Braided Delta Research Articles

Sedimentary infill of Early-Middle Jurassic in the southeastern Tarim Basin and its constraints on the evolution of the Altyn Tagh Fault in the Northeast Tibet Plateau

The Altyn Tagh Fault (ATF) is the boundary between the Tarim Basin and the Tibetan Plateau, which is crucial for the tectonic evolution and uplift of the Tibetan Plateau. However, the Mesozoic tectonic evolution of the ATF remains controversial. This study reconstructed the Early-Middle Jurassic palaeogeographic pattern of the southeast depression of the Tarim Basin (SDTB) and revealed potential source-sink relationships between the SDTB and its adjacent mountains to constrain the Mesozoic tectonic evolution of the ATF based on sedimentology and provenance analyses. The results indicated that the SDTB was infilled with alluvial fan facies during the Early Jurassic and the late Early Jurassic, giving rise to the braided-river delta to lacustrine facies. During the Middle Jurassic, the lacustrine basin expanded with the wide development of lacustrine facies, and the lacustrine basin contracted and was primarily infilled with braided delta facies in the late Middle Jurassic. The Jurassic sedimentary center, the SDTB situated in front of the Altyn Tagh Mountain and East Kunlun Mountain, played a crucial role in the development of three types of hydrocarbon source rocks: charcoal mudstone, coal beds, and dark mudstone. Combined with seismic reflection data interpretations, the Early Jurassic the STDB is a rift basin that underwent a rift-depression transition in the late Early Jurassic, solidifying the characteristics of the depression basin in the Middle Jurassic. The extensional environment in the SDTB resulted from the tectonic effects of slab break-off following the closure of the Tethys Ocean and the spreading of the distal stress field of the Bangonghu–Nujiang Ocean. The analyses of gravel component and heavy mineral indicated that the Jurassic source of the basin was traced back to the ancestral Altyn Mountains, the East Kunlun Mountains, and the internal palaeo-uplift of the basin, which demonstrated that the SDTB and Western Qaidam Basin (WQB) shared a common lacustrine basin during the Jurassic, suggesting that the ATF did not initiate strike-slip movements during this period. This study significantly enhanced our understanding of the Mesozoic tectonic evolution in Northwest China, providing valuable insights into exploring Mesozoic oil-bearing basins in the northern Tibetan Plateau. Furthermore, it served as a useful reference for constraining the timing of the initial slip of similar plate boundary strike-slip faults.

Tectono-stratigraphic architecture, depositional systems and salt tectonics to strike-slip faulting in Kribi-Campo-Cameroon Atlantic margin with an unsupervised machine learning approach (West African margin)

Located in the Gulf of Guinea, the Kribi-Campo sub-basin belongs to the Aptian salt basins along the West African Margin. In this paper, we investigated the tectono-stratigraphic architecture of the basin, focusing on the role of salt tectonics and strike-slip faults along the Kribi Fracture Zone with implications for reservoir prediction.Using 2D seismic data and well data interpreted through sequence stratigraphy with integrated seismic attributes analysis with Python Programming and unsupervised Machine Learning, at least 6 s-order sequences, indicating three main stages of tectono-stratigraphic evolution were determined: pre-salt syn-rift, post-salt rift climax and post-rift stages. The pre-salt syn-rift stage with KTS1 tectonosequence (Barremian-Aptian) reveals a transform rifting along NE-SW transfer faults associated with N–S to NNE-SSW syn-rift longitudinal faults bounding a NW-SE half-graben filled with alluvial to lacustrine-fan delta deposits. The post-salt rift-climax stage (Lower to Upper Cretaceous) includes 2 s-order tectonosequences (KTS2 and KTS3) associated to the salt tectonics and Campo High uplift. During the rift-climax stage, the growth of salt diapirs developed syncline withdrawal basins filled by early forced regression, mid transgressive and late normal regressive systems tracts. The early rift climax underline some fine grained hangingwall fan or delta deposits and coarse grained fans from the footwall of fault scarps. The post-rift stage (Paleogene to Neogene) contains at least three main tectonosequences KTS4, KTS5 and KTS6-7. The first one developed some turbiditic lobe complexes considered as mass transport complexes and feeder channel-lobe complexes cutting the unstable shelf edge of the Campo High. The last two developed submarine Channel Complexes associated to lobes towards the southern part and braided delta to tidal channels towards the northern part of the Kribi-Campo sub-basin.The reservoir distribution in Kribi-Campo sub-basin reveals some channels, fan lobes reservoirs and stacked channels reaching up to the polygonal fault systems.

Depositional model of clastic and carbonate rocks in a continental rift basin: a case study of the Paleogene Shahejie Formation in the Cangdong Depression, Bohai Bay Basin

The study of a clastic and carbonate rock depositional model in a continental rift basin is helpful to better understand the sedimentary processes active in continental rift basins. However, the spatiotemporal evolution characteristics and controlling factors of clastic and carbonate rocks in continental rift basins are still unclear. Therefore, the sedimentary characteristics of the clastic and carbonate rocks in the Paleogene Eocene Shahejie Formation (Es) are analysed via seismic, well log, core, thin section, and geochemistry data. Then, the impacts of tectonic movement, sediment supply and hydrologic conditions on sedimentary characteristics are discussed, and a depositional model is finally established. Five 3rd-order sequences, named SQ1–SQ5 from bottom to top, are identified in the Es. Fan delta, braided delta, meandering river delta, and lake sedimentary systems are identified in the Es; clastic rock sedimentary systems are identified in SQ1–SQ4, while concurrent clastic and carbonate rock sedimentary systems are identified in SQ5. During the SQ1–SQ4 period, the palaeogeomorphology formed by differential faulting activity or fault interactions and the inherited palaeogeomorphology resulted in different distributions of the sedimentary systems. The changes in accommodation space and sediment supply caused by tectonic movement led the delta to prograde or disappear. During the SQ5 period, the clastic rock sedimentary systems underwent a transition into clastic rock and carbonate rock sedimentary systems as a result of rift weakening, broad crustal thermal subsidence, sediment supply reduction and high-salinity lake water. This study provides a typical case for the evolution characteristics of the clastic rock and carbonate rock systems in continental rift basins, identifies changes in the tectonic, sediment supply, and hydrologic characteristics during their mutual transformations, and provides an important reference for hydrocarbon reservoir prediction.

Depositional model, facies analysis and reservoir quality in a wave-modified clastic lacustrine system: The early cretaceous Tongbomiao formation of the Tanan Depression, Tamtsag Basin, Mongolia

The Tanan Depression has experienced multi-phase tectonic movements and formed various types of reservoirs, which has brought some difficulties to proper oil exploration. This study aims to characterize the depositional facies and petrophysical properties of clastic reservoirs in the Tanan Depression and to determine the most prolific areas for exploration based on the analyses of core samples, thin sections, logging data, petrophysical properties, seismic profiles, and paleontological data. A wind-wave controlling sedimentary model and seven primary depositional facies, namely: 1) alluvial fan; 2) braided delta plain; 3) braided delta front; 4) wave-modified fan-delta front; 5) shoreface; 6) storm deposits; and 7) sublacustrine fan, were revealed in this study. The predominant wind direction during the deposition of the upper Tongbomiao Formation was from the southeast (SE), and the resulting wave-modified sandbodies mainly extended along the paleoshore line of the lake in a NE-SW direction. Facies of weakly-to not-modified depositional settings (alluvial fan, braided channel, braided delta front, and sublacustrine fan) were mainly characterized by high matrix content, poor grain sorting, and poor maturity. Conversely, facies related to wave-dominated depositional settings (wave-modified fan delta front, shoreface and storm deposits) are characterized by low matrix content, higher grain sorting, and higher maturity. A positive correlation between reservoir quality and wave-modified intensity of sandstones is evidenced in the study area. Sandbodies that have experienced wave modification, early carbonate cementation, and dissolution served as the main reservoirs for oil accumulation. Therefore, future oil exploration in the Tanan Depression should focus on clastic sedimentary bodies reworked by waves. Moreover, the facies distribution model and high-quality reservoirs forming mechanism in the Tanan Depression could provide a significant reference for oil exploration and development in other similar basins.

The Upper Triassic Braided River Thin-Bedded Tight Sandstone in the Yanchang Formation, Ordos Basin: Sedimentary Characteristics, Seismic Forecasting Method, and Implication

In the Ordos Basin, Chang 81, a Member of the Yanchang Formation, features the development of braided river thin-bedded tight sandstones. These sandstones constitute one of the main production layers of tight oil and gas in the Yanchang Formation within the basin. This study integrates data from core samples, drilling, and seismic information to identify braided river thin-bedded sandstones in the Chang 81 Member at Daijiaping, Ordos Basin, using a method of constrained correlation between seismic waveform and seismic facies. This approach aids in determining the sedimentary microfacies types and reservoir characteristics of thin-bedded tight sandstones. We establish a quantitative fitting formula for the width-to-thickness ratio of braided channel sand bodies to finely characterize sand body stacking patterns and spatial distribution of thin-bedded tight sandstones in braided channels. Braided delta plain deposits in the Chang 81 Member at Daijiaping mainly comprise four types of sedimentary microfacies: braided channels, crevasse channels, floodplains, and swamps. The thickness of the reservoir sand body of Chang 81 member is mainly concentrated between 5–25 m, with low porosity and permeability, making it a typical thin-bedded tight sandstone reservoir. A method of constrained correlation between seismic waveforms and seismic facies was employed to identify sand bodies of braided river thin-bedded sandstones in the Chang 81 Member, summarizing four sand body stacking patterns: longitudinal incision type, longitudinal separation type, lateral shifting type, and single channel type. Furthermore, a quantitative forecasting formula of width-to-thickness ratio was established for the river channel scale, providing accurate guidance for well deployment. Horizontal wells deployed from the sand body’s side towards its center in a river channel yield a production 1.8 times higher than that of horizontal wells deployed in the opposite direction. Thin-bedded tight sandstones in braided channels, characterized by flat-top and convex-bottom lenticular seismic facies, hold practical significance in guiding the deployment of horizontal well patterns for tight oil and enhancing oil and gas recovery.

The Facies Analysis, Evolution, and Coal-Bearing Source Rock Features of the Middle–Late Triassic Shallow-Water Delta in the North Carnarvon Basin, Northwest Shelf of Australia

The sedimentary facies, architecture, and depositional mechanism of deltaic systems have been one of the global research hotspots in recent decades; however, the detailed distribution, sedimentary evolution, source rock potential, and major control factors of the coal-bearing shallow-water delta are still unclear. A typical shallow-water braided delta with coal-bearing source rocks developed in the Middle–Late Triassic Mungaroo Formation of the North Carnarvon Basin, which can be a good study area for an analysis of coal-bearing source rocks. In this study, the sedimentary facies, distribution and evolution, and coal-bearing source rock features of the Triassic strata were analyzed based on the integrated study of wireline logs, drilled cores, thin sections, seismic facies and attributes, and geochemical data. A range of shallow-water delta sedimentary facies was identified, including the proximal delta plain channel/interdistributary bay, distal delta plain channel/interdistributary bay, and the delta front. The coal-bearing shallow-water delta system of the Middle–Late Triassic Mungaroo Formation was characterized by the largest scale delta system with relatively broad proximal and distal delta plains and relatively narrow delta front subfacies. The scale of the delta system showed a trend of increasing from the Early Triassic Locker Shale to the Middle–Late Triassic Mungaroo Formation and then decreasing to the Late Triassic Brigadier Formation. The distal delta plain subfacies of the Mungaroo Formation should have the highest potential coal-bearing source rock, and the proximal delta plain also can be a favorable target for source rock evaluation. The major control factors of the coal-bearing source rocks of the Mungaroo shallow-water delta mainly included the Triassic megamonsoon climate, the topographic features, eustatic changes, and provenance supply. The proximal and distal delta plains of the shallow-water delta system with thin coal seams, carbonaceous mudstone, and dark mudstone lithologies’ association could be a favorable source rock exploration facies for the next stage of natural gas field exploration.

A record of enhanced water cycle in the late Paleozoic icehouse

The late Paleozoic ice age was the longest icehouse of the Phanerozoic with complex terrestrial ecosystems and metazoan life. Simulations of late Paleozoic climate have confirmed the coupled relationship between Gondwanan ice dynamics and paleotropical climate on a multitude of timescales. However, it is still unclear how precipitation in the tropical zone would respond to changes in paleoclimate during the late Paleozoic ice age. Here, we present detailed sedimentological and biostratigraphic works on a late Carboniferous mixed carbonate-siliciclastic succession from the southeastern South China Block to explore sedimentary responses to paleoclimate variations. Conodont and fusulinid biostratigraphy indicate that the Outangdi succession formed during the middle to late Moscovian. Detailed facies analysis suggests that the succession was deposited on a carbonate ramp, episodically punctuated by braid delta sedimentation. Given the relatively stable tectonic setting and well-correlated, coeval siliciclastic deposition in the paleotropical peri-Paleotethys regions, the abrupt increase in siliciclastic influx in the late Moscovian resulted from an enhanced precipitation event associated with the glacial-to-interglacial shift. • An abrupt siliciclastic influx were recorded in tropical peri-Paleotethys regions in the late Moscovian • Water cycle would be enhanced in the paleotropical zone responding to the demise of glacial interval • Anomalous siliciclastic deposits could have been driven by climate factors.

Implication Linkage among Microfacies, Diagenesis, and Reservoir Properties of Sandstones: A Case Study of Dongying Formation, Nanpu Sag, Bohai Bay Basin

The reservoir quality of sandstones is significantly impacted and transformed by sedimentation and diagenesis. It is necessary to clarify the internal relationship among them to precisely predict the sweet reservoir. In this study, five types of sedimentary microfacies are recognized through core observation and logging data: submerged distributary channel (fan delta), submerged interdistributary bay, submerged distributary channel (braided delta), distal bar, and turbidite fan. The major diagenetic processes, including compaction, cementation, and dissolution, have been analyzed based on petrography, scanning electron microscopy, and X-Ray diffraction. The dominant diagenetic cement includes calcite, smectite, kaolinite, illite, and I/S mixed-layer minerals, with small quantities of chlorite, pyrite, siderite, feldspar, and quartz cement. The reservoir quality is best in the submerged distributary channel (fan delta) sandstones, followed by submerged distributary channel (braided delta). Submerged interdistributary bay, distal bar, and turbidite fan are of poor reservoir quality. The grain size is the primary reservoir quality controlling factor, highly affected by sedimentary microfacies. Subsequent controls are diagenetic processes such as mechanical compaction, clay minerals formation, grain replacement, and dissolution that collectively influence the porosity and permeability.

Dinosaur tracks in a Cretaceous (lower Albian) braid delta system (Basque–Cantabrian Basin, western Pyrenees): linking trace fossils suites and short-term preservation windows

Abstract Albian dinosaur tracks from the Monte Grande Formation (Basque–Cantabrian Basin) are described. Sedimentary succession shows seven different facies associations (FAs) with four track-bearing levels (levels 1–4) of a braid delta system. FA 4, a proximal crevasse subdelta, preserves in levels 2 and 3 theropod, possibly sauropod and undetermined dinosaur tracks, as well as Ophiomorpha and Skolithos traces. FA 5, a middle crevasse subdelta, presents undetermined cross-section tracks in levels 1 and 4, besides Ophiomorpha , Skolithos and locally Teichichnus in other layers. Both FAs have the same general ichnoassociation, although with preservational variations. The sedimentary succession represents an area with periods of crevasse subdelta progradation and colonization of opportunistic organisms, and other periods of subdelta abandonment represented by short sedimentary hiatuses in which the water sheet would be shallow or even absent and where dinosaurs would be roaming. Particularly, the traces of levels 2 and 3 represent the same ichnocoenosis constituting two different suites and may be envisaged as penecontemporaneous, produced within a short-term preservational window. This tracksite is one of the scarce Albian dinosaur ichnological records in the southwestern European island palaeoarchipelago and one of the few worldwide localities where Albian dinosaur tracks have been found in deltaic facies.

The depositional model of the Lower Cretaceous Nantun Formation and its implications for oil accumulation in the northern Wuerxun Sag of Hailar Basin, NE China

The northern Wuerxun Sag is a lacustrine rift basin which experienced multiple faulting and subsidence during the Lower Cretaceous. The tectonic evolution with uplifted regions as the provenances caused the complex sedimentary fill of the Lower Cretaceous Nantun Formation. In this paper, we use well log, core and 3D seismic data to characterise the sedimentary facies and depositional environment of the Nantun Formation, and consider how this affects the distribution of hydrocarbons within the basin. Four depositional systems are recognised in this study: fan deltas located along the steep slopes; braided deltas along the gentle slopes; sublacustrine fans and lacustrine in the centre of the basin. Fan deltas are mainly composed of coarser sediments with lower sediment maturity than braided deltas, and are mainly located in steep slopes with rift border faults. Sublacustrine fans form lenticular depositional bodies in the basin centre, with higher sediment maturity as a result of reworking. Three sediment-transport pathways were recognised which governed the sediment deposition in this basin, which are (1) the steep rift border fault in the northwest, (2) the NE–SW Surennuoer strike-slip fault zone (SSFZ), and (3) the major faults in the south-eastern gentle slope. Most oil is found in sublacustrine fan sandstones which were transported along the SSFZ mainly from shallower fan deltas, with underlying mudstones (SQN1) as effective source rocks and overlying thick and laterally continuous lacustrine mudstones (SQD1) as seal rocks. The good source–reservoir–seal assemblages enable prolific near-source oil distribution in this area, suggesting the sedimentary controls on oil accumulation, which may guide future oil exploration and production activities in this area.

Sedimentary Characteristics of the Neogene in the Eastern Liaodong Area, Bohai Sea, China

The sedimentary characteristics of the Neogene in the eastern Liaodong area are poorly studied, which restricts the oil and gas exploration in this area. Based on the current seismic, logging, drilling, and core data, we have studied the sequence stratigraphy and sedimentary characteristics of the Neogene in the eastern Liaodong area. Ultimately, we identified the controlling factors of sedimentation and established the depositional model. In this study, six sequence boundaries of the Neogene were identified and can be divided into five third-order sequences. The sedimentary evolution can be divided into four stages: braided river delta, meandering river delta with a low sand-stratum ratio, meandering river delta with a high sand-stratum ratio, and meandering river delta. Both the palaeotectonic movements and paleoclimate changes of the Neogene influence the periodical fluctuation of the lake level and then control the vertical evolution of the sedimentary types. The paleogeomorphology controls the macroscopic distribution of the sedimentary facies zone, the paleo-valleys that are associated with the strike-slip faults control the channel strike, and the fault slopes at the edge of the lacustrine basins control the sand redistribution of the delta front.

Sedimentology and Stratigraphy of a Large, Pre-Vegetation Deltaic Complex

The operation of depositional processes in fluvial systems prior to the development of land-based, macrophyte vegetation has undergone considerable research. The differences in sedimentation in the channels and floodplains compared to post-vegetation systems should result in differences also existing in the deposits developed where the rivers debouch their loads into standing bodies of water. These deltaic deposits have been studied relatively less. The 1.4 Ga Kama Hill and Outan Island Formations of the Sibley Group, northwestern Ontario, provide a site to further investigate these potential differences. Four totally cored drill holes intersect this 340 m thick, prograding, deltaic to fluvial succession. It is divisible into eight lithofacies associations representing environments ranging from prodelta to the delta-top floodplain-channel assemblage. The sub-aqueous portion of the succession is composed of two thick progradational delta lobes probably caused by: deposition induced regression followed by a condensed sequence due to channel avulsion with distal repositioning, and finally channel avulsion with proximal repositioning driving another depositional induced regression. Large-scale sequence stratigraphic nomenclature does not properly describe this succession. At a smaller scale, members of progradational parasequence sets decrease in thickness from the prodelta to upper distributary-mouth bars. The deltaic deposits are compared to the modern, large Mississippi deltaic system. Sub-aqueous sub-environments are similar to this, except: 1) the lack of infauna bioturbation leads to better preservation of Sibley Delta sedimentary structures; 2) scarce organic sediment does not allow reducing diagenetic reactions, thus preserving the highly oxidized nature of the fluvial sediments; 3) the Sibley distributary-mouth bars have more medium- and coarse-grained sand than the Mississippi bars; 4) the Sibley distributary-mouth bars have abundant thick, massive, non-graded, normal graded and reverse graded sandstones, likely the result of high-discharge slurry-flows from the fluvial channels; 5) the lack of macrophytes (coastal marshes) complicates interpreting the transition from bar tops to sub-aerial environments; 6) the abandonment phase of bay-fill progradational parasequences is further condensed due to the lack of organic deposits. However, the considerable similarities between the Sibley Delta and the Mississippi Delta denote that not all pre-vegetation deltas conform to general braid and fan delta models.

Braided deltas and diagenetic control on tight sandstone reservoirs: A case study on the Permian Lower Shihezi Formation in the southern Ordos Basin (central China)

The Lower Shihezi Formation, situated in the south of the Chinese Ordos Basin, is a tight sandstone reservoir that is generally influenced by its sedimentary facies and diagenesis, corresponding to distinct petrophysical reservoir properties for each lithofacies type. The existing lithofacies types within the Lower Shihezi Formation were determined through observations in field profiles and drill cores that were combined with the analysis of logging data. Diagenetic processes and their effects were analyzed by using different methods, such as conventional thin section, cast thin section, and scanning electron microscope (SEM) analyses. The results of this contribution indicate that the Lower Shihezi Formation in the southern Ordos Basin is dominated by braided delta facies and locally developed lacustrine facies. During diagenesis of this studied formation, compaction and cementation led to a reduction in porosity with compaction probably forming the main factor. Existing cementation is mainly manifested in the form of clay mineral cements, siliceous and carbonate cements, and induced a somewhat lower pore reduction effect on the reservoir compared to compaction. Minor dissolution led to the break-down of rock fragments, feldspars, and other soluble minerals, and thereby may have caused a subsequent formation of secondary pores, thus effectively improving the reservoir property. Consequently, due to the compositional differences and corresponding unique diagenetic effects, the distributary channels and underwater distributary channels display the highest reservoir properties. These are followed by the mouth bars and sand sheets. The lowest reservoir properties are shown by the natural levee microfacies.

Influencing Factor Analysis on the Fractured Tight Sandstone Gas Reservoir Characteristics: A Case Study of Bozi 3 Gas Reservoir in the Tarim Basin

The Bozi 3 gas reservoir in the Tarim Basin is a typical ultra-deep fractured tight sandstone gas reservoir, in which the main formations are the Cretaceous Bashijiqike Formation and the Baxigai Formation. The reservoir has the characteristics of deep burial, tight reservoir matrix, well-developed fractures, large differences in well productivity, and regional pressure distribution. These characteristics bring development challenges in well deployment, production allocation, gas production rate optimization, and production improvement. The main reservoir of Bozi 3 is deposited in the braided river delta, fan delta, and shallow lake environment with adequate provenance and a strong hydrodynamic environment, which provides favorable depositional conditions for the formation of thick sandbodies. Although the buried depth of the Bozi 3 gas reservoir is over 6,000 m, it has undergone an evolutionary process of long-term shallow burial in the early stage and rapid deep burial in the late stage, which protects the primary pore, and the finally formed reservoir has the characteristics of an extra-low pore and ultra-low permeability pore structure. Late tectonic movements caused the Bozi 3 gas reservoir to experience strong uplift under a strong nappe-thrust environment, forming a large number of high-angle fractures around the faults. Although the fracture distribution is inhomogeneous, tensile fractures at the top anticline have a low-filling degree and good effectiveness, which effectively improve Bozi 3 reservoir properties. Through analysis of the reservoir characteristics and influencing factors of the Bozi 3 gas reservoir, it is concluded that the difference in reservoir characteristics is the main reason for the large difference in production of adjacent wells and zonal distribution of formation pressure at the early development. This study can also provide a reliable geological basis for well deployment, production allocation rationalization, and gas production rate optimization in the study area.

The earliest <i>Ancistrolepis</i> (Gastropoda: Buccinidae) and its geologic implications

The buccinid gastropod Ancistrolepis carolineae Squires, 1984 is the earliest known and shallowestmarine species belonging to the extant genus Ancistrolepis Dall, 1895, which is endemic to the NorthPacific region. This species is of late early Eocene (Ypresian Stage) age and is about 7 million years older than the previously oldest known record (late Eocene) of the genus. This rare species occursat several localities in a 1-m thick bed (“Stewart bed”) of fossiliferous shallow-marine sandstone within the Llajas Formation, on the north side of Simi Valley, Ventura County, southern California.The “Stewart bed” contains a rich fauna of subtropical mollusks and other invertebrates, which lived just below effective wave base, at the distal edge of a braid delta, immediately adjacent to an upper bathyal prodelta/slope environment, where a rich microfauna of calcareous nannofossils and benthic foraminifers lived. Dispersal of A. carolineae would have been either via drifting of buoyant “pouches” containing its non-planktonic eggs or by rafting of its adults in buoyant plant material debris. The dispersal of A. carolineae northward coincided with the cooling of the NorthPacific waters during the late Eocene, thereby providing the opportunity for Ancistrolepis to adapt to living in bathyal waters and also to living in association, in some cases, with chemosynthetic (methane) cold seeps.

Lithologic Hydrocarbon Accumulations in the Upper Jurassic Kalazha Formation in the Foreland Slope, the South Marginal Junggar Basin, Northwest China

The foreland slope in the south marginal Junggar Basin (SMJB) is geologically favorable for mass lithologic oil and gas reservoir accumulation. Based on outcrops, thin sections, individual-well facies, and cross-well stratigraphic correlation, the stratigraphic distribution and features of the Upper Jurassic Kalazha Formation were investigated in the SMJB. There are two sedimentary sources in the southwest and southeast Kalazha Formation, which thin out from south to north and vanish in the northern slope. Fan deltas occur in the southwest, and braided river deltas occur in the southeast. Kalazha reservoir rocks and sedimentary facies were predicted using joint 2D and 3D seismic impedance inversion. According to the overall analysis, the Kalazha Formation in the foreland slope is favorable for hydrocarbon accumulation, considering structural highs as the destination of long-term hydrocarbon migration and the updip wedge-out zone in the north with potential lithologic traps. The accumulation model was established with source rocks, reservoir rocks, caprocks, and migration systems for the lithologic hydrocarbon reservoirs in the Kalazha Formation in the SMJB. As per the model and regional stratigraphic and structural features, Kalazha lithologic hydrocarbon reservoirs mainly formed in the annular updip wedge-out zone in the foreland slope. Our deliverables may provide useful information for mass lithologic hydrocarbon accumulation in the foreland slope and deep to ultra-deep hydrocarbon exploration in the SMJB.

The sedimentary characteristics and depositional evolution in the Middle Jurassic Xishanyao Formation, Southeastern Xiayan Rise, Junggar Basin, NW China

The significant hydrocarbon discoveries have been found during the past few years in Middle Jurassic Xishanyao Formation in Southeastern Xiayan Rise, Junggar Basin. The braided delta front deposits and shore‐shallow lacustrine deposits are well preserved in Xishanyao Formation, their history being documented by integrated analysis of seismic volumes and borehole datasets (wireline logs, cores, grain size etc). The results reveal that five sedimentary microfacies are identified. Among them, the braided delta front deposits include subaqueous distributary channel, inter‐distributary channel and mouth bar, while shore‐shallow lacustrine deposits consist of lacustrine mud and swamp. In addition, depositional evolution of braided delta front deposits and shore‐shallow lacustrine deposits from bottom to top in Xishanyao Formation is characterized by retrogradation and progradation during depositional period. The depositional evolution during depositional period of Xishanyao Formation is controlled by interactions of the source and distribution of provenance, the catchments, the lacustrine fluctuations and the climates.

Lithofacies paleogeography restoration and its significance of Jurassic to Lower Cretaceous in southern margin of Junggar Basin, NW China

In view of the difficulties in the study of lithofacies paleogeography and the low reliability of the distribution range of sedimentary sand bodies in the prototype basin caused by less deep drilling, complex seismic imaging and low degree of exploration in the southern margin of Junggar Basin, NW China. A new method based on the source to sink idea was used to restore lithofacies paleogeography and predict glutenite distribution. In the restoration, apatite fission track age was used to define range and uplift time of macro-provenance; the range of provenance area and the migration process of lake shoreline were restored based on the quantitative relationship between gravel diameter and transportation distance, tectonic shortening and other geological parameters; drilling cores and field outcrop sedimentary structures were analyzed, and a series of maps of lithofacies paleogeographic evolution and distribution range of glutenite bodies were compiled. It is concluded that from Early Jurassic to Early Cretaceous, in the southern margin of Junggar Basin, the provenance area gradually expanded from south to north, the lake basin expanded, shrunk and expanded, and the paleoclimate changed from humid to drought to humid. The western section always had proximal fan delta deposits from the southern ancient Tianshan provenance developed, and in the middle and eastern sections, the provenance areas evolved from far source to near source, mainly river-delta, braided delta, fan delta and other sediments developed. The boundary between provenance areas of the western and middle sections is speculated to be Hongche fault zone. In an angle open to the northwest with the current basin edge line, the restored ancient lake shoreline controlled the heterogeneity of reservoirs in the delta plain belt and delta front belt on its both sides. The ancient lake shoreline, current stratigraphic denudation line and current basin margin line limit the types and scope of favorable reservoirs. This understanding provides an important geological basis for oil and gas exploration in the deep lower source-reservoir assemblage at the southern margin of Junggar Basin.

Study on Sedimentary Characteristics of Chang 9 Member of Triassic in Ordos Basin

The Triassic Yanchang Formation in the Ordos Basin is one of the main exploration targets. Based on the detailed exploration of the outcrop at the southern margin of the Ordos Basin, combined with previous results, a systematic analysis of lithology, rock assemblage, sedimentary facies characteristics and temporal evolution is carried out. The results show that the Changchang 9 oil-bearing group on the southern margin of the Ordos Basin can be divided into 9 lithofacies types, and three types of sedimentary facies, meandering river, meandering river delta, and braided river delta are developed. The meandering river facies develops riverbed retention deposits, beaches, and There are four types of microfacies in natural dikes and rupture fans. There are five types of microfacies in meandering river deltas: underwater distributary channels, underwater natural dikes, divert bays, estuary dams, and remote sand dams. Braided river deltas develop underwater distributary channels, There are two kinds of microfacies between distributary channels; the Chang 9 oil-bearing group has a coarse-fine-coarse compound rhythm in the vertical direction; meandering river sand bodies and delta front sand bodies are the main places for oil and gas migration and accumulation.

Estimating Reservoir Properties from 3D Seismic Attributes Using Simultaneous Prestack Inversion: A Case Study of Lufeng Oil Field, South China Sea

Summary Lufeng oil field was discovered in the 1980s in the Pearl River Mouth Basin (PRMB) of the South China Sea and has entered the stage of secondary oil recovery. One major problem that has restricted the subsequent exploration of the oil field is the unclear regional reservoir and caprock distribution, because practitioners have been using post-stack attributes and acoustic impedance inversion to analyze the distribution of sandstone (reservoir) and mudstone (caprock). In current geophysical research on reservoirs and caprocks, prestack inversion has been widely used because of its advantage over post-stack inversion. However, the accuracy of using single P/S-wave velocity ratio (VP/VS) or density (Vden) inverted by prestack to predict lithology is still insufficient. In this study, we created a new attribute VRDEN, the sum of VP/VS and the weighted Vden, to capture the lithology variation of the reservoir. We integrated 3D seismic data and well log data and applied simultaneous prestack inversion and multiattribute regression analysis to determine reservoir properties, such as sand thickness, effective porosity, and distribution of sandstone and mudstone of the Lufeng oil field. Then, we calculated the new attribute VRDEN from VP/VS and Vden obtained from simultaneous prestack inversion to determine the lithology variation. The multiattribute regression analysis, combining prestack attributes and post-stack attributes, indicates the effective porosity and sand volume in the Enping Formation, which contains the main oil-bearing reservoirs in the Lufeng oil field. Results show that when the sandstone thickness is greater than 12.5 m, the prediction error of VRDEN is the lowest compared with VP/VS and Vden. In En-2 member of the Lower Enping Formation, medium- to high-porosity (14 to 17%) sandstone (19.5 m thickness) is widely distributed in the west and middle of the study area, with an area of nearly 300 km2. The high-porosity sand zones stretch from the east of the Lower Enping Formation to the west of the Upper Enping Formation, which is the result of westward progradation of the braided delta. Our workflow used a novel seismic attribute VRDEN in the simultaneous prestack inversion and multiattribute regression process to provide a more predictive spatial distribution of reservoir-nonreservoir features. The improved reservoir understanding will allow more efficient exploitation of the Lufeng oil field, and the improved workflow will facilitate exploration of other oil fields in the world.