SEDIMENTATION, DIAGENESIS AND RESERVOIR CHARACTERISTICS OF THE UPPER CARBONIFEROUS CARBONATE ROCKS IN THE MIDDLE YANGTZE AREA

The petrographic and sedimentological data show that the Carboniferous strata in the Middle Yangtze area are assigned to the littoral facies, carbonate tidal-flat subfaeies, carbonate platform facies, platform-shoal subfacies, open platform subfacies and restricted platform subfacies, among which the carbonate tidal-flat subfacies and platform-shoal subfacies together with the open platform subfacies are believed to be the favourable reservoir facies zones.
The diagenesis influencing the reservoir characteristics of the carbonate reservoirs comprises cementation, erosion, compaction, pressure solution and dolomitization, among which dolomitization, burial erosion and pressure solution are interpreted as the most important processes which are responsible for the formation of the pore spaces created in the oil penetration stage.
The modes of occurrence of bitumen and fluorescence analysis have revealed that the reservoir pore spaces of the Upper Carboniferous carbonate rocks in the study area include intercrystalline pores, intercrystalline solution pores, matrix solution pores, intergranular solution pores and stylolitic fissures. The reservoir rocks are composed of silty-fine crystalline dolostones, micritic-sparry grainstones and dolostones, micritic limestones, blue algae-foraminiferal dolostones, and the best reservoir members consist of the lower and apper members of the Huanglong Formation.