1. Giant gas discovery in the Precambrian deeply buried reservoirs in the Sichuan Basin, China: Implications for gas exploration in old cratonic basins.
- Author
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Zhu, Guangyou, Wang, Tongshan, Xie, Zengye, Xie, Banghua, and Liu, Keyu
- Subjects
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PRECAMBRIAN , *RESERVOIRS , *NATURAL gas prospecting , *THERMAL efficiency , *GEOCHEMISTRY - Abstract
An estimated trillions of cubic meters of natural gas have recently been discovered in the Sinian dolomite reservoirs in the Sichuan Basin, China. The 20–26 m thick reservoirs were found in deep structural–lithologic traps of 5000–5500 m deep. The Sinian gas is thought to have been derived mainly from the Sinian source rocks, and partly from the Cambrian source rocks. The gas is thermally matured, with a dryness ratio (C 1 /∑C 1–4 ) of 0.997–0.9998, C 2 H 6 content <0.2%, and H 2 S content of 0.5–2.75%. The stable carbon isotope compositions (δ 13 C) of CH 4 and C 2 H 6 are similar in all the reservoirs, with values of −33.5‰ to −31.5‰ and −36.0‰ to −27.0‰ (PDB), respectively. The Sinian gas reservoirs are believed to have been accumulated by the following processes: initial paleo-oil accumulated during the Silurian–Devonian; gas accumulation after oil cracking of the paleo-oil accumulations during the Jurassic–Early Cretaceous; and the current gas accumulations formed by paleo-gas remigration and adjustment since the Late Cretaceous. This new gas accumulation model is consistent with large-scale gas accumulations for deeply buried paleocratonic basins worldwide, in which the source rocks initially underwent thermal oil generation from kerogen, followed by gas generation via oil cracking. The gas discovery in the Sinian dolomite reservoirs has led to a new wave of hydrocarbon gas exploration in the ancient cratonic basin containing the mostly thermally matured source rocks in China. We conducted a series of geochemical analyses of the H 2 S in the gases. The sulfur isotope compositions (δ 34 S) of gypsum, H 2 S, and secondary pyrite sampled from the Sinian Dengying Formation showed that H 2 S was generated by a moderate thermochemical sulfate reduction (TSR) during the Jurassic–Early Cretaceous periods. TSR was terminated by the depletion of SO 4 2− in the formation water, resulting in a modest H 2 S content of 5–10% in the natural gases. Because the dolomites are rich in cations, a large quantity of H 2 S may have been bound to form metal sulfides. We infer that H 2 S contents of 0.8–10% are present widely in the deeply buried Sinian reservoirs. Therefore, measures should be taken to prevent H 2 S-related accidents during gas exploration in the deep strata. [ABSTRACT FROM AUTHOR]
- Published
- 2015
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