Controls of sedimentary facies and sealevel fluctuation on dolomitization: The Lower Cambrian Longwangmiao Formation in Sichuan Basin, China

Abstract

The Lower Cambrian Longwangmiao Formation is a significant deep carbonate (dolomite) reservoir that has great hydrocarbon potential in the Sichuan Basin, South China. However, the origin of the Longwangmiao dolomite and the mechanism of dolomitization remain a debate that needs better understanding due to its crucial importance in determining reservoir quality. In the current study, a multi-technique approach of petrography, geochemistry (C- and O-isotopes, major and trace elements including rare earth elements), and microthermometry has been applied to reconstruct the mechanisms of dolomitization. Petrographic examinations revealed three types of matrix dolomite: (1) near-micritic (dolomicrite), fabric retentive texture (Md1, 10–50 μm), (2) finely to medium crystalline dolomite, subhedral to euhedral crystals (Md2, 60–120 μm), and (3) medium to coarsely crystalline dolomite, subhedral to nonplanar crystals (Md3, 100–300 μm). The similarity of PAAS-normalized REE patterns of the investigated dolomites to that of their precursor lime mudstone and their δ13C (C1 = 0.3 ± 0.3‰, Md1 = −1.3 ± 0.6‰, Md2 = −0.6 ± 1.6‰, and Md3 = 0.3 ± 1.5‰) and δ18O (C1 = −9.4 ± 0.2‰, Md1 = −7.8 ± 0.8‰, Md2 = −8.7 ± 0.5‰, and Md3 = −8.9 ± 0.6‰) values suggest that the parent dolomitizing fluids of Md1, Md2, and Md3 dolomites were likely modified seawater. The significant enrichment of Na (1266 ± 1316 ppm) and K (4649 ± 3199 ppm), tight structure and presence of moldic pores in Md1 and its slightly elevated δ18O values suggest that it formed in a near-surface (i.e., mesosaline to penesaline) environment from slightly evaporated seawater, which is consistent with its Sr/Ca molar ratios (0.0075−0.0090). The Md2 dolomite is crosscut by stylolites and has a coarser grain size, higher Mn (295 ± 172 ppm), NdSN/YbSN (1.33 ± 0.06) and U (0.49 ± 0.11 ppm), and lower Ce/Ce* (0.83 ± 0.02) values than Md1. This suggests a shallow to mid-burial dolomitization with possible downwards seepage-reflux of Mg2+-rich evaporated brines. The Md3 dolomite has the largest crystal size and mosaic contacts, lowest Ce/Ce* (0.81 ± 0.02), and highest U (0.64 ± 0.15 ppm) and Mn (487 ± 240 ppm) values, which imply a deeper dolomitization environment than Md2. Thus, it precipitated likely in a mid-to deep burial setting at a higher temperature (Th = 120−135 °C), from the residual Mg2+-rich fluids stored in the strata pores. Considering the regional geology, we suggest the sedimentary facies and paleogeomorphology controlled the spatial distribution of early dolomites, while sealevel fluctuations influenced the initial time and scale of dolomitization.