Formation mechanisms , potentials and exploration practices of large lithologic gas reservoirs in and around intracratonic rifts : Taking the Sinian-Cambrian of Sichuan Basin as an example

By examining structures, sediments, reservoirs, and accumulation assemblages in the Deyang-Anyue rift and its surrounding area, four new understandings are obtained. First, during the initiation period of Deyang-Anyue rift, multiple groups of faults developed in the rift due to the effect of tensile force, bringing about multiple mound and shoal belts controlled by horsts in the second member of Dengying Formation; in the development stage of the rift, the boundary faults of the rift controlled the development of mound and shoal belts at the platform margin in the fourth member of Dengying Formation; during the shrinkage period of the rift, platform margin grain shoals of Canglangpu Formation developed in the rift margin. Second, four sets of large-scale mound and shoal reservoirs in the second member of Dengying Formation, the fourth member of Dengying Formation, Canglangpu and Longwangmiao Formation overlap with several sets of source rocks such as Qiongzhusi Formation source rocks and Dengying Formation argillaceous limestone or dolomite developed inside and outside the rift, forming good source-reservoir-cap rock combinations; the sealing of tight rock layers in the lateral and updip direction results in the formation model of large lithologic gas reservoirs of oil pool before gas, continuous charging and independent preservation of every gas reservoir. Third, six favorable exploration zones of large-scale lithologic gas reservoirs have been sorted out through comprehensive evaluation, namely, mound and shoal complex controlled by horsts in the northern part of the rift in the second member of Dengying Formation, isolated karst mound and shoal complex of the fourth member of Dengying Formation in the south of the rift, the second and fourth members of Dengying Formation and the superimposed area of multi-stage platform margin mounds and shoals in Canglangpu Formation in the north slope area, the platform margin mounds and shoals of the second and fourth members of Dengying Formation in the west side of the rift, and the platform margin mound and shoal bodies of the fourth member of 石油勘探与开发