The characterization of a marine shale gas reservoir in the lower Silurian Longmaxi Formation of the northeastern Yunnan Province, China

Abstract

Shale gas of Longmaxi Formation, in the northeast of the Yunnan Province, has great potentialities but at a relatively immature exploration stage presently. Geological conditions and reservoir pore structure of the Longmaxi shale gas were clearly studied based on our researches. In the study area, the thickness of Longmaxi black shale gradually increases from southeast to northwest and the maximum is more than 120 m. For 46 shale samples from measured profiles, the total organic carbon (TOC) content and mineral composition of the shale gas reservoir were investigated. The TOC content increases from south to north, with an average content of 0.96% and higher TOC content in the lower segment of the Longmaxi Formation. The main minerals are clays and quartz, with an average content of 45.67% and 27.08% respectively. High pressure mercury intrusion (HPMI), low pressure nitrogen gas adsorption (N2-GA), carbon dioxide gas adsorption (CO2-GA) and scanning electron microscopy (SEM) methods were used to quantitatively and qualitatively analyse the pore structure of the shale gas reservoir. The openness and connectivity of macropores and mesopores gradually decrease from bottom to top in the Longmaxi Formation, and macropores and a few mesopores contribute to the dominant pore volume. Micropores with 0.30–0.40 nm and 0.45–0.60 nm diameters and mesopores of approximately 4 nm in diameter are strongly developed. Interparticle pores (Interp P), intercrystal pores (Interc P), intraparticle pores (Intra P) and organic matter pores (OMP) were observed. The shale porosity is in the range of 1.35–7.49%, with a mean of 3.98%. Porosity generally increases with increasing TOC and clays contents and decreases with increasing carbonate contents. More pore volume and surface area are always developed in shales with high TOC and clays contents and low carbonate contents. The micropore surface area is more developed in shales with high illite/smecnite mixed layers and kaolinite contents, whereas it is less developed in shales with high illite and chlorite contents. Therefore, the Longmaxi shale gas geological conditions are promising in the northeast of Yunnan Province.