Carbonate cement characteristics and porosity evolution model of the Oligocene reservoir in the middle block of X Oilfield in Weixinan Sag

To search for favorable development areas and guide the exploration of remaining oil of Weixinan Sag in Beibu Gulf Basin, South China Sea, this research focused on the carbonate cementation of the third member of the Oligocene Weizhou Formation in the middle block of X Oilfield (E₃w₃) between two main areas in the sag center and near fault, respectively. The type, content, and distribution characteristics of carbonate cements of the clastic rock reservoir in the E₃w₃ were quantitatively analyzed by means of laser particle size analysis, slice microscopic identification, cathode luminescence, scanning electron microscopy, conventional physical property analysis of core, numerical simulation of diagenesis of single well, and the porosity evolution modeling. The results show that, the carbonate cements of the reservoir are mainly of calcite and dolomite, which is mainly formed in the early stage of diagenesis. Under the influence of fracture distribution and thermal fluid activity, the distribution of carbonate cements in horizontal and vertical directions showed obvious regularity. The carbonate cement near fault was more developed, the porosity loss was 22.6%, which accounted for 67.0% of the total porosity loss caused by cementation, while those in the sag center were weaker, being 7.9% and 39.9%, respectively. The clastic rock reservoir of weak cementation area in the sag center showed better physical properties, and thus can be used as a target of favorable area for oil and gas reservoir development.