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Successful Delineation of a Deep Sour Gas Play in the NE Sichuan Basin: Puguang and More Recent Discoveries*
Yongsheng Ma1
Search and Discovery Article #10156 (2008)
Posted October 9, 2008
*Adapted from oral presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23, 2008 Editorial Note: Viewer is referred to “Evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the Puguang gas field, Sichuan Basin, China,” by Fang Hao et al., AAPG Bulletin, v. 92, no. 5 (May 2008), p. 611-637.
1
China Petrochemical Corporation, 
Exploration
Company, Beijing, China
([email protected])
The recent discovery of the giant Puguang gas field in northeastern Sichuan Basin, SW China, resulted from the successful application of a new play concept and new technology in a mature basin. The field is a combination structural-stratigraphic trap closed by lateral depositional change and fault closure. The trap evolved from a paleo-oil reservoir originating in the Triassic-Jurassic. The entrapment of thermal gas, which was derived from Lower-Middle Silurian and Permian source rocks, occurred during deep burial in the Jurassic-Cretaceous. Tertiary-Quaternary compression transformed the paleotrap into the present gas reservoir. Gas is contained in the Lower Triassic Feixianguan and the Upper Permian Changxing reservoirs, which consist predominantly of dolomitized oolites deposited in shelf and platform-margin shoal and backreef environments. Reservoir quality is characterized by porosity of 1-29% and permeability of 0.01-9664 md, with buried depth greater than 5000 m. The discovery has not only broadened the exploration scope in the Sichuan Basin, but also provided an excellent analog for exploration in other fold-thrust belts worldwide. This presentation reviews the exploration history of the Sichuan Basin, discusses the key scientific and technological advances that have been achieved in the field discovery process, and provides an update on more recent exploration successes in this sour gas play following the initial discovery.
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Petroleum
The Geological and Evolution History The Puguang Gas Field was discovered by Well Puguang-1, in which the depth to top of the Feixianguan Formation is 1400m deeper than that in Well Chuyue 83, previously drilled on the structure high (Figure 4). Proven gas reserves are greater than 3,812x108 m3(13.47 TCF). Gas in this field is mainly contained in the Lower Triassic and Upper Permian. The best reservoirs are oolitic dolomites of platform margin shoal/reef facies. The Lower Triassic Feixianguan Formation is the main reservoir rocks , which developed mainly in the platform-margin shoal and evaporitic platform environments (Figure 5). Abundant solid bitumens were found in all cored wells in marine carbonate reservoirs. The maximum cumulative thickness of solid bitumen-bearing carbonate reservoirs in a single well is greater than 300 m, suggesting that the Puguang gas field evolved from a large paleo-oil reservoir (Figure 6). The hydrocarbon gases display relatively constant C1/C2 values and significant increases in C2/C3 ratios; they should be interpreted as secondary cracking products. The formerly accumulated oil in the marine carbonates from which the solid bitumen and hydrocarbon gases originated was confirmed to have generated from Upper Permian source rocks. Gases in the field have relatively high CO2 and H2S contents. CO2 and H2S were derived from thermochemical sulfate reduction (TSR). TSR in the NE Sichuan basin significantly altered the chemical composition of petroleum hydrocarbons by forming sulfur-rich pyrobitumen and increasing the non-hydrocarbon gas contents, the dryness of hydrocarbon gases, and the d13C values of light hydrocarbons. The marine carbonate reservoirs in the Puguang gas field were buried to a depth of about 7000 m and experienced maximum temperature up to 210°C before uplift to the present-day depth (Figure 7). Oil accumulation took place mainly in Early Jurassic, and thermal cracking of formerly accumulated oils occurred mainly in Late Jurassic and Early Cretaceous (160 to 120 Ma). As a result of different uplift and erosion during the late tectonic movements, the morphology of the sealing surface changed significantly, which resulted in fluid re-migration in this field to the present configuration (Figure 8). During the Late Triassic and Early Jurassic, oil generated from the Upper Permian source rocks migrated through faults and dolomites and accumulated in the Puguang-Dongyuezhai structure (Figure 9(A)). From about 160 to 145 Ma, increasing reservoir temperature resulted in thermal cracking of accumulated oils and thermochemical sulfate reduction (Figure 9(B)). From about 140 Ma, changes in three-dimensional morphology of the cap rocks, gases in the field re-migrated and finally re-accumulated, forming the Puguang gas field (Figure 9(C)).
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