关键词: 准噶尔盆地, 碱湖, 混积型页岩, 自生硅, 细粒岩, 岩石分类, 二叠系风城组, 玛北地区

Abstract: The Permian Fengcheng Formation is a key stratigraphic unit for shale oil exploration in the northern Mahu sag (Mabei area) of Junggar Basin. Exploration breakthroughs in multiple wells have confirmed the substantial shale oil potential in Mabei area, Due to unique geological settings, the lithological classification schemes for global analogues cannot be directly applied to Fengceng Formation shale, thus posing challenges to hydrocarbon accumulation mechanisms research and sweet-spot assessment in this area. Based on a substantial dataset of petrographic identification and X-ray diffraction (XRD) quantitative mineralogical analyses, combined with extensive exploration practice, this study establishes a new lithological classification scheme for the hybrid shales of Fengcheng Formation in Mabei area. The research results show as follows. (1) Applying the classification scheme based on a ternary diagram that incorporates carbonate minerals, clay minerals, and felsic minerals (containing authigenic silica and reedmergnerite) as end-members, shales are categorized into four primary types: argillaceous shale, carbonate-rich shale, felsic shale, and hybrid shale. For felsic lithologies, a secondary ternary classification system based on the relative contents of feldspar, quartz, and reedmergneriteis established for further subdivision. (2) Argillaceous shale is not observed in the Fengcheng Formation of Mabei area. The shale lithologies predominantly include calcareous shale, dolomite-bearing calcareous shale, dolomitic shale, calcite-bearing dolomitic shale, siliceous shale, feldspathic shale, reedmergnerite shale, felsic shale, clay-bearing felsic shale, carbonate-bearing felsic shale, and hybrid shale. (3) Siliceous shale, felsic shale, and reedmergnerite shale are the most favorable lithologies for hydrocarbon generation and accumulation, with high potential to form self-generated and self-accumulated petroleum systems within the shales. In contrast, carbonate-rich shales exhibits the poorest hydrocarbon generation capacity. These findings provide positive guiding for the exploration and evaluation of shale oil in Fengcheng Formation of Mabei area.

Key words: Junggar Basin, alkali lake, hybrid shale, authigenic silica, fine-grained rock, rock classification, Permian Fengcheng Formation, northern Mahu sag