ABSTRACT

The Walker Creek field, located in Lafayette and Columbia Counties, Arkansas, is the largest stratigraphic trap yet discovered in the Smackover State Line Trend. The porosity at Walker Creek is developed in an Upper Smackover oolite sequence thought to represent a regressive, high-energy shoreline deposit modified by contemporaneous structural movements associated with salt swells. The southern Persian Gulf shelf is seen as its Holocene analogue. The upper Smackover oolite reservoir is a continuous sequence of very well sorted lime grainstones containing no interstitial, low-energy lime muds. Porosity occlusion and ultimate trap formation is the result of early cementation associated with meteoric water table conditions developed during periodic exposure of the Smackover during its depositional history. The porosity-occluding early carbonate cements formed in the meteoric phreatic zone, immediately beneath the water tables, while primary porosity was being preserved in the overlying meteoric vadose zones. This primary porosity has been preferentially preserved over the active structures because vadose conditions persisted across these topographic highs for longer periods of time. Porosity distribution within the Smackover at Walker Creek, thus, is not controlled by original depositional processes--such as the pinchout of a porous sand into a lagoonal clay--but is the direct result of the early cementation history of a carbonate sand sequence that exhibited little variation in original porosity.

The demonstration that Walker Creek is a "diagenetic trap" rather than a true stratigraphic trap gives the explorationist and production engineer in the Arkansas-Louisiana Smackover trend a valid alternative model to conceptualize potential reservoir characteristics.