Oxygen isotope clue to migration of dolomitizing fluid as exampled by the Changxing Formation dolomite at Panlongdong, northeastern Sichuan

Abstract

The Upper Permian Changxing dolomite reservoirs serves as one of the most important gas and oil reservoirs in the NE Sichuan Basin. Determining the dolomitizing fluid’s pathway is regarded as the key to solve the “dolomite problem” and further petroleum exploration. Outcrop samples from Upper Permian Changhsingian Panlongdong Section were studied using oxygen isotopic analysis, cathodoluminescence (CL) and major element analysis, in an attempt to determine the migration path way and properties of the dolomitizing fluid. Of the Changxing dolomite, the δ 18O values ranged from -3.494‰ to -5.481‰, which decreased from the top layer to the bottom in the section; the MgO contents varied from 9.24% to 21.43%, CaO contents from 28.65% to 39.87%, the CaO/MgO ratio from 1.40 to 4.31 and the MnO contents from 0.004% to 0.009 8%. The MgO contents showed a downwardly decreasing trend in the section, while the CaO/MgO showed an opposite rule. All of the dolomites looked dull or dark when they were exposed to the electron beam of the cathodoluminescence device. None of the fine- to medium grained dolomite showed a banded structure. Given that dolomitizing fluid’s salinity decreased during the dolomitization process in its pathway, we concluded that the dolomitizing fluid migrated downwardly in Changxing Formation after excluding the possibility of deep burial or meteoric-marine mixing-water influences. As the dolomitizing fluid’s pathway has always been difficult to be determined in highly dolomitized Formation, this study showed an important application of oxygen isotope values in resolving this problem.