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Albian Sunniland Formation in Southwest Florida and Lower Glen Rose Formation in Central Texas: Spectrum of Depositional Facies

Abstract

The best quality carbonate reservoirs commonly exist within grainy platform-interior settings. Characterization of shoal complexes and patch reefs within this interior setting is critical to understand reservoir connectivity and flow in 3D. A detailed comparison of Albian deposits both within the subsurface and in outcrop in the Gulf of Mexico provides insight into patch-reef deposition. The Albian Sunniland trend in southwest Florida was deposited in a moderately restricted, shelf-interior platform setting approximately 320 km behind the Stuart City-equivalent shelf margin. Cycles are composed of carbonate mudstones through grainstones/boundstones. Evaporite units are also present. The lower portion of the Sunniland Formation (also called the Rubble Zone) is made up of predominantly miliolid/mollusk wackestones, peloidal mudstones with rooted zones, and laminated wackestones with rip-up clasts. At a regional scale the upper Sunniland Formation formed a mosaic of facies responding to position on the shelf. The lower energy facies are miliolid/mollusk lime and dolowackestones and less common packstones. Some contain toucasids, a low-energy, mud-associated rudistid. The higher energy facies are skeletal lime grainstones and dolograinstones. The composition of the grainstones range from miliolid to mollusk to rudistid dominated sediments. Several grainstone intervals contain ooids. In the Alico-2 core, a caprinid dolobafflestone patch reef can be observed. Some of the sediment fill displays a geopetal-fill texture. Outcropping equivalents to the Sunniland patch reef systems occur in the Lower Glen Rose of South Texas, specifically Red Bluff Creek. Peloidal wackestones and toucasid rudstones define a platform interior facies tract while caprinid rudstone build-ups with well-developed flank beds form a high-energy ramp-crest belt that is backstepped 48 km from the previous Sligo shelf margin. The paleoenvironmental conditions shift from Central Texas around to east on the tip of the Florida Peninsula. The similarities in fauna and grain types are expected given similar geologic age. The insight into reservoir performance lies in their differences based upon the role of paleotopography and marine conditions with periods of restriction. The Sunniland Trend contains reservoir-quality grainstones and rudistid buildups on the topographic highs. The Glen Rose Formation is a broader, with more sheet-like deposits with rudistid buildups forming larger patch reefs.