Enrichment theory of large area and high abundance marine shale gas and its geological evaluation technology progress and application

The enrichment characteristics and main controlling factors of marine shale gas, geological evaluation and comprehensive area selection of sweet spots are the core content and key to the breakthrough of shale gas exploration and development in Sichuan Basin and its periphery. On the basis of intensive study of geology and geophysical multidisciplinary joint research, field investigation, survey and sampling analysis of key outcrops, core observation and fine description of key exploration wells, this paper presents the enrichment theory and connotation of geological evaluation technology of large area and high abundance shale gas as well as its recent advances, field applications and effectiveness of the Wufeng Longmaxi Formations in southern Sichuan. The progress of enrichment theory of large area and high abundance shale gas includes: the indexes of large area and high abundance shale gas and their corresponding geological connotations were determined, and it was concluded that the development of sulfide anoxic shelf environment controls the distribution of organic rich sediments and organic rich shale in southern Sichuan; the booming of silicon-rich and calcium-rich marine organisms controls the distribution of high organic matter abundance of shale gas, and the sedimentary coupling of multiple geological events controls the sweet spot section and high resource abundance distribution of organic rich shale; it was proposed that the high-quality reservoir lamina and organic matter nano pore throat were developed, and the banded silty sand lamina high-quality shale reservoir had high gas bearing property, high brittleness, network fracture and the best properties of lithology, electrical property, physical property, geochemical property, gas bearing property and compressibility; the evolution model of overpressure accumulation and differential enrichment of shale gas in 4 stages were established. The progress of geological evaluation technology of large area and high abundance overpressure shale gas includes: the small layer correlation and evaluation technology of black graptolite shale formation based on “sequence stratigraphy, chemical stratigraphy and biostratigraphy” is established; five sets of key parameter test devices and test analysis technology for shale reservoir characterization are developed; and the quantitative characterization and evaluation technology of shale gas reservoir and the “double thickness, multi parameter” sweet spot area optimization evaluation technology is formed. Favorable target areas of 36 were selected and total geological resources of shale gas was 10.35×10¹² m³. The application shows that this set of theories and technologies effectively guides and supports PetroChina shale gas geological selection, resource evaluation and reserve discovery, comprehensive evaluation and construction of demonstration area, and provides important support and decision-making basis for scale development and production construction deployment of marine shale gas and the development of shale gas industry.