Shiyou shiyan dizhi (May 2023)
Cross-formational hydrocarbon accumulation of Cambrian sequences and its significance for natural gas exploration in eastern Sichuan Basin
Abstract
It has not been clearly understood whether the hydrocarbon generated from the argillaceous sourcerocks of the Lower Cambrian Qiongzhusi Formation could upward pass through the regional cap rocks of the Lower-Middle Cambrian gypsum rocks and accumulate in the Middle-Upper Cambrian Xixiangchi Group reservoir, namely the cross-formational hydrocarbon accumulation, in the eastern Sichuan Basin. Therefore, the studies on the reservoir solid bitumen in the Xixiangchi Group and the sealing performance of the regional cap rocks of the Lower-Middle Cambrian gypsum rocks were carried out, and the cross-formational hydrocarbon accumulation of the Cambrian oil and gas and its significance for natural gas exploration were discussed. It was shown by synthetic studies that the reservoir solid bitumen in the Xixiangchi Group mainly originated from the argillaceous source rocks of the Lower Cambrian Qiongzhusi Formation. The sealing integrity of the regional cap rocks of the Lower-Middle Cambrian gypsum rocks was damaged to some extent by the reactivated basement faults and the large nascent faults in the Middle Permian to Middle Triassic though it had good sealing performance in geological times, which could provide convenient vertical pathways for the Cambrian cross-formational hydrocarbon accumulation in the eastern Sichuan Basin. Three evolutionary stages were built up about the Cambrian hydrocarbon migration and accumulation as follows. Stage 1 is the formation of the Cambrian pre-salt early paleo-reservoirs in the period ofthe Late Silurian to the Early Permian (S3-P1), stage 2 is the one of the Cambrian cross-formational hydrocarbon accumulation in the period of the Middle Permian to the Middle Triassic (P2-T2), and stage 3 is the formation of the Cambrian late paleo-gas reservoirs in the period of the Late Triassic to the Early Cretaceous (T3-K1). The cross-formational hydrocarbon accumulation is an important feature of hydrocarbon accumulation in multi-cycle superimposed basins, and the reactivated basement faults and the large nascent faults in the period of the MiddlePermian to the Middle Triassic are important clues to the Xixiangchi Group primary paleo-gas reservoirs. Identification and evaluation of the Xixiangchi Group primary paleo-gas reservoirs will be the key points and the main challenges to natural gas exploration in the study area.
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