Abstract:

The Upper Paleozoic marinecontinental transitional shale is important gasrich shale in the Ordos Basin. A series of studies have been carried out on the gas generation and resource potential, but it lacks of detailed reservoir characterization and its impact on the gas content. This constrains the shale gas exploration and exploitation. A large number of shale samples were collected from the Upper Paleozoic Benxi Formation and Shanxi Formation to investigate the geochemical and mineral composition, pore type and distribution. Field canister gas desorption and isothermal adsorption experiments were performed to measure gas content and gas bearing capacity of shale samples. The results show that the Upper Paleozoic shale contains high TOC (0.5%11%) and high thermal maturity (1.0%3.0% Ro), and its organic matter belongs to type III kerogen. These shale samples are characterized by high level of clay mineral (up to 60%) and low porosity (2%8%). Some samples are relatively microfracturedeveloped. The pore is dominated by mesopore, which accounts for over 90% of all pore surface area and volume. TOC content also has close relationship to the pore surface area and volume, suggesting the organic matter pore makes significant contribution to all pore in the shale. The canister adsorption shows the gas content varies in a wide range from 0.591 m3/t to 4.05 m3/t, isothermal adsorption measurement shows the Langmuir gas volume ranges from 0.05 m3/t to 14 m3/t, both of which were seriously impacted by the TOC content. Besides, the clay mineral content might also contribute to the gas bearing capacity of shales.

Key words: Upper Paleozoic, the distribution of shale, aperture, content of gas, Yanchang exploration area, Ordos Basin