Abstract: Determining Between-Well Reservoir Architecture in Deltaic Sandstones Using Only Well Data: Oligocene Frio Formation, Tijerina-Canales-Blucher Field, South Texas

Paul R. Knox

Accurate prediction of compartment architecture and intracompartment heterogeneity is necessary to locate and recover the estimated 15 billion barrels of mobile oil remaining in U.S. fluvial-dominated deltaic reservoirs. Complex architecture and rapid lateral variability in such reservoirs complicate subsurface prediction, particularly in mature fields where well logs are the only available subsurface data. A genetic-stratigraphy-based methodology has been developed that improves between-well prediction of deltaic reservoir architecture and, thus, reduces risks associated with infill-drilling.

In the area of Tijerina-Canales-Blucher (T-C-B) field, which lies on the northeast margin of the Oligocene-age Norias delta, the productive 3rd-order Lower Frio unit was subdivided into eight 4th-order genetic units. Delta-front positions were identified on the basis of regional and subregional cross sections. The 4th-order units (30 to 80 ft thick) were subdivided into two to five 5th-order units (10 to 30 ft thick). Log patterns and net sandstone maps were used to identify facies, which include (1) distributary channels (up to 25 ft thick, <1,000 to >8,000 ft wide, and commonly narrower than 40-acre well spacing), (2) mouth bars (up to 15 ft thick, ranging in size from 40 to 640 acres in area, commonly <320 acres), (3) bayfill splays (up to 10 ft thick, 20 to 700 acres in a ea, and commonly <160 acres), (4) wave-reworked delta fronts (up to 35 ft thick, and >5,000 ft wide), and (5) washover fans (up to 10 ft thick, and 7,000 ft wide). Many reservoir compartments, including the prolific 21-B interval, contain a significant degree of stratigraphic trapping caused by updip pinchout of delta front or washover sandstones or convex-updip segments of meandering distributary channel sandstones.

The methodology and results of this study are directly applicable to other Gulf Coast fluvial-deltaic reservoirs in the Frio Formation and Wilcox Group, as well as to deltaic reservoirs throughout the U.S. The general methodology should be applied to develop remaining reserves in mature fields where geophysical data are lacking. Further study of T-C-B reservoirs may include stratigraphic analysis of 3-D seismic data and infill drilling to confirm between-well interpretations.

AAPG Search and Discovery Article #90950©1996 AAPG GCAGS 46th Annual Meeting, San Antonio, Texas