Pore Structure And Shale Oil Potential Of Qingshankou Formation Shale In Songliao Basin

Shale oil is an important research and exploration field of unconventional oil and gas resources in the world.As an important continental petroliferous basin in China,Songliao Basin has great potential for shale oil exploration and development,thus it is an important direction for the sustainable development of Daqing Oilfield after the depletion of conventional oil and gas resources.In this study,the mineral and maceral compositions,vitrinite reflectance?Ro?,total organic carbon?TOC?content,rock-eval pyrolysis,biomarker parameters,pore types and pore structures characteristics of the Qingshankou Formation shales from seven boreholes in the area of central depression of Songliao Basin were investigated.The relationships between petrology and geochemical parameters and oil contents of these shales were analysed,hoping to comprehensively discuss the influence of geochemical characteristics,sedimentary conditions and pore structure of these organic-rich shales on the formation and enrichment of shale oil.The major conclusions are as follows:?1?The organic matter type of the organic-rich shales of Qingshankou Formation belongs to type I,which is at low maturity to maturity stage at present,characterized by a large amount of liquid hydrocarbon generation.Based on the analysis of the correlations between the original TOC₀ content and geochemical parameters of these shales,it is interfered that the formation of the organic-rich shales is related to the conditions of clay mineral adsorption,nutrient element input,algal bloom and strong reducing bottom water environment.The organic-rich shale at maturity stage of oil window is the basic condition for the formation and enrichment of shale oil.?2?The pore types of these shales are mainly interparticle pores associated with minerals,and a small number of organic pores and microfractures.The number of pores in shale is mainly small pores with diameter less than 10nm,while the pore volume is mainly contributed by large pores with diameter greater than 10nm.Small pore is the main pore throat of shale,the more developed of small pore,the more complex of the pore structure and the better connectivity of pore network;Large pore is the main pore space of shale,the more developed of large pore,the simpler complex of the pore structure and the worse connectivity of pore network.?3?The influence of organic matter on pore structure of these shales within the oil window is reflected in two aspects:on the one hand,the liquid hydrocarbons could fill and block mineral pores and microfractures,that is disadvantage for shale pore development;on the other hand,the solid OM could develop some bubble-shaped macropores and shrinkage fractures,that is advantage of shale pore development.Different types of minerals have various influences of pore structures,for examples,illite is the major contributor of pore throats;chlorite fills the pores and blocks the pore throats;quartz is of advantage for the development of interparticle pores;feldspar contain some dissolution pores or recrystallization in large pores;and carbonate minerals destroy the pores in the form of cements.?4?Shale oil content of Qingshankou formation shows great laterally and vertically heterogeneity.Oil content of the K2qn²+3+3 section is significantly lower than that of the K2qn¹ section,and Oil content in the area of Gulong depression is higher than that in other areas.Generally,Shale oil content increases with the increase of depth.When the buried depth is more than 1700m,the oil saturation index?OSI?value begin to exceed100mg/g,indicating that this depth interval shows potential of shale oil resource.The chemical composition of soluble organic matter in the mature shales of Gulong sag is basically the same as that of the crude oil from this area,which indicates that the shale oil mobility is good and show potential for exploitation.?5?The enrichment of shale oil is affected by both organic matter and mineral compositions.Shale oil content increases with the increase of TOC content and maturity,indicating that hydrocarbon generation is the main factor controlling shale oil enrichment.Shale oil content increases with the increase of illite and quartz content,due to the fact of that these clastic minerals are the main contributors of shale pore space.Shale oil content decreases with the increase of chlorite and feldspar content,related to that these minerals could filling and blocking pore throat in the form of secondary minerals.?6?The pore structures have a significant impact on shale oil enrichment.The OSI content decreases with the increase of pore volume measured by N₂ adsorption,and increases with the increase of pore volume measured by mercury intrusion,indicating that the pores in different size range has various influence on shale oil enrichment.The free oil mainly occurs in the pores larger than 10nm,while the adsorbed hydrocarbon mainly exists in the pores smaller than 10nm.There is a significant positive correlation between OSI content and Hg retention ratio,indicating that the worse the pore connectivity,the better condition for the retention of shale oil.?7?Oil bitumen is mainly developed in low mature shale,and there are a lot of hydrocarbon inclusions in mature shale.According to fluorescence color and fluorescence lifetime,hydrocarbon inclusions can be divided into three types.Different types of oil bitumen and inclusions are closely related to hydrocarbon generation and evolution of source rocks,representing shale oil trapped at different maturity stages.According to the close relationship between fluorescence lifetime and the density of hydrocarbon inclusions,it is predicted that the density of shale oil produced from Gulong sag area is between 0.75-0.80g/cm³,while that produced from Fuyu uplift area is between 0.93-0.94g/cm³.