Petrographic, Mineralogical and Geochemical Constraints on the Origin of Regional-Scale Dolomitization: A Case Study of the Cambrian–Lower Ordovician Dolomites in the Bachu Area, Central Tarim Basin, China

Abstract

The Cambrian and Lower Ordovician strata of the Bachu area in central Tarim Basin are characterized by extensively dolomitized, which have significant potential for favourable hydrocarbon reservoirs. The study herein combined petrographic, mineralogical and geochemical analyses to investigate the formation environments of these dolomite units and utilized Mg isotopic profiles to constrain the changes in the fluid flow trajectory. Positive excursions of δ¹⁸O and upward-increasing δ²⁶Mg profiles were observed in samples of the Wusonger formation (Є₁w), which may result from evaporation. Moreover, based on high ⁸⁷Sr/⁸⁶Sr ratios, high total REE concentrations and the occurrence of specific minerals, including quartz, barite, gypsum and pyrite, we interpret an evaporitic near-surface origin for the Є₁w dolomites. Samples of the Xiaoerblak Formation (Є₁x) are the product of seepage-reflux dolomitization, suggested by upward-decreasing δ²⁶Mg profiles and relatively lower ⁸⁷Sr/⁸⁶Sr ratios than the Є₁w samples. The overlapping isotope (δ¹⁸O, δ¹³C and ⁸⁷Sr/⁸⁶Sr) values between samples of the Shayilik (Є₂s) and Awatagh (Є₂a) Formation, in addition to the petrography evidence and Mg isotope variations, suggest that the formation of these dolomites were due to the shallow-burial dolomitization. The occurrence of the Xiaqiulitagh (Є₃ql) and Penglaiba (O₁p) dolomites along stylolites, together with their large crystals with opaque cores and clear rims and vertically constant δ²⁶Mg profiles, indicate that their formation may be related to pressure dissolution by mechanical compaction during deep-burial dolomitization. Therefore, the assembling of multiple petrographic, mineralogical and geochemical methods can shed light on the complicated generation processes of regional-scale dolomite within Tarim Basin or elsewhere.

Keywords

genesis of dolomites / isotopes / magnesium isotope / Tarim Basin / Cambrian–Lower Ordovician

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