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Regional facies analysis of San Andres/Blaine (Guadalupian, Permian) strata of the Palo Duro and Dalhart basins by use of cores, cuttings, and well logs is of interest for hydrocarbon exploration. San Andres rocks are composed of dolomite, anhydrite, and salt and exhibit basinward (southerly) facies changes from supratidal to subtidal. Supratidal facies reflect many features of modern, low-relief coastal sabkhas.
Lower San Andres strata include five mappable cyclic units; each cycle is comprised of (1) subtidal to intertidal shelf carbonates (basal transgressive facies), (2) lower sabkha, nodular and bedded anhydrite, and (3) upper sabkha salt formed in brine ponds and evaporating pans. Cycles represent repetitive progradation of facies to the south through time. Salt beds pinch out in the central and southern parts of the Palo Duro basin and mark the basinward limit of the upper sabkha evaporating terrane. Upper San Andres intertonguing anhydrite
beds and salt, representing lower to upper sabkha progradational couplets, reflect dominance of sabkha sedimentation during upper San Andres deposition. The overall genetic aspect of the stratigraphy is a general southerly facies shift through time.
Porosity is best developed in subtidal dolomite facies. Facies mapping delineates areas of potential porosity preservation and is important to explorationists predicting updip San Andres porosity trends and pinch-outs.
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