ABSTRACT
Fan deltas prograded repeatedly onto a shallow, carbonate-shelf environment in the Anadarko basin during Late Pennsylvanian (Missourian) time. These fan-delta sandstones were part of a thick wedge of coarse elastics which were derived from the Amarillo uplift. Similar fault-bounded basement uplifts found throughout the west-central United States at this time formed an intracratonic tectonic setting in which alluvial-fan and fan-delta sedimentation was common.
Missourian reservoirs at Mobeetie field consist of three alternating limestone and sandstone units. Limestones accumulated along strike in elongate, phylloid-algal mounds that separated the open shelf from a more restricted lagoonal environment. Algalmound cores consist of algae-foraminifer wackestone and packstone, and mounds are capped by coral boundstone and encrusting algae-foraminifer wackestone. Progradation of coarse-grained terrigenous elastics onto the carbonate shelf halted carbonate production. Braided streams breached low areas along the algal-mound trend and deposited fan-delta sediments on the shelf basinward of the mounds. Distal-fan-delta deposits in the two younger sandstones were reworked along strike into spits and bars. Carbonate deposition recommenced following waning of clastic influx and subsidence of the fan-delta sediments. Algal mounds were reestablished preferentially on shallow platforms formed by terrigenous elastics of the previous cycle.
The principal trapping mechanism in Mobeetie field is structural draping of Pennsylvanian beds over an underlying horst block, forming a doubly plunging anticline. However, within the area of closure, stratigraphy controls reservoir quality and production. Updip fluvial sandstones and undisturbed distal-fan-delta deposits have good porosity and permeability, and produce oil. Reworked fan-delta deposits are not productive, even within the area of closure, because of the loss of porosity by calcite cementation.
