High-Resolution Sequence Stratigraphy: The Key to Identifying Compartment Styles in Frio Formation Fluvial-Deltaic Reservoirs, T-C-B Field, Jim Wells County, Texas

Paul R. Knox, Lee E. McRae

An estimated 15 BSTB of mobile oil remains in fluvial-deltaic reservoirs in the U.S., mostly in untapped and incompletely drained compartments in mature reservoirs. To recover this resource, we must understand their reservoir compartment style. Using high-resolution sequence-stratigraphic analysis of reservoirs within Tijerina-Canales-Blucher(T-C-B) field, we identified three compartmentalization styles representative of Gulf Coast Frio fluvial-deltaic reservoirs.

Lower Frio reservoirs consist of reservoirs within a 4th-order sequence that are structurally compartmentalized by growth faults and stratigraphically compartmentalized by vertical stacking of deltaic sandstones. Individual 5- to 15-ft-thick sandstones form 5th-order reservoir sequences that are vertically isolated by 5-10 ft of prodelta mudstones and laterally compartmentalized by mudclast-rich basal distributary-channel deposits. These low-permeability layers bound the bases and margins of dip-oriented distributary-channel reservoir sandstones, effectively inhibiting lateral fluid migration into or out of surrounding strike-oriented delta-front reservoir sandstones.

The middle Frio Formation is composed of 20- to 30-ft-thick 5th-order fluvial reservoirs that represent two different compartmentalization styles. Sandstone-rich, 80-ft-thick 4th-order sequences display laterally stacked architecture, where successive sandstone bodies are in vertical contact with, and isolated laterally by, floodplain facies. Scour surfaces between stacked sandstones are commonly overlain by thin mudclast-rich intervals that form partial, leaky seals between reservoirs. In contrast, sandstone-poor, 70-ft-thick 4th-order sequences exhibit vertically stacked architecture, with individual sandstone bodies vertically isolated from one another by 5 ft or more of floodplain shale. Both reservoir types are internally compartmentalized by shale-prone bounding surfaces separat ng channel and splay sandstones. Stratigraphic complexity of these fluvial reservoirs has contributed to their low oil recovery rates (<10 %). Recovery efficiency in all three reservoir styles can be greatly improved by combining petrophysical and volumetric analyses with compartment mapping to delineate compartment boundaries and target recompletions and infill drilling in untapped and incompletely drained compartments.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995