摘要
Both Sulige gas field in the Ordos Basin and Xujiahe gas field in the central Sichuan Basin are large low-porosity and low-permeability gas fields composed of conventional and unconventional gas accumulations,and are 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 condition,mechanism and distribution.The similarities include:(1) large-scale development of accumulation conditions and close source-reservoir contacts provide basis for large-scale accumulations;(2) large-scale energy storage during source kitchen burial and large-scale gas expulsion during uplift provide important conditions for large-scale natural gas accumulation;(3) coexisting volume flow and diffusion flow give rise to the co-existence of conventional gas reservoirs and unconventional gas accumulations;and(4) both are clustered lithologic gas accumulations on a large scale.The differences are:(1) different source-reservoir-seal assemblages give birth to two kinds of accumulations,i.e.extensive accumulations and large-scale patchy accumulations;(2) different source kitchen continuity and gas supply potentials lead 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 are conducive to avoid risks and improve exploration success ratio and benefits.