Sublacustrine Fan Research Articles

Depositional Models of Deep-Water Gravity-Flow in Lacustrine Basin and Its Petroleum Geological Significance—A Case Study of Chang 6 Oil Group in Heshui Area, Ordos Basin, China

Gravity-flow can carry a large number of sediments and organic matters from shallow water to deep lakes with its strong transporting energy, directly or indirectly facilitating the formation of deep-water tight reservoirs and shale reservoirs. Therefore, studying the genetic types, dynamic mechanisms, and depositional models of gravity-flow deposits is essential in the exploration of unconventional petroleum in large lacustrine basins. This research studied the genetic types, dynamic mechanisms, and sedimentary models of the gravity-flow deposits of the Chang 6 oil group in the Heshui Area, Ordos Basin, China, aiming to reveal its petroleum geological significance. Core observation, microscopic thin section identification, particle size analysis, and determination of rare earth elements were carried out. As a result, three types of gravity-flow deposits are detected, namely, slide-slump, sandy debris flow, and turbidity current. A certain slope gradient in bed form is the necessary geomorphic condition for gravity flow formation, and determines its development level, distribution range, and flow transformation efficiency. Sufficient provenance lays the material foundation and determines its depositional composition and development type. Other factors include earthquakes, volcanoes, and floods, which serve as triggering forces. In addition, fragmentation, liquefaction, and fluid mixing are the main dynamic mechanisms driving flow transformation. Based on the flow type of gravity flow, particle size characteristics, gravity-flow transformation relations, development mechanism, and spatial distribution pattern, we distinguished two depositional gravity-flow models, i.e., slump turbidite body and sublacustrine fan. Re-portrait the spatial distribution of deep-water gravity flow in the study area. From the perspective of sedimentology, explain the genesis of sand bodies in the northeast and southwest. The sandy debris flow in the middle fan braided channel microfacies of the sublacustrine fan sways the development of thick massive sand bodies in the study area. Hybrid event beds formed by the fluid transformation in a slump turbidite are the potential dessert area for deep-water tight oil and gas.

Geomorphology and Architecture of Paleogene Sublacustrine Fans in Eastern Slope of Liaoxi Uplift, Bohai Bay Basin, East China

Geomorphology and Architecture of Paleogene Sublacustrine Fans in Eastern Slope of Liaoxi Uplift, Bohai Bay Basin, East China

Seismic sequence stratigraphy of the Permo- Triassic Karoo Group and its implication for hydrocarbon exploration, southwestern Ogaden Basin, Ethiopia

The Ogaden Basin is the largest sedimentary basin in Ethiopia and one of the underexplored basins in eastern Africa. It has an area of approximately 350,000 sq. km and an estimated sediment thickness in excess of 7 km in the deepest part. The Karoo sequence which lies at the base of the sedimentary section is a Permian to Triassic age terrigenous clastics deposited in tensional to trans-tensional tectonic setting. The sequence has an estimated total thickness of 3000 m and contains proven gas reserve in excess of 6 TCF. Previous studies focused primarily on the sedimentology and lithostratigraphy of the section. This work provides the first integrated sequence stratigraphic analysis of the Karoo section based on seismic reflection and well data. In this study, the Karoo super-sequence is divided into four second-order sequences and at least four higher order sequences in the uppermost sequence. The transgressive systems tract and the overlying highstand systems tract separated by a maximum flooding surface constitute the bulk of these sequences. The dominant depositional systems in systems tracts are shallow to deep lacustrine, fluvio-deltaic and possibly sublacustrine fans and turbidites. Our study suggests that the major control in creation of accommodation space is related to wrench tectonics which gave rise to narrow pull apart sub-basins and their surrounding steep flanks. These structures are oriented NE-SW to E-W. Organic rich lacustrine shales commonly associated with transgressive systems tract and prodelta to lacustrine shales of highstand systems tract, charging deltaic and fluvial sand reservoirs of the highstand systems tract as well as potential targets such as turbidites, and sealed by transgressive surfaces make up potential hydrocarbon plays of the Karoo section. This study is one of the first detailed analysis of the Karoo sequence using sequence stratigraphic approach to predict an enhanced hydrocarbon potential in the Ogaden Basin.

Sedimentological and diagenetic impacts on sublacustrine fan sandy conglomerates reservoir quality: An example of the Paleogene Shahejie Formation (Es4s Member) in the Dongying Depression, Bohai Bay Basin (East China)

A section of the northern Yanjia nearshore subaqueous fan sandy conglomerate was investigated to understand high-quality reservoirs' distribution and controlling factors in the retrogradation and progradation sequences. These conglomerates have been demonstrated to have strong heterogeneity, tightly cemented, and feature low porosity and permeability. To provide an inclusive understanding of the formation mechanisms and factors governing the distribution of the high-quality reservoirs, we utilized integrated methods including seismic, cores description, wireline, petrography, scanning electron microscopy, cathodoluminescence, and mercury intrusion capillarity. The results showed that the retrogradation rock types are dominated by arkose and lithic arkose and the progradation by lithic arkose and feldspathic litharenite. The dominant diagenesis events are compaction, carbonate cementation, and dissolution. The mechanical compaction effect is similar in the progradation and retrogradation sequence. The average compaction index of 0.70 and 0.68 for the transgressive system tract and highstand system tract, respectively, revealed that compaction is the dominant mechanism in reducing reservoir quality in the nearshore subaqueous fan.The carbonate cementation is more extensive in the highstand system tract than the transgressive system tract. The braided channel sandy conglomerate from transgressive system tract and highstand system tract have the best reservoir quality and features type I and II mercury intrusion capillarity cures with large pore throat and good physical properties. However, the braided channel reservoir quality from the highstand system tract developed during the progradation phase is relatively better than the braided channel of the transgressive system tract in the retrogradation phase. These microfacies are characterized by a substantial dissolution of rock fragments and feldspar. The main channel and the interchannel deposited during the transgressive and highstand system tract, respectively, represented by type III mercury intrusion capillarity curves with small pore throats, are non-reservoirs. The formation mechanism of the high-quality reservoirs in the highstand system tract is guided by sedimentary facies, compaction resistant ability, dissolution of rock fragment, and feldspar. The genetic mechanism of the high quality in the transgressive system tract is governed by the sedimentary facies, compaction resistant ability, carbonate and feldspar dissolution. This study revealed that the progradation sequence retained the favorable high-quality in the nearshore subaqueous fan. This study proved that, for adequate assessment of reservoirs in the Yanjia area and similar reservoirs elsewhere, reservoir characterization must link sequence stratigraphy, sedimentary facies, and diagenesis to comprehend the high-quality reservoir distribution to help the predrill evaluation of reservoirs.

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.

Architecture and depositional processes of sublacustrine fan systems in structurally active settings: An example from Weixinan Depression, northern South China Sea

Sublacustrine fans developed in structurally active settings are commonly characterized by complicated depositional characteristics, which create great challenges to hydrocarbon exploration. To advance understanding their sedimentary architecture and depositional processes, a large-scale sublacustrine fan system in the Eocene Weixinan Depression, northern South China Sea has been identified and investigated through the comprehensive analysis of 3-D seismic, well-logging, and core data. This fan was deposited during a lake-level falling stage, covering an area of 55 Km2. It can be divided into two sections in the vertical direction by the sedimentary characteristics. The lower section mainly consists of thin layers of well-sorted fine sandstones with parallel bedding, which are interpreted as deposits of turbidity currents. By contrast, the upper section comprises thick layers of gravelly coarse sandstones with various deformation structures and widespread floating clasts, which should be the deposits of cohesive debris flows. Provenance analysis suggests that the development of complicated sedimentary characteristics has a close relationship with the occurrence of multiple sediment supply directions. The lower section was sourced by the western delta with long-distance transported well-sorted sediments, while the northern fan delta sourced the upper section with short-distance transported coarse-grained sediments. Its depositional process was mainly controlled by paleo-geomorphology, sediment supply, and relative lake level changes. The lower section, controlled by the flexure slope belt, is a typical sand-rich sublacustrine fan characterized by the low gradient (<1°–1.5°), while the upper section, controlled by the steep slope fault belt, is a gravel-rich sublacustrine fan dominated by intense faulting and high gradient (3.2°–4.5°). In addition, during the depositional period of the upper section, the relay ramps between en-echelon faults provided major entry points for coarse-grained sedimentary flows into the depression. Therefore, a deep-lacustrine model is proposed for the sublacustrine fan system, showing the detailed “triggering-transportation-deposition” process. This study outlines the complexity of the sedimentary architecture of sublacustrine fans in structurally active settings, and the proposed model may be applicable in other lacustrine basins.

Paleogene lake deep water sedimentary facies in the northern zone of the Chezhen Sag, Bohai Bay Basin, China

This study researches an important genetic type of Paleogene sublacustrine fan deposits in widely developed glutenite bodies in the northern Chezhen Sag. The Paleogene sublacustrine fans in the Bohai Bay Basin are primarily distributed in Es3 (The third member of Paleogene Shahejie Formation). There are many sedimentary types in Chezhen depression, but understanding of the reservoir is too limited to support a development strategy. Therefore, a detailed study on the core characteristics, sedimentary structure, sedimentary microfacies and reservoir properties has been completed in order to reduce uncertainty. It is confirmed that the sublacustrine fan deposits are mostly characterized by a positive cycle of upward thinning, and the base is composed of conglomerate supporting conglomerate and massive gravel sandstone, which reflect the sedimentary characteristics of traction and gravity currents. Finally, the influence of sedimentary process on properties was assessed by establishing the correlation between microfacies and reservoir physical parameters, such as porosity, permeability, detrital particle. It is revealed that the sublacustrine fan glutenite body formed in the deep depression period is interspersed with deep- to semi-deep lake source rocks. The superior source–reservoir–caprock combination is one of the most important exploration targets in the northern zone.

Sedimentary characteristics and geological significance of sublacustrine fan in the Houdomoer tectonic belt

At present, the sedimentary characteristics of the sublacustrine fans in the Houdomoer tectonic belt have not been clearly understood, which restricts further oil and gas exploration. In this research, the sedimentary characteristics of the sublacustrine fans and their oil and gas geological significance are studied based on the data of core, well logging, logging and earthquake. It is found that the sublacustrine fans in the study area can be divided into three types. The sublacustrine fan of the Houdomoer tectonic belt is located in the stratum next to the best source rock in Nantun Formation. Oil and gas are injected into the sublacustrine fan sand body from the bottom and side. Generally, oil and gas accumulation is mainly controlled by the physical characteristics of the reservoir. Experimental analysis shows that the reservoir capacity of different sedimentary microfacies is quite different. The braided channel microfacies of the middle fan is the most favorable type of reservoir facies belt, and the main channel microfacies of the inner fan is the second most favorable reservoir. The practice of oil and gas exploration also proves that the high-yield wells are mostly located in the braided channel microfacies of the middle fan. Therefore, this microfacies type has a broad exploration prospect.

The Lithological Features of Sublacustrine Fans and Significance to Hydrocarbon Exploration: A Case Study of the Chang 7 Interval of the Yanchang Formation, Southeastern Ordos Basin, North China

The Chang 7 interval of the Upper Triassic Yanchang Formation in the Ordos Basin represents a typical deep lacustrine depositional sequence. On the basis of field outcrops, cores, well logs, light/heavy mineral provenance analysis, and petrological studies, we evaluated the characteristics of deep-water gravity flow deposition of the Chang 7 interval and constructed a depositional model. The sediments mainly came from the northeast of the study area, and multiple sublacustrine fans were deposited in the center of the basin. Different from the deep-marine fan, the sublacustrine fan in the study area develops under the background of gentle slope without any erosional canyon between the fan and delta front. Gravity flow deposits in the study area can categorised into three groups: sand debris flow deposits, turbidity current deposits, and deep-water mudstone deposits. The main channel and branch channel are mainly developed with thick massive sandy debris sandstone, while the channel lateral margin and branch channel lateral margin are mainly developed with middle massive sandy debris sandstones and turbidite sandstones, which from bottom to top, the thickness of sand layer becomes thinner and the grain size becomes smaller. Thin mudstone is developed between channels; the lobe fringe includes sheet-like turbidite sandstones and deep lake mudstones. The widely distribute, good quality source rocks ( TOC = 2 % – 6 % ) developed in deep lacustrine have attained the peak stage of oil generation ( R o = 0.9 % – 1.2 % ). The superimposition of the sublacustrine fan sand bodies and the wide distribution of good quality source rocks favor the formation of large lithologic reservoirs characterized by source–reservoir integration, self-generation and self-storage, and near-source accumulation.

Interpretation and Reconstruction of Depositional Environment and Petroleum Source Rock Using Outcrop Gamma-ray Log Spectrometry From the Huai Hin Lat Formation, Thailand

Gamma ray logs are most useful in identifying subsurface lithology and interpreting depositional environments. This study highlights the use of outcrop gamma-ray logs along with outcrop observations and total organic carbon (TOC) analysis to provide the stratigraphic framework of the organic-rich rocks of Huai Hin Lat Formation in central Thailand. The study reveals five sedimentary facies including (1) structureless sandstone, (2) structured sandstone, (3) interbedded sandstone and siltstone, (4) interbedded mudstone and siltstone and (5) calcareous mudstone. These facies can be grouped into two facies associations; mudstone-dominated and sandstone-dominated facies associations. The depositional environment was interpreted as lacustrine basin-fill subdivided into deep lacustrine environment and sublacustrine fan associated with the turbidity currents. The total gamma-log characteristics are closely related to the lithologies controlled primarily by clay mineral compositions. Whist, the use of spectral gamma-ray can reveal more details on depositional environments and conditions. In this study, U concentrations is proven to be useful in highlighting organic-rich rocks in low K and Th concentration successions due to its ability to be fixed in clay minerals and organic materials under an anoxic condition. Thus, the U spectral gamma ray is suggested to combine with conventional gamma ray log for depositional environment and recognition of organic-rich rocks.

Deep-water gravity flow deposits in a lacustrine rift basin and their oil and gas geological significance in eastern China

The types, evolution processes, formation mechanisms, and depositional models of deep-water gravity flow deposits in a lacustrine rift basin are studied through core observation and systematic analysis. Massive transport of slide and slump, fluid transport of debris flow and turbidity currents are driven by gravity in deep-water lacustrine environment. The transformation between debris flow and turbidity current, and the transformation of turbidity current between supercritical and subcritical conditions are the main dynamic mechanisms of gravity flow deposits in a lake basin. The erosion of supercritical turbidity current controls the formation of gravity-flow channel. Debris flow deposition gives rise to tongue shape lobe rather than channel. Deep-water gravity flow deposits are of two origins, intrabasinal and extrabasinal. Intrabasinal gravity flow deposits occur as single tongue-shape lobe or fan of stacking multiple lobes. Extrabasinal gravity-flow deposits occur as sublacustrine fan with channel or single channel sand body. However, the nearshore subaqueous fan is characterized by fan of stacking multiple tongue shape lobes without channel. The differential diagenesis caused by differentiation in the nearshore subaqueous fan facies belt results in the formation of diagenetic trap. The extrabasinal gravity flow deposits are one of the important reasons for the abundant deep-water sand bodies in a lake basin. Slide mass-transport deposits form a very important type of lithologic trap near the delta front often ignored. The fine-grained sediment caused by flow transformation is the potential “sweet spot” of shale oil and gas.

Linkage and growth of the independent and coherent faults: Insight into the effect of relay ramps on sedimentation patterns in the northern Bonan Sag, Bohai Bay Basin

The influence exerted by the linkage and growth of fault segments on the sedimentation pattern in a lacustrine rift subbasin, the northern Bonan Sag in the Jiyang Depression in the Bohai Bay Basin, is studied by integrating drilling cores, wireline logs and 3D seismic data. The NW-trending Guxi Fault formed through the linkage of three fault segments, which display a roughly en echelon arrangement in map view and are probably related to early-stage regional transtensional and slip-strike stress. Between the vertical displacement troughs of normal faults, two narrow relay ramps, attributed to the coherent fault linkage-and-growth model, formed through the linkage of the three fault segments. A relatively wide relay ramp, attributed to the isolated fault linkage-and-growth model, developed due to the linkage of the NW-trending Guxi Fault and E-W-oriented segmented Chengnan Fault. The sedimentation pattern was strongly controlled by the geometry and evolution of the relay ramps. The sediment routing system was dominated by the relay zone, and fan-delta and sublacustrine fan depositional systems developed in the early stage of relay ramp formation. Lateral breaching of the relay ramps through extensive faulting and rifting probably caused an increase in the vertical throw and resulted in deposition of a coarse-grained nearshore subaqueous fan in front of the normal faults. The relay zone that formed from the linkage of independent faults (the Chengnan and Guxi faults) is associated with a broad drainage area and fan-delta and sublacustrine fan deposits, which contain the most effective hydrocarbon reservoirs in this deeply buried setting.

Bed type and flow mechanism of deep water sub-lacustrine fan fringe facies: an example from the Middle Permian Lucaogou Formation in Southern Junggar Basin of NW China

Submarine or sub-lacustrine lobe deposits are important reservoirs, but the fan fringe deposits form heterogeneities within deep water fan deposits. Fan fringe facies records the complex sediment gravity flow types. By understanding of the bed types and flow mechanisms, we can identify the fan fringe deposit, which aids in the reconstruction of deep water fan and reservoir evaluations. The Jiucaiyuanzi and Dalongkou sections in the West Bogda Mountains preserve well-exposed 536-m and 171-m thick successions, respectively, of a deep water lacustrine depositional system from the Middle Permian Lucaogou Formation. Bed types of the Lucaogou Formation include high-density turbidite, low-density turbidite, incomplete Bouma-type turbidite, hybrid event beds, and slump deposits. The Lucaogou Formation is interpreted here as a fan fringe facies due to the thin bed thickness that characterize turbidites and hybrid event beds, as well as the predominance of the isolated sheet architecture. Previous studies suggest that these deposits were considered as deposited in a deep water setting due to the absence of wave-related structures. The presence of abundant mud clasts in massive medium-coarse grained sandstone beds reflects the significant erosional capability and interactions between high-density turbidity currents and lake floor. The fan fringe facies here contains amalgamated and thick-bedded homolithic facies (~ 30%) and thin-bedded heterolithic facies (~ 70%). The examination of the bed type is of wider significance for facies prediction and reservoir heterogeneity in the sub-lacustrine fan fringe facies.

Sedimentary architecture and provenance analysis of a sublacustrine fan system in a half-graben rift depression of the South China Sea

Research on the sedimentary architecture and provenance of sublacustrine fans is crucial not only to better understand the gravity flow evolution in lacustrine basins but to successfully locate hydrocarbon reservoirs. This study targets at a sublacustrine fan with a large petroleum reserve that has been revealed by drilling within an Eocene lacustrine succession in the Fushan Depression, northern South China Sea. The sedimentary architecture of the sublacustrine fan was investigated through the integration of core, well logs and 3D seismic data, and its provenance was identified by detrital zircon UPb dating. The examination of 117 m long continuous drilling core reveals four major lithofacies in the sublacustrine fan deposits, which are interpreted as deposits of two types of gravity flows (cohesive debris flows and turbidity currents). The cohesive debris flows developed in the lower section are mainly composed of massive gravelly mudstones with various floating clasts and high clay content, whereas the turbidity currents in the upper section are characterized by parallel to ripple bedding and normal graded bedding. These observations reflect a transformation from cohesive debrites to turbidites from deep to shallow water environments. In addition, provenance analysis of 648 detrital zircon UPb ages yield a well-defined relationship between potential provenance areas and sublacustrine fan deposits in the deep lake. The UPb age pattern of the fan is similar to that of northern fan delta deposits, but different from sediments from the western and southern deltas, suggesting that the sublacustrine fan was sourced from the north. Therefore, a deep-lacustrine depositional model is proposed for the sublacustrine fan, showing a gravity-driven evolution process from fan delta front in the northern slope into cohesive debrites and then turbidites in the lake center. This was mainly controlled by fault-related topography and sediment supply. This study highlights the effective use of the detrital zircon UPb dating method to identify the provenance direction of sublacustrine fans, and the proposed depositional model may be applicable to reservoir prediction in other lacustrine basins.

Reservoirs properties of slump-type sub-lacustrine fans and their main control factors in first member of Paleogene Shahejie Formation in Binhai area, Bohai Bay Basin, China

Abstract High-yielding oil wells were recently found in the first member of Paleogene Shahejie Formation, the Binhai area of Qikou Sag, providing an example of medium- and deep-buried high-quality reservoirs in the central part of a faulted lacustrine basin. By using data of cores, cast thin sections, scanning electron microscope and physical property tests, the sedimentary facies, physical properties and main control factors of the high-quality reservoirs were analyzed. The reservoirs are identified as deposits of slump-type sub-lacustrine fans, which are marked by muddy fragments, slump deformation structure and Bouma sequences in sandstones. They present mostly medium porosity and low permeability, and slightly medium porosity and high permeability. They have primary intergranular pores, intergranular and intragranular dissolution pores in feldspar and detritus grains, and structural microcracks as storage space. The main factors controlling the high quality reservoirs are as follows: (1) Favorable sedimentary microfacies of main and proximal distributary gravity flow channels. The microfacies with coarse sediment were dominated by transportation and deposition of sandy debris flow, and the effect of deposition on reservoir properties decreases with the increase of depth. (2) Medium texture maturity. It is shown by medium-sorted sandstones that were formed by beach bar sediment collapsing and redepositing, and was good for the formation of the primary intergranular pores. (3) High content of intermediate-acid volcanic rock detritus. The reservoir sandstone has high content of detritus of various components, especially intermediate-acid volcanic rock detritus, which is good for the formation of dissolution pores. (4) Organic acid corrosion. It was attributed to hydrocarbon maturity during mesodiagenetic A substage. (5) Early-forming and long lasting overpressure. A large-scale overpressure compartment was caused by under-compaction and hydrocarbon generation pressurization related to thick deep-lacustrine mudstone, and is responsible for the preservation of abundant primary pores. (6) Regional transtensional tectonic action. It resulted in the structural microcracks.

Microbially induced dolomite precipitates in Eocene lacustrine siliciclastic sequences in the Dongying depression, Bohai Bay Basin, China: Evidence from petrology, geochemistry, and numerical modeling

This study investigates the origin of dolomite by examining thick lacustrine sequences of evaporites, mudstones, and deep-water sublacustrine fan sandstones in the lower unit of the fourth member of the Eocene Shahejie Formation (Es4x) in Bohai Bay Basin, China. This petrographic, geochemical, and numerical modeling study recognizes evidence for microbially mediated dolomite precipitates in the Es4x sandstones, including the following: (1) spherical or elliptical dolomite aggregates consisting of submicrometer-sized crystals; (2) negative δ13CVPDB (VPDB refers to Vienna Peedee belemnite) values of the dolomite cements (–6.56‰ to –3.35‰); (3) relatively low precipitation temperatures of dolomite cements (39°C–65°C [102°F–149°F]) based on δ18OVPDB values from –8.76‰ to –4.65‰; and (4) negative 34SVCDT (VCDT refers to Vienna Canon Diablo troilite) values of associated framboidal pyrite (–3.9‰ to +5.7‰) with respect to an anhydrite precursor (+21.2‰ to +37.8‰). One-dimensional reactive transport modeling incorporating biodegradation kinetics was conducted to evaluate the link between biodegradation of organic matter and geochemical reactions of inorganic minerals during microbial sulfate reduction (MSR). Microbial metabolism in the model creates favorable geochemical conditions for dolomite precipitation by increasing pH, carbonate alkalinity, and Mg2+/Ca2+ ratio. The modeling results reproduce petrographic observations associated with the MSR process and indicate that the kinetics of microbial growth can significantly modify the geochemistry of simulated waters and enhance microbially mediated dolomite precipitation. Therefore, this study illustrates the effect of MSR in dolomite precipitation in a semiclosed and reducing continental lacustrine basin.

Tectono-stratigraphy characteristics of the Lower Cretaceous Saihantala Sag in the Erlian Basin, China: Normal faulting and controls on the depositional variability

Tectonism is one of the most important factors controlling the formation of sedimentary stratigraphy in lacustrine rift basins, but the controlling effects need to be further clarified. Based on 2D and 3D seismic data, well logs, logging data, and core data from the Saihantala Sag, our study aimed to analyse the fault system of the sag, determine its tectonic evolution process, establish the faulting pattern of the sag, recognise its depositional systems, and finally discuss the relationships between the tectonic factors and the basin evolution and sediment filling process.Four types of depositional systems including fan-delta, braided-delta, near-shore subaqueous fan and lacustrine deposits were recognized in the Saihantala Sag based on an integrated analysis of the well logs, cores and seismic data. The results indicated that 1) The Xilin fault is the eastern boundary fault of the Saihantala Sag, and its subsections and connections controlled the stratigraphic pattern of the sag. 2) tectonic evolution controlled the evolution of the lake basin and depositional systems in the Saihantala Sag, especially the deposition of near-shore subaqueous fan and the connection and retrogradation of braided-delta and fan-delta systems. 3) The tectonic evolution also controlled the filling pattern of the Saihantala Sag, which features two sets of braided-delta deposits in its gentle slope belt and one set of fan-delta deposits and two sets of near-shore sub-lacustrine fan deposits in its steep slope belt. 4) All three tectonic belts (Zhabu, Yihe and Saisi) mainly experienced a middle-late activated single-cycle faulting pattern, which controlled the distribution of braided-delta front deposits and lenticular sandy bodies.

Seismic geomorphology, architecture and genesis of unusual confined and semiconfined sedimentary units on the northern slope of the Bonan uplift, Bohai Bay Basin, China

Studying the fundamental influence of basin-floor topography on the growth and morphology of sedimentary systems has largely focused on submarine fans; however, in lacustrine settings, only a limited number of studies are primarily concerned with confined sublacustrine fans, although such fans can form high-quality hydrocarbon reservoirs. By using recently acquired three-dimensional seismic data, a series of unusual confined and semiconfined sedimentary units with seismic reflection patterns different from those of previous studies have been identified. The identified sedimentary units are developed in the Paleogene succession on the northern slope of the Bonan uplift, Bohai Bay Basin, China, and are characterized by either vermicular-like chaotic progradational reflections or coupled vermicular-like and progradational seismic reflections in the dip direction and channel-shaped bidirectional onlap reflections along strike directions. Through restored paleogeomorphology, six elongate incised valleys (V-1 to V-6) with diverse scales on the uplift slope are clearly documented. The identified sedimentary units were deposited within these incised valleys. The units identified in this study are considered “confined” or “semiconfined” based on the following several aspects, they: 1) were deposited within elongate depocenters along dip directions; 2) are clearly separated from each other by topographic highs along strike directions; 3) have belt-shaped sediment dispersal patterns in planar view; and 4) have the thickest deposits primarily distributed in the middle and distal parts. Through comparing sedimentary landforms, feeder systems, seismic reflection characteristics and spatial distribution patterns between sedimentary systems, the confined and semiconfined systems identified in this study display both similarities to and profound differences from previously studied confined sublacustrine fans and are here considered a new unusual type. We anticipate this work may serve as a useful reference for confined sedimentary system research in other types of sedimentary basins.

Rift-related sedimentary evolution and its response to tectonics and climate changes: A case study of the Guaizihu sag, Yingen-Ejinaqi Basin, China

Distinguished from most existing rift basin evolutionary models, the succession of deep-water deposits in the Guaizihu sag were mainly developed during rift initiation instead of the greatest tectonic subsidence phase. This paper focuses on the detailed analysis of rift-related sequence stratigraphy and sedimentary evolution in this continental rift sag, and discusses tectonic and climatic controls on filling patterns. The deposits of the Guaizihu sag can be ascribed to four major depositional systems: alluvial fan and fan delta, braided delta, sublacustrine fan and shallow to deep lacustrine deposits. They are in part contemporaneous and some succeeding each other, forming five sequence units and corresponding substages of basin evolution: (i) rift initiation, (ii) early rift climax, (iii) middle rift climax, (iv) late rift climax, and (v) late syn-rift substages. With the onset of fault movement and hot-humid climate conditions, the first substage is characterized by a number of poorly linked depocenters, which consist of alluvial fan, fan delta and deep lacustrine deposits with organic-rich mudstones. The second substage is associated with an enhanced activity of border faults and block rotation. This unit is dominated by lacustrine deposits, but fan or braided delta and sublacustrine fans also can be observed. The third substage is marked by the maximum subsidence rate and shallow lakes fringed by fan and braided deltas. The arid environment is likely to be the main factor controlling the lithofacies and filling characteristics of this unit. During the fourth substage two early sub-sags gradually converged to form a uniform one. The relative tectonic quiescence and climate fluctuations (sub-humid to sub-arid) favored carbonate deposition. The late syn-rift unit, which is dominated by fine-grained deposits, is a consequence of degradation of source area (mostly tectonic). This study shows that the effects of tectonics and climate on lacustrine sedimentation are intimately interwoven. Tectonism determines the sag structure, stratigraphic architecture, and spatial distribution pattern of depositional systems. Climatic conditions superimposed on tectonics tends to be the major control on sediment supply, lithofacies types and sedimentary features.

Sedimentary characteristics and model of gravity flows in the eocene Liushagang Formation in Weixi'nan depression, South China Sea

Gravity flows have been well investigated in deep-marine settings but less so in deep-lacustrine environments. To further understand the sedimentary characteristics and processes of gravity flows in a lacustrine rift basin, sublacustrine fans in the Weixi'nan Depression in the South China Sea were identified and studied by integrating core, well logging and 3D seismic data. Several aspects of gravity flow deposits were discussed in this study: (1) classification: seven lithofacies types were further grouped into four lithofacies assemblages/associations associated with emplacement by slumps, gravel-rich debris flows, high-density turbidity currents (which may transform into muddy debris flows) and low-density turbidity currents. (2) Proximal vs. distal position: the quantitative analysis of the lithofacies associations indicated that the gravity flow deposits are dominated by gravel-rich debris flow deposits and high-density turbidites in the proximal parts of the sublacustrine fans, whereas low-density turbidites dominate in the basin center. (3) Major triggers (humid climate and/or slope failures): the presence of active structures and a humid climate favored the formation of gravity flows, which originated from delta collapses and/or floods. (4) Lateral and vertical lithofacies distributions: the coarse-grained deposits represent a spectrum of different categories of gravity flows. In proximity to the slope break zone, slumps transformed into gravel-rich debris flows through liquefaction and deformation. As the gravity flows advanced toward the basin, the gravel-rich debris flows gradually became diluted with the surrounding water and transformed into high-density turbidity currents. When the flows reached the deep basin plain, the high-density turbidity currents transformed into low-density turbidity currents because the coarse grains could no longer remain in suspension. (5) An alternative mechanism: when the flows eroded the muddy substrate/basement, an alternative mechanism involving reverse-flow transformation occurred. In some cases, high-density turbidity currents transformed into muddy debris flows because the eroded light material became segregated toward the rear of the turbidity current and inhibited turbulence. This sedimentary model of gravity flows can be used for oil exploration in the study area and similar lacustrine rift basins.