Abstract:

The Yingshan Formation of the Middle-Lower Ordovician is a potential exploration replacement series in the Central Uplift of the Tarim Basin, however, the genesis of dolostone is still unclear, which hinders dolomite exploration in the basin. The ongoing dolomatization in the Yingshan Formation, as evidenced by the sequential, bottom-to-top developments of dolostone, dolomitic limestone and limestone, makes it feasible to study the dolostone reservoirs by Mg isotope analysis that has shown good results in tracing the source and migration pathway of dolomitic fluid. In this study, we systematically collected dolostone, dolomitic limestone and limestone from the Yingshan Formation and carried out comprehensive petrological, trace elemental and isotope (C, O, Mg) analyses. Six types of dolostone were identified in the Yingshan Formation: dolomicrite/micro-crystalline dolostone (D1), powder/fine-crystalline dolostone (D2), granular dolostone (D3), medium/coarse-crystalline dolostone (D4), dolomitic limestone (DL) and limestone (L). The δ¹³C_V-PDB value of carbonate rock ranged between -2.10‰ to -0.37‰ (average -1.37‰), δ¹⁸O_V-PDB value between -7.51‰ to -3.54‰ (average -5.41‰), and δ²⁶Mg value between -4.03‰ to -1.28‰ (average -2.55‰), and certain degree of correlation in the vertical direction was observed between δ²⁶Mg, Na, Sr/Ba, Mn/Fe, δ¹³C_V-PDB, δ¹⁸O_V-PDB, paleosalinity (Z) and plaeotemperature (T). Results on dolomite genesis revealed (1) the δ²⁶Mg variation trend in the vertical direction was closely related to sedimentary cycles, where the top layer was identified as the source area of Mg-rich fluid and the layer interface as the migration channel. (2) There were four migration pathways of dolomitic fluid identified based on the Mg isotopic variation trends: L→DL→D, D1→D2, D2→D3→D4→L and D4→L/DL. (3) The formation of dolostone was mainly related to the periodic fluctuation of sea-level: with decreasing sea level, Mg-rich fluid formed in the confined water via evaporation migrated vertically downward along the high-porosity, high-permeability granular limestone to promote continuous dolomitazation, or it encountered low-porosity argillaceous limestone and failed to enter the rock lattice, thus terminating dolomitization; whereas rising sea-level destroyed the source area of Mg-rich fluid, thus weakening or interrupting dolomitization to form calcareous dolostone and limestone. (4) Multi-stage metasomatism, recrystallization and hydrothermal processes in the deep-burial stage were also conducive to the formation of dolostone. (5) Dolomitization had significant impact on the sedimentary reservoirs, where early dolomitization was conducive to the inheritance and preservation of pores while late-burial and hydrothermal dolomitization destroyed the reservoirs.

Key words: Mg isotope, dolostone origin, Ordovician Yingshan Formation, Tarim Basin, Central Uplift