Abstract: Application of Sequence Stratigraphy to the Prioritization of Incremental Growth Opportunities in Mature Reservoirs: An Example from Frio Fluvial-Deltaic Sandstones, T-C-B Field, South Texas

Paul R. Knox, Lee E. McRae

The bulk of the estimated 15 billion barrels of mobile oil remaining in fluvial-deltaic reservoirs in the United States lies in mature fields characterized by heterogeneous reservoirs. Detailed geological investigations necessary to identify the residency of the remaining resource in such reservoirs are inhibited by perceived high finding costs of remaining reserves and limited manpower. To convert remaining resources to economic reserves, operators must focus their manpower on reservoirs having the highest reserve-growth potential. These are most often the reservoirs with the greatest internal heterogeneity. We are applying lessons learned in outcrop studies to subsurface reservoirs to identify reservoir heterogeneity and reserve-growth potential in relation to position within high-frequency stratigraphic cycles. Initial studies have focused on fluvial upper-delta-plain reservoirs in the mature T-C-B field of South Texas.

The Scott-Whitehill reservoir interval in T-C-B field is a 160-ft-thick fourth-order genetic unit that represents a fifth-order progradational-retrogradational depositional cycle. It is composed of four stratigraphic units ranging from 25 to 50 ft in thickness. These fifth-order units show an upward progression of increasing fluvial-channel sandstone content and increasing reservoir heterogeneity concomitant with increasing accommodation space. Reservoir-quality sandstones in the lowermost cycle occur in dip-oriented meanderbelts as much as to 5,000 ft wide with net sandstone thicknesses ranging from 15 to 20 ft, rarely reaching 30 ft. The reservoir bodies are internally homogeneous and are laterally isolated from equivalent meanderbelt deposits by nonreservoir floodplain facies. In contrast, channel sandstones within the uppermost cycle occur in a single meanderbelt more than 2 mi wide. Internally, these meanderbelt deposits are very heterogeneous, being composed of multiple individual channel deposits, each up to 15 ft thick, that are commonly vertically and laterally amalgamated. Outcrop studies suggest that thin mudclast-rich layers at the base of such channel deposits combined with increased preservation potential of mud-rich upper-channel-fill deposits create low-permeability baffles to fluid flow where adjacent channel deposits are in contact. The abundance of these baffles in the uppermost cycle creates a high degree of internal heterogeneity and may be a key contributing factor to oil recovery rates of less than 10 percent.

These results support observations from outcrop studies that indicate reservoir heterogeneity is predictable within a sequence stratigraphic framework. Such predictability is a powerful tool in prioritizing reservoirs for studies aimed at identifying the residency of remaining resources. Where applied by operators it shows great promise in focusing their efforts on those reservoirs with the best potential for incremental growth, thus reducing finding and development costs and increasing ultimate reserves.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana