Multiphase dolomitization and hydrothermal alteration of the Upper Cambrian-Lower Ordovician carbonates in the Gucheng uplift, Tarim Basin (NW China)

The thick Upper Cambrian-Lower Ordovician carbonates of the Gucheng uplift (southeastern Tarim Basin, northwestern China) have been extensively dolomitized, leading to the formation of massive dolomite reservoirs. The origins of different dolomites and their original fluids were studied based on an integrated analysis of petrology, geochemical data (C, O, and Sr isotopes) and fluid inclusion assemblages (FIAs). The D1 (very finely to finely crystalline planar-e/s dolomite) and the D2 (finely to medium crystalline planar-s/a dolomite) dolomites were formed from the mesosaline to penesaline seawater in the subsurface to shallow burial stage. The increasing temperatures and changes of dolomitizing fluid properties led to the formation of different textures and distributions. Both D1 and D2 dolomites, compared to the matrix limestone, show no obvious improvements in porosity, which indicates that early dolomitizations did not create or preserve the primary pores. CD1 (finely to medium crystalline nonplanar-e/s dolomite) and CD2 (medium to coarsely crystalline nonplanar saddle dolomite) dolomites are dolomite cements of hydrothermal alteration at the deep burial stage. Due to changes in temperature and Mg2+ concentrations of the hydrothermal fluids, the precipitation of CD1 and CD2 dolomites exhibited different textures and filling sequences. In the limited space of matrix dolomite, hydrothermally altered dolomite was dominated by D3 (medium to coarsely crystalline nonplanar-a dolomite). The late calcite fillings were precipitated on account of decreasing Mg2+ concentration at the end of the hydrothermal alteration. Hydrothermal fluids, depleted in Mg2+ and enriched in silica, could be deep-basinal brines driven by the strongly transpressional faults from the Late Caledonian to Early Hercynian. Compared to D1 and D2 dolomites, the higher porosities in hydrothermally altered dolomite indicate that hydrothermal fluids are constructive to the formation of reservoirs. Hydrothermal alteration products showed obvious differences between the dolomite and limestone, and led to different performance in physical characteristics. Hence, the hydrothermally altered reservoirs in the eastern Tarim Basin were mostly distributed in the dolomite but were less developed in the limestone. This study provides new insights into the evolution of dolomitizing fluids that are responsible for multiple dolomitization events and the formation of the hydrtothermal dolomite in the Tarim Basin and elsewhere around the world.