Subaqueous Distributary Channel Research Articles

Impacts of sedimentary characteristics and diagenesis on reservoir quality of the 4th member of the Upper Triassic Xujiahe formation in the western Sichuan basin, southwest China

The 4th member of Xujiahe Formation (Xu4) in the western Sichuan Depression is a typical porous tight sandstone reservoir. Finding reservoirs with relatively high porosity controlled by sedimentation and diagenesis is crucial for successful tight gas exploration. In this study, the Xu4 sandstone was studied by diverse methods, including outcrop and core observation, logging data interpretation, thin section and cathodoluminescence examination, scanning electron microscopy analysis, and X-ray diffraction, to determine sedimentary and diagenetic characteristics and to discuss their impact on the sandstone reservoir quality. During the depositional period of Xu4, subaqueous distributary channel sandstone of braided river delta front developed extensively under controls of bidirectional provenance (LASP and SASP), mainly consisting of litharenite and sublitharenite, with low feldspar and high rock fragments volume, respectively. Strong compaction and carbonate cementation have largely destroyed primary porosity, while secondary pores constitute the main pore space in the Xu4 sandstone. The sediment provenance, sedimentary microfacies and A/S determine the clastic composition of sandstone, genetic type and thickness of sand bodies, furthermore affect the subsequent diagenetic variations. The larger the thickness of the sand body indicates a higher-energy depositional environment, coarser grain size, better sorting, and lower shaliness content of the sandstone during deposition period. This results in stronger resistance to compaction of the sandstone, which is conducive to the preservation of primary pores and dissolution of organic acids, resulting in better physical properties of sandstone. Among them, the physical properties of the sandstones of the Xu4s and Xu4x are better than those of the Xu4z; moreover, LASP's sandstone is superior to SASP. It is worth noting that subaqueous distributary channel sandstones with thickness exceeding 13 m in Xu4s and Xu4x from LASP represent favorable areas for high-quality reservoirs. The work can be used as a reference for the exploration and evaluation of similar tight sandstone gas in the world.

Genetic mechanisms of high-quality tight siliciclastic reservoirs: A case study from the Upper Triassic Xujiahe Formation in the Yuanba area, Sichuan Basin, China

This study analyzed the petrological characteristics, diagenesis, pore types, and physical properties of the tight coarse-grained siliciclastic sequences in the third member of the Upper Triassic Xujiahe Formation (also referred to as the Xu-3 Member) in the western Yuanba area in the northeastern Sichuan Basin, China, based on the results of 242.61-m-long core description, 292 thin-section observations, scanning electron microscopy (SEM), and 292 physical property tests. The types and genetic mechanisms of high-quality tight coarse-grained siliciclastic reservoirs in this member was determined thereafter. The research objective is to guide the exploration and development of the tight coarse-grained siliciclastic sequences in the Xu-3 Member. The results of this study are as follows. Two types of high-quality reservoirs are developed in the coarse-grained siliciclastic sequences of the Xu-3 Member, namely the fractured fine-grained sandy conglomerate type and porous medium-grained calcarenaceous sandstone type. Hydrodynamic energy in the sedimentary environment is the key factor controlling the formation of high-quality reservoirs. These high-quality reservoirs are developed mainly in the transitional zone with moderately high hydrodynamic energy between delta-plain braided channels and delta-front subaqueous distributary channels. The dolomitic debris (gravel) content is the main factor affecting the reservoirs’ physical properties. The micritic algal debris and sandy debris in the dolomitic debris (or gravels) tend to recrystallize during burial, forming intercrystalline pores within. In the medium-grained calcarenaceous sandstones, intercrystalline pores in the dolomitic debris are formed at the early diagenetic stage, and a pore system consisting of structural fractures connecting intergranular pores, intergranular dissolution pores, and kaolinite intergranular micropores is developed at the late stage of diagenesis. The formation of intercrystalline pores in dolomite gravels and gravel-edge fractures, a pore system connected by gravel-edge and tectonic fractures, is closely related to the dolomite gravels in the sandy fine-grained conglomerates.

Sedimentary texture differences between lithofacies types and their control on the physical property in fan-delta conglomerate reservoirs: A case study from the Triassic Baikouquan Formation in Mahu Sag, Junggar Basin, Northwestern China

Conglomerate reservoir developed in fan-delta environment is characterized by the diverse lithofacies types and strong heterogeneity. Lithofacies type is the main primary factor that controls the reservoir physical properties. However, quantitative sedimentary texture parameters of lithofacies types are scarcely studied, and their influence on the reservoir physical properties are still poorly understood. In this paper, lithofacies type, sedimentary texture and pore type, porosity and permeability of the Baikouquan Formation in Mahu Sag were studied to reveal the control of sedimentary texture parameters on the reservoir physical properties.The results showed that (1) lithofacies types in Baikouquan Formation can be classified into Gcm1, Gcm2, Gcm3, Gcm4, Gc, Sm1, Sm2, MSm, SSr, SSp and Mh; (2) different sorting coefficient and matrix content are present in Gcm, Gc, Sm1, Sm2 and MSm. These two texture parameters are highest in Sm2 and MSm, below Gcm and Gc, and the lowest in Sm1; (3) intra-gravel microfracture, gravel-edge microfracture and intra-matrix microfracture develop in conglomerates, which can improve their permeability; (4) porosity in Gcm, Gc, Sm1, Sm2 and MSm and permeability in Sm1, Sm2 and MSm have negative correlations with their sorting coefficient and matrix content. However, permeability in Gcm and Gc have poor correlations with these two texture parameters due to microfractures; (5) Gc and Sm1 have relatively high porosity and permeability due to their good sorting, low matrix content and consequently strong dissolution. By contrast, Gcm, Sm2 and MSm have relatively low porosity and permeability due to their bad sorting, high matrix content and consequently weak dissolution. These results indicate that Gc and Sm1 deposited in subaqueous distributary channels and lobes should be the first target in hydrocarbon exploration of fan-delta conglomerate reservoir.

Coupling Relationships between Sedimentary Microfacies, Sand Bodies, and Tectonic Fracture Characteristics in Braided River Deltas: A Case Study of the Bashijiqike Formation in the Keshen 2 Area

Fractures are crucial as main natural gas transport channels in tight sandstone reservoirs. In order to reveal the correlation between the combination of fractures in Block Keshen 2 and sandstone, we have collected drilling core data, logging curve data, imaging logging data, and rock thin-section data from the Bashjiqike Formation in the Keshen 2 area, and by classifying and statistically analyzing the different influencing factors of fractures, we have established a correlation between the development of fractures and sandstone thickness, lithology, and sedimentary microfacies. The results reveal the following: (1) frequent vertical superposition and lateral migration occur in the sedimentary sand bodies of the Bashijiqike Formation. Three types of patterns of sand bodies have been identified according to the changes in microfacies. Type I refers to the patterns of sand bodies developed in main subaqueous distributary channels, type II refers to the patterns of sand bodies developed in secondary subaqueous distributary channels or mouth bars, and type III refers to the patterns of sand bodies developed in isolated subaqueous distributary channels; (2) three types of fracture patterns have been described in the various sand bodies of the Bashijiqike Formation in the Keshen area, including high–medium-angle branch-like fracture patterns, medium-angle reticular fracture patterns, and isolated fractures; (3) the coupling relationship among sedimentary microfacies, sand body patterns, and fracture patterns has been established. The high–medium-angle branch-like fracture patterns mainly develop in the main underwater distributary channel and the type I sand body patterns. The medium-angle reticular fracture patterns mainly develop in the secondary underwater distributary channel and mouth bars, as well as the type II sand body patterns. Isolated fractures can occur in all sedimentary microfacies but are sporadically distributed within the three types of sand body patterns. The research results present the regularity of fracture development in fractured reservoirs, which can be applied to oil and gas fields with the same background, providing certain geological evidence for exploration and development.

The delta-fluvial evolution of a lacustrine basin with gentle slope and low sedimentation rate: A case study of the Fudong Slope, Junggar Basin, Northwest China

Gentle slope belts of lacustrine basins are important areas for the generation of stratigraphic petroleum reservoirs controlled by sedimentary systems. Based on coring, well logging, 3D seismic and analytical test data, the sedimentary system and sandbody distribution characteristics of the Jurassic Toutunhe Formation (J2t) in the Fudong slope of the Junggar Basin were analysed. The results indicated that the Fudong slope showed the characteristics of low sediment input. Sediments mainly originated from the Shaqi uplift, and the sediment input gradually weakened from the early to the late J2t stages. The sedimentary system evolved from a delta to a meandering river, resulting in a delta front facies in the 1st member (J2t1) and a meandering river facies in the 2nd (J2t2) and 3rd (J2t3) members of the Toutunhe Formation. A sedimentary evolution pattern for the gentle slope with low sediment input was established. Thick deltaic sandbodies developed locally in the J2t1 stage. However, meandering rivers with high sinuosities predominantly developed and resulted in a striped geometry in the J2t2 and J2t3 members. Moreover, the meandering sandbodies exhibited an anastomosing morphology after multistage superposition. The channel and point bar facies in J2t2 were more developed than those in J2t3 because of sediment input differences. Under low-sediment input conditions, the sedimentary environment of the gentle slope belt in the lacustrine basin was controlled by changes in the palaeoclimate, palaeogeomorphology, gradient (slope) and base level. Palaeoclimate was the main factor controlling the evolution of the sedimentary environment. Palaeogeomorphology and slope controlled the distribution of sedimentary microfacies, channel sinuosity and sandbodies. Base-level change restricted the sandbody superimposition style. Under the low sediment input conditions of the Fudong slope, the subaqueous distributary channels of the delta environment and the point bars of the meandering river environment in the transitional belt of the Fudong slope are the main potential targets for stratigraphic petroleum reservoirs, whilst the 2nd member of the Toutunhe Formation (J2t2) is the main favourable interval for stratigraphic reservoir exploration.

Diagenesis, Diagenetic Facies and Their Relationship with Reservoir Sweet Spot in Low-Permeability and Tight Sandstone: Jiaxing Area of the Xihu Sag, East China Sea Basin

The optimization of reservoir sweet spots is the key to the efficient exploration and development of low-permeability and tight sandstone gas reservoirs. However, offshore deep, low-permeability and tight sandstone has the characteristics of large burial depth, large diagenesis heterogeneity and prominent importance of diagenetic facies, which make it difficult to predict reservoir sweet spots. This work comprehensively used logging data, core observation, conventional core analysis, thin section, powder particle size analysis, clay X-ray diffraction analysis, cathode luminescence analysis, scanning electron microscopy and energy spectrum analysis and carried out the study of diagenesis, diagenetic facies and reservoir sweet spots of low-permeability and tight sandstone of H3 and H4 (the third and fourth members of Huagang Formation) members in the Jiaxing area of the Xihu Sag. The results show that the H3 and H4 sandstones were divided into five diagenetic facies types, and chlorite-coated facies and dissolution facies were favorable diagenetic facies belts. The H3 member mainly develops chlorite-coated facies, dissolution facies and quartz-cemented facies, whereas the H4 member primarily develops quartz-cemented facies and chlorite-coated facies. The percentages of type I sweet spot, type II1 sweet spot and type II2 sweet spot in the H3 reservoir are approximately 21%, 23% and 26%, respectively, whereas the percentages of type I sweet spot, type II1 sweet spot and type II2 sweet spot in the H4 reservoir are about 16%, 15% and 16%, respectively. The distribution rules of reservoir sweet spots were investigated. Type I sweet spot was mainly developed in the areas of chlorite-coated facies and dissolution facies of medium sandstone and coarse sandstone in the channel bar and braided channel sedimentary microfacies. Type II sweet spot was primarily distributed in the areas of quartz-cemented facies, chlorite-coated facies and minor dissolution facies of medium sandstone, fine sandstone and sandy conglomerate in the braided channel, subaqueous distributary channel and channel bar sedimentary microfacies. Type III sweet spot was chiefly developed in the areas of tightly compacted facies, calcite-cemented facies and quartz-cemented facies of fine sandstone, siltstone and a small amount of sandy conglomerate in the subaqueous distributary channel sedimentary microfacies.

Identification of Reservoir Water-Flooding Degrees via Core Sizes Based on a Drip Experiment of the Zhenwu Area in Gaoyou Sag, China

In order to identify the degree of water flooding in a reservoir and to discover any remaining oil-enriched areas, in this paper, a systematic study on the water flooding of cores in obturated coring wells is carried out. With observations and testing data of the cores, based on the notion of sedimentary facies, the water-flooding degrees of 4–7 sand groups in member one of the Paleogene Sanduo Formation (E2s14–7) of the Zhenwu area in the Gaoyou Sag are determined. Overall, the results show that the study area is formed under the background of lake regression, with various sedimentary systems, mainly including delta facies, braided fluvial facies, and meandering fluvial facies. The degree of water flooding is determined using a point-by-point drip experiment of the core. Combined with the testing results of the core, the water-flooding degrees of the different sedimentary facies are quantitatively determined. Identification standards for the water-flooding degree of delta facies, braided river facies, and meandering river facies are established. The water-flooding degree of the delta sand body is generally weak, with an oil saturation rate of 24.1–40.2%, essentially indicating no water flooding or weak water flooding. The water-flooding degree of the braided fluvial sand body significantly changes, and the variation range of the oil and water saturation measurement results is also large. The water-flooding degree of the meandering fluvial sand body is weaker than that of the braided fluvial sand body, which is mostly not flooded or weakly flooded. The water-flooding degree is obviously controlled by the sedimentary rhythm and the sedimentary type. The top of the positive rhythm, the bottom of the sludge bed in the braided fluvial point bar, the deltaic front subaqueous distributary channel, and the point bar in the meandering fluvial have relatively low water-flooding degrees. They are the subjects of subsequent development adjustment and the remaining oil potential tapping.

Fine structure interpretation and reservoir forming characteristics analysis of Jurassic Badaowan Formation in Madong area of Xiayan fault zone

Fine structural interpretation is one of the effective methods to find favorable structural traps and targets. Aiming at the fault interpretation of Jurassic Badaowan Fm in Madong area of Xiayan fault zone is not fine and structural characteristics are not implemented, the structural characteristics of Jurassic Badaowan Fm are implemented and favorable traps are found through the study of seismic, well logging and geological comprehensive methods. The results show that the structure of Jurassic Badaowan Fm in Madong area is high in the north and low in the south. The first member of Badaowan Fm is a delta front subaqueous distributary channel deposit, which is a hydrocarbon enrichment zone. Oil and gas are adjusted along the relay of deep and shallow faults, and the great fault zone and the inherited uplifting zone are favorable accumulation areas of Jurassic Badaowan Fm in Madong area. In this paper, the relationship between deep and shallow fault system and hydrocarbon distribution of Badaowan Fm in Madong area is systematically reviewed for the first time. Combined with the palaeohigh distribution, three favorable hydrocarbon accumulation zones are identified, which provides favorable targets for exploring the hydrocarbon accumulation potential of Badaowan Fm.

Microscopic Characteristics and Formation Mechanism of Effective Reservoirs in the Xihu Depression, China: The Important Role of the Poikilotopic Calcite Cements in Tide-Dominated Delta Systems

The Xihu depression is an offshore sag located on the East China Sea Shelf Basin, which is currently one of the major oil and gas basins along the coast of China. In this study, an integrated approach using thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence (CL), high-resolution 3D CT core scanning and stable isotope analysis was applied to examine the diagenetic evolution and investigate the microscopic characteristics and formation mechanisms associated with effective reservoirs. Four types were distinguished: upper conventional reservoirs (UC reservoirs), lower conventional reservoirs (LC reservoirs), “bottom calcium” low-permeability reservoirs (“bottom calcium” reservoirs) and “MI clay” low-permeability reservoirs (“MI clay” reservoirs). Poikilotopic calcite cements play an important role in the diagenetic alterations and reservoir quality evolution, precipitating during early eogenesis, provided a framework that retards the adverse impacts of UC reservoirs by compaction. Conversely, in LC reservoirs, with limited poikilotopic calcite, secondary porosity is mostly due to the dissolution of feldspar or unstable rock fragments. UC reservoirs normally develop in the middle of tidal channels and in subaqueous distributary channels, with the base of the sand-body being extensively cemented by carbonate cements, such as late calcite, Fe-calcite and dolomite, which formed the “bottom calcium” reservoir. Combined evidence from petrographic and geochemical analyses suggests that calcite precipitates from diagenetic fluids of mixed marine and meteoric waters, with additional external sources from calcareous siltstones and bioclasts. The carbon sources of calcite mostly originate from the dissolution of carbonates clacts or bioclasts within sandstone beds or adjacent silty mudstones, while dolomite cements have an isotopic composition that is more comparable to the generation of biogenic methane. This study demonstrates how poikilotopic calcite, developed in tide-dominated delta systems, affects the vertical heterogeneity. The results can be used to improve the reservoir evolution model of tide-dominated delta systems and provide a basic understanding for researchers conducting reservoir studies of similar sedimentary systems. Our results can act as a geological basis for further oil and gas exploration.

Evaluating the sedimentological and diagenetic impacts on terrestrial lacustrine fan delta sandy conglomerates reservoir quality: Insights from the Triassic Baikouquan Formation in the Mahu sag, Junggar Basin, Western China

The terrestrial lacustrine fan delta conglomerate reservoirs in the Triassic Baikouquan Formation (BF) are the primary hydrocarbon exploration targets in the Mahu sag, Junggar Basin, Western China. However, the varying lithofacies and complex diagenesis controlling the reservoir quality remain poorly understood. For the sake of revealing how reservoir quality is associated with sedimentary processes and diagenetic alterations, we used integrated approaches like petrographical analysis, cores description, wireline, X-ray diffraction (XRD), scanning electron microscopy (SEM), mercury intrusion capillary pressure (MICP) and measured petrophysical properties. The results showed that sedimentary processes have considerable control over facies distribution, grain size, and clay content, causing spatial variation of reservoir quality within the fan delta deposits. The coarser grain and high clay content fraction at the base of gravity flow will first settle in the fan delta plain near provenance, whereas the finer grains at the top of flows progressively deposit in the fan delta front areas. From proximal to distal fan delta, a progressive conversion from cohesive muddy debris flow into traction current can be inferred from the variations of lithofacies associations. A total of 8 types of microfacies were identified where the conglomerates of the BF were deposited. The grayish-green fine-grained conglomerates concentrated in the subaqueous distributary channels provide the best reservoir performance, whereas debris flow deposits dominated primarily by muddy debrites occupy the worst reservoir quality because of their highest clay content and coarsest grain size.The dominant diagenetic events include mechanical compaction that continued throughout the entire burial, two phases of quartz cementation, dissolution of unstable components, early and late carbonate cementation, precipitation of authigenic clay minerals, and two phases of hydrocarbon charging. It is unveiled that compaction is the predominant mechanism negatively affecting reservoir quality in the fan delta complex, while the effect of cementation on pore reduction of the conglomerate reservoir is weaker than compaction. Simultaneously, a complex relationship exists between reservoir petrophysical properties within different clay minerals. Dissolution does not significantly improve the permeability of the reservoir because of the partial dissolution which primarily generated intragranular pores with strong heterogeneity and poor connectivity. The most favorable reservoirs occur primarily in the subaqueous distributary channels in the fan delta front, which comprise well-sorted fine-grained conglomerates with well-developed residual intergranular pores and secondary dissolution pores and have undergone two phases of oil charge. This work may provide a useful reference for hydrocarbon exploration of the fan delta systems and regions which have encountered similar sedimentary and diagenetic processes.

Grain Size Curve Characteristics of 2nd Member of Sangonghe Formation in Qianshao Area and Its Indicative Significance of Hydrodynamic Environment

There are mainly two sedimentary types of the 2nd member of Jurassic Sangonghe Formation (J1s2) in the Qianshao area: shallow-water delta and sandy debris flow. In order to accurately understand the characteristics and further identify the difference of sedimentary microfacies, the characteristics of the grain size curve are analyzed. The results show that the cumulative probability curve of sandstone particle size in the research area mainly includes two basic types: tractive current type and sandy debris flow type. The tractive current type is mainly developed in the shallow-water delta front. Among them, the inner front subaqueous distributary channel microfacies develop two patterns: tri-segment pattern and one bouncing segment-one suspension segment-one complex transitional zone pattern. The transitional section is widely developed. There are two patterns in the subaqueous distributary channel microfacies of the outer front (bi-segment pattern and one bouncing segment-one suspension segment-one transitional zone pattern) and two patterns in the estuary bar microfacies (bi-bouncing segment-one suspension segment-one transitional zone pattern and one bouncing segment-one suspension segment-one transitional zone pattern). The main tractive current characteristics are higher bouncing component content and slope and lower suspended component content. The sand debris flow type mainly develops two patterns: one bouncing segment-one suspension segment-one transitional zone pattern and one bouncing segment-one suspension segment-one complex transitional zone pattern with lower jump component content and slope and higher suspended component content.

Impacts of Sedimentation and Diagenesis on Deeply Buried Reservoir Quality of a Rift Basin: A Case Study of Wenchang Formation in the Lufeng Depression, Pearl River Mouth Basin, China

The deeply buried reservoirs of Wenchang Formation in the Lufeng Depression, Pearl River Mouth Basin, display strong heterogeneity, and the major controls for the development of high-quality reservoirs remain unclear. To address these issues, we conducted a series of experiment analyses, including petrographic microscope, scanning electron microscopy, and X-ray diffraction, and analyzed the impacts of sedimentation and diagenesis on the quality of deeply buried reservoirs. The results demonstrate that the sandbodies of subaqueous distributary channel and mouth bar deposited in lowstand systems tract (LST) and highstand systems tract (HST), as compared to the beach-bar and subaqueous fan sandstones deposited in transgressive systems tract (TST), have coarser grain size, higher quartz content, and lower muddy matrix content, which induced stronger anti-compaction capability, higher preservation of intergranular pore spaces, and thus better reservoir qualities. The reservoir types developed in subaqueous distributary channel and mouth bar are mainly types I, II, and III with medium-low porosity and low-ultra low permeability, while beach-bar and subaqueous fan mainly developed type III reservoir with low-porosity and ultra-low permeability. The reservoirs developed in E2w of the study area have undergone strong compaction, intense dissolution, but weak cementation. The subaqueous distributary channel and mouth bar reservoirs in LST are adjacent to Ew4 source rock in spatial distribution, resulting in strong organic acid dissolution, and developed numerous dissolved pores. The charging of hydrocarbons before deep burial further inhibited the later compaction and cementation and protects the preservation of residual primary intergranular pores and secondary dissolved pores. The combination of these factors leads to the development of the abnormally high porosity and high-quality reservoirs in LST. The results of this study reveal the genetic mechanism of deep, high-quality reservoirs in the rift basin and guide the selection of high-quality reservoirs in the later stage.

Seismic diagenetic facies prediction of tight sandstone in the offshore sparse well area: An example from the Xihu Depression of the East China Sea Basin

The purpose of this study is to forecast the spatial distribution of diagenetic facies (DF) of tight sandstone in the offshore sparse wells area. Analytical test data, rock physics experiment data and prestack seismic data were used to study the spatial distribution of seismic diagenetic facies (SEISDF) of tight sandstone of the second (H4b) submember of the fourth (H4) member of the Huagang (E3h) Formation in the N gas-field, Xihu Depression, East China Sea Basin. Based on diagenesis and diagenetic minerals, core-based DF of the H4b sandstone were divided into chlorite-coated facies (CHF), calcite-cemented facies (CAF), dissolution facies (DIF), tightly compacted facies (TIF), and quartz-cemented facies (QUF). Various seismic elastic parameters were obtained by prestack elastic parameter inversion method, and sensitive seismic elastic parameters were selected according to the seismic identification principle of DF. In order to predict the spatial distribution of DF, four DF and sensitive seismic elastic parameters were integrated to establish a SEISDF prediction model based on supervised learning, and obtain a diagenetic facies comprehensive index (DFCI) that can distinguish four DF. Then, the 90° phase conversion technique of seismic lithology was used to realize the consistency of the DFCI between isochronous stratigraphic interfaces with core-based DF. The stratal slicing technique of seismic geomorphology was used to make stratal slices of the DFCI between isochronous stratigraphic interfaces. Representative stratal slices were singled out, and the spatial distribution of the SEISDF was interpreted. This study shows that the H4b1 sublayer primarily develops the TIF, while the H4b2 and H4b3 sublayers chiefly develop the QUF, CHF, and DIF. Compared with the H4b2 and H4b3 sublayers, the areas of the QUF and DIF in the H4b4, H4b5, and H4b6 sublayers slightly increase. The QUF are mainly distributed in the subaqueous distributary channel and a small number of sheet sand and channel bar depositional facies in the form of continuous sheets. The CHF are primarily developed in lenticular shape in the channel bar and a small amount of sheet sand and subaqueous distributary channel depositional facies. The most favorable DIF are chiefly developed in the subaqueous distributary channel, channel bar, and sheet sand depositional facies near the fault, which are distributed in strips. This study proposes a very valuable method for the SEISDF prediction of tight sandstone, which can provide a reference for the study of DF of similar strata in other sedimentary basins.

Factors controlling reservoir quality of a retreating delta-front in shallow-water lacustrine in the Songliao Basin, Northeast China

Shallow-water lacustrine delta sand bodies with large areas of high-quality reservoirs are widespread in the first member of Yaojia Formation (k2y1) of the Upper Cretaceous in Qijia–Gulong area, central depression of the Songliao Basin. The analysis of elements affecting reservoir quality has always been a focus and difficulty of oil and gas exploration and development. Seismic profile interpretations, well log analysis, lithological associations, core observations, and heavy mineral assemblage determinations are used to interpret sequence stratigraphic framework and sedimentary evolution of K2y1. The latter is interpreted as a transgressive systems tract comprising three fourth-order sequences (SQ1–SQ3). Based on core observations and wireline logs, K2y1 primarily contained a shallow lake, distal bars, sand sheets, interdistributary bay, and subaqueous distributary channel deposits. The sand ratio and scale of the channels in the formations from SQ1 to SQ3 decreased which was reflecting the continuously retrograding sedimentary evolution of deltas to the lakeshore, as observed based on mutual calibration of single well sedimentary facies and root mean square amplitude attributes. Based on petrographic analysis of thin sections, particle sizes, scanning electron microscope (SEM) as well as X-ray diffraction, the effects of diagenesis and sedimentary facies on reservoir quality are investigated. In the process of delta-front retrogradation, the changes in reservoir quality in different sequences are primarily a result of the following factors: (1) the fine sandstone has better reservoir quality and more primary pores than the siltstone, which has higher argillaceous content. Additionally, in the fine sandstone, more primary pores can be preserved in the compaction process with higher reservoir quality. (2) Less authigenic illites have little influence on reservoir quality. When the authigenic illite content is >6%, the flocculent characteristics of authigenic illite crystals easily separate the intergranular pores into tinier micropores, thereby reducing the reservoir physical properties. (3) More than 3% of Pompon-like authigenic chlorites may occupy the pore throats of sandstone reservoirs. (4) The feldspars and rock fragments in the sandstone reservoir adjacent to the source rock easily form numerous secondary dissolution pores and improve the reservoir quality. The present study reveals that large-areas of subaqueous distributary channel sand bodies with stable lateral distribution, high reservoir quality, and significant petrophysical characteristics are the best-qualities of reservoir within the early stage of retreating delta-front system.

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.

Discussion of seismic diagenetic facies of deep reservoir in the East China Sea Basin

Evaluation of seismic diagenetic facies is an effective interpretation procedure to characterize the spatial distribution of diagenetic facies in deep tight sandstones in offshore sparse well areas. Based on petrographic and seismic data, research on seismic diagenetic facies was used in the proposed workflow to predict the spatial distribution and distribution characteristics of seismic diagenetic facies for the third (H4c) sub-member of the fourth (H4) member of the E3h Formation in the Xihu Sag, East China Sea Basin, Eastern China. Herein, two discriminant functions between the core-derived diagenetic facies and the seismic elastic parameters of shear modulus, p-wave impedance, and porosity data volume were established using the supervised learning method, and seismic data X and seismic data Y containing diagenetic facies information were evaluated. Based on the linear function between seismic data X and seismic data Y, a new seismic data volume was calculated and a comprehensive index of diagenetic facies was proposed to describe the distribution of diagenetic facies. The comprehensive index of diagenetic facies can better identify the spatial distribution of diagenetic facies. The results of seismic diagenetic facies research mainly focused on (1) the spatial distribution of diagenetic facies and (2) the distribution characteristics of diagenetic facies. Based on core-derived diagenetic facies, typical stratal slices, and sedimentary microfacies, the spatial distribution of seismic diagenetic facies in the two sublayers of the H4c sandstone was accurately interpreted. The northern part of the H4c1 and H4c2 sublayers mainly developed quartz-cemented facies and a small amount of chlorite-coated quartz facies. The southern part and the surrounding areas of the H4c1 and H4c2 sublayers primarily developed tightly compacted facies. The area near four (F1, F2, F3, and F4) faults in the H4c1 sublayer mainly developed a small amount of dissolution facies, whereas the area near the F2 and F4 faults in the H4c2 sublayer primarily developed a small amount of dissolution facies. Furthermore, the distribution characteristics of seismic diagenetic facies were investigated from the perspective of sedimentary microfacies and diagenetic facies. The chlorite-coated facies is mainly distributed in the subaqueous distributary channel sedimentary microfacies. The dissolution facies is primarily distributed in the subaqueous distributary channel and sheet sand sedimentary microfacies near the faults. This study shows that the research of seismic diagenetic facies using petrographic data and seismic data can better predict the spatial distribution of diagenetic facies of deep tight sandstone.

Sediment gravity-flow deposits in Late Cretaceous Songliao postrift downwarped lacustrine basin, northeastern China

The Songliao Basin is a large Cretaceous nonmarine petroliferous postrift downwarped basin in northeastern China. Deep lake channel-levee-lobe and slope fan deposits are identified within six depositional sequences that span from the Turonian to early Coniacian upper part of the fourth member of the Quantou Formation and Qingshankou Formation to the early Campanian second member of the Nenjiang Formation. Research on the sediment gravity-flow systems in the Songliao Basin has broad application for understanding the sedimentary processes and pattern of sediment gravity-flow systems, and predicting the presence of good sandstone reservoirs within postrift downwarped lacustrine basins. Research results show that the channel-levee-lobe deposits are present within sequences in the first member of the Qinshankou Formation and the first member of the Nenjiang Formation. The straight channel levees and slope fans were on high-gradient slopes, whereas the sinuous channel levees were on low-gradient slopes. The straight-to-sinuous channel levees, fed by northeastern deltaic subaqueous distributary channels, extend for 15–70 km, predominantly within transgressive-to early highstand system tracts, which terminated in bifurcated or small lobes. The channel-levee-lobe and slope fan deposits sourced from western deltas were deposited basinward of the syndepositional flexural slope-break zones controlled by deep-seated faults. The falling trajectories of the paleoshorelines during the highstand and lowstand systems tracts and large sediment fluxes during the transgressive systems tracts resulted in the development of the slope fans and channel-levee-lobes. Lakebed paleorelief influenced the channel forms. Sand bodies within the channel-levee-lobes and slope fans, encased within organic-rich source rocks, represent new significant targets for hydrocarbon exploration.

Physical property and hydrocarbon enrichment characteristics of tight oil reservoir in Chang 7 division of Yanchang Formation, Xin'anbian oilfield, Ordos Basin, China

Xin'anbian Oilfield of the Ordos Basin is the large tight oilfield to be first exploration discovery in china. The production of tight oil increased significantly in recent years. It shows great exploration potential of Chang7 tight oil. But the physical property and hydrocarbon enrichment characteristics of Chang 7 tight oil reservoirs were rarely studied, The forming conditions of tight oil reservoirs are systematically summarized and analyzed through the study of hydrocarbon generation, sedimentary reservoirs and hydrocarbon migration and accumulation based on production and core experimental data. The result shows that, The porosity of the Chang 7 2 reservoir mainly distributed in 5.0–11.0%, average at 7.9%, The permeability mainly distributed in 0.04–0.18 × 10 −3 μm 2 , average at 0.12 × 10 −3 μm 2 , The pore diameters of the tight oil reservoir distributed in 2–8 μm. The high-quality Chang 7 3 source rocks and the micropsammite of Chang 7 2 subaqueous distributary channel were widely distributed in the study area. The lenticular or banded sand bodies are distributed among mudstone or hydrocarbon source rocks and have the advantage of migration distance for hydrocarbon accumulation. The reservoir space is composed of micro-nanometer pores and throat, that is formed in the process of increasing pressure during hydrocarbon generation and hydrocarbon accumulation. The Chang 7 tight oil was generated in the early Cretaceous and injected into the sand of the subaqueous distributary channel driven by continuous hydrocarbon generation supercharging. The formation and accumulation of tight oil reservoirs are mainly controlled by source rocks, sedimentary microfacies and reservoirs of good quality.

Sedimentary evidence for a period of rapid environmental change in the Yangtze River Delta, China around 150 years ago

The mechanism of channel bifurcation in tide-dominated deltas is poorly understood. Here, we evaluate the function of extreme events on the channel bifurcation by examining the lithology, grain size, and organic carbon stable isotope (δ13C) composition, as well as the AMS 14C and OSL ages in two sediment cores (KZ01-A and KZ02) collected close to a site of a shipwreck of a known age at the offshore section of the North Channel in the Yangtze River mouth, China. The sedimentary sequence in both cores is mostly typical of deltaic succession, changing vertically from a prodelta to a subtidal mouth shoal, and finally to a subaqueous distributary channel. In KZ01-A, a shipwreck layer underlies the succession of the subaqueous distributary channel, and the porcelain collected from the sunken ship constrains the formation of the North Channel to post-1862, which is consistent with historical literature and marine charts though not with the OSL ages of the shipwreck layer, which are overestimated, likely owing to the rapid deposition during the event. After the formation of the North Channel, the consistently more negative δ13C values in the facies of subaqueous distributary channel in KZ01-A evidence the prevalence of fluvial input. Based on the shipwreck's location in the flood-dominated tidal channel of the outer delta front platform, the ship likely sank due to a large typhoon rather than a river flood. In core KZ02, significant fluctuations in both grain size and δ13C values, such as shifting from less negative to more negative δ13C in the mouth shoal facies, indicate periods of frequent river floods and typhoon events before formation of the channel. We therefore suggest that the frequent extreme events including both typhoons and river floods in the late 19th century played an important role in the geomorphological evolution of the North Channel in the tide-dominated Yangtze River Delta.

Sedimentary system and sand bodies distribution of the second member of the Xujiahe Formation in the Xinchang area, Western Sichuan Depression, China

The Xujiahe Formation of the Late Triassic in the Western Sichuan Depression contains abundant gas reservoirs. Influenced by the thrust tectonic movement of the foreland basin, the fluvial-delta sedimentary system supplied by multiple provenances formed the Xu2 Formation of the Xinchang area. We used detailed description of drilling wells and cores to define the sequence stratigraphic framework and sand body types. We used stratal slices through the seismic texture model regression (TMR) attribute volume to map the evolution of the sedimentary system and the sand body distribution. Our results indicate that the Xu2 Formation exhibits a complete long-term base-level cycle and that there are six sand body deposit types: distributary channel, interchannel, subaqueous distributary channel, interdistributary bay, mouth bar, and sheet sand. Stratal slices through the seismic TMR attribute volume at different levels map the spatial variation of sand and mudstone, which we use to construct a sedimentary filling evolution model. This model indicates that during the time of deposition of the lower submember, the main provenance supply came from the northwest direction, resulting in the sand bodies mainly being deposited in the west. During the time of deposition of the central submember, the sediment supply was large and came from the northwest and northeast directions, resulting in large, laterally extensive, thick sands. During the time of deposition of the upper submember, the sediment supply was from the northeast direction, with the sand bodies more developed in the east. The flow direction of the (subaqueous-) distributary channels indicate that they migrated from northwest to northeast. There are significant differences in the channel energy, sedimentary characteristics, and reservoir physical properties in the three submembers, which cause differences in oil and gas productivity in the reservoir of the Xu2 Formation. We believe that detailed mapping of the spatial distribution of sedimentary systems can provide critical guidance not only to explore but also to develop in high-quality oil and gas reservoirs such as the Xu2 Formation.