Depositional Environments and Reservoir Compartmentalization Within the Frio Zone 21-B Reservoir, Tijerina-Canales-Blucher Field, South Texas

REISTROFFER, JAMES R., and NOEL TYLER, University of Texas at Austin, Austin, TX

Subsurface study of five sandstone units comprising the Frio zone 21-B reservoir at Tijerina-Canales-Blucher (T-C-B) field determined the environments of deposition and yielded a detailed three-dimensional portrait of the zone's heterogeneous reservoir compartments. Detailed isolith and log-facies maps, based on closely spaced production wells, integrated with maps of resistivity and determination of regional depositional setting, form the basis for interpretation of zone 21-B.
The 100-ft thick 21-B reservoir at T-C-B field documents two late-stage progradational events occurring within a larger scale lower Frio depositional episode which encompasses 800 ft of sediment in which 21-B occurs in the upper quarter. The earliest depositional event is reflected in units 4 and 4a of 21-B which record the establishment of a barrier island/lagoon system. Reservoir facies include a complex mosaic of barrier core, shale, shoreface, flood-tidal delta, tidal channel, crevasse-splay, washover fan, and barrier-flat facies. Boundaries between these facies define reservoir compartments that are substantiated by anomalous resistivity values.

The onset of the depositional episode's first-order, major transgressive phase is evident in the landward retrogradation of shore-zone deposition observed in unit 3. Analysis of five imbricately stacked subintervals composing unit 3 illustrates how shore-zone styles evolved from an early, more landward, V-shaped, estuary-conformed chenier plain to a more seaward, dominantly marine-influenced, thin, broad beach-shoreface sand belt. Unit 2 records the establishment of two imbricately stacked barrier island systems and is notable for its preservation of shore-parallel, migrated tidal-inlet facies and the prominence of two fluvial systems which had progressed seaward from their position in unit 3. By unit 1, as burial accommodation space became more constricted, the shore zone evolved int a strand plain.

The shore-zone sand wedges of units 1, 2, 3 (subunits p and q only), 4, and 4a pinch out uniformly along a tectonic hinge line in the center of the reservoir. Landward of this linear feature, an ancient estuary is apparent from funnelled, more dip-aligned sand contours, from the recognition of estuarine facies such as bayhead delta and fluvial systems and from the relative percentages of mudstone throughout the field area. Because shore-zone facies east of the hinge line were subjected to greater and more sustained wave energies, they contain lower percentages of mudstone and exhibit higher hydrocarbon saturations than reservoir compartments in the western area. Shore-zone environments of each of the five units consist of offlapping barrier-core (or beach) ridges, each of which repres nts a separate reservoir compartment.

Because of its huge volumes of initial oil and gas in place (157 million bbl of oil and 327.4 bcf of gas), and because it is in the late stage of its productive life, T-C-B (zone 21-B) may be a candidate for a program of recompletions and infill drilling to capture bypassed reserves resulting from the complex reservoir facies architectures as delineated in this study.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)