Fine-grained gravity flow deposists have some important implications for palaeogeography and palaeoclimate. The genetic mechanism and distribution of fine-grained sediments are crucial to palaeoenvironment reconstruction and stable carbon cyde, but their deposits processes are more complicated than that of coarse-grained gravity flow sediments. Detailed outcrop study shows that fine-grained gravity flow deposits are mainly devebpem in the Upper Triassic Kache Formation in the Zhuoni area of the West Qinling orogenic belt. Fine-grained gravity flow deposits of the Kache Formation in the study area can be divided into four lithofacies,corresponding to two deep-water sedimentary processes(i.e. fine-grained turbidity current and fine-grained hyperpycnal flow). Among them,fine-grained hyperpycnite is dominated by siltstone and argillaceous siltstone with suspended load(single-stage sedimentary thickness is 0.05-0.60 m),current ripple and argillaceous transitional ripple(type I)or current ripple and hummocky cross-stratification(type Ⅱ)appear alternately in the vertical direction,showing the overlapping characteristics of multi-stage positive rhythm and composite rhythm in the vertical direction. Fine-grained turbidites are also developed in the study area(single-stage sedimentary thickness is 0.05-0.2 m),which is generally characterized by Ta,Te or Ta,Tc of Bouma sequence. The two important sedimentary processes developed in the study area constitute a complex gravity flow sedimentary system,which has important implications for the sedimentology of fine-grained gravity flows in the shallow-water narrow continental shelf-upper slope of active continental margin of the Zhuoni area in the West Qinling orogenic belt.

The oil and gas resources stored in deep-water gravity-flow sediments are extremely abundant,but there are still many problems in understanding the distribution of deep-water gravity-flow sedimentary sand bodies and high-quality reservoirs at present. These problems restrict the exploration and development of oil and gas. The transition from supercritical to subcritical turbidity current under the control of hydraulic jumping and the transformation of turbidity current to muddy debris flow under the control of turbulence damping are two key issues that form the new understanding of the distribution of gravity flow deposits. Deep-water gravity flow deposits have numerous architectural elements,including channel and levee,channel-lobe transition zone,and lobe. The detailed analysis of identification marks of different architectural elements is critical to understanding of the distribution of gravity flow deposits. The strong erosion of substrate by hydraulic jump during the transition from supercritical to subcritical turbidity current is the new understanding for the formation of gravity flow channels. Tectonic activity,sediment supply, and the interaction between climate and sea(lake)level are external factors of sedimentary basins that control the distribution of gravity flow deposits. Water density of sedimentary basin,topographic slope, and basin bottom topography are internal factors that determine the distribution of gravity flow deposits. The synergistic diagenesis of sand and mud during the burial and diagenetic evolution of deep-water gravity flow deposits plays an important role in the development of high-quality reservoirs with relatively high porosity and permeability. Research on the depositional process that emphasizes both ancient outcrops and modern-day monitoring,deep-water gravity flow sedimentary architectural elements and inner structure,deep-water gravity flow-caused fine-grained sedimentation and shale oil and gas research,deep-water gravity flow sedimentary sand body diagenesis are expected to be the next step of sedimentological research prospects of deep-water gravity flow sedimentary oil and gas exploration.

Deep-water channel is an important reservoir for oil and gas storage in deep-water environment. At present,there are many reports on the sedimentary architecture and classification of deep-water channels,but few studies on the classification of single channel were carried out. Current classification schemes are largely based on the erosive capacity of the channel,however,due to the limitation of seismic resolution and outcrop integrity,there are great limitations in practical application. Based on 26 outcrops in the world and the drilling core data of a deep-water basin in West Africa,different gravity flow facies with different fluid dynamics are identified,including the high-density turbidity current facies(HTL),low-density turbidity current facies(LTL)and debris flow facies(DL). According to the proportion of different gravity flow facies in the channel,nine types of single channel can be divided: single high density turbidity filling channel(HTL>70%),single low density turbidity filling channel(LTL>75%),single debris flow filling channel(CL>60%),massive sandy mixed filling channel(HTL=40%~70%,LTL=40%~20%,CL<30%),gravelly sandy mixed filling channel(HTL=40%~70%,LTL<30%,CL=15%~50%),layered sandy mixed filling channel(HTL=5%~60%,LTL=40%~75%,CL<20%),clastic sand mixed filling channel(HTL<40%,LTL=40%~75%,CL=20%~40%),isophase mixed filling channel(HTL=20%~40%,LTL=20%~40%,CL=20%~40%),sandy gravel mixed filling channel(HTL<50%,LTL<60%,CL=40%~60%). Based on field outcrops and the practical application of division scheme in deep-water oilfield,we found that the vertical distribution law of different types of channels are as follows: the channels filled by debris flow lithofacies are often developed at the bottom of channel system,and the channels filled by high-density turbidity flow lithofacies are close to the middle and lower part,and the channels filled by low density turbidity current lithofacies is mainly located in the middle and upper part. According to the percentage of different gravity flow lithofacies within channel,different channel types are clearly defined. This division scheme has better applicability and maneuverability,and is beneficial to deep-water channel reservoir prediction and reservoir quality evaluation in practice.

Deep-water deposits are well developed in the Early Paleozoic Pingliang Formation of the Ordos Basin. The investigation on the sedimentary system types and characteristics of the Pingliang Formation is not only helpful to understand the paleogeographic pattern and tectonic characteristics,but also has an important implication for oil and gas exploration. Based on the field survey and indoor comprehensive study of the Pingliang Formation and its contemporaneous strata in the southwest margin of the basin,we have systematically studied the Pingliang Formation in the Ordos Basin from the perspectives of stratigraphic spatial distribution,rock types,structure and texture,sedimentary facies,organic geochemical characteristics,etc. Our work indicated that there are deep-water autochthonous deposits,traction current deposits,gravity current deposits,pyroclastic rocks and seismites deposits in the Pingliang Formation,forming a sedimentary assemblage of front platform slope and deep-water basin facies. Combined the tectonic setting with sedimentary facies assemblage,the deep-water sedimentary model of the Pingliang Formation was established. The water depth of the Pingliang Formation was shallow at the initial stage of sedimentation with undeveloped contour current deposits,while the gravity flow deposits were dominant. Towards to the middle and late developed period,the gravity flow weakened with the deepening of water level,which leaded to rework of the preexisting deposits by contour currents. In addition,the deep-water deposits of the Pingliang Formation are conducive to oil and gas accumulation. Based on stratum thickness,rock type,sedimentary facies,organic matter type and content,as well as thermal evolution degree,favorable oil and gas exploration areas in the Pingliang Formation are selected in this research,which are mainly distributed in south of Wuhai,the east of Shizuishan-Helan and the west of Otog Qianqi. The area is about ten thousand square kilometers and is banded distributed in north-south direction. Finally,it is found that the key area of exploration is mainly distributed in part of the Tianhuan depression,east of Helan.

Gravity flows,including coarse-grained and fine-grained gravity flows,are the most important sedimentation processes in deep-lake environments. Fine-grained gravity flow sediments hold significant potential as unconventional shale oil “sweet spot intervals”. The distribution characteristics of fine-grained gravity flow sediments within sequence stratigraphic frameworks in lacustrine basins are of great significance to the prediction of shale oil “sweet spot intervals”. Through systematic comparative study of gravity flow deposits and sequence frameworks in major lacustrine basins in China,it is found that gravity flow sedimentary systems are mainly deposited in high accommodation sequences. In the lowstand systems tracts(LST)of the faulted/rifted lacustrine basins,coarse-grained gravity flow deposits are developed at proximal sublauctrine fans and thin fine-grained gravity flow sediments are deposited at distal sublaucustrine fans. The distribution of sublacustine fans is controlled by valley and syn-sedimentary structural slope breaks. Fine-grained channel-levee-lobe systems are developed in transgressive systems tracts(TST). Unconfined-channel sublacustrine fans are developed in highstand systems tracts(HST),and fine gravity flow deposits are distributed in distal sublacustrine fans. Large fine-grained channel-levee-lobe systems influenced by flooding occurred in transgressive systems tracts(TST)of down-warped lacustrine basins. On the steeper slope of syn-depositional flexural breaks,fine-grain sublacustrine fan systems are present. Both fine-grained sublacustrine fan systems and fine-grained channel-levee-lobe systems can be deposited in highstand systems tracts(HST)and lowstand systems tracts(LST). Fine-grained gravity flow deposits at distal sublacustrine fans and channel-levee-lobe systems can form “sweet spot intervals” of shale oil. Mudflow deposits form “sweet spot intervals”of foliation type,fine-grained debris flow,fine-grained turbidity current,fine-grained hyperpycnal flow and fine-grained transition flow deposits form “sweet spot intervals”of lamination type and bedding type,while fine-grained concentrated density flow deposits only form “sweet spot intervals”of bedding type. Fine-grained gravity flow deposits are favorable “sweet spot intervals” for shale oil exploration and development.

The lacustrine fine-grained gravity flow sand body is a favorable target area for exploration in deep-water basins. Based on core,drilling,well logging,mud logging and thin section identification,the sedimentary types,characteristics,sedimentary evolution process and sedimentary model of fine-grained gravity flow are studied in the Member 7 of Yanchang Formation in the Ningxian area,Ordos Basin. The findings results show the presence of ten lithofacies,six lithofacies associations,and five types of deep-water sediments,including slide-slump deposits,sandy debris flow deposits,hybrid event bed,turbidity current deposits and deep lacustrine muddy deposits. According to the lithofacies and their combination characteristics,the semi-deep to deep lake subfacies can be further divided into six sedimentary microfacies: confined channel,unconfined channel,levee,lobe,inter-channel and semi-deep to deep lake mud. The delta front sediments destabilized under the action of external forces,and experienced five evolution stages of sliding,slumping,sandy debris flow,hybrid event bed,and turbidity currents. With the“fluid transformation and separation”and“hydroplaning”,the gravity flow deposits gradually evolved from confined channel-levees,unconfined channel-levees,to lobes toward the center of basin. This study will help to deepen the understanding of lacustrine fine-grained gravity flow deposits and provides guidance for deep-water oil and gas exploration and development.

Modern marine geological surveys show that there are contourites along the slopes of carbonate platforms. However,it is difficult to identify contourite drifts preserved in the strata. In this study,we document contourites from the Lower Silurian Xiaoheba and Shiniulan Formations in the southeastern Sichuan Basin based on the field investigation and microscopic observation. Symmetrical graded beddings and banded structures with periodic variations are found from the Xiaoheba Formation at such sections as Sanquan in Nanchuan County,Jinzhu in Shizhu County,Huangcaochang in Wulong County. Nodular limestones and argillaceous strip-banded limestones are developed in the lower Shiniulan Formation,while thin-bedded sandstones are found in the upper Shiniulan Formation at Baiyangshu and Podu sections in Tongzi County. Compared with modern carbonate platforms at Bahama and Maldivian,and with consider of the typical deep-water isobathic sedimentary features,it is suggested that contourites predominate the sandstones of Xiaoheba Formation. Likewise,the thin-bedded sandstones yielding from the upper Shiniulan Formation might be the contourites controlled by periodic changes of sea level. Meanwhile,the striped limestones in the lower Shiniulan Formation are recognized as the deposits controlled by tractive current(i.e.,contour current). This study provides an actualistic example for ancient contourites which might enhance our understanding of the Lower Silurian palaeogeographic evolution and of oil/gas reservoirs in the southeastern Sichuan Basin.

Bottom currents are common phenomenon in the deepwater setting of continental margins,which have important impacts on the deepwater sedimentary process and sandstone distribution. Extensive studies have been focused on the mechanisms and sedimentary products of gravity flow-bottom current interaction,but it remains much unknown regarding the submarine-fan reservoir architecture model modified by bottom currents. The Rovuma Basin offshore East Africa is a typical region to study the interaction between sediment gravity flows and bottom currents. Taking the representative submarine channel systems from the Lower Eocene as an example,this paper characterizes the reservoir architecture by integrating cores,well logs and 3D seismic data,with an aim to establish the submarine channel architecture model under interaction of sediment gravity flows with bottom currents. Results show that the target channel system developed three types of architectural elements,channel,overbank,and lobe,where channel deposits contain channel complexes, individual channels,and other smaller hierarchical units. Fine-grained materials could be transported by bottom currents forming asymmetrical distribution of overbanks deposits. This forced channel complexes to migrate laterally against bottom currents,with shale barriers easily preserved. Individual channels show downstream migration or lateral migration against bottom currents. In downstream migration,channels incised each other forming good sand connectivity,while indined shale baffles may develop in lateral migration. As influenced by sediment gravity flows,individual channel-fills evolved from mixed sand-shale to sand-prone channels,and in the mapview sandbodies are mostly distributed in the axis of channel bends.

Fine-grained gravity flows can transport clastic sediments and particulate organic matter from shallow water to deep lakes,forming distinctive sediments finer than 0.0625 mm. During the depositional period of the upper section of the Lower Ganchaigou Formation(E²₃),a sub-lacustrine fan system and chemically precipitated carbonates developed in a saline semi-deep to deep lake in the Yingxiongling area of the Qaidam Basin. Seven types of lithofacies associations(LA)are identified in the fan fringe sub-environment on the basis of core observations: LA1: association of mud flow,mud turbulent flow,and suspension fallout deposits in quiet water;LA2: association of chemical deposits interbedded with event deposits in semi-deep lake;LA3: association of slump,fine-grained debris flow,and mud flow deposits;LA4: fine-grained hyperpycnal flow deposits;LA5: fine-grained concentrated density flow deposits;LA6: association of fine-grained debris flow,fine-grained transitional flow,and fine-grained turbidity current deposits;LA7: association of fine-grained concentrated density flow,fine-grained turbulent flow,fine-grained transitional flow,and mud flow deposits. These lithofacies associations are attributed to various fine-grained gravity flows and their transformations which are suggestive of complex flow processes. Our study also shows that the fine-grained deposits are influenced by paleoclimate,tectonic activity,and lake-level fluctuations. In an arid environment and during humid periods,lake-level rise promotes the formation of favorable source rocks and fine-grained gravity flow deposits. Conversely,in times of drought,lake-level decrease is more conducive to the development of chemically precipitated carbonates and slump-derived fine-grained gravity flow deposits. Mud flow,mud turbulent flow,and fallout deposits in quiet water(LA1)and chemical deposits interbedded with event deposits(LA2)are the most favorable “sweet sections”in the saline semi-deep to deep lake.

The interplay of bottom currents and gravity flows is an important deep-water sedimentary process,which plays an important role in redistributing submarine sediments and reshaping seafloor bedforms. At present,research on bottom currents and gravity flows interaction in deep-water channels is relatively mature,but little work has been done on the sedimentary characteristics of lobes influenced by bottom currents. Based on an integrated study of seismic facies and seismic attributes using 3D seismic and logging data,this study documents the distribution and lithological characteristics of the Miocene lobes in the Rovuma Basin(East Africa). The effect of bottom currents and gravity flows interaction on lobe sedimentation is revealed through quantitative restoration of gravity flow velocity,combined with regional ocean current data analysis. The results indicate that,early channels migrated southward due to the northward-directed bottom current,and their northern levees are well developed with a negative terrain present to the south of the channels,which led subsequent lobes to deposit to the south of these channels. Deposition of the lobes is influenced by bottom current flow direction and gravity flows forming the lobes. Velocity of the gravity flows is much higher than that of the bottom currents. Therefore,when the directions of bottom currents and the lobe propogation are opposite and oblique,the influence of bottom currents is suppressed on the side against bottom current flow direction(south side),and the lobe gradually thins from the axis to the margin,accompanied by increasing mud content;On the side downstream of bottom current flow direction(north side),gravity flows intersect obliquely with bottom currents,and the mud at the top of turbidity currents is stripped off by the bottom current,leaving sand deposited in the lobe. When bottom current flow direction and lobe propogration direction are close to orthogonal,the influence of bottom current on the lobe is consistent on the whole,and the mud at the top of gravity flows is stripped. Therefore,the lobe is mainly composed of thick sandy deposits at the side against the bottom current flow direction,and the sand content decreases towards the side downstream of the bottom current,accompanied by an increasing mud content.

Abundant sublacustrine fan(SLF)gravity flow deposits are developed in various tectonic evolution stages of lacustrine rift basins. Through 3D seismic fine interpretation combined with drilling/logging analysis,the genetic type and distribution of gravity flow fans in lacustrine rift basins are discussed and the depositional models are summarized for the Shahejie Formation of the Baxian Sag,Bohai Bay Basin. The research shows that three genetic types include slumping-,flood-,and storm-induced gravity flow SLF are developed in the Paleogene Shahejie Formation,and can be further divided into five specific types. Based on the sediment source of gravity flow SLF,bottom structures of the sag on the sediment transport path,and plane distribution characteristics between them,it can be divided into three plane combination types: provenance area-fault steep slope zone-SLF,(progradational)delta-slope break zone-SLF,underwater low bulge(beach bar)-SLF. Four depositional models are established including storm gravity flow SLF in the initial rift period,flood SLF in the strong rift period,flood SLF in the stable rift period,and slump SLF in the post rift return period. The study shows that gravity flow SLF deposits of different genetic types are developed in each tectonic evolution stage of lacustrine rift basin. The triggering mechanism of gravity flow SLF is mainly related to delta instability in slope area,paroxysmal flood and lake wave action. The architecture and distribution of gravity flow SLF are controlled by fault activity and bottom structures of the sag,and its size is mainly controlled by triggering mechanism. The gravity flow SLF wrapped by dark mudstone and oil shale formed in deep and semideep lake has good source reservoir cap rock configuration and oil/gas accumulation conditions,which is a sweet spot for oil and gas exploration. Seismic slice analysis based on well data can effectively guide the exploration of thin sandstone sheets.