The Cretaceous Gulong shale oil reservoirs in the Songliao Basin are composed of organic-rich continental shales with high clay content,interbedded with minor amounts of thinly laminated calcareous sandstones and dolomites.Currently,there is a lack of studies on the pore-fissure system and shale oil enrichment pattern of these reservoirs.Based on the data from experiments and analyses including argon ion milling-field emission scanning electron microscopy(FE-SEM),energy-dispersive X-ray spectroscopy(EDS),high-pressure mercury injection analysis,low-temperature nitrogen adsorption experiment,fluorescence thin section observation,X-ray diffraction(XRD)mineralogy of whole rock,and geochemical analysis,we investigate the organic-inorganic pore-fissure system in the Gulong shale and its relationship with shale oil enrichment.The results are as follows:(1)The Gulong shale hosts a dual-porosity reservoir system consisting of matrix pores and microfissures.Matrix pores serve as shale oil enrichment spaces,while microfissures provide both storage spaces and seepage pathways for shale oil;(2)Influenced by multiple factors such as mineral evolution,hydrocarbon generation,and cracking and conversion of crude oil,the Gulong shale exhibits varying pore-fissure combinations at different evolutionary stages.At the mature stage,the shale predominantly contains micron-scale dissolved pores and organo-clay complex pores/fissures(i.e.,pores/fissures with clay minerals as framework and formed as a result of hydrocarbon generation).In contrast,the highly mature stage is characterized by nano-scale organo-clay-complex pores/fissures and bedding fissures;(3)There exists a coupling relationship between the shale oil enrichment and the evolution of pore-fissure combinations for the Gulong shale.The shale oil primarily accumulates within inorganic intergranular and intercrystalline pores at the low mature stage,while it is relatively heavy and predominantly concentrates in dissolved pores and organo-clay complex pores/fissures at the mature stage.At the highly mature stage,the shale oil becomes lighter and largely gets enriched in organo-clay complex pores/fissures and bedding fissures.

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.