Geochemical characteristics and genetic mechanism of dolomite reservoirs in the Dazhuoma area, Qiangtang Basin

Abstract

Dolomite is a major reservoir rock in many petroliferous basins, and has long been a central research focus for oil and gas resource evaluation and exploration. Here, we investigate the petrological and elemental geochemical characteristics of the Middle Jurassic Buqu Formation dolomite reservoir in the Dazhuoma area, Qiangtang Basin, to constrain its genetic mechanism. Our results show that Buqu Formation dolomite is dominated by homogeneous crystalline textures, and can be divided into three main textural classes: residual-grained dolomite, mosaic crystalline dolomite, and medium to coarse-grained hydrothermal euhedral dolomite. The average molar Ca/Mg ratio of the studied dolomite is 1.8, consistent with an origin via dolomitization of pre-existing limestone. Relative to coeval limestones, the dolomite is enriched in Mg, Ba and Mn, and depleted in Ca, Sr and Si, indicating that element migration during diagenesis was governed by dolomitization and subsequent burial diagenetic processes. We identify for the first time a vertically segmented pattern in the carbon (δ ¹³ C) and oxygen (δ ¹⁸ O) isotope compositions of Buqu Formation carbonate strata: the upper segment retains coupled δ ¹³ C and δ ¹⁸ O values that preserve primary seawater precipitation signatures, whereas the lower segment exhibits decoupled isotopic behavior, likely reflecting overprinting by burial hydrothermal processes or deep-circulating crustal fluids. This new isotopic zonation provides independent evidence for post-depositional dolomitization modification. Integration of petrological, elemental geochemical, and C–O–Sr isotopic data supports a multi-stage superimposed dolomitization genetic model for the studied dolomite reservoir.