Timing and stages of the Permian oil-gas accumulations in northeastern Ordos Basin

Abstract：Geochronology of oil-gas accumulation (OGA) is a key research frontier for petroleum reservoir-forming dynamics in the multi-cycle superimposed basin. The OGA timing and stages of the Permian reservoirs in northeastern Ordos Basin (OB) are constrained in this paper by K-Ar dating of authigenic illite (AI) and indirect dating of fluid inclusions (FI) from oil-gas-bearing sandstone core samples of the Lower-Upper Permian period. AI dating results of the Permian samples show a wide time span of 178～108 Ma and a spatial decreasing trend from 178～122 Ma in the south to 160-108 Ma in the north. The distribution of the AI ages generally reveals 2-stage primary OGA of the Permian reservoirs, which were mainly developed in the time spans of 175～155 Ma and 145～115 Ma respectively with 2-peak ages of 165 Ma and 130 Ma. Additionally, the FI temperature peaks of the samples and their projected ages on AFT thermal path of the FI-host rocks not only statistically present two groups with a low and a high peak temperatures in ranges of 90～78 °C and 125～118 °C, respectively corresponding to 2-stage primary OGA processes of 162～153 Ma and 140～128 Ma in the Permian reservoirs; nevertheless, there is also a medium temperature group with the peak of 98 °C in agreement with a secondary OGA process of ca. 30 Ma in the Upper Permian reservoirs. An integrated analysis of the AI and FI ages with the regional tectonic thermal evolution reveals that the Permian reservoirs in the northeast OB mainly experienced 2-stage primary OGA processes of 165～153 Ma and 140～128 Ma during the Mid-Early Mesozoic multi-cycle burial heating processes, and then the Upper Permian reservoirs further experienced 1-stage secondary OGA of ca. 30 Ma in accordance with a critical tectonic conversion from the slow to rapid uplift-cooling process during the Late Cretaceous-Neocene period.