Dolomitization by penesaline sea water in Early Cambrian Longwangmiao Formation, central Sichuan Basin, China

Abstract

The Lower Cambrian Longwangmiao Formation shoal dolostone reservoir in Sichuan Basin is currently an exploration and research highlight in China. Reservoir rocks mainly consist of crystalline dolomite with residual grain texture, and dolarenite of which the arene is mainly composed of muddy to micritic dolomite with some crystalline grain directionally aligned. The trace element indicates that the dolomites of Longwangmiao Formation may be related to the high salinity of sea water. The oxygen isotope values of crystalline dolomite and dolarenite are both similar to that of the Early Cambrian marine dolomites, and the carbon isotope values of every kind of dolomites are completely overlapped with that of the seawater in Early Cambrian, indicating the dolomitization fluid is originated from the Early Cambrian sea water. The restricted marine biological communities and a small amount of gypsum pseudonodule seen in muddy to micritic dolomite indicate that the sea water in Early Cambrian was restricted and evaporated. However, the general lack of massive evaporite mineral and gypsum karst breccia indicates that the salinity of sea water during dolomitization was lower than the value of gypsum's precipitation. The Longwangmiao Formation consists of several high-frequency sedimentary cycles, indicating frequent sea level changes. This study indicates that massive dolomitiza-tion may also occur in underwater palaeohigh in carbonate platform through the reflux of penesaline sea water driven by a combination of high- and low-frequency sea-level changes. This kind of dolomiti-zation can explain the generation of massive dolomites in the absence of evaporite precipitation, and further indicates that replacement dolomites can be produced by sea water with a wide range of salinity (normal, penesaline to hypersaline).