Abstract:

To determine the genesis of dolomite in the lower Yingshan formation-Penglaiba formation of the Middle-Lower Ordovician in northern Tarim basin, this paper investigates the dolomitization in the target interval through the observations of core samples and thin sections and the analysis of cathodoluminescence, X-ray diffraction order degree, stable carbon and oxygen isotopes, strontium isotopes, and rare earth element compositions and partition patterns, and by combining petrological characteristics with geochemical characteristics. The results show that silty to microcrystalline dolomites and silty to fine-grained anhedral dolomites were formed from syndepositional dolomitization in high-salinity seawater which was primarily originated from the seawater under low-temperature surface evaporation; silty to fine-grained euhedral dolomites were formed from shallow-burial dolomitization in early diagenetic period, with fluids sourced from Ordovician seawater and an increasing temperature with the increase of burial depth; and saddle-like dolomites were formed from hydrothermal dolomitization in early diagenetic period, with fluids sourced from Ordovician seawater as well as later deep-seated magmatic hydrothermal fluid. Reservoir spaces can’t generate from syndepositional high-salinity seawater dolomitization, but may be formed after the dissolution of the precipitated gypsum due to regional constraints and intense evaporation. Eeuhedral dolomite can form under early diagenetic shallow-burial dolomitization, which promotes the development of intercrystalline pores where dissolution fluid may easily enter in late diagenetic stage, forming intercrystalline dissolution pores and dissolved pores. Early diagenetic hydrothermal dolomitization is unfavorable for the formation of reservoir spaces.

Key words: Tarim basin, Tabei area, Ordovician, Yingshan formation, Penglaiba formation, dolomite, dolomitization, reservoir space