Facies Heterogeneity, Pay Continuity, and Infill Potential in Barrier-Island, Fluvial, and Submarine-Fan Reservoirs
Authors: Ambrose William A. , Tyler Noel , Parsley Matthew J. 
Published: November 2010

Abstract
An enormous mobile-oil resource (35 Bbbl in Texas and 80–100 Bbbl in the United States) is trapped in inefficiently drained, mature reservoirs. Much of this oil can be recovered at low cost through conventional infield exploration and development strategies that target untapped compartments in these reservoirs. Three-dimensional facies architecture exerts the primary control over the distribution and continuity of reservoir compartments. Reservoirs with complex internal architecture contain large volumes of remaining mobile oil; those with a simpler internal architecture are efficiently drained.
Three Texas reservoirs, representing differences in internal architecture, illustrate variations in reservoir-pay continuity: barrier-island (41-A reservoir in West Ranch field in south-central Texas), fluvial (Jim Wells and Brooks reservoirs in La Gloria field in South Texas), and submarine fan (reservoirs in the Spraberry Trend in the Midland Basin). Methods of estimating pay continuity based on facies geometry and variations in permeability between wells can be used to describe reservoir heterogeneity and indicate areas for infield exploration in each of these reservoirs.
Although most barrier-island reservoirs are considered to be relatively homogeneous, the 41-A barrier-island reservoir in West Ranch field contains wide, dip-oriented belts of lenticular tidal-inlet facies that disrupt reservoir continuity in the main barrier-core facies. Additionally, the tidal-inlet facies are internally less continuous than the barrier-core facies. Other reservoir compartments that exhibit low continuity in the 41-A reservoir occur in flood-tidal-delta sandstones partly encased in lagoonal mudstones updip of the barrier core. Infill wells drilled into the lower-continuity facies in the 41 - A reservoir at well spacings less than the conventional 20-acre spacing can contact additional oil, resulting in substantial reserve additions.
Fluvial reservoirs have a higher degree of internal complexity than do barrier-island reservoirs, and they 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 floodplain mudstones. Recompletions in bypassed stringers in La Gloria field contact channel-fill sandstone compartments that are isolated vertically by floodplain mudstones.
Mud-rich submarine-fan deposits are extremely heterogeneous and may have the greatest potential for infill drilling to tap isolated compartments in clastic reserviors. The Spraberry Trend in West Texas contains thin, discontinuous reservoir sandstones deposited in a complex midfan channel- and levee-system. Although facies relationships in Spraberry reservoirs are similar to those in fluvial resevoirs in La Gloria field, individual pay stringers are thinner and more completely encased in low-permeability mudstone facies.