Abstract

The 74 year old McCamey Field, located in the Southeast corner of Upton County, Texas, is composed of five separate and distinct oil reservoirs. Each structurally trapped, yet stratigraphically controlled reservoir exhibits its own unique set of reservoir properties and consequent challenges for secondary oil recovery.

Reservoir heterogeneity is controlled by the grainstone shoal, back-shoal subtidal and intertidal distribution within the Grayburg Formation (U. Guadalupian) and cave-collapse and fill of the San Andres Formation (L. Guadalupian). In the northeastern portion of the field (“Shoal Area”), the Grayburg embodies several stacked, prograding, grainstone shoal complexes bounded up-dip by intertidal muds, forming a stratigraphically trapped oil reservoir. The center portion of the field (“San Andres High Area”) is dominated by a heavily karsted, 100-250 ft thick, San Andres interval, structurally trapped by the non-reservoir, intertidal and supratidal Grayburg. The San Andres responded to the intense karstification both brittlely in the mud-supported facies and ductilely in the grain-supported facies, creating regions of brecciated cave collapse and cave fill. Immediately to the west of the “San Andres High Area” lies the “West End Island Area,” consisting of both Grayburg subtidal and a series of stacked San Andres grainstone shoal reservoirs. Present within the San Andres interval exists a shallowing upward middle shelf to shoal complex capped by a 25 ft thick, organic shale rich interval representing an island exposure surface with mangrove-type swamps. Similar to the “West End Island Area,” the south and southeastern edge of the field (“South Side,” and “East Side”) are also composed of both Grayburg and only slightly karst modified San Andres reservoir. Complicated diagenetic history and multiple fluid migrations have added a layer of complexity to the over-all reservoir characterization.