Impact of Late-Accumulated Mantle-Derived CO2 on Quality of Paleogene Clastic Reservoir at Actic Area, Qin Nan Sag

Abstract

In this paper, the origin and accumulation time of CO2, and the transformation of clastic reservoir bed by CO2 in the A2 structure at the actic area of Qin Nan Sag were studied. The elevated δ13CCO2, R/Ra values and 40Ar/36Ar values showed that CO2 was mantle derived. CO2-bearing inclusions were associated with hydrocarbon inclusions, and CO2-bearing inclusions included hydrocarbon components, showing migration and filling of CO2 and hydrocarbon at the same stage. Basin modeling and homogenization temperature of inclusions indicated that the time of hydrocarbon and CO2 accumulation was mainly less than 3 Ma. CO2 late filling resulted in strong dissolution of feldspar and carbonate cement, with a high content of intragranular dissolution pores and a high proportion of intergranular primary pores, mostly from carbonate cement dissolution. Acidic environment inhibited illite growth, and dissolution of feldspar and acidic environment promoted formation of kaolinite. CO2 late filling brought out obvious thermal fluctuation, which was in favor of redistribution of dissolved matter, and top strong cementation was found at rock cores. Because of late accumulation of CO2, acidic environment of reservoir bed lasting for a long time, reservoir bed at a high temperature, undergrown dawsonite showed weak influence on reservoir bed quality. Dissolution of feldspar and carbonate cement, inhibition of illite growth, a high content of authigenic kaolinite, a low content of dawsonite, and redistribution of dissolved matter played a decisive role in improving reservoir quality, with rarely seen high permeability.