Genesis of Dolomite Reservoir in Ediacaran Chigbrak Formation of Tarim Basin, NW China: Evidence from U–Pb Dating, Isotope and Element Geochemistry

Abstract

The Chigbrak Formation in the Upper Ediacaran is one of the important exploration targets in the Tarim Basin, NW China. However, no significant discoveries have been made in this field, and unclear reservoir genesis is one of the important factors restricting exploration breakthrough. This study examined the outcrops of the Aksu area in northwestern Tarim Basin by using systematic descriptions of petrologic features in the Upper Ediacaran Chigbrak Formation. Samples were selected for tests of stable carbon and oxygen isotopic compositions, strontium isotopic compositions, rare earth elements, LA–ICP–MS element mapping and U–Pb dating. It was found that (1) the Chigbrak Formation is mainly composed of dolomitic microbialite, with average values of δ13C (PDB), δ18O (PDB) and 87Sr/86Sr of 3.50‰, 2.95‰ and 0.709457, and has similar geochemical characteristics to the coeval seawater. The dolomites have also been characterized by a medium degree of cation ordering (avg. 0.68), a low content of ΣREEs (avg. 9.03 ppm) and a chondrite standardized curve of REEs showing enrichment of LREE and depletion of HREE. The U–Pb ages range from 538 to 618 Ma, corresponding to the age of Ediacaran period. (2) Dolomitization occurred in a marine diagenetic environment during the penecontemporaneous period, with seawater as the dolomitization fluid. (3) Vugs are the dominant pore type of Chigbrak Formation, and they are the products of the dissolution of meteoric water in penecontemporaneous period. (4) The main controlling factors of reservoir were lithofacies, meteoric water dissolution controlled by fourth– or fifth–order sequences and tectonic movement, and early dolomitization. The research results are of great significance to the dolomite reservoir prediction of the Upper Ediacaran Chigbrak Formation of the Tarim Basin.