Petrology and Reservoir Characteristics of Smackover Formation, Hatter's Pond Field--Implications for Smackover Exploration in Southwestern Alabama: ABSTRACT

ABSTRACT Hatter's Pond Field in northern Mobile County, Alabama has produced 11 million barrels of condensate and 43 Bcf of gas since its discovery in 1974. Production is from multiple pay zones in the Upper Jurassic Norphlet and Smackover Formations. The trapping mechanism in the field is a highly complex, combination structural and stratigraphic trap involving salt movement in association with normal faulting. The Smackover in the Hatter's Pond Field area is enigmatic for the Smackover in Alabama for two principal reasons. One, the Smackover is very thin (less than 200 feet) in comparison to thicknesses to the northwest and southeast. Secondly, the Smackover does not show the characteristic lower Smackover-upper Smackover lithologic subdivision so apparent throughout south Alabama and the Gulf Coast. These unique features are a product of the field's position on the northwest flank of the Wiggins uplift. Smackover deposition was significantly affected by the uplift which maintained the Hatter's Pond area as a subaerial high, while lower Smackover carbonates were being deposited in the deeper areas of the Mississippi Interior Salt Basin and Conecuh embayment. It was not until near maximum transgression that the seas covered the Hatter's Pond area and deposited shallow-water upper Smackover lithologies. These lithologies were later massively dolomitized by mixing-zone dolomitization during the subsequent Buckner regression. This dolomitization almost completely masked depositional textures, but was largely responsible for the development of reservoir grade porosity in the Hatter's Pond area. Six major lithofacies can be identified in the Smackover in the Hatter's Pond Field: anhydritic mudstone, skeletal-pelodial packstone/grainstone, oolitic grainstone, microcrystalline dolomite, finely-crystalline dolomite, and coarsely-crystalline dolomite. The microcrystalline dolomite is commonly associated with bedded and nodular anhydrites and is interpreted to represent early replacement in a sebkha environment. Both the finely- and coarsely-crystalline dolomites are secondary in nature and represent replacement of low energy skeletal-peloidal packstones and high energy oolitic grainstones respectively. The majority of the reservoir porosity in the Smackover is late stage vuggy and/or moldic and is facies selective and preferential to the coarsely-crystalline dolomite. This porosity, which commonly ranges from 4 to 22% with permeabilities of 2 to over 100 millidarcies, is a product of mesogenetic leaching related to migration of CO2-charged fluids during the early stages of hydrocarbon maturation. The porosity is facies selective to the coarsely-crystalline dolomite since this lithology possessed the greatest porosity and permeability at the time of migration of the CO2-charged solutions. Evidence suggests the oolitic grainstones, which were the precursors of the coarsely-crystalline dolomites, were deposited as a series of linear bars along the flanks of the Wiggins uplift. If this is the case, and more study is needed to document this definitively, the coarsely-crystalline dolomite should occur in elongate mappable trends. Hydrocarbon exploration in this area and all along the flanks of the Wiggins uplift should involve location and mapping of these trends with the greatest success occuring in areas where the trends are superimposed over structural highs produced by faulting and/or salt diapirism.