Petrographic, mineralogical and geochemical constraints on the fluid origin and multistage karstification of the Middle-Lower Ordovician carbonate reservoir, NW Tarim Basin, China

Abstract

The Middle-Lower Ordovician carbonate formation is the most important hydrocarbon-bearing unit in the Tarim Basin, China. Previous studies have predominately attributed the fracture-cave carbonate reservoirs to epigene karstification, and few involved polygenic karstification. Until now, mechanism and evolution model of the fracture-cave reservoirs has not been very robust. Based on detailed petrographic, mineralogical, karstological and geochemical analysis at the outcrop, this research provides new insights into the characteristics of the karst reservoir, origin of diagenetic fluids and multistage karstification in the Middle-Lower Ordovician. The results indicate that matrix porosity and permeability of the reservoir rock are low, ranging from 0.03% to 1.35% (mean 0.46%) and from 0.000864 mD to 9.15 mD (mean 0.33 mD), respectively, and that karst cave and fracture systems account for the dominant secondary reservoir porosity in the Middle-Lower Ordovician. Meteoric water and hydrothermal fluids are the two most important diagenetic fluids in the Middle-Lower Ordovician, and two stages of dissolution are responsible for the development of the fracture-cave reservoirs in the region. Extensive dissolution by meteoric water resulted in the formation of large volumes of fracture-cave reservoirs during the Middle Caledonian. Hydrothermal dissolution induced by the hydrothermal event during the Early Permian formed additional fracture-cave reservoirs in the Middle-Lower Ordovician carbonate as well. The meteoric dissolution is primarily controlled by the karst unconformity, and the fracture-cave system resulting from meteoric dissolution has a stratiform distribution below the unconformity. The fracture-cave system formed by hydrothermal fluids is mainly controlled by the deep-seated faults and the previous permeable layers, which determine that its distribution is closely related to faults. Moreover, hydrothermal dissolution may occur at high temperature and at great depths, while mineral precipitation may be the dominate process at relatively low temperature and at shallow depths. In this study, a reservoir development model of multistage karstification was established that could further enhance the knowledge of karstification process and assist in petroleum exploration in deep carbonate strata. • Two types of diagenetic fluids are revealed by integration of Geochemical, Petrographic and Mineralogical analysis. • The effects of multistage karstification on the carbonate reservoir are determined. • Hydrothermal karstification plays a positive role in the deeply burial environment. • A development model of the carbonate reservoir covered by the overlying impermeable strata is established.