Abstract
Determining the pore structure characteristics and influencing factors of continental shale reservoir in the oil generation stage is of great significance for evaluating the shale oil reservoir space and analyzing shale oil enrichment mechanism. In this paper, shale from the first member of the Upper Cretaceous Qingshankou Formation (K2qn1) in the Songliao Basin was selected. X-ray diffraction (XRD), Rock-Eval pyrolysis, total organic carbon content (TOC), scanning electron microscopy (SEM), nitrogen gas adsorption (N2GA), and high-pressure mercury injection (HPMI) were used to clarify the composition characteristics of inorganic minerals and organic matter and determine the influencing factors of pore development in the K2qn1 shale. The results show that intergranular pores related to clay minerals and quartz, intragranular dissolution pores related to feldspar, and other mineral intragranular pores are developed. The organic matter pore is less developed, mainly composed of intragranular pores and crack pores of organic matter. Mesopores related to clay minerals are widely developed, rigid quartz particles can protect and support mesopores and macropores, and carbonate cementation can inhibit pore development. Although the TOC contents of shale are commonly less than 2.5%, it has a good positive correlation with porosity; TOC is greater than 2.5%, and the increase of residual oil fills part of the pores, leading to a decrease in porosity with the increase of TOC. Three types (types I, II, and III) of the reservoir space were classified by the combined pore size distribution diagram of N2GA and HPMI. By comparing the characteristics of pore structure parameters, it is found that Type I reservoir space is favorable for shale oil enrichment. It provides scientific guidance for shale oil exploration in the Songliao Basin.
Highlights
With the massive consumption of conventional oil and gas resources, the contradiction between fossil energy supplies is increasing sharply; the exploration and development of traditional conventional oil and gas resources are gradually changing to unconventional ones
The United States and Canada are the main countries that realize the commercial development of shale oil and gas such as Bakken, Barnett, and Eagle Ford Marine shale formation in the United States and a proven unconventional shale oil and gas reservoir in the Western Canadian Sedimentary Basin (WCSB)
The mineral composition of selected shale samples was analyzed by X-ray diffraction (XRD)
Summary
With the massive consumption of conventional oil and gas resources, the contradiction between fossil energy supplies is increasing sharply; the exploration and development of traditional conventional oil and gas resources are gradually changing to unconventional ones Mineral composition and organic matter characteristics are the main controlling factors for the physical properties and pore structure of shale. In the process of hydrocarbon generation, the decarboxylation of organic matter generates CO2, which combines with H2S to form carbonic acid, phenolic acid, and so on [22] It has certain dissolution effect on feldspar and calcite, which could increase the pore space and effectively improve porosity of shale [21]. The main purposes of this study include (1) identifying the main pore types of shale in the K2qn of Songliao Basin, (2) determining the influence of mineral composition and TOC content on the pore structure of K2qn shale, and (3) classifying and evaluating the shale reservoir space of the. K2qn Formation is the main source of rock formation in the basin and the main development section of shale oil ([11]; Figure 1)
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