Eocene Dongying Depression Research Articles

Variability of sequence architecture in a rift margin controlled by tectonic activity: The Eocene Dongying Depression, Bohai Bay Basin, China

AbstractSequence architectures along the margins of rift basins are still poorly documented compared to passive continental margin settings. The Eocene Shahejie Formation on the rift margin of the Dongying Depression records a complex sequence stratigraphic distribution of conglomerate, sandy conglomerate, sandstone and mudstone. These facies have been mainly attributed to fan delta and marginal subaqueous fan depositional settings that developed during segmented fault activity along the rift margin. We utilize three‐dimensional (3D) seismic data, conventional cores, and wireline log data to dissect the overall wedge‐shaped upper sub‐member strata of the Shahejie Formation's fourth member. The study interval is a third‐order sequence formed between 45.4 and 42.5 Ma and contains a lowstand–transgressive systems tract (LST–TST) and a highstand systems tract–falling‐stage systems tract (HST–FSST). We found that the LST–TST developed several huge amalgamated depocenters along the Chennan border fault; whereas, these depocenters gradually diminished or even disappeared during the HST–FSST period, illustrating less significant control by the border fault. Through calculations of strata growth rates, we confirmed that the segmented activity of the border fault influences the stratigraphic distribution and facies evolution during these two periods. Specifically, deep‐water depositional systems, represented by marginal subaqueous fans were widely developed in the LST–TST period and were influenced by overall strong tectonic activity, including retrogradational (R) and aggradational (A) patterns. However, the aggradational to progradational (AP) deltas and progradational to degradational (PD) deltas dominated the generally muted tectonic activity setting during the HST–FSST. Additionally, influenced by localized segmented fault activity, these systems tracts exhibit incomplete vertical development, resulting in spatial variability in stratigraphic stacking patterns.

Sediment provenance and dispersal in the early Eocene Dongying Depression, Bohai Bay Basin, Eastern China: Evidence from detrital zircon geochronology, geochemistry and petrology

Comprehensive provenance studies of syn-rift basin fills are required to better understand possible sources of clastic detritus and sediment routing systems. The Eocene fill of the Bohai Bay Basin in eastern China represents a syn-rift succession, where subsurface datasets permit investigation of sediment sources and sinks. New detrital zircon U–Pb samples (441 detrital zircon grains) from six wells were combined with elemental geochemical analysis of siliciclastic sediment, sandstone petrography and palynology to investigate depositional ages and provenance. This study demonstrates the importance of integrating geochronometry, geochemistry, petrology and palynology datasets to fully unravel syn-rift sediment provenance and routing. Zircons of the Dongying Depression were derived principally from an active continental margin island-arc setting characterized by felsic acid magmas, and subordinately from a recycled orogenic belt. The Shicun and Binnan faults controlled the relationship between sediment routing systems and source areas, explaining spatial differences of provenance signals in the depression. The routing system around the Shicun fault is characterized by a dominance of late Paleozoic and Paleoproterozoic zircons and subordinate Mesozoic zircons. In contrast, increased Mesozoic zircons in samples from south of the Binnan fault provide evidence of Mesozoic magmatism in this area. The early Eocene sediments record the signal of early Cretaceous magmatism in the North China Craton, but lack a record of substantial syn-depositional magmatic activity since the Paleogene in the Dongying Depression. Zircons from early Cretaceous strata and pre-existing zircons from the Xing-Meng Orogenic Belt and Inner Mongolia paleo-uplift were transported to the Yanshan and Luxi areas together. These sediments entered the Dongying Depression in the early Eocene, following a period of recycling. In addition, a Precambrian basement signal indicates possible denudation of Neoarchean to Proterozoic rocks in the Luxi uplift and from the Binnan fault footwall. Overall, the provenance signals of the basin reflect zircon recycling from an Early Cretaceous succession, denudation of ancient magmatic rock masses and the absence of syn-sedimentary magmatism.

Reconstruction of Lake-Level Changes by Sedimentary Noise Modeling (Dongying Depression, Late Eocene, East China)

The late Eocene succession of the Dongying Depression forms a highly productive hydrocarbon source. However, due to lack of an unambiguous fine chronostratigraphic framework for the late Eocene stratigraphy, it is challenging to understand the paleolake’s evolution and the driven mechanism of lake-level variation, a limitation which hinders hydrocarbon exploration. In this work, high-resolution gamma-ray logging data were analyzed to carry out the cyclostratigraphic analysis of the third member (Es3) of the Shahejie Formation in the Dongying Depression. Significant 405-kyr eccentricity cycles were recognized based on time series analysis and statistical modeling of estimated sedimentation rates. We abstracted ~57 m cycles of the GR data in the Es3 member, which were comparable with the long eccentricity cycles (~405-kyr) of the La2004 astronomical solution, yielding a 6.43 Myr long astronomical time scale (ATS) for the whole Es3 member. The calibrated astronomical age of the third/fourth member of the Shahejie Formation boundary (41.21 Ma) was adopted as an anchor point for tuning our astrochronology, which provided an absolute ATS ranging from 34.78 ± 0.42 Ma to 41.21 ± 0.42 Ma in Es3. According to the ATS, sedimentary noise modeling for the reconstruction of lake-level changes was performed through the late Eocene Es3. The lake-level changes obtained based on sedimentary noise modeling and spectrum analysis reveal significant ~1.2 Myr cycles consistent with global sea level variations which were related to astronomical forcing. Potential driven mechanisms of marine incursion and/or groundwater table modulation were linked to explain the co-variation of global sea level changes and regional lake level changes. Our results suggest global sea level fluctuations may have played an important role in driving the hydroclimate and paleolake evolution of the late Eocene Dongying Depression.

Fibrous calcite veins record stepwise, asymmetric opening and episodic hydrocarbon expulsion from organic-rich shales

Episodic fluid expulsion through fractures is widely expected during hydrocarbon generation, yet direct evidence for this process is lacking in the case of organic-rich shales. We investigated the formation of antitaxial, bed-parallel fibrous calcite veins hosted in organic-rich shales of the Eocene Dongying Depression, Bohai Bay Basin, China. Our results from detailed, in situ geochemical traverses show that while some symmetric veins exhibit broadly synchronous and steady-state opening, other asymmetric veins consist of two geochemically distinct generations of calcite to either side of the median zone, suggesting asymmetric and asynchronous growth in two discrete episodes during hydrocarbon expulsion. Thus, we argue that each asymmetric vein recording two stages of opening implies that hydrocarbons were expelled from shales episodically.

Fibrous calcite veins record stepwise, asymmetric opening and episodic hydrocarbon expulsion from organic-rich shales

AbstractEpisodic fluid expulsion through fractures is widely expected during hydrocarbon generation, yet direct evidence for this process is lacking in the case of organic-rich shales. We investigated the formation of antitaxial, bed-parallel fibrous calcite veins hosted in organic-rich shales of the Eocene Dongying Depression, Bohai Bay Basin, China. Our results from detailed, in situ geochemical traverses show that while some symmetric veins exhibit broadly synchronous and steady-state opening, other asymmetric veins consist of two geochemically distinct generations of calcite to either side of the median zone, suggesting asymmetric and asynchronous growth in two discrete episodes during hydrocarbon expulsion. Thus, we argue that each asymmetric vein recording two stages of opening implies that hydrocarbons were expelled from shales episodically.

Differences of sedimentary triggers and depositional architecture of lacustrine turbidites from normal regression to forced regression: Eocene Dongying depression, Bohai Bay Basin, East China

Sedimentary processes of marine turbidite systems under sea-level changes have been the focus of deep-water sedimentology. Compared with well-studied marine basins, the products of deep-water sediment gravity flows responding to lake-level changes in closed lacustrine basins are still poorly understood. In this study, we integrate cores of 10 exploratory wells, logs of 280 development wells, 3D seismic data and geochemical elements from the Eocene Dongying Depression, Bohai Bay Basin, East China to investigate sedimentary mechanisms and depositional architecture of lacustrine turbidite systems under lake-level changes. The studied strata are divided into three parasequence sets (PSS4 ~ PSS2), corresponding to early highstand normal regression with ascending trajectories, late highstand normal regression with flat trajectories, and forced regression with descending trajectories, respectively. From early highstand normal regression to forced regression, the ratios of Fe/Mn and U/Th reveal that the humid climate shifted to the arid climate. More lithofacies clues indicate that turbidites in highstand normal regression were triggered by river floods, whereas counterparts in forced regression were related to sediment failures. There are different architectural features of turbidite systems in three regression stages. PSS4 developed a series of channel belts within which individual channel elements are aggraded vertically. PSS3 is composed of a channelized lobe formed by individual distributary channels migrating laterally. PSS2 is characterized by a suite of compensational stacking debrite tongues. Climate forcing is a crucial factor controlling depositional architecture of the turbidite systems. A relatively humid climate during highstand normal regression led to rising lake-level and frequent floods, which are conducive to the formation of hyperpycnal-fed channel-lobe systems. Nevertheless, during forced regression under an arid climate, the strong progradational clinothems were prone to failure and sediment remobilization, resulting in debrite tongue complexes. This study highlights depositional differences of lacustrine turbidites during regression stages under the control of climate. Meanwhile, it also provides a new predictive model for deep-water hydrocarbon exploration and production in lacustrine basins worldwide.

Features of the Early Hydrocarbon Accumulation and Its Effects on the Late Accumulation in Sandstone Reservoir: A Case Study of the Eocene Dongying Depression, Bohai Bay Basin

Multi-stage hydrocarbon generation and accumulation are very common in petroliferous basins. Also, determination of hydrocarbon accumulation stages and each related diagenetic feature is important for understanding the distribution of current oil and gas resources in one individual basin. However, in the case of conventional sandstone reservoirs, few studies have focused on these resulting reservoir features (e.g., physical property and wettability) that are caused by the first accumulation of hydrocarbons and their geological implications. In this study, taking the reservoir from the Eocene Dongying Depression as an example, we investigated the features of the early hydrocarbon accumulation and its effects on the late accumulation, by analyzing the early accumulation-related paragenesis, fluid inclusions, clay minerals content, reservoir physical property, and wettability in sandstones. Our results show that the early stage accumulation of the overpressured hydrocarbon fluid could support and offset some loads of overburden, weaken the normal compaction, and preserve some original pores. Also, the early hydrocarbon accumulation resulted in an alternated, diagenetic fluid environment from the initial acidic to subsequent alkalic conditions, with the overall dissolution intensity stronger than the cementation, and formed a large number of secondary pores. As a result, the early accumulation was beneficial to reservoir space development and provided more space for the late hydrocarbons. In addition, the early hydrocarbon accumulation could cause the alteration of rock wettability and gradually formed a lipophilic migration pathway from the source rock to the reservoir. Also, the late stage of hydrocarbons would preferentially choose this dominant pathway for their migration (i.e., inheritance of migration pathway).

Sequence Stratigraphy Variation and Its Implication in Shale Oil Exploration: A Case Study of the Eocene Dongying Depression, Eastern China

Organic matter (OM) and carbonate are the important organic and inorganic components, respectively, in the shales of the upper fourth member of the Eocene Shahejie Formation, Dongying Depression, Eastern China. Their enrichment affects the hydrocarbon generation capacity and reservoir development of shales. The development mechanism of sequence stratigraphy and the influence of sequence stratigraphy on OM enrichment and carbonate deposition are investigated in this study. Based on the petrological, mineralogical, and geochemical analyses, a third-order sequence is recognized, including the lowstand systems tract (LST), transgressive systems tract (TST), and highstand systems tract (HST). The total organic carbon (TOC) content is 0.11–9.05 wt% (average 2.2 wt%), and the carbonate content is 3–95 wt% (average 51 wt%). Organic matter and carbonate are enriched in TST and gradually reduced in HST and LST. The decrease in the relative lake level in LST results in the deposition of abundant siltstone, leading to both poor OM preservation and carbonate precipitation. The rapid rise in the relative lake level in TST may be triggered by marine transgression, promoting OM enrichment in terms of both production and preservation. Seawater input brought abundant Ca2+ and Mg2+, which facilitated the precipitation of carbonates. The enrichment of OM and carbonate improves the capacity of hydrocarbon generation and reservoir performance of shales in the TST. The decrease in the relative lake level in HST causes a slight increase in the terrigenous clastic content, which, on the one hand, accelerates the sedimentation rate and OM deposition and, on the other hand, reduces carbonate precipitation. This study not only explains the differential distribution of OM and carbonate but also helps to improve the accuracy of the evaluation of shale oil sweet spots.

Variation of lacustrine carbonate deposition in the Eocene Dongying Depression and its comparison with Holocene environments

AbstractThe sedimentary characteristics and preservation potential of lacustrine carbonates provide fundamental information on climate change. The lacustrine carbonate deposition in the Eocene Dongying Depression was investigated using a combination of mineralogical, petrological and geochemical analyses. Micritic calcite/dolomite, granular calcite, columnar calcite, calcareous shell fragments and reworked detrital calcite were identified. Varying patterns of carbonates (VPC) including lithofacies, geochemical indicators and carbonate distribution were revealed in the Dongying Depression: (i) carbonates hardly precipitate in the nearshore area (average 12 wt %); (ii) carbonate content is high (average 53 wt %) in the shallow lake and (iii) gradually decreases to reach a minimum (average 24 wt %) in the deeper part of the lake. Comparison of VPC in four Holocene lakes (the Qinghai Lake and Barkol Lake in China, Oro Lake in Canada and Montcortès Lake in Spain) with the Dongying Depression suggests that four distinct lake stages were developed, namely the terrigenous clastic/gypsum-rich, carbonate-rich, carbonate-decreasing and carbonate-poor stages. A depositional model of lacustrine carbonates influenced by detrital influx, climate, palaeoproductivity and salinity is developed. This study contributes to the understanding of the genetic mechanisms of lacustrine carbonate deposition to reconstruct environmental changes.

Soft-sediment deformation structures in a lacustrine depositional context: An example from the Eocene Dongying Depression in the Bohai Bay Basin, East China

The study documents soft-sediment deformation structures (SSDSs) in the lacustrine Dongying Depression, a Paleogene synrift basin located in the southeastern corner of the Bohai Bay Basin, East China. The focus of this study is the lower part of the upper 4th member of the Eocene Shahejie Formation (Es4U), characterized by littoral to sublittoral deposits in the southwestern Dongying Depression. Nine lithofacies are grouped into the upper shoreface facies associations (FA1-1), lower shoreface facies associations (FA1-2), and sublittoral facies associations (FA2). Fine-grained calcareous sandstone was mainly deposited in FA1-1, with thinly alternating calcareous mudstone and siltstone or sandstone in FA1-2, while deposition in FA2 was dominated by mudstone, containing sediment gravity flow deposits. The SSDSs widely encountered in the lower Es4U can be classified into seven categories: (1) load structures, (2) water-escape structures, (3) folds and convolute lamination, (4) sediment-injection structures, (5) microfaults, (6) autoclastic breccias, and (7) slumps. Detailed observations of SSDSs from cores indicate liquefaction and fluidization as the main deformation mechanisms in the sands and muds. Moreover, rupturing of cohesive deposits also contributes to the soft-sediment deformation. The types and morphologies of the SSDSs are closely related to the rheologies of the sediments (e.g., the sand content), influenced by the facies that determine the lithologies and stratigraphic stacking patterns. Except for some deformation structures related to subaqueous sediment gravity flows (e.g., muddy debrite flows and slumps), most SSDSs are ascribed to rift-related seismicity due to the activity of the adjacent synsedimentary faults when the Dongying Depression underwent quick subsidence during the Eocene.

Depositional process and dispersal pattern of a faulted margin hyperpycnal system: The Eocene Dongying Depression, Bohai Bay Basin, China

There has been an increased attention on hyperpycnal flows due to its importance in delivering large volumes of sediments into deep-water. The process and products of hyperpycnal flow in tectonically-active margins are still poorly understood, and potentially constitute one of the most important deep-water mechanisms in rift basins. This study integrates core data, well-logging and 3D seismic data to investigate the hyperpycnal flow process and dispersal pattern on the Eocene Dongying rift margin. 17 facies, including 5 conglomerate facies, 9 sand facies and 3 mud facies are identified, interpreted as the product of debris flows, traction currents, turbidity currents, transitional flows and lofting plumes, and suggesting the complex blend in flood-triggered hyperpycnal flow on rift margin. Two different hyperpycnal flow types are identified and a related process model is proposed based on facies sequence, distribution, transport mechanism and flood discharge analysis, including seasonal-flood triggered hyperpycnal flow (SHF) and outburst-flood triggered hyperpycnal flow (OHF). The evolution of the hyperpycnal system suggests two dispersal styles, including proximal sublacustrine fan dominated by OHF and distal sublacustrine fan dominated by SHF, respectively. Climate and tectonic movements are suggested to be the main factors controlling hyperpycnal flow generation and deposition on rift margins. The relatively arid climate enhanced seasonal-flood activity and associated sustained and stable SHF, which further prompt distal sublacustrine fan development during a weak rifting period. On the other hand, the generation of outburst-floods can be attributed to the enhanced fault activity, which corresponds to the periodical tectonic movements in the basin. As a result, proximal sublacustrine fans tend to develop in near-shore topographic lows down-dip of syn-depositional faults due to increased tectonic activities, accommodation and enhanced OHF. A deep-water depositional model is proposed for hyperpycnal systems on rift margins by emphasizing the variety in sedimentary process and dispersal patterns controlled by climate and tectonics forces, which may provide new insights into hyperpycnal flow theories and deep-water exploration in world rift basins.

Impact of deep‐time palaeoclimate on the sedimentary records and morphology of lacustrine shoal‐water deltas, Upper Eocene Dongying Depression, Bohai Bay Basin, China

AbstractShoal‐water deltaic systems are predominant during the deposition of the second member of the Eocene Shahejie Formation (Es2) in the Dongying Depression, Bohai Bay Basin, China, and their presence is ascribed to low slopes, shallow water bodies and extremely slow tectonic subsidence. Integrated analyses of sedimentological, seismic geomorphological, palaeohydraulic, palynological and element‐geochemical data reveal the influence of palaeoclimate on the morphology and depositional processes of distributary networks in shoal‐water deltaic strata. Two contrasting palaeoclimatic stages were identified based on palynological and geochemical evidence, namely the earlier warm and humid climatic stage, and the later arid climatic stage. Compared with the earlier humid stage, the arid stage features much more abundant xerophytic species (Labiatae and Ephedra) as well as much higher Mg/Ca, Sr/Ba, and Th/U ratios and B content, indicating extensive evaporation. A third‐order sequence stratigraphic unit was recognized, consisting of a transgressive systems tract with landward stepping parasequence sets, a poorly developed highstand systems tract, and a falling stage systems tract with basinward stepping parasequence sets, which were products of the humid and arid climatic stages, respectively. A series of distributary channel complexes exhibiting reticular patterns was identified in the strata corresponding to the humid climatic stage, with amalgamated, multi‐storey distributary channel sandbodies showing a sinuosity range of 1.1 to 1.3. In contrast, the strata corresponding to the arid climatic stage are dominantly composed of distributary channel systems exhibiting dendritic forms, with prevalent single‐storey channel fills and a low sinuosity range of 1.0 to 1.2. The dendritic system is more stable than the reticular counterpart due to the effects of frequent incision and limited vertical aggradation. Typical modern shoal‐water deltas formed in different climatic conditions (i.e. Poyang Lake delta and Neales River delta) serve as analogues to the proposed conceptual depositional models of distributary networks related to palaeoclimate.

Origin of Bedding-Parallel Calcite Veins from Lacustrine Shale in the Eocene Dongying Depression, Bohai Bay Basin, China

Calcite veins, which developed parallel to the bedding, are widespread in laminated source rocks in the Eocene Dongying Depression. However, there is a lack of systematic description and classification of the veins. This study presents a systematic characterization of the calcite veins, host rocks, and micritic carbonate laminae by applying petrographic and geochemical methods to understand vein-forming mechanisms. Antitaxial and syntaxial veins are examined. Antitaxial veins contain typical fibrous crystals with the most intense fluorescence, and the median zone of these veins is often the micritic carbonate. Calcite crystals in syntaxial veins develop a blocky morphology of various sizes, indicating obvious growth competition. Data of rare earth elements and trace elements obtained from the micritic laminae, host rocks, and calcite veins are very similar. This indicates that the vein-forming nutrients originated from the carbonate in the host rocks and micritic laminae. The minor difference in C and Sr isotopes between calcite veins and micritic carbonate within the host rock and the negative shift in O isotopes in the veins are caused by ion exchange and dehydration of swelling clay minerals in the burial environment. This further proves that the calcite veins are formed in a closed system. Geochemical analysis suggests that the rocks are in the oil window and have good hydrocarbon potential. Thermal evolution of the acidic fluids generated from organic matter (OM) resulted in the dissolution of carbonate and formed fluid overpressure in the rocks. Fluid overpressure induced the formation of fractures in the interlayer and expanded the veins with the force of crystallization due to fibrous calcite growth. Blocky crystals grow in the fractures from the margins toward the center. Hydrocarbon expulsed via OM maturation in the host rock fills the intercrystalline pores. Moreover, shale with bedding-parallel calcite has the characteristics of high-quality shale oil reservoirs. These characteristics will probably provide guidance for shale oil exploration.

An eogenetic karst in lacustrine carbonates and its influence on reservoir development: A case study of the Eocene Dongying Depression, Bohai Bay Basin, East China

Research on eogenetic karsts has mostly focused on oceanic settings but rarely targeted lacustrine settings. This study describes an eogenetic karst formed in a saline lacustrine setting in the upper 4th member of the Shahejie Formation (Es4s), Dongying Depression, China. Limestones were deposited on a landmass-attached carbonate platform characterized by boundstones and grainstones interbedded with micrites. Tectonic uplift at the end of Es4s caused the karstification in a saline lake shore setting over approximately 2 Ma. During the exposure period, pervasive macroporosity developed in intervals within the top 20 m below the unconformity. Caves formed along the periphery of the platform, and they are mainly at 0.2–1.3 m in height and are filled with wackestone sediments without turbulent flow features. The caves are interpreted as the result of mixing dissolution on the margin of a mixing zone of saline lake-water and fresh-water. Sponge-like intervals occur in interior areas and range from 0.5 to 2.5 m in height, and they present a spongy porosity with pyrite crusts, pervasive pyrite framboid disseminations and occasional cements. The sponge-like dissolution pores are analogous to the Swiss-cheese texture forming in modern and fossil mixing zones and are probably caused by microbially-mediated dissolution in a mixing zone where mixing rate is limited because of reduced circulation in saline lake phreatic water. A small number of sub-vertical conduits (turbulent flow paths) and pervasive fissures are generated due to dissolution by meteoric water. The dissolution effects are strongly influenced by primary lithofacies. Spongy dissolution and caves develop almost in boundstone and grainstone layers, which is probably because they maintained a high permeability after shallow burial and thus become the preferential diffuse flow paths. According to this model, the boundstone and grainstone intervals that developed along the periphery of the platform could be favorable reservoirs for future petroleum exploration.

Direct evidence for fluid overpressure during hydrocarbon generation and expulsion from organic-rich shales

Abstract Fluid overpressures are widely expected during hydrocarbon generation and expulsion from source rocks, yet direct evidence for this phenomenon is lacking in the case of organic-rich shales. Here we show that formation of bed-parallel fibrous calcite veins in mature laminated organic-rich shales in the Eocene Dongying depression, Bohai Bay Basin, east China, occurred in direct response to fluid overpressure due to hydrocarbon generation. The evidence for overpressure is recorded by coexisting primary aqueous and petroleum inclusions in the calcite fibers. Our results show that all analyzed fluid-inclusion assemblages record variable degrees of overpressure during vein dilation, ranging from only modestly in excess of hydrostatic, to approaching and perhaps exceeding lithostatic. Thus, our results indicate that fluid pressures during dilation of horizontal veins are not necessarily equal to the opposing force of overburden throughout the history of opening. This suggests that at least some of the vein dilation is accommodated by concomitant narrowing of the adjacent wall-rock laminae, likely by scavenging (dissolution and reprecipitation) of CaCO3 from the adjacent wall rock.

Alternating of aggradation and progradation dominated clinothems and its implications for sediment delivery to deep lake: The Eocene Dongying Depression, Bohai Bay Basin, east China

Abstract Sediment routing in deep-water settings can be significantly different in lacustrine basins from their well-studied marine counterparts, which has been largely overlooked during the past decades. The seismically well-imaged lacustrine clinothems in the Eocene Dongying Depression provide a good opportunity to bridge the deep-water sediment partitioning with clinothem architecture and shelf-edge trajectories, which have been studied through the integration of 3D seismic data and well-logging data. In the study area, the Eocene delta-complexes are architecturally manifested as alternating aggradation dominated (AC) and progradation dominated clinothems (PC). The Eocene Dongying clinothem architecture has been documented in terms of shelf–edge trajectory angle (θsht), edge-to-toe distance (ETT), aggradation height (Ah), progradation distance (Pd) and maximum foreset angle (γmfa). Aggradation dominated clinothems are characterized by ascending shelf-edge trajectories with thick and widespread sand-prone bottomsets, suggesting higher efficiency of sediment delivery into the outlying lake floor. Progradation dominated clinothems, in contrast, are characterized by flat to slightly descending trajectories and generally by a lack of bottomsets. Alternations of rising and flat to slightly descending trajectories accompanied with different clinothem types probably indicate frequent relative lake level changes (on time-scales of 100s of kyr). Further analysis of seismic images and extensive well-logs suggest the distal aggradational bottomsets are generally consist of sub-lacustrine channel-lobe systems, whereas sediment slide, slump blocks occur in slope to slope toe areas of progradation dominated clinothems, which indicate varied processes of deep-water sediment transport associated with different clinothem types and shelf-edge trajectories. Lake-level, sediment supply, water salinity and density change induced by humid-arid climate cycle are suggested to be main factors controlling clinothems stacking pattern and sediment dispersal into deep-lacustrine. Humid climate are considered to be related with strongly aggradational clinothems, during which sediment can be directly transported to the distant bottomset areas due to increased sediment concentration and energy in river discharge. Whereas during dry periods, progradation dominated clinothems start to build under stillstand or falling lake level conditions, sediment generally accumulates at river mouth and collapses due to sediment overload or fault activity. Therefore, a relationship between deep-lacustrine depositional process and climatic signature was established which may provide new thoughts in both the study of deep-water sequences and deep-water exploration in lacustrine basins.

Geomorphology, lithofacies and sedimentary environment of lacustrine carbonates in the Eocene Dongying Depression, Bohai Bay Basin, China

Abstract The Eocene lacustrine carbonates in the western Dongying Depression, Bohai Bay Basin were examined based on integrated analysis of 3D seismic data, cores, thin-sections, and geochemical testing. Seismic profiles and geomorphological mapping revealed a landmass-attached (semi-isolated) carbonate platform restricted by marginal syn-sedimentary faults and surrounded by three deep sags. 3D topographic analysis of the platform revealed six districts characterized by variable buried depths, topographic gradient, and fault combinations, including two flat-topped areas (PFW and SD; topographic gradient

Formation of bedding-parallel, fibrous calcite veins in laminated source rocks of the Eocene Dongying Depression: A growth model based on petrographic observations

Bedding-parallel, fibrous calcite veins (commonly referred to as “beefs”) are widely developed within Eocene, lacustrine, laminated organic-rich source rocks in the Dongying Depression of Bohai Bay Basin, East China. Understanding the formation of this type vein is essential to elucidate the diagenetic evolution and fluid migration in muddy source rocks. This study investigates the mechanism(s) of growth of bedding-parallel veins in hydrocarbon source rocks of the Es4 upper and the Es3 lower units in the Dongying Depression, based on systematic observations of macro- and micro-scale features both in veins and their wall-rocks, combined with X-ray diffraction and analyses of geochemical indicators.The source rocks are heterogeneous in both composition and structure. The beefs mainly occur in laminated rocks consisting of organic-rich laminae, which occur together with horizontally-continuous banded lamalginites and rare carbonate-rich laminae, overall corresponding to high TOC, low carbonate contents, and low maturity (average Ro = 0.57%). Most beefs are embedded in the neighboring lamalginites of organic-rich laminae with low clay-mineral contents. The beefs are products of diagenesis accompanying burial, and both the initial fracture opening and subsequent dilation represent early stages of hydrocarbon generation and expulsion. The presence of bedding-parallel bituminite veins and the median plane (or “parting”) is evidence for horizontally oriented hydrofracturing due to fluid overpressure. In turn, fluid overpressure reflects the generation and primary migration of hydrocarbon-bearing fluids. Hydrocarbon generation (organic matter evolution) and clay-mineral dehydration (illitization of smectite) were responsible for the fluid overpressure. Crystal growth within the beefs was a continuous process with an antitaxial growth direction, and there was no growth competition between adjacent fibres. These observations indicate that the crystal growth and fracture dilation were synchronous and growth space was limited. The force exerted by crystallizing vein fill contributed to the high fluid pressure, suggesting that fibrous vein minerals actively promoted further dilation as the veins grew. Vein dilation accompanied the migration of hydrocarbon-bearing fluids, evinced by the occurrence of primary hydrocarbon inclusions found in the fibrous calcites. Therefore, the hydrocarbon-bearing fluids were present at the time of horizontal fluid flow, from the start of fracture opening to the end of the dilation.

Using of stratal slicing in delineating delta-turbidite systems in Eocene Dongying depression, Bohai Bay Basin: Insights for the evolution of multi-source delta-turbidite systems in a fourth order sequence

Delta-turbidite systems represent one of most significant targets for hydrocarbon exploration and development. Due to complex stratigraphic architecture of deltas and small scale of delta-fed turbidites, the prediction and characterization of them pose a challenge to successful exploration for hydrocarbons. Through seismic sedimentology approach, 3D seismic and borehole data were used to improve reservoir prediction of the delta-turbidite systems in the Eocene Shahejie Formation, eastern Dongying depression, Bohai Bay Basin, China. A highstand systems tract (HST) is recognized in the Eocene Shahejie Formation in eastern Dongying depression. It is composed of six fourth-order sequences (PSS5∼PSS10), each of which corresponds to a given period of delta progradation. Using stratal slicing of 90°-phase seismic data, mapping the fourth stage of delta development (PSS8), as an example, highlights the distribution and evolution of multi-source deltas and their associated thin-bedded turbidites within a fourth-order sequence. Interpretations of stratal slices indicate that the formation of delta-fed turbidites is closely related to base-level changes, sediment supply and syndepositional faults. Development of early larger-scale turbidites fed by north fan delta and Yongan delta suggests that enough sediment supply, rather than base-level falling, plays a more fundamental and critical role for turbidite formation. At the end period of a highstand fourth order sequence, the largest-scale turbidites developed in the front of Dongying delta are jointly driven by both rapid sediment supply and base level falling. This research also proves that the constraint of fourth-order sequences is necessary for ensuring the isochronism of stratal slices, which provide effective reservoir prediction of small-scale delta-fed turbidites and further insights into evolution of multi-source delta-turbidite systems.

Delta-fed turbidites in a lacustrine rift basin: the Eocene Dongying depression, Bohai Bay Basin, East China

ABSTRACTSediment avalanche from delta ramp is one of the significant development mechanisms for a turbidite system in a lacustrine basin. To advance our understanding of deep-water sedimentary processes in a lacustrine delta ramp, delta-fed turbidites in the Eocene Dongying depression of the Bohai Bay Basin were studied using core data, 3-D seismic data and well log data. Sandy debris flows, muddy debris flows, mud flows, turbidity currents, slides, sandy slumps and muddy slumps were interpreted based on the identification of lithofacies. Data indicates that deep-water sedimentary processes in the study area were dominated by debris flows and slumps, which accounted for ∼68% and 25% (in thickness) of total gravity flow deposits, respectively; turbidity-current deposits only accounted for ∼5%. Mapping of turbidites showed that most were deposited after short-distance transportation (<20 km), restricted by the scale of deep-water areas and local topography. Channels, depositional lobes, debris flow tongues, muddy turbiditic sheets, slides and slumps were identified in a delta-fed ramp system. Slides and slumps were dominant at the base of slopes or at the hanging walls of growth faults with strong tectonic activity. Channels and depositional lobes developed in gentle, low-lying areas, where sediments were transported longer distances. Sand-rich sediment supply, short-distance transportation and local topography were crucial factors that controlled sedimentation of this ramp system. Channels generally lacked levees and only produced scattered sandstones because of possible hydroplaning of debris flow and unstable waterways. In addition to lobes, debris flow tongues could also be developed in front of channels. These findings have significant implications for hydrocarbon exploration of deep-water sandstone fed by deltas in a lacustrine basin.