Print this pageReservoir Compartmentalization in Lowstand Deltas and Shorefaces
HART, BRUCE S.
Studies of modern and ancient lowstand complexes reveal several stratigraphic, diagenetic and structural features that can contribute to reservoir compartmentalization. Sediment supply, river mouth and basinal dispersal processes, basin physiography, structural setting and rate of change of relative sea level control the external geometry and internal facies architecture of lowstand shoreline reservoirs. Where river mouths deposit sediment faster than it can be reworked, lobate deltas form (e.g., Pleistocene Gulf of Mexico). Conversely, extensive reworking of river mouth deposits by waves alongshore results in elongate sandbodies (e.g., Cretaceous Western Interior Seaway). Shingled sandbodies formed by river mouth switching or high-frequency RSL fluctuations may or may not be in full fluid communication with each other in a shore normal direction if sand formed a subordinate portion of the sediment supplied by the river and reservoir sands are separated by shales, or if once-continuous shoreface or delta front sands are truncated by lowstand erosion, leaving only discontinuous inner-shelf/upper slope sands. Shifting deltaic distributaries can lead to the vertical stacking of lobes; the internal down lap surfaces between lobes can act as seals to vertical fluid movements. Diagenesis at exposure surfaces can cause vertical permeability barriers. Within shorefaces, changes in sediment grain size and sorting along strike can create enhanced permeability trends. Pinch outs onto syndepositional structural highs can act as traps, and post-depositional faulting can further compartmentalize reservoirs. Full integration of subsurface (log, core), seismic and production data is the key to characterizing and efficiently draining these complex reservoirs.