The Uncertainty in Assessing Fault Seal Analysis in Carbonate Reservoir

Abstract

One of the undissolved topics of carbonate reservoir is the relation between faults and permeability. Generally, fault seal carbonate analysis is applying sandstone reservoir algorithm, which is using Volume of shale (Vsh) value obtained from log calculation. However, the value may not reflects the exact condition on carbonate reservoir analysis where in most cases carbonate reservoir does not have clay mineral. The study object is focused on the Miocene limestone reservoir of Kais formation in the Salawati Basin, Northwestern part of the Bird's Head, West Papua. Based on structural framework analysis and subsurface map, generally three main structural patterns can be identified within the study area: strike-slip fault system trending NE-SW, normal fault system trending NNE-SSW, and normal fault system trending N-S. The classification of Kais limestone facies consists of Mudstone, Mudstone-Wackestone, Wackestone-Packstone, Packstone-Grainstone, Floatstone-Rudstone, and Boundstone while the classification of Miocene depositional environments generally consists of lagoonal patch reef or mounds and lagoonal pinnacle reef. The purpose of this study is to determine the appropriate method in calculating Shale Gouge Ratio (SGR) using various Vclay calculation methods to conduct FSA in carbonate reservoirs. Petrophysical analysis of well data is used to determine the clay fraction in each stratigraphic unit. This information then compiled to estimate the possibility of Volume of clay (Vclay) between wells. The Vclay attribute of the wells are mapped at the footwall side and the hanging wall side by interpolating Vclay log from wells. The Vclay determination is using three types of methods, which are the deterministic method, the non-linier Larionov method, and the Multimineral model method. The different methods in determining Vclay produce the different values of SGR because it is strongly influenced by the percentage of shale or clay in the rock interval which displaced by the fault. In this study, SGR calibration was carried out based on the pressure difference between K1 and K2 well which separated by fault. The K1 well is located in the hanging wall while K2 well is located in the foot wall. The calibration results show that SGR cut off 13% using Vclay Multimin method can be used to determine the fault seal capacity in Kais limestone reservoir. The drilling results show that the hydrocarbons only accumulate in the hanging wall and are not continuous up to the footwall. This evidence is supported by the failure of K2 well to find hydrocarbons in the footwall area.

AAPG Datapages/Search and Discovery Article #90354 © 2019 AAPG Asia Pacific Technical Symposium, The Art of Hydrocarbon Prediction: Managing Uncertainties, Bogor, (Greater Jakarta), Indonesia, August 7-8, 2019