Sedimentary Characteristics Of Silurian Longmaxi Black Shale And Its Significance For Shale Gas In Southeast Of Chongqing

The study area is located in the southeastern margin of Sichuan basin, and it is one of the first shale gas resources strategy guide experimental investigation area in China. In order to evaluate the accumulation potential of shale gas in this area, based on the outcrops, cores observation, chip identification, scanning electron microscope, field emission scanning electron microscope, X-ray diffraction, and rock property analysis etc., we analysed the sedimentary characteristics, rock mineral elements, geochemical characteristics and oil and gas geological features of Longmaxi black shale. And then, accumulation potential of shale gas in Longmaxi formation of Lower Silurian was evaluated by the method of "hydrocarbon source rocks-reservoir-gas content".The Lower Silurian Longmaxi Formation in the southeast of Chongqing can be divided into a third-order sequence, including transgressive systems tract and high stand systems tract. Transgressive systems tract is mainly composed of deep shelf and bathyal facies, while the high stand systems tract is mainly composed of shallow shelf and deep shelf facies. Mineral and element analysis results show that Longmaxi black shale is mainly composed of clay mineral with average content41.7%, quartz with average content38.7%, feldspars with average content11.1%, carbonate minerals with average content8.2%and pyrite etc. The reducibility of sea water in work area experienced two increase-decrease cycle changing processes in the early Longmaxi stage, and palaeosalinity is relatively high in northern part of the area with average palaeosalinity23.1‰, and in western part of the work area, average palaeosalinity is22.1‰. The sea water of Longmaxi stage is brackish and semi-brackish. Organic matter in Longmaxi black shale is mainly composed of type Ⅰ. Longmaxi balck shale, as a hydrocarbon source rock which has average content of TOC2.05%and average Ro2.68%, has entered into a high mature to overmature stage. As a reservoir, the average effective porosity is2.806%, and the average permeability is0.0199×10-3μm2in Longmaxi black shale and it is belong to ultra-low porosity and ultra-low permeability reservoir. Multiple types of pores and cracks, including intergranular pores, dissolution pores, pores in crystal clay mineral, expulsion residual pores in organic matter, etc. developed well in Longmaxi black shale. Moreover, fractures including diagenetic fractures, structural fractures and diagenetic-structural fractures also developed well, which are filled and semi-filled. Considering various geological conditions, shale gas accumulation potential is relatively favorable, especially in middle and northern part of the work area, and preliminary evaluation shows that the geological resources can reach13.6trillion m3in work area according to "hydrocarbon source rocks-reservoir-gas content" evaluation system and principles of volumetric evaluation.