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

- Three types of dolomite phases are distinguished.
- Multiple types of evidence constrained the dolomite origins.
- Dolomitizing fluid for type-D2 originated from seawater-like compaction fluid.
- Burial-compaction model is proposed for type-D2 dolomite.
- Potential high-quality reservoirs likely occur in shoals on the opposing margins.

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.