Abstract
In addition to marine and marine-continental transitional strata, the continental ones are also widely distributed in various oil and gas-bearing basins in China. The continental shale generally provides favorable material bases for hydrocarbon generation, such as wide distribution, large thickness, multiple series of strata, high TOC content, nice organic matter type, and moderate thermal evolution. Part of such shale contains shale oil, but the pore space characteristics for the occurrence of this oil are not thoroughly studied. In order to accurately and quantitatively characterize the pore space where the continental shale oil in different types of lithofacies occurs, we sampled the rock cores from the Middle Jurassic Lianggaoshan Formation in the southeastern Sichuan Basin of the Upper Yangtze Area. The TOC content and mineral composition were analyzed, and we also carried out experiments on CO2 and N2 adsorptions, high-pressure mercury injection, and wash oil. Results show significant differences in pore space characteristics for the occurrence of shale oil in different types of lithofacies. In organic-rich mixed and clayey mudstones with the highest TOC content, the free shale oil, occupying the largest reservoir space, mainly occurs in macropores and mesopores, and the adsorbed shale oil, occupying the largest reservoir space, mainly occurs in mesopores. In the organic-bearing clayey mudstone, which has a higher TOC content, the free shale oil takes a larger reservoir space and mainly occurs in macropores, followed by mesopores, and the absorbed one, occupying a larger reservoir space, mostly occurs in micropores and then the mesopores. The organic-bearing mixed mudstone has a moderate TOC content, in which the free shale oil occupies a smaller reservoir space and primarily occurs in mesopores, followed by macropores, and the absorbed one, which takes a larger reservoir space, all occurs in mesopores. In the fine sandstone, the free shale oil occupies a smaller reservoir space and primarily occurs in mesopores, while the absorbed one occupies a smaller reservoir space and all occurs in mesopores.
Highlights
The change in geological concepts and the achievements made in horizontal well drilling and fracturing technologies in recent years brought a remarkable breakthrough in China’s exploration of marine shale gas, and various shale gas fields have been successively built at Jiaoshiba, Weiyuan, Changning, FushunYongchuan, Zhaotong, Luzhou, etc. [1–7] Besides, continental strata are widely distributed in China’s oil and gasbearing basins, including Sichuan, Junggar, and Bohai Bay Basins
(1) It is classified into 4 types according to the mineral composition: clayey shale, calcareous shale, and mixed shale (Figure 3). (2) It is classified into 3 types according to the TOC content: organic-lean shale (TOC content = 0% ~ 1%), organic-bearing shale (TOC content = 1% ~ 2%), and organic-rich shale (TOC content ≥ 2%)
The shale samples at the same depth were divided into two groups, one of which was analyzed to quantitatively characterize the full-scale pore size of samples containing the shale oil by characterizing the distribution features of micropores, mesopores, and macropores through experiments on CO2 and N2 adsorptions and highpressure mercury injection, respectively [46–55]
Summary
The change in geological concepts and the achievements made in horizontal well drilling and fracturing technologies in recent years brought a remarkable breakthrough in China’s exploration of marine shale gas, and various shale gas fields have been successively built at Jiaoshiba, Weiyuan, Changning, FushunYongchuan, Zhaotong, Luzhou, etc. [1–7] Besides, continental strata are widely distributed in China’s oil and gasbearing basins, including Sichuan, Junggar, and Bohai Bay Basins. The continental shale in Jianghan Basin is featured with wide distribution, large thickness, multiple series of strata, high TOC content, moderate thermal evolution, and good kerogen type. Oil companies, such as PetroChina and the Sinopec Group, have attached great importance to the geological research on continental shale oil and gas and have planned to exert more efforts in exploration during China’s “14th Five-Year Plan” [8–14]. To reveal the effects of shale reservoir characteristics on the movability of shale oil and its action mechanism in the lower third member of the Shahejie Formation, Ning et al selected samples with different features and analyzed them using N2 adsorption, high-pressure mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR), high-speed centrifugation, and displacement image techniques [17]. The data before and after extraction were compared to obtain the pore space characteristics
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