Genetic mechanism of Cambrian dolostone in Bachu-Tazhong area, Tarim Basin, western China

Abstract

Abundant energy resources that are found in Tarim Basin are hosted in pervasively dolomitized carbonates of the Cambrian age. The thick dolostone is distributed widely, deeply buried and quite complex in diagenesis. The middle Cambrian gypsum sequence is widespread in west of the Tarim basin. From bottom to top, the Cambrian is divided into pre-salt, inter-salt and post-salt with various diagenesis types and degree causing different sealing properties. Subsurface core samples from the three systems were studied using cathodoluminescence (CL), isotope studies (δ13C, δ18O and 87Sr/86Sr), fluid inclusion, and elemental analysis for rare earth elements (ΣREE) to determine the origin of the dolomitizing fluids. Trace elements and isotope data show that high salinity seawater was most likely responsible for dolomitization of pre-salt system. Likewise, the pre-salt system has been the result of an open hydrothermal fluid transformation system. The inter-salt system was sealed by the upper gypsum layer and lower gypsum layer. The gypsum layer not only enhanced the sealing property of inter-salt strata but also impeded dolomitization, where the evaporating seawater caused dolomitization by reflux-seepage process. Therefore, the inter-salt system was a nearly closed evaporation system dominated by the seawater. The geochemical parameters reveal that the post-salt system has a variety of fluid sources and mainly interacts with meteoric water and seawater, since the C isotope was clearly negative and Y/Ho ratio was found low(＜30).