Origin of dolomite in the third member of Feixianguan Formation (Lower Triassic) in the Jiannan area, Sichuan Basin, China

Abstract

The third member of Feixianguan Formation, Jiannan area consists of marine carbonates, parts of which were dolomitized to form high-quality reservoirs. Based on petrographical features, three types of dolomite phases are distinguished, including very finely–finely crystalline dolomite (type-D1), medium crystalline dolomite (type-D2), and coarse crystalline saddle dolomite (type-D3).Type-D1 dolomite has characteristics of micritic to fine crystals (<5–60 μm) with dull to no cathodoluminescence (CL). It commonly preferentially replaced the matrix components between grains in partially dolomitized limestone. These features suggest that type-D1 had an early, near-surface origin. However, the δ18O values imply partly diagenetic stabilitization during burial. Type-D2 dolomite has characteristics of mainly medium (100–250 μm) subhedral crystals with zoned to uniform dull red CL. The estimates from δ18O values suggest precipitation at depths of 760–1100 m. Values of δ13C and 87Sr/86Sr of type-D2 are similar to values of host limestone, suggesting that dolomitization was mediated by T1f3 seawater-like fluids. Type-D3 is strictly associated with type-D2. High homogenization temperatures and limited distributions indicate that type-D3 formed in a deep, closed environment.Dolostone made of type-D1 dolomite is volumetrically minor and is characterized by micropores and low permeability, while type-D3 as a void-filling cement slightly reduces the reservoir porosity. High-quality reservoirs are only intimately associated with dolostone made of type-D2. A burial-compaction model is proposed for type-D2. Mg2+ was derived from seawater-derived, compaction fluid that was expelled from the trough strata. The potential high-quality reservoirs are likely to occur in opposing margin shoals.