Dynamic evaluation on sealing capacity of caprocks of the Meso-Neoproterozoic reservoirs in Ordos Basin, China

The Meso-Neoproterozoic is a new play in the Ordos Basin. A deeper understanding about the dynamic relationship between the caprocks and the source rocks is needed. Based on the comprehensive analysis of hydrocarbon source development characteristics of the Meso-Neoproterozoic and its overlying strata, as well as the formation contact relationships, lithology characteristics and exploratory drilling data, it is recognized that the Meso-Neoproterozoic contains two types of petroleum accumulation assemblage, that is, the “self-sourced indigenous” and “upper source rock-lower reservoir” assemblages. The former is mainly controlled by the development and distribution of source rocks of the Changcheng System, with the Lower Cambrian shale sequence as its caprock. The later is controlled by the superposition between the Meso-Neoproterozoic and its overlying source rocks and this assemblage is mainly distributed in Hangjinqi and Pingliang areas with the Carboniferous-Permian shale sequence as its caprock. The dynamic evaluation on the displacement pressure serves to reconstruct the displacement pressure history of the caprock. The results show that the shale sequence of the Cambrian Maozhuang Formation in well XY 1 in the southern Ordos Basin has possibly acquired the ability of sealing natural gas since the early of Late Triassic. Its displacement pressure increased rapidly up to 20 MPa during the Late Triassic-Jurassic and keeps at 9.2 MPa at present, indicating fair sealing ability. The Carboniferous-Permian caprocks in Hangjinqi area could have acquired the ability to seal natural gas in the Late Jurassic-Early Cretaceous, and the present-day displacement pressure is 9–12 MPa, indicating good sealing ability. The upper Paleozoic caprock in Pingliang area has been able to seal natural gas since the Early Jurassic, with a maximum displacement pressure of 23 MPa during the Cretaceous period and a current value of 17–20 MPa, indicative of strong ability to seal natural gas. The sealing ability of caprocks of both the “self-sourced indigenous” and “upper source rock - lower reservoir” assemblages has come into being earlier than or at least no later than the peak gas generation of the source rocks and therefore the caprocks are dynamically effective in geohistory. The Meso-Neoproterozoic reservoirs in the Ordos Basin are well preserved and probabally of better potential for exploration in terms of the caprock-source rock combination.