Formation mechanisms and exploration breakthroughs of new type of shale gas in Cambrian Qiongzhusi Formation, Sichuan Basin, SW China

Abstract

The basic geological characteristics of the Qiongzhusi Formation reservoirs and conditions for shale gas enrichment and high-yield were studied by using methods such as mineral scanning, organic and inorganic geochemistry, breakthrough pressure, and triaxial mechanics testing based on the core, logging, seismic and production data. (1) Both types of silty shale, rich in organic matter in deep water and low in organic matter in shallow water, have good gas bearing properties. (2) The brittle mineral composition of shale is characterized by comparable feldspar and quartz content. (3) The pores are mainly inorganic pores with a small amount of organic pores. Pore development primarily hinges on a synergy between felsic minerals and total organic carbon content (TOC). (4) Dominated by Type I organic matters, the hydrocarbon generating organisms are algae and acritarch, with high maturity and high hydrocarbon generation potential. (5) Deep- and shallow-water shale gas exhibit in-situ and mixed gas generation characteristics, respectively. (6) The basic law of shale gas enrichment in the Qiongzhusi Formation was proposed as “TOC controlled accumulation and inorganic pore controlled enrichment”, which includes the in-situ enrichment model of “three highs and one over” (high TOC, high felsic mineral content, high inorganic pore content, overpressured formation) for organic rich shale represented by Well ZY2, and the in-situ + carrier-bed enrichment model of “two highs, one medium and one low” (high felsic content, high formation pressure, medium inorganic pore content, low TOC) for organic-poor shale gas represented by Well JS103. It is a new type of shale gas that is different from the Longmaxi Formation, enriching the formation mechanism of deep and ultra-deep shale gas. The deployment of multiple exploration wells has achieved significant breakthroughs in shale gas exploration.