Characteristics of Microorganisms and Origins of Organic Matter in Permian Shale in Northwestern Sichuan Basin, South China

Permian shale gas, a resource-rich energy source, has garnered significant attention in recent years regarding its organic matter enrichment characteristics. This study conducted detailed observations via scanning electron microscopy (SEM) and optical microscopy to clarify the differences in the types and assemblages of hydrocarbon-generating organisms across Permian shale formations in Northwestern Sichuan, as well as to determine the characteristics of organic matter sources. The types and combinations of hydrocarbon-generating organisms in the Gufeng Formation, Wujiaping Formation, and Dalong Formation in Northwestern Sichuan are systematically summarized. Based on this information, the primary sources of organic matter in the Permian shale were analyzed. Hydrocarbon-generating organisms in the Permian shales of the study area are predominantly acritarchs (a type of planktonic algae), followed by higher plants and green algae. In the Gufeng Formation, acritarchs constituted the vast majority of hydrocarbon-generating organisms, with smaller amounts of higher plants and green algae. At the bottom of the Wujiaping Formation, the relative acritarch content decreases significantly, while that of higher plants substantially increases. In the Dalong Formation, acritarchs regain dominance, and higher plants decline, resembling the Gufeng Formation in microorganism composition. The relative content of green algae shows minimal variation across all layers. Overall, the organic matter sources of Permian shale in the study area were mainly acritarchs (derived from planktonic algae), followed by green algae, and terrestrial higher plants. During the Gufeng Formation period, the sea level was relatively high. The Kaijiang–Liangping Trough in Northwestern Sichuan was generally a siliceous deep shelf. The main source of organic matter was aquatic planktonic algae, containing a small amount of terrigenous input. At the bottom of the Wujiaping Formation, the sea level was relatively low, resulting in the overall coastal marsh environment of the Kaijiang–Liangping Trough, which was characterized by mixed organic matter sources, due to an increase in terrigenous organic matter content. The sedimentary environment and organic matter sources of the Dalong Formation were similar to those of the Gufeng Formation. This research can provide a theoretical basis for exploration and development of Permian shale gas.