Basically, the hydrocarbon bearing reservoirs are usually indicated by high resistivity on well log measurement based on response to hydrocarbon occurrence in the sediment pore space. However, the hydrocarbon bearing reservoir of Ngrayong glauconitic shaly sandstone in the Trembul Field not related to high resistivity. The low resistivity reservoir occurred in this area which characterized by low reading of deep resistivity logs is commonly < 6 Ohm.m but proven flowed gas based on well tested data.
This paper discusses the physical and chemical characteristics of low resistivity reservoir of Res-O and Res-Q and the factor controlled the low resistivity phenomenon in pay zone which has been proven flowed gas hydrocarbon in the well LMG-1. Integrated analysis has conducted based on comprehensive study of wireline log data, master log, core analysis (SCAL, RCAL, petrography, SEM and XRD) and also well testing data to identify and evaluate the pay zone in the low resistivity interval. Furthermore, this paper also provides suggestion for proper water saturation method to calculate the optimized result water saturation.
The reservoir of Ngrayong sandstone in the study area exhibit the funnel(Res-O) and blocky pattern(Res-Q)of gamma ray log with low resistivity reading are 2–4 Ohmm in the Res-O and 1–6 Ohmm in the Res-Q. The reservoirs have been proven hydrocarbon and flowed up to 1.679 mmscf dry gas. Some key reasons to explain this phenomenon in this area are the sedimentology aspects which includes facies and the mineralogy aspect. The reservoir rocks are typically coarse silt to fine grained sands with interbedded shales, moderately sorted, angular-subrounded, and consist of quartz, k-feldspar, calcite and high amount of clay minerals. The wireline log identification indicated the proven reservoir characterized by high porosities (> 20%) but low permeabilities (< 14 mD).
