The Walker Creek Field: A Smackover Diagenetic Trap
Jack W. Becher, Clyde H. Moore, 1979. "The Walker Creek Field: A Smackover Diagenetic Trap", Geology of Carbonate Porosity, Don Bebout, Graham Davies, Clyde H. Moore, Peter S. Scholle, Norman C. Wardlaw
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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.
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Geology of Carbonate Porosity
In clastic situations, primary porositv is a direct function of texture and fabric, including size, sorting and shape (Fig. 1). Grain size, sorting, fabric, as well as sedimentary structures are related directly to sedimentary processes acting at the time of deposition (Fig. 1). Each depositional environment is characterized by a distinct suite of processes distributed across the active sediment water interface in a pattern unique for that environment (Fig.2). This suite of processes gives rise to a group of products, including sediment texture, fabric, and structures distributed across the active sediment water interface in a pattern unique for each depositional environment (Figs. 1 and 2). In a prograding or regressive situation, when sedimentation is taking place at the active sediment-water interface, a vertical sequence of sediments is formed which reflects, in an orderly fashion, from deepest at the base, to shallowest at the top, the progressive changes in texture, fabric and sedimentary structures resulting from the progressive changes in processes found along this interface from shallow to deep water (Fig. 3). Each sedimentary environment then, can be characterized by a unique vertical sequence of sediment textures, fabrics and sedimentary structures. It is this unique suite of characteristics that is commonly used for the identification of depositional environments in ancient rock sequences, and most importantly, is used to predict the presence and detailed distribution of the most porous (best sorted, coarsest) potential reservoir facies (Fig. 3).
In a regional setting, the recognition of distinct sedimentary environments and knowledge of logical lateral relationships is the keystone for prediction of the lateral extension or even presence of potential reservoir facies.