Xie Xinong

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.

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.

Assessment on Redox Conditions and Organic Burial of Siliciferous Sediments at the Latest Permian Dalong Formation in Shangsi, Sichuan, South China

ABSTRACT The redox sensitive elements, molybdenum (Mo) and uranium (U), in marine sediments from the latest Permian Dalong Formation at the Shangsi Section, Northeast Sichuan, South China, were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) to determine their response to a range of redox conditions, and to estimate the organic carbon burial rate. On the basis of the correlation between authigenic Mo abundance and organic carbon content in modern oceans, the organic carbon burial rates were calculated for the rocks at Dalong Formation, ranging from 0.48–125.83 mmol/(m2·d), which shows a larger range than the mineralization rate of organic carbon at the continental margins (1.6–4.23 mmol/(m2·d)). The Zr-normalized Mo and U abundances show large fluctuations in the entire section. The maxima of Zr-normalized Mo abundance and thus the maxima of the organic carbon burial rates were observed at the interval between the 155th and 156th beds (404–407 m above the base of Middle Permian). A decrease (the minimum) in U/Mo ratios is present in this interval. It is speculated that the oxygen-limited conditions and ultimately anoxia or euxinia may develop within this depth interval. In contrast, an enhanced enrichment of Zr-normalized U abundance is found, in association with less enrichment in Zr-normalized Mo abundance in the interval from the 151st to 154th beds (395–404 m above the base of Middle Permian), inferring the dominance of a suboxic/anoxic depositional condition (denitrifying condition), or without free H2S. The presence of small quantities of dissolved oxygen may have caused the solubilization and loss of Mo from sediments. It is proposed that the multiple cycles of abrupt oxidation and reduction due to the upwelling at this interval lead to the enhanced accumulation of authigenic U, but less enrichment of Mo. A decrease in the contents of U, Mo, and TOC is found above the 157th bed (407 m above the base of Middle Permian), in association with the enhanced U/Mo ratio, suggesting the overall oxic conditions at the end of the Dalong Formation.

Lateral migration pathways of petroleum in the Zhu III subbasin, Pearl River Mouth basin, South China Sea

Abstract The discovery of the WC13-1 and WC13-2 oil fields in the Zhu III subbasin of the Pearl River Mouth basin in the South China Sea has led to debate as to the oils on Horst Qionghai (HQH) are sourced from Half-graben Wenchang-A (HGWC-A) or Half-graben Wenchang-B (HGWC-B). Using the previous findings of sequence stratigraphers, this paper further analyzes the sedimentological characteristics of the Zhuhai and Zhujiang Formations and points out that the sandstones deposited on the unconformities (basement, T 6 and T 7 ) possess good porosity, permeability and lateral connectedness for hydrocarbon migration. The depositional systems and facies for the major reservoir rocks had gradually changed from fan-delta and semi-enclosed bay via barrier islands and washover to tidal flats, tidal channels and tide-influenced shoreface in response to the rise in sea level in the early Miocene. The sandy carrier beds of the transgressive systems tract in the Zhujiang Formation, especially in Zhujiang Formation II, act as the principal migration conduits for the WC13-1 and WC13-2 oil fields. Measurement of sandstones in drilling holes shows an increase in porosity in the Zhujiang Formation II and implies that the oil had ever passed through. Furthermore, the structural morphologies of T 5 and T 4 demonstrate that there are two salient structural noses connecting HQH with the HGWC-B and plunging into HGWC-B. T 5 , a regional muddy seal, seals the porous standstones underneath and accounts for the key role played by the sandy carrier beds in transporting the oils generated in HGWC-B to HQH. Correlation of oils in HQH and HGWC-B supports the plausibility of this migration pathway.

Conduit System and formation mechanism of heat fluids in diapiric belt of Yinggehai basin,China

The conduit system of heat fluids in diapiric belt of Yinggehai basin is dominantly vertical faults and fractures. Detailed research on the formation mechanism and their occurrence features shows that the faults and fractures can be classified into three types: intrastratal dispersive hydrofracture, puncturing fault and upwarping-extensional fault. The development of the fault and fracture system not only resulted in the changes of the temperature and pressure fields in the basin, but also affected the hydrocarbon migration in the overpressured system. These faults and fractures constituted the main pathways for vertical hydrocarbon migration, and opening and closing intermittently led to episodic expulsion of overpressured fluid compartment. Thus there formed the pool-forming model of multi-source mixing and ploy-stage migration and accumulation for hydrocarbons in the Yinggehai basin.

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.

Provenance of Central Canyon in Qiongdongnan Basin as evidenced by detrital zircon U-Pb study of Upper Miocene sandstones

Deep-water canyon systems can provide important sandstone reservoirs for deep-water oil and gas exploration in the South China Sea; however, the sedimentary provenance of the Central Canyon in the Qiongdongnan Basin remains controversial. In this work, detrital zircon grains from three drilling sandstones in the Upper Miocene Huangliu Formation in the western part of the Central Canyon were analysed by LA-ICP-MS for U-Pb ages, in order to constrain their provenance. One hundred and ninety-one zircon grains yield concordant U-Pb ages ranging from 28.6 to 3285 Ma. Most of them show oscillatory or linear zoning in CL-images and high Th/U ratios (>0.1), suggesting that they are magmatic zircons. Three major age clusters at about 30 Ma (N=6), 220–270 Ma (N=29), and 420–440 Ma (N=13), and five minor age clusters at 70–110 Ma (N=7), 150–170 Ma (N=4), 800–850 Ma (N=11), 1800–2000 Ma (N=16), and 2400–2600 Ma (N=7), can be identified in the age spectrum, which are very similar to those of the Upper Miocene sandstones and modern river sands in the Red River area, but different from those of other nearby regions (e.g., Hainan Island, the Pearl River area, and the Mekong River area) in Southeast Asia. The major age peak at about 30 Ma in our samples is consistent with the timing of tectonothermal events in the Red River Fault Zone. Therefore, we suggest that the provenance of the western part of the Central Canyon, in the Qiongdongnan Basin, was fed dominantly by the Paleo-Red River system during the Late Miocene.

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.