Cased-hole Gamma Ray Log Assessment and Application to Support Reservoir Characterization at Salak Field in Indonesia

Abstract

Gamma Ray (GR) logging is a common wireline activity conducted in the oil & gas industry to identify sand and shale bodies. In the geothermal industry, studies have shown that the GR count has a close correlation with the silica content of rocks. Generally, rocks with high silica content (i.e., felsic) will have relatively higher GR count than rocks with low silica content (i.e., mafic). In the reservoir section of geothermal field, very few (to almost none) rocks cuttings are obtained because mud loss normally occurred. Therefore, another dataset is required to identify lithologies/formations and GR log is one of the continuous datasets in the reservoir hole section that may help to solve this problem. At Salak, open-hole GR logs have been obtained during well completion, but these are few compared to the hundreds of wells drilled. In 2017, the GR tool was included in the bottom-hole assembly (BHA) of wireline surveys at Salak, such as Pressure-Temperature (PT) and Pressure-Temperature-Spinner (PTS) and thus, providing an opportunity to collect infill cased-hole GR data. Evaluation of the cased-hole GR logs indicated good quality and correlation with open-hole GR logs and used to support lithology/formation interpretation, especially for the Rhyodacite Marker (RDM) and Marine Sediments and Volcaniclastic (MSV) Formations as both the formations have distinct GR response compared to other formations at Salak.