Abstract:

Through research on the Ordovician sequence stratigraphy,sedimentary facies and carbonate reservoirs in Tarim basin,this paper discusses the distribution of platform marginal facies and main controlling factors of reservoirs in the Ordovician.Four stages of platform marginal high-energy facies belts are identified,including the Penglaiba Formation of the lower Lower Ordovician,the Yinshan Formation of the upper Lower Ordovician to the lower Middle Ordovician,the Yijianfang Formation of the middle to upper part of the Middle Ordovician,and the Lianglitage Formation of the middle Upper Ordovician.The main factors controlling the effective formation of carbonate reservoirs in these marginal facies are that the high-energy bank facies grainstone serves as the main material basis for the effective formation of reservoirs,followed by the barrier reefs and micrite mounds;the fresh water dissolution,mixed-water dolomitization and mixed-water dissolution,caused by contemporaneous exposure of marginal carbonate due to Milankovitch or the third-rank sea level dropping,are the chief constructive diagenesis for the formation of effective reservoirs;secondary vugs and cavities were generated by the karstification of the Ordovician marginal carbonate during the Silurian and the Devonian;and the organic acid dissolution and magma hydrothermal fluid dissolution during deep burial of the Permian and the open fault-fracture system parallel or small-angle obique to the major compressional stress are constructive for forming effective reservoirs in the Ordovician marginal carbonate.

Key words: karst, marginal facies, reservoir, carbonate rock, Ordovician, Tarim Basin