Sedimentary Characteristics Of The Lower Silurian And Optimizing Evaluation Of Favorable Target Areas Of The Shale Gas In The Eastern Sichuan Basin

The black shale of Lower Silurian Longmaxi Formation is widely regarded as a potential shale gas in middle-lower Yangtze region. However, there are only few studies on it. the study of its sedimentary characteristics and optimizing evaluation of black shale is still relatively weak,also underexplored. Field outcrop observation and sampling were carried out, available drilling data and core samples were collected, also with laboratory microscopic observation, organic geochemistry, X-ray diffraction, SEM and various tests in research goals. Based on these works, the sedimentary features of the black shale in the Lower Silurian Longmaxi Formation were studied in the paper, and the parameters for evaluating the shale gas were analyzed, favorable target areas were perferably selected for the synthetic evaluation systematically.The overall sedimentary environment of Wufeng to Longmaxi Formation is the continental shelf, which can be subdivided into five periods: ○1 Jiancaogou Formation(Linxiang Formation) is calcareous shallow-water shelf, ○2 the lower part of Wufeng is muddy deep-water shelf, ○3 Guanyinqiao is gray-muddy shallow-water shelf, ○4 the lower Longmaxi is muddy deep-water shelf, ○5 the upper Longmaxi is sand-muddy shallow shelf deposits. Deep water shelf gradually transit to bathyal-deep sea basin on a plane from Heishui to Tianba. Longmaxi shale thickness gradually thickens from cross-section along the ends to the center.In the study area, the types of kerogen of black shale of Longmaxi Formation is dominated by I-II types mainly; The TOC contents from 1% to 4%, and it averages about 1.7%; Vitrinite reflectance(Ro) values greater than 2.2%, usually in 2.4% to 4.3%, has been in over mature evolutionary stages. The Ro in the core region was larger than 3.6%(maximum up to 4.3%). The TOC was gradually decreased from Wufeng to the lower of Longmaxi Formation in vertical and similar with structure contour, distributed in the direction of SW-EN in ring pattern.Studies have shown that the black shale minerals mainly includes quartz, clay minerals, followed by pyrite, carbonate rocks in the Lower Silurian. The contents of clay mineral ranged from 24.2% to 60.6%, with an average content of 44.14%, and clay mineral ingredients are eamonn interlayer, illite and chlorite, with the highest content of illite. Quartz mineral contents ranged from 27.5% to 60.9%, with an average content of 40.01%. The BET surface area is 8.156m2/g~30.54m2/g, with an average of 14.796m2/g.The shale porosity is poor, mainly in the 4% to 8%, with an average of 6.17%. The micro-pore is open, and good connectivity. Intergranular pore, intragranular pore, organic matter and microfractures pore were recognized in the black shale of the Longmaxi Formation. Intergranular pore subdivided into intergranular framework pore, the pore between the clay, granules edged pore, supergene dissolved pore, and intragranular pore include the pores within pyrite grains of strawberry spherical, inner hole between clays and holes of diagenesis.On the basis of the division standard of favorable target area, by integrating shale effective thickness, organic carbon content, organic matter maturity, shale outcrops and depth and favorable structures and others, three favorable target areas were delineated and evaluated in the east Sichuan: Xishui block, Shizhu block, Wuxi block.