Abstract:

The Xihu Sag in the East China Sea is one of the largest offshore basins. In recent years, major oil and gas discoveries have been made in the reservoirs of the mid-deep Huagang Formation. However, due to the large thickness, strong heterogeneity, low porosity and low permeability, and complex pore structure of the reservoir, it brings great difficulties to fine evaluation and prediction of sweet spots. This paper classified 6 types of micro-scale lithofacies units for the reservoirs in the study area based on the identification of lithology and bedding structure by using high-resolution electrical imaging data, namely trough cross bedding fine-medium sandstone, plate cross bedding fine-medium sandstone, trough cross bedding fine sandstone, plate cross bedding fine sandstone, massive bedding sandstone and calcareous cemented sandstone. And furthermore, NMR logging data was used to finely characterize the pore structure of different lithofacies units. The results show that the three lithofacies of trough cross bedding fine-medium sandstone, plate cross bedding fine-medium sandstone, and trough cross bedding fine sandstone have a drainage pressure less than 0.5 MPa and an average pore throat radius greater than 0.5 μm. The lithofacies of plate cross bedding fine sandstone has a displacement pressure between 0.5 MPa and 1.0 MPa, and an average pore throat radius between 0.25 μm and 0.5 μm. The above four types of lithofacies have coarser lithology, more developed cross bedding, lower displacement pressure, and larger average pore throat radius, which are favorable sweet spots in the study area. Based on the above studies, a classification scheme suitable for fine reservoir evaluation and sweet spots identification in the study area has been established, which provides a reliable basis for further exploration and development in the study area.

Key words: Xihu Sag, mid-deep formation, electrical imaging, lithofacies, pore structure, sweet spot