ABSTRACT: Reservoir Heterogeneity in Middle Frio Fluvial Sandstones: Case Studies in Seeligson Field, Jim Wells County, Texas

Lee A. Jirik

Detailed evaluation of middle Frio (Oligocene) fluvial sandstones reveals a complex architectural style potentially suited to the addition of gas reserves through recognition of poorly drained reservoir compartments and bypassed gas zones. Seeligson field is being studied as part of a Gas Research Institute/U.S. Department of Energy/State of Texas-sponsored program, with the cooperation of Oryx Energy Company and Mobil Exploration and Producing U.S., Inc.

Four reservoirs, Zones 15, 16D, 16E, and 19C, were studied in a 20 mi² area within Seeligson field. Collectively, these reservoirs have produced more than 240 bcf of gas from wells within the study area. Detailed electric log correlation of individual reservoirs enabled subdivision of aggregate producing zones into component genetic units. Cross sections, net-sandstone maps, and log-facies maps were prepared to illustrate depositional style, sand-body geometry, and reservoir heterogeneity.

Zones 15 and 19C are examples of laterally stacked fluvial architecture. Individual channel-fill sandstones range from 10 to 50 ft thick, and channel widths are approximately 2500 ft. Crevasse-splay sandstones may extend a few thousand feet from the main channel system. Multiple, overlapping channel and splay deposits commonly form sand-rich belts that result in "leaky" reservoir compartments that may be incompletely drained. Zones 16D and 16E are examples of vertically stacked fluvial architecture, with discrete, relatively thin and narrow channel and splay sandstones generally encased within floodplain muds. This architectural style is likely to form more "isolated" reservoir compartments.

Although all of these reservoirs are currently considered nearly depleted, low-pressure producers, recent well completions and bottom-hole pressure data indicate that untapped or poorly drained compartments are being encountered. Preliminary geophysical and reservoir engineering analyses support these characterizations and help to define potential for incremental recovery through recompletion of bypassed reservoirs and/or infill drilling.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990