Algal mounds of the western Orogrande basin present an excellent opportunity for studying reservoir behavior because these mounds are superbly exposed and of reservoir scale. Upper Paleozoic algal bioherms form significant reservoirs both domestically and internationally but are difficult to exploit owing to characteristically pronounced facies and diagenetic heterogeneity. Establishing predictive relationships between petrophysical properties and stratigraphy is key to enhancing exploitation success in these systems. This article focuses on a well-exposed, reservoir-scale algal mound complex of the western Orogrande basin (New Mexico) to construct an outcrop-based reservoir model for this and analogous systems.
An accurate sequence stratigraphic framework is key for meaningful modeling but is difficult to establish in the subsurface. The studied mound consists of multiple high-frequency sequences with elevated porosity and dolomitization proximal to sequence boundaries, particularly in mound-flank positions. The porosity model indicates that the mound flanks may form a stratified reservoir, whereas the mound core is tight and, therefore, acts as a lateral seal. Early marine cement occluded primary porosity in outcrop mound-core facies.
Forward seismic modeling shows that the internal stratigraphic architecture of the mound complex is below conventional seismic resolution. Rock-physics measurements on 20 outcrop samples, however, demonstrate a relationship between dolomite content and the ratio of compressional-wave to shear-wave velocities at in-situ conditions. For a given dolomite percentage, seismic impedance decreases as porosity increases. Accordingly, surface seismic data could potentially be used to estimate gross reservoir properties.