Stratigraphy and Depositional Mosaics of Lower Clear Fork and Wichita Groups (Permian), Northern Midland Basin, Texas

Abstract

The stratigraphy, depositional environments, and reservoir potential of the Wichita Group and overlying Lower Clear Fork (Permian) in the northern Midland basin are described on the basis of seismic interpretations, log correlations, and lithologic analyses. The Wichita in this area is divided into a lower member of slope to basinal limestones and shales, and an upper member of shelf dolomites, varicolored shales, and evaporites. Only a starved lower Wichita section is present in areas to the south. Fusulinid evidence and physical correlations suggest that the Wolfcampian-Leonardian boundary occurs within the Wichita and ascends stratigraphically toward the north. These data, in conjunction with the upward-shoaling aspect of the Wichita, establish a progradational relatio between shelf facies of the upper member and slope to basinal facies of the lower member. The Lower Clear Fork beneath the Tubb member records continued southward progradation of similar shelf facies. These lithologies are represented in the basin by limestones and shales beneath, and possibly including lowermost Dean sandstones. Depositional systems of the Wichita and pre-Tubb Clear Fork sections are carbonate-dominated, rimmed shelves adjoining deeper basins. Periods of rapid progradation and offlapping shelf-edge accretion were cyclic, being coincident with periods of low sea level. The Tubb section records a change to a mixed siliciclastic and carbonate, rimmed shelf system. Deposition of shelf Tubb and basinal Dean facies was related to a mosaic of cyclic, reciprocal siliciclastic-carbonate sedimentation. Rapid shelf outbuilding occurred at low sea-level stands during the siliciclastic phases of sedimentation. The formation of the shallow North platform was more a consequence of constructional-sedimentary processes during the late Wolfcampian and early Leonardian than of structural influences. Over much of the northern Midland basin study area, potential stratigraphic hydrocarbon reservoirs are in Wichita and Lower Clear Fork strata. Such reservoirs occur as separate offlapping shelf-marginal (dolomitized) oolite grainstone shoals rather than organic buildups as was described in earlier studies.