Discovery of nano organo-clay complex pore-fractures in shale and its scientific significance: A case study of Cretaceous Qingshankou Formation shale, Songliao Basin, NE China

Abstract

A new pore type, nano-scale organo-clay complex pore-fracture was first discovered based on argon ion polishing-field emission scanning electron microscopy, energy dispersive spectroscopy and three-dimensional reconstruction by focused ion-scanning electron in combination with analysis of TOC, R_o values, X-ray diffraction etc. in the Cretaceous Qingshankou Formation shale in the Songliao Basin, NE China. Such pore characteristics and evolution study show that: (1) Organo-clay complex pore-fractures are developed in the shale matrix and in the form of spongy and reticular aggregates. Different from circular or oval organic pores discovered in other shales, a single organo-clay complex pore is square, rectangular, rhombic or slaty, with the pore diameter generally less than 200 nm. (2) With thermal maturity increasing, the elements (C, Si, Al, O, Mg, Fe, etc.) in organo-clay complex change accordingly, showing that organic matter shrinkage due to hydrocarbon generation and clay mineral transformation both affect organo-clay complex pore-fracture formation. (3) At high thermal maturity, the Qingshankou Formation shale is dominated by nano-scale organo-clay complex pore-fractures with the percentage reaching more than 70% of total pore space. The spatial connectivity of organo-clay complex pore-fractures is significantly better than that of organic pores. It is suggested that organo-complex pore-fractures are the main pore space of laminar shale at high thermal maturity and are the main oil and gas accumulation space in the core area of continental shale oil. The discovery of nano-scale organo-clay complex pore-fractures changes the conventional view that inorganic pores are the main reservoir space and has scientific significance for the study of shale oil formation and accumulation laws.