Dolomite genesis and dolomitization mechanisms of the Ordovician lower Yingshan Formation, Gucheng area, Tarim Basin, China

Abstract

The massive dolomite reservoirs of the Ordovician formations in the Tarim Basin have significant gas exploration potential, but their occurrence and distribution are not clearly understood. This study collected core samples covering regional dolomite reservoirs from the lower Yingshan Formation in the Gucheng area. A combination of petrology, fluid-inclusion assemblage (FIA) microthermometry, and isotope geochemistry data were used to define the dolomitization process and its impact on reservoirs. Petrological and geochemical analyses reveal that various dolomites have different dolomitization mechanisms. In (pene)contemporaneous (PC) stage, on the western side of the east Tarim Platform, laminated dolomicrite (LDm) of the third Member (3rd Mm) was formed by microbial and sabkha dolomitization, whereas dolomicrite (Dm) of the fourth Member (4th Mm) generated from sabkha dolomitization in the slightly elevated salinity setting in an evaporative sea environment. The residual structural dolomite (RSD) of the 3rd and 4th Mm was characterized by reflux fluids on the central platform margin. The shallow-middle burial (SMB) stage was dominated by reflux dolomitization, precipitated structureless dolomite (SD) primarily associated with RSD. Bimodal geochemical characteristics indicate that dolomitizing fluids gradually transitioned to pore water consisting of evaporated seawater reserved in the strata. In the middle-deep burial (MDB) stage, hydrothermal burial effects were superimposed and transformed the dolomites near the fractures. Leopard dolomite (LD) in the 4th Mm is limited to faults and has seawater and hydrothermal geochemical characteristics, controlled by the superimposition of pore water in SMB stage and hydrothermal fluids in MDB stage. Massive dolomite (MD) in 3rd Mm, which is not completely limited to the fracture system, was derived from multiple superimposed dolomitization events and transformed by terrestrial water. Finally, the effect of dolomitization on reservoirs was investigated. It is concluded that the sabkha and reflux dolomitization in PC stage was the primary dolomitization mechanism and was the basis for the pore formation.