Similarities and differences between natural gas accumulations in Sulige gas field in Ordos basin and Xujiahe gas field in Central Sichuan Basin

Both Sulige gas field in the Ordos Basin and Xujiahe gas field in Central Sichuan Basin are large low-porosity and low-permeability gas fields composed of conventional gas reservoirs and unconventional gas accumulations, representing typical cases of low-grade natural gas accumulations in China. Studies show there are both similarities and differences between these two fields in terms of forming conditions, mechanism and distribution. The similarities include: (1) large-scale development of accumulation conditions and close source-reservoir contacts, providing basis for large-scale accumulations; (2) large-scale energy storage during source kitchen burial and large-scale gas expulsion during uplift, providing important conditions for large-scale natural gas accumulation; (3) coexisting volume flow and diffusion flow, giving rise to the co-existence of conventional gas reservoirs and unconventional gas accumulations; and (4) clustered lithologic gas accumulations on a large scale. The differences lie in: (1) different source-reservoir-seal assemblages, giving birth to two kinds of accumulations, i.e. large area accumulations and large-scale patchy accumulations; (2) different source kitchen continuity and gas supply potentials, leading to different enriched gas-bearing layer series and different abundances. Timely understanding of the similarities and differences between large-scale natural gas accumulations with low porosity and permeability in these two basins would be conducive to avoid risks and improve exploration success ratio and benefits.