Characteristics of Lower Cambrian marine black shales and evaluation of shale gas prospective area in Qianbei area, Upper Yangtze region

Based on the test materials including the measurement of a large number of shales in the field geological section, the component analysis of the rock mineral samples and the result of organic geochemistry experiment, the characteristics of development and distribution about Lower Cambrian (Niutitang Formation) black shale in Upper Yangtze region (Qianbei area) has been systematically discussed. Besides, the prospective area of shale gas exploration in Qianbei area has been successfully predicted combining with the geological condition of marine shale formation in North America and the evaluation standard of prospective area of shale gas in China. The results show that the Lower Cambrian (Niutitang Formation) marine black shales in Qianbei area have the characteristics of large thickness and wide range of distribution. The water level was deeper in the early transgressive system tract (TST), the centre of deposition and settlement located in the southeast area, the main sources were supplied from the northwest area, and the sedimentary facies were sandy shelf facies, shallow water shelf facies and deep water shelf facies (distributed in the southeast part of study area and developed in the northwest-southeast direction). The water level got lower in the late highstand system tract (HST), the range of shelf sand bodies expanded with the descend of relative sea level, and the range of shallow water shelf and the deep water shelf narrowed down to the southeast part of study area. The organic material type of black shale is type III, and the average value of organic carbon content is high(4. 88%), which generally greater than 2%; the overall evolution level is high with the higher average value of organic material maturity(2. 7%); the mineral compositions of the black shale are the detrital mineral and the clay mineral. Black shale mainly composed of detrital minerals and clay minerals. The average content of detrital minerals is 61. 3%, the ingredients are mainly quartz and a small quantity of feldspar; the average content of clay minerals is 31. 1%, the ingredients are mainly illite and smectite mixed layer minerals. Compared to the indicators of the main shale gas production area in American, Lower Cambrian marine black shale in Qianbei area of Upper Yangtze region has a larger thickness, higher average abundance of organic matter, higher thermal evolution degree, lower brittleness mineral content, lower shale porosity and the characteristic of medium clay mineral content. This study area which has the basic geological conditions of producing shale gas is one of the main marine shale gas exploration areas in China. This paper makes comprehensive evaluations to predict the prospective areas and favorable areas of Lower Cambrian(Niutitang Formationg) black shale in Qianbei area.