Bedding texture types and the combinations of fine-grained rocks, and their reservoir significances: a case of the Permian Lucaogou Formation in Jimsar Sag, Junggar Basin

Abstracts: The lamination and bedding structures in fine-grained rocks are key factors affecting the quality of shale oil reservoir. The Permian Lucaogou Formation in the Jimsar Sag, Junggar Basin, NW China was studied for this. Based on drilling wells, cores, thin sections, and physical property data of the area, the characteristics, genesis, and reservoir significance of bedding structures were analyzed. Thickness of 1cm was defined as the boundary: less than 1cm referred to a lamina, and greater than 1 cm referred to a bedding. Results show that, both laminae and beddings are in sedimentary origin, showing differences in hydrodynamic condition and thickness. Two types were identified. The first one is thick-bedded bedding-lamination type, often shown in fine-grained sandstone, with vertical development of upward-coarsening/finning bedding, massive bedding, cross bedding, climbing ripple bedding, and irregular lamination, indicating the strong hydrodynamic origin such as hyperpycnal channels. The second type is laminae-dominated one usually seen in muddy siltstone, consisting of vertical upward-coarsening/finning bedding and irregular lamination, which is formed in hyperpycnal lobe indicative of week hydrodynamic origin. The first type generated large single-layer thickness and cumulative thickness, and good reservoir properties, the second type resulted in smaller single-layer thickness and cumulative thickness, and relatively poor reservoir properties. This understanding provides valuable references for study in sedimentology of fine-grained rocks, and for shale oil exploration and development practices.