Effective approaches to improving oil recovery from any reservoir must be based on an accurate model of the reservoir framework. Key to the construction of the framework is a detailed understanding of the sequence architecture, facies, cyclicity and facies-stacking, and diagenesis. Although some of these data are obtainable from subsurface data (cores and wireline logs), accurate reservoir frameworks must be based on outcrop models.

To develop the most accurate model of reservoir architecture in the Fullerton Clear Fork field, we conducted detailed outcrop studies of reservoir-equivalent deposits in the Sierra Diablo of West Texas (Figure 3). The sequence architecture (Figure 4) defined from these outcrop studies provides a crucial basis for sequence-based reservoir architecture. Key elements of this architecture are (1) a basal Leonard (L1) sequence characterized by top-lapping, clinoformal outer platform fusulinid-rich wackestones, (2) karst-related cave and sinkhole formation, sediment infill, and collapse associated with exposure at the L1-L2 sequence boundary, (3) three high-frequency sequences in the lower Clear Fork (L2) characterized by upward-shallowing successions of outer to inner ramp facies, and (4) a siliciclastic-rich, basal transgressive leg of the L3 sequence (equivalent to the Tubb unit in the subsurface). Interpretation of subsurface data in light of outcrop models shows that the sequence architecture at Fullerton field (Figure 5) is very similar to that expressed in outcrop.

Outcrop studies also provide information on styles of facies- and cycle-stacking, porosity distribution, and cycle continuity that are crucial for identifying such patterns in the subsurface. A key observation from outcrop studies is that cycles (Figure 6) are typically composed of basal, lower porosity, mud-rich facies and capping, grain-rich facies having higher porosity. Core studies (Figure 7) reveal that these relationships among facies, cyclicity, and porosity are key features of the reservoir architecture and that these relationships form the fundamental basis for correlation and development of a cycle-based reservoir framework.