Fine-grained sedimentary characteristics and evolution model of Permian Fengcheng Formation in Hashan area, Junggar Basin

Abstract

The geological conditions of Hashan area on the northwestern margin of the Junggar Basin are complicated and it is difficult to explore. It is of great significance to clarify the sedimentary and evolutionary characte-ristics of source rocks in the Permian Fengcheng Formation in this area for expanding oil and gas exploration on the basin margin. Based on the analysis of whole-rock XRD, elemental geochemistry, organic matter abundance, thin section identification, and lithofacies association characteristics, this paper conducted a comparative analysis of the sedimentary characteristics, lithology, and lithofacies association characteristics of the Fengcheng Formation in the Mahu Sag and Hashan area, restored the ancient sedimentary environment evolution sequence in the study area, and established a model of ancient sedimentary evolution. The research results show that the lithofacies association characteristics and ancient sedimentary environment evolution sequence of the Fengcheng Formation in the Hashan area are highly similar to those in the Mahu Sag, which are generally fine-grained sediments mixed from multiple sources in an alkaline lake with a volcanic background, and various lithofacies associations are developed. The sedimentary paleoenvironment has phased evolution characteristics. The large number of typical alkaline minerals in the second member of Fengcheng Formation in Hashan area reveals the development of another lake basin center in addition to the Mahu Sag. During the deposition of the first member of Fengcheng Formation (P1f1), the lake level was relatively higher, the water salinity was low, the climate was semi-arid, and volcanic sedimentation was developed, with lithofacies mainly composed of organic-rich blocky tuffaceous limestone. In the lower part of the second member of Fengcheng Formation (P1f2), the basin began to shrink, the climate became relatively drier, the water became saltier, the environment became more limited, and the lithofacies were mainly organic-rich layered dolomitic mudstone. In the top of the P1f2 and the lower part of the third member of Fengcheng Formation (P1f3), the environment was relatively closed, the water salinity was higher, a large number of alkaline minerals were developed, and the lithofacies were mainly organic-rich layered alkaline dolomitic mudstone and organic-rich layered mixed shale. In the upper part of P1f3, the input of terrigenous debris increased, salinization weakened, a fan-delta system was developed, and the lithofacies were mainly organic-rich blocky fine sandstone. The sedimentary environment of the Fengcheng Formation has a controlling effect on organic matter enrichment. Overall, a deep-water environment with less input of debris, warm and humid conditions, and relatively lower salinity is more conducive to organic matter enrichment.

Keywords

• fine-grained sediment
• lithofacies
• sedimentary environment
• fengcheng formation
• hasan area
• junggar basin
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