Source/reservoir characteristics and shale gas “sweet spot” interval in Shahezi mudstone of Well SKII in Songliao Basin, NE China

Abstract

Samples from the Shahezi mudstone of Well SongkeII (SKII) in the Songliao Basin were analyzed for mineral composition, source rock geochemistry, and reservoir pore structure. It is revealed that the Shahezi mudstone is a typical clay rock, with an average TOC of 2.12% (dominantly Type II–III organic matters) and a vitrinite reflectance Ro of 2.28–2.34%. The pore structure of the Shahezi mudstone reservoir mainly comprises intergranular pores and interlayer fractures in clay minerals, and also organic pores (including fractures at the particle-organic edges and also pores in some unstructured organic matters). The Shahezi mudstone has a specific pore volume of 0.01–0.04 cm3/g. Correlation analysis of specific pore volume and mineral composition reveals that the pore development of the Shahezi mudstone is mainly controlled by inorganic minerals. The nitrogen adsorption and high-pressure mercury intrusion tests were combined to obtain the full-scale pore size distribution and specific pore volume of the Shahezi mudstone, and the shale gas “sweet spot” interval is inferred to be 3,500–3,700 m. The study results can be referential for understanding the pore characteristics of highly-mature continental mudstone and locating the “sweet spot” of shale gas globally.