Geological conditions and controls of gas content of Carboniferous shale gas reservoirs in western Guizhou

Abstract

The Carboniferous Jiusi Formation is an important organic-rich shale stratum developed in Guizhou province, yet no breakthrough has been achieved for shale gas exploration and development. To study the geological conditions and controlling factors of shale gas accumulation in the Jiusi Formation, core and outcrop samples from the western Guizhou were analyzed using a series of methods, including organic geochemistry, XRD, FE-SEM, under overburden pressure, in-situ desorption and high pressure isothermal. The organic matters in the Jiusi shale are primarily type Ⅱ kerogen with a high organic carbon content, and are at the early stage of over maturity. Quartz and clays are the dominant minerals. The brittle mineral content is high, which is conducive to hydraulic fracturing. The reservoir is characterized by ultra-low porosity and permeability. There are mainly inter- and intragranular pores, organic pores, and micro-cracks. Small pore diameter, well-developed nano-scale pores, big specific surface area and total pore volume provide favorable conditions for shale gas enrichment and preservation. High in-situ desorption gas content (with an average value of 1.95 m3/t) and strong adsorption capacity (with a mean value of 3.10 m3/t) suggest good potential for shale gas. The adsorbed gas quantity is positively correlated with TOC, Ro, clay mineral content, porosity, specific surface area and total pore volume, and negatively correlated with average pore size.

Keywords

• shale gas
• accumulation characteristics
• gas-bearing property
• jiusi formation
• carboniferous
• western guizhou