Geochronological Records of Oil-Gas Accumulations in the Permian Reservoirs of the Northeastern Ordos Basin

Geochronology of oil-gas accumulation (OGA) is a challenging subject of petroleum geology in multi-cycle superimposed basins. By K-Ar dating of authigenic illite (AI) and fluid inclusion (FI) analysis combined with apatite fission track (AFT) thermal modeling, a case study of constraining the OGA times of the Permian reservoirs in northeast Ordos basin (NOB) has been conducted in this paper. AI dating of the Permian oil-gas-bearing sandstone core-samples shows a wide time domain of 178-108 Ma. The distribution of the AI ages presents 2-stage primary OGA processes in the Permian reservoirs, which developed in the time domains of 175-155 Ma and 145-115 Ma with 2-peak ages of 165 Ma and 130 Ma, respectively. The FI temperature peaks of the samples and their projected ages on the AFT thermal path not only present two groups with a low and a high peak temperatures in ranges of 90-78 degrees C and 125-118 degrees C, respectively corresponding to 2-stage primary OGA processes of 162-153 Ma and 140-128 Ma in the Permian reservoirs, but also appear a medium temperature group with the peak of 98 degrees C in agreement with a secondary OGA process of c.approximate to 30 Ma in the Upper Permian reservoirs. The integrated analysis of the AI and FI ages and the tectono-thermal evolution reveals that the Permian reservoirs in the NOB experienced at least 2-stage primary OGA processes of 165-153 Ma and 140-128 Ma in agreement with the subsidence thermal process of the Mid-Early Jurassic and the tectono-thermal event of the Early Cretaceous. Then, the Upper Permian reservoirs further experienced at least 1-stage secondary OGA process of c.approximate to 30 Ma in coincidence with a critical tectonic conversion between the slow and the rapid uplift processes from the Late Cretaceous to Neogene.