Contrast of reservoir-forming conditions of marine-continental transitional shale gas in different sedimentary environments in the Lower Yangtze area of China

In terms of the organic-rich shale of Longtan Formation in different sedimentary environments,some geologic parameters,such as thickness and distribution characteristics,sedimentary facies,organic geochemistry,reservoir char- acteristics and gas-bearing properties,were tested by means of rock pyrolysis analysis,organic element analysis,physi- cal property measurement and X-ay diffraction. The results prove that the organic-rich shale of Longtan Formation in the Lower Yangtze area are widely distributed in strand plain,barrier-lagoon,littoral delta and shallow sea. The average content of total organic carbon ( TOC) of shale is about 2. 43% . The organic matter types are dominated by Ⅱ and Ⅲ. Organic maturity (Ro ) varies from 0. 55% to 2. 66% and the organic matters of shale are at medium-high mature stage. The average value of hydrocarbon potential ( S1 +S2 ) is 2. 96 mg / g. Hydrocarbon potential of shale is at the stage of massive gas generation. The physical property of shale in the study area is poor,which belongs to the special low porosity and permeability reservoir. Content of brittle minerals,such as quartz,feldspar and calcite,is abundant and it is conducive to the development of fracturing of shale gas reservoir. A conservative estimate of adsorption gas content of Longtan Formation is from 0. 34 m3 / t to 0. 96 m3 / t and the storage capacity of shale gas is further enhanced by capillary sealing and hydrocarbon concentration sealing. In the Lower Yangtze area,the comprehensive studies show that the favorable shale gas accumulation environment is in the barrier-lagoon by comparing the reservoir-forming con- ditions of shale gas in different sedimentary environments.