The age and dolomitization processes in the Paleo-oil reservoir zone, which is composed of massive dolostones found in the Qiangtang Basin (SW China), are still debated. In this research, the Long’eni-Geluguanna Area was selected. Macroscopic information, thin sections, and geochemical methods were used to investigate the dolomitization characteristics and the processes that controlled dolomitization. Five types of replacive dolomites and two types of dolomite cement were observed. Some of the dolomites displayed ghosts of primary sedimentary structures. Saddle dolomites were prevalent, occurring in the interparticle and moldic pores of the limestone which should have been filled at an early diagenetic stage. Ten microfacies types were identified. The foraminifera assemblage provides evidence that the studied interval is of Early Jurassic age. The δ¹³C values are similar to the contemporaneous seawater signature. The REE+Y patterns of limestones and dolostones exhibit similarities to that of seawater. The mean Na and Sr values are comparable to those of other near-normal seawater dolomites. The δ¹⁸O values of all lithologies are markedly depleted. The dolomitization started penecontemporaneously, with deposition. A general sand shoal setting with patch reefs developed. The dolomitizing fluids, near-normal seawater, was probably formed by slight evaporation on top of the shoal. Saddle dolomites in the interparticle and moldic pores might indicate hydrothermal activity, which also caused the recrystallization of some pre-existing dolomites. The recrystallization might have slightly increased the crystal size, demolished the ghost structures, formed saddle dolomites, and altered the REE+Y patterns. The recrystallization extent diminished with increasing distance from the fluids-providing fracture. Furthermore, the existence of protected areas within the sand shoal settings could enhance the vertical and horizontal heterogeneity of dolostone reservoirs.

The “dolomite problem” has long been a significant challenge in sedimentology, particularly regarding the determination of dolomite formation temperatures, a subject that remains highly debated. Magnesium (Mg) isotopes, due to their stability in dolomite during diagenesis and the strong relationship between isotopic equilibrium fractionation and temperature, present a promising tool for estimating the formation temperatures of dolomite. In this study, we analyzed the Mg isotope composition (δ²⁶Mg) of various dolomite samples from the Ediacaran to Ordovician in the Tarim Basin, China, to assess the potential of Mg isotopes as a thermometer for dolomite formation. The δ²⁶Mg values of micro-fine crystalline dolomites (D1) ranged from − 1.98 to − 1.69‰, fine-medium crystalline dolomites (D2) from − 1.68 to − 1.33‰, and medium-coarse crystalline dolomites (D3) from − 2.35 to − 1.99‰. Based on the Mg isotope temperature equilibrium fractionation equations, Δ²⁶Mg_dol−fluid = − 0.1554(± 0.0096) × 10⁶/T² or Δ²⁶Mg_dol−cal=0.1453(± 0.0106)×10⁶/T², the calculated formation temperatures for D1 ranged from 45.0 to 65.4 °C, for D2 from 53.1 to 73.9 °C, and for D3 from 142.4 to 173.9 °C or 156.4 to 189.0 °C. These temperature estimates align closely with those derived from fluid inclusions and clumped isotopes in previous literature studies of the same formations, supporting the reliability of Mg isotopes as a method for determining dolomite formation temperatures. This study introduces an innovative approach for assessing dolomite formation temperatures using Mg isotopes, offering crucial insights into the resolution of the “dolomite problem.”