Diagenesis and origin of porosity formation of Upper Ordovician carbonate reservoir in northwestern Tazhong condensate field

Abstract The Upper Ordovician Lianglitage Formation in Tazhong has been proven to be productive reef-shoal carbonate reservoirs, being of great significance for oil and gas exploration in Tarim Basin. Based on petrographic features, cathodeluminescence and fluid inclusion measurements, pore types and diagenetic processes of Lianglitage Formation are analyzed. In attempt to define the origin of porosity formation, paragenetic sequence and additional geochemistry evidences were then illustrated. The result of this study demonstrates that pore space of Lianglitage Formation in northwestern Tazhong is mainly composed of intraparticle pores, channels, intracrystal pores, vuggy pores, intercrystal pores and fractures. Furthermore, five stages of calcite cementation (Calcite-1 to Calcite-5) and five stages of dissolution of skeletal allochems or cements (D-1 to D-5) are identified in this study. The origin of porosity mechanism of Lianglitage Formation in northwestern Tazhong is related to three types of geofluids: meteoric water, mixing water and hydrothermal fluid. Meteoric water dissolution (D-2) and associated fracturing during 2nd Phase of Middle Caledonian tectonic movements control present-day porosity generation. Although mixing water dissolution (D-1) in high-energy sedimentary facies generated fabric-selective porosity during eogenetic diagenetic stage, the contribution to porosity creation and preservation is restricted. Hydrothermal fluid in mesogenetic diagenetic stage is evidenced to occur, but it mainly leads to precipitation of late diagenetic calcites and porosity occlusion rather than porosity enhancement. This is especially useful for further understanding reservoir formation and development, and thus has significant implications for hydrocarbon exploration.