Paleogene lacustrine dolomitization, Xingou region, southern Qianjiang Depression, China

Abstract

The lacustrine dolomicrites of Unit II of the lower Xingouzui Formation deposited in the southern part of the Qianjiang Depression provide a good “tight carbonate” hydrocarbon reservoir. The dolomicrites are typically interbedded with and sourced from organic-rich argillite, which together form a source-reservoir assemblage characterized by thin single-layer thicknesses, far lateral extent and continuous lateral distribution. Systematic sedimentological and geochemical studies indicate that the depositional environment and organic matter evolution of the dark argillites (with TOC of 0.5–2%) are favorable for formation of the dolomicrites. The dolomicrite was mainly formed in a shallow-water evaporative environment; crystals are anhedral under SEM, in non-surface contact, and include a number of honeycomb intergranular micropores associated with replacement. The dolomicrite shows orange-orangered light by cathodoluminescence, indicative of association with minor terrigenous debris; X-ray diffraction results indicate that the dolomicrite is characterized by high calcium content (CaCO₃ molar contents of 52.28–52.28%), low degree of orders of 0.2–0.4), and high content of Fe. The dolomicrite has δ¹⁸O values of 2.47–2.47‰ PDB, more positive than micrite, and δ¹³C values as moderately negative as 9.4–4.21‰ PDB, similar to micrite. Both values indicate the dolomicrite formed in a low-temperature genetic zone. Together, these characteristics indicate that the dolomicrite in the study area was formed in a high-salinity, confined lake basin, characterized by low temperature, shallow burial, and rapid nucleation and crystallization. The dolomicrite experienced a contemporaneous period with formation of dark mudstone, shale and calcite, and a penecontemporaneous period with formation of dolomicrite via seepage-reflux and a diagenetic period when ferrodolomite was formed via burial.