Mineralogical composition and organic matter characteristics of lacustrine fine-grained volcanic-hydrothermal sedimentary rocks: A data-driven analytics for the second member of Permian Lucaogou Formation, Santanghu Basin, NW China

The volcanic and/or hydrothermal activities are considered as important factors affecting the sedimentation processes of fine-grained sedimentary rocks in some cases. Recent works demonstrate that the Permian Lucaogou Formation in Santanghu Basin (NW China) was so strongly influenced by synsedimentary volcanic and hydrothermal activities that some researchers classify it as fine-grained volcanic-hydrothermal sedimentary rock. In this work, we compile mineralogical and organic geochemical data on 566 samples from 183 m continuous cores of Well L1 in Santanghu Basin and combine descriptive statistical method to dissect the second member of Permian Lucaogou Formation fine-grained volcanic-hydrothermal sedimentary rocks. The dataset shows that the second member of Lucaogou Formation lacustrine fine-grained volcanic-hydrothermal sedimentary rocks in Well L1 can be divided into three distinct intervals based on mineralogical composition and organic matter characteristics, although considerable vertical variability exists. The most striking differences in mineralogical composition among the three intervals are the vanishing of glauberite in the upper interval and the missing of plagioclase in the lower interval. Combined with published genetic mineralogy studies, the mineralogical composition dataset suggests that the second member of Lucaogou Formation in Santanghu Basin was deposited in an alkaline, reducing lake. The middle and upper intervals experienced three and two synsedimentary volcanic cycles, respectively, while lower interval was significantly influenced by hydrothermal fluid input prior to volcanic eruption. In addition, the lower and middle intervals were deposited in saline conditions, while the upper interval was deposited in freshwater conditions due to increased freshwater input. Based on the correlations analysis among the mineral contents and organic geochemical parameters, we suggest that the volcanic activity in the second member of Lucaogou formation is an important driver for the development of high-quality source rocks. By contrast, the hydrothermal activity has a dual effect on organic matter accumulation. In detail, the anoxic environments related to the hydrothermal activity is conducive to organic matter preservation, while the accompanying high-rate chemical precipitation of dolomite has an effect of dilution on organic matter accumulation. Moreover, the second member of Lucaogou Formation lacustrine fine-grained volcanic-hydrothermal sedimentary rocks in Well L1 are characterized by considerable oil generation potential and good fracability, but low oil content. This suggests that the fine-grained sedimentary rocks with good oil-generating capacity and good fracability may not guarantee the occurrence of shale oil sweet spot zone, and a comprehensive assessment in combination with oil retentive capacity is needed.