Main controlling factor and mechanism of gas-in-place content of the Lower Cambrian shale from different sedimentary facies in the western Hubei area, South China

The Lower Cambrian shale gas in the western Hubei area,South China has a great resource prospect,but the gas-in-place(GIP)content in different sedimentary facies varies widely,and the relevant mechanism has been not well understood.In the present study,two sets of the Lower Cambrian shale samples from the Wells YD4 and YD5 in the western Hubei area,representing the deep-water shelf facies and shallowwater platform facies,respectively,were investigated on the differences of pore types,pore structure and methane adsorption capacity between them,and the main controlling factor and mechanism of their methane adsorption capacities and GIP contents were discussed.The results show that the organic matter(OM)pores in the YD4 shale samples are dominant,while the inorganic mineral(IM)pores in the YD5 shale samples are primary,with underdeveloped OM pores.The pore specific surface area(SSA)and pore volume(PV)of the YD4 shale samples are mainly from micropores and mesopores,respectively,while those of the YD5 shale samples are mainly from micropores and macropores,respectively.The methane adsorption capacity of the YD4 shale samples is significantly higher than that of the YD5 shale samples,with a maximum absolute adsorption capacity of 3.13 cm^(3)/g and 1.31 cm^(3)/g in average,respectively.Compared with the shallow-water platform shale,the deep-water shelf shale has a higher TOC content,a better kerogen type and more developed OM pores,which is the main mechanism for its higher adsorption capacity.The GIP content models based on two samples with a similar TOC content selected respectively from the Wells YD4 and YD5 further indicate that the GIP content of the deep-water shelf shale is mainly 34 m^(3)/t within a depth range of 1000—4000 m,with shale gas exploration and development potential,while the shallow-water platform shale has normally a GIP content of<1 m^(3)/t,with little shale gas potential.Considering the geological and geochemical conditions of shale gas formation and preservation,the deep-water shelf facies is the most favorable target for the Lower Cambrian shale gas exploration and development in the western Hubei area,South China.

Facies-controlled prediction of dolomite reservoirs in the Middle Permian Qixia Formation in Shuangyushi,northwestern Sichuan Basin

The Middle Permian Qixia Formation in the Shuangyushi area,northwestern Sichuan Basin,develops shoal-facies dolomite reservoirs.To pinpoint promising reservoirs in the Qixia Formation,deep thin shoal-facies dolomite reservoirs were predicted using the techniques of pre-stack Kirchhoff-Q compensation for absorption,inverse Q filtering,low-to high-frequency compensation,forward modeling,and facies-controlled seismic meme inversion.The results are obtained in six aspects.First,the dolomite reservoirs mainly exist in the middle and lower parts of the second member of Qixia Formation(Qi2 Member),which coincide with the zones shoal cores are developed.Second,the forward modeling shows that the trough energy at the top and bottom of shoal core increases with increasing shoal-core thickness,and weak peak reflections are associated in the middle of shoal core.Third,five types of seismic waveform are identified through waveform analysis of seismic facies.Type-Ⅰ and Type-Ⅱ waveforms correspond to promising facies(shoal core microfacies).Fourth,vertically,two packages of thin dolomite reservoirs turn up in the sedimentary cycle of intraplatform shoal in the Qi2 Member,and the lower package is superior to the upper package in dolomite thickness,scale and lateral connectivity.Fifth,in plane,significantly controlled by sedimentary facies,dolomite reservoirs laterally distribute with consistent thickness in shoal cores at topographical highs and extend toward the break.Sixth,the promising prospects are the zones with thick dolomite reservoirs and superimposition of horstegraben structural traps.

An improved deep dilated convolutional neural network for seismic facies interpretation

With the successful application and breakthrough of deep learning technology in image segmentation,there has been continuous development in the field of seismic facies interpretation using convolutional neural networks.These intelligent and automated methods significantly reduce manual labor,particularly in the laborious task of manually labeling seismic facies.However,the extensive demand for training data imposes limitations on their wider application.To overcome this challenge,we adopt the UNet architecture as the foundational network structure for seismic facies classification,which has demonstrated effective segmentation results even with small-sample training data.Additionally,we integrate spatial pyramid pooling and dilated convolution modules into the network architecture to enhance the perception of spatial information across a broader range.The seismic facies classification test on the public data from the F3 block verifies the superior performance of our proposed improved network structure in delineating seismic facies boundaries.Comparative analysis against the traditional UNet model reveals that our method achieves more accurate predictive classification results,as evidenced by various evaluation metrics for image segmentation.Obviously,the classification accuracy reaches an impressive 96%.Furthermore,the results of seismic facies classification in the seismic slice dimension provide further confirmation of the superior performance of our proposed method,which accurately defines the range of different seismic facies.This approach holds significant potential for analyzing geological patterns and extracting valuable depositional information.

Application of 9-component S-wave 3D seismic data to study sedimentary facies and reservoirs in a biogasbearing area:A case study on the Pleistocene Qigequan Formation in Taidong area,Sanhu Depression,Qaidam Basin,NW China

To solve the problems in restoring sedimentary facies and predicting reservoirs in loose gas-bearing sediment,based on seismic sedimentologic analysis of the first 9-component S-wave 3D seismic dataset of China,a fourth-order isochronous stratigraphic framework was set up and then sedimentary facies and reservoirs in the Pleistocene Qigequan Formation in Taidong area of Qaidam Basin were studied by seismic geomorphology and seismic lithology.The study method and thought are as following.Firstly,techniques of phase rotation,frequency decomposition and fusion,and stratal slicing were applied to the 9-component S-wave seismic data to restore sedimentary facies of major marker beds based on sedimentary models reflected by satellite images.Then,techniques of seismic attribute extraction,principal component analysis,and random fitting were applied to calculate the reservoir thickness and physical parameters of a key sandbody,and the results are satisfactory and confirmed by blind testing wells.Study results reveal that the dominant sedimentary facies in the Qigequan Formation within the study area are delta front and shallow lake.The RGB fused slices indicate that there are two cycles with three sets of underwater distributary channel systems in one period.Among them,sandstones in the distributary channels of middle-low Qigequan Formation are thick and broad with superior physical properties,which are favorable reservoirs.The reservoir permeability is also affected by diagenesis.Distributary channel sandstone reservoirs extend further to the west of Sebei-1 gas field,which provides a basis to expand exploration to the western peripheral area.

Study on the Fine Configuration of Reservoir in River Facies Oilfield in Bohai Sea Area

The geological conditions of offshore shallow water delta oil reservoirs were complex, with limited well data and large well spacing. Taking A Oilfield in the Bohai Sea Area, China as an example, the target sand body was formed in a shallow water delta sedimentary environment, with well-developed underwater distributary channels and frequent branching and diversion. The reservoir was strong non-uniformity and uneven plane water cut pressure. To this end, based on the existing work of predecessors, combined with seismic, logging, and production dynamics data, and based on the genesis mechanism of shallow water delta reservoirs, the boundary of composite river channels was identified through seismic facies, and logging facies were used to subdivide them into single river levels within the composite river channels. Then, seismic waveform characteristics were applied to track and characterize the plane distribution of single river channels, guiding the efficient development of offshore shallow water delta oil fields and achieving increased storage and production in Bohai Oilfield, China.

Subaqueous volcanic eruptive facies,facies model and its reservoir significance in a continental lacustrine basin:A case from the Cretaceous in Chaganhua area of southern Songliao Basin,NE China

The conventional lithofacies and facies model of subaerial and marine pyroclastic rocks cannot reflect the characteristics of subaqueous volcanic edifice in lacustrine basins.In order to solve this problem,the lithofacies of subaqueous eruptive pyroclastic rocks is discussed and the facies model is established by taking the tuff cone of Cretaceous Huoshiling Formation in the Chaganhua area of the Changling fault depression,Songliao Basin as the research object.The results indicate that the subaqueous eruptive pyroclastic rocks in the Songliao Basin can be divided into two facies and four subfacies.The two facies are the subaqueous explosive facies and the volcanic sedimentary facies that is formed during the eruption interval.The subaqueous explosive facies can be further divided into three subfacies:gas-supported hot pyroclastic flow subfacies,water-laid density current subfacies and subaqueous fallout subfacies.The volcanic sedimentary facies consists of pyroclastic sedimentary rocks containing terrigenous clast subfacies.A typical facies model of the tuff cone that is formed by subaqueous eruptions in the Songliao Basin was established.The tuff cone is generally composed of multiple subaqueous eruption depositional units and can be divided into two facies associations:near-source facies association and far-source facies association.The complete vertical succession of one depositional unit of the near-source facies association is composed of pyroclastic sedimentary rocks containing terrigenous clast subfacies,gas-supported hot pyroclastic flow subfacies,water-laid density current subfacies and subaqueous fallout subfacies from bottom to top.The depositional unit of the far-source facies association is dominated by the subaqueous fallout subfacies and contains several thin interlayered deposits of the water-laid density current subfacies.The gas-supported hot pyroclastic flow subfacies and the pyroclastic sedimentary rocks containing terrigenous clast subfacies are favorable subaqueous eruptive facies for reservoirs in continental lacustrine basins.

Logging curves characteristics and response mechanisms of basaltic facies and sub-facies: A case study from eastern sag of Liaohe depression, Northeast China

Based on five types of conventional logging curves including GR,RLLD,CNL,DEN and AC,and 39 core samples from 30 representative boreholes,the logging characteristics and lithofacies and sub-facies of the basaltic rocks were studied.Three basaltic facies and four sub-facies are recognized from the well logs,includ-ing volcanic conduit facies(post intrusive sub-facies),explosive facies,and effusive lava flow facies(tabular flow,compound flow and hyaloclastite sub-facies).The post intrusive,tabular flow and compound flow sub-facies logging curves are mainly controlled by the distribution of vesiculate zones and vesiculate content,which are characterized by four curves with good correlation.Post intrusive sub-facies are characterized by high RLLD,high DEN,with a micro-dentate logging curve pattern,abrupt contact relationships at the top and base.Tabular flow sub-facies are characterized by high RLLD,high DEN,with a bell-shaped log curve pattern,abrupt contact at the base and gradational contact at the top.Compound flow sub-facies are characterized by medium-low RLLD,with a micro-dentate or finger-like logging curve pattern,abrupt contact at the base and gradational contact at the top.Explosive facies and hyaloclastite sub-facies logging curves are mainly controlled by the distribution of the size and sorting of rock particles,which can be recognized by four kinds of logging curves with poor cor-relation.Explosive facies are characterized by low RLLD,medium-low CNL and low DEN,with a micro-dentate logging curve pattern.Hyaloclastite sub-facies are characterized by low RLLD,high CNL,low DEN and high AC,with a micro-dentate logging curve pattern.The present research is beneficial for the prediction of basaltic reser-voirs not only in the Liaohe depression but also in the other volcanic-sedimentary basins.

Impacts of Depositional Facies and Diagenesis on Reservoir Quality:A Case Study from the Rudist-bearing Sarvak Formation,Abadan Plain,SW Iran

An integrated geological-petrophysical analysis of the rudist-bearing sequence of the Cretaceous Sarvak Formation is given one giant oilfield,and provides an improved understanding of this main reservoir in the Abadan Plain,in the Zagros Basin,SW Iran.The main objective of this study is to evaluate reservoir potential of the Sarvak Formation,and then to utilize the calibrated well log signature to correlate reservoir potential in un-cored wells.Eight main facies are recognized and categorized in five facies groups:lagoon,shoal,rudist-biostrome,slope,and shallow open marine,deposited on a shelf carbonate platform.Given the distribution of diagenetic products and their effects on pore systems,three diagenetic facies namely,(DF-1)low dissolution and cementation;(DF-2);high dissolution;and(DF-3)high cementation are differentiated.The initial sedimentary characteristics in combination with distribution of diagenetic products play an important role in reservoir quality heterogeneity.The effect of diagenetic processes related to disconformities mainly depends on the facies nature below these surfaces.Grain-dominated facies of shoal and rudist debris,observed below the Cenomanian–Turonian disconformity,are mostly characterized by high dissolution and interconnected pore systems.Finally,depositional and diagenetic facies in the studied wells are correlated by petrophysical well log data,leading to distribution of the reservoir zones.Data obtained can be utilized for efficient reservoir characterization of the Sarvak Formation and its equivalent units in the Arabian Plate.

Evolutions of sedimentary facies and palaeoenvironment and their controls on the development of source rocks in continental margin basins:A case study from the Qiongdongnan Basin,South China Sea

Hydrocarbon resources in the Qiongdongnan Basin have become an important exploration target in China.However,the development of high-quality source rocks in this basin,especially in its deep-water areas,are still not fully understood.In this study,evolutions of sedimentary facies and palaeoenvironment and their influences on the development of source rocks in diverse tectonic regions of the Qiongdongnan Basin were investigated.The results show that during the Oligocene and to Miocene periods,the sedimentary environment of this basin progressively varied from a semi-closed gulf to an open marine environment,which resulted in significant differences in palaeoenvironmental conditions of the water column for various tectonic regions of the basin.In shallow-water areas,the palaeoproductivity and reducibility successively decrease,and the hydrodynamic intensity gradually increases for the water columns of the Yacheng,Lingshui,and Sanya-Meishan strata.In deep-water areas,the water column of the Yacheng and Lingshui strata has a higher palaeoproductivity and a weaker hydrodynamic intensity than that of the Sanya-Meishan strata,while the reducibility gradually increases for the water columns of the Yacheng,Lingshui,and Sanya-Meishan strata.In general,the palaeoenvironmental conditions of the water column are the most favorable to the development of the Yacheng organic-rich source rocks.Meanwhile,the Miocene marine source rocks in the deep-water areas of the Qiongdongnan Basin may also have a certain hydrocarbon potential.The differences in the development models of source rocks in various tectonic regions of continental margin basins should be fully evaluated in the exploration and development of hydrocarbons.

Epidemiological Facies of Human Rabies Deaths in Côte d’Ivoire from 2016 to 2022

Introduction: Rabies is a highly fatal viral disease outside the post-exposure prophylaxis (PEP). Data from previous studies and reports from the rabies center show that cases of death from human rabies are regularly reported, despite training measures for health workers, awareness-raising and communities, and the availability of vaccines. Objectives: The aim of this study was to analyse the determinants of human rabies deaths in Côte d’Ivoire from 2016 to 2022. Method: A retrospective cross-sectional qualitative study was carried out from January to August 2023 on all the records of patients who died of human rabies at the Treichville Rabies Center from 2016 to 2022. An analysis of the rabies center’s annual activity reports from 2016 to 2022 was also carried out in order to complete the information gathered from the files. Results: The socio-demographic characteristics showed that out of 148 deaths, children accounted for 47.97% (71) and 66.22% (98) of rabies deaths were male, with an M/F sex ratio of 1.96. In terms of clinical characteristics, in 97.30% (144) of cases, the deceased had presented with furious rabies, and analysis by the Pasteur Institute of Côte d’Ivoire laboratory confirmed 101 cases (92.66%) of human rabies. Geographical distribution showed that cases of human rabies were recorded in 56 health district, and 64% (94) of the deceased lived in rural areas. The main causes of the occurrence of rabies were essentially non-use of post-exposure prophylaxis (PEP) and missed vaccination due to ignorance. Conclusion: In view of these results, control strategies and interventions need to be adapted to the epidemiological characteristics of the disease, using a community-based approach with a view to eliminating dog-transmitted rabies by 2030.

Automatic discrimination of sedimentary facies and lithologies in reef-bank reservoirs using borehole image logs

Reef-bank reservoirs are an important target for petroleum exploration in marine carbonates and also an essential supplemental area for oil and gas production in China. Due to the diversity of reservoirs and the extreme heterogeneity of reef-banks, it is very difficult to discriminate the sedimentary facies and lithologies in reef-bank reservoirs using conventional well logs. The borehole image log provides clear identification of sedimentary structures and textures and is an ideal tool for discriminating sedimentary facies and lithologies. After examining a large number of borehole images and cores, we propose nine typical patterns for borehole image interpretation and a method that uses these patterns to discriminate sedimentary facies and lithologies in reeI^bank reservoirs automatically. We also develop software with user-friendly interface. The results of applications in reef-bank reservoirs in the middle Tarim Basin and northeast Sichuan have proved that the proposed method and the corresponding software are quite effective.

Seismic facies response of tectonics and direction of block movement in the northeastern South China Sea

The close relationship between the structure,evolution,and resources environment of the South China Sea is a current research focus and also a focal point of our study.In this paper,we use regional seismic sections to analyze the relationship between seismic facies and the structure and tectonic evolution of the northeastern South China Sea and propose new points about the structure and the direction of block（land mass） motion.First,the tectonic features are clear and can be divided into five different structural units which are both linked and independent of each other.Second,it doesn＇t matter if the South China Sea basin pattern is extensional,weakly compressive,or strongly compressive,the regional tectonic stress field is unified.For the first time we find that two shallow subduction zones are recognized in the seismic profiles.All the tectonic blocks have accordion-fold-style structures,converging in the east,and the South China Sea exhibits different stages of basin development：growth, maturity,end,and termination.The block subduction and regional block dip directions are all aligned with the regional stress field.

Pre-stack-texture-based reservoir characteristics and seismic facies analysis

Seismic texture attributes are closely related to seismic facies and reservoir characteristics and are thus widely used in seismic data interpretation.However,information is mislaid in the stacking process when traditional texture attributes are extracted from poststack data,which is detrimental to complex reservoir description.In this study,pre-stack texture attributes are introduced,these attributes can not only capable of precisely depicting the lateral continuity of waveforms between different reflection points but also reflect amplitude versus offset,anisotropy,and heterogeneity in the medium.Due to its strong ability to represent stratigraphies,a pre-stack-data-based seismic facies analysis method is proposed using the selforganizing map algorithm.This method is tested on wide azimuth seismic data from China,and the advantages of pre-stack texture attributes in the description of stratum lateral changes are verified,in addition to the method＇s ability to reveal anisotropy and heterogeneity characteristics.The pre-stack texture classification results effectively distinguish different seismic reflection patterns,thereby providing reliable evidence for use in seismic facies analysis.

Generating geologically realistic 3D reservoir facies models using deep learning of sedimentary architecture with generative adversarial networks

This paper proposes a novel approach for generating 3-dimensional complex geological facies models based on deep generative models.It can reproduce a wide range of conceptual geological models while possessing the flexibility necessary to honor constraints such as well data.Compared with existing geostatistics-based modeling methods,our approach produces realistic subsurface facies architecture in 3D using a state-of-the-art deep learning method called generative adversarial networks(GANs).GANs couple a generator with a discriminator,and each uses a deep convolutional neural network.The networks are trained in an adversarial manner until the generator can create "fake" images that the discriminator cannot distinguish from "real" images.We extend the original GAN approach to 3D geological modeling at the reservoir scale.The GANs are trained using a library of 3D facies models.Once the GANs have been trained,they can generate a variety of geologically realistic facies models constrained by well data interpretations.This geomodelling approach using GANs has been tested on models of both complex fluvial depositional systems and carbonate reservoirs that exhibit progradational and aggradational trends.The results demonstrate that this deep learning-driven modeling approach can capture more realistic facies architectures and associations than existing geostatistical modeling methods,which often fail to reproduce heterogeneous nonstationary sedimentary facies with apparent depositional trend.

Seismic Facies in a Deepwater Area of a Marine Faulted Basin:Deepwater Area of the Paleogene Lingshui Formation in the Qiongdongnan Basin

In recent years, deep water areas have become popular exploration fields because of their abundant hydrocarbon resource potential. There are only relatively poor planar seismic profiles and no wells for deepwater areas of the Lingshui Formation in the Qiongdongnan Basin. A lot of faults developed and strata are fragmented due to high temperatures and high pressure, and this has resulted in dim sequence boundaries. Based on seismic data of the deepwater area and well data of bordering shallow water areas, Lingshui Formation was divided into four third class sequences; namely SI, SII, SIII and SIV, and the three-dimensional isochronous stratigraphic framework of the Lingshui Formation in the studied area was shaped. Based mainly on seismic attributes such as amplitude, continuity, internal structure and external shape, six typical seismic facies were identified, including mat-shaped, filling, wedge-shaped, foreset, moundy-shaped and lenticular-shaped, and a seismic facies distribution map was subsequently drawn. With studies on wells of bordering shallow water areas, regional sedimentary characteristics, and isopach map as references, sedimentary planar distribution features were analyzed. The deepwater area of the Lingshui Formation has mainly developed littoral and shallow sea. Sandstone bodies of fan delta, braided river delta, slope fan, basin floor fan, and turbidite fan are at an interdigitate junction to marine hydrocarbon source rocks and thus are favorable prospecting targets.

Garnet-bearing Granulite Facies Rock Xenoliths from Late Mesozoic Volcaniclastic Breccia, Xinyang, Henan Province

This paper presents the primary results of petrologic, mineralogical and petrochemical studies of garnet-bearing granulite facies rock xenoliths from Xinyang, Henan Province. These xenoliths, which are found in a pipe of late Mesozoic volcaniclastic breccia, are of high density (3.13–3.30 g/cm3) and high seismic velocity (Vp = 7.04–7.31 km/s), being products of underplating of basaltic magmas and had experienced granulite facies metamorphism. The underplating and metamorphism took place before the eruption of the host rock. Petrographical studies and equilibrium T-P calculations show that these xenoliths were captured at a 49 km depth and experienced at least a 16 km uplift before they were captured. The dynamics of the uplift could be related to the continent-continent collision between the North China plate and the Yangtze plate during the Triassic.

Neoproterozoic Molar-tooth Structure and Constraint of Depositional Facies and Environment in the North China Platform in Jiangsu, Anhui and Liaoning, Eastern China

Molar tooth structure （MTS） represented by complex ptygmatical shapes is widely distributed in the Proterozoic of the world. MTS filled by fine, equant sparry calcite （or dolomite） displays an abrupt contact with hosting rocks, which are mainly composed of carbonaceous micrites and fine-grained carbonates with local silts and stormdominated deposits with graded, cross or wave beddings, numerous erosional surfaces and truncated and fills or guttered bases. Occurrence of MTS suggests a result of the constraint of sedimentary facies, and the storm-base in ramp settings is the maximum depth for the formation of MTS. Vertical succession of MTS-bearing carbonates shows a deposition stacked by high-frequency shallow subtidal and peritidal cycles. An individual cyclic MTS-bearing sequence is characterized by thinning, shallowing and dynamic decreasing-upward, and peritidal caps of purple red iron and organic carbonaceous sediments with more complicated shapes of MTS are common on the top of individual MTS-bearing sequences.

Genetic Types of Meter-Scale Cyclic Sequences and Fabric Natures of Facies Succession

Different genetic types of meter-scale cyclic sequences in stratigraphic records result from episodic accumulation of strata related to Milankovitch cycles. The distinctive fabric natures of facies succession result from the sedimentation governed by different sediment sources and sedimentary dynamic conditions in different paleogeographical backgrounds, corresponding to high-frequency sea-level changes. Naturally, this is the fundamental criterion for the classification of genetic types of meter-scale cyclic sequences. The widespread development in stratigraphic records and the regular vertical stacking patterns in long-term sequences, the evolution characters of earth history and the genetic types reflected by specific fabric natures of facies successions in different paleogeographical settings, all that show meter-scale cyclic sequences are not only the elementary working units in stratigraphy and sedimentology, but also the replenishment and extension of parasequence of sequence stratigraphy. Two genetic kinds of facies succession for meter-scale cyclic sequence in neritic-facies strata of carbonate and clastic rocks, are normal grading succession mainly formed by tidal sedimentation and inverse grading succession chiefly made by wave sedimentation, and both of them constitute generally shallowing upward succession, the thickness of which ranges from several tens of centimeters to several meters. The classification of genetic types of meter-scale cyclic sequence could be made in terms of the fabric natures of facies succession, and carbonate meter-scale cyclic sequences could be divided into four types: L-M type, deep-water asymmetrical type, subtidal type and peritidal type. Clastic meter-scale cyclic sequences could be grouped into two types: tidal-dynamic type and wave-dynamic type. The boundaries of meter-scale cyclic sequences are marked by instantaneous punctuated surface formed by non-deposition resulting from high-frequency level changes, which include instantaneous exposed punctuated surface, drowned punctuated surface as well as their relative surface. The development of instantaneous punctuated surface used as the boundary of meter-scale cyclic sequence brings about the limitations of Walter's Law on the explanation of facies distribution in time and space, and reaffirm the importance of Sander's Rule on analysis of stratigraphic records. These non-continuous surface could be traced for long distance and some could be correlative within same basin range. The study of meter-scale cyclic sequences and their regularly vertical stacking patterns in long-term sequences indicate that the research into cyclicity of stratigraphic records is a useful way to get more regularity from stratigraphic records that are frequently complex as well as non-integrated.

Research on Diageneses of Cambrian Shoal Facies Carbonate Rocks in the Xiadong Area, Hubei Province

With continuous outcrops, developed shoal facies rocks, complete types of diagenesis and changeable diagenetic environments, Cambrian strata are well developed in the Xiadong area, Yichang, Hubei Province. Under the combined influence of numerous diageneses, secondary pores can be formed, which result in better reservoir properties of the rock strata.

Sedimentary facies and depositional model of shallow water delta dominated by fluvial for Chang 8 oil-bearing group of Yanchang Formation in southwestern Ordos Basin,China

A systematic analysis of southwestern Ordos Basin's sedimentary characteristics,internal architectural element association styles and depositional model was illustrated through core statistics,well logging data and outcrop observations in Chang 8 oil-bearing group.This analysis indicates that shallow water delta sediments dominated by a fluvial system is the primary sedimentary system of the Chang 8 oil-bearing group of the Yanchang Formation in southwestern Ordos Basin.Four microfacies with fine grain sizes are identified: distributary channels,sheet sandstone,mouth bar and interdistributary fines.According to the sandbody's spatial distribution and internal architecture,two types of sandbody architectural element associations are identified: amalgamated distributary channels and thin-layer lobate sandstone.In this sedimentary system,net-like distributary channels at the delta with a narrow ribbon shape compose the skeleton of the sandbody that extends further into the delta front and shades into contiguous lobate distribution sheet sandstone in the distal delta front.The mouth bar is largely absent in this system.By analyzing the palaeogeomorphology,the palaeostructure background,sedimentary characteristics,sedimentary facies types and spatial distribution of sedimentary facies during the Chang 8 period,a distinctive depositional model of the Chang 8 shallow water fluvial-dominated delta was established,which primarily consists of straight multi-phase amalgamated distributary channels in the delta plain,net-like distributary channels frequently diverting and converging in the proximal delta front,sheet sandstones with dispersing contiguous lobate shapes in the distal delta front,and prodelta or shallow lake mudstones.