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The Cisco Group is a mixed terrigenous clastic and carbonate rock stratigraphic unit deposited on the Eastern shelf, a late Paleozoic constructional platform developed on the margin of the sediment-starved Midland basin. Detailed facies mapping of the Harpersville format, a boundary-defined unit within the Cisco Group, outlines 3 depositional systems that are differentiated by gross lithologic composition and position relative to the equivalent shelf edges. They are the (1) Cisco fluvial-deltaic system, (2) Sylvester shelf-edge bank system, and (3) Sweetwater slope system. The Cisco fluvial-deltaic system is composed of dip-fed fluvial and deltaic facies and associated strike-fed interdeltaic embayment facies. The Sylvester shelf-edge bank system consists of an offlapping series of elongated, prismatic limestone banks that lie along the shelf margin. The Sweetwater slope system is composed of numerous slope wedges, or fans, which include shelf-margin, slope-trough, and distal-slope sandstone facies. The eastern shelf prograded into the Midland basin by local upbuilding through fluvial, deltaic, and shelf-edge bank deposition contemporaneous with outbuilding by slope-fan deposition.
Oil pools are found in all 3 depositional systems. Productive facies include fluvial, distributary channel, and distributary-mouth bar sandstones of the fluvial-deltaic system and distal-slope and shelf-margin sandstones of the slope system. Production is concentrated in areas where 2 broad, subparallel, structurally-related NE-SW trends intersect the mapped fluvial-deltaic lobes. The complex, lenticular geometry of these thin deltaic sandstones affords maximum opportunity for development of stratigraphic and combination traps.
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