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The mixed deposition constitutes the external clastic, intrabasinal, and pyroclastic components in both marine and lacustrine environments. The finegrained mixed lacustrine source rocks are rich in organic matter and present good unconventional energy resources. However, the mixed lacustrine source rock systems have extremely low permeabilities and need hydraulic fracturing to stimulate oil from the complex nanoscale matrix. The role of rock fabric, organic matter, and mineral compositions in full-scale pore structures is still unclear, especially for the source rock from a mixed sedimentary environment. This restricts sweet spot identification. Here, we use three groups of Permian Lucaogou source rock samples with fine laminated, thick laminated, and massive rock fabric to investigate the relationship between rock fabric, organic matter, and pore structure using a combination of mineralogy, organic geochemistry, low-pressure nitrogen adsorption, micro-CT, and mercury injection capillary pressure data. The results indicate that the Lucaogou source rocks mainly contain type I and type II kerogen and show good to excellent hydrocarbon generation potential. The source rock was deposited in a mixed environment with high contents of carbonate and less siliceous minerals, showing good frackability. The mineralogy-based ternary classification shows that the source rock mainly belongs to high total organic carbon (TOC > 4%) mixed carbonate mudstone and high TOC mixed mudstone. For mixed lacustrine source rock, the full-scale pore-fracture distribution shows that the average percentage values of pore volume for micropores (<10 nm), transitional pores (10−100 nm), mesopores (100−1000 nm), and macropores (fracture) (>1000 nm) are 11.14, 21.62, 10.77, and 56.47%, respectively. However, the average percentage values of pore surface area for the abovementioned pores are 62.45, 30.45, 5.47, and 1.63%, respectively. Both quartz and terrigenous clast present a weak-medium unimodal correlation with the TOC content. Both terrigenous clast and clay minerals control the source rock hydrocarbon generation potential. The carbonate and terrigenous clast mineral content play a significant role in micropores and transitional pores, while the clay mineral presents a negative impact on macropore development. The effect of rock fabric on shale oil potential is negligible compared with organic matter abundance. Shales with thick parallel laminae and medium TOC (2% < TOC < 4%) possess favorable shale oil potential.

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The Permian Lucaogou Formation (LCG Fm.) in the Junggar Basin is an organic-rich source rock interval formed in a salinized paleolake, and organic-rich laminated and massive shales are broadly deposited. However, the paleoenvironment difference between laminated and massive shales is still unclear, and the effect of this difference on petrology, pore structure, and organic matter enrichment is significant to shale oil resource evaluation. In this study, organic and element geochemistry, mineralogy, and nitrogen adsorption are used to analyze key differences between laminated and massive shales. The results show that most shale samples present mature thermal stage and oil-prone type II kerogen. Felsic igneous rocks in the continental island arc are their primary mineral component sources. The differences between laminated and massive shales are mainly from their silica origin, paleoclimate, and salinity. The silica origin in laminated shale is primarily from the terrestrial debris influx, while the massive shale is mixed with terrestrial debris and biogenic origin. The silica origin from hydrothermal activities is negligible. The laminated shale prefers to be deposited in a hot and dry climate with weak weathering and relatively higher salinity. However, the massive shale is mainly deposited in a warm and humid climate with moderate weathering and lower salinity conditions. For both laminated and massive shales from the LCG Fm., the warm and humid climate is beneficial to organic matter (OM) accumulation. Paleoproductivity presents an increasingly positive impact on source rocks when the rock fabric transforms from massive to thick laminae. Overall, lower salinity, humid climate, and strong terrigenous clastic input jointly enhance organic matter (OM) accumulation in both laminated and massive shales. Arid and semiarid climates are beneficial to improving the fractability of both laminated and massive shales. The laminated shale presents a relatively wider average nanopore diameter and lower pore volume than massive samples. These findings provide an important insight into the correlation between organic enrichment, laminae, pore structure, and depositional environment. This study has profound implications for understanding the formation mechanisms of laminated and massive shales in the lacustrine paleoenvironment.

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