Abstract

Upper Morrowan reservoirs in the northwestern shelf of the Anadarko basin are primarily valleyfill sandstones that formed as part of transgressive-regressive cycles controlled by sea-level changes. The lowering of the sea level resulted in the basinward migration of the shoreline and the development of incised river valleys. Lag conglomerate composed of shale intraclasts was the major facies formed during this episode. On the other hand, valley-fill sediments composed of fluvial, estuarine, and flood-plain facies were deposited during major relative sea-level rises. The complex depositional pattern and resultant diverse lithofacies are the main factors that contributed to the heterogeneity of these reservoirs. Depositional style within these channels was the major factor that affected reservoir quality.

Fluvial dominated sandstones are typically coarser grained, contain less calcite cement, and are good-quality reservoirs. Marine-dominated sandstones contain abundant skeletal grains and carbonate cement that occluded porosity. Finer grained (tidal-dominated) estuarine sandstones are often poor-quality reservoirs due to biogenic modification that destroyed primary porosity.