Fracture filling mechanisms in the carbonate buried-hill of Futai Oilfield in Bohai Bay Basin, East China

Abstract

Genesis and distribution of fracture fillings of the Cambrian-Ordovician carbonate buried-hill in Futai Oilfield of Bohai Bay Basin were investigated based on observation of core samples and thin sections, analysis of inclusions in fracture fillings, stable isotopic compositions of carbon and oxygen as well as trace element compositions in the fillings and surrounding rocks. There are many kinds of fractures developed in Futai buried-hill structure. Generally, the fracture spaces are filled over 60% by calcite, mostly sparry calcite and formed once time in the fractures. The homogenization temperatures of inclusions in calcite fillings in the fractures are higher than those in the buried-hill reservoirs. Oil-bearing fluid inclusions were found in the calcite fillings in the fractures, which means that the fracture filling is at the same time as oil-pool formation. Analysis of compositions of stable carbon and oxygen isotopes and rare earth elements showed that calcite mother fluids (including oil-pool forming fluid) were hydrothermal fluids coming from deep strata in the basin, and the hydrothermal fluids dissolved the limestone and dolomite and then deposited calcite in the fracture networks in the shallow part of the buried-hill structure. The hydrothermal genesis of calcite fillings in the fractures is useful in understanding the distribution of the calcite filling contents in the buried-hill structure, that is, the fractures near Chengnan and Chengnan Ertaijie faults have higher filling contents, while the fractures far from these faults have fewer fillings. Key words: carbonate buried-hill, fracture filling mechanism, deep thermal fluid, Futai Oilfield, Bohai Bay Basin