Petrophysical Analysis Using Deterministic and Probabilistic Methods in Low Resistivity Zone of Gumai and Talang Akar Formations in Jambi Subbasin South Sumatra

Abstract

The South Sumatra Basin is the most productive hydrocarbons producer within the back-arc basin on the east coast of Sumatra. To maintain a balanced supply and demand, hydrocarbon findings are optimized in zones previously not considered prospective, known as low resistivity reservoirs. Zones of interest to be studied are chosen based on their hydrocarbon shows in mud logs but with wireline log resistivity of less than 5 Ωm. The research will analyze core data from petrography, X-Ray Diffraction (XRD), and Scanning Electron Microscope (SEM) to find the causes of low resistivity. In addition, the applications of deterministic and probabilistic methods are compared to identify petrophysical parameters, namely shale volume (Vsh) by the Stieber method, porosity using neutron-density method, and water saturation determined by the Indonesian method. Results from this research are also matched with routine core analysis, mud log, and drill stem test data, indicating that the low resistivity in prospective zones is due to the presence of clay minerals composed of kaolinite, illite, and chlorite. Formation evaluation from well data estimates for shale volume of 7.85 – 37.8%, porosity of 18.22 – 23.08%, and medium water saturation of 38.68 – 52.78%. In conclusion, by combining core analysis and petrophysical evaluation, this interval was improved as potential reservoir zones and pay zone. The probabilistic method is considered more effective compared to the deterministic method, because it can construct multi-mineral and multi-fluid from several lithologies and carry out two stages of validation, from core and statistics.