Facies Heterogeneity, Pay Continuity, and Infill Potential in Barrier-Island, Fluvial, and Submarine Fan Reservoirs: Examples from the Texas Gulf Coast and Midland Basin
W. A. Ambrose, N. Tyler
Recognizing variations in facies architecture can improve development strategies for heterogeneous clastic reservoirs. Three reservoirs representing different depositional environments--barrier island (West Ranch field, south-central Texas), fluvial (La Gloria field, south Texas), and submarine fan (Spraberry trend, Midland basin)--illustrate variations in reservoir continuity. Pay continuity methods based on facies geometry and variations in permeability and thickness between wells can quantify reservoir heterogeneity in each of these examples.
Although barrier-island reservoirs are relatively homogeneous, West Ranch
field contains wide (1,000-5,000 ft or 300-1,500 m) dip-parallel belts of
lenticular inlet-fill facies that disrupt reservoir continuity in the main
barrier-core facies. Other reservoir compartments in West Ranch field are in
flood
-tidal delta deposits partly encased in lagoonal mudstones updip of the
barrier core.
Fluvial reservoirs have a higher degree of internal complexity than
barrier-island reservoirs. In La Gloria field, reservoirs exhibit significant
heterogeneity in the form of numerous sandstone stringers bounded vertically and
laterally by thin mudstone layers. Successful infill wells in La Gloria field
contact partly drained reservoir compartments in splay deposits that pinch out
laterally into flood
-plain mudstones. Recompletions in vertically isolated
sandstone stringers in La Gloria field contact other reservoir compartments.
Submarine fan deposits are extremely heterogeneous and may have the greatest potential for infill drilling to tap isolated compartments in clastic reservoirs. The Spraberry trend contains thin discontinuous reservoir sandstones deposited in complex mid-fan channels. Although facies relationships in Spraberry reservoirs are similar to those in fluvial reservoirs in La Gloria field, individual pay stringers are thinner and more completely encased in low-permeability mudstone facies.
AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.