Comparative study of gas accumulations in the Permian Changxing reefs and Triassic Feixianguan oolitic reservoirs between Longgang and Luojiazhai-Puguang in the Sichuan Basin

Abstract

Reef and oolitic gas reservoirs in the Permian Changxing and Triassic Feixianguan formations have been discovered in both the Luojiazhai-Puguang and Longgang areas of the Sichuan Basin, but the accumulation scale, abundance and distribution patterns differ greatly. In fact, the two areas have more differences than they have similarities. Similarities include: (1) the lithology, properties and types of porosity of the reef and oolitic reservoirs are primarily consistent in the two areas; (2) well-developed regional caprocks that seal the fluid system; and (3) accumulation process histories, with liquid hydrocarbon charging at an early stage, cracking into gas at a high to over-mature stage, adjustment of gas reservoirs at an uplifting stage, and final reservoir generation at a late stage. Differences include: (1) gas accumulations in the Luojiazhai-Puguang area that are dominated by oil-type cracked gas, with multiple and sufficient gas sources, contrasting with gas accumulations in the Longgang area that are dominated by coal-sourced gas with insufficient gas sources; (2) heterogeneity and horizontal diversity of reservoirs in the Longgang area that are stronger than in the Luojiazhai-Puguang area; and (3) the development of deep and large faults in the Luojiazhai-Puguang area that link source, reservoir and migration pathways, contrasting with poorly developed faults in Longgang area, where laterally variable fractures are the major migration pathway. Because of these differences, abundant large-scale structurally-controlled gas accumulations are developed in the Luojiazhai-Puguang area, whereas lithology plays a more important role in gas accumulation in the Longgang area where accumulations are smaller-scale and have lower gas-bearing abundance. We suggest that future exploration for reef and oolitic gas accumulations should be strengthened in NE Sichuan and NW Sichuan Jiulongshan. In particular, the transitional zone between Longgang and Kaijiang-Liangping Bay has the best potential for new discoveries.advantage transport of fault pathway, non-equilibrium transport of fracture pathway, integral accumulation with high abundance, heterogeneous accumulation with low abundance, Sichuan Basin

Organic geochemistry of marine source rocks and pyrobitumen-containing reservoir rocks of the Sichuan Basin and neighbouring areas, SW China

“…The main Paleozoic marine source rocks in the Sichuan Basin include the Lower Sinian Doushantuo Formation (Z1ds, e.g., Xu et al, 2012;Yang et al, 2012;Wei et al, 2013), the third section Cross section AB modified from Xu et al, 2012; Cross section CD modified from Zhao et al, 2011. of the Upper Sinian Dengying Formation (Z2dn3, e.g., Xu et al, 2012;Wei et al, 2013), the Lower Cambrian Jiulongdong Formation (Є1j) and its age-equivalents, i.e., Niutitang Formation (Є1n) and Qiongzhusi Formation (Є1q, e.g., Zhu et al, 2006;Liang et al, 2008Liang et al, , 2009, the Lower Silurian Longmaxi Formation (S1l, e.g., Zhu et al, 2006;Liang et al, 2008Liang et al, , 2009, the Lower Permian Liangshan Formation (P1l, e.g., Liang et al, 2008Liang et al, , 2009 Modified from Ma et al, 2007Ma et al, . 2008Ma et al, , 2009, and the Upper Permian Dalong Formation (P2d, e.g., Wu et al, 2012;Chen et al, 2013a).…”

“…Although the Sinian and Lower Cambrian source rocks cannot be excluded as the source for the paleo-oil and hydrocarbon gases in the reservoirs within the Middle Carboniferous, Upper Permian and Lower Triassic strata in eastern Sichuan based on geology and molecular data (Fig. 1, Table 2, Zhao et al, 2011), they were unlikely to be the major source based on the isotopic data (Table 1, Fig. 4).…”

“…Several marine source rocks have been identified from Sinian to Permian in the Sichuan Basin. The major source rocks are within the Upper Sinian, Lower Cambrian, Lower Silurian and Upper Permian strata (e.g., Zhu et al, 2006;Liang et al, 2008Liang et al, , 2009Ma et al, 2010;Zhao et al, 2011;Xu et al, 2012;Wei et al, 2013). All of the marine hydrocarbon gases are very dry, with dryness (C 1 /SC 1e 4 ) ratio values >0.975 and mostly >0.99 due to high maturities of the marine source and reservoir rocks Liu et al, 2013).…”

Generation of isotopically and compositionally distinct water during thermochemical sulfate reduction (TSR) in carbonate reservoirs: Triassic Feixianguan Formation, Sichuan Basin, China

“…At least some of the high porosity has been proposed to be the result of late stage dissolution and is thus secondary porosity; this dissolution has been linked to TSR (Cai et al, 2014;Ma et al, 2008;Zhao et al, 2011). The dilution of pre-existing formation water by up to four times (by volume) with new water resulting from TSR may have insights into naturally elevated carbonate reservoir quality during deep burial environments (Ehrenberg et al, 2012).…”

Genetic origin of gas condensate in Permian and Triassic strata in the southern Sichuan Basin, SW China

“…The Sichuan Basin, covering an area of 180 Â 10 3 km 2 , is a large superimposed basin (Zhao et al, 2011). The basin is diamond shaped and trends northeast, and is located in the northwest of the Yangtze craton (Fig.…”