Sedimentary Processes and Depositional Characteristics of Coarse-grained Subaqueous Fans along Steep Slopes in a Lacustrine Rift Basin:A Case Study from the Dongying Depression,Bohai Bay Basin,China

Coarse-grained subaqueous fans are vital oil and gas exploration targets in the Bohai Bay basin,China.The insufficient understanding of their sedimentary processes,depositional patterns,and controlling factors restricts efficient exploration and development.Coarse-grained subaqueous fans in the Yong′an area,Dongying Depression,are investigated in this study.These fans include nearshore subaqueous fans,and sublacustrine fans,and their sedimentary processes,depositional patterns and distribution characteristics are mainly controlled by tectonic activity and paleogeomorphology.Nearshore subaqueous fans developed near the boundary fault during the early–middle deposition stage due to strong tectonic activity and large topographic subsidence.Early sublacustrine fans developed at the front of the nearshore subaqueous fans in the area where the topography changed from gentle to steep along the source direction.While the topography was gentle,sublacustrine fans did not develop.During the late weak tectonic activity stage,late sublacustrine fans developed with multiple stages superimposed.Frequent fault activity and related earthquakes steepened the basin margin,and the boundary fault slopes were 25.9°–34°.During the early–middle deposition stage,hyperpycnal flows triggered by outburst floods developed.During the late deposition stage,with weak tectonic activity,seasonal floods triggered hyperpycnal flows,and hybrid event beds developed distally.

Reclassification and distribution patterns of discovered oils in theDongying Depression, Bohai Bay Basin, China

The Dongying Depression is an important petrolifeous province,with diverse source rocks and complex petroleum distribution patterns.A total of 32 crude oils were analyzed by the gas chromatographyemass spectrometry and isotopic compositions to better understanding the petroleum systems in the study area.Three oil types were classified by hierarchical cluster analyses.Type I and II oils have closely correlation with the discovered source rocks,which have been confirmed to be mainly derived from the lower third and upper forth member of the Eocene Shahejie Formation source rocks(Es3^(L) and Es4^(U)),respectively.Obviously,type III oils contain abundant gammacerane,tricyclic terpanes and C_(29) steranes and have lower values of δ13C than type I and II oils,indicating a completely different source rock and biological origins.Until recently,type III oils fail to match any of the discovered source rock,which contains main contribution of aquatic organism or/and bacteria inputs.In addition,the spacial distribution of these three oil types were discussed.Type I oils mainly distributed in the Es3 and Es4 reservoirs that closed to the generative kitchens.Type II oils occurred in the Es4 reservoirs in the sourthern slope of the depression,which probably caused by lateral migration along the horizontal fractures and sandstone layers within the Es4 interval.Differently,type III oils in the sourthern slope of the depression were mainly discovered in the Eocene Kongdian or Ordocician reservoirs,which suggests great exploration potential of deep underlying strata.

Depositional Patterns and Oil／Gas Accumulation Features of Sha-3 Member Turbidites in Dongying Depression, Bohai Bay Basin

Recent exploration results indicate that a significant exploration potential remains in the Dongying Depression of the Bohai Bay Basin and the undiscovered oil and gas are largely reservoired in subtle traps including turbidite litholigcal traps of the Sha-3 Member. In order to effectively guide the exploration program targeting turbidites, this study will focus on the depositional models of the Sha-3 Member turbidites and oil/gas accumulation characteristics in these turbidites. Two corresponding relationships were found. One is that the East African Rift Valley provides a modern analog for the depositional systems in the Dongying Depression. The other is that the depositional models of line-sourced slope aprons, single point-source submarine fan and multiple source ramp turbidite, established for deep-sea turbidites, can be applied to interpret the depositional features of the turbidite fans of three different origins: slope turbidite aprons, lake floor turbidite fans and delta-fed turbidite fans in the Sha-3 Member. Updip sealing integrity is the key factor determining whether oil/gas accumulates or not in the slope aprons and lake floor fans. The factors controlling oil/gas migration and accumulation in the delta-fed turbidite fans are not very clear. Multiple factors rather than a single factor probably played significant roles in these processes.

Diagenetic Evolution and Formation Mechanisms of High-Quality Reservoirs under Multiple Diagenetic Environmental Constraints:An Example from the Paleogene Beach-Bar Sandstone Reservoirs in the Dongying Depression,Bohai Bay Basin

The diagenetic environment, diagenetic responses, diagenetic transformation model and formation mechanisms of high-quality reservoirs（beach-bar sandstones of the Paleogene fourth member） in the Dongying depression were studied through the analysis of fluid inclusions, thin section and burial evolution history. The diagenetic fluids of the beach-bar sandstone reservoirs evolved from early high salinity and weak alkalinity to low salinity and strong acidity, late high salinity and strong alkalinity and late low salinity and acidity, which were accompanied by two stages of oil and gas filling. The fluids at the margins of the sandbodies were continuously highly saline and strongly alkaline. The western（eastern） reservoirs experienced early open（closed）, middle open, and late closed diagenetic environments during their burial history. The flow pattern was characterized by upwelling during the majority of the diagenesis（in the east, a non-circulating pattern transitioned into an upwelling current）. Due to the evolution of the diagenetic fluids, the diagenetic sequence of the beach-bar reservoirs was as follows： early weak carbonate cementation; feldspar and carbonate cement dissolution and authigenic quartz cementation; late carbonate and anhydrite cementation, authigenic feldspar cementation, and late quartz dissolution; and late carbonate cementation, feldspar dissolution, and authigenic quartz cementation. The diagenetic strength during these stages varied or was absent altogether in different parts of the reservoirs. Due to the closeness of the diagenetic environment and the flow pattern of the diagenetic fluids, the diagenetic products are variably distributed in the sandstones interbedded with mudstones and in the fault blocks. The evolution of multiple alternating alkaline and acidic diagenetic environments controlled the distribution patterns of the reservoir diagenesis and reservoir space, and the reservoir quality index, RQI, increased gradually from the margins to the centers of the sandstones. The closeness of the diagenetic environment and the flow patterns of the diagenetic fluids controlled the differences in the reservoir properties among the fault blocks. With increasing distance from the oil-source faults, the RQI values in the west gradually decreased and in the east initially increased and then decreased.

Formation and Accumulation of Hydrocarbons in the Central Uplift,Dongying Depression

A large number of crude oil and rock samples from various oil pools of the Central Uplift in Dongying Depression were investigated to reveal the origin and accumulation of hydrocarbons. The results showed that the composition and areal distribution of hydrocarbons vary with the location of the oil pools. An increasing trend of gammacerane/C30-hopane and steranes/hopanes but a decreasing trend of prystane/phytane （Pr/Ph） and 18（H）-/17（H）- trisnorhopane （Ts/Tm） ratios were observed from southwest to northeast. This indicates a variation of source rocks and hydrocarbon properties. An increasing trend of water salinity with much more input of algae-rich microorganisms in the deposition environment of the source rock was observed from southwest to northeast. However, the thermal maturity of the source rocks showed a decreasing trend in the same direction. Oil-to-oil and oil-to-source rock correlations showed that most oils were sourced from Es3 and Es4 members with a burial depth of more than 3,000-3,150 m and 2,700 m respectively. The oils in the Shinan and Ying 11 sand bodies, which were generally mixed with the Es4 sourced oils, came mostly from Es3 mudstones. It is quite significant that the Es4 derived oils migrated vertically for hundreds of meters and accumulated in an overlying lithologic pool of the Es3^m, i.e., Ying 11 synclinal lithologic pool, which suggests that the mechanisms of migration and accumulation for subtle oil pools in the Dongying Depression are more complex than that of the previously expected.

Depositional Environment of Es_4~u and Es_3~l Shales Based on Biomarkers from the Boxing Sag of Dongying Depression, East China

The Boxing Sag is located in the southwest of Dongying Depression, southern Bohai Bay Basin of East China. It is one of the main petroliferous sags in the Dongying Depression, and has two major source rocks, namely the upper 4th Member （Es4u） and lower 3rd Member （Es31） shales of Eocene Shahejie Formation, which are the new exploration targets in recent years. In this study, 16 core samples were collected from Es4u and Es31 shales in the Boxing Sag, and the saturate hydrocarbons were analyzed by gas chromatography （GC） and gas chromatography-mass spectrometry （GC-MS）, respectively. The results show that Es4u shale has obvious phytane and gammacerane predominance, higher concentration of tricyclic terpanes and regular steranes, and very low concentration of 4- methly steranes; Es31 shale has pristane predominance, lower concentration of tricyclic terpanes, gammacerane and regular steranes, and higher concentration of 4-methly steranes. The Es4u shale can be further divided into two types based on the distribution of n-alkanes in gas chromatograms： normal distribution and double peak pattern. The biomarker characteristics and sedimentary facies distribution show that Es4u shale was deposited in the saline-hypersaline semi-deep （Type A Es4u shale, sag center） to shallow （Type B Es4u shale, sag edge） lacustrine environments, Es31 shale was deposited in the freshwater-brackish semi-deep-deep lacustrine environments, and the former sedimentary facies maps of Es4u and Es31 in the Boxing Sag are further modified.

Distribution Patterns of Calcite Cement in the Turbidite Sandstones of the Shahejie Formation in the Dongying Depression,Eastern China

Calcite cements are volumetrically dominant among the most diagenetic constituents in turbidite sandstones of the Paleogene Shahejie Formation in the Dongying Depression.The results show carbonate cements mainly occur in three phases:Calcite I(pre-compaction),Calcite II(post-compaction)and ankerite in the turbidite sandstone.The isotopic composition of Calcite I and Calcite II range from−4.3‰to 4.4‰,−3.5‰to 4.3‰PDB,respectively,suggesting that the calcite cements are mainly derived from the interbedded shales.The early calcite cement precipitated from the mixing of lacustrine and meteoric waters at 38-69℃.The late-phase calcite precipitated from pore waters modified by water-rock interactions at 64-126℃.Two distribution patterns of calcite cements can be identified,depending on their position within the sandstone.The peripheral cementation of the sandstones is near the sand-shale contact.The development of cement is controlled by sandstone thickness and the properties of the interbedded shale.For scattered cementation,the distribution of the calcite cement is not controlled by the sand-shale contact.Calcite cement was preferentially distributed in coarse-grained sandstones,indicating that sandstones with high original permeability are the preferred pathway for the migration of early diagenetic fluids.

A novel redox indicator based on relative abundances of C_(31) and C_(32) homohopanes in the Eocene lacustrine Dongying Depression, East China

A suite of oils and bitumens from the Eocene Shahejie Formation(Es) in Dongying Depression was geochemically characterized to illustrate impact of source input and redox conditions on distributions of pentacyclic terpanes. The Es_(4) developed under highly reducing, sulfidic hypersaline conditions, while Es_(3) formed under dysoxic, brackish to freshwater conditions. Oils derived from Es_(4) are enriched in C_(32) homohopanes(C_(32) H), while those from Es_(3) are prominently enriched in C_(31) homohopanes(C_(31) H). The C_(32) H/C_(31) H ratio shows positive correlation with homohopane index(HHI), gammacerane index, and negative correlation with pristane/phytane ratio, and can be used to evaluate oxic/anoxic conditions during deposition. High C32 H/C_(31) H ratio(> 0.8) is an important characteristic of oils derived from anoxic environments, while low values(< 0.8) indicate dysoxic conditions and extremely low values(< 0.4)indicate strong oxic conditions. The C_(32) H/C_(31) H ratio can be applied for deposition condition diagnosis because carboxyl group of C_(32) hopanoic acids might be reduced to C_(32) homohopanes under anoxic conditions, and oxidized to C_(31) homohopane under oxic conditions. Advantages to use C_(32) H/C_(31) H ratio as redox condition proxy compared to the HHI and gammacerane indexes are wider valid maturity range,less sensitive to biodegradation influence and better differentiating reducing from oxic environments.

Syndepositional Fault Control on Sandbody Distribution at Two Eocene Confluence Deltas in the Dongying Depression,Bohai Bay Basin(Eastern China)

The prediction of sandstone distribution is the main target for hydrocarbon exploration and petroleum reservoir engineering,especially in a rift basin with a complex geological setting.During the Middle Eocene,the Dongying and Yong’an deltas converged in the Dongying depression of the Bohai Bay Basin,eastern China.So far,studies on the control of the existing syndepositional faults on the deltaic sandbodies of the two confluence deltas have been largely neglected.In this contribution,the sedimentary characteristics and spatial distribution of the two deltas,converging in the Dongying depression,were determined.In addition,the morphodynamic controlling factors of the deposits of the two deltas were identified based on core examination,log analysis,and seismic data analysis.During delta progradation,the sandbodies of the initially separated deltas were affected by the development of syndepositional tensional faults,which eventually also influenced the distribution of the sandstone reservoirs.The results of this study can not only be applied to petroleum reservoir engineering but also provide important new information for the studies on delta sandbody all over the world.

A Discussion on the Origin of Shale Reservoir Inter-Laminar Fractures in the Shahejie Formation of Paleogene, Dongying Depression

This paper was conducted on the shale reservoir in the Dongying depression in the Shahejie Formation of Paleogene. Based on detailed core description, rock thin sections, argon ion polishing, scanning electron microscopy（SEM） analysis, CT scanning and carbon-oxygen isotopic test, the formation mechanism of the inter-laminar fractures（or seams） and their development conditions were discussed by comprehensive analysis of the diagenetic evolution features and the reservoir space evolution characteristics. The results show that the laminated composite pattern in Dongying depression was dominated by vertical distribution of laminated calcite and clay minerals. Contrasted to the traditional understanding, development degree of inter-laminar fractures was much lower. Pure fluid between layers cannot exist without framework minerals, and the pre-existing well connected pore or horizontal overpressure micro seam was favorable paths of fluid migration. From the points of inter-laminar fractures origin mechanism and its evolution, the so called seams, were much more complex than traditional understanding that hydrocarbon generation raised the pore pressure and led to the parallel microfractures. But the inter-laminar fractures had little relationship with that whether the lamination was developed or bedding boundary was clear or not. In fact, the shale reservoir inter-laminar fractures were not rigorous fracture. They were controlled by pre-existing inter-granular pore（or seam） or dissolved pore, which distributed along carbonate laminae boundary and were connected by later super pressure, dissolution and the seepage force. And the development conditions were burial depth（bigger than 3 000 m） and the Ro value（〉0.6%）. Discussion on its origin and evolution has important petroleum geological significance on optimizing reservoir segment and favorable target areas selection.

Division and Characteristics of Shale Parasequences in the Upper Fourth Member of the Shahejie Formation, Dongying Depression, Bohai Bay Basin, China

Shale parasequence analysis is an important part of sequence stratigraphy sudies. This paper proposed a systematic research method for analyzing shale parasequences including their delineation, division, characteristics and origins. The division method is established on the basis of lithofacies. Multi-method analysis and mutual verification were implemented by using auxiliary indicators（such as mineral compositions, geochemical indicators and wavelet values）. A typical shale parasequence comprises a lower interval of deepening water-depth and an upper interval of shallowing water-depth（e.g., a shale parasequence including a high-total organic carbon（TOC） shale-low-TOC limy shale）. Abrupt increases in pyrite content, TOC value, relative hydrocarbon generation potential（（S1＋S2）/TOC）, and wavelet values are indicative of parasequence boundaries. The proposed research method was applied to study the upper fourth member of the Shahejie Formation in the Dongying depression, Bohai Bay Basin. Results show that there were seven types of parasequences developed. A singular and a dual structured parasequences were identified. Three factors controlling the development of the shale parasequences were identified including relative lake level change, terrestrial input and transgression. The development of high-TOC（〉2%） shale parasequences was mainly controlled by biological and chemical sedimentation. The low-TOC（〈2%） shale parasequences were mainly deposited by chemical sedimentation. The diversities of shale parasequences were caused by four major controlling factors including climate, relative lake level change, terrestrial input and emergency（e.g., transgression）.

Oil Cracking of Deep Petroleum in Minfeng Sag in North Dongying Depression,Bohai Bay Basin,China:Evidence from Natural Fluid Inclusions

A fluid inclusion fluorescence and microthermometric study was performed on sandstones from the deep Es4 reservoir rocks of the Minfeng （民丰） sag, north of Dongying （东营） depression. Two types of oil inclusions （yellow and blue white fluorescence）, one type of gas inclusions （blue white fluorescence）, and bitumen inclusions （no fluorescence） were detected within quartz and feldspar minerals. The evolution of hydrocarbon fluid inclusions in the lower Es4 sequence indicates that present oil accumulation was predominantly thermal stress controlled. Homogenization temperatures of aqueous fluid inclusions coexisting with gas-bearing and bitumen-bearing fluid inclusions indicate that oil cracking occurred at temperatures up to 160 ℃, primary condensate or wet gas genera- tion occurred during 170-195℃. Oil has cracked into condensate or wet gas in the depth of 4 300- 4 410 m and dry gas and abundant pyrobitumen in the depth of more than 4 410 m in the geological history based on the fluid inclusion extrapolation. Secondary oil cracking is undergoing in present day when the depth of reservoir is more than 4 150 m whose temperature is the threshold temperature of oil cracking （160 ℃）. However, because of the consumption of oil in the first oil cracking process, it may have few chances to find liquid petroleum, and only natural gas can be found when the depth of reservoir is more than 4 410 m, where oil cracks into condensate gas or wet gas according to present-day formation temperature. This study is preliminary but foreshadows a new insight into oilcracking using natural fluid inclusions to trace hydrocarbon evolution in sedimentary basins.

Space-Time Evolution and Controlling Factor of Fine-Grained Sediments in the Upper Fourth Member of Shahejie Formation, Dongying Depression

Fine-grained rocks（FGR） are the important source rocks and reservoirs of shale hydrocarbon which is the prospect hotspot at present. Widely distributed fine-grained sediments（FGS） of the upper fourth member of Shahejie Formation in Dongying depression are taken as an example to study the space-time evolution and controlling factor of FGS in this paper. Based on the analysis of well cores, thin sections, inorganic and organic geochemistry indicators, FGR are divided into 7 types of lithofacies. Through the study of ‘point-line-plane＇, this study shows that FGS has the characteristics of rhythum, diversity and succession. The first stage is characterized by clayey FGS（massive claystone）. The second stage is characterized by carbonate FGS（low-TOC laminated limestone） and dolomitic FGS（dolomitic-silty shale） formed by transgression. The third stage is characterized by organic-rich carbonate FGS（middle/high-TOC laminated limestone） distributed in cycle. The fourth stage is characterized by FGS mixed carbonate and siliciclastic sediments（calcareous-silty shale）. A variety of space-time evolution of FGS are controlled by multiple factors including tectonism, climate and lake conditions.

3D geocellular modeling for reservoir characterization of lacustrine turbidite reservoirs:Submember 3 of the third member of the Eocene Shahejie Formation,Dongying depression,Eastern China

3D geocellular modeling is increasingly essential in the petroleum industry;it brings together all petroleum disciplines,and it is commonly used in simulation and production forecast.However,modeling slope and deep-water turbidite reservoirs using conventional modeling methods pose a significant challenge due to the structural complexity and thin-beds associated with these reservoirs.Through the innovative modeling technology of PaleoScan,the reservoirs in Sub member 3 of the third member of the Shahejie Formation are modeled to understand the structural framework.The resulting model is populated with petrophysical properties i.e.,porosity and permeability to predict their lateral and vertical distribution within these sandstone reservoirs.The study suggests that the reservoir in the highstand system tract(HST)is characterized by the clinoforms configuration framework.The reservoir is highly faulted mainly in the northern and southeastern parts of the depression.The sedimentary layers are deposited across the slope and downlapping,thinning,and terminating toward to the west.The two isochore surface maps reveal sediment thickness variation and depositional trends within each individual depositional layer.The zones or areas that corresponds to low values on the thickness maps suggest minor uplifts associated with intensive faulting in the Eocene period.These topographical highs played a fundamental role in distributing the sediments delivered to the basin from distant sources.The maps reveal that sediments that filled the basin appear to come from different source points,primarily delivered from the north,southeast,and northeast of the basin with varying depositional trends.The modeled porosity and permeability indicate that the delta fed turbidite reservoirs are characterized by medium to high porosity values of 10e20%and low to medium permeability values of 30-410mD,respectively.The porosity values increase to the southeast and toward the basinwards(west)while permeability varies within the individual sedimentary layers.The distribution of porosity and permeability is not uniform vertically.This suggests the presence of mixed none-reservoir layers with locally and periodically deposited sandstone reservoirs within the stratigraphic during rapid delta progradation.The HST is characterized by six different delta progradation cycles;each phase produced locally deposited lacustrine turbidite sandstones in the basin,which are essential reservoirs in this Formation.The innovative PaleoScan interpretation technology has successfully created a high-resolution 3D reservoir model of this complex geology-such innovative technology is vital to similar complex geology globally.

Distribution pattern of deltaic sand bodies controlled by syn-depositional faults in a rift lacustrine basin

Take the lacustrine delta in the second member of Paleogene Shahejie Formation in block Wang43, Dongying depression, Bohai Bay Basin as an example, the deposition architectural characteristics of lacustrine deltaic sand bodies controlled by syn-depositional faults in complex fault blocks of rift basin are examined from the aspect of the tectonic-deposition response, using cores, well logs and three-dimensional seismic data. The small-scale syn-depositional faults in complex fault blocks are dense and different in dip, the activity along the strike of syn-depositional fault varies in different positions, and all these control the sedimentary process of deltaic sand bodies. Influenced by syn-depositional faults, the deltaic distributary channel is more likely to pass through the position with weak fault activity, and be deflected or limited at the position with strong fault activity. In downthrown side of a single syn-depositional fault or micro-graben areas, sand bodies increase in thickness and planar scale, and sand bodies of multiple stages are likely to stack over each other vertically. In micro-horst areas controlled by syn-depositional faults, the sand bodies decrease in abundance, and appear in intermittent superimposed pattern vertically. This study can provide new research ideas and theoretical basis for exploration and development research in complex fault blocks.

A Quantitative Method for Characterizing Transport Capability of Compound Hydrocarbon Carrier System

The occurrence of hydrocarbon migration in petroliferous basins depends on the balance of driving force and resistance of carriers, which restricts mostly the quantity and positions of hydrocarbon accumulation. The driving forces of hydrocarbon migration have been quantitatively studied, whereas the migration pathways and carriers were only qualitatively discussed up to now. Establishing a compound hydrocarbon carrier system and quantitatively characterizing its transport capability are significant for understanding the dynamic process of hydrocarbon migration and revealing the hydrocarbon accumulation characteristics. It has become an innovatory trend and also a difficult topic in study of hydrocarbon migration. In this article, a method is described for using displacement pressure to quantitatively characterize the transport capability of the compound carrier system, which composed of sandstone carriers, unconformities and faults. When the weathered and leached zone rarely developed, the basal conglomerate or transgressive sandstone of unconformities can be treated as part of sandstone carriers. An empirical relationship among core porosity, air permeability, and the pore aperture radius corresponding to a mercury saturation of 10% （r 10 ） can be obtained by multiple regression. Using porosity and permeability inversed by seismic data, the displacement pressure of sandstone carriers can be calculated by the empirical relationship and Washburn Equation. Displacement pressure of fault plane can be estimated by the regression formula between fault opening index （FOI） and hydrocarbon column height it can support. This method is applied in the eastern part of south slope in Dongying （东营） depression, Bohai （渤海） Bay Basin, China, to quantitatively characterize thetransport capability of the compound carrier system of Shahejie （沙河街） Formation. The results have good agreement with data from drilling wells. This method may be a step further in study of compound hydrocarbon carrier system in petroliferous basins. It may provide the basis of coupling expulsion quantity, migration driving force and hydrocarbon carrier system to simulate hydrocarbon migration and accumulation. Therefore this will help predict hydrocarbon migration pathways and the locations ofhydrocarbon accumulation.