Reflux Dolomite Crystal Size Variation in Cyclic Inner Ramp Reservoir Facies, Bromide Formation (Ordovician), Arkoma Basin, Southeastern Oklahoma

Abstract

In the subsurface Arkoma Basin of southeastern Oklahoma, the Bromide Formation (Upper Ordovician) consists of shallow-water inner ramp dolomitized facies deposited on the low-angle South Ozark Platform. The Bromide Formation is composed of repetitive, thin (3-10 ft. thick), shallow subtidal to peritidal, shallowing-upward cycles. Typically, cycles have a basal thin quartz sandstone unit, overlying subtidal dolomitized mudstones to grainstones, and capping peritidal facies. Peritidal dolomitized facies include stromatolitic and thrombolitic bindstones and diverse mudstones to grainstones with fenestral fabrics, dessication cracks, geopetal vadose silt pore fillings, collapse breccias, squared-off crystal molds, and sparse remnant enterolithic bands of former evaporites. Peritidal dolomitized packstones and grainstones are common and composed mostly of dark peloids and small mudstone intraclasts, many of which are microbial in origin. Oolitic grainstones are moderately common. There is a significant increase in Bromide dolomite crystal sizes and porosity in a downdip direction on the ramp. In the more updip Red Oak Field, the Bromide dolostones are mostly composed of fabricretentive microcrystalline dolomite, and have only microporosity. In the more downdip Wilburton Field area, the dolostones are composed of fine- to medium crystalline replacive dolomite in which grains are recrystallized beyond recognition or moldic, and have intercrystalline, skelmoldic, vuggy pores. The dolomite crystal size change downdip is thought to be a result of variation in the reflux dolomitization process. The dolomitization of the updip microcrystalline Bromide by locally-derived supersaturated brines was penecontemporeous to very early diagenetic and was completed relatively quickly. The brines that refluxed through more downdip ramp facies became somewhat depleted (i.e., less saturated with Mg) and dolomitization proceeded more slowly, resulting in coarser planar dolomite crystallization. Later burial diagenesis served mainly to occlude macroporosity, but not the microporosity.