| Four sets of Palaeozoic shales were developed in the Sichuan Basin, and they were characterized by abundant organic content, good organic type and widely distributed, and were therefore regarded as major shale gas strata for exploration and development in China. In this study, we choose three fresh outcrop and one drilling Upper Ordovician-Lower Silurian sections in Sichuan Basin and surrounding areas and collected a total of 314 samples. Based on the organic geochemistry study of high-over matured Wufeng Formation-Longmaxi Formation shale, the type and abundance of organic matter were recoved; By studying organic carbon isotopes, and element geochemistry characteristics, controlling factors of organic matter contents in Wufeng Formation-Longmaxi Formation shale were analyzed, and formation model for high quality source rocks was proposed. In addition, according to the relationship between gas content and organic matter, pore characteristics, mineral composition, regional structural features, we discussed the controlling factors of gas contents in Wufeng Formation-Longmaxi Formation shale, explored favorable areas of Wufeng Formation-Longmaxi Formation shale in Sichuan Basin, and draws the following conclusions.Residual organic matter abundance is higher in Wufeng Formation and lower part of Longmxi Formation, especially occur a peak at the Wufeng Formation-Longmaxi Formation boundary, and then decrease in the upper part of the Longmaxi Formation with the stable values of ～1%. Those observations are a typical variation of organic matter contents under shelf depositional environment. The main type of organic matter for Wufeng-Longamxi shale was type I, and the organic matters were derived from lower hydrobiont and algae.The thermal evolution has reached a high-over mature stage for Wufeng-Longmaxi shale in Sichuan Basin, and the vitrinite reflectance is generally greater than 2.0%. The original organic matter abundance of Wufeng Formation-Longmaxi Formation shale is clearly different from residual organic matter abundance after restoration. The original organic matter contents is 1.65 times as high as the residual TOC after restoration; the thickness of original organic matter contents>2%interval is twice that of residual TOC>2% interval. All Wufeng-Longmaxi shales have the original organic matter contents of above 2% in Shizhu area.Strongly reducing condition caused by tectonic activity and sea level rising controlled organic matter enrichment for Wufeng Formation and Longmaxi Formation bottom shale. The organic-rich shales of Wufeng-Longmaxi mainly were deposited in strongly reducing euxinic deepwater environment, which favored the preservation of organic matter. The relative sea level fall caused by tectonic uplift in Late Longmaxi resulted in the increase of terrigenous detrital material input and oxic conditions, which account for the decreased TOC content in Upper Longmaxi Formation. The formation pattern of Wufeng- Longamxi bottom organic-rich shale belongs to "deep-water shelf" and "underwater hypoxia". The sea water is deeper in Southern Sichuan deep watter shelf area, Eastern Sichuan deep water shelf area and Northern Sichuan deep water shelf area, which surround Chuanzhong uplift and Qianzhong uplift in Wufeng and early Longmaxi period. Dissolved oxygen content decreased from surface water to sea bottom, so the sediment - water interface was in an euxinic (extreme hypoxia) environment. During this period, the organic matter enriched in sediments due to favorable preservation conditions.The TOC contents are positively correlated with gas content, indicating the organic matter abundance is the key factor for controlling shale gas potential. Microporous, mesoporous and macroporous have developed in Wufeng Formation-Longmaxi Formation shale and the development of organic nanopores in high-over mature shale provide space and specific surface area for shale gas storage and adsorption. Wufeng-Longmaxi shale is rich in brittle minerals which are mainly composed by quartz, which are favorable for the hydraulic fracturing. Tectonic activity also has some impact on shale gas potential. For example, the tetonic uplift can lead to the partial loss shale gas so shale gas potential is generally low in faulting zone and near the outcrop area.Wufeng Formation-Longmaxi Formation shale in Sichuan Basin has good accumulation condition. The thickness of Wufeng-Longmaxi bottom organic-rich shale is mainly depended on the distance from depocenter and decrease from the depocenter to the margin. Three depocenter (Zhenba to Chengkou area, Shizhu to Wanxian area and Luzhou to Yibin area) are the favorable areas for quality shale (TOC>2%) development, and also are the preferred areas for shale gas exploration and development. Wufeng Formation and Longamxi bottom section are the favorable intervals for shale gas reservoir. |
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