The Challenges for Carbonate Petrophysics in Petroleum Resource Estimation

An examination of the core and log analysis of carbonate reservoirs has confirmed that identified shortcomings are rooted in disparate pore character. Many of the interpretation methods were developed for elastic rocks, which typically show an intergranular porosity, sometimes augmented by fracture porosity. In carbonate reservoirs, the primary pore system comprises interparticle porosity that coexists with a highly variable secondary system of dissolution voids and/or fractures. As a consequence, carbonate reservoirs are markedly heterogeneous from pore to reservoir scales, and this variability poses significant challenges to data acquisition, petrophysical evaluation, and reservoir description. For example, the ranges of carbonate facies and their pore character often control the distributions of net pay, porosity, and hydrocarbon saturation. Putting these matters together, conventional petrophysical practices that exclusively use reservoir zonation based on lithology/mineralogy have limited application in carbonates. Instead, recourse is made to a zonal discrimination that draws upon the distribution of microporosity and its connectivity with macroporosity and fractures. The discrimination scheme uses downhole technologies such as high-resolution imaging and magnetic resonance logs, supported by advanced core analysis. On this basis, a value-adding workflow is proposed to increase confidence in those petrophysical deliverables that are used in static volumetric estimates of petroleum Resources.