Geological conditions and distributional features of large-scale carbonate reservoirs onshore China

Based on well cores and thin section observations of more than 300 wells from major exploration target areas and formations in the Tarim, Sichuan and Ordos Basins, combined with seismic, well logging and testing data, the types and characteristics of carbonate reservoirs as well as the geologic conditions for their extensive development are analyzed systematically, and their distribution features are summarized. All varieties of marine carbonate reservoirs are developed in China, including three types of large-scale effective reservoirs, which are (1) depositional reef-shoal and dolomite reservoirs, (2) epigenetic dissolution-percolation reservoirs and (3) deep burial-hydrothermal altered reservoirs. Besides sedimentary facies, paleoclimate and paleogeomorphy, other factors controlling the development of deep large-scale effective reservoirs include interstratal and intrastratal dissolution-percolation and burial dolomitization which can be impacted by hydrothermal processes. Large effective reservoirs in deep carbonate rocks are distributed along unconformities and hiatuses in sedimentation, while reservoirs of epigenetic dissolution-percolation type extend from paleohigh uplift zones to lower slope reliefs. The reservoirs are widely distributed in stratified planar forms, and are superposed by multi-stage karstification processes vertically and have obvious heterogeneity controls. Burial dolomitization is restricted by primary sedimentary facies, and can form extensive effective reservoirs in deep layers in layered or stratified shapes. Hydrothermal related reservoirs are always distributed along deep, large faults, forming effective reservoirs in the form of a bead string in vertical direction and band-rod horizontally, which are not restricted by burial depth. Key words: carbonates, reservoir, extensive development, deep, distribution feature, Tarim Basin, Sichuan Basin, Ordos Basi