The Petrophysic Role of Low Resistivity Pay Zone of Talang Akar Formation, South Sumatera Basin, Indonesia

Abstract

Abstract The South Sumatra Basin is one of the most hydrocarbon prolific Indonesian Tertiary back-arc basins today. Identifying, evaluating, and testing Low Resistivity Pay Zone (LRPZ) in South Sumatera Basin, is a challenge for increasing hydrocarbon resources in term of exploration in Indonesia. The objective of this study was to formulate the petrophysical paramater reference of LRPZ of Talang Akar Formation in South Sumatera Basin, validated by well test data. The methods used in this study included regional well correlation, petrophysical analysis, core and cutting description, petrography analysis, petrophysical cross plot and cut-off analysis, and well test data analysis. Regional well correlation and petrophysical analysis was conducted for 110 well data in all of sub basin in South Sumatera Basin, those are Jambi, Central Palembang, North Palembang, and South Palembang Sub Basin. Core, cutting description, and petrography analysis needed to observe Low Resistivity Pay mineralogy content. Result of petrophysical cross plot and cut-off parameter analysis would be validated by well test data to formulate the role of LRPZ. Based on well data, LRPZ was controlled by stratigraphy on sequence boundary of SB-T4 till Maximum Flooding Surface of MFS-T5 in Jambi Sub Basin, and on Maximum Flooding Surface of MFS-T4 till sequence boundary of SB-T5 in South Palembang Sub Basin. Core and cutting description indicated presence of pyrite and glauconite mineral in LRPZ of Talang Akar. Furthermore, petrography analysis indicated presence of pyrite and siderite mineral. These mineral were responsible to increase conductivity on formation. Based on cross plot, it was observed that LRPZ of Talang Akar Formation contained dispersed-laminated shale. It indicated that presence of LRPZ in South Sumatera Basin was caused by presence of fine grained reservoir or superficial porosity. Cross plot two parameter, RPD (Relative Porosity Difference) and resistivity log cross plot, defined cut off parameter is 5 Ohmm for LRPZ of Talang Akar Akar. Based on well test data validation, the water saturation (Sw) cut off on LRPZ is 70%. The observation result concluded that Low Resistivity Pay Zone (LRPZ) of Talang Akar Formation, South Sumatera Basin, was characterized by low gamma ray value, presence of neutron-density log crossover, and presence of hydrocarbon evidence on total gas reading and oil show on cutting, but very low resistivity (<5 Ohm.m). This petrophysic role of Low Resistivity Pay Zone would be useful to help geoscientist to determine well test interval technically for successful hydrocarbon discovery in Low Resistivity Pay Zone of Talang Akar Formation, South Sumatera Basin, Indonesia.