Abstract
Tight gas accumulations, commonly characterized by low permeability, low porosity, and complicated pore structure, are widely distributed in the Sichuan Basin. Recent exploration in the Chengdu Sag, Western Sichuan Basin has proven that Jurassic tight-sandstone reservoirs attach significant gas potential. However, long distance migration between source and reservoir intervals entangles understanding of the tight-gas accumulation mechanism. It is unclear whether producible gas in Jurassic intervals is either from “simple sweet-spots in a continuous accumulation” or “conventionally trapped accumulations in low-permeability reservoir rocks”. To identify the regionally active gas system and characterize the charging pattern, a geochemical study was performed by interpreting the gas molecular and carbon isotope compositions in Jurassic and conducting gas–source correlations as well as gas migration distance calculation with the relationship among δ13C1 vs. Ro vs. H (burial depth). Research results indicate that the Jurassic tight gases in Majing-Shifang areas are coal-derived dry gases generated by the primary cracking of kerogen. Gas/source correlation and gas migration distance calculation reveal that gases are mainly sourced from the Upper Triassic humic source rocks (T3 x5, the fifth member of the Xujiahe Formation). Gas accumulations in the Jurassic Penglaizhen Formation were formed with an original vertical migration of about 2–3 km and then a long-distance lateral migration within tight sand layers, which is verified by the decreasing δ13C1 and the general increasing iC4/ nC4 in the Penglaizhen Formation. The Jurassic tight-sandstone reservoirs in Majing-Shifang areas occur in low-porosity and low-permeability reservoir rocks in conventional lithological traps, which are not continuous-type gas accumulations or basin-centered gas systems. The faults in Majing area serve as dominant vertical conducting pathway and the relatively permeable intervals within Jurassic and microfractures play an important role in the development of the conventionally trapped natural gas accumulations.
Highlights
Tight gas reservoirs were firstly discovered several decades ago and have been raising attention of geologists, since exploitable conventional reserves are to be exhausted (Dai et al, 2012; Jiang et al, 2006; Law, 2002; Law and Spencer, 1993; Masters, 1979; Rose et al, 1984; Schmoker, 2002)
Jurassic tight gas reservoirs in the Western Sichuan Basin (WSB) are characterized by longdistance primary migration (Cai and Liao, 2000; Qin et al, 2007; Shen et al, 2008; Tang et al, 2013; Wu et al, 2001)
The molecular and carbon isotopic compositions analysis indicates that Jurassic tight gases in Majing-Shifang areas are coal-derived dry gases generated by the primary cracking of kerogen
Summary
Tight gas reservoirs were firstly discovered several decades ago and have been raising attention of geologists, since exploitable conventional reserves are to be exhausted (Dai et al, 2012; Jiang et al, 2006; Law, 2002; Law and Spencer, 1993; Masters, 1979; Rose et al, 1984; Schmoker, 2002). Jurassic tight-sandstone reservoirs in the Western Sichuan Basin (WSB) have been emerging as a significant supply of natural gas in the last several years. These resources differ from the typical scenario of the reservoir and source intervals with close proximity, as observed from most tight gas intervals around the world (Zou et al, 2012). Yang and Zhu (2013) considered Jurassic tight gas reservoirs as superposed tight sandstone gas-bearing zones It is currently unclear whether producible gas in Jurassic intervals either represents ‘‘simple sweet-spots in a continuous accumulation’’ or ‘‘conventionally trapped accumulations in very low-permeable reservoir rocks’’. It is necessary and meaningful to investigate the gas genesis, migration and accumulation for this atypical tight-sandstone gas, which will provide evidences for better understanding of accumulation mechanisms of tight-sandstone gas reservoirs as to the typical ones (e.g. continuous, basin-centered, or anticlinal ones)
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