Abstract
Tight sandstone gas characterized by a wide distribution, local enrichment and a complex gas–water distribution has high exploration potential. This study, using the Xujiahe Formation in the Guang’an gas field as an example, aims to determine the main controlling factors of the enrichment of tight gas through comprehensive analyses of the source rock and reservoir characteristics, pressure evolution and structural effects by using various methods including well logging, geochemistry, mercury injection, reservoir physical properties and formation pressure. The results show that the proximal-source, interbedded hydrocarbon accumulation results from a dispersed hydrocarbon supply, which is the root cause of the widely distributed tight sandstone gas. The abnormally high reservoir pressure caused the enrichment of tight sandstone gas even under insufficient hydrocarbon generation dynamics; in addition, natural gas preferentially accumulated in the relatively high-quality reservoirs under the same hydrocarbon supply, which means that differences in the reservoir physical properties control gas charge in the reservoir. Structure controls the gas–water differentiation under the stable tectonic background, and the higher the structure is, the more abundant the gas–water differentiation is, and the easier pure gas reservoirs form. Therefore, the accumulation and enrichment of tight sandstone gas in the Xujiahe Formation is controlled by source rocks, abnormally high reservoir pressure and the physical properties and structure of the reservoir.
Highlights
The Xujiahe Formation source rocks are composed of mudstone and coal seams within Members 1, 3 and 5 (Zhao et al.,2011)
Analyses of the 30 source rock samples showed that the content of organic carbon in Member 1 ranged from 0.61% to 2.54%, with an average of 1.11%, Member 3 ranged from 0.49% to 2.83%, with an average of 1.23% and Member 5 ranged from 0.59% to 2.97%, with an average of 1.66%, which indicates better hydrocarbon generation potential than that of Members 1 or 3
The light hydrocarbons in the gas show similar characteristics to the nearby strata, which means that the Xu-6 natural gas is sourced from Xu-5 source rocks and the Xu-4 gas is from Xu-3 source rocks
Summary
With its organic-rich source rocks and wide distribution, has become a focus of unconventional gas exploration (Chen et al, 2014, 2015; Jia, 2017; Law, 2002; Lu et al, 2017; Ma et al, 2009; Qin et al, 2017; Wei et al, 2017; Zou et al, 2009a, 2012, 2014, 2015; Wei et al, 2016). Keywords Guang’an area, Xujiahe Formation, tight sandstone gas, abnormally high reservoir pressure, reservoir structure, main controlling factors The Xujiahe Formation source rocks are thick, have high gas generation potential, and developed mainly from humic organic matter with a high abundance of mature to early high-mature organic matter.
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