Preliminary Geology Model of Muara Laboh: Facies and Porosity Assessment

Abstract

In geothermal fields, the distribution of different lithologies in the reservoir is a major control over pore volume (porosity) variation, which is an important factor in making reserves calculations. Porosity is also effected by open fracture width and density, and hydrothermal alteration; both which can also vary with lithology. Alteration control over porosity is complex as it can both increase or decrease porosity. Fractures increase porosity but less so than increases related to alteration (Rejeki et al., 2005). During the exploration phase, well data is too limited to fully interpret rock type distribution, but can be supplemented by applying a facies model analog, in this case an andesite strato-volcano facies model. A volcanic facies model was constructed at Muara laboh by using four strategically located wells. Rock types were identified from cuttings, cores, image logs, and drilling parameters from these wells. Lateral facies distribution was obtained from surface mapping and remote image interpretation. After the volcanic facies model has been constructed, porosity values assigned according to the rock distribution. Porosity values are derived statistically from other strato-volcanic hosted fields in Indonesia and derived from core analysis (using both mercury and helium analysis technique). For this study, an average 2-4 % porosity is used for lava and 6-8% for pyroclastic rocks.