PORE STRUCTURE OF TRANSITIONAL SHALES IN THE ORDOS BASIN, NW CHINA: EFFECTS OF COMPOSITION ON GAS STORAGE MECHANISM

Pore structure and effects of organic and inorganic composition on development of the Upper Paleozoic transitional shale pore system were investigated by combination of geochemical measurements and petrological techniques using both low pressure nitrogen and carbon dioxide adsorption methods. Pore structure controls the occurrence, storage mechanism and transport behavior of shale gas. Three different transitional shales deposited in marsh-lagoon and coastal delta settings from Ordos Basin, NW China were collected to illustrate the roles of organic matter (OM) in development of shale pore structure of transitional shales in Yanchang area, Ordos Basin.

The transitional shales comprises of Upper Permian Shanxi shales (average TOC 1.58 wt. %, Type III kerogen, average Ro 2.6%) and Upper Carboniferous Benxi shales (average TOC 1.91 wt.%, Type III kerogen, average Ro 2.74%) at over mature stage or dry gas window. mature. One of the most significant characteristics of Benxi and Shanxi shales is a large proportion of clay minerals, average 68.9% and 54% of shale composition respectively. The content of quartz of Benxi and Shanxi shales, average 16.7% and 39.7%, ranked the second. Low temperature nitrogen adsorption experiments shows that Benxi and Shanxi shales mainly comprise of silt-shaped pores with development of ink-bottle-shaped pores, characterized by ultra-low porosity and permeability.

The results of low pressure nitrogen and carbon dioxide adsorption isotherms show that OM mainly contributes to mesopores of less than 10 nm diameter of transitional shales in Yanchang area. Micropore volume nonlinearly increases with increasing OM, although the contribution of organic micropore volume is limited and mesopore and macropore volumes of inorganic compositions contribute mostly to total pore volume of transitional shales. However, organic micropore specific surface area accounts mostly for total specific surface area of transitional shales. For thermally over mature transitional shales, water saturation could be low, and clay minerals can be the dominant controlling factor of specific surface area and pore volume as well as absorbed gas and free gas.