Summary

Located in the central west of China, the Sichuan Basin is abundant in natural gas resources. It is the earliest basin where natural gas was discovered and utilized in the world. After more than 60 years' exploration, many gas fields have been found in the Paleozoic - Mesozoic carbonate and clastic formations. The basin has become a critical production base in China for its cumulative proved recoverable gas reserves of more than 800 bcm, and gas production over 15 bcm in 2010. During the past decade, many tight gas reservoirs have been found in the Xujiahe coal measures of the upper Triassic continental deposits. Based on lithology, this suite of formation can be divided into six members from bottom to top. The source rocks are the coal beds and carbon-bearing mudstones in Xu1, Xu3 and Xu5 members with relatively high organic carbon contents and type III kerogen; the reservoir rocks are the tight sandstones in Xu2, Xu4 and Xu6 members. The source rocks and the reservoirs distribute alternatively and widely in "sandwiched" structure, providing favorable conditions for natural gas accumulating near source. As the formations are gentle and lack of structural traps, the lithologic gas reservoirs dominate the Xujiahe tight sandstones. Both coal-measure source rocks and sandstone reservoir distribute in strong heterogeneity, leading to thin gas-layers in the reservoir, poor continuity in plane, and varying full-up ratio and gas saturation in the gas reservoir. Within the 80,000 km2 area, the Xujiahe Fonnation has the features of widespread gas-bearing beds and local gas enrichment. The current high-yield gas wells are mainly distributed in the tectonic highs or fractured zones in the areas with effective source-reservoir assemblages. The resources assessment is made considering the tight gas accumulating intensively into reservoir. It reveals the favorable gas-bearing area up to 60,000–70,000 km2 and the estimated recoverable gas reserves of 2–3 tcf in the Xujiahe Formation.

Introduction

Located in the southwestern China, the Sichuan Basin is prolific in natural gas. Extensive exploration began from the 1950s, to find structural and fractured gas reservoirs initially. However, most of the gas reservoirs discovered were small. In the past decade, along with the progress of exploration and development technologies, many lithologic-stratigraphical reservoirs were discovered, making the gas reserves up to several hundreds to thousands billions of cubic meters. The discoveries of some large gas fields (such as Puguang and Luojiazhai) in marine carbonate reservoirs represent significant breakthroughs of exploration in the Sichuan Basin. Recently, Guang'an, Hechuan and other 108m3-level gas fields were found in the Xuj iahe continental tight sandstones, with 3P gas reserves more than trillion cubic meters, which made this formation the critical target in the Sichuan Basin and even China. The tight gas exploration area and scale are still expanding.

URTeC 1576565

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