang River, Yunyan River, and Yan River). The results reveal the characteristics, the sublacustrine fan development, and the significance of the gravity flow sediments for exploration in the Chang 7 interval in the southeastern Ordos Basin. The research results provide a theoretical basis for further exploration and development in the southeastern Ordos Basin.

Geological Setting
The Ordos Basin extends from the central to the western part of the North China Craton and is surrounded by the Helan-Liupan Mountain north-south tectonic belt to the west, the Pacific tectonic region in the east, and the Qilian-Qinling fold tectonic belt and the Tianshan-Xingmeng fold tectonic belt in the north and south, respectively ( Figure 1). It is the second largest onshore sedimentary basin in China, with an area of about 37 × 10 4 km 2 [58][59][60][61][62][63]. Under the influence of collision orogeny in Qinling area, the strata of the Middle and Late Triassic basin are generally high in the north and low in the south, thin in the north and thick in the south. The water body is characterized by a shallow north and a deep south, and the sedimentary range is much larger than that in the present basin. The Yanchang Formation consists of 1000-1300 m (3280-4265 ft) fluvial, delta, and lacustrine sediments [64,65]. The evolution of the Yanchang Formation involves a period of early subsidence and extension (Chang 10-Chang 8 intervals), a peak stage (Chang 7 interval), and a late basin shrinkage stage (Chang 6-Chang 2 intervals) that continued and was followed by uplift and filling of the lake basin in the Late Triassic (Chang 1 interval) [64,65]. Fluvial and lacustrine facies dominate the sediments, with thickness ranging from 1000 m to 1600 m [12,49,[66][67][68][69] (Figure 2). The Chang 7 interval represents the peak stage of the Triassic lake development in the Ordos Basin. The basin experienced major subsidence during this period, affecting an area of over 5 × 10 4 km 2 , with water depth in the deep lake attaining 150 m ( Figure 2). Deposition included dark mudstones and thick and abundant black shales named the "Zhangjiatan shale." These constitute the principal Mesozoic petroleum source rocks in the basin, with layer thickness of 15 m-30 m. Organic geochemistry data support the dark shale as the primary source rock for reservoirs in the basin because of the potential for hydrocarbon generation and expulsion associated with its high organic carbon content [70][71][72][73][74][75].
The study area lies in the southeast of the Ordos Basin bound in the south by Yan'an, in the north by Tongchuan, in the west by Yichuan, and in the east by Heshui and covers an area of about 4:4 × 10 4 km 2 . It belongs to the Yishan slope and partly to the northern margin of the Weibei uplift ( Figure 1). The eight outcrops studied are mainly in the eastern and southern margins of the study area.

Data and Methods
This study is based on 240 wells' log data, 22 well cores, and several outcrops to interpret the gravity flows in the study area ( Figure 1(b)). The outcropping sediments deposited during the Late Triassic, which were described in detail to illustrate the vertical characteristics of gravity flow deposits and interpret lithofacies. Then, the interval thickness and the ratio of sandstone thickness to interval thickness can be calculated through the integrated application of well logs, including gamma-ray, acoustic, and resistivity logging. Furthermore, casting thin section (CTS) observation and scanning electron microscopy (SEM) were used to analyze reservoir characteristics of sublacustrine fan sand bodies.

Provenance Direction.
We analyzed the characteristics of light minerals, heavy minerals, and detritus in outcrop sections and wells and divided the area into northeast, southern, and central portions, reflecting two provenance directions. The northeast and the southwest are indicated as the dominant provenance directions. The heavy minerals in the Chang 7 reservoir strata mainly include zircon, garnet, leucosphenite, tourmaline, rutile, and subordinate amounts of chloritoid, epidote, and sphene [76][77][78][79][80]. The Fuxian-Yan'an area is the northeast provenance control area mainly comprises fine-grained lithic feldspar sandstones and feldspathic sandstones. The heavy minerals are the zircon-garnet assemblage, with generally over 50% garnet. The light minerals are dominated by feldspar containing over 50% and less than 30% quartz, suggesting low maturity sandstones. The Zhengning-Xunyi area is the south provenance control area, and its heavy minerals belong to the garnet-leucosphenite-zircon assemblage. The content of stable minerals such as zircon is generally over 40% and increases from the southwest to the center of the basin, whereas unstable minerals such as epidote decrease and are absent in some areas. The light minerals generally comprise 40%-50% quartz and about 20% feldspar, with an overall high compositional maturity. The zircon-tourmaline-rutile (ZTR) contour map displays increasing ZTR indices from the northeast, southwest, and south to the central region ( Figure 4). Several heavy minerals occur in the central region, and the heavy mineral contents vary significantly. This indicates proximity to the sedimentary depocenter and is characteristic of a turbidity convergence zone affected by multiple provenances.  The grain size distribution curves for turbidity current deposits in the study area are characterized by straight line or approximate straight line (Figures 6(a) and 6(b)). The absence of rolling or saltation size populations, the high total suspended particle content, and the poor degree of sorting are typical particle size Deep to semi-deep lake subaqueous fans  In this paper, the log facies characteristics of several sedimentary microfacies in the study area are presented on the basis of data including core descriptions and sedimentary structures (Table 1, Figure 7).

Lithofacies Association and Facies
Types. Field observations revealed the presence of Bouma sequences and that the longitudinal thickness of a layer of the sublacustrine fan sand body ranges from a few centimeters to a few meters in the study area. The horizontal distribution of the sand body is stable, and core observations confirm common thin mudstone and argillaceous siltstone intercalations of the sand body. The core characteristics and evolution from the sedimentary structures of the Chang 7 interval reveal subfacies 5 Geofluids types including main channel, the branch channel, the interchannel, lobe fringe, and deep lake plain (Figure 7).

Main Channel Filling Deposits.
The main channel filling deposits are close to the provenance area and includes the main channel and overflow deposits. The main channel filling deposits in the study area is not well developed than branch channel deposits. The main channel facilitated the transport of sediments along the slope characterized by the development of massive sandstones. The thickness of a sand body is over 1 m, whereas the thickness of a multistage superimposed sand body exceeds 10 m. The lithology is dominated by fine sandstones with graded bedding from bottom to top, with scour structures at the bottom. The Bouma sequence is characterized by repeated occurrence of the A or AB member. The planar distribution of the main channel reflects the topography, the flow direction, and the scale of the transported objects. The lateral thickness of the lenticular main channel sandstones changes rapidly, and the overflow deposits comprise mostly dark siltstones and mudstones (Figures 8(a) and 8(b)).

Branch Channel Filling
Deposits. The branch channel filling deposits are generally developed in the middle and upper parts of sublacustrine fans, and the microfacies types include the branch channel and the interchannel. The branch channel is the extension of the main channel characterized by a weaker hydrodynamic condition. The sand body therefore is thinner, ranging from 0.3 m to 1 m, and the lithology is mainly fine sandstone. The branch channel microfacies represents the main facies of the sandstone reservoirs in the study area, with the Bouma sequence showing overlap of the AB and A members. The interchannel microfacies are mainly fine-grained sediments revealed by core observations to include irregular sand-shale interbeds, argillaceous siltstones, and silty mudstones, with a CDE Bouma sequence assemblage (Figures 8(b) and 8(c)).

Facies Distribution.
Taking the Ch-72 period as an example, gravity-flow deposits developed widely in the deep lake, with sediments mainly accumulated in the northeast and south (Figures 9 and 10). The scale of sublacustrine fans increased as sublacustrine fans from different directions converged in the center of the study area, with sand bodies spreading smoothly.
The sand bodies of the sublacustrine fan in the northeast deep water area extend far, and six large fan bodies can be identified (Figures 9 and 10). The extension distance of the sublacustrine fan sand bodies in the southern deep water area is slightly shorter, and two large fans are developed (Figures 9  and 10). The main channel and branch channel were stripshaped in plan, and the thickness of single sand bodies varies from 0.5 to 11 m, but composite sand bodies can reach more than 20 m. The channel sand bodies with single layer thickness more than 2 m are mostly caused by sandy debris flow, which can form reservoirs. The channel lateral margin sand bodies were strip-shaped, which are mostly a deposit combination of sandy debris flow, muddy debris flow, and turbidity current, with a thickness ranging from 0.2 m to 2.5 m. The lobe fringe was sheet-like in plan and developed multilayered

Geofluids
The sublacustrine fan is lobed and tongue shaped in planar view, with a wide front at the fan margin and is lenticular in the longitudinal direction. The grain size of the fan sediments decreases away from the source.
Gravity flow deposits can be developed on both gentle and steep slopes of the lake basin, and the sedimentary types are controlled by both slope and provenance. In the northeast of the study area, the slope is gentle, and the provenance supply is sufficient. So the lacustrine fan developed in the deep water area is large, and most of the sand bodies are vertically superimposed along the provenance direction and extend to the center of the lake basin. On the other hand, the southern slope of the lake basin is steeper than that of the northeast, and the sublacustrine fan is also developed in the deep water area, but the scale is relatively limited, and the extension distance to the center of the lake basin is slightly shorter. The sublacustrine fan formed at the junction of the delta front and the slope belt. Factors such as earthquakes, volcanoes, and floods triggered stability loss of the sand body originally in the delta front, causing gravity flows including debris, turbidity, and grain flows. These flows transported large quantities of debris to deep portions of the lake area, with turbidity flow dominating during rapid transportation and terminating with the formation of sublacustrine fans in the relatively low-lying part of the lake basin. As the terrain slope becomes gentle, the main channel forks to form multiple branch channels, and the branch channels are easy to overlap with each other. After the branch channels enter the sublacustrine plain, with the further decrease of the terrain slope, the erosion of the gravity flow sediment to the underlying argillaceous sediment gradually weakens, and the channels disappear. In front of the channel, a wide lobed area is formed, and most of them are argillaceous siltstone and silty mudstone with thin sands.  10 Geofluids  The principal reservoirs currently in the study area occur within or adjacent to Chang 7 source rocks. The close relationship between reservoirs and source rock distribution suggests a "source control."  14 Geofluids

Reservoir Characteristics of Sublacustrine Fan Sand Bodies
(1) Distribution of Sand Bodies in the Sublacustrine Fan. The extensive sublacustrine fan sand bodies accumulated in the semideep areas of the lake and constituted an important reservoir of the "tight oil near-source" accumulation. Turbidite sand bodies accumulated on a large scale in the Chang 7 interval of the study area, with reservoirs widely distributed from the delta front slope to the center of the basin. The sand bodies exhibit transverse and longitudinal superimposition in spatial distribution and constitute tight reservoirs. The sand bodies of the 1st layer of the Chang 7 reservoir are the most developed in the study area ( Figure 13). The thickness of the sand body varies from 5 m to 25 m. From the 3rd layer to the 1st layer of the Chang 7 reservoir, the lake shoreline gradually decreased and allowed the sand body to advance toward the center of the deep lake, thereby increasing the extent of the sublacustrine fan. Concurrently, the multistage sand bodies were superimposed and continuously deposited. The wide distribution of sublacustrine fans and significant thickness of the sand bodies are suitable for the formation of tight reservoirs.

Geofluids
(2) Pore Structure and Petrophysical Properties of Sublacustrine Fan Sand Bodies. The reservoir rock types are feldspathic sandstone and lithic feldspar sandstone, with moderate sorting and subangular grains. The main reservoir spaces of the Chang 7 reservoirs are primary intergranular pores, dissolved intergranular pores, dissolved intragranular pores, micropores, and microfracture pores (Figure 14), which are effective transport avenues for tight reservoirs.
The measured porosity varies from 6% to 8%, with an average value of 6.83%, and the permeability ranges from 0:1 × 10 −3 to 0:5 × 10 −3 μm 2 , with an average value of 0:24 × 10 −3 μm 2 ( Figure 15). The reservoirs of the Chang 7 interval are tight, with the main channel and branch channel sandstones showing good petrophysical properties primarily hosting the tight reservoir oils.

Effective Source-Reservoir Relationship as an Important
Factor for Accumulation. The horizontal and vertical distribution of turbidite sand bodies and source rocks partially controls the distribution of reservoirs. In the process of hydrocarbon generation, pressure build-up, and 17 Geofluids hydrocarbon expulsion, the generated oil first migrates to the adjacent horizons. The sublacustrine fan sand bodies extended to the basin center and established direct contact with the source rocks. The expelled oil accumulated in the reservoirs following short distance migration ( Figure 16). The vertical superimposition of the thick sand bodies deposited by the main and branch channels formed large scale reservoirs and provided favorable conditions for oil accumulation. The dark mudstones in the Chang 7 interval were suitable for cap rock.
The relationship between the source rocks and the tight sandstone reservoirs in the Chang 7 interval in the study area reveals three types of source-reservoir assemblages including (1) thick sand bodies with multiple source rocks assemblage, (2) source rocks overlain by multiple medium-thick sandstone assemblage, and (3) source rocks overlain by multiple thin sandstone assemblage. The first two assemblages are the most favorable source and reservoir combinations and represent favorable exploration areas for tight oil. 18 Geofluids

Conclusions
(1) The main body of the Chang 7 interval of the Yanchang Formation in the southeastern Ordos Basin is a deepwater gravity flow system of semideep to deep lacustrine environment. The sublacustrine fan in the study area is a sublacustrine fan with gentle slope, without any erosion canyon between the sublacustrine fan sand bodies and delta front sand bodies. The sublacustrine fan is lobed and tongue shaped in planar view, with a wide front at the fan margin, and lenticular in the ver-tical direction. The grain sizes of the sublacustrine fan sediments decrease gradually away from the source (2) Gravity flow deposits in the study area can categorised into three groups: sand debris flow deposits, turbidity current deposits, and deep-water mudstone deposits. The main channel and branch channel are mainly developed with thick massive sandy debris sandstone, while the channel lateral margin and branch channel lateral margin are mainly developed with middle massive sandy debris sandstones and  19 Geofluids turbidite sandstones, which from bottom to top, the thickness of sand layer becomes thinner and the grain size becomes smaller (3) The source rocks in the Chang 7 interval have generated and expelled sufficient oil for the formation of tight oil reservoirs. At the same time, the turbidite sand bodies of the Chang 7 interval overlying the source rock show properties suitable for tight reservoirs. The source-reservoir assemblages are capable of "self-generation, self-storage, and self-capping." The main and the branch channels are favorable exploration targets for tight oil and gas reservoirs

Data Availability
The data used in this manuscript available upon the author reasonable request.

Conflicts of Interest
The authors declare that they have no competing interests.