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