Following the analysis of cores, outcrops, well log, and seismic sections, we have studied the seismic reflection configuration and depositional history of the hydrocarbon-rich Yanchang Formation in the Ordos Basin. We divided the seismic reflection configurations into five types: subparallel reflection, parallel reflection, tangential progradational reflection, shingled progradational reflection, and chaotic reflection. Based on our study results, we concluded that the slopes exhibit differences in the different regions of the Ordos Basin during the sedimentary period of the Yanchang Formation: The slope with the largest gradient of approximately 10°–20° occurred in the southwestern basin, followed by the northwestern basin (with a slope of approximately 1.6°–3.3°), but the slope was relatively gentle in the northeastern basin (approximately 0.8°–1.2°). We also found that the paleocurrent direction of the basin mainly includes two directions: The paleocurrent direction of the southwest region is 186°–259°, which indicates the provenance came from the southwestern region, whereas the paleocurrent direction of the northeast region is 10°–79°, which indicates that the provenance came from the northeastern region. In addition, the Ordos Basin was under isostatic subsidence as a whole during this period, and its sedimentary infilling evolution underwent five stages: the initial depression, intense depression, progradational filling, uplifting and denudation, as well as shrinking and extinction stages, just corresponding with the Chang 10-Chang 9, the Chang 8-Chang 7, the Chang 6-Chang 4+5, the Chang 3-Chang 2, and the Chang 1 depositional age, respectively.
In recent years, new understandings on stratigraphic age identification, sequence division, restoration of prototype basin, and distribution of sedimentary sand bodies were obtained after plenty of studies on the Yanchang Formation in the Ordos Basin by means of data analysis of geophysical prospecting, well logging, geochemistry, and geology (Li et al., 2006a, 2006b, 2009, 2012; Chen et al., 2007; Zhang et al., 2010; Fu et al., 2015; Wang et al., 2016). Yang (2004) analyzes the interaction among the boundary properties, provenance characteristics, sedimentary facies distribution patterns, and the orogenic activities in Qinling during the Indosinian movement in the Ordos Basin. Chen et al. (2006) divide the Yanchang Formation into four long-term cycles with stable regional distribution and summarize the characteristics of its filling responses. Wang et al. (2010) believe that the lacustrine basin has undergone a complete process of initial depression, intense depression, uplifting in return, and shrinking and extinction with the development of Indosinian movement during the Yanchang sedimentary period, featuring rapid subsidence, slow filling, and constant migration of the depocenter southwestward. Zhao et al. (2011) analyze the developmental types and filling process of the sequence architecture and establish the distribution pattern of the sequence architecture of the large inland down-warped basin in the Yanchang Formation of the Ordos Basin. However, few studies on the filling structure of the Yanchang Formation have been made using seismic data. Therefore, we used seismic data to analyze the seismic reflection configurations and we investigate the seismic sedimentary characteristics of the fluvial, fan delta, and lacustrine facies during the Yanchang sedimentary period. In addition, the progradational reflection was used to calculate the paleoslope and paleocurrent, so as to determine the deep water scope in the lacustrine basin, and to summarize the sedimentary filling mode of the Yanchang Formation. These will be of great referential value for further studies on sedimentary filling in lacustrine basins of the same type, and the hydrocarbon exploration and development in the study area.
The Ordos Basin ranks second among the petroliferous basins in China. It crosses five provinces of Northern China (Shaanxi, Gansu, Ningxia, Inner Mongolia, and Shanxi) covering an area of approximately 250,000 km2 (Figure 1b). Topographically, the western margin of the Ordos Basin features a big slope gradient due to rapid subsidence, whereas a wide and gentle slope is featured at the eastern margin. Rich mineral resources including oil and gas, coal, and uranium have been discovered in the basin, and its Middle and Upper Triassic Yanchang Formation is an important reservoir for hydrocarbon exploration (Li et al., 2006a, 2006b; Deng et al., 2011; Fu et al., 2012).
The North China Craton can be divided into three parts, the eastern, western, and central Orogenic belts, respectively, due to the tectonic deformation in 1.85 Ga (Kusky and Li, 2003; Kusky, 2011). The Ordos Basin is located in the Western Block as an intraplate cratonic basin (Liu et al., 2006), and it is quite different from the rift basins in Eastern China and the foreland basins in Eastern China. The rift basins in Eastern China have undergone lithospheric thinning, and the foreland basins in Western China are characterized by crustal shortening and lithospheric thickening throughout the Cenozoic (Harrison et al., 1992; Owens and Zandt, 1997; Yin and Harrison, 2000). Indochina movement during the Late Triassic caused the denudation and deformation of the entire basin, which entered a relatively stable depression stage since the Early Jurassic, thus initiating the formation of the Ordos Basin.
The interior part of the Ordos Basin can be mainly subdivided into six secondary tectonic units: the Yimeng uplift in the north, the central Yishan slope, the Tianhuan depression in the west, the thrust belt at the western margin, the Weibei uplift in the south, and the Jinxi flexural fold zone in the east (Figure 1b). Four stages of tectonic evolution are evident there, and so are the four stages of deposition in the basin (T2-K1). Middle-Lower Jurassic Fuxian and Yan’an stages are the most prosperous periods when the broad lake received extensive deposits with an area of over two times compared to previous periods; thus, the main hydrocarbon-bearing strata were well developed; then the basin entered the rapid subsidence stage during the Triassic-Middle Jurassic, when a vast area of oil shale and sandstone reservoirs was formed; under the intense tectonic activities during the Late Jurassic-Early Cretaceous, the thrust-nappe zone was developed at the western margin of the basin along with the conglomerate supply deposited in the foredeep of the eastern western margin in various depths, whereas denudation occurred in the central and eastern basin. As of the Early Cretaceous, the locations of depocenter and accumulation center were nearly the same, that is, on the central-southern part of the western basin (Liu et al., 2005, 2006; Yang and Zhang, 2005).
The formations encountered in the Ordos Basin from top to bottom include Quaternary, Tertiary, Cretaceous, Jurassic, Triassic, Permian, Carboniferous, Ordovician, and Cambrian. The Triassic strata are the main oil-bearing formations in the basin and are composed of three groups: Upper, Middle, and Lower Triassic. Specifically, the Lower Triassic includes the Liujiagou Formation and the Heshanggou Formation from bottom to top, the Middle Triassic is the Zhifang Formation, and the Middle-Upper Triassic is the Yanchang Formation (Figure 2a). More specifically, the Yanchang Formation can be subdivided into 10 segments from bottom to top (Figure 2b). Besides, the sedimentary facies is complex for the Yanchang Formation, including alluvial fan, channel, delta, and lacustrine facies. The delta facies is characterized by compartmentalization and zonation: Braided river delta is mainly distributed on steep slope zones and meandering river delta is mainly distributed on gentle slope zones. The sedimentary systems described above converged at the Huachi-Heshui area, Gansu Province (Fu et al., 2010). The maximum flooding happened during the Chang 7 period, along with which a set of organic-rich dark mudstone and oil shale was deposited in the central basin, characterized by a larger sedimentary thickness and wider distribution range, becoming the main hydrocarbon source rocks of the Mesozoic Ordos Basin (Zhao et al., 2003; Yang and Zhang, 2005; Yang and Deng, 2013; Yang et al., 2016). The oil shale zone is also called a hydrocarbon-rich sag (Figure 1c) (Liu et al., 2006).
Materials and methods
Two-dimensional seismic lines and log data were used in this study. The seismic section is an offset superimposed section under zero-phase procession in the time domain, with a main frequency of 30 Hz, and the sampling interval of the seismic data is 2 ms. Two lines were used for seismic faces analysis (Figure 1), four lines were used to analyze the boundary of deep water and the paleoflow direction (Figure 4), and the data are generally excellent throughout and are eligible for the relevant study. In total, 42 wells were included in this paper, including 35 wells for seismic facies analysis, seven wells for sedimentary phenomena analysis, and two outcrop profiles. The logging curves including natural gamma ray (GR), acousticlog, and spontaneous potential of the 35 wells in the study with a 0.125 m sampling interval, which can clearly show the lithology of the formation.
The seismic reflection configuration refers to the extension of the seismic event itself, and the interactive relationship of the seismic events in the sequence on the seismic section. It is the most reliable seismic facies parameter for revealing the seismic pattern of the sedimentary system (David and Richard, 2009; Peng et al., 2012; David and Bruce, 2016; Frauke et al., 2016; Mustafa et al., 2017). Factors such as the internal structure, appearance, amplitude, and continuity of the seismic reflection, to identify the seismic facies and progradational reflection configurations, are particularly important to interpret the sedimentary setting (Berg, 1982; Zeng et al., 2015). The seismic facies during the Yanchang sedimentary period is analyzed at every stage; then, GR curves are used to summarize the sedimentary facies features and analyze the correlation between the seismic and sedimentary facies for each stage.
We can estimate the paleoslope gradient of sedimentary water bodies through a combination of geologic data (including paleontology and bathymetric data) and progradational reflection on seismic sections (Pekar and Kominz, 2001; Feng et al., 2010; Zeng et al., 2015). This method is more applicable to the modern sedimentation, whereas due to the larger thickness of the overlying strata and compaction for complicated continental basins, it is necessary to carry out compaction correction and adopt the corrected stratigraphic thickness and measured length of the seismic section to calculate the paleoslope gradient. For each seismic section, we select and measure the readily identifiable segments with continuous progradational reflection events to calculate the measured formation length (Table 1, ) and thickness (Table 1, ). Also, the decompaction correction on the measured stratigraphic thickness (Table 1, ) (the correction coefficient of the stratigraphic compaction is 0.3 for the Yanchang Formation in the Ordos Basin; Li et al., 2014) facilitates the calculation of the paleoslope gradient during the Yanchang sedimentary period in the Ordos Basin (Table 1).
Many methods are used to study paleocurrents at present, such as the application of various sedimentary structures and the anisotropy of magnetic susceptibility (Parés et al., 2007; Sounthone et al., 2015), as well as the analysis of heavy minerals (Wolfgang et al., 2016), and the distribution of sedimentary facies in the depositional system. However, each method has its unique advantages and disadvantages under specific conditions. In the study, we use the progradational structure to calculate the azimuth of the paleocurrent (Pu, 1994; Zhang et al., 2008). To this end, we select two intersecting seismic lines with well-defined characteristics of progradational reflection: The azimuth of line1 is x1, and the azimuth of line2 is x2; the apparent dip of the progradational reflection interface along the direction of line1 is , and the apparent dip along the direction of line2 is ; and the angle between line1 and line2 is , and the angle of the progradational reflection trend line with line1 is , whereas that with line2 is (-) (Figure 3). The spatial geometric relationship of the progradational reflection interface can be used to calculate and then obtain the trend of the progradational reflection interface through x = x1 + , that is, the paleocurrent direction (Zhang et al., 2008).
The progradational reflection is not limited to where “water flow” is present, and the key to its initiation is the sedimentary slope, which may come into being in the progradational process of water flow, and also mostly in association with other factors, such as the margin of carbonate rock platform, the margin of the inland basin, or the continental slope (Pu, 1994; Johannessen and Steel, 2005; Magyar et al., 2013). In the seismic section, the bottom of the foreset overlap with the bottom of lacustrine basin, and the bottom boundary of the deep waters is the intersection of the foreset top interface and the bottom of lacustrine basin. Once the boundaries for the deep waters of the same period identified on the seismic section get connected, we obtain the boundary of the deep waters of the lacustrine basin, which will facilitate the further determination of the distribution scope of the deep waters of the lacustrine basin during different periods.
Seismic reflection configurations of the Yanchang Formation
The hydrocarbon-rich Yanchang Formation in the Ordos Basin exhibits six to eight pairs of seismic events with a total thickness of approximately 1300 m. Its seismic reflection configuration comes in five types in general: subparallel, parallel, tangential progradational, shingled progradational, and chaotic reflection, which are characterized below:
Subparallel reflection: The subparallel reflection mainly occurs in the Chang 10-Chang 8 members, Chang 3-Chang 1 members in the steep slope zone, and Chang 3-Chang 1. It usually occurs in the gentle slope zone, featuring low frequency, moderate amplitude, and continuity, and exhibiting approximately parallel contact with the upper and lower reflection layers (Figure 4d, DD′). The subparallel reflection occurring in the Chang 10-Chang 8 members includes three to four pairs of seismic events, whereas the reflections occurring in the Chang 3-Chang 1 members in the steep and gentle slope zones include two to three pairs of seismic events (Figure 4d).
Parallel reflection: In the study area, the Chang 7 member mainly developed parallel reflection, with a pair of seismic events, characterized by strong amplitude and high continuity. They exhibit approximately parallel contact with the lower reflection layer and downlap contact with the upper reflection layer (Figure 4b and 4d, Tt7).
Tangential progradational reflection: The Chang 6-Chang 4+5 members in the steep slope zone developed good tangential progradational reflection with moderate amplitude, relatively continuous seismic events, well-defined topset and bottomset, and diagonally crossing tangential contact between foreset and topset. The progradation complexes include one to two pairs of seismic events and grow thinning from the provenance (southwest) to the center of the basin (northeast) (Figure 4b, AA′). The type of reflection configuration was developed widely in the southwestern steep slope zone of the basin, and it exhibits a fan-shaped distribution toward the interior of the basin.
Shingled progradational reflection: The Chang 6-Chang 4+5 members in the gentle slope zone developed good shingled progradational reflection with moderate amplitude and relatively good continuity of seismic events. They are multiple groups of approximately parallel shingled reflection events, with a well-defined topset and foreset but no obvious bottomset. The progradation complexes include two to three pairs of seismic events, and they thicken from the provenance (northeast) to the center of the basin (southwest) (Figure 4b, BB′ and Figure 4d, BB′). This type of reflection configuration was widely developed in the northeastern gentle slope zone of the basin and distributed in a fan-shaped pattern toward the interior of the basin.
Chaotic reflection: The Chang 3-Chang 1 members at the center of the lacustrine basin usually developed a chaotic reflection, characterized by low frequency, weak amplitude, poor continuity, and a chaotic or discontinuous reflection with the upper and lower reflection layers (Figure 4b, CC′). The chaotic reflection of the Chang 3-Chang 1 members at the center of the lacustrine basin includes two to three pairs of seismic events.
Calculation of the paleoslope gradient and paleocurrent direction with progradational reflection
Calculation of the paleoslope gradient: The results show that during the Yanchang sedimentary period, the maximum grade laid in the southwestern basin (the southwestern steep slope zone with a gradient of approximately 10°–20°) (Table 1, Figure 5a), followed by the northwestern basin (with a gradient of approximately 1.6°–3.3°) (Table 1, Figure 5a), whereas the slope was relatively gentle in the northeastern basin (northeastern gentle slope zone with a gradient of approximately 0.8°–1.2°) (Table 1, Figure 5a). In addition, we have restored the paleobathymetry of the lacustrine Ordos Basin during the Yanchang sedimentary period thereby (Figure 5b).
Calculation of the paleocurrent direction: Through the calculation process, we have found that the paleocurrents in the basin tended to flow toward two directions: 186°–259° in the southwestern region, indicating the provenance origin for the region, and 10°–79° in the northeastern region, indicating the provenance origin for this region.
Demarcation of the deep waters in the Yanchang Formation in the lacustrine basin
The Ordos Basin entered the continental basin development stage in the Late Triassic, and the provenance of the basin was mainly derived from two directions: southwest and northeast. Since the Chang 7-Chang 4+5 sedimentary period, a series of highly constructive sedimentary systems of fluvial-lacustrine delta facies occurred in the southwestern steep and northeastern gentle slope zones, which have developed multiple progradational reflections on the seismic section. We have interpreted several seismic sections of the hydrocarbon-rich basin and identified the typical progradational reflections. Combined with the well drilling data, we have also identified the intersecting point between the progradational reflection interface and the bottom of the lacustrine basin at different periods, and we classified and noted the intersecting points between the progradational reflection interface orienting toward the southwest and northeast and the bottom of the lacustrine basin on each seismic section. In connecting the intersecting points of the different lines of the same periods, the scope of the deep waters of the lacustrine basin was obtained during the Chang 7-Chang 4+5 sedimentary periods (Figure 6), the same as previous results (Qiu et al., 2009). The lacustrine basin was broad in area during the Chang 6 sedimentary period, and the deep waters of it mainly exhibited a northwest–southeast-trending distribution. During the Chang 4+5 sedimentary periods, the lacustrine basin gradually decreases in area and depth, so did the deep waters compared to the Chang 6 sedimentary period, and the deep waters still trended toward the northwest–southeast. Since the Chang 3 sedimentary period, the lacustrine basin died out gradually along with the basement uplift. Based on the analysis above, the Chang 7-Chang 3 members are the periods when the lacustrine basin during the Yanchang sedimentary period grew shrinking from the maximum scale to extinction, during which different oil-rich formations were in conformable contact.
Sedimentary filling characteristics of the Yanchang Formation
Fluvial-shore-shallow lacustrine sediments of subparallel reflection: Topographically, the fluvial facies, shallow delta facies, shore-shallow lacustrine facies, and swamp facies were developed in the basin during the Chang 10-Chang 8 sedimentary period (Chen et al., 2019). The lithofacies consists of coarse-grained sandstones with parallel bedding, fine-grained sandstones (Figure 7a), fine-grained sandstones with cross bedding, siltstones with wavy cross bedding (Figure 7b), and mudstones with horizontal bedding. Lag deposits were developed at the bottom of the channels (Figure 7a), with plant carbon and biological debris seen locally (Figure 7c), and the inferior coal seams are often seen in the mudstone (Deng et al., 2008). The seismic section is characterized by moderate continuity and subparallel reflection, and it shows three to four pairs of seismic events (Figure 4b and 4d), indicating a deposition dominated by vertical aggradational deposits and featuring isostatic subsidence and relatively large accumulation space. The well-tie cross section indicates that there were hardly any sand bodies developed during the Chang 10 sedimentary period. However, sand bodies in isolated channels and distributary channels were developed well during the Chang 8-9 sedimentary periods. The interwell continuity of the sand bodies is poor, usually extending to 4–10 km or so (Figure 8b and 8d).
Semideep to deep lacustrine sedimentation of parallel reflection: Late in the Chang 8 sedimentary period, the scale of the lacustrine basin expanded somewhat and the water body grew deep, exhibiting shore-lacustrine paludification over large areas. During the Chang 7 sedimentary period, the water body of the lacustrine basin got deeper sharply with the basin reaching its peak in development, and a set of organic-rich dark mudstone and oil shale with a large thickness was developed (Figure 7f) (the Zhangjiatan Shale). In terms of the seismic section, the set of oil shale shows the characteristics of parallel reflection — high continuity, strong amplitude, and one pair of seismic events developed (Figures 4 and 8). The thickness of the oil shale varies between 20 and 40 m, and its natural GR value is more than 180 American Petroleum Institute (API) (the natural GR value of the adjacent mudstones is approximately 100 API); it is easy to identify single wells, and the interwell continuity is good (Figure 8b and 8d). The set of shale may serve as the major hydrocarbon source rocks in the Mesozoic Ordos Basin, with a distribution extending to Dingbian in the north, Xunyi in the south, the Huanxian-Zhenyuan area in the east, and the Zhidan-Fuxian area in the west (Figure 1).
During the middle-to-late Chang 7 sedimentary period, the sand bodies deposited at the delta front in the vicinity of the semideep lakes underwent secondary transportation and deposition under the impact of external forces, including tectonic events and waves, forming the arenaceous debris flow sand bodies and turbidite sediments over large areas in the deep waters at the bottom of the lake (Xian et al., 2018) (Figure 8b and 8d). In particular, the dominant lithofacies is the massive fine-grained sandstone facies for the debris-flow sand bodies (Figure 7d). The basal/top and mudstone contacts of debris flows are sharp, and the argillaceous agglomerates and mudstone clasts are seen locally. The turbidite sediments were mainly the siltstones with wavy cross bedding alternating with mudstones with horizontal bedding (Figure 7e), and the thickness of monolayers ranges between 10 and 100 cm.
Braided river delta sedimentation of tangential progradation: During the Chang 6-Chang 4+5 sedimentary periods, the basin basement slowed down in subsidence, the periphery of the basin began to rise. The provenance supply increased gradually, and the supply flux of sediments was greater than the available accumulation space, leading to the formation of a series of highly constructive fluvial-lacustrine delta sedimentary systems in the basin. The braided river delta-dominated and meandering river delta in the southwestern steep slope zone are just some examples (Wang et al., 2010).
In terms of the seismic section of the southwestern steep slope zone, the Chang 6-Chang 4+5 members developed good tangential progradational reflections (Figure 4b, AA′), and we have identified at least four progradational reflections that may just represent four episodes of the braided river delta progradation. The progradational reflection layers are trending toward the southwest–northeast and dipping to the northeast indicating that the provenance was derived from the southwest and prograded toward the northeast where the lake was situated (Figures 4a and 8a). The sand bodies of braided river delta were well developed in Chang 6-Chang 4+5, and the lithofacies are mainly fine-grained sandstones with cross bedding (Figure 7g), fine-grained sandstones with parallel bedding, siltstones with wavy cross bedding (Figure 7j), and mudstones with horizontal bedding. The well-tie cross section indicates that the braided river delta shows obvious progradational features, the topset is mainly resulted from braided channel deposits, and the foreset is from the sediments in the underwater distributary channels at the delta front. Under the impact of such external forces as gravity and waves, the sand bodies at the delta front underwent secondary transportation and deposition, resulting in arenaceous debris flow sand bodies distributed over large areas (Figure 8b and 8d).
Meandering river delta sedimentation of shingled progradation: In terms of the seismic section of the northeastern gentle slope zone, shingled progradational reflections were well developed in the Chang 6-Chang 4+5 members (Figure 4b, BB′), and we have identified at least three progradational reflections that may just represent three episodes of meandering river delta progradation. The progradational reflection layers are all trending toward the northeast–southwest, with a tendency toward the southwest, indicating that the provenance was derived from the northeast, and it prograded to the southwest where the lake was situated (Figures 4a and 8a). The meandering river delta in the Chang 6-Chang 4+5 members developed widespread sand bodies, and the lithofacies mainly include the fine-grained sandstones with cross bedding (Figure 7i), fine-grained sandstones with parallel bedding, siltstones with wavy cross bedding, and mudstones with horizontal bedding. The meandering river delta is relatively extensive on the northeastern gentle slope zone. Its outcrop sections cover the outcrops of Qingjianhe (Figure 7i), Yanhe, Shiwanghe, Yunyanhe, Huangling, Tongchuan, Yaoxian, and Xuefengchuan (Xian et al., 2018). These deposits are usually channels filled with sandstone and the fine-grained sediments of flood plain with a ratio close to one, and the normally graded sequence deposition is well-defined and exhibits a dual structure. The well-tie cross section indicates strong characteristics of meandering river delta progradation, with the topset mainly being the sediments of the meandering river and delta plain distributary channels and the foreset being the sediments of the underwater distributary channels at the delta front. With the impact of external forces, such as gravity and waves, the sand bodies at the delta front suffered secondary transportation and deposition, which caused the sand bodies of arenaceous debris flow distributed over large areas (Figure 8b and 8d).
Fluvial-shore shallow lacustrine sedimentation of subparallel chaotic reflection: Under the progradational effect during the Chang 6-Chang 4+5 sedimentary periods, the basin decreased substantially in scale and it began to die out during the Chang 3 sedimentary period (Wang et al., 2010). In terms of the seismic section, discontinuous progradational reflections of small scale are identified on the southwestern steep and the northeastern gentle slope zones (Figure 8a and 8c). The periphery of the lacustrine basin exhibits the characteristics of subparallel reflection, and the center of the basin shows the seismic facies of chaotic reflection with weak amplitude and poor continuity (Figure 8a and 8c), which may just represent the characteristics of fluvial-shore shallow lacustrine sedimentation. During the Chang 2 sedimentary period, the basin experienced intense uplifting and denudation: The southwestern formation was corroded completely, and the northwestern part suffered erosion of scouring from the Jurassic paleochannels as a result. The residual formation is relatively thin (Figure 8b and 8d). During the Chang 1 sedimentary period, the coal seams were developed widely in the Ordos Basin due to the swamping; meanwhile, the formation of Chang 1 in the northeast was also incomplete under later erosion and scouring of seasonal floods (Figure 8b and 8d). The major lithofacies during the Chang 3-Chang 1 sedimentary periods include fine-grained sandstones with parallel bedding (Figure 7k), fine-grained sandstones with cross bedding (Figure 7i), siltstones with wavy cross bedding, and mudstones with horizontal bedding. Moreover, scoured surfaces were well developed in channels (Figure 7k). The well-tie cross section indicates that sand bodies of isolated and distributary channels were well developed in the Chang 3-Chang 1 members, but the interwell continuity of the sand bodies is poor, extending to approximately 4–10 km (Figure 8b and 8d).
Sedimentary filling pattern of the lacustrine basin during the Yanchang sedimentary period
The Ordos Basin entered the stage of continental basin development in the Late Triassic, and water systems around the basin during the Yanchang sedimentary period were well developed to extend into the lacustrine basin, generating plenty of sediments of fluvial, delta, semideep to deep lacustrine, and turbidity current facies advancing toward the center of the basin (Zhao et al., 2011).
The Yanchang Formation in the Ordos Basin was generally dominated by isostatic subsidence as a whole; its sedimentary filling evolution has undergone five stages: initial depression, intense depression, progradational filling, uplifting and denudation, and shrinking and extinction. During the Chang 10-Chang 8 sedimentary periods, the lacustrine basin began its initial depression, during which the basin was relatively flat in topography, and generally in sedimentary settings of fluvial, shallow water delta, shore-shallow lacustrine, and swamp facies. The sand bodies were hardly developed during the Chang 10 sedimentary period (Chen et al., 2019), whereas during the Chang 9 and Chang 8 sedimentary periods, sand bodies were widely developed in isolated and distributary channels, with a poor continuity of the interwell sand bodies, extending to approximately 4–10 km.
The lacustrine basin was in the intense depression stage during the Chang 7-Chang 8 sedimentary periods. It was expanded somewhat with the water body growing deeper during the Chang 8 sedimentary period, resulting in shore-lacustrine paludification over large areas. The water body got rapidly deeper, and the lacustrine basin reached its peak in development during the early Chang 7 sedimentary period, with a thick set of organic-rich dark mudstone and oil shale (the Zhangjiatan Shale) being generated. During the middle-to-late Chang 7 sedimentary period, the sand bodies deposited at the delta front in the vicinity of the semideep to deep lakes underwent secondary transportation and deposition, forming a wide distribution of arenaceous debris flow sand bodies and turbidite sediments in the deep waters at the bottom of the lake.
During the Chang 6-Chang 4+5 sedimentary periods, the basin entered the progradational filling stage: The basement subsidence slowed down, and the periphery of the basin began to rise. The provenance supply increased gradually, and the supply flux of sediments was greater than the available accumulation space, leading to the formation of a series of highly constructive fluvial-lacustrine delta sedimentary systems in the basin (Wang et al., 2010). Four episodes of braided river delta progradation with tangential progradational reflections were deposited on the southwestern steep slope zone (Figure 9a), and three episodes of meandering river delta progradation with shingled progradational reflections were deposited on the northeastern gentle slope zone (Figure 9b), at least.
The Chang 3-Chang 2 sedimentary periods represent the stage of uplifting and denudation for the lacustrine basin, resulting in obvious shrinking in the basin scope. The fluvial-shore-shallow lacustrine facies became the dominant facies during the Chang 3 sedimentary period.
The center of the lake started moving to the north during the Chang 1 sedimentary period, when large areas of swamp and coal seams were developed in the basin. The lacustrine basin entered the stage of silting, shrinking, and extinction. The southern, especially the southwestern, basin was subjected to strong uplifting and denudation, the upper strata of which were almost gone; whereas the northwestern part suffered scouring erosion from the Jurassic paleochannels and the thickness of the residual strata is relatively thin.
Differential uplifting and denudation of the lacustrine basin during the Yanchang sedimentary period
The entire evolutionary stage is relatively stable for the Yanchang Formation in the Ordos Basin. There are no large-scale unconformities among the strata, and the seismic section has shown no obvious truncation. However, the degree of denudation varies in the different regions inside the basin, and we called this the differential uplifting and denudation of the basin. Chen and Ni (2006) adopt the stratigraphic correlation to determine that the denudation thickness was not large for the top of the Yanchang Formation at the end of the Triassic and that the southwestern basin was a region having undergone the most intense denudation. Gao and Ren (2006) conclude that the denudation thickness came to its peak in the southern basin according to the ln (Ro)-H regression equation of certain single wells in the basin and the distribution of restored denudation thickness. Li et al. (2016) estimate the stratigraphic denudation thickness of the Triassic using the compaction curve extrapolation and the stratigraphic trend comparison method, and they suggest that tectonic subsidence, uplifting and denudation, should jointly control the structural evolution of the Yanchang Formation. In the study, given the restoration of the sedimentary section of the Yanchang Formation through the analysis of seismic section and drilling data (Figure 8). Based on the drilling data and the calculation of paleoslope and paleoflow direction, we restored the sedimentary process of the Yanchang Formation, and we have obtained the following understanding:
The results show that the maximum slope located in the southwestern Ordos Basin had a gradient of approximately 10°–20°, and the flow direction was 186°–259° during the sedimentary period of the Yanchang Formation, which also indicates that the provenance area is located in the southwestern Ordos Basin; the minimum slope located in the northeastern Ordos Basin had a gradient of approximately 0.8°–1.2°, and the flow direction was 10°–79°, indicating the provenance area from this region. The source direction that we got is essentially consistent with previous research results (Wei et al., 2003; Cao et al., 2008; Luo et al., 2008).
The denudation degree reached its peak on the southwestern formation of the hydrocarbon-rich basin; the residual formation in Chang 2 was relatively thin (Figure 8b); part of the basin was subject to the downcutting of the Jurassic paleochannels; the strata in Chang 2 were gone from scouring, and the strata in Chang 3 also experienced a certain degree of denudation (Figure 8d). The denudation rate was relatively low for the strata in the northeastern basin, with only Chang 1 suffering denudation and its residual strata being relatively thick (Figure 8d). By comparing multiple east–west-trending seismic sections from south to north, we found that the variation in the topset thickness is relatively large for the progradational reflection late in the Yanchang sedimentary period situated to the west side of the hydrocarbon-rich basin: The topset is relatively thin in the progradational reflection in the southwestern basin, whereas the topset is relatively thick in the progradational reflection in the northwestern basin (Figure 8), which just indicates that the southwestern strata went through higher degree of erosion compared to the northwestern equivalents. This is because at the end of the Late Triassic, intense compression happened between the Yangtze Plate and the North China Plate, causing strong uplifting and denudation for the southwestern margin of the basin. This geodynamic process has been confirmed by paleomagnetic data (Wang et al., 2010).
The seismic reflection configurations of the Yanchang Formation mainly come in five types: subparallel reflection with low frequency, moderate amplitude, and moderate continuity; parallel reflection with strong amplitude and high continuity; tangential progradational reflection with moderate amplitude and relatively continuous seismic events; shingled progradational reflection arranged in imbricate fashion with moderate amplitude and relatively continuous seismic events; and chaotic reflection with low frequency, weak amplitude, and poor continuity.
During the Yanchang sedimentary period, the slope gradient came to its top for the southwestern region of the basin (the southwestern steep slope zone of approximately 10°–20°), followed by the northwestern basin (with a gradient of approximately 1.6°–3.3°), whereas the slope gradient was relatively gentle in the northeastern basin (the northeastern gentle slope zone of approximately 0.8°–1.2°). The paleocurrents of the basin tended to flow toward two directions. The paleocurrents in the southwest region trended toward 186°–259°, indicating where the provenance was derived from for the region, whereas the paleocurrent direction in the northeastern region was 10°–79°, also indicating the provenance origin for this region.
The Yanchang Formation in the Ordos Basin is generally dominated by isostatic subsidence, and its sedimentary filling evolution has experienced five stages: initial depression, intense depression, progradational filling, uplifting and denudation, and shrinking and extinction, corresponding to the Chang 10-Chang 9, Chang 8-Chang 7, Chang 6-Chang 4+5, Chang 3-Chang 2, and Chang 1 sedimentary periods, respectively.
This study was supported by National Science and Technology Major Project of China (2016ZX05026-007-007), Natural Science Basic Research Plan in Shaanxi Province of China (2017JM4013), and Doctoral Research Project of Yan’an University (YDBK2018-31). The authors would like to thank the anonymous reviewers for their critical reviews, which have led to much improvement of this paper.
Data and materials availability
Data associated with this research are available and can be obtained by contacting the corresponding author.