Pore systems and their correlation with oil enrichment in various lithofacies of saline lacustrine shale strata

Abstract

The potential of China's saline lacustrine shale oil resources is enormous. Currently, the storage space of saline lacustrine shale oil remains unclear, posing significant challenges for commercial development. In this study, we selected the samples from the typical saline lacustrine shale strata of the Lucaogou Formation, Jimusar Sag, Junggar Basin and investigated the pore systems and their relationship with oil content S1 of different lithofacies. The pore structures were quantified by using the combination of low-pressure N2 adsorption and mercury intrusion. TOC assessment, rock pyrolysis, and XRD were utilized for characterizing the organic geochemical and mineralogical parameters. The results showed that the organic matter comprising type III mainly appears in the clay-enriched lithofacies (types I2, II1 and III2), while the organic matter types I, II1 and II2 can be found in other lithofacies. Regarding the nine lithofacies, siltstone has the highest oil content S1, followed by the felsic-enriched lithofacies (types III3 and III1). The enrichment of oil in siltstone, carbonate rocks, and felsic-enriched lithofacies (types III3 and III1) is primarily attributed to macropores. Conversely, in clay-enriched lithofacies (types I2, II1 and III2), the oil content S1 is attributed to both the mesopore fractal dimensions (D1 and D2) and the TOC content. Moreover, the higher the complexity of the mesoporous structure (D2) and the larger the macropore surface area in clayey carbonate felsic shale (II3), the greater the oil content S1. It is further observed that, macropores with size range between approximately 60 nm and 3000 nm are abundant in siltstone and felsic mineral-enriched lithofacies (i.e., carbonate felsic shale III3 and felsic shale III1). Furthermore, the siltstone shows the widest oil-rich macropore size range (range of 70–1000 nm), followed by felsic shale (III1) (range of 150–1000) and carbonate felsic shale (III3) (range of 100–110 nm). Siltstones and felsic-enriched shales are optimal for exploiting saline lacustrine shale deposits.