Contribution of moderate overall coal-bearing basin uplift to tight sand gas accumulation: case study of the Xujiahe Formation in the Sichuan Basin and the Upper Paleozoic in the Ordos Basin, China

Abstract

Tight sand gas is an important unconventional gas resource occurring widely in different petroleum basins. In coal-bearing formations of the Upper Triassic in the Sichuan Basin and the Carboniferous and Permian in the Ordos Basin, coal measure strata and tight sandstone constitute widely distributed source-reservoir assemblages and form the basic conditions for the formation of large tight sand gas fields. Similar to most tight gas basins in North America, the Sichuan, and Ordos Basins, all experienced overall moderate uplift and denudation in MesoCenozoic after earlier deep burial. Coal seam adsorption principles and actual coal sample simulation experiment results show that in the course of strata uplift, pressure drops and desorption occurs in coal measure strata, resulting in the discharge of substantial free gas. This accounts for 28 %-42 % of total gas expulsion from source rocks. At the same time, the free gases formerly stored in the pores of coal measure source rocks were also discharged at a large scale due to volumetric expansion resulting from strata uplift and pressure drop. Based on experimental data, the gas totally discharged in the uplift period of Upper Paleozoic in the Ordos Basin, and Upper Triassic Xujiahe Formation in the Sichuan Basin is calculated as (3-6) 9 10 8 m 3 /km 2 . Geological evidence for gas accumulation in the uplift period is found in the gas reservoir analysis of the above two basins. Firstly, natural gas discharged in the uplift period has a lighter carbon isotope ratio and lower maturity than that formed in the burial period, belonging to that generated at the early stage of source rock maturity, and is absorbed and stored in coal measure strata. Secondly, physical simulation experiment results at high-temperature and high-salinity inclusions, and almost actual geologic conditions confirm that substantial gas charging and accumulation occurred in the uplift period of the coal measure strata of the two basins. Diffusive flow is the main mode for gas accumulation in the uplift period, which probably reached 56 9 10 12 m 3 in the uplift period of the Xujiahe Formation of the Sichuan Basin, compensating for the diffusive loss of gas in the gas reservoirs, and has an important contribution to the formation of large gas fields. The above insight has promoted the gas resource extent and potential of the coal measure tight sand uplift area; therefore, we need to reassess the areas formerly believed unfavorable where the uplift scale is large, so as to get better resource potential and exploration prospects.

Formation pressure evolution and its causes for the reservoirs of the upper Triassic Xujiahe formation in the northeast portion of the Sichuan basin, China

“…Among these, the first, third, and fifth members of the Xujiahe Formation represent the main source rocks, whose lithologies are mainly dark mudstone and carbonaceous mudstone, while coal rocks exist only in local sections (Zhang et al, 2009;Zhao et al, 2011). The type of organic matter is mainly II 2 -III, which is in a stage of high overmaturity (Bian et al, 2015;. The second and the fourth members are the main reservoirs, whose lithologies are dominated by lithic sandstone and feldspar lithic sandstone (Figure 2), which experienced strong compaction and cementation, resulting in the abnormally tight reservoirs Zhang et al, 2016).…”

Sequence architecture and sedimentary characteristics of a Middle Jurassic incised valley, western Sichuan depression, China

“…During the depositional period of the Shaximiao Formation, the LMTB was relatively static, whereas the Micang-Daba Mountains Tectonic Zone (MDMTZ; Fig. 1) was more active because of the ''scissor-type'' subduction of the Yangtze Block (Deng 2007;Jin et al 2008;Liu et al 2010;Yuan et al 2012;Bian et al 2015). As a result, the foredeep depression was transferred from the northern section of the Longmen Mountains to the Micang-Daba Mountain front (Chen et al 2008;Luo et al 2011).…”

Pore structure and reservoir physical properties for effective development of tight sandstone gas: A case study from the Central Sichuan Basin, China

“…The reservoirs in the second, fourth, and sixth members of the Xujiahe Formation discovered in Central Sichuan Basin are nonconventional tight reservoirs, with porosity of generally less than 10% and permeability of less than 1 mD. Many researchers have made significant contributions to the understanding of the Xujiahe Formation with respect to sedimentology, stratigraphy, reservoir characteristics, and diagenetic evolution (Bian et al, 2015; Li, Chen, Qiu, Su, & Wu, 2019; Yu et al, 2019; Zhang, 2021; Zhong, Huang, Ye, Lan, & Liu, 2020), including the impacts of sequence stratigraphy, sedimentary facies, and diagenesis on reservoir quality and the strong control of the changes in diagenesis (including coMPaction, cementation, and corrosion) on the heterogeneity of tight reservoirs.…”

“…The currently discovered tight sandstone gas reservoirs in the Xujiahe Formation are mostly composite lithological‐structural gas reservoirs, with generally low gas saturation of 45%–65% and generally high water saturation of more than 40%. Low‐permeability and high water saturation pose challenges to the development of such gas reservoirs (Bian et al, 2015; Qin, Li, Li, Zhou, & Zhou, 2018; Sun et al, 2021; Yue et al, 2018). Therefore, it is significant for the development of such gas reservoirs to further understand the pore‐throat structure characteristics of the tight reservoirs in the Xujiahe Formation and the gas and water movability characteristics of reservoirs with different pore structures under certain water saturation (Gong, Zhao, Zhuo, & Liu, 2020).…”