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.

The sedimentary facies and dynamic environment of the Diaokou lobe in the modern Huanghe River Delta of China

The Huanghe River captures the Diaokou River in 1964 and forms a deltaic lobe in the subsequent 12 a. The progradational process of the Diaokou lobe is in associated with complicated evolution of riverine sheet flooding, merging, and swinging. On the basis of 11 borehole cores and 210 km high resolution seismic reflection data set, the sedimentary sequence and dynamic environment of the Diaokou lobe Cone subdelta lobe of the modern Huanghe River Delta） are studied. The stratigraphy of the lobe is characterized by an upward-coarsening ternary structure and forms a progradational deltaic clinoform. Totally six seismic surfaces are identifiable in seismic profiles, bounded six seismic units （SUs）. These SUs correspond to six depositional units （DUs） in the borehole cores, and among them, SUs 4-6 （DUs D to F） consist of the modern Diaokou lobe. Lithological and seismic evidences indicate that the delta plain part of the Diaokou lobe is comprised primarily by fluvial lag sediments together with sediments from sidebanks, overbanks, fluvial flood plains and levees, while the delta front part is a combination of river mouth bar sands （majority） and distal bar and deltaic margin sediments （minority）. As a result of the high sedimentation rate and weak hydrodynamic regime in the Huanghe River Delta, the sediments in the delta front are dominated by fine-grained materials. The grain size analysis indicates the Huanghe River hyperpycnal-concentrated flow shows the suspension, transportation and sedimentation characteristics of gravity flow, and the sediment transportation is primarily dominated by graded suspension, while uniform suspension and hydrostatic suspension are also observed in places. The strength of the hydrodynamic regime weakens gradually offshore from riverbed, river mouth bar, sidebank, distal bar subfacies to delta lateral margin and flooding plain subfacies.

The Features of Sedimentary Facies and Copper Enrichment Metallogenic Regularities of Kuzigongsu Group in Sareke Glutenite Type Copper Deposits,Wuqia,Xinjiang

1 Introduction Sareke glutenite-type copper deposit is the large size copper deposit discovered in recent years,and it is located Sarekebayi intracontinental pull-apart basin in the western margin of the Tarim basin.Conglomerate of

Sequence Stratigraphy and Sedimentary Facies of Feixianguan Formation in the Kaijiang-Liangping Area of Sichuan Basin, China

The Feixianguan formation in the Kaijiang-Liangping basin has been the focus of extensive research on multiple aspects. Based on field survey, core observation, laboratory analysis and seismic data interpretation, the sequence stratigraphy and sedimentary facies of the Kaijiang-Liangping area are studied. Four sequence boundaries and three maximum flooding surfaces of the Feixianguan formation are detected in this area. Three third-order sequences are identified as first sequence (SQ1), the second sequence (SQ2), and the third sequence (SQ3) in which SQ1 corresponds to the member 1 of the Feixianguan formation, SQ2 corresponds to the member 2, and SQ3 corresponds to the member 3 and member 4. Member 1, 2, 3 and 4 are lithologic sections divided by predecessors. On the basis of this sequence division and their sedimentary marks, the facies of the Feixianguan formation can be divided into open platform and evaporate platform categories. The open platform is composed of three subfacies identified as platform bank or basin marginal bank, interbank, and platform basin. Thus, a sedimentary evolution model is established. According to the sedimentary and seismic characteristics of the Kaijiang-Liangping area, we determine that two oolitic bank models, the aggradation model and the progradation model, are developed in this area. The aggradation model is developed in the Longgang region, which includes the basin marginal bank as a favorable exploring zone. The progradation model is developed in the Jiulongshan and Longhui areas, besides the basin marginal bank, the favorable exploration zones also include the oolitic bank developing areas of the inner basin.

Sedimentary Facies Architectures of Third-Order Sequences from Early to Middle Triassic in Nanpanjiang Basin

There are complex and regular changes on sedimentary facies from the Early to the Middle Triassic in the Nanpanjiang basin. After the obvious drowned event of carbonate platforms in the transitional period between Permian and Triassic, carbonate platforms have evolved into the ramp type from the rimmed shelf type. The differentiation of sedimentary facies becomes clearer in space, which are marked by the changes from an attached platform to a turbidity basin and several isolated platforms in the basin. The striking characteristics are the development of oolitic banks on isolated platforms in Nanning and Jingxi and the reef and bank limestones in the margin of the attached platform in the Early Triassic. Despite the difference of the time span and the architecture of facies succession of third order sedimentary sequences, the process of the third order relative sea level changes reflected by the sedimentary facies succession of the third order sequences is generally synchronous. Therefore, six third order sequences could be discerned in the strata from the Early to the Middle Triassic in the Nanpanjiang basin. Using two types of facies changing surfaces and two types of diachronisms in stratigraphic records as the key elements, the sedimentary facies architectures of the third order sequences that represent sequence stratigraphic frameworks from the Early to the Middle Triassic in the Nanpanjiang basin could be constructed.

Lower Jurassic Sequence Stratigraphy and Sedimentary Facies on the North Slope of Mount Qomolangma

The Lower Jurassic is subdivided in ascending order into the Wulong, Kangdui and Yongjia Formations on the north slope of Mount Qomolangma, with a total thickness of 1362 m. They are thought to have been deposited respectively in the environments of the carbonate ramp fault-bounded basins and carbonate platform, with six sedimentary facies and six sub-facies. During the Early Jurassic, the Qomolangma area experienced strong faulting and subsidence, and was of a matured rift basin. The Lower Jurassic consists of eleven 3rd-order sequences, which can be grouped into three 2nd-order sequences and form a large transgressive-regressive cycle. The 3rd-order sequences and the corresponding sea-level changes recognized in the area can be correlated quite well with those set up in the western Tethys, and may have been caused by the eustatic fluctuations, while the 2nd-order sequences seem to be more closely related to the basement subsidence and the variation in sedimentary influx, indicating the evolution of the eastern Neotethys and the movement of the plates on its two sides.

Sedimentary Facies Controls on the Upscaling of Petrophysical Properties from Core to Log Scales and Its Implications to Petroleum Exploration

The clastic sedimentary realm comprises a number of genetically distinct depositional systems, which are dominated by distinct depositional processes. A variogram and a Levy-stable probability distribution-based geostatistical method have been applied to analyze petrophysical properties from well logs and cores from a variety of depositional environments in sedimentary basins from Australia to quantify the heterogeneity and upscaling range of different depositional systems. Two reservoir sequences with contrasting sedimentary facies, depositional processes and a diagenetic history are investigated for their petrographic, petrophysical and log characters and their scaling behaviour. The microscopic derived petrophysical parameters, including visual porosity, grain size, sorting and amount of matrix, core plug measured porosity and permeability and log-derived V-shale, porosity and permeability, have been found to be well correlated (|R|=0.72 to 0.91) across all the scales for the reservoir sequence deposited under a single predominant depositional process and a gradational change of the energy regime (Bilyara-1). In contrast, for the reservoir sequence (East Swan-2), which was deposited in heterogeneous processes and underwent diagenetic alteration, the cross-correlation of the petrophysical properties derived from the three different scales is extremely poor (|R|=0.01-0.54). Log-derived porosity and permeability for a thinly bedded reservoir sequence with an individual bed thinner than one metre can therefore be affected by the intrinsic averaging effects of the logging tools.

Application of multi-attribute matching technology based on geological models for sedimentary facies:A case study of the 3rd member in the Lower Jurassic Badaowan Formation,Hongshanzui area,Junggar Basin,China

Regarding high drilling costs,an effort should be made to substantially reduce the drilling operation.To achieve this goal,exploration and development stages should be carried out precisely with maximum information acquired from the reservoir.The use of multi-attribute matching technology to predict sedimentary system has always been a very important but challenging task.To resolve the challenges,we utilized a quantitative analysis method of seismic attributes based on geological models involving high resolution 3D seismic data for sedimentary facies.We developed a workflow that includes core data,seismic attribute analysis,and well logging to highlight the benefit of understanding the facies distribution in the 3 rd Member of the Lower Jurassic Badaowan Formation,Hongshanzui area,Junggar Basin,China.1)Data preprocessing.2)Cluster analysis.3)RMS attribute based on a normal distribution constrains facies boundary.4)Mapping the sedimentary facies by using MRA(multiple regression analysis)prediction model combined with the lithofacies assemblages and logging facies assemblages.The confident level presented in this research is 0.745,which suggests that the methods and data-mining techniques are practical and efficient,and also be used to map facies in other similar geological settings.

Research on Thin Layer Structure Identification and Sedimentary Facies of Middle and Deep Layers Based on Reflection Coefficient Inversion—By Taking Dongying Formation of CFD Oilfield in Bohai Offshore as an Example

The sand layer B of Dongying Formation of CFD oilfield in Bohai offshore belongs to the middle deep layer of buried hill overlap deposit. Its reservoir distribution has the characteristics of large burial depth, thin thickness and rapidly lateral change. Because of low resolution of seismic data and overlying sand layer. It is difficult to identify and interpret the structure of sand layer accurately. The uncertainty of structure and reservoir restricts the fine development of B sand layer. In order to identify the top surface of reservoir effectively. The seismic data are processed by using the reflection coefficient inversion method. The results show that the inversion resolution of reflection coefficient is significantly higher than that of original data. The top surface of sand layer B and its overlying sand layer can be well identified and traced. Carrying out structural interpretation of B sand layer based on reflection coefficient inversion data and the microstructure and the formation tip extinction point are implemented. Based on the constraint of new interpretation level, the sedimentary facies plane distribution of B sand layer is described and make prediction of dominant reservoir development area in detail combining with sedimentary paleogeomorphology, along layer attribute section and limited drilling data. The research shows that the study area is mainly from the northwest material sources, the slope belt in the northwest is close to the lake shoreline with a gentle slope and shallow water depositional environment, which is located on the main transport and deposition channels. The shallow water gentle slope landform is suitable for forming large-area sand bar deposition, mainly composed of underwater distributary channel and debouch bars facies, which is the dominant reservoir development area. The research conclusion guides the deployment and implementation of the development well location effectively.

Fine Single Channel Identification Controlled by Sedimentary Facies—Taking KLA Oilfield as an Example

The Neogene fluvial reservoir in the Bohai oilfield is one of the leading development horizons for increasing reserves and production in the Bohai oilfield. However, the development of offshore fluvial reservoirs is faced with the problems of thin reservoir thickness, narrow plane width, rapid lateral change, and thin well pattern. Taking the KLA oilfield as an example, this paper discusses the nuanced characterization and configuration of a single channel controlled by sedimentary facies to guide developing offshore river facies’ narrow channel main control oilfield. Firstly, based on a large number of core data, the acceptable sedimentary facies identification is realized, the sedimentary model of the study area is established, the delicate calibration of logging facies and seismic facies is realized under the constraint of the sedimentary model, and a set of technical methods for nuanced reservoir characterization guided by seismic sedimentology is summarized, to realize the boundary identification of composite channel configuration and further realize the nuanced characterization of the single narrow channel. Based on this set of technology, it guides the smooth implementation of horizontal wells in the oilfield. The drilling encounter rate of the reservoir in the horizontal section of the single well exceeds 90%, ensuring the injection production connectivity and increasing the reserve production rate by more than 10%.

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.

Construction of a fluvial facies knowledge graph and its application in sedimentary facies identification

Lithofacies paleogeography is a data-intensive discipline that involves the interpretation and compilation of sedimentary facies.Traditional sedimentary facies analysis is a labor-intensive task with the added complexity of using unstructured knowledge and unstandardized terminology.Therefore,it is very difficult for beginners or non-geology scholars who lack a systematic knowledge and experience in sedimentary facies analysis.These hurdles could be partly alleviated by having a standardized,structured,and systematic knowledge base coupled with an efficient automatic machine-assisted sedimentary facies identification system.To this end,this study constructed a knowledge system for fluvial facies and carried out knowledge representation.Components include a domain knowledge graph for types of fluvial facies(meandering,braided and other fluvial depositional environments)and their characteristic features(bedforms,grain size distribution,etc.)with visualization,a method for query and retrieval on a graph database platform,a hierarchical knowledge tree-structure,a data-mining clustering algorithm for machineanalysis of publication texts,and an algorithm model for this area of sedimentary facies reasoning.The underlying sedimentary facies identification and knowledge reasoning system is based on expert experience and synthesis of publications.For testing,17 sets literature publications data that included details of sedimentary facies data(bedforms,grain sizes,etc.)were submitted to the artificial intelligence model,then compared and validated.This testing set of automated reasoning results yielded an interpretation accuracy of about 90%relative to the published interpretations in those papers.Therefore,the model and algorithm provide an efficient and automated reasoning technology,which provides a new approach and route for the rapid and intelligent identification of other types of sedimentary facies from literature data or direct use in the field.

The sedimentary facies characteristics and lithofacies palaeogeography during Middle-Late Cambrian,Sichuan Basin and adjacent area

Combined with the regional strata filling characteristics of Middle-Upper Cambrian,the present paper conducts a systematic research on sedimentary facies in the basin and its peripheral area by utilizing 164 field outcrops and drilling and coring data.Further,the method of“multi-factor comprehensive synthesis based on single-factor analysis”was employed to investigate the sedimentary facies and palaeogeography of the study area and establish the sedimentary facies model.Stratigraphic reveals that the study area represents the pattern of thin-northwest and thicksoutheast by stretching northeast-southwest.Within the present basin,the pattern of“one thin and two thick”predominates,while outside the basin“four thin and three thick”filling feature was found.Sedimentary facies shows that the study area was featured by rimmed carbonate platform.Specifically,carbonate platform,slope and northeastern corner Qinling paleooceanic Basin and southeastern corner Jiangnan Bain was identified from the west to the east.The carbonate platform contains restricted platform,evaporation-restricted platform,semi-restricted platform and the platform margin.Single factor analysis and lithofacies palaeogeographic characteristics manifests that during Middle-Late Cambrian,the western Old land evolved into peneplain stage,and that the eastern and southwestern sub-sags remained connected to the open-sea to some extent.At the time,the shllow seawater circulation was relatively restricted,while the ancient seabed tended to be flat and evaporation characteristics significantly diminished.Secondary sea-level fluctuation intensively influenced the development of scaled grain beach.It is suggested that tide marginal beach,intraplatform shoal subfacies zone,along with Shiqian-SangZhi in southeast and Zhenba-Xinshan in northeast platform-margin beach subfacies zone to be preferable targets for the favorable reservoir facies zone and potential oil and gas reservoir area.

Sedimentary Facies of Marine Shale Gas Formations in Southern China： The Lower Silurian Longmaxi Formation in the Southern Sichuan Basin

Sedimentary facies is an important factor influencing shale gas accumulation. It not only controlls hydrocarbon generation, but also affects reservoir characteristics and distribution. This paper discusses the Lower Silurian Longmaxi Formation in the south of the Sichuan Basin. Outcrop, core, drilling and logging data identify the sedimentary facies of the formation as continental shelf facies, which is divided into two subfacies： an inner shelf and an outer shelf subfacies. These two subfacies can be further divided into seven microfacies： muddy silty shallow shelf, calcareous silty shallow shelf, muddy limy shallow shelf, storm flow, muddy deep shelf, silty muddy deep shelf and contour current microfacies. Vertical and horizontal distribution of microfacies establishes a sedimentation model of the continental shelf facies. Combined with analization or calculation of geochemical, mineralogical, physical and gas-bearing properties of samples, sedimentary microfacies is evaluated using nine parameters： total organic carbon content, effective shale continuous thickness, vitrinite reflectance, kerogen type, mineral components, porosity, permeability, water saturation and gas content. The evaluation revealed that the most favorable facies for shale gas exploration and development are the muddy deep shelf and part of the silty muddy deep shelf microfacies, with TOC more than 2%, siliceous component over 50%, clay less than 30%, porosity more than 3%, water saturation lower than 40%, gas content greater than 2 m3/t. These results provide a theoretical basis for deciston-making on the most promising areas for shale gas exploration in the Sichuan Basin and for marine shale gas exploration and development in South China.

Sequence Stratigraphy and Sedimentary Facies in the Lower Member of the Permian Shanxi Formation, Northeastern Ordos Basin, China

The Lower Permian Shanxi （山西） Formation is one of the main gas-bearing stratio graphic units in northeastern Ordos （鄂尔多斯） Basin, China. Based on an integrated investigation of well logs, cores, and outcrop, we delineated the sedimentary facies of the lower member of the Shanxi Formation and divided the succession into three third-order sequences from base to top as SQ1, SQ2, and SQ3. The lower region of Shanxi Formation was deposited in the following sedimentary facies or subfacies： subaqueous braided channel, subaqueous interdistributary, mouth bar, swamp and shelf in the Daniudi （~z~=J~） Gas Field and braided channel, and shelf and lake at Heidaigon （~J） outcrop. Braided-river deposits form the lowstand systems tract （LST） in each sequence. Braided channels mark the sequence boundaries at Heidaigou outcrop. A shelf and lake depositional environment with dark gray mudstone forms the transgressive systems tract （TST）. The location where dark gray mudstone first appears above the braided channel marks the first flooding surface （FFS）, and the end of that marks the maximum flooding surface （MFS）. The highstand systems tract （HST） deposits are fine-grained sediments with an aggradational parasequence at Heidaigou outcrop and swamp coalbed in the Daniudi Gas Field. Mouth-bar sand bodies in braided delta front, which form the LST in each sequence, form excellent reservoirs in the Daniudi Gas Field.

Sequence Stratigraphy and Sedimentary Facies of Lower Oligocene Yacheng Formation in Deepwater Area of Qiongdongnan Basin,Northern South China Sea:Implications for Coal-Bearing Source Rocks

For unveiling coal-bearing source rocks in terrestrial-marine transitional sequences, the sequence stratigraphic framework and sedimentary facies of Lower Oligocene Yacheng Formation of Qiongdongnan Basin were investigated using seismic profiles, complemented by well bores and cores. Three third-order sequences are identified on the basis of unconformities on basin margins and correlative conformities in the basin center, namely SQYC3, SQYC2 and SQYC1 from bottom to top. Coal measure in Yacheng Formation of Qiongdongnan Basin were deposited within a range of facies associations from delta plain/tidal zone to neritic sea, and three types of favourable sedimentary facies associations for coal measure were established within the sequence stratigraphic framework, including braided delta plain and alluvial fan, lagoon and tidal flat, and fan delta and coastal plain facies associations. Results shown that, in the third-order sequences, coal accumulation in landward areas（such as delta plain） of the study area predominantly correlates with the early transgressive systems tract（TST） to middle highstand systems tract（HST）, while in seaward areas（such as tidal flat-lagoon） it correlates with the early TST and middle HST. The most potential coal-bearing source rocks formed where the accommodation creation rate（Ra） and the peat-accumulation rate（Rp） could reach a state of balance, which varied among different sedimentary settings. Furthermore, intense tectonic subsidence and frequent alternative marine-continental changes of Yacheng Formation during the middle rift stage were the main reasons why the coal beds shown the characteristics of multi-beds, thin single-bed, and rapidly lateral changes. The proposed sedimentary facies associations may aid in predicting distribution of coal-bearing source rocks. This study also demonstrates that controlling factors analysis using sequence stratigraphy and sedimentology may serve as an effective approach for coal-bearing characteristics in the lower exploration deepwater area of South China Sea.

Some new thoughts of definitions of terms of sedimentary facies:Based on Miall's paper(1985)

In recent years,the Journal of Palaeogeography(Chinese Edition and English Edition)continually received manuscripts,in which some authors are from China and some are from Pakistan,Lithuania,Morocco,South Africa,etc.The authors of these manuscripts,according to the viewpoint and method of Miall’s paper(1985),selected rocks from each bed in the clastic sections of their study areas and induced some rock types,such as conglomerates,sandstones and fine-grained stones,and considered them as lithofacies.It does not conform to the definition of lithofacies.I wrote some papers,i.e.,Feng(2018,2019,2020),to point out the problems and hope that the authors worldwide,especially Chinese authors,will not continually cite,spread and follow Miall’s paper(1985)viewpoint and method blindly.Prof.Miall is the first person who considered rocks(in fact,the sediments)as lithofacies and proposed a facies analysis method.His viewpoint and facies analysis method confused the definition of facies and facies analysis method.My current paper is a special article to discuss the principal problems of Miall’s paper(1985),i.e.,he considered the sediments as lithofacies and utilized lithofacies to analyze facies,but not to discuss the contributions and less strictness of architectural elements of his paper.Here,I have to declare that the facies in my current paper is the facies of sedimentary petrology,but not of other geological disciplines,such as igneous petrology,metamorphic petrology,palaeontology,stratigraphy,geophysics,geochemistry.Certainly,the definitions of facies and lithofacies are controversial.My current paper will adhere to the policy of“A hundred flowers blossom and a hundred schools of thought contend”.I hope that through academic discussions,contends and geological practice,these problems will be solved gradually.

Evolution of sedimentary facies and fossil communities in the Middle Permian Maokou Formation in Zigui County, Hubei Province,South China

A geological event that happened at the end of the Middle Permian resulted in different levels of erosion in a shallow-water platform of South China, which led to diverse geological records in different sections. The Lianziya section in western Hubei Province of South China has a well-exposed stratigraphic sequence of the late Middle Permian rocks with abundant fossils and sedimentary facies types, providing new evidences for understanding the evolution of marine biota and environment in the late Middle Permian. Our study shows that four fossil communities can be recognized with the change of sedimentary facies in the late Middle Permian: Foraminiferal-algal community, phylloid algal community, bryozoan community and Ungdarella community. The foraminiferal-algal community is dominated by fusulinids and calcareous algae, but was soon replaced by the phylloid algal community. With an increase of terrestrial input, the phylloid algal community was again replaced by the bryozoan community. Near the end of the Middle Permian, with a decrease of terrestrial input, the bryozoan fossil community disappeared while the Ungdarella community became dominant. A 10 cm-thick weathered crust occurred at the top of the Middle Permian limestone, marking a large amplitude fall in sea-level and the beginning of a terrestrial erosion stage. The sedimentary facies and fossil community changes in the Lianziya section indicates that the Middle Permian crisis was a gradual process while the sea-level fall mainly occurred in the latest Middle Permian.

Sedimentary facies and lithologic characters as main factors controlling hydrocarbon accumulation and their critical conditions

Taking more than 1000 clastic hydrocarbon reservoirs of Bohai Bay Basin, Tarim Basin and Junggar Basin, China as examples, the paper has studied the main controlling factors of hydrocarbon reservoirs and their critical conditions to reveal the hydrocarbon distribution and to optimize the search for favorable targets. The results indicated that the various sedimentary facies and lithologic characters control the critical conditions of hydrocarbon accumulation, which shows that hydrocarbon is distributed mainly in sedimentary facies formed under conditions of a long lasting and relatively strong hydrodynamic environment; 95% of the hydrocarbon reservoirs and reserves in the three basins is distributed in siltstones, fine sandstones, conglomerates and pebble-bearing sandstones; moreover, the probability of discovering conventional hydrocarbon reservoirs decreases with the grain size of the clastic rocks. The main reason is that the low relative porosity and permeability of fine-grained reservoirs, lead to small differences in capillary force compared with surrounding rocks and insufficiency of dynamic force for hydrocarbon accumulation; the critical condition for hydrocarbon entering reservoir is that the interfacial potential in the surrounding rock（ Un） must be more than twice of that in the reservoir（ Us）; the probability of hydrocarbon reservoirs distribution decreases in cases where the hydrodynamic force is too high or too low and when the rocks have too coarse or too fine grains.

Cenozoic uplift of the Tibetan Plateau:Evidence from the tectonic-sedimentary evolution of the western Qaidam Basin

Geologists agree that the collision of the Indian and Asian plates caused uplift of the Tibet Plateau. However, controversy still exists regarding the modes and mechanisms of the Tibetan Plateau uplift. Geology has recorded this uplift well in the Qaidam Basin. This paper analyzes the tectonic and sedimentary evolution of the western Qaidam Basin using sub-surface seismic and drill data. The Cenozoic intensity and history of deformation in the Qaidam Basin have been reconstructed based on the tectonic developments, faults growth index, sedimentary facies variations, and the migration of the depositional depressions. The changes in the sedimentary facies show that lakes in the western Qaidam Basin had gone from inflow to still water deposition to withdrawal. Tectonic movements controlled deposition in various depressions, and the depressions gradually shifted southeastward. In addition, the morphology of the surface structures in the western Qaidam Basin shows that the Cenozoic tectonic movements controlled the evolution of the Basin and divided it into （a） the southern fault terrace zone, （b） a central Yingxiongling orogenic belt, and （c） the northern fold-thrust belt; divided by the XI fault （Youshi fault） and Youbei fault, respectively. The field data indicate that the western Qaidam Basin formed in a Cenozoic compressive tectonic environment caused by the India--Asia plate collision. Further, the Basin experienced two phases of intensive tectonic deformation. The first phase occurred during the Middle Eocene--Early Miocene （Xia Ganchaigou Fm. and Shang Ganchaigou Fro., 43.8- 22 Ma）, and peaked in the Early Oligocene （Upper Xia Ganchaigou Fro., 31.5 Ma）. The second phase occurred between the Middle Miocene and the Present （Shang Youshashan Fro. and Qigequan Fro., 14.9-0 Ma）, and was stronger than the first phase. The tectonic--sedimentary evolution and the orienta- tion of surface structures in the western Qaidam Basin resulted from the Tibetan Plateau uplift, and recorded the periodic northward growth of the Plateau. Recognizing this early tectonic--sedimentary evolution supports the previous conclusion that northern Tibet responded to the collision between India and Asia shortly after its initiation. However, the current results reveal that northern Tibet also experi- enced another phase of uplift during the late Neogene. The effects of these two stages of tectonic activity combined to produce the current Tibetan Plateau.