TSR-Related Calcite Cements in Triassic Dolomite, NE Sichuan Basin, China

Abstract

Calcite cement is a significant and widespread component in the Upper Triassic dolomites in northeastern Sichuan Basin, China, and has been previously interpreted as a product of dedolomitization. However, this conclusion was based on questionable data analysis and this therefore calls for a reevaluation of the geochemical information recorded in these calcite cements. The major objective of this study is to determine the major reason for calcite precipitation, specifically, whether these calcites are related to thermochemical sulfate reduction (TSR). Here, we have investigated eighteen dolomite samples from wells HB-1 and L-2 using petrography, X-ray diffraction, stable isotopes, major and trace element rock chemistry, mineral chemistry, and fluid inclusions. In thin section, the calcite occur as 1) extremely small crystals scattered in the intercrystal pores of dolomite matrix and 2) coarsely crystalline blocky cements. Fluid-inclusion homogenization temperatures suggest about 110^OC to 240^O C at the time of this authigenic calcite formation. Sixteen samples of finely-crystalline dolomite yielded bulk dC values ranging around 1.6‰ to 7.0‰ and bulk d¹⁸O values ranging around −4.4‰ to −2.8‰ PDB. These samples contain 1.3 to 17.8 wt.% calcite, 87 to 385 ppm strontium, 12 to 71 ppm manganese, and 383 to 4,487 ppm iron. In contrast, two samples of pure, coarsely crystalline calcite (with calcite content up to 98.8 wt.% and 98.3 wt.%) are characterized by highly negative dC values of −20.2‰ and −16.2‰, relatively low d¹⁸O values of −8.5‰ and −7.9‰, and more strontium contents (1,564 and 3,400 ppm). However, cross-plots of dC, d¹⁸O and Sr contents versus percent of calcite show good correlations for dolomite samples. Thus, the end-member data can be evaluated by linearly extrapolating to 100% calcite, and the results (dC=−14.0‰, d¹⁸O=−11.0‰, Sr=1,432 ppm) are close to the measured ones for coarsely crystalline calcite. The low dC values indicate that carbon incorporated in calcite cement was partly derived from oxidation of organic matter. The homogenization temperatures, in conjunction with d¹⁸O data, suggest a thermochemical origin for calcite precipitation. The high Sr contents reflect high Sr/Ca ratios in the diagenetic fluid, which may be due to inhibition of celestite precipitation at the expense of SO₄²⁻. The formation of calcite cements may thus be the result of TSR rather than dedolomitization.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014