Reservoirs in Delaware Mountain Group fields in West Texas have low average recovery of original oil in place (<14 percent) because of vertical and lateral heterogeneity caused by depositional processes and postdepositional diagenetic modification. Ford Geraldine unit in Reeves and Culberson Counties, Texas, produces from the Ramsey sandstone, the most prolific horizon in the Bell Canyon Formation. The Ramsey reservoir at Ford Geraldine unit is composed of a 0- to 60-ft-thick sandstone bounded by laminated siltstones. On the basis of core descriptions, log correlations, and study of an outcrop analog, Ramsey sandstones are interpreted as having been deposited by sandy high- and low-density turbidity currents. The sands were deposited in a basin-floor setting in a channel-levee system with attached lobes. Channel facies are approximately 1,200 ft wide and 15 to 35 ft deep. They consist of massive and crossbedded sandstones interpreted to have been deposited from high-density turbidity currents. Channel margins, characterized by rippled and convoluted sandstones interbedded with minor siltstones, are interpreted as channel levees formed by overbanking of low-density turbidity currents. Lobe sandstones are interpreted as being deposited at the mouth of the channel by high-density turbidity currents. They were identified by massive and graded sandstones with load and dewatering structures that include flame structures, dish structures, and vertical pipes. Lobe sandstones are 3 to 30 ft thick and have a sheetlike geometry.

Reservoir heterogeneity caused by siltstone beds and extensively calcite-cemented sandstones constrains producibility of the Ford Geraldine unit. Progradation, aggradation, and retrogradation of the system resulted in lateral and vertical offset of channel, levee, and lobe facies. Laminated siltstones provide the greatest amount of depositional heterogeneity because of the grain size and permeability contrast between sandstones and siltstone facies. A laminated siltstone that is continuous across the northern part of the unit subdivides the Ramsey reservoir into two sandstones. These sandstones are in turn further subdivided by smaller siltstones that are of sufficient extent to be correlated in several wells. The calcite-cemented zones apparently are not large enough to correlate between wells, but the complexity that they add to the reservoirs probably contributes to reduced sweep efficiency.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas