Yan Jiaxin

Main Controlling Factors of Organic Matter Richness in a Permian Section of Guangyuan, Northeast Sichuan

ABSTRACT A complete Permian section in Guangyuan, Northeast Sichuan, has been investigated, to explore the variation in organic matter richness and its main controlling factors. The research results of the detailed lithological description and organic/inorganic geochemical analysis of about 325 samples indicate that a high content of total organic carbon (TOC) occurs in calcareous mudstones and laminated marls or thin-bedded limestones. In carbonate rocks, the TOC content is negatively related to the thickness of the massive beds; the thinner the bed is, the higher the TOC content is. Marine organic matter is enriched in the outer shelf and relatively shallow basin floor environments with the maximum TOC contents of 5.07% and 14.6%, respectively. The main factors that affect the quantity and quality of marine organic matter include primary productivity, depositional processes, and redox conditions during deposition and the early diagenesis stage. Three intervals of marine good quality source rocks are identified in this section; they are lower Chihsia Formation, topmost Maokou Formation, and the middle segment of the Dalong Formation. They are formed in anoxic environments, in association with high primary productivity. Among these, high productivity in the lower Chihsia interval may originate from an upwelling flow area, whereas, in the rest of the intervals, it is inferred to be related to hot fluid activity due to volcanic eruption. The results of this study suggest that good quality marine source rocks in the Permian strata offer significant hydrocarbon potential.

Architectural units and heterogeneity of channel reservoirs in the Karamay Formation, outcrop area of Karamay oil field, Junggar basin, northwest China

Reservoirs of the Karamay Formation occur as oil sandstones in outcrop in the Karamay oil field. Two types of high-quality reservoirs are present in this field: low-sinuosity channel sandstones and distributary-channel sandstones, each having different sediments, bounding surfaces, and architectural units. The low-sinuosity channel has coarse-grained sediments, a smaller scale, and relatively simple architectural units composed of intrachannel units and lithofacies that occur in a vertically stacked pattern. The distributary channel has fine-grained sediments, a larger scale, and relatively complex architectural units, which are reflected in the following three ways. (1) The distributary channels have three multiorder architectural units: an intrachannel unit, a macroform accretion unit, and a lithofacies unit. (2) Isolated barrier beds, whose permeability is lower, occur near third-order and fifth-order surfaces. (3) The evolution of the distributary channels has two stages. In the flooding stage, the sediments were coarse grained, and the intrachannel units were in a vertically stacked pattern. In the waning stage, the sediments were fine grained, and the intrachannel units were in a laterally stacked pattern. Reservoir heterogeneity of distributary-channel sandstones is revealed in the following three scales. (1) In a microscale range, the growth of pores is associated directly with lithofacies. The pore diameter and the throat width both increase with the coarsening of the grain size in the sediments. The anisotropic pore structure is reflected in the variation of permeability. The horizontal permeability parallel to the paleocurrent is the largest; the horizontal permeability perpendicular to the paleocurrent is intermediate; and the vertical permeability is the smallest. (2) At a channel scale, the fluid-flow units are demarcated according to the isolated barrier bed and porosity and permeability inside the distributary channel. The fluid-flow unit, distributed mainly in the middle–lower interval of the distributary channel or the intrachannel unit, is associated closely with third-order or fifth-order surfaces. The porosity and permeability inside the fluid-flow unit decrease gradually from its center to the margin and upper interval, depending on the spatial distribution of lithofacies. In this kind of lithofacies, which is characterized by a low matrix content and by coarse-grained sediments with large trough cross-beds, high porosity and permeability are commonly found. (3) In a lacustrine delta system, the distributary-channel sandstone has the best porosity and permeability and is the best reservoir.

Subdivision of Permian Fossil Communities and Habitat Types in Northeast Sichuan, South China

ABSTRACT Recent achievement in hydrocarbon exploration in Northeast Sichuan demonstrated that Permian calcareous and argillaceous deposits are the major contributing source rocks. Reevaluation on the hydrocarbon potential of the Permian strata over the whole Yangtze region is thus to be desired. A comprehensive corroboration was carried out at the Shangsi Section, Guangyuan, Northeast Sichuan, which is believed to be another promising area in South China. This article deals with the ecological and depositional conditions of the Permian strata in the section, including the Chihsia, Maokou, Wujiaping, and Dalong formations, which share some similarities with those occurring in the broad Yangtze carbonate platforms. Five fossil communities of Mizzia-Permocalculus, Hayasakaia, Inozoan, Ostracod, and Crinoid were identified in the Yangtze region and described in detail including their components and occurrence. On the basis of the fossil communities and depositional features, 23 habitat types, mainly occurring from inner to outer shelves, were recognized at the Shangsi Section in order to reconstruct the depositional conditions and accordingly to evaluate the paleoproductivity. In addition, the subdivision of Chihsia Formation in the section was revised, which would be of significance for the reconstruction of the Chihsian paleogeography in the study area.

Permo-Triassic paleogeographic, paleoclimatic and paleoceanographic evolutions in eastern Tethys and their coupling

Based on reconstructions of paleogeography, paleoclimate and paleoceanography of the Chihsian, Wujiapingian, Anisian and Norian intervals in the eastern Tethys, the multiple intrinsic relationship and interaction among the lithosphere, the hydrosphere and the atmosphere as well as their evolutions are discussed. It was demonstrated that paleogeographic change of the eastern Tethys and the northward shift of the Pangea during the Permo-Triassic periods governed the coeval paleocurrent pattern and its evolution, which in turn seems to be a key contributor forcing the Permo-Triassic megamonsoon to the climax.Based on reconstructions of paleogeography, paleoclimate and paleoceanography of the Chihsian, Wujiapingian, Anisian and Norian intervals in the eastern Tethys, the multiple intrinsic relationship and interaction among the lithosphere, the hydrosphere and the atmosphere as well as their evolutions are discussed. It was demonstrated that paleogeographic change of the eastern Tethys and the northward shift of the Pangea during the Permo-Triassic periods governed the coeval paleocurrent pattern and its evolution, which in turn seems to be a key contributor forcing the Permo-Triassic megamonsoon to the climax.

Biogeochemical Records at Shangsi Section, Northeast Sichuan in China: The Permian Paleoproductivity Proxies

ABSTRACT The marine primary producers assimilate the atmospheric CO 2 to form the organic carbon in surface water. The organic carbon then settles down through the water column and is removed from the oceans by final preservation in sediments in the form of petroleum or nature gases. The reconstruction of paleoproductivity will thus improve our understanding of the biological processes in the formation of fossil energy resource and help to locate new sites for future exploration. In this study, biorelated elements P, Cd, Al, Ba, as well as redox sensitive element Mo, were analyzed in the 448 rock samples collected from Permian strata at the Shangsi Section, Guangyuan, Northeast Sichuan in China. On the basis of the Ti content, the nondetrital contents of P, Ca, and Al, denoted as P xs , Cd xs , and Al xs , were calculated and found to coincide with the TOC content throughout the whole section, with some enrichment intervals being found in the middle part of Chihsia Formation, topmost Maokou Formation, and Dalong Formation. This suggests that the biorelated elements could be used as proxies for the paleoproductivity here in this section. Ba xs , a paleoproductivity indicator widely used in the paleoceanography, shows insignificant correlation with TOC, P xs , Cd xs , and Al xs , probably arising from the loss of biological barium in anoxic conditions. Compiled with the data of TOC content and P xs , Cd xs , and Al xs , three episodes of enhanced paleoproductivity were identified in Permian strata including the middle part of Chihsia Formation, topmost Maokou Formation, and Dalong Formation.

Sequence Stratigraphic Delineation and Correlation of the Dongying Formation in the Nearshore and Adjacent Sea Areas, Bohai Bay Basin

Abstract Based on the analysis of well logs, seismic data, core studies, the Dongying () Formation along the nearshore and adjacent sea areas, Bohai () Bay basin has been subdivided into three third-order sequences from base to top: namely, sequences SQ1, SQ2 and SQ3. The three sequences have different wireline (SP) log responses, showing triple-section characteristics with SQ1 being characterized by primarily flat baseline with intercalation of relatively low spontaneous potential, SQ2 generally exhibiting weak or moderate amplitude spontaneous potential with finger-shaped peaks, and SQ3 having relatively high spontaneous potential with funnel-shaped log curves. On the basis of the triple-section characteristics, the stratigraphic sequences can be correlated consistently throughout the entire study area. A stratigraphic and sedimentary model for sequences SQ1 to SQ3 of the Dongying Formation in the study area has been proposed. The accommodation space change in the two sides of the asymmetrical basin was examined and the asymmetrical basin has the feature of the asymmetrical accommodation space change. The asymmetrical physiography is a vital factor to influence the accommodation changes in additional to the lake level change, tectonism and sediment supply. This may have important implications to similar basins in other parts of China or elsewhere.

The geobiological formation process of the marine source rocks in the Middle Permian Chihsia Formation of South China

The Chihsia Formation is one of the four sets of regional marine hydrocarbon source rocks from South China. In the past two decades, detailed geochemical and sedimentological studies have been carried out to investigate its origination, which have demonstrated that the high primary productivity plays a primary role in the deposition of sediments enriched in the organic matter. However, the mechanism of this high productivity and the path of the deposition and burial of the organic matter have always been a mystery. Based on the previous studies on the Shangsi Section in Guangyuan City, Sichuan Province, we proposed that the development of the equatorial upwelling due to the sea level rise is responsible for the relatively high productivity in the Chihsia Formation. The sea waters with high nutrient were transported by the sub-surface currents along the equator. High organic carbon flux was deposited on the deeper shelf, and then decomposed by bacteria, leading to the occurrence of anaerobic respiration. The metabolism of the microorganisms consumed the dissolved oxygen in waters, which was in favor of the preservation of the organic matter. This suggested geobiological model integrating with paleoclimatology, paleoceanography and geomicrobiology will help us to understand the causes of this particular sedimentary sequence.

Depositional and Ecological Features of Permian Oxygen Deficient Deposits at Shangsi Section, Northeast Sichuan, China

Because oxygen deficient conditions enhance the preservation of depositional organic matter, analysis on paleooxygenation conditions of depositional environments becomes a routine work in evaluations of potential hydrocarbon source rocks. The article focuses on depositional and ecological features relating to oxygen deficient shelfal environments at the Shangsi Section, as a part of multidiscipline collaboration to reevaluate the hydrocarbon potential of the Middle and Upper Permian, Guangyuan, Northeast Sichuan Province. Ichnofabric Zoophycos, sepiolite-bearing limestones (SBL) were interpreted as indicators of dysaerobic environments. Laminated calcareous and/or siliceous mudstones with pelagic ammonites and radiolarians were believed to be the deposits of anaerobic environments. When rhythmic succession was considered, average strategy was adopted for the oxygenation explanation of a given interval. The anaerobic condition in this Permian section was observed in the upper part of the latest Permian Dalong Formation, in which siliceous mudstones with ammonite, radiolarians develop in association with lower U/Mo ratio, lower biomarker ratio of Pr/Ph, and the highest TOC content. The topmost Maokou Formation, featured by thin-bedded calcareous and siliceous mudstones with ammonite, thin-shelled bivalves, and laminations would be deposits of the quasianaerobic condition. The middle part of the Members I and III of the Chihsia Formation is proposed to be dysaerobic condition as indicated by occurrences of SBL and ichnofabric features, with the Member III being the severe dysaerobic condition.