Differences between Laminated and Massive Shales in the Permian Lucaogou Formation: Insights into the Paleoenvironment, Petrology, Organic Matter, and Microstructure

Abstract

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.

Classification and Control Factors of Reservoir Types in Hybrid Sedimentary Rocks: A Case Study on Lucaogou Formation in the Jimusaer Sag, Junggar Basin

“…Three main types of diageneses have been identified in the rocks of the Lucaogou formation, 25 namely, compaction, dissolution, and cementation, all of which affect the reservoir quality. The HSR of the Lucaogou formation, buried at depths ranging from 2000 to 4800 m, experience significant compaction, 26 resulting in a denser rock matrix and a dramatic decrease in porosity with increasing burial depth. As observed between the rock particles ( Figure 12 ), stylolites indicate compaction-induced changes.…”