Petrologic and Geochemical Constraints on the Origin of Dolomite of Feixianguan Formation, NE Sichuan Basin, China

In the Northeast Sichuan Basin, some of the large gas reservoirs occur in dolostones from the Lower Triassic Feixianguan Formation. These dolostones are concentrated on a semi-isolated carbonate platform, which was separated to the east from a large, shallow, carbonate platform by the Kaijiang-Liangping sea trough. The Feixianguan Formation consists of three basic end-member textures of replacement dolomite: 1)micritic dolomite replacing the lime mudstone and wackestone, 2)finely crystalline, grain-dominated dolomite with good fabric preservation, and 3)fine to medium crystalline dolomite that partially to completely obliterated precursor limestone textures. These three dolomite types form the top, middle-upper part, and lower part of a shallowing-upwards carbonate cycle, respectively. The micritic dolomite is intergrown with abundant gypsum and anhydrite that appear to be cogenetic, indicating it formed in a sabkha-type tidal-flat environment. The grain-dominated dolomite is characterized by relatively low manganese concentration (21 to 124 ppm), low strontium concentration (124 to 300 ppm), and very few gas-liquid inclusions. This dolomite have δ¹⁸O values ranging from -2.7 to -4.6‰ PDB, which is similar to those expected from Early Triassic marine dolomite, and it also show ⁸⁷Sr/⁸⁶Sr ratios overlap with sea-water values for the Feixianguan period. These geochemical characteristics of the grain-dominated dolomite indicate contemporary sea water was again a likely dolomitizing fluid. Instead, crystalline dolomite yielded δ¹⁸O values ranging from -4.3‰ to -6.4‰ PDB, this is depleted relative to that of contemporary marine dolomite. Fluid-inclusion homogenization temperatures suggest about 70°C to 170°C at the time of this dolomite formation. This dolomite contains very low manganese concentration (8.3 to 30 ppm), indicating that fluid(s) responsible for the formation of dolomites did not react with aluminosilicate rocks. Strontium-isotope determinations on the this dolomite yield ⁸⁷Sr/⁸⁶Sr ratios between 0.70865 and 0.70901, which is slightly enriched compared to strontium-isotope composition of seawater of Feixianguan time, but correspond to those between Jialingjiang and Leikoupo time. These data may suggest that reflux circulation of younger, hypersaline seawater associated with thick evaporite layers of Jialingjiang and Leikoupo Formation can form dolomitization at intermediate burial depth.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California