# Diagnosis of the Permian Fengcheng Formation in the Mahu Sag, Junggar Basin, China
## 1 Introduction  
Traditional early and middle diagenesis is mainly conducted in acidic diagenetic environments, which are characterized by carbonate mineral, feldspar, and clay minerals being in an unstable state and easy to be dissolved [1; 2; 3; 4; 5]. When the reservoir experiences an alkaline diagenetic environment, reservoir property is often changed due to the role of alkaline formation water, quartz dissolution, feldspar enlargement, and carbonate mineral replacement becoming the most distinctive diagenetic phenomena [6; 7; 8; 9; 10; 11]. The theory of alkaline diagenesis has enriched, improved, and deepened the traditional theory of diagenesis. At present, the understanding of alkaline diagenesis is relatively insufficient compared to traditional diagenesis. The conditions of alkaline diagenesis generally includea sedimentary environment, climate, organic acids, and thermal fluid evolution [12; 13]. Alkaline diagenesis can have constructive effects on reservoirs [14; 15]. For example, mica interlaminar pores and albite pores formed under the action of alkaline fluids are good oil storage spaces. The diagenetic phenomena of alkaline diagenesis are diverse, including alkali lacustrine salt rocks [16], alkaline evaporative salt minerals [17], and quartz dissolution under the action of alkaline groundwater [18]. In addition, alkaline diagenesis may also provide a new explanation for the silicification of metal deposits, the origin of siliceous cementation, and the reddening and fragmentation of rocks [11; 12; 13; 14; 15; 16; 17; 18; 19]. The Junggar Basin is located in northwestern China (Figure 1a), with an area of about 1393 square kilometers [20]. The basin is divided into six first-level structural units: Ulungu sag, Luliang uplift, Western uplift, Central sag, Eastern uplift, and North Tianshan thrust band [21] (Figure 1a). The Mahu sag, located northwest of the Central sag of the Junggar Basin in an NE trend, is the most petroliferous sag (Figure 1). The most important source rock in the northwestern part of the Mahu sag is the lower Permian Fengcheng Formation [3; 21; 22]. The Fengcheng Formation consists of a mixed shale series comprising carbonate rocks, alkaline intermediate to basic volcanic rocks, and terrigenous detrital rocks. It contains alkaline minerals such as kanemite, bradleyite, and eitelite, suggesting formation in an alkaline lake environment [23]. The rocks of the Fengcheng Formation also contain bands and clumps of the uncommon mineral reedmergentier [Na(BSi\({}_{3}\)O\({}_{8}\))], indicating that it has undergone complex diagenetic processes [4; 5; 6]. The Fengcheng Formation is a fair-to-good source rock development horizon with high organic matter content, which has the characteristics of multi-phase hydrocarbon generation peak and large-area hydrocarbon generation [23; 24]. Meanwhile, the Fengcheng Formation has good exploration prospects and development value for oil and gas exploration, which has been widely concerned in the oil field recently. Studies show that the alkali lake of the Fengcheng Formation has experienced five stages of drought-wet-dry-heat-continuous dry-heat-humid [25], forming major reservoir types, including dolomite, clastic rock, and volcanic rock, and the formation of alkaline minerals is closely related to the evaporative salinization of lake brine [26]. The study of the alkaline lake environment and its diagenesis is still in the initial stage and needs to be further studied. Focused on the Fengcheng Formation, this study tried to obtain (1) the petrological and mineralogical characteristics, (2) diagenetic types, (3) and sedimentary diagenetic models, and this study is helpful for promoting the theoretical innovation of the Fengcheng Formation for further oil and gas exploration purposes.