Geochemical Characteristics and Formation Environment of Mudstones in the Xueyuanhe Formation in the Raggyorcaka...

The Permian source rocks in the Qiangtang Basin exhibit good potential for oil and gas exploration. Elucidating their organic matter enrichment mechanism and sedimentary paleoenvironmental characteristics is of great significance for evaluating the basin's Paleozoic oil and gas resources. This paper presents a systematic study of the Middle Permian mudstones from the Xueyuanhe Formation in the Raggyorcaka area of the North Qiangtang Depression (NQD), integrating petrological, organic geochemical, and elemental geochemical analyses. The key objectives are to characterize the hydrocarbon source rock potential, paleoenvironment, provenance, and tectonic setting of these mudstones, and thereby reconstruct their organic matter enrichment mechanism. The total organic carbon and chloroform bitumen "A" contents of these mudstones are in the range of 0.42% to 0.60% (avg. of 0.52%) and 0.006-0.014% (avg. of 0.009%), respectively. They are interpreted as poor source rocks. Kerogen index (TI), kerogen H/C and O/C atomic ratios indicate that the organic matter type is predominantly type II₂ kerogen, with minor type III kerogen. Vitrinite reflectance (Ro) and pyrolysis peak temperature (T _max) of samples exhibit a high degree of thermal evolution. Medium chemical index of alteration (CIA) values of samples indicates a warm and humid climate. Redox indexes (V/Cr, Ni/Co, U/Th, Mo_EF-U_EF) reveal that these mudstones were deposited in oxic conditions. Low Sr/Ba and Ca/(Ca+Fe) ratios indicate a low-salinity water column during the Xueyuanhe Formation mudstones deposition. Fe/Ti-Al/(Al + Fe + Mn) diagram and Al-Fe-Mn ternary diagram indicate that these mudstones were not affected by hydrothermal activity. Terrigenous flux proxies (Al, Zr, Ti_EF, and Zr_EF) indicate that there was less input of terrigenous detritus during sedimentation period. Relatively low Si_bio, Ba_bio, and P/Ti values reveal that the level of primary productivity in the surficial water column was relatively low. Analysis of sediment provenance and tectonic setting reveals that the Xueyuanhe Formation mudstones are derived mainly from felsic volcanic rocks, with possible minor inputs from basaltic and sedimentary sources, the source area corresponding to a continental island arc environment. The warm and humid paleoclimate during deposition of the Xueyuanhe Formation favored organic matter preservation. However, the combination of low primary productivity, oxic water conditions, slow sedimentation rates, and continuous input of felsic terrigenous detritus inhibited the enrichment of organic matter, ultimately resulting in the poor hydrocarbon generation potential of these mudstones.