Study on main controlling factors of high-felsic shale formation in Qiongzhusi Formation in Jingyan and Ziyang areas

The Lower Cambrian Qiongzhusi Formation in the Jingyan and Ziyang areas is an important shale gas replacement system in the Sichuan Basin, characterized by high-felsic content and favorable reservoir physical properties. However, the main controlling factors of the formation of high-felsic shale are not yet clear, which constrains reservoir evaluation and exploration deployment for this system. In the study, two typical wells in the middle section of the elongated trough in Jingyan and Ziyang areas were taken as the research objects. Based on experiments including scanning electron microscopy, X-ray diffraction mineral analysis, geochemical element analysis, particle size analysis, and total organic carbon (TOC) content determination, systematic research on the mineral composition characteristics and the controlling factors for the hign content of felsic minerals was conducted. The research results indicated that: (1) the Qiongzhusi Formation in the study area developed in a semi-deep-water to deep-water transitional trough sedimentary system, characterized by an oxygen-poor to anoxic state (U/Th>0.75), relatively high paleoproductivity (Babio>500 μg/g), and significant variations in sedimentation rates.(2) From shallow-water ramps to deep-water ramps, the felsic content decreased from 66.7% to 61.4%, and TOC content increased from 0.51% to 2.96%. The mineral grain size in shallow-water ramps (average > 35 μm) was coarser, and the sorting was poorer compared to that in deep-water ramps (average < 30 μm). The high-felsic shale of the Qiongzhusi Formation in the study area was mainly controlled by relatively proximal provenance, an oxygen-poor to anoxic environment, and differential sedimentation rates. The shallow-water ramps, due to their closer provenance and higher sedimentation rate, had coarser and poorer-sorted mineral grains. The distance from the source, redox conditions, and sedimentation rate jointly controlled the formation of high-felsic shale, establishing a development model of "low TOC in shallow-water ramps and high TOC in deep-water ramps with high-felsic contents". The findings can provide theoretical support for the evaluation, prediction, and subsequent exploration and development of shale gas reservoirs in the Sichuan Basin.