Abstract

Effective exploitation of the remaining oil resource in San Andres reservoirs is often hampered by an incomplete knowledge of the controls of reservoir development. This is largely due to the insufficient resolution of these shallow-water carbonates provided by typical subsurface data sets. In actuality, careful analysis of these reservoirs using modern specialized logging tools and an appreciation of outcrop-derived depositional models can result in a greatly improved picture of the reservoir architecture and the identification of new opportunities for incremental oil recovery.

The San Andres–Grayburg reservoir at Fuhrman-Mascho field comprises three distinctly different reservoir successions. The basal Grayburg reservoir is dominated by highly permeable, cycle-based siltstone-sandstone beds that are continuous over large areas. Because these beds are easily defined with conventional logs, Grayburg reservoir architecture can be well-constrained for purposes of designing effective incremental oil production strategies. By contrast, definition of the boundaries and architecture of the underlying upper and lower San Andres reservoirs requires the integration of data from outcrops with cores or specialized wireline logs. These data show that reservoir quality in the upper San Andres reservoir is controlled by depositional facies: poor-quality tidal-flat carbonates dominate over much of the field but are progressively replaced by better quality subtidal rocks down structural dip. Reservoir development in the lower San Andres is the result of diagenesis below a sequence boundary that formed during a major sea-level fall during the middle San Andres. Although an excellent target for advanced recovery operations, this broadly continuous zone may have been overlooked in many San Andres reservoirs.