The genesis and sedimentary architecture of lacustrine gravity flow deposits include ongoing questions affecting the exploration and development of oil and gas, which require attention and investigation. Based on the core description, logging characteristics, and seismic facies in the third member of Paleogene Shahejie formation from the southern Bohai Bay Basin, this study provides some insights regarding lacustrine gravity flows by analyzing the characteristics, distribution patterns, and sedimentary processes of lacustrine gravity flow deposits. Twenty lithofacies are classified into eight bed types which are caused by cohesive flows, inflated flows, concentrated-density flows, or turbidity currents. The characteristics and distributions of different bed types in five sublacustrine fans with two provenance directions and in a slump olistolith reflect two sedimentary processes influenced by sediment supply, basin structure, and climate, including cohesive flow deposits covered by inflated sandflow deposits transforming into concentrated-density flow deposits into hybrid event beds and finally into turbidity current deposits from inner to outer fan, and cohesive flow deposits transforming into inflated sandflow deposits covered by turbidity current deposits into concentrated-density flow deposits into hybrid event beds finally into turbidity current deposits from proximal to distal lobe. Based on the results, a depositional model of lacustrine gravity flow deposits is established, which highlights the change from superimposed channels to anastomosing channels, the distributions and characteristics of hybrid event beds, and distinctive facies in different elements.

Given the importance of gravity flow deposits in hydrocarbon exploration, they have been extensively investigated in marine setting for understanding their sedimentary process and architecture [1,ib2,ib3,ib4,ib5-6]. Many gravity flow types and their sedimentary patterns have been put forward, including sandy or muddy debris flows [7], liquefied flows [8], hyper-concentrated density flows [9], low-density turbidity currents [10], cohesive flows [11], hybrid event beds (HEBs) [12], subaqueous fan system, and mass-transport deposition (MTD) system [13].

Gravity flow deposits are important hydrocarbon reservoirs in lacustrine basins just like in deep-marine settings [14, 15]. In recent years, many sedimentologists have described the sedimentary characteristics and proposed processes and models of gravity flows in deep-lake setting by reference to sedimentary patterns of deep-marine gravity flows. For example, Zavala et al. [16] and Xian et al. [17] have documented the sedimentary characteristics of lacustrine gravity flow deposits (LGFDs). Dodd et al [18] and Yang et al [19] have pointed out that hyperpycnal flow deposits caused by floods are widely developed in deep-lake setting, and LGFDs are affected by confining palaeobathymetry. Niu et al. [20] have proposed a sedimentary process including HEBs and highlighted the LGFD controlled by climate. It is worth noting that lacustrine basins are characterized by more provenances, shorter transportation distance of debris, and smaller area as opposed to the marine setting. However, only very little publications discussed the sedimentary characteristics, processes, and models of LGFD with multiple sources and two trigger mechanisms at the same time. Expounding the facies and sedimentary architectures and establishing a clear model of LGFD are crucial to fill the gap of knowledge about the sedimentary characteristics of widely existent multisource lacustrine gravity flows [17, 18, 21, 22].

During the third member of Shahejie formation in Paleogene (Es3), the LGFD with multiple sources and multiple periods formed in the Gubei sag of eastern China [23, 24]. Identifying the geneses and dispositional model of LGFD in the Es3 in Gubei sag will improve knowledge about sedimentary characteristics of multisource lacustrine gravity flows. In addition, understanding the LGFD characteristics will provide useful information for optimizing oil and gas development strategies in Gubei sag.

This study analyzes cores, logging data, and seismic data from the Gubei sag to (a) determine the sedimentary characteristics of multisource LGFD, (b) point out the types and origins of gravity-driven flows, the sedimentary process between different flow types, as well as the controlling factors of deposition in deep-lacustrine setting, and (c) establish a depositional model of multisource LGFD for lacustrine rift basins, in the hope of injecting some insights into the studies on gravity flows in continental lakes and optimizing oil and gas development strategies.

The Bohai Bay Basin is a typical pull-apart basin with petroliferous in eastern China [23]. During the Mesozoic, the basin evolved into back-arc basin and then developed into a rifted basin during the Cenozoic [25]. Extensional stress along three mantle uplift belts caused rifting and subsidence and also resulted in the formation of a variety of grabens and half grabens. The Gubei sag is a Cenozoic faulted lake basin developed by the Yanshan movement and Himalayan orogeny in the northeast of the Jiyang depression, Bohai Bay Basin (Figure 1 [26]). It is bordered by the Zhuangxi buried hill to the north, the Gudao salient to the south, the Chengdong fault to the west, and the Changdi fault to the east (Figure 1(c)). The Gubei sag is a U-shaped half graben with three steep fault belts in the north, east, and west and a gentle slope belt in the south (Figure 1 [27, 28]). The fault strike mainly exhibits N–S, SW–NE, and W–E orientations. (Figure 1(c)).

Paleogene strata are in unconformable contact with underlying Cretaceous and overlying Neogene strata in Gubei sag. The Mesozoic strata mainly consist of neutral-acid volcanic rocks, gray conglomerates, and coalbeds. The sedimentary succession mainly contains Paleogene and Neogene strata (Figure 1(d)). The Guantao Formation is featured by sand-rich deposits. Covering the Kongdian Formation (Ek), Shahejie Formation (Es), and Dongying Formation (Ed), the Paleogene strata are the primary oil-bearing strata [29]. The Es is extensively distributed in the faulting-subsidence sag and is further divided into four members, Es4–Es1 (bottom to top). The Es3 has been interpreted to represent an early or middle stage of lake basin development, during which the differential subsidence of the faulted depressions occurred and deep-lake basins formed [25, 27]. The argillaceous limestones and oil shales reflect deep-water setting in Es3 (Figure 1(d)). During the lower group of Es3 times (Es3L), the study area was interpreted as being under warm and humid climates (Figure 1(d) [25]). Clastic sediments in areas of sharp relief have been regarded as being transported toward the sag, leading to the formation of lake-floor fans and turbidite deposits [28, 30]. Underlain and overlain by thick mudstones, the sand-rich gravity flow deposits in the late of Es3L are the object of this study.

3.1. Data and Methods

A set of drilling cores, well logs, and three-dimensional seismic data provide insights into the sedimentary characteristics of gravity flows. Over 200 well logs and 1700 m core samples from the Gubei sag are investigated to identify lithology, document sedimentary structures, explain sedimentary elements, and define the genetic types of gravity flows. These data provide the basis for the interpretation of the depositional model of lacustrine gravity flows. In this study, the depositional stratigraphic division and correlation method established by Cao et al. [31] (i.e., the widely distributed thick-bedded mudstones present in well correlations and seismic sections are used for the division and correlation of depositional units composed of well-organized fining- and thinning-upward facies sequences) have been leveraged to divide LGFD in Es3L of the study area to ensure that the LGFD under analyses is a successive deposition.

3.2. Terminology

Different flow types of gravity-driven flow have been defined by using examples from deep-marine settings. Middleton et al. [10] classified gravity flows as turbidity currents, fluidized sediment flows, grain flows, and debris flows. Shanmugam [7] divided gravity flows into sandy and muddy debris flows based on clay content and into high- and low-density turbidity currents based on turbulence concentration. And according to different triggering mechanisms, gravity flows also are classified as hyperpycnal flows (extrabasinal) and slump-induced gravity flows (intrabasinal) [17, 32, 33]. Four terms (cohesive flows, inflated sandflows, concentrated-density flows, and turbidity currents) proposed by Pickering et al. [11] are used in this paper frequently to interpret the causes of different lithofacies and bed types. The cohesive flow is viscoplastic fluid dominated by cohesive strength [34]. Grain collision and pore pressure are the primary support mechanisms of inflated sandflow deposition [11]. The inflated sandflow is approximately equivalent to liquefied flow [8], density-modified grain flow [35], cohesionless debris flow [36], and sandy debris flow [7]. The terms “high-concentration turbidity current” and “high-density turbidity current” [7] approximately amount to the term “concentrated-density flow” dominated by grain collision. Turbidity currents are turbulences, which are approximately equivalent to turbidity flows [9, 37] and low-density turbidity currents [7]. The term “bed types” is used to document different facies associations and HEBs in gravity flow deposits [20].

In addition, the terms “sublacustrine fan” containing channel elements and “slump olistolith” with the characteristics of MTDs are used to document different sedimentary systems in deep-lake setting. The terms “feeder channel” and “distributary channel” are used to distinguish the geometry feature of channel elements in inner fan and middle fan subenvironments, respectively. The term “feeder channel” is similar to “feeder system” proposed by Dodd et al. [18].

4.1. Facies Analysis

Sedimentary rocks in the research region mainly comprise conglomerates, pebbly sandstones, sandstones, siltstones, and mudstones (Figures 2,3-4). Twenty lithofacies are distinguished according to lithology, grain size, texture, sedimentary structure, and grading (Table 1). Eight sedimentary bed types presenting different sedimentary elements in deep-lake setting are summarized and interpreted based on the stacking pattern of different lithofacies (Figure 5).

4.2. Bed Types

4.2.1. Type 1 Beds: Dominated by Mud-Rich Massive Conglomerate

Description

Type 1 beds covered by Fm consist of mud-rich conglomerate (Gmm) with an erosive base in the lower part, Gcm or Mps in the middle part, and Ngps in the upper part (Figure 5(a)). The boundaries are sharp within these beds. A small amount of dispersed mud clasts is seen within Gcm/Mps. Phytodetritus can be observed.

Interpretation

The presence of phytodetritus indicates that type 1 beds can be induced by floods (Figure 2(b) [38]). Numerous black mud clasts within Gmm can be transported by gravity-driven cohesive flows. However, it has been confirmed that gravity-driven cohesive flows are barely able to form an erosive base [31], and the bypassing of gravity flow deposits cannot form the successive stacking of Gmm and Gcm/Mps (Figure 5(a) [39]). Therefore, it is concluded that numerous black mud clasts within Gmm with erosive base source from muddy substrates [40]. Muddy substrates are scoured in deep-lake setting, increasing high-concentrated flows’ mud content, thus transforming it into cohesive flows to form Gmm [15, 41,ib42-43]. With chaotic small mud clasts and pebbles being present, Gcm/Mps is inferred to be deposited through en masse freezing of inflated sandflows [11]. Coarse-grained clastic rocks (Gcm and Mps) without erosive base and the overlying Ngps caused by concentrated-density flows imply that the decelerating high-concentrated flows transform into inflated sandflows (sandy debris flows) when gravity-driven flows enter into lake setting [7, 11, 19]. Therefore, type 1 beds representing proximal channel elements are regarded as resulting from the erosion of the lower part of concentrated-density flows, the deceleration of the middle part of concentrated-density flows, and the rapid vertical suspension fallout from the upper part of concentrated-density flows [11, 34]. In addition, the structureless sandy deposits in the middle of these beds could be Mps or Gcm, depending on the transportation distance.

4.2.2. Type 2 Beds: Massive Conglomerate and Pebbly Sandstone

Description

These beds, which commonly overlie mudstones, can be divided into structureless sandy clastic rocks (Gcm and Mps) and overlying graded sandy clastic rocks (Ngc, Ngps, Igps, Sps, Ngs, Ps, and Sp; Figure 5(b)). Phytodetritus scatters among Gcm and the top of Ngc (Figure 5(b)). Unlike Type 1 beds, these beds don’t have a great amount of mud clasts at the bottom. The successive deposition of Nps, Ps, and Sp covered by mudstones shows Ta–Tc intervals of Bouma sequence at the top of these beds.

Interpretation

Type 2 beds are regarded as channel deposits [32, 44]. Phytodetritus, Igps, and Sps, within type 2 beds indicate that these beds may be caused by sustained hyperpycnal flows [16]. Although Gcm can be induced by layer-by-layer deposition from the lower part of a concentrated-density flow, dispersed phytodetritus and nonerosive base suggest that Gcm at the bottom of type 2 beds is caused by the en masse freezing of inflated sandflows [7, 32, 34]. Graded or stratified pebbly sandstones (Ngps, Igps, and Sps) caused by concentrated-density flows within type 2 beds imply that the inflated sandflows induced by the deceleration of sustained hyperconcentrated flows are the cause of the formation of Gcm and Mps [11, 19]. The vertical stacking pattern of normally graded and stratified sandstones (Nps, Ps, and Sp) suggests that the top of type 2 beds is brought on by low-density turbidity currents [7, 45]. The mix of ambient water can reduce the flow concentration, which makes concentrated-density flows transform into turbidity currents to form deposits with Bouma sequence. Considering the stacking pattern of lithofacies and the absence of mud interlayers, type 2 beds are regarded as feeder channel deposits [43]. In addition, medium- to fine-grained sandstones in these beds suggest an increase of transportation distance.

4.2.3. Type 3 Beds: Graded Pebbly Sandstone and Graded Sandstone

Description

Normally graded type 3 beds are dominated by Ngps, Igps, Sps, Ngs, Ps, and Sp, which mainly show a repeated stacking of Ta–Tb Bouma sequence in the lower part and Tb–Td Bouma sequence in the upper part (Figure 5(c)). The erosional feature is observed at the bottom of Ngps or Nps, occasionally.

Interpretation

Due to the existence of Igps and Sps, these beds are regarded as resulting from hyperpycnal flows (a special type of concentrated-density flow) induced by flooding [16, 38, 46]. The vertical succession of Igps, Sps, Ngps, and Ngs indicates that these deposits form by concentrated-density flows [34, 44]. The vertical stacking of Ngs, Ps, and Sp in the upper beds shows Tb–Tc Bouma sequence, suggesting that the upper beds are formed by turbidity currents [45]. Type 3 beds reflect channel elements with relatively long transport distance [47,ib48-49].

4.2.4. Type 4 Beds: Graded and Stratified Sandstone/Siltstone

Description

Type 4 beds are composed of Ngs, Ps, Sp, Ngss, Pss, Rss, Fl, and Fm, mainly showing repeated stacking of the upper Bouma sequence or complete Bouma sequence (Figure 5(d)). Igs with sharp boundaries is seen occasionally, which is covered by normally graded sandstones. Very thin couplets of mudstone and sandstone can be observed in these beds.

Interpretation

These beds primarily presenting the upper Bouma sequence indicate low-density turbidity current deposits in lobe elements [18, 45, 50,ib51-52]. The vertical stacking of inversely to normally graded sandstone–mudstone couplets can be interpreted as the deposition of distal hyperpycnal flows or the transformation of turbulences [12, 16, 46, 49].

4.2.5. Type 5 Beds: Dominated by Deformed Sandstone

Description

Featured by the mixing and deformation of sands and muds, type 5 beds are composed of Ds and Fm with deformation structures (Figure 5(e)). Fine-grained Igs and Pss with deformation structures can be observed in these beds as well (Figure 5(e)).

Interpretation

Ds and Fm with deformation structures confirm that type 5 beds are the product of cohesive flows induced by slumping [53]. Distorted normally graded and stratified sandstones are likely caused by the reworking of turbidity current deposits [54, 55].

4.2.6. Type 6 Beds: Dominated by Massive Sandstone/Siltstone

Description

Type 6 beds mainly consist of Sm/Mss, thin-bedded normally graded and stratified siltstones (Ngss, Pss, and Rss), and dark gray mudstones (Fl and Fm; Figure 5(f)). Covered by mudstones, siltstones overlie massive sandstones/siltstones with dispersed small mud clasts and are in sharp contact with other beds.

Interpretation

These beds indicate MTDs [56]. Massive sandstones or massive siltstones can be formed by debris flows or the aggradation of high-density turbidity currents [32]. However, the great amount of dispersed mud clasts suggests that resembling the deposits of quasi-laminar plug flows, sandy/silty deposits (Sm and Mss) in type 6 beds are caused by en masse freezing of debris flows (inflated sandflows) [57]. Thin-bedded normally graded stratified siltstones and overlying Fl at the upper of these beds jointly represent turbidity current deposits.

4.2.7. Type7 Beds: Normally Graded and Stratified Sandstone/Siltstone

Description

Type 7 beds are composed of Ngs, Ngss, Sp, Ps, Pss, Rss, Fl, and Fm, which mainly show the upper Bouma sequence or complete Bouma sequence (Figure 5(g)). Unlike type 4 beds, type 7 beds with erosion structures lack Igs. The stacking couplets of Ps/Pss–Sp/Rss can be seen.

Interpretation

The stacking pattern of sandstones and siltstones indicates that these beds result from turbidity currents. The erosive base of Ngs/Ngss and the absence of Igs jointly suggest surge-like turbidity current deposits in slumps [19, 20].

4.2.8. Type 8 Beds: HEBs

Description

Type 8 beds with the sedimentary characteristics of HEB can be divided into two subtypes, including binary event beds and ternary event beds (Figure 5(h)). The average gain size of Ngs above binary event beds seems larger than that of pure sandstones at the bottom of binary event beds. Covered by Ngs, binary event beds are composed of pure sandstones (Ngs) with water escape structures (equivalent to the “H1” division of HEB) and overlying Cas (equivalent to the “H3” division of HEB). A great amount of black mud clasts is dispersed in Cas which is slightly thicker than Ngs/Ngss in binary event beds (Figure 5(h)). The boundaries of Cas and Ngs/Ngss are sharp. Ternary event beds with erosive base are mainly composed of fine-grained sandstones/siltstones featuring the upper part of Bouma sequence, normally graded muddy siltstones (Cas), and overlying black mudstones (Figure 5(h)). Normally graded muddy siltstones in ternary event beds can be divided into the lower part with mud parches and the upper part with siltstone injections and dispersed mud (ca 20%–60% visually), which are similar to “H3a” and “H3b” subdivisions described by Dodd et al. [40], respectively. Brown siltstones with parallel lamination (representing the “H4” division of HEBs) overlie Cas and underlie mudstones (representing the “H5” division of HEBs) in ternary event beds.

Interpretation

Type 8 beds are a subset of HEBs; therefore, they result from complex processes including erosion, deceleration, and suspension fallout of high- or low-density turbidity currents, en mass freezing of cohesive flows, as well as the transformation between turbulence and plastic flows [12, 57, 58]. Binary event beds are induced by the upward buoyant movement of lower-density clasts. Normally graded sandstones (Ngs) in binary event beds are induced by the suspension fallout of concentrated-density flows, aggradation of decelerating turbidity currents, or lofting of hyperpycnal flows [11, 46]. And fine-grained sandstones or siltstones with water escape structures in binary event beds indicate that particles moved upward through the action of pore pressure and buoyancy during deposition. The upward movement of lower-density debris (clasts) leads to an increase in the mud content in the upper turbulence, then to the dampening of turbulence and the transformation to cohesive flows (forming Cas) [57,ib58,ib59-60]. Binary event beds are similar to the “HEB3” bed types described by Pierce et al. [61]. Considering the relatively large average gain size, Ngs covering binary event beds is not regarded as the “H4” division of HEB [12]. Ternary event beds are regarded as a product of the erosion of turbidity currents. The erosive base of normally graded sandstones/siltstones showing Tc–Td Bouma sequence indicates that muddy substrates are eroded by low-density turbidity currents. The injection of mud clasts and advanced vertical/longitudinal segregation processes caused by substrate erosion can induce the formation of a quasi-laminar plug within (and/or behind) the upper turbidity current, leading to the formation of Cas [62,ib63-64]. The longitudinally fractioned flow transformation process and the shear jointly promote the along-flow segregation of Cas into two parts in ternary event beds [40]. The dilute turbulent wake and the suspension fallout jointly cause the deposition of stratified sandstones/siltstones covering Cas and of the overlying mudstones in ternary event beds [12, 65].

4.3. Characteristics of Architecture Elements

4.3.1. Sublacustrine Fan

The bed types and their distributions in sublacustrine fan system of the study area are similar to that in the sublacustrine fan system of Dongying sag documented by Cao et al. [31]. What’s more, the channels present different stacking patterns in different areas. Thus, this paper adopts the classification scheme proposed by Cao et al. [31] to classify sublacustrine fan system into inner fan, middle fan, and outer fan, of which the details are as follows.

4.3.1.1. Inner Fan
Description

Wedge-shaped and lenticular chaotic-hummocky reflections, single U-shaped bidirectional onlap reflection, and the thinning of seismic reflection axis at both sides of the lenticular bodies are visible in seismic profiles (Figure 6). Type 1 and type 2 beds locate in the slope characterized by onlap reflection, with the latter taking a higher proportion than the former (Figure 6). In steep-slope zones, gamma ray (GR) logging curve is in shape of serrated box, serrated bell, and funnel (Figure 6(b)). Among them, the box- and bell-shaped GR logging curve mainly correspond to the vertical stacking of massive conglomerates and fining-upward pebbly sandstones. Funnel-shaped GR logging curve corresponds to the vertical stacking of coarsing-upward pebbly sandstones and sandstones.

Interpretation

Wedge-shaped and lenticular chaotic-hummocky reflections indicate sublacustrine fan deposits [19]. The feeder channel element is represented by the single U-shaped bidirectional onlap reflection and the box- and bell-shaped GR logging curve, and the levee element located at both sides of the lenticular bodies is represented by the thinning of seismic reflection axis. Featuring massive conglomerates (Gcm and Gmm) and pebbly sandstones (Mps and Ngps), type 1 and 2 beds predominantly originate from debris flows (cohesive flows and/or inflated sandflows) and overlying concentrated-density flows [7, 11], reflecting that the sandy debris is not transported effectively [15]. The vertical stacking of funnel-, box-, and bell-shaped GR logging curve suggests progradation and aggradation of the feeder channel [19, 39]. In steep-slope zones, LGFD dominated by debris flow deposits and concentrated-density flow deposits consists of progradational and aggradational feeder channel deposits and levee deposits, which represent a high-energy environment and are interpreted as inner fan deposits. Similar to the fan system proposed by Cao et al. [31], inner fan deposits near the source area are dominated by the stacking of massive pebbly sandstones and massive conglomerates.

4.3.1.2. Middle Fan

Description

The vertical stacking of type 3 beds with graded and stratified sandy deposits (Ngps, Ngps, Igps, Sps, Ngs, Ps, and Sp), together with serrated box-shaped GR logging curve and serrated bell-shaped GR logging curve, corresponds to the middle section of the wedge geometry present in seismic profiles (Figure 7). Thin mudstones are covered by type 3 beds (Figure 7(b)). Anastomosing or parallel-arranged U-shaped bidirectional onlap reflections are visible in seismic profiles. Type 8 beds with binary sedimentary structure can be clearly observed (Figure 7(c) and 7(g)).

Interpretation

Featuring vertical stacking of box- and bell-shaped GR logging curve, the vertical stacking of type 3 beds is accounted for as deposits of concentrated-density flows and overlying turbidity currents [11, 16], which indicates the distributary channel element [19]. Anastomosing or parallel-arranged U-shaped gullies in seismic profiles represent the migration and anastomosis of channels. Situated in the middle section of the wedge geometry in seismic profiles, the migration and anastomosis of channel deposits (i.e., distributary channel deposits) and inter-channel deposits indicated by thin mudstone interlayers jointly constitute middle fan deposits. Similar to the sedimentary characteristics of the middle fan proposed by Cao et al. [31], the middle fan deposits in the study area are characterized by graded sandstones. As opposed to inner fan deposits, the absence of structureless sandy deposits and the finer grains in middle fans jointly indicate that the debris has been transported in relatively long distances. Compared with the feeder channel in inner fans, the GR logging curve of the distributary channel has a lower amplitude, and the thickness of the single-channel sediment event bed decreases. In addition, type 8 beds with binary sedimentary structure imply the transformation of turbulence in middle fans.

4.3.1.3. Outer Fan

Description

In seismic profiles, the enhancement of seismic amplitude at the lower section of steep-slope zones can be observed, which corresponds to the funnel- or finger-shaped GR logging curve and type 4 beds composed of fine-grained deposits with Bouma sequence (Figure 7). Type 8 beds with ternary sedimentary structure can be clearly observed (Figure 7(c) and 7(g)).

Interpretation

Low-density turbidity current deposits indicated by type 4 beds constitute lobe deposits [11, 16]. Composed of multiple lobe elements, outer fans characterized by fine-grained deposits with Bouma sequence are situated in the lower section of steep-slope zones. Compared with the inner and middle fan, the shale content and mudstone thickness of the outer fan are much higher (Figure 7), indicating a relatively low-energy environment. Type 8 beds with ternary sedimentary structure suggest that the transformation of turbulence occurs in outer fans.

4.3.2. Slump Olistolith

The sedimentary characteristics of slump olistolith deposits are similar to those of MTD [66, 67]. The slump olistolith dominated by types 5, 6, and 7 beds consists of proximal and distal lobes according to the sedimentary locations and characteristics.

4.3.2.1. Proximal Lobes

Description

In the seismic profile, lenticular chaotic-hummocky reflections related to the hangingwall of normal faults as well as an increase in the amount of seismic reflection axis are visible (Figure 8). The GR logging curve is relatively smooth, most of which is box- or bell-shaped (Figure 8(a)). In the middle of lenticular geometry, LGFD is dominated by types 5 and 6 beds composed of Ds, Sm, Mss, and Fm with deformation structure, silty deposits with the upper Bouma sequence and Fl (Figure 8(d) and 8(f)). It is worth noting that parallel laminations and inverse grading are observed in deformed sandstones (Figure 8(d)).

Interpretation

The deformation structure forms in the proximal area of slumps with lenticular geometry present in seismic profiles [19]. Thus, LGFD characterized by deformation structure and lenticular geometry can be regarded as proximal lobe deposits in slump olistoliths. The shape of GR logging curve may depend on the characteristics of the original unconsolidated sediments. Slump deposits are situated in front of sublacustrine fans, which can be suggested by the parallel laminations and inverse grading within deformed sandstones and the location of core data (Figure 8(d)). The slumping of channel deposits in sublacustrine fans may result in the formation of box- or bell-shaped GR logging curve in proximal lobes. In Gubei sag, the proximal lobe deposits represented by types 5 and 6 beds are related to the hangingwall of normal faults (Figure 8), implying that the slumps are likely to be caused by tectonic activities.

4.3.2.2. Distal Lobes

Description

In the fringe of lenticular geometry, type 7 beds, funnel-shaped GR logging curve with relatively high value and slightly progradational seismic axis can be observed (Figure 8). Type 8 beds characterized by ternary structure are visible.

Interpretation

Given the absence of deformation structure and the evolution of flows (i.e., with the transportation distance increasing, debris flows gradually transform into low-density current flows [11]), deposits resulting from high- or low-density turbidity currents in the fringe of lenticular geometry are interpreted as distal lobe deposits. The Ds is the symbol for distinguishing proximal lobes and distal lobes.

4.4. Source Direction and Depositional Unit

Identifying the source direction of LGFD is beneficial to determine flow trends as well as the sedimentary distribution of gravity flows and to analyze the origin of LGFD. Progradational reflection configuration with nearly S–N trend and a northward inclining slope observed in the seismic profile jointly indicate a nearly S–N source direction of LGFD (Figure 9(a)). In the SE–NW seismic profile, a wedge-shaped LGFD pinches out to the northwest and downlaps at the bottom of a slope (Figure 9(b)), which suggests a source direction from southeast to northwest. Based on the features of wedge-shaped LGFD and progradational reflection configuration in seismic profiles, two provenance directions (nearly S–N trend and SE–NW trend) are recognized in Gubei sag (Figure 9), which indicates the interactions between flows with different directions and suggests that there are two sublacustrine fans at least.

In order to accurately determine the sedimentary process and model of lacustrine gravity flows, it is necessary to divide LGFD’s depositional units to ensure that the LGFD under analyses is a successive deposition. From the genetic perspective, the sedimentary event caused by gravity flows can be regarded as a depositional record in a weak-strong-weak energy cycle [20]. Thus, a widely distributed thick-bedded mudstone serving as a sedimentary record of a low-energy environment indicates the end of the LGFD. Referring to the depositional stratigraphic division and correlation criteria established by Cao et al. [31], the widely distributed mudstone bed is regarded as a symbol for dividing LGFD into different depositional units. A widely deposited mudstone bed (average thickness >5 m) underlying and overlying sandy deposits with normal grading is recognized in Gubei sag, according to which the LGFD in Gubei sag has been divided into the early depositional unit (SQ1) and the late depositional unit (SQ2; Figure 9(i)). The sedimentary process and model of lacustrine gravity flows in SQ1 have been discussed in this study.

4.5. Spatial Distribution

4.5.1. Sublacustrine Fan

Feeder channel deposits in inner fans are thinner than distributary channel deposits in middle fans and lobe deposits in outer fans (Figure 10). Vertically, the feeder channel deposits overlie and underlie the dark mudstones (thicker than 5 m); lobe deposits can overlie or underlie amalgamated distributary channel deposits (Figure 10(a)). Type 4 beds covered by mudstone beds locate always at the top of sublacustrine fan systems. The amount of mudstone interlayers in the feeder channel elements with amalgamated type 1 and 2 beds is smaller than that in amalgamated distributary and lobe elements, increasing from south to north in Gubei sag (Figure 10(a)). In a single well dominated by distributary channel deposits and lobe deposits, the thickness and amount of mudstone interlayers decrease at first and then increase as the depth decreases (Figure 10(a)).

In the view of planform, five elongated sublacustrine fans in the same period with interconnected distributary channels are identified (Figure 11(a)–11(d)). Along the feeder channel trends suggested by two provenance directions, the proportion of type 4 beds increases, the proportion of type 3 beds increases at first then decreases to zero, and the types 1 and 2 beds gradually extinct (Figure 11(d) and 11(e)). Type 8 beds are generally located in lobes (Figure 11(d) and 11(e)); however, it is noted that they are also located in interconnecting middle fan area and the terminal area of distributary channels, indicated by their existence in wells z62 and z52-9.

In addition, the distribution patterns of channel elements are different in inner fans and middle fans. In inner fans, the vertical stacking of feeder channels is shown in the seismic profile. (Figure 11(a) and 11(d)). In middle fans, the distribution pattern of distributary channels can be divided into two types, including the horizontally migrating channels in near-inner fan area and the anastomosing and meandering complex channels in near-outer fan area (Figure 11(a)–11(c)).

4.5.2. Slump Olistolith

From south to north, the thickness of proximal lobe deposits rapidly decreases as the thickness of distal lobe deposits increases (Figure 10). The distal lobe deposits can cover proximal lobe deposits in well z23-16-15 located in the junction of the proximal lobe and distal lobe (Figure 10(b)).

There is a slump olistolith at the front of sublacustrine fan in northern Gubei sag (Figure 11(d)). Type 5 and 6 beds gradually extinct as the proportion of type 7 beds increases from the proximal lobe to the distal lobe (Figure 11(d) and 11(e)). As shown in Figures 10(b), 11(d),, type 8 beds with a mudstone cap exist in the distal lobe of slump system.

5.1. Flow Types and Evolutionary Process of Gravity Flows

The purpose of this study is to understand the sedimentary characteristics and distributions of LGFD and, in turn, to recognize the characteristics of reservoirs in a hope to promote the exploration and development of oil and gas. Different flow types form different facies and bed types; hence, accurately identifying flow types is conducive to grasp the sedimentary characteristics and origins of LGFD events. The interpretation of flow types is also a prerequisite for interpreting the evolutionary processes of lacustrine gravity flows. As evolutionary processes influence the characteristics and distributions of LGFD, an accurate understanding on the evolutionary processes of lacustrine gravity flows will contribute to the recognition of the LGFD’s distribution and, in turn, favor the prediction of reservoirs. All in all, the interpretation of flow types and evolutionary processes of lacustrine gravity flows are important for achieving the purpose of this study.

5.1.1. Flow Types

Based on lithofacies analysis, four gravity-driven flow types are identified (Figure 12): cohesive flow (nonNewtonian Bingham plastic flow) indicated by Gmm and Ds which are consolidated en masse on account of cohesive freezing [44, 61, 66]; inflated sandflow indicated by Gcm with sharp boundaries, Mps, Sm, and Mss with little dispersed mud clasts, of which the deposits are formed by frictional “freezing” [11]; concentrated-density flow mainly indicated by type 3 beds with graded and stratified pebbly sandstones/sandstones [16, 19]; and turbidity currents (turbulent Newtonian flows) with the upper part of Bouma sequence indicated by types 4 and 7 beds which are mainly composed of normally graded and stratified sandstones/siltstones [9, 15, 36]. According to different triggering mechanisms, turbidity currents can be further classified into two types: quasi-steady turbidity currents triggered by flooding, of which the deposits are characterized by Igs covered by Ngs; surge-like turbidity currents triggered by sediment failure, of which the deposits feature Bouma-like sequences and erosive base [9, 32]. However, the results from a high-resolution advanced numerical computational fluid dynamics simulation suggest that there is no clear macroscopic recognition to distinguish surge-like turbidity current deposits and quasi-steady turbidity currents [68]. Therefore, unlike Yang et al. [19] and Niu et al. [20], this paper does not further divide turbidity currents. Being found in fan and slump systems as a special product of gravity-driven flow, HEB with stacking of cohesive flow deposits and turbidity current deposits is involved in the sedimentary processes of lacustrine gravity flows [12, 57, 69, 70].

5.1.2. The Sedimentary Processes of Gravity Flows

Sublacustrine fan system caused by flooding and slump system caused by slumping both exist in Gubei sag indicated by type 3 and type 5 beds, respectively (figures 5, 6, 8; [15]). Compared with the lacustrine gravity flows’ sedimentary processes proposed by Yang et al. [19] and Niu et al. [20] in the two systems, the sedimentary processes put forward in this paper highlight flow transformation when floods enter into lake, HEB’s involvement, and turbidity current deposits which overlie debris flow deposits (Figure 13).

5.1.2.1. LGFD Induced by Flooding

The change of dominated bed types from the inner fan to out fan (type 1 and type 2 beds–type 3 beds–type 4 beds) documents a widely recognized evolution process of gravity flows triggered by flooding in lacustrine basins, which is evolving from debris flows to concentrated-density flows finally into turbidity currents (Figure 13 [18, 19, 71]). However, the gravity flow evolution process suggested by the distributions and different bed types in Gubei sag varies from the existing evolution patterns. The preferential emplacement of cohesive flows caused by the erosion of concentrated-density flows occurs at first as floods enter, which is indicated by type 1 beds’ proportion rapidly decreasing and type 2 beds’ proportion increasing with flow run-out in inner fans (Figures 11(d), and 11(e) [16, 44]). The facies in the upper type 1 beds covering Gmm is similar to type 2 beds, which suggests that the inflated sandflows transformed from decelerated concentrated-density flows cover cohesive flows [11]. The concentrated-density flow deposits (Nps, Sps, and Igs) covered by turbidity current deposits (Ngs with the Tc interval of Bouma sequence) overlie inflated flow deposits (Gcm) in type 2 beds, which indicates the coexistence of inflated sandflows at the bottom, concentrated-density flows at the middle and turbidity currents at the top. The absence of turbidity currents at the top of type 1 beds suggests that turbidity currents are a product of concentrated-density flows caused by flow dilution as the transport distance increases [42, 43]. Therefore, the changes of type 1 and 2 beds’ proportion and the characteristics of type 1 and 2 beds jointly suggest an initial evolution process of gravity flows, including cohesive flows getting covered by inflated sandflows, and then transforming to inflated flows along with the top of concentrated-density flows transforming to turbidity currents as distance increases and ambient waters entrain (Figure 13 [72]). HEB can exist in the interconnected middle fan area, indicating HEB forms after the complete transformation from debris flows to concentrated-density flows (Figures 10(a) and 11(d) and [12, 13]). In addition, outer fan deposits dominated by type 4 beds suggest that turbidity current is the end type in lacustrine gravity flows’ evolution process. In the sublacustrine fan system, the evolution process of gravity flows is summarized as a set of flow transformation from cohesive flows covered by inflated sandflows to inflated sandflows covered by concentrated-density flows to concentrated-density flows covered by turbidity currents then to hybrid flows (causing HEB), and finally into turbidity currents (Figure 13).

5.1.2.2. LGFD Induced by Sediment Failures

The changes of types 5, 6, and 7 beds from the proximal lobe to the distal lobe indicate a recognized gravity flows’ evolution process in the slump system, which is from cohesive flows to inflated sandflows/concentrated-density flows finally into turbidity currents [17, 19, 20]. Based on the characteristics and distributions of types 6 and 8 beds, this paper has refined this evolution process. Stratified siltstones (Pss), Fl, and underlying massive sandstones/siltstones with dispersed mud clasts in type 6 beds indicate that turbidity currents cover inflated sandflows (Figure 5(f)). The ambient waters’ fluctuation caused by slumping and the upward movement of interstitial waters may induce the dilution of the top of inflated sandflows to form turbidity currents and traction structures. Normally graded sandstones presenting Ta–Tb interval of Bouma sequence are in sharp contact with overlying very fine-grained sandstones/siltstones with Tc–Td interval of Bouma sequence, which indicates that turbidity currents cover concentrated-density flows (Figure 5(f)). HEB showing erosion structure and the Tc interval of Bouma sequence suggests that it is a product of turbidity currents and flow transformation [12]. Therefore, it is inferred that HEB forms after the formation of turbulences. In the slump system, the evolution process of gravity flows is summarized as a set of flow transformation from cohesive flows to inflated sandflows (covered by turbidity currents) to concentrated-density flows then to hybrid flows (causing HEB), finally into turbidity currents (Figure 13).

5.2. Controlling Factors

The main factors that can influence the formation and distribution of LGFD are paleotopography, lake-level fluctuation, trigger mechanism, and climate [73]. In Gubei sag, sediment supply, basin structure, and climate are regarded as the three main controlling factors of LGFD’s distributions and sedimentary characteristics.

5.2.1. Sediment Supply

Sediment supply determines the initial concentration, components, and types of gravity flows [72, 74]. In Gubei sag, two source directions (nearly S–N trend and SE–NE trend) determine gravity flow run-outs’ trends and, in turn, cause the formation of sublacustrine fans extending north and extending northwest (Figures 9 and 11). The amount and duration of fan depositions controlled by sediment supplies influence the thickness and area of LGFD. The initial concentration and detrital component of gravity flows fed by flooding determine the initial gravity flow type in the sedimentary processes and the characteristics of LGFD [52]. In the sublacustrine system of Gubei sag, coarse-grained and sand-rich detritus lead to a higher proportion of inflated sandflow deposits than cohesive flow deposits, the existence of pebbly sandstones in middle fans, and promote the transformation from decelerated concentrated-density flows into inflated sandflows (Figures 5,6-7). In a slump system, the source of deposits determines the size of grains.

In addition, sediment supply controls the formation of HEB. In the sublacustrine fan system of Gubei sag, gravity flows from different directions mix in the center of the lake basin, which brings about a ramp-up in debris concentration and fluctuation of flows, then may lead to the transformation from turbulence to plastic flow to form the HEB [12, 75]. In the interconnecting middle fan area, a plenty of sediments supplied from different sublaustrine fans result in the formation of HEB without mudstone cap as well as a reduction in the amount of mudstone layer (Figures 10(a), 11(d),). Meanwhile, the formation of HEB with ternary structure is also attributed to the increasing mud content caused by erosion in turbulence.

5.2.2. Basin Structures

The topographic characteristics of lake basins and the development of faults are factors that control LGFD [42]. On one hand, topographic changes affect LGFD’s distribution and hydrodynamic changes [18, 39, 74]. In the Gubei sag, sublacustrine fans sourced from gentle slope areas cover a greater distance compared with the situation in steep slope areas (Figures 1 and 11). A high slope angle can increase hydrodynamic force to induce the bypassing of coarse-grained debris, which may serve as a cause for the existence of pebbly sandstones in the terminal of distributary channels (Figure 11). On the other hand, different hydrodynamic forces and loads can modify topographic characteristics. Weak-energy flows and the scouring of high-energy flows fill and modify local topography, respectively, which influences the distribution pattern of LGFD [41].

A slump olistolith is located in front of sublacustrine fan and is near a fault (Figures 1 and 11(d)), implying that faults as an element of basin structures are a salient factor for the formation of slumps. In addition, the fault action can lead to the formation of a slope with a high angle to influence the long-distance LGFD transportation or promote bypassing.

5.2.3. Climate

Climate controls the development and sedimentary characteristics of LGFD [20, 73]. Warm and humid climate tends to trigger floods which carry subaerial sediments into lakes to form LGFD containing phytodetritus [38]. In Gubei sag, the warm and humid climate indicated by the high climate index (higher than 0.4) during Es3L [76] enriches the vegetation in flood discharge areas, increases the frequency of flood, and enhances hydrodynamic force and, in turn, leads to a wide distribution of flood-induced LGFD which contains phytodetritus, Igs, Igps, and Sps. In addition, climate can influence the grain size of LGFD. A humid climate featuring high precipitation and high discharge of rivers facilitates the transportation of coarse-grained clasts and, in turn, causes the development of LGFD with a high proportion of pebbly sandstones in Gubei sag. Although the formation of slumps is associated with humid climate, there is no evidence to verify that the slumping of sublacustrine fan deposits is controlled by climate.

5.3. The Distinctive Depositional Model

In general, the depositional model established in this paper is similar to a combination of the submarine fan model proposed by Walker [77] and the MTD model proposed by Embley [13], which exhibits the characteristics of the two systems (Figure 14). In the sublacustrine fan system, the inner fan composed of feeder channel and levee elements is dominated by cohesive and noncohesive debris flow deposits (Gcm, Gmm, and Mps); the middle fan is dominated by distributary channels with structural sandy deposits (Ngps, Igps, Sps, Ngs, Igs, Ps, and Sp); and the outer fan is composed of turbidity current deposits with Bouma sequence. In the slump system, the proximal lobe is dominated by cohesive flow deposits (Ds) and inflated sandflow deposits (Sm and Mss); the distal lobe is dominated by fine-grained turbidity current deposits with Bouma sequence (Figure 14).

It is worth noting that the new depositional model of LGFD has many distinctive characteristics which highlight the concentrated-density flow deposit in inner fans, the distribution pattern and stacking of HEB and channel elements, the amalgamated graded sandy deposits in middle fans, as well as the stacking facies of superimposed sublacustrine fans and slumps. The stacking pattern of different lithofacies in feeder channel elements shows that inflated sandflow deposits (Gcm or Mps) above cohesive flow deposits (Gmm) are covered by concentrated-density flow deposits (Ngc or Ngps) in proximal inner fans and that concentrated-density flow deposits (Ngps, Igps, and Sps) above inflated sandflow deposits (Mps) are covered by turbidity current deposits with Bouma sequence in distal inner fans (Figures 5, 11 and 14). In a middle fan, thin mudstone interlayers exist, which indicates the lateral migration of channels. Anastomosing distributary channels induced by multiple flood-produced gravity flows in the interconnecting middle fan area is characterized by amalgamated graded sandy deposits (concentrated-density flow deposits and overlying turbidity current deposits) and the absence of mudstone interlayers (Figures 11 and 14). The stacking pattern of channels in different parts of sublacustrine fan system exhibits a change from superimposed feeder channels in the inner fan to horizontally migrating distributary channels in the middle fan, and finally to anastomosing and superimposing distributary channels in the interconnecting middle fan area (Figures 11 and 14). In addition, HEB without mudstone cap is located in the distal interconnecting middle fan area and HEB with erosive base, and mudstone cap is mainly situated in the lobes of sublacustrine fan system and distal lobes of slump system (Figures 10, 11 and 14).

In Gubei sag, the slump olistolith located at the end of sublacustrine fans is characterized by the stacking of deformed sandstones with laminations (Ds) and deformed mudstones (Ms) covering turbidity current deposits in the initial end of the proximal lobe, inflated sandflow deposits (Sm and Mss) covered by Fl or Pss in the terminal end of the proximal lobe, as well as fine-grained sandy deposits and silty deposits both with Bouma sequence in the distal lobe. The pattern of lobes of sublacustrine fan system and distal lobes of slump system both present overlapped turbidity current deposits featuring the upper Bouma sequence, which is similar to the depositional model proposed by Yang et al. [19].

5.4. Implications

The sedimentary characteristics and distributions of LGFD have implications for the exploration and development of oil and gas. Wedge-shaped or lenticular LGFD together with the underling and overlying mudstones jointly constitute updip pinch-out traps. The variety of both bed types and lithofacies recognized in this study reveals the complexity of reservoirs’ characteristics and distributions. Poorly sorted conglomerates and pebbly sandstones mainly developed in inner fan subenvironment imply low-permeability and heterogeneous reservoirs. In the lobe subenvironment, sandy deposits with relatively high clay content and the formation of HEB also suggest heterogeneous reservoirs with low permeability. As opposed to the reservoirs in other subenvironments, the channel elements mainly comprising pure sandstones in middle fans may be homogeneous with relatively high porosity and permeability, which are regarded as favorable reservoirs. In addition, anastomosing and superimposing channels in middle fans can increase the connectivity of reservoirs and decrease heterogeneity. It is worth noting that the formation of HEB in the tail end of middle fans can cause heterogeneity and local low permeability, which can have negative influences on the reservoir quality. The deformed sandstones (Ds) and high clay content in slumps sourced from the front of sublacustrine fans may imply homogeneous reservoirs with low permeability, so sandy deposits in slumps are not deemed as favorable reservoirs. Nevertheless, given that sandy deposits in slumps are located near sources rocks, they may still favor the accumulation of unconventional hydrocarbon.

  1. The LGFD consists of twenty lithofacies in Gubei sag. There exist five sublacustrine fans with two provenance directions composed of types 1–4 beds, as well as a slump olistolith composed of types 5–7 beds located in the terminal end of sublacustrine fans. HEB exists in the distal part of both sublacustrine fan system and slump system.

  2. Four flow types are recognized, including cohesive flows, inflated sandflows, concentrated-density flows, and turbidity currents. The LGFD sedimentary processes are defined as cohesive flow deposits covered by inflated sandflows transforming into concentrated-density flow deposits into HEBs and finally into turbidity current deposits in sublacustrine fan system, and cohesive flow deposits transforming into inflated sandflow deposits covered by turbidity current deposits into concentrated-density flow deposits into HEBs finally into turbidity current deposits in slump system.

  3. The distinctive depositional model of lacustrine gravity flows influenced by sediment supply, basin structure, and climate documents the coexistence of cohesive flow deposits, inflated sandflow deposits, and concentrated-density flow deposits in proximal inner fans, depicts the change of distribution pattern from superimposing channels to anastomosing and superimposing channels, exhibits the contrast between HEBs without mudstone cap forming in the interconnecting middle fan area and HEBs with mudstone cap forming in outer fans and distal lobes, and presents the evolution process from cohesive flow deposits to turbidity current deposits.

Data supporting the results can be found in figures and tables within the manuscript.

The authors declare no conflict of interest.

This work is supported by the National Science and Technology Major Project (Grant No. 2016ZX05033). Our thanks go to the Shengli Oilfield Company of the SINOPEC Group for their support in data collection and rock core observation.

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