Characteristics of Tight Reservoirs in the Oligocene Huagang Formation, Xihu Sag, East China Sea Basin
The Xihu Sag in the East China Sea Basin can be divided into five structural belts. From west to east, they are western slope belt, western sub-sag, central inversion tectonic belt, eastern sub-sag and eastern fault and fold belt. The petrographical, diagenetic and petrophysical properties of the Oligocene Huagang Formation were studied in terms of the structural belts. The Huagang Formation is characterized by the depositional facies of delta and lacustrine facies. The main rock types are feldspathic litharenite and lithic subarkose with low compositional and texture maturity. Based on the scanning electronic microscopy (SEM) and thin section observations, it was found that the reservoir rock went through moderate compaction, and reservoir porosity decreased with increasing depth. The cement of the sandstones is relatively simple, containing mainly carbonate minerals, such as calcite, ferrocalcite, dolomite and ankerite, together with some kaolinite and authigenic quartz. Carbon and oxygen isotope data suggested that the calcite cement in the reservoir was generated from the early diagenetic to the mid diagenetic stages associated with meteoric water. The average porosity in the different structural belts of the study ranged from approximately 3% to 13%, with 20% porosity zones being rare. The permeability ranged from 0.1×10-3μm2 (mD) to 10×10-3μm2 (mD), which indicates the Huagang Formation belongs to a low to ultra-low permeability reservoir. Reservoir development of the Huagang Formation was mainly influenced by deposition and diagenesis. The depositional characteristics of the environment was the primary factor in controlling petrophysical properties of the reservoir, whereas the diagenetic processes were the secondary factor in controlling quality of the reservoir. The strong extent dissolution results in the development of secondary porosities, such as intergranular pores, feldspar intra-granular pores and the kaolinite crystalline pores. This study also found that the secondary porosity resulted from dissolution is more than the porosity loss due to the precipitation of authigenic minerals.
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Faculty of Graduate Studies and Research, University of Regina