Shale Oil and Gas Generation Process and Pore Fracture System Evolution Mechanisms of the Continental Gulong Shale, Songliao Basin, China

Continental shale oil is a new source of oil, and its formation and evolution mechanism is one of the most important scientific problems in its effective exploration and development. In this work, the hydrocarbon generation mechanism, occurrence of oil and gas, and pore structure characteristics were studied through a series of pyrolysis experiments using an improved closed experimental system, combined with chloroform extraction, two-dimensional nuclear magnetic resonance, and computed tomography, for the continental Gulong Shales, Songliao Basin. The results show that hydrocarbon generation from organic matter of continental Gulong shale follows a sequential reaction model, with adsorbed oil as an "intermediate" at Ro of 0.9–1.1%. The free oil reaches the generation peak and begins to convert to natural gas, and the gas/oil ratio increases rapidly when Ro > 1.3%. These suggest that the favorable oil generation window for Gulong shale is that Ro is between 0.9 and 1.6%. With oil generation, the shale pore structure and permeability change with maturity accordingly. The corresponding Ro for rapid porosity and pore volume increase is 0.9–1.2%, while Ro is greater than 1.2% for rapid permeability change as a result of the large amount of adsorbed oil converting into free oil. It is found that hydrocarbon generation and pore-forming materials are mostly lamalginite and the organic-clay complex, and their volumes shrank while generating hydrocarbon, forming pores and fractures along the layers and "sponge-like" pores, respectively. When Ro > 1.2%, the shale oil generation peak, pore development peak, and overpressure evolution peak are coupled, providing favorable conditions for shale oil and gas enrichment in continental Gulong shale. These experimental findings mutually prove and confirm the practice of Gulong shale oil exploration, and it may have important theoretical and practical significance for continental shale oil exploration and discovery in other basins.