Introduction
In recent years, high-resolution seafloor images and three-dimensional seismic data have been widely used in the study of the deep-sea gravity flow sedimentary system on the continental margin, and have made substantial progress in the geomorphology of the deep-sea gravity flow system, the distribution of sedimentary units, sedimentary formation, stratigraphy and other aspects [1⇓-3]. A number of fine-grained channel-levee-lobe systems have been found in the deep-water basin of the continental margin [4⇓⇓-7], in which the gravity flow sediments are the reservoirs or sweetspots of lithologic oil and gas reservoirs and shale oil and gas reservoirs, attracting widespread concerns from geologists [8⇓-10].
Similar to what happens in the sea basin, the gravity flow system is also widely developed in the lacustrine basin. In the Miocene of Dacian Basin, Romania, thin fine grained turbidite deposits are developed on the deep lake slope, thick coarse grained turbidite is developed in the lake bottom channels, and sandy debris flow deposits, argillaceous mass-transport deposits and mixed event layers are visible in the underwater fan [11]. In the Lower Cretaceous of the North Falkland Basin, Antarctica, turbidite, slump deposit and mixed event layer are developed in some confined channel - sublacustrine fans [12]. The sandy mixed event layer is mainly developed in the channel - fan transition zone, and the fine mixed event layer is developed in the fan edge [13]. The study of gravity flow sediment in the Jiyang depression of the Bohai Bay shows that the third grade slope zone (slope break) developed on the Chengnan low uplift of Zhanhua depression controls the development of the fan delta-gravity flow system of the Oligocene Dongying Formation (E3d), the second grade steep slope zone developed the unconfined channel coarse-grained gravity flow sublacustrine fan system, and the third grade gentle slope to the flat lake bottom developed the fine-grained channel-levee-lobe systems [14]. The gravity flow deposition related to river flooding and the gravity flow system caused by the slump of the front delta have developed in the third member of Eocene Shahejie Formation in Dongying Sag (E2s3 for short) [15-16]. Silty fine-grained gravity flow channel-levee-lobe system closely related to flood events has been found in the first member of the Upper Cretaceous Qingshankou Formation (referred to as Qing 1 Member, K2qn1 for short) and the first member of the Nenjiangkou Formation (referred to as Nen 1 Member, K2n1 for short) in the northern part of the Songliao Basin[17⇓-19]. Argillaceous slump and mass-transport deposits developed in the second member of the Upper Cretaceous Nenjiangkou Formation (referred to as Nen 2 Member, K2n2 for short) [20]. Slump, sandy debris flow [21-22], turbidity current and density current deposits [22-23] have been found in the sublacustrine fan of the Triassic Yanchang Formation (Chang 7 for short, T3yc7) in the Ordos depression lacustrine basin, and muddy gravity flow deposits have also been found in the muddy front delta slope [22,24]. The above research shows that the density current deposits related to flooding and gravity flow deposits caused by the slump of delta sediments can be developed in the lacustrine basin. The steep slope and gentle slope of the lacustrine basin develop sublacustrine fan system that is not limited to the confined channel, while the front end of the confined channel develops sublacustrine fan. The fine-grained gravity flow deposits developed at the flat lake bed can form a long-extended channel-lobe system. Similar to the deep-water basin, there are many types of gravity flow rheology in the lacustrine basin, as well as the transformation of gravity flow rheology properties. In the traditional view, fine-grained sediments are mainly deposited in quiet water bodies through suspension and fall. However, recent studies have found that fine-grained flocculent particles and silts formed by biochemical processes can be transported thousands of kilometers to deep water basins in the form of bottom flow [25-26], and fine-grained sediments such as silts and argillaceous can be transported and deposited in deep water in the form of gravity flow [27]. Fine-grained gravity flow deposition is mainly formed via the channel-levee-lobe system. Gravity flow is one of the important deposition and transportation methods for the formation of deep-water fine-grained silty and muddy sediments [10]. At present, scholars have done much research on the sedimentary characteristics and genetic mechanisms of the mass-transport deposits (MTD), debris flow, high-density turbidity current, sandy debris flow, low-density fine-grained turbidity current, mixed event layer and flood-related density current developed in the lacustrine basin and in related to the slump of the sedimentary slope [12⇓⇓⇓⇓⇓-18,23,28⇓⇓⇓ -32]. In addition to fluvial and delta facies tight sandstone oil and shore-shallow lake mixed tight oil, the shale oil sweet section developed in the deep lake shale series is mostly fine-grained gravity flow sediment. The fine-grained gravity flow sediment composed of felsic, clay, carbonate minerals and organic matter with grain size less than 0.0625 mm in the lacustrine basin is an important sweet section for shale oil and gas enrichment and high yield.
Due to the complexity and variety of deposition of fine-grained gravity flow in lakes and the limitation of data, the study of deposition process and sweet section controlling of fine-grained sediment system in large lacustrine basins is not sufficiently elaborated [33-34]. Based on the latest progress in the study of fine-grained gravity flow deposits, this paper systematically investigated and sorted out the fine-grained gravity flow system in lacustrine basins, studied and characterized the sediment characteristics and deposition process of fine-grained gravity flow system in typical lacustrine basins in China, thoroughly analyzed the main control factors, trying to establish the deposition model of fine-grained gravity flow in different types of lacustrine basins, so as to deepen the understanding and recognition of the deposition of fine-grained gravity flow in lacustrine basins. It is of great theoretical and practical significance to enhance the evaluation of shale oil sweet section and the accuracy of prediction in the lacustrine basin in a scientific way.
1. Classification and terminology system of gravity flow
The fine-grained gravity flow deposition is the main fine-grained deposition type of shale series. The classification and terminology applicable to the study of coarse gravity flow can no longer satisfy fine gravity flow sedimentology. The fine-grained gravity flow sediments developed in the deep lake shale series, mainly silty and muddy, are not well reflected in the traditional classification and terminology of coarse gravity flow. In order to carry out in-depth research on the sedimentology of fine-grained gravity flow in lacustrine basins, this paper adopts the classification scheme of gravity flow based on sediment concentration and deposition process proposed by Mulder and Alexander [35] and the classification scheme of transitional flow proposed by Baas et al. [36-37], and establishes a classification and terminology system suitable for the study of fine-grained gravity flow deposition in deep lakes.
In 2001, Mulder and Alexander [35] proposed a simple classification scheme of sediment gravity flow (Fig. 1 ). According to the concentration percentage of gravity flow sediment, gravity flow is classified into four types from low to high: turbidity flow, density flow, hyperconcentrated density flow and cohesive flow. Cohesive flow includes debris flow and mud flow (Fig. 1a ). On this basis, the turbidity current is further divided into three subtypes according to its duration: (1) quasi-steady turbidity current, namely hyperpycnal flow, (2) surge-like turbidity current, (3) turbidity flow - surge. The concentrated density flow and hyperconcentrated density flow are divided into two end elements according to their support types (Fig. 1b ).
Fig. 1. Illustration of the density flow type (a) named according to the percentage of sediment volume concentration and definition of underwater sediment density flow (b) (according to Reference [33]). |
The transitional flow is not included in the classification scheme of Mulder and Alexander [35]. In this paper, the term of transitional flow based on Baas et al. [36-37] is used, which refers to the transitional flow between turbulence and mud flow generated by the mixing of clay minerals and the enhancement of fluid viscosity, which modulates the turbulence of fine-grained turbidity flow. The transitional flow includes (1) turbulence enhanced transitional flow (TETF), (2) lower transitional plug flow (LTPF) and (3) upper transitional plug flow (UTPF). Turbulence (TF) and quasi-layered plug flow (QLPF) are two end-member components. The former belongs to turbidity flow and the latter belongs to mud flow [36⇓⇓-39].
In this paper, in addition to the surge-like turbidity current and mud flow deposits (which are fine sediment), the gravity flow deposits with grain diameter less than 0.0625 mm are defined as fine gravity flow deposits.
2. Fine-grained gravity flow sediments and their sedimentary processes in the lacustrine basin
Petroleum exploration and outcrop research show that gravity flow system is generally developed in deep lake environment, in which coarse gravity flow sediments mainly composed of sandstone, pebbly sandstone and conglomerate are mainly developed in the gravity flow channel on steep or gentle slope of the basin, the transitional zone between channels and sublacustrine fan, the middle part of the fan, or the near end of the sublacustrine fan with unconfined channels, and the sediments are mainly composed of hyperconcentrated density flow, concentrated density flow [35] and slump and debris flow deposits [12⇓-14,21,29]. According to the research of predecessors [13-14,18] on the Permian Fengcheng Formation of 1st member of Qingshankou Formation, 1st member of Nenjiangkou Formation of Songliao Basin and Mahu depression of Junggar Basin, the development of fine- grained gravity flow sediments with grain size less than 0.0625 mm, mainly silty and argillaceous, in the large depression lacustrine basin represented by Songliao Basin, is related to the channel - levee - lobe system caused by flooding and the unconfined channel - sublacustrine fan formed by the slump of meandering river delta [18]. In the fault-depressed lacustrine basin represented by the Fengcheng Formation of Mahu Sag and the Sha3 and Dongying Formation of the Bohai Bay Basin, the fine- grained gravity flow deposits are mainly developed in the fan margin of the sublacustrine fan [13] or in the channel - levee - lobe system developed on the flat lake bed [14].
According to the observation results of lithofacies, the fine-grained gravity flow sediments in the continental lacustrine basin mainly include surge-like turbidity flow deposits, fine-grained hyperpycnal flow deposits, fine- grained concentrated density flow deposits, fine-grained debris flow deposits, mud flow deposits, and fine-grained transitional flow deposits.
2.1. Turbidity flow deposits
Turbidity flow deposits are the most common fine- grained gravity flow deposits in continental lacustrine basin deposits. There are two main types that can be recognized: surge-like turbidity flow deposits and fine- grained hyperpycnal flow deposits.
2.1.1. Surge-like turbidity flow deposit
The surge-like turbidity current is a kind of turbidity current whose grains are supported by turbulence, which can develop into a turbulent fluid that is both uneven and unstable. The fluid has a long duration, including a turbulent head and a considerable length of fluid section. This fluid can be caused by the conversion of flow pattern, or by the slump of sediment suspended cloud, which tends to transport particles with grain size smaller than sand grade. Most of the sediments develop Tb-Td segments of the Bauma sequence (b, c, d segments of the Bauma sequence) [35]. The surge-like turbidity current deposit is mainly developed in the quiet deep lake environment, and is a sedimentary combination mainly composed of silty sand and organic-rich mudstone. In the Qing 1 section of Gulong Sag, Songliao Basin, the surge-like turbidity sedimentary assemblage (Fig. 2a ) is composed of massive coarse siltstone facies (SS1, Fig. 3a , 3b ), parallel-lamination fine siltstone facies (SS3, Fig. 3d ), wavy cross-lamination fine siltstone facies (SS4, Fig. 3e ), wavy parallel-lamination fine siltstone facies (SS5, Fig. 3f ) and wavy-lamination argillaceous siltstone facies (AS2, Fig. 4c ).
Fig. 2. Sedimentary assemblage of fine-grained gravity flow in Qing 1 Member, Gulong Sag, Songliao Basin. M—mudstone; ssm—silty mudstone; mss—argillaceous siltstone; fss—fine siltstone; css—coarse siltstone; AS1—convolute laminar argillaceous siltstone facies; AS2—corrugated laminar argillaceous siltstone facies; AS3—injection structure argillaceous siltstone facies; SM1—silty mudstone facies with silty clasts mixed in argillaceous matrix; SS1—massive coarse siltstone facies; SS2—coarse siltstone facies with directional arrangement of mud and plant debris; SS3—fine siltstone facies with planar-lamimation; SS4—fine siltstone facies with wavy cross-laminae; SS5—wavy-parallel laminar fine siltstone facies. |
Fig. 3. Photos of fine-grained gravity flow core in the first section of Qinghe Formation, Well Y47, Gulong Sag, Songliao Basin. (a) Abrupt contact between massive coarse-grained siltstone and underlying mudstone (SS1a), 2368.02 m. (b) Flame structure (SS1b) developed at the bottom of massive coarse-grained siltstone, 2363.62 m. (c) Coarse siltstone (SS2) with oriented arrangement of mud debris and plant fragments, 2359.47 m. (d) Parallel-laminated fine siltstone (SS3), 2367.52 m. (e) Wavy cross-laminated fine siltstone (SS4), 2360.47 m. (f) Wavy-parallel laminar fine siltstone (SS5), 2357.72 m. |
Fig. 4. Photos of Qingshankou Formation (K2qn), Gulong Sag, Songliao Basin, and the second member of Fengcheng Formation (P1f2), Mahu Sag, Junggar Basin, fine-grained gravity flow sedimentary cores. (a) Well L242-1, 1827.9 m, the second member of Qingshankou Formation (K2qn2), surrounded by laminar argillaceous siltstone (AS1). (b) Well Y38, 1728.88 m, the first member of Qingshankou Formation (K2qn1), the lower part is silty mudstone mixed with silt and mud (SM1); the upper part is wavy laminar argillaceous siltstone (AS2). (c) Well L242-1, 1818.70 m, K2qn2, sand wall injected into structural argillaceous siltstone (AS3). (d) Well Y38, 1722.78 m, K2qn1, silty mudstone mixed with silt and mud (SM1). (e) Well Maye 2 in the Mahu Sag, 3859.60 m, the second member of Fengcheng Formation (P1f2), the upper part is quasi-laminated silty mudstone (SM2) containing silt debris and mud debris; the lower part is wavy-parallel laminar silt injection structure argillaceous siltstone (AS4). |
The surge turbidity flow can be generated by the slump of the delta front, or the high-density current can be generated by the dilution of the lake water [25,35,40]. This set of sediment can be developed on the side of the channel axis of the channel - levee - lobe system, the channel edge and the near end of the levee and lobe [18]. If the sediment assemblage contains rich plant debris, it is out-of-basin turbidity flow, which can be interpreted as weakened density current deposition [41-42].
2.1.2. Fine-grained hyperpycnal flow deposits
Fine-grained hyperpycnal flow is a quasi-steady, suspended load hyperpycnal turbulence or turbidity flow, and its particles are mainly silt. The hyperpycnal flow sediment is closely related to flooding, and the significant difference between it and the surge turbidity flow is that its fluid maintains longer than the surge turbidity flow, lasting for several days to several weeks, and it is more likely to occur in lakes, forming a meandering gravity flow channel - levee - lobe system [35,40 -41]. Taking the Qing 1 member of Gulong Sag in Songliao Basin as an example, the fine-grained hyperpycnal flow sedimentary sequence consists of a coarsening sedimentary sequence from the lower part to the upper part and a thinning sedimentary sequence from the lower part to the upper part. The scouring surface can be developed between these two sedimentary sequences, sometimes the scouring surface is blurred. The sedimentary sequence coarsening from bottom to top is composed of lithofacies SS5 (Fig. 3f ), SS4 (Fig. 3e ), SS3 (Fig. 3d ) and SS2 (Fig. 3c ). The upward-fine sedimentary sequence consists of SS1 (Fig. 3a , 3b ), SS3 (Fig. 3d ), SS4 (Fig. 3e ) and SS5 (Fig. 3f ) lithofacies (Fig. 2b ). The silty density current deposits developed in the Chang 73 sub-member of the Ordos Basin are composed of positive and negative progressive siltstones separated by scouring surfaces [23]. This upwardly coarsening and upwardly thinning sedimentary sequence belongs to fine-grained hyperpycnal flow deposits or fine-grained hyperpycnal rocks [22,40,43]. The presence of argillaceous debris near the scouring surface can be interpreted as the erosion of the argillaceous bed by the hyperpycnal flow, making the mud debris enter the fluid. In a low-magnitude flood event, the maximum flow should be greater than the critical flow for continuous generation of hyperpycnal flow, thus forming hyperpycnal flow. A complete hyperpycnal flow sedimentary sequence should include a pair of positive and negative progressive, or upward thickening and upward thinning sedimentary sequences. When the particles are very fine, the boundary scouring surface is not clear [40,43]. The hyperpycnal flow deposition is mainly developed in the channel - levee - lobe system of the Qing 1 section of the Gulong Sag in the Songliao Basin, which is located on the side of the channel axis and the sedimentary environment of the channel edge [18].
2.2. Fine-grained cohesive flow deposits
The fine-grained cohesive flow deposits can be divided into fine-grained debris flow deposits and mudflow deposits. These two types of fine-grained cohesive flow deposits can be seen in the Qing 1 Member of Yingtai slope in Gulong Sag, Songliao Basin.
(1) The fine-grained debris flow deposition is a viscous density flow supported by particles on a cohesive matrix. Debris flow deposits are composed of poorly sorted sediments, generally containing more than 5% gravel and sandy, silty and muddy components with large content changes, which can transport huge soft sediment fragments. The fine-grained debris flow deposition referred to in this paper refers to the silty debris flow deposition supported by the argillaceous matrix, including the argillaceous siltstone facies (AS1, Fig. 4a ) and argillaceous siltstone facies (ASS2, Fig. 5b , 5c ) with the development of wrapped bedding.
Fig. 5. Photos of core of fine-grained concentrated density flow deposit (CSS1, CSS2), transitional flow deposit (ASS1, FSS1, FSS2), mud flow deposit (SSM1) and debris flow deposit (ASS2) in Maye 1H well, Mahu Sag, Junggar Basin. (a) Well Maye 1 in the Mahu Sag, 4690.88 m, the second member of the Fengcheng Formation (P1f2), the lower part is coarse siltstone with fine gravel massive bedding (CSS1); the middle part is parallel laminar argillaceous siltstone (ASS1); the upper part is argillaceous siltstone (SSM1) with mixed mud and silt debris. (b) Well Maye 1H in the Mahu Sag, 4574.34 m, P1f2, the lower part is graded bedding coarse siltstone (CSS2) with argillaceous tear debris directional arrangement; the upper part is argillaceous siltstone (ASS2). (c) Well Maye 1H in the Mahu Sag, 4574.93 m, the third member of the Fengcheng Formation (P1f3), the lower part is wavy bedding fine siltstone (FSS2) with thin muddy bands; the middle part is shale-bearing clastic argillaceous siltstone (ASS2); the upper part is wavy bedding fine siltstone (FSS1). |
(2) Mudflow deposition is a viscous density flow with gravel content less than 5% and sediment ratio greater than 1:1 [35]. When the clay content is less than 25%, it can be called Silty Mud Flows. If the clay content is more than 40%, it can be called clay-rich mud flows [35]. It can be developed into silty mudstone facies (SM1, Fig. 4d ) with silty debris mixed in the argillaceous matrix, or it can be expressed as silty mudstone facies with small silty particles arranged along the plastic flow shear plane to form quasi-layered mudflow deposits with silty debris mixed in the argillaceous matrix [36] (SM1, SM2, Fig. 4b-4e , SSM1, Fig. 5a ).
The Qing 1 Member (K2qn1) of Yingtai slope in Gulong Sag of Songliao Basin, fine-grained debris flow and mudflow sedimentary sequence (Fig. 2c ) are composed of AS1 lithofacies (Fig. 4a ), argillaceous siltstone facies with developed sand wall injection structure (AS3, Fig. 4 c) and SM1 lithofacies (Fig. 4d ). The sedimentary sequence is characterized by quasicontemporaneous deformation structure or soft sediment deformation structure, sand and mud debris mixed structure, silt-wall injection structure and silty block deformation and elongation. This sedimentary sequence is developed at the edge of the channel, the distal end of the sublacustrine fan and the lobe [18]. In this sedimentary facies sequence (Fig. 2c ), AS1 (Fig. 4a ) can be interpreted as clayey debris flow deposit, and SM1 (Fig. 4d ) can be interpreted as clayey mud flow deposit [35]. AS3 (Fig. 4c ) can be interpreted as a silt-injection complex associated with soft sediment slump during shallow burial. The slump of soft sediment causes its own liquefaction and injects silt into the extended fracture system [44-45]. Some silty sand injection bodies are also interpreted as seismic rocks, which are the result of the instantaneous liquefaction of soft sediments caused by earthquakes [46]. These debris flow, mud flow and silty injection complex sediments mostly developed in the sublacustrine fan on the front delta slope, which forms an unconfined channel. They are caused by slope instability and subsequent slump caused by high deposition rate [47-48], and can also be caused by earthquakes [47].
2.3. Fine-grained concentrated density flow deposits
Fine-grained concentrated density flow is a sediment density flow supported by turbulence, particle interaction and buoyancy. The fine-grained concentrated density flow deposits discussed here is limited to coarse siltstone with a small amount of fine gravel and coarse siltstone with mud chips. The fluid contains the deposition of particles caused by the deceleration of turbulence, which can cause meaningful separation and produce clean coarse siltstone section [35]. The block coarse siltstone facies with fine gravel (CSS1, Fig. 5a ) and the graded bedding coarse siltstone facies with argillaceous tearing debris (CSS2, Fig. 5b ) can be seen in the second member of Fengcheng Formation (referred to as the second member of Fengcheng Formation, P1f2) of Well Maye 1, Mahu Sag, Junggar Basin. They are respectively characterized by the directional arrangement of block coarse siltstone facies with fine gravel and argillaceous debris, and can be considered as concentrated density flow deposits. The massive coarse siltstone facies with fine gravel represents the rapid suspended turbulent deposition. Directionally arranged mud debris represents the erosion of concentrated density flow on the mud bed and is the result of mud debris deposition after entering the fluid [42,48].
2.4. Fine-grained transitional flow deposits
According to the different turbulence intensity and plug development degree caused by the mixing of clay minerals in the transitional flow, fine-grained transitional flow deposits can be further divided into three types: fine-grained turbulent enhanced transitional flow deposits, fine-grained lower transitional plug deposits and fine-grained upper transitional plug deposits.
(1) Fine-grained turbulent enhanced transitional flow deposits. When turbulence is mixed with a small amount of clay minerals, turbulent enhanced transitional flow occurs. Compared with turbid flow (turbulence) with similar velocity, the fluid in the enhanced transitional flow has stronger turbulence intensity. Turbulence enhancement comes from the Kelvin-Helholmes instability effect produced by the shear layer developed at the bottom of the fluid, so the sediment can develop large sand ripple bedding and erosion of sand ripple backflow surface [49-50]. The wavy bedding fine siltstone facies (FSS1, Fig. 5c ) of Well Maye 1H in the second member of Fengcheng Formation in the Mahu Sag of the Junggar Basin is pure and the ripples are eroded, which may belong to turbulent enhanced transitional flow deposition.
(2) Fine-grained lower transitional plug deposits. Compared with fine-grained turbulent enhanced transitional flow, the lower transition plug flow is formed under higher clay mineral concentration, and its fluid turbulence is initially suppressed to form a plug flow section. In fact, there is no or only weak turbulence in this plug section. It first forms at the place where the shear force is the weakest near the water surface, and then expands downward with the increase of clay mineral concentration. The enhancement of turbulence comes from the Kelvin-Hermas instability effect generated by the fluid bottom shear, which makes the turbulence strengthen near the bottom bed, thus making the gradient of the turbulence intensity between the bottom and the top of the fluid maximum. The lower transitional plug sediment develops into a double-layer structure: the siltstone section with large wavy bedding in the lower part and the plug section composed of silty mudstone or mudstone in the upper part [36-37]. The wavy bedded fine siltstone facies (FSS2, Fig. 5c ) with thin argillaceous strips and the parallel-lamination argillaceous siltstone facies (ASS1, Fig. 5a ) developed in the third member of the Fengcheng Formation of the Maye 1H well in the Mahu Sag of the Junggar Basin may belong to the lower transitional plug flow deposit. This transitional flow deposit is developed between the CSS1 lithofacies of fine-grained concentrated density flow deposit (Fig. 5a ) and the SSM1 lithofacies of mudflow deposit (Fig. 5a ), or under the muddy siltstone containing debris deposited by clastic flow deposit (ASS2, Fig. 5b ), which belongs to fine-grained lower transitional plug flow deposit generated after turbulence is modulated.
(3) Fine-grained upper transitional plug deposits. With the increase of clay concentration, the lower transitional plug flow will be changed into the upper transitional plug flow, making the plug flow section of the transitional flow thicker. At the same time, when the viscosity of the suspended clay exceeds the turbulent force, its bottom shear layer will become a weak turbulent source, which will further inhibit the whole turbulence and form the upper transition plug flow. The fine-grained upper transition plug flow is an intermediate transition type between the fine-grained lower transition plug flow and the quasi-layer transition plug flow (mud flow). Compared with the fine-grained lower transitional plug flow deposit, the thickness of the plug flow section composed of silty mudstone or mudstone in the sediment is larger, and the siltstone section develops low-width bed sand-like bedding [38,49]. For example, the argillaceous siltstone facies (AS4, Fig. 4e ) with wavy-parallel laminated siltstone injection structure developed in the second member of the wind of the Maye 2 well in the Mahu Sag of the Junggar Basin can be interpreted as the upper transitional plug-flow deposit, in which the thin wavy-parallel laminated siltstone is the lower turbulent section deposit, and the thick layer of silty mudstone containing silt chips is the plug-flow section deposit. Then upward, it transits into quasi-layered silty mudstone containing silt and mud chips (SM2, Fig. 4e ), belonging to quasi-layered plug flow deposit or stratified mud flow deposit [35].
When the concentration of clay minerals further increases, the turbulent flow at the bottom of the transition plug flow at the upper part of fine particles stops developing, forming a quasi-layered plug flow, namely, mud flow. It’s characterized with the laminar plug flow section moving above the thin shear layer, which makes the shear layer have residual turbulence [36-37]. Its sediment is represented by laminated silty mudstone, with silty spots arranged along the laminae (SM2, Fig. 4e ). The quasi- layered plug flow, also known as mud flow, is developed on the upper transitional plug flow deposit (AS4, Fig. 4e ), reflecting the transition from the upper transitional plug to the quasi-layered plug flow (mud flow).
3. Depositional model of fine-grained gravity flow in the lacustrine basin
The gentle and deep lacustrine environment is the main place for the development of fine-grained gravity flow deposition. A variety of micro-paleogeomorphic units control the distribution of the gravity flow deposition system [18]. Based on the deposition of fine-grained gravity flow in lacustrine basin, the distribution of sedimentary system and its controlling factors, this paper discusses the deposition model of fine-grained gravity flow in faulted lacustrine basin and large depression lacustrine basin.
3.1. Depositional model of fine-grained gravity flow in the faulted lacustrine basin
The Fengcheng Formation of Mahu Sag in Junggar Basin and the Sha3 and Dong3 members of Jiyang depression in Bohai Bay Basin belong to the faulted (fractured) lacustrine basin under the conditions of warm and humid - dry heat or warm and humid climate during the sedimentary period, and both developed the gravity flow sedimentary system [14-15,51 -52]. According to its gravity flow sedimentary characteristics, deposition process and distribution, the fine grain gravity flow depositional model of fault basin under such climatic conditions can be summarized as follows:
Gullies and 2-3 fault step zones develop at the edge of the steep slope of the lacustrine basin, which can form 2-3 slope break zones. The valley at the basin margin determines the development position of the fan delta, and the first slope break zone at the outermost controls the distribution of the fan delta plain and front subfacies. The second slope break controls the distribution of the sub-lacustrine fan with steep slope and unconfined channels caused by the rapid decline of the lake level. In the fan, dense and high-density flow deposits dominated by sandy conglomerate are developed, and the fan margin develops fine-grained concentrated density flow, fine- grained clastic flow, turbulence and mud flow deposits. The third slope break controls the distribution of small confined channel sub-lacustrine fans. The gentle slope of the lacustrine basin mainly develops river delta and sub-lacustrine fan system. The development of slope break zone is similar to that of steep slope. The first slope break controls the distribution of the delta plain, the second slope break controls the distribution of the delta front, and the third slope break controls the distribution of the confined and unconfined channel sub-lacustrine fan. The rapid decline of lake level (LST) led to the down-cutting of the channel above the slope break and the subsequent deposition and filling of coarse-grained dense current, and the formation of sub-lacustrine fan below the slope break. Coarse-grained concentrated and high-density current deposits are developed in the sub-lacustrine fan, and fine-grained surge turbidity flow, fine-grained clastic flow and mud flow deposits and incomplete fine-grained mixed event layer are developed in the fan margin. The fine-grained concentrated density flow caused by the flood during the lake level rise period (TST, lacustrine Transgressive Systems Tract) can form a channel - lobe system in the deep lake area with flat terrain. During the slow declining of lake level (HST, Highstand Systems Tract), due to the high deposition rate or the instability of the sedimentary slope caused by earthquakes, the sandy and muddy sub-lacustrine fans developed in the unconfined channel of the front delta slope may form, and the fine-grained gravity flow deposits develop in the marginal facies of the sandy sub- lacustrine fan and the muddy sub-lacustrine fan (Fig. 6 ).
Fig. 6. Depositional model of fine-grained gravity flow in faulted lakes. |
3.2. Depositional model of fine-grained gravity flow in the large downwarped lacustrine basin
Under the warm and humid environment, the under-compensated depression lacustrine basin is the main place for the development of gravity flow, especially fine-grained gravity flow deposits. Its overall characteristics are as follows: (1) The meandering river delta depositional system mainly composed of silts transported over a long distance is mainly developed in the direction of the long axis of the lacustrine basin, and then developed on both sides of the lacustrine basin. These silty delta sediments and fine-grained sediments carried by floods become the material basis for the deep lake fine-grained gravity flow sedimentary system formed by the re-transportation of the depression lacustrine basin due to the delta slump and flooding, which makes the deep lake area of the large depression lacustrine basin mainly develop fine-grained gravity flow sedimentary system. (2) The deep lake area of the depression lacustrine basin is open, and the bottom bed of the lacustrine basin is flat. It is easier to develop the fine-grained gravity flow sedimentary system of large meandering channels, which is closely related to flooding [18,28]. (3) The flexure slope break is the boundary dividing the shallow lake and deep lake in the depression lacustrine basin. The steep slope and gentle occurrence control the difference of the size and type of the fine-grained gravity flow.
The 1st member of Qingshankou Formation and 1st member of Nenjiangkou Formation of the Songliao Basin represent the fine-grained gravity flow deposition model of the meandering river delta-deep lake sedimentary environment under the background of the flat slope of the large depression lacustrine basin. The Chang 73 sub-member of the Ordos Basin represents the depositional model of mixed-source fine-grained gravity flow under the background of the flexural slope break of the depression lacustrine basin. The difference between the two models depends on the gentle slope of the lacustrine basin bottom bed and the participation of volcanic activity and underwater hydrothermal fluid.
3.2.1. Depositional model of fine-grained gravity flow in downwarped lacustrine basin in flat topography slope
During the depositional period of the first member of Qingshankou Formation and the first member of Nenjiangkou Formation in Songliao Basin, the lacustrine basin belongs to a large depression lacustrine basin under warm and humid climate conditions. The bottom bed of the lacustrine basin is generally flat, and only low amplitude flexure slope breaks are locally developed. Based on the study of the sedimentary characteristics and controlling factors of its deep lake channel - levee - lobe and sublacustrine fan system, a depositional model representing the meandering river delta - deep lake fine-grained gravity flow in a large depression lacustrine basin is constructed (Fig. 7 ). The depositional period of the first member of Qing Formation and the first member of Nenjiangkou Formation respectively corresponds to two flood events in the Late Cretaceous [53-54]. The rapid rise of lake level (Transgressive Systems Tract, TST) and the high sediment flow caused by flooding are conducive to the development of the channel - levee - lobe system. The straight channel supplied by the Northern and Western delta front is filled with dense current, sandy (coarse) density current, surge turbidity flow and fine-grained clastic flow sediments. The straight channel is developed on the high palaeogeomorphic slope controlled by the flexure slope break zone, or is related to the front delta slope. The meander channel system is distributed on the lower slope of the ancient slope, and has deposited fine-grained silty density current and surge turbidity flow deposits, and the end is a lobe or bifurcation. The near end of the embankment and the lobe develops silty surge turbidity flow deposits, while the far end of the lobe develops fine-grained clastic flow and mud flow deposits, which are mainly distributed in the transgressive and high early systems tracts. These waterways, embankments and leafy systems can extend 15 to 70 km. In the period of lake level decline (regressive systems tract (RST) or low-stand and highstand systems tract (LST and HST)), the unconfined channel sublacustrine fan composed of deposits caused by delta slump is distributed under the front delta slope or flexure slope break, mainly composed of fine-grained clastic flow deposits, mud flow deposits, and sand wall complex (Fig. 7 ) [18].
Fig. 7. Depositional model of fine-grained gravity flow in the meandering river delta-deep lake environment in the gentle slope background of the first member of Qingshankou Formation and the first member of Nenjiangkou Formation in the Songliao depression lacustrine basin (according to Reference [18]). |
3.2.2. Depositional model of fine-grained gravity flow in the downwarped lacustrine basin with flexure slope break
The research shows that during the sedimentary period of the Chang 73 sub-member of the Ordos Basin, the tectonic subsidence rate is greater than the sediment supply rate, the lacustrine basin is in the lake flooding period (Carnian), and the lake area reaches the maximum. Compared with the sedimentary period of the first member of Qinghe Formation in Songliao Basin, the basin tectonic activity in this period is more active. In the southwest of the lacustrine basin, there is a relatively steep flexural slope break, while in the eastern slope, there is a relatively gentle flexural slope break (Fig. 8 ). The active volcanic eruption in the uplift area caused the tuff to fall into the lake, which increased the productivity of the ancient lake while forming the sedimentary tuff. Sulfates from volcanoes promote microbial reduction, and H2S produced makes water form a strong reduction environment, which is conducive to the preservation of organic matter. Tectonic activities caused the underground hydrothermal fluid to rise along the fault into the lake, and the prosperity of lake algae and plankton promoted the development and preservation of organic matter in the Chang 73 sub-member, forming high-quality source rock series[55].
Fig. 8. Depositional model of mixed-source fine-grained gravity flow with flexural slope break of Chang 73 sub-member of the Ordos depression lacustrine basin (modified according to Reference [56]). |
During declining of lake level (RST), the river delta from the northeast and southwest of the basin pushed towards the lake, and the seismic and sedimentary slope instability caused by volcanic activities led to the development of sublacustrine fans with unconfined channels and sublacustrine fans with confined channels at the flexure slope break (Fig. 8 ). The proximal part of these fans is mainly composed of high-density current and high-density current deposits, while the distal part of the fan margin is composed of fine-grained dense current, fine-grained clastic flow, surging turbidity flow and fine-grained transitional flow deposits. During the transgressive systems tract period (TST), when the lake area expands, the density current caused by flood can form a channel - levee - lobe system. Among them, coarse gravity flow deposits, including sandstones deposited by concentrated density flow, are developed in channel axis microfacies. Fine-grained gravity flow deposits, such as surge turbidity flow deposits, fine-grained debris flow, mud flow, fine-grained density current deposits, fine-grained transitional flow deposits, etc., are developed in the channel edge, levee, and lobe microenvironment (Fig. 8 ).
In short, the sedimentary structure of the first member of the Qinghe Formation in the Songliao large depression lacustrine basin is relatively stable, and the bottom bed of the lacustrine basin is flat. The axial meandering gravity flow system and the unconfined channel sublacustrine fan controlled by the lateral deflection slope break are mainly developed, and the sediments are mainly fine-grained gravity flow deposits. In contrast, during the sedimentary period of the Chang 73 sub-member of the Ordos Basin, the structure was relatively active, which was characterized by volcanic eruption, hydrothermal activity, and the development of the flexure slope break with a steep slope, which made the development of the gravity flow sedimentary system controlled by the flexure slope break, and the gravity flow sediments of the sublacustrine fan were distributed under the slope break. The density current caused by flood can form a channel - levee - lobe system, which is distributed in the flat and deep lake area, but the unit size is small. These differences are caused by the differences in paleogeomorphology and tectonic activities of the lacustrine basin bed.
4. Influence and significance of fine-grained gravity flow deposition on the development of shale oil sweet sections
4.1. Influence of fine-grained gravity flow deposition on the development of shale oil sweet sections section
The various depositions of gravity flow can form conventional reservoirs, tight reservoirs and shale oil sweet sections in shale formations, among which fine-grained gravity flow deposition is more likely to form shale oil sweet sections.
Coarse-grained dense density flow and fine-grained dense density flow detrital particles are relatively coarse, containing no or very little clay minerals, and the interaction and turbulence between the particles can suspend the clay minerals between the fluid particles, and the sediment. After being sorted, clean sandstone and siltstone with good physical properties are formed. Taking Well X125 in the first member of Nenjiangkou Formation located in the Daqing placanticline, Songliao Basin as an example, the coarse-grained concentrated density flow deposited pebble-bearing sandstone developed from the channel axis to the flanks has an average porosity of 19.85% and a permeability of 15.04×10-3 μm2, which belong to high-quality sweet sections or conventional reservoirs (Table 1 ). For fine-grained hyperpycnal flow and surge-like turbidity or turbulent flow, due to the low concentration (sediment volume concentration less than 10%), finer particles and clay minerals can be separated from the fluid during the flow, resulting in meaningful sorting [35], forming clean siltstone. Taking Well Y47 in the first member of Qinghe Formation in Gulong Sag, Songliao Basin as an example, the coarse siltstone deposited by the fine-grained hyperpycnal flow and surge-like turbidity flow developed on the flanks of the channel axis can form a porosity of 15.7% and an air permeability of 6.2×10-3 μm2, which are high-quality sweet sections or conventional reservoirs (Table 1 ). The fine-grained hyperpycnal flow and the surge-like turbidity flow developed on the edge of the channel and the embankment have a porosity of 5.90%-9.05%, and an air permeability of (0.020-0.037)×10-3 μm2, belonging to the better-good sweet section (Table 1 ) [18].
Table 1. Gravity flow deposition process, sedimentary microfacies and relationship between the first member of Qingshankou Fromation and the 1st member of Nenjiangkou Formation in the Daqing placanticline, Gulong Sag, Songliao Basin [18] |
| Layer | Well | Depth/m | Lithology | Porosity/% | Permeability/10-3 μm2 | Gravity flow deposits | Sedimentary microphase | Sweet spot type | ||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| min | max | Average | min | max | Average | |||||||
| 1st member of Qingshankou Formation | Y47 | 2 358.00- 2 359.88 | Fine siltstone | 4.7 | 7.6 | 5.90 | 0.02 | 0.25 | 0.02 | Silty hyperpycnal flow deposits | Waterway edge | Better |
| Y47 | 2 360.27 | Argillaceous siltstone | 6.4 | 6.4 | 6.40 | 0.02 | 0.02 | 0.02 | Surge turbidity deposits | Embankment | Better | |
| Y47 | 2 362.03- 2 362.69 | Fine siltstone | 5.7 | 12.4 | 9.05 | 0.11 | 0.62 | 0.37 | Surge turbidity deposits | Waterway edge | Good | |
| Y47 | 2 363.07- 2 366.20 | Coarse siltstone | 7.4 | 21.3 | 15.70 | 0.01 | 58.50 | 6.20 | Surge-like turbidity flow deposits and silty hyperpycnal flow deposits | Flank of the channel axis | High-quality/ conventional reservoir | |
| Y51 | 2 270.48- 2 275.35 | Argillaceous siltstone | 7.5 | 8.9 | 8.03 | 0.08 | 0.11 | 0.09 | Fine debris flow deposition | Non-confined channel lake bottom fan | Better | |
| 1st member of Nenjiangkou Formation | X125 | 883.30- 885.80 | Pebbled sandstone | 15.8 | 23.9 | 19.85 | 1.47 | 28.6 | 15.04 | Concentrated density flow deposition | Channel axis to axis flank | High-quality/ conventional reservoir |
| P44 | 805.00- 810.20 | Argillaceous siltstone | 16.1 | 16.4 | 16.25 | 0.13 | 0.14 | 0.14 | Fine debris flow deposition | Far-end of lobe | Good | |
The plug flow section (mud flow) of fine-grained debris flow, mud flow and transitional flow deposits belongs to cohesive flow deposits. The formation of these cohesive flows is associated with the slump of fine-grained sediments and the addition of ambient water at the far end of the delta front, and their evolution into fine-grained debris flows, surge-like turbidity flow flows [40], and turbidity (turbulent) current flows. Due to the addition of clay minerals, it is related to the evolution of transitional flow and mudflow [36-37]. Cohesive flow can form argillaceous siltstone, silty mudstone and mudstone with silt clastics with complex sedimentary structures (such as soft sediment deformation structure, silt clusters and mud clusters mixed or distributed in layers). Cohesive flow sediments are high in shale and are supported by a shaly matrix, but can also form sweet sections. Taking Well Y51 of the first member of Qinghe Formation in Yingtai Slope of Gulong Sag and Well P44 of 1st member of Nenjiangkou Formation in the Daqing placanticline as examples, the argillaceous siltstone developed in the sublacustrine fan of the unconfined channel and the fine-grained debris flow developed in the distal end of the lobe, the porosity is 8.03%, 16.25%, and the air permeability is 0.09×10-3 μm2 and 0.14×10-3 μm2 respectively, forming sweet sections respectively (Table 1 ) [18].
The Qemscan scanning analysis of a dark gray silty mudstone facies sample (SSM2) (Fig. 9 ) and a quiet water body constructed through the development of mud debris, silt debris and silty agglomerates distributed in layers in a mud flow deposit in the Chang 73 sub-member of the Ordos Basin and the deposited dark gray lamellar mudstone facies sample (M1) (Fig. 9 ) reveals that the surface porosity of SSM2 is as high as 8.77%, and the pores are irregular, dominantly dissolution pores with a short axis width of about 2-3 μm and a long axis of about 5-10 μm, and interlayer micro-fractures of 1-2 μm in width and 30-50 μm in length. Most of the pores are disconnected (Fig. 9 ), while the surface porosity of M1 is only 3.19%, mainly isolated and sporadic dissolution pores with a short axis of 1-2 μm and a long axis of 4-6 μm (Fig. 9 ).
Fig. 9. Quantitative scanning electron microscopy (Qemscan) images of dark gray silty mudstone facies (SSM2) deposited by mud flow deposits in the Chang 73 submember of the Ordos Basin and dark gray lamellar mudstone facies (M1) deposited in quiet waters of deep lakes. (a) SSM2: dark gray silty mudstone facies with mud debris, silt debris and silty masses distributed along the bedding, Well Zheng 70, Chang 73 sub-member, 1 645.87 m; (b) M1: dark gray laminae shaped mudstone facies, Well Yan 285, Chang 73 submember, 2854.0 m. |
The reason for the large difference in porosity between SSM2 and M1 facies is mainly due to the sedimentary process of lithofacies. The SSM2 lithofacies of mud flow deposits with mud debris, silt debris and silty agglomerates distributed along the bedding structure is caused by events. It is transformed by the reduction of mud deposits and the increase of shale content. The plastic shear flow of the mud flow makes the silt debris, mud debris and silty agglomerates and pyrite particles contained in the fluid distribute along the bedding and form a bedding structure. Its compositional maturity is low, and it is rich in feldspar minerals that are easy to dissolve. At the same time, the plastic shear flow of the mud flow forms shear laminae. In contrast, the felsic clasts in the M1 phase deposited by hydrostatic airfall (Fig. 9 ) may have come from the monsoon and floating and subsidence of the lake surface, and are richer in quartz. Since feldspars are more likely to be dissolved by acidic media, shear laminae are more likely to form interlayer micro-fractures than airfall interfaces, so that the SSM2 rocks deposited by mudflows have higher porosity and more sweet sections are easily formed (Fig. 9 ). Existing data show that the fine-grained gravity flow sediments have higher porosity (greater than 5%) than the dark gray lamellar mudstone deposited by hydrostatic cavitation, are rich in felsic, and have lower lithofacies composition and structural maturity, which can be as the sweet section of shale oil, such as the shale oil in the first member of Qinghai Formation in Gulong Sag, Songliao Basin.
4.2. Lateral distribution of shale oil sweet sections formed by fine-grained gravity flow and its development in the sequence
For large depression lacustrine basins such as Songliao Basin and Ordos Basin, deep lake fine-grained gravity flow deposits are mainly developed in the channel-levee- lobe gravity flow system related to lake flooding and the unconfined channel lake bottom controlled by bending slope breaks Fan system (Figs. 7 and 8 ). In faulted lacustrine basins, deep lake fine-grained gravity flow deposits are mainly developed in the small channel-levee-lobe gravity flow system of the fan edge of the sublacustrine fan and the flat area in front of the fan controlled by the slope break of the synsedimentary structure (Fig. 6 ). The development of the fine-grained gravity flow system in the lacustrine basin is controlled by changes in the size of the lake plane, lake flooding, high sediment flow, and paleogeomorphology of the lake bed.
Taking the Fengcheng Formation in the Mahu Sag as an example, the lateral distribution of sweet sections of fine-grained gravity flow shale oil and its development characteristics in the sequence were discussed. The concentrated density flow deposits developed in the steep- slope sublacustrine fan of Well Maye 2 are consisted of pebble-bearing sandstone and fine sandstone, with a porosity of 0.8%-3.0% and a permeability of less than 0.1×10-3 μm2 (Fig. 10 ). This lacustrine basin belongs to the saline-alkali lake environment, with relatively deep burial of sediments and strong diagenesis, making it a favorable facies belt for the development of tight reservoirs and tight oil (Fig. 10 ). The subfacies of the proximal and distal fan margins of sublacustrine fans mainly develop fine-grained gravity flow deposits dominated by dolomitic siltstone, dolomitic mudstone, and tuffaceous mudstone, with a porosity of 0.4%-1.5% and a permeability of less than 0.1×10-3 μm2, TOC value is 0.5%-1.5%, which is a shale oil sweet section and a favorable shale oil development facies belt (Fig. 10 ).
Fig. 10. East-west sedimentary facies profile of Fengcheng Formation in Mahu Sag, Junggar Basin. |
The fine-grained gravity flow deposition in the deep lake environment mostly occurs in the stage of lake level decline, including the highstand systems tract (HST), low-stand systems tract (LST) or regressive systems tract (RST) of the sequence. This is because the tectonic subsidence of the lacustrine basin or the drought can not only lead to the drop of the lake level, but also lead to the occurrence of gravity flow. During the tectonic calm period, the climate was humid, and the water ingress caused the lake level to rise further. In addition to inducing hyperpycnal flow, organic-rich shale layers were also deposited. The transgression (TST)-regression (RST) cycle of the lacustrine basin can form multiple sets of source-reservoir combinations in the shale formation (Fig. 10 ). Further research shows that the transgressive systems tract of the fourth-order sequence of the Fengcheng Formation corresponds to the section with high shale and high TOC value. At the top of the transgressive systems tract, the TOC value reaches its maximum. The regression systems tract of the fourth-order sequence corresponds to the low shale, low TOC value section and high carbonate rock content section. The corresponding relationship between the water advance-regression cycle of the third-order sequence and its TOC value and shale content is similar to that of the fourth-order sequence. This phenomenon reflects that the expansion of the lake area caused by calm structure and humid climate is conducive to the development of high muddy and high TOC value sections. The arid climate and tectonic subsidence of lacustrine basin shrunk and retreated favor the development of fine-grained gravity flow deposits and sections with high carbonate content. Fine-grained gravity flow and carbonate rock deposition can form sweet sections of shale oil, such as, thin-bed siltstone sweet sections deposited by fine-grained concentrated density flow, fine-grained hyperpycnal flow, and turbidity flow; dolomite sweet section; the sweet section of interbedded mudstone and silt sand deposited by fine-grained transitional flow; the quasi-horizontal bedding mud shale sweet section containing silt debris and mud debris formed by fine-grained debris flow and mud flow wait.
Unconventional shale oil in the lacustrine basin is the main replacement area for the future oil exploration and production of China. The recognition of the distribution of shale oil sweet sections under the constraints of the depositional model of the fine-grained gravity flow system in the lacustrine basin is crucial to improving the prediction accuracy of sweet sections. It has important application value to reduce the blind drilling and further effectively guide and promote the increase of shale oil and gas reserves and production in key basins such as Ordos [57⇓-59].
5. Conclusions
The deep-water environment of the lacustrine basin develops fine-grained concentrated density flow, surge-like turbidity flow, fine-grained hyperpycnal flow, fine-grained debris flow, mud flow, and fine-grained transitional flow.
In the deep lake area of the faulted lacustrine basin: during the period of rapid lake level decline, the unconfined channel sublacustrine fan system and the small fine-grained channel limited channel sublacustrine fan system developed under the slope break of the steep slope belt of the lacustrine basin. Sublacustrine fans with unconfined channels and sublacustrine fans with limited channels developed under the gentle slopes and breaks of the lacustrine basin. During the period of slow lake level decline, high deposition rate or instability and slump of prodelta sedimentary slopes caused by earthquakes, sublacustrine fans with unconfined channels can be formed, and the fan margin subfacies of these sublacustrine fans all develop fine-grained gravity flow deposits. During the rising period of the lake level, a channel- bank-leaf system developed mainly by fine-grained hyperpycnal flow caused by floods. In the deep lake area of large-scale depression lacustrine basins, during the period of lake level rise, the hyperpycnal flow caused by flooding formed a large-scale channel-bank-leaf fine-grained gravity flow system. During the period of lake level decline, the flexural slope break and prodelta sedimentary slope developed fine-grained unconfined channel sublacustrine fan system.
The transgression of the fourth-order sequence is accompanied by the development of organic-rich mudstone and fine-grained hyperpycnal flow deposits. The retreat is accompanied by the deposition of fine-grained concentrated density flow, surge-like turbidity flow, fine-grained debris flow, mud flow and fine-grained transitional flow. These fine-grained gravity flow depositions affect the development of shale oil sweet sections. The various fine- grained gravity flow deposits in the lacustrine basin formed the thin-layer siltstone sweet section formed by fine-grained concentrated density flow, fine-grained hyperpycnal flow, and surge-like turbidity flow deposition, and the mudstone and siltstone layer formed by fine- grained transitional flow. The sweet section of interbedded sand is a horizontally bedding mud shale sweet section containing silt debris and mud debris formed by fine-grained debris flow and mud flow. Multiple sets of source-reservoir assemblages formed in multiple fourth- order sequences in the lacustrine shale series with water ingress-regression cycles.