Collapsed-Paleocave Systems in the Permian San Andres Formation, Central Basin Platform, West Texas, Revealed by Integrated Seismic Characterization
Karst controlled collapsed-paleocave systems frequently cause carbonate reservoir problems, including complex reservoir compartmentalization and highly variable hydrocarbon production. Although collapsed-paleocave systems have long been recognized in the Permian San Andres Formation of West Texas and New Mexico, their geologic development and spatial distribution is still under-documented by seismic data. A complex subsurface collapsed-paleocave system related to paleokarst in the Permian Upper San Andres Formation of the Central Basin Platform of West Texas is revealed by detailed seismic characterization integrating core, well logs and rock physics data. Our study shows that the expression of the multi-stage Upper San Andres paleocave system varies by location. In the platform interior, relatively small-scale (~12m) paleocave collapsed features are interpreted from core, and include irregularly developed brecciation, cave fill and porosity occluding anhydrite cementation. High acoustic impedance of the fine-scale paleocave interval, invisible in seismic section, enables us to apply model-based seismic impedance inversion methods to delineate the 3D extent of these platform paleocave facies. In contrast, along the platform margin, large-scale feature of the Upper San Andres collapsed-paleocave system is observable in seismic images, and is marked by sags and small vertical faults. Two seismic geometric attributes, coherence and curvature, are used to map the 3D spatial distribution of the shelf margin collapsed-paleocave systems. The large-scale collapsed-paleocave features are distributed parallel to the strike of the shelf margin. The platform interior collapsed-paleocave systems exhibit wide variation in thickness, as interpreted from seismic inversion, but is thicker at the high position of paleotopography.
Using an analogy to hydrological environments within modern carbonate islands, a carbonate platform hydrological model is proposed to explain the spatial development of the Upper San Andres collapsed-paleocave system. According to this model, it is expected that additional collapsed-paleocave systems should also exist in structurally high areas of the Lower San Andres Formation, within the Guadalupian-3 and Guadalupian-4 high frequency sequences. Better understanding of the relation of collapsed-paleocave systems development to platform location could help in understanding reservoir behavior and in making sound reservoir management decisions.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009