The 8th Member of Permian Lower Shihezi Formation (He 8 Member) in the Ordos Basin is a main gas production layer of the Sulige large gas province^[1]. As a natural gas producing pay of great importance, its sedimentary features have been studied extensively. According to the previous studies, which focused on the provenance systems in the north of the basin, the He 8 Member is generally considered as a set of deposits of braided river-shallow delta facies. From north to south, the Ordos Basin was reported to develop the deposits of alluvial fan, alluvial plain, delta plain, delta front and shallow lake, and the frequent migration of channels resulted in large-scale distribution of sand bodies in the northern basin^[2,3,4,5,6]. The basis of this conclusion is that there was a unified catchment lake basin in the south-central basin, and the rivers in the north formed delta after reaching the lake area. Some researchers think that the He 8 Member is mainly composed of beach-bar deposits in the basin, where the SN-trending pulsatile lacustrine transgression continuously transformed the terrigenous clastic materials and formed the shore-lake beach-bar sand bodies of wide distribution all over the basin from east to west^[7]. As the natural gas exploration of Upper Paleozoic strata in the Ordos Basin extended to the south in recent years, the south-central basin has become the study focus correspondingly, and the existence of provenances in the south during the depositional period of He 8 Member and the distribution of provenance systems in the north and south have been extensively discussed. The concepts of "catchment area" and "interaction area" of the north and south provenance systems were proposed^[8]. It is considered that the delta front belts in the northern and southern delta systems were joined in the central south of the basin, separating the unified catchment lake basin into several catchment areas. Others argued that the whole Ordos Basin was covered by rivers without lacustrine deposits when the He 8 Member was deposited, and the catchment lake then was outside the present Ordos Basin^[9]. The latest exploration results show that sand bodies still develop widely in the south-central basin, but are more complex than those in the north. It is difficult to explain their formation with the early braided river-shallow delta and single channel or beach bar deposits. Therefore, the development characteristics of He 8 Member sedimentary systems in the Ordos Basin need to be examined and rediscovered further.

During the Late Paleozoic period, the Ordos Basin was on the slope zone in the west of North China Platform, and its tectonic and sedimentary evolution was closely related to that of the North China Basin^[10,11]. Thus, in this study, the sedimentary features of He 8 Member in this area are investigated from a broader view of the North China platform. On the basis of the development and distribution characteristics of Ordos Basin and Late Paleozoic sedimentary systems in North China Platform obtained from previous researches, combined with the data of more than 1000 wells and 32 outcrops in Ordos Basin and North China Basin, the sedimentary features of He 8 Member in Ordos Basin are analyzed and re-recognized systematically from the perspective of the sedimentary background and lithofacies paleogeographic pattern of the larger North China Basin, and its sedimentary and evolution process is reproduced by conducting flume experiment, and its sedimentary model is established.

The Ordos Basin (Ordos area) refers to the vast area south of Yinshan Mountain, north of Qinling Mountain, east of Helan Mountain and west of Lüliang Mountain today, and was located in the western North China Platform in Paleozoic (Fig. 1)^[10]. In the Late Paleozoic, the platform of North China area was gentle and open, the basin bottom was almost flat with very small relative height difference. As a part of the North China Platform, the Ordos area had tectonic and sedimentary evolution consistent to the North China Platform during the Late Paleozoic, which was controlled by the expansion and subduction of Xing'an- Mongolian Trough in the north, Qinqi Trough and its associated Helan Aulacogen on the southwest margin^[10,11]. During the Late Carboniferous period, with the slow subsidence of the whole platform, the regional structure was characterized by the SN paleouplift running across the basin in the middle, the Helan rift zone in the west and the shallow depression in the craton in the east. Since the sedimentation period of Shanxi Formation in the Early Permian, influenced by the southward subduction of Xing'an-Mongolian Trough in the north, the regional stress field had been dominated by the NS compressive stress, resulting in the uplifting of the northern region, gradual seawater retreat to the south and southeast, and dominance of terrestrial deposits in the Ordos area with residual sea remained locally in the south. To the beginning of the deposition of Lower Shihezi Formation (He 8 Member), the sea level began to decline rapidly, the coastline migrated to the southern North China, and the sea water completely retreated from the Ordos area. The climate changed from warm and humid to hot and dry, and a set of grey - yellowish green terrigenous clastic formations deposited^{[10,11,12,13]}.

The development of massive fluvial sedimentary sand bodies in the He 8 Member in northern Ordos area has been widely accepted by researchers^[2,3,4,5,6]. However, due to the low degree of exploration in the south-central Ordos area, the type and distribution of sedimentary facies there have always been controversial. The dominant viewpoint for a long time has been that the He 8 Member in the south-central Ordos area is a braided river-shallow delta system^{[2-6, 8]}. With the deepening of exploration and the enrichment of geological data, this viewpoint was doubted.

(1) Massive distribution of gravel-bearing sandstone all over the area. The He 8 Member sandstone in Ordos area, also known as the Luotuobozi sandstone, is a set of light gray gravel-bearing coarse sandstone of alluvial fan-braided river delta origin, grayish medium-coarse grained sandstone and grayish-green lithic quartz sandstone^{[2, 13-15]}. Statistics of nearly 1000 exploration wells in the Ordos Basin show that the sandstone has a short-axis lenticular cross-section, overlaps vertically and appears as a large piece laterally like a net blanket. In addition, the gravel interlayers in the sandstone are widely distributed in the whole Ordos area (Fig. 2). After review of 799 wells in the area, the gravel-bearing layer was found in a total of 377 wells. The gravel-bearing sandstone interlayers are originated from the formation of debris flow and transportation by tractive current in high-energy waterway during flooding^[16]. As shown in the overlap of He 8 Member sandbody distribution and the thickness of the gravel-bearing interlayers, the gravel-bearing sandstone interlayers are very well-developed well and coarse in grain size in the north of Sulige, Uxin Banner and Yulin in the northern basin, gravels are developed in Qingyang area in the southwest and developed in Yanchang in the central basin and Yichuan and Xunyi in the south. The widespread distribution of gravel interlayers in the He 8 Member from north to south in the Ordos area indicates that high-energy channels were widespread across the basin during the deposition of He 8 Member. The large-scale meshwork-carpet distribution of gravel-bearing sandstone all over the area, especially the wide distribution of gravel interlayers in the sandstone cannot be explained reasonably if the south-central basin was a river-shallow delta system and existing catchment lake basins, as the shallow lake facies is low-energy sedimentary environment.

(2) Undefined division of sedimentary facies belts. The inaccurate differentiation of delta plain and delta front is another problem in the viewpoint that this area was of braided river-shallow delta system. Previously, researchers used to identify water oxidation environment and underwater reduction environment according to the color of mudstone, and then distributary channel and underwater distributary channel. However, according to the sedimentary facies map^{[2,3,4,5,6,7,8]} that reflects the delta in this area, the division of delta plain and delta front facies belts is not unified, and the delta front belt is more than 100 km, which is inconsistent with the viewpoint accepted in academia that a shallow delta is characterized by large plain and small front, and can’t explain that the underwater distributary channel extended over 100 km.

After comprehensive analysis of the geological background, drilling data and marginal outcrops of Ordos Basin, it is concluded that the alluvial-fluvial systems were widely distributed during the deposition of He 8 Member, several separated seasonal lakes developed in the south-central area, and the whole area was not covered by monotonous fluvial or beach-bar deposits, which are based on the followings.

(1) Multiple provenances, strong supply and development of multiple water systems. The Late Paleozoic was a giant cratonic basin, which opened in the east and southeast and connected to the ocean and was surrounded by ancient lands or paleouplifts to the north, west and south, and its continental slope was speculated to locate in the Japanese Sea and the East China Sea to the east of Korea^[17]. During the depositional period of He 8 Member in the Lower Shihezi Formation, the Ordos area was adjacent to the paleocontinents in the north, west and south, and connected with the vast North China Craton Basin to the east. With the amalgamation of Hercynian fold belt and the strong collision between Siberian plate and North China plate, the orogeny around North China platform gradually strengthened, which aggravated the weathering erosion of parent rocks in the provenances, thus source material supply increased. The Ordos area at this point were near provenances in the north, west and south, and had multiple sand supply sources and multi-water systems^{[2, 13]}, which provided the necessary conditions for wide development of alluvial-fluvial systems in the whole area.

(2) Flat ancient landform, shallow sedimentary water body. The He 8 Member in Ordos area doesn't have big variation, with thickness difference of less than 30 m across the whole area, indicating open and flat landform. Statistics of mudstone color of He 8 Member shows the He 8 Member mudstone in Ordos area comes in gray, light grey, grey green, grey brown, mixed-color, and brown etc, in which light grey, grey green and mixed-colored mudstone are in dominance with barely dark ones (Fig. 2a and 2c). From the thickness of mudstone, the pure mudstone layers in the He 8 Member are generally 5-10 m thick each, and 5-25 m thick cumulatively. The high-value and low-value regions of mudstone cumulative thickness distribute alternately, and are mutually complementary with the sandstone thickness (Fig. 2b), which indicates that the He 8 Member was deposited in a very shallow water environment with frequent fluctuation of water level in the seasonal dry climate, and the whole area was lack of deep- water deposits and the sediments frequently exposed to the surface.

(3) Distribution of fluvial sand bodies all over the area. The He 8 Member sandstone in the south-central Ordos area is composed of poorly sorted gravel-bearing coarse sandstone, coarse sandstone, medium sandstone and fine sandstone, of low composition maturity and texture maturity, with gravel interlayers in some wells, massive bedding, parallel bedding, lateral accretion cross-bedding, tabular bedding, and characteristic scouring structure at the sandstone bottom (Fig. 3a, 3c, 3e). Generally, the sandstone takes on box-shape assemblages, box-bell-shape assemblages and bell-shape assemblages with medium-high amplitude on GR curve (Fig. 4). Its lithofacies, sedimentary structures and GR curves are basically similar to those of fluvial sand bodies in the north. Comparing the cores of He 8 Member gravel-bearing interlayers from 8 typical wells in the northern and south-central Ordos area shows that the gravel size in the south-central Ordos area is smaller than that in the northern part, and the composition and texture maturity of gravel and fillings between gravels are higher, which should be the result of a long-distance transport by river (Fig. 3d, 3e and Table 1).

(4) Existence of several separated shallow lakes. The He 8 Member sandstone and mudstone in the south-central Ordos area are mostly light gray, gray and grayish green, with a small amount of variegated and brown mudstone. The mud-stones in Well HT 2 are grayish green and variegated, with reddish-brown mud gravels, reflecting the sedimentary environment was shallow in water depth, and the interchannel mudstone exposed repeatedly due to seasonal drought (Fig. 3f). The mudstone layers in the He 8 Member are thick in a few wells. For example, in Well LA1, there is a continuous grey mudstone layer of about 30 m thick, which must be deposits formed in local relatively stable shallow lake. Moreover, the areas with mudstone layers of combined large thickness are not distributed widely and are separated by areas with thick sandstone (Fig. 2a and 2b), which indicates there are shallow lake deposits discontinuous in distribution, appearing as multiple independent seasonal lakes separated by multi- channel systems.

During the deposition of He 8 Member, Ordos area was at the early stage to early transform to complete terrestrial deposition, and developed alluvial fan-alluvial plains dominated by the alluvial-fluvial systems due to flat ancient landform, adequate source supply and frequent flooding. The south-central area was characterized by a sedimentary framework of channels, floodplains alternated with interchannel overflow lakes without a unified large-scale catchment lake. Meanwhile, with the regional regression in the North China Basin during the late deposition stage of Shanxi Formation, the water systems in the north, west and south continued to advance and extend along the regression direction to the southeast after converging in the south-central relatively low-lying regions. To verify this inference, it is necessary to look and examine the development and distribution of sedimentary systems in Ordos area from the broader paleogeographic background of North China Basin.

According to the regional stratigraphic correlation with reference to the previous research results, we tried to reconstruct the paleogeographic pattern during the deposition of He 8 Member in North China based on the lithofacies characteristics analysis, types and distribution of sedimentary facies. Limited by deficient data available, the North China area selected in this study covers the area from Tanlu fault in the east, to west margin thrust belt in the west, from Yinshan ancient land in the north to Qinling fold belt in the south.

The identification and correlation of isochronostratigraphic units are the basis of paleogeographic reconstruction. According to the view of three parts in the Permian^[18,19], and the stratigraphic division proposed by "Stratigraphic Guide of China and Specifications of Stratigraphic Guide of China"^[20], the Lower Shihezi Formation in Ordos area was formed in the Early Permian, while the He 8 Member at the bottom of Lower Shihezi Formation was deposited in the early period of Middle Permian. Previous studies on division and correlation of the Permian strata in North China have basically reached a basic consensus ^{[18,19,20,21,22]} that the Lower Shihezi Formation is characterized by the development of Middle Paleozoic cathaysian flora including Taeniopteris, Cathaysiopteris whitei, Emplectopteris, Emplectopteridium and Gigantonoclea. The central and northern areas of North China take the Permian in Qiligou, in Xishan, Taiyuan as the standard section. The bottom and top of Lower Shihezi Formation in the lower part of Middle Permian are the bottom of Luotuobozi mudstone and top of Taohua mudstone. The southern part of North China takes Fangshan, Yuxian in the west of Henan Province as the standard section. The bottom and top of contemporaneous sedimentary strata of Lower Shihezi Formation are the bottom of Luoguoyao mudstone and the top of Daziban mudstone. Based on this, the division and comparison of He 8 Member in North China were carried out in this study according to lithofacies features and regional marker beds (such as coal seam) of sedimentary strata in the early stage of Middle Permian.

The Lower Shihezi Formation in the lower part of Middle Permian in North China generally is 40 to 170 m thick in total. In the ancient land adjacent to the basin margin, it is thinner, ranging from 40 m to 80 m. It has a thickness difference of less than 40 m within the basin, suggesting that the whole cratonic basin was stable. The He 8 Member at the bottom of Lower Shihezi Formation in North China is composed of pebbly sandstone and gravel-bearing sandstone, light gray sandstone, grayish green fine-siltstone, mudstone and transitional argillaceous sandstone and sandy mudstone, and the sandstone is generally coarse in the north and fine in the south. The lithofacies types mainly include massive bedding gravel-bearing coarse sandstone facies, tubular bedding medium-coarse sandstone facies, normal grading bedding medium-coarse sandstone facies, wedge-shaped bedding medium-coarse sandstone facies, parallel bedding medium-coarse sandstone facies, ripple siltstone facies, massive argillaceous sandstone facies and shale facies.

The sand content of He 8 Member in North China is high in the north and low in the south. The areas with sand content of greater than 40% are mainly distributed in the north of North China. Among them, Wuhai, Baotou, Zhungeer, Datong and Jinzhou have the highest sand content of 70% to 92%, where several belts with high sand content extend from north to south, and in Ordos, Luliang, Wuxiang and Lingchuan, the sand content is 40% to 70%. In Jinzhou, Tianjin, Beijing and Xinwen, the sand content is 40% to 90%. In addition, in Yaoxian and Hancheng in the south of the area, the sand content ranges from 40% to 50%, and in the southern North China the sand content is the lowest, generally ranging between 5% and 20%.

The He 8 Member mudstone in the central and northern parts of North China contains plant stem and leaf fossils but scarcely, and the number of plant fossils increase in the southern North China. In the south of Jiaozuo - Zhengzhou - Xuzhou, the He8 Member has multiple thin coal seams and coal streaks, which is called as the Third Coal-Bearing Member in the Regional Geology. As shown by the distribution map of coal seams, the cumulative thickness of coal seams in the north of Pingdingshan and south of Pingmu is more than 3 m. From Huaibei to Huainan, the coal seams increase in number and single layer thickness, and reach 4.5 m thick cumulatively in Huainan (Fig. 5). The black shales are mainly developed in Pingdingshan, Fangshan, Pingmu in the southeast of Henan Province and Huaibei and Huainan of Anhui Province, and coal seams and black shales are almost absent in the area north of Yellow River and Ordos area.

Referring to the previous study results^{[2-9, 23-26]}, the sedimentary environment and facies of the He 8 Member in North China were identified and analyzed based on the drilling cores, outcrops, logging curves and rock assemblages. The sedimentary environments identified include alluvial fan, alluvial plain river, coastal plain, delta and sedimentary subfacies identified include mid-fan, braided river, flood plain, meandering river, overflow flat, flood lake (interchannel overflow lake), delta, tidal flat, lagoon and coastal marshand etc (Fig. 6 and Table 2). Regionally, the He8 member in North China has fan facies and alluvial plain facies of continental alluvial system, and deposits of delta and coastal plain of transitional sedimentary system from north to southeast. In the north of Wuhai - Hangjin Banner - Ordos, the deposits of piedmont alluvial fan dominate, while in the south, massive fluvial deposits of alluvial plain are in dominance.

Based on the comprehensive analysis of stratigraphic lithofacies, types and distribution of sedimentary facies, drawing on previous research results, the paleogeography of the early period of Middle Permian (depositional period of He 8 Member) in North China was mapped (Fig. 7). This map shows that the North China Basin was characterized by sea in the south and land in the north. As the Yingshan paleo-land in the north uplifted and suffered increasingly severe denudation, providing sufficient source material, the sea level fell rapidly, the sea water receded to the southeast, and the coastline retreated to the southern North China. The northern and central North China Basin were arid, so clastic sediments deposited aggressively, inhibiting plant growth and peat accumulation, forming the inland alluvial plain without coal seams, so alluvial fan-river- interchannel overflow lake facies developed from north to south. The southern North China Basin was an offshore sedimentary environment with warm and humid climate, where thin coal seams or recoverable coal beds formed, and transitional facies including delta, tidal flat, lagoon and swamp developed.

According to the reconstructed paleogeography of North China, the Ordos area was located in the west of the Great North China Basin during the depositional period of the He 8 Member, and was mainly an alluvial fan-alluvial plain environment, where the sedimentary system was controlled by the multiple provenances jointly, including Yingshan paleo-land in the north, Alashan in the northwest, Longxi paleo-land in the southwest and Qinling-Dabie in the south. Waters along provenance directions converged in the south and then flew out to the southeast, and the Ordos area during this period was only a "water-pass-by area” rather than a final catchment area.

By using the flume simulation experiment device provided by Changjiang University, the research group carried out the flume simulation experiment for the depositional evolution of Upper Paleozoic Shanxi Formation-He 8 Member in the Ordos Basin. The designed experimental device and process were reported in detail in Ref. [23]. Limited by space, this paper only introduces and analyzes the design parameters and simulation experimental results of He 8 Member.

According to the provenance characteristics during the deposition of He 8 Member, three water inlets were installed on one side of the flume to simulate two provenances in Yingshan paleo-land in the north and one provenance in Alashan paleo-land in the northwest margin, two water inlets were installed on the other side to simulate the provenances of Qinling paleo-land in the south and Liupanshan paleo-land in the southwest. Among them, three provenances in the north were the main provenances with coarser sand, and the other two provenances were secondary provenances with finer sand. The outlet was installed in the middle of the flume to simulate the outflow channel and control the sedimentary base level. In addition, since flooding events happened frequently during the depositional period of He 8 Member, the simulation process was divided into four stages. The first and third stages simulated the depositional process during flood period, and the second and the fourth stages simulated the sedimentary process during dry season, respectively. The ratio of sand addition during flood period to that during dry season was set at 6:1.

The simulation results show that the sedimentary systems and sandbody distribution formed by the north and south provenances are controlled by paleotopography, water velocity, flow rate and base level fluctuation. The ancient terrain low in the middle and high in the north and south makes waters to concentrate in the mid-south from all directions, and the local difference of ancient terrain controls the flowing direction of waters. The sediments are carried southward rapidly in flood season, and a large amount of coarse debris deposits, expanding the distribution area of sand bodies and reducing the catchment area gradually. In dry season, with the decrease of flow rate, the sand quantity carried by water dropped greatly, and fine debris deposit, and later floods would wash away and transport debris deposited earlier, leading to continuous forward migration of sand bodies. The water level of outlet controls the ultimate the advance distance of the provenance system. Before the He 8 Member deposited, a unified catchment area between the north and south alluvial systems existed due to the high water level of outlets. With the fall of water level, the catchment area shrank gradually. When the base level dropped to the lowest, the continuous transportation by multi-period floods resulted in the convergence of alluvial systems in the north and the south, the unified catchment area was separated by sands and shrank, and finally the whole area was occupied by alluvial sand bodies (Fig. 8).

Based on the above experiment results and paleogeographic background analysis, the large-scale regression in the Ordos Basin at the late depositional period of Permian Shanxi Formation caused the face-to-face advancing of sedimentary systems in the north and south and the alluvial fan-alluvial plain rich in gravels all over the basin during the deposition of He 8 Member. Before the deposition of He 8 Member, a unified offshore catchment lake basin existed in the Ordos area. With the rapid recession of the North China Sea to the southeast, the base level fell on the whole, and flood events occurred frequently, the alluvial systems of the provenances in the north and south carrying massive detrital materials pushed toward each other and finally intersected, and the connected sand bodies separated the original unified lake basin into several independent catchment areas. To the deposition end of He 8 Member, the alluvial plain eventually dominated the whole area, forming extensive pebbly sandstone, and only in the south-central area remained several seasonal flooding lakes (interchannel overflow lakes) divided by multiple channels. Based on this, the sedimentary model of "alluvial river & floodplain lake" of He 8 Member has been established. During the depositional period of He 8 Member, the Ordos area was dominated by alluvial plain, and alluvial fan, braided river, and flood plain developed in the north. To the south- central area, the sedimentary facies changed to less tortuous meandering river, floodplain and flooding lake (interchannel overflow lake), appearing as alternated distribution of main channels, flooding plains and interchannel flooding lakes, with secondary channels entering into or passing through the flooding plain and interchannel flooding lake (Fig. 9).

The large gentle-slope delta developed in the Upper Paleozoic in the northern Ordos Basin provides favorable reservoir conditions for the widely developed tight gas reservoirs in this area. As the exploration of the basin expanded to the south in recent years, it has been confirmed that there are still gas- bearing sand bodies in the He 8 Member of the southern basin. For example, in the southern Sulige, the sand bodies of He 8 Member are 15-30 m thick, mainly composed of medium- coarse quartz sandstone with pebbly sandstone interlayers, contain largely dissolved pores, and have an average porosity and permeability of 8.7% and 0.83×10^-3 μm² respectively, and an average gas layer thickness of 14.6 m^[27]. Progress has been made in exploration in the southwestern basin recently, and gas layers of quartz sandstone with pebbly sandstone interlayers still were discovered in the He 8 Member of Wells QT1, QT2, ZT2 and LN2. Gas production was tapped from the He 8 Member in Wells QT2 and LN2 through testing, and Well QT2 obtained an industrial gas flow of 11.15×10⁴ m³/d.

In the Sedimentary model of "alluvial river & floodplain lake", we argue that the He 8 Member in Ordos area was formed in the background of large-scale regression in North China Craton, where the progradation and convergence of multi-source alluvial systems along the regression direction led to the large-scale network-distribution of channel sand bodies in the whole Ordos area; the long-distance continuous transportation of multi- period high-energy floods resulted in the occurrence of coarse sand bodies in the south-central basin. This mode changed the traditional sedimentary model and explained the characteristic of "basin full of sand" in the He 8 Member, especially the origin of the distribution of pebbly sandstone interlayers across the whole area, expanded the distribution range of sand bodies in the south-central Ordos Basin. Thus, the He 8 Member in the southern Ordos Basin still has substantial exploration potential.

During the deposition period of the Permian He 8 Member, the Ordos Basin area had open and flat terrain, arid-semi-arid climate, abundant multi-source supply and multiple water systems, providing favorable conditions for the formation of alluvial fan-alluvial plain. The whole area had a sedimentary pattern characterized by alternate distribution of channels, flood plains and interchannel overflow lakes. The paleogeographic reconstruction shows that the Ordos area was the "water-pass-by area" in the western North China Basin during the depositional period of He 8 Member. The multiple provenance-oriented water systems converged in the south and then flew out uniformly toward southeast. The area was mainly covered by the alluvial plain, with the alluvial fan, braided river, flood plain, low-tortuosity meandering river and flooding lakes (interchannel overflow lake) developed. With the rapid southeastward recession of the North China Sea, the base level fell overall and flooding events happened frequently, the alluvial systems from the north and south provenance in the Ordos area carrying large amount of coarse clasts pushed toward each other rapidly, resulting in the abundance of gravels all across the area. The sedimentary model of "alluvial river & floodplain lake" reasonably explains the origin of the large-scale distribution of pebbly sandstone in the He 8 Member in the Ordos Basin, and provides sound geological basis for expanding exploration towards the southern basin.