Research paperPetrological and geochemical constraints on porosity difference between Lower Triassic sour- and sweet-gas carbonate reservoirs in the Sichuan Basin

- Dolomites have much higher secondary porosity and H2S contents than limestones.
- Anhydrite and dolomite show dissolution and replacement by TSR calcite.
- The dolomite dissolution resulted in CO2δ13C values heavier than TSR calcite.
- Vug-filling TSR calcite shows lower FeO and SO3 contents than replacive calcite.
- TSR may have improved or redistributed porosity in dolostones.

Petrographic features, carbon and oxygen isotopes, and trace elements were determined, and fluid inclusions were analyzed on late diagenetic cements and vug-fillings from the Lower Triassic carbonates to account for much higher porosities in the sour dolomite reservoirs than in the sweet limestones in northeastern Sichuan Basin. In the NE side to the Kaijiang-Liangping Trough, dolostone reservoirs have porosities mainly from 6 to 12%, up to 28% and contain 9-20% H2S. The dolomite includes pre-bitumen re-crystallized silt-sized crystals without occurrence of fluid inclusions, and late burial dolomite. The late burial dolomite has been measured to have fluid inclusions homogenization temperatures (HTs) from 72 to 128 °C. Part of the dolomites with HTs up to 121 °C show corroded edges or replacement by TSR calcites. TSR calcites show δ13C values from −2.3 to −18.9‰, FeO from null to 810 μg/g and SO3 from 50 to 18,700 μg/g. Vug-filling TSR calcites have much lower FeO and SO3 than the replacement calcites of anhydrite, suggesting that dissolved Fe2+ released from the dissolution of dolomite and anhydrite as a result of TSR may have reacted with TSR-H2S and precipitated as pyrite. The dissolution may have released 13C-rich CO2 resulting in the present-day CO2 having δ13C values significantly heavier than those of the TSR calcites as light as −18.9‰. The dolomite and anhydrite dissolution during late stage of TSR may have enlarged or redistributed porosity in the NE side dolostones. In contrast, oolitic limestone reservoirs in the SW side contain significantly lower sulfates and thus no significant late-diagenetic dissolution and TSR. The porosity difference in the two sides resulted from original physical property due to early dolomitization and subsequent improvement from anhydrite and dolomite dissolution as a result of TSR.

1) NE side dolomite reservoirs have much higher secondary porosity (A) and H2S contents than SW side limestones (B), and is considered to result from the dissolution of anhydrite and dolomite as a result of TSR. 2) Present-day CO2 in the NE side dolomite reservoirs shows heavier δ13C values than TSR calcites, indicating that 13C-rich CO2 was released from dolomite dissolution.