Integrated Geological and Geophysical Studies of Low Permeability Carbonate Rock as an Unconventional Reservoir: Baturaja Formation, Palembang Basin, Indonesia

The early Miocene carbonate Baturaja Formation in Sumatra, Indonesia, is a gas-charged tight reservoir characterized by very low permeability (mostly below 0.1 mD) and porosity (mostly below 5%). Historically, the formation has been regarded as a conventional reservoir; however, it has required acid fracturing to enable hydrocarbon production approaching an economically viable rate. Changing the exploitation strategy of the formation to that of an unconventional carbonate may improve gas recovery. The research presented herein integrates geological and geophysical methods to better characterize the Baturaja Formation, treating it as an unconventional reservoir. The first part of the dissertation defines the carbonate platform type, the influence of siliciclastic input to the different carbonate facies comprising the platform, and the porosity distribution. This is followed by the development of a scheme based on available well-log data to classify the reservoir quality using a combination of Lamé parameters and elastic moduli. In the final part of the research, the rock-quality classification is extrapolated across the field using rock parameters extracted from an amplitude variation with offset (AVO) inversion of an industry-provided seismic dataset. The depositional setting of the early Miocene Baturaja Formation is herein suggested to be a carbonate ramp, dominated by oligopothic biota in the inner ramp and mid ramp, and by aphotic biota in the proximal outer ramp. The analysis of RMS amplitudes from the seismic dataset identified siliciclastic influences to the carbonate platform, and thereby allowed the definition of two carbonate facies (A and B) based on the abundance of siliciclastic input. Hydrocarbons (gas and condensate) are likely to be trapped near lateral boundaries between the carbonate facies due to large contrasts in acoustic impedance and porosity. Six carbonate reservoir-quality classes were defined within Baturaja Formation. Reservoir quality was determined based on inferred rock parameters, including brittleness, porosity, TOC and mineral content. The rock-quality class boundaries were determined using a well-based carbonate classification template. Very good to moderate reservoir quality dominates the inner ramp facies due to the high terrigenous siliciclastic content. Lower moderate to poor quality reservoirs dominate the outer ramp, due to extensive cementation. Integration of stratigraphic interpretation and carbonate rock quality was used to suggest a prospective interval for acid fracturing and to test the potential unexploited resources in the reservoir. The inverted P-impedance explains the characteristics of the progradational, aggradational and retrogradational stratigraphic zones related to marine cementation, siliciclastic input, and the effects of carbonate drowning. The seismic-based classification scheme produces spatially-contiguous lateral and vertical distributions of reservoir quality classes across the field. The reservoir quality classification can be used to guide appropriate location of infilling wells for the purpose of increasing future gas production from Baturaja Formation