TRIASSIC DOLOMITES IN CARBONATE PLATFORMS OF SOUTH CHINA: CONSTRAINTS ON MODELS FOR DOLOMITIZATION

Carbonate platforms host about half of conventional petroleum reserves. Dolomite with intercrystalline porosity is a major reservoir type. The goal of this project is to investigate the processes that generated dolostones within the carbonate platforms of the Nanpanjang Basin (NPJB) of south China. The NPJB is a large foreland basin in the southern margin of the South China plate that developed during the Triassic convergent tectonics. The basin contains several isolated carbonate platforms. Dolostone forms massive stratiform deposits in the platform interior and slope facies. In this study, we examined the distribution and petrogophy of dolostone in the platform interior and slope environments of the Great Bank of Guizho (GBG), the northernmost isolated platform in this basin. Differences in spatial distributions and petrographic characteristics between end-member models of dolomititzation include: a) stratiform and microcrystalline in the tidal flat model, b) cross-cutting distribution and variable crystallinity and association with evaporites in the evaporative reflux model, c) local island restriction in the mixing zone model, and d) massive thick, coarsely crystalline and fabric destructive textures in the burial model.

Field observations reveal two types of dolostone developed in the GBG: 1) microcrystalline tidal flat dolostone that affects peritidal cycle caps with syndepositionally reworked dolomite and 2) coarse, massive, fabric destructive dolostone that affects strata 100s of meters thick. The coarse, massive dolostone cuts across bedding, consistent with either the reflux or burial models. Platform interior and margin oolite facies are consistently dolomitized, whereas large blocks transported to the basin along a collapsed margin are preserved as limestone (i.e., they were never dolomitized). This observation rules out the possibility that reflux dolomitization was the sole mechanism, because the platform would have dolomitized before shedding blocks to the slope.

Geochemical differences between dolostones of different origin (oxygen isotopes, trace chemicals, and fluid inclusions) depend upon the origins of dolomitizing fluids. Further work will be done to evaluate mode of dolomitizaton using petrography, oxygen isotope geochemistry, and fluid inclusion geothermometry.