Characteristics and main factors controlling development of tectonicFractures In Oil-Shale Member of Upper Triassic Tanzhuang Formation In Jiyuan Basin

The Oil-Shale Member in the Upper Triassic Tanzhuang Formation in the Jiyuan Basin is about 140 m thick and generally buried at a depth of 3 000 m and 7 000 m. It is mainly composed of oil shale，mudstone，silty mudstone，and siltstone. This is the most favorable shale deposit for oil and gas exploration and exploitation in the Jiyuan Basin. Identifying the development characteristics of structural fractures and their main controlling factors in the target interval in this areacan provide the basis for the exploration and development of shale oil and gas in the region. In this paper，through outcrop observation，sampling and laboratory testing，the source rock characteristics，tectonic fracture characteristics，and the main controlling factors of fractures of the target oil-shale Member in the study area were studied. The results show that the brittle-mineral contents of the oil shale in the oil shale section of the Upper Triassic Tanzhuang Formation in the Jiyuan Basin ranges from 37.4%-77.9% with an average of 53.7%，and the total organic carbon content is from 0.36% to 10.33%，with an average of 4.13%; The reflectivity of vitrinite ranges from 1.01% to 2.12%，with an average content of 1.33 %. The organic matter type is mainly humic-saprolite type. There are four types of tectonic fractures in the study area，e.g. shear，tensile，bedding-slip，and compound fractures. These fractures are mostly vertical or intersect the bedding at high angles. It is dominated by partially filled or unfilled fractures with a width of 0.01-2 cm; the average linear density of fractures is 6.29 strip/m，and it increases with the finer grain size of the rock and with the increase of the brittle mineral content，decreases with the increase of rock thickness，and the closer to the fault，the more developed. The results show that the oil shale layers in the target formation in the study area has high gas generation potential and strong reformability，and the relatively developed structural fractures provide space for the accumulation and migration of oil and gas.The Oil-Shale Member in the upper Triassic Tanzhuang Formation in the Jiyuan Basin has a good prospect for shale gas exploration and development.