Bromide Dolomite (Ordovician) Inner Ramp Depositional Cycles, Arkoma Basin, Southeastern Oklahoma
Gregory P. Wahlman1, Vince L. Felt2, and Brad D. Moon2
1 BP America, Houston, TX
2 North American Gas Business Unit, Arkoma Asset Team, BP America, Houston, TX
The Bromide Formation (early Late Ordovician, Mohawkian) is known primarily from normal marine ramp limestone and shale facies that outcrop in the Arbuckle Mountains of southern Oklahoma, and very little subsurface carbonate lithofacies data on the unit are available. In the subsurface Arkoma Basin of southeastern Oklahoma, Bromide Dolomite gas reservoirs consist of very shallow-water inner ramp facies deposited on the low-angle South Ozark Platform. Cores through the Bromide Dolomite section are composed of repetitive thin (3-10 ft. thick), very shallow subtidal to peritidal shallowing-upward cycles. Typically, the basal unit in a cycle consists of a thin quartz sandstone composed of mature well-rounded fine- to medium-grained sands that were transported from the north by wind and water during sealevel lowstands, and then reworked during subsequent marine transgressions and storms. Quartz sand continues upward throughout most cycles with variable abundance, but dolostones usually dominate the middle and upper parts of the cycles. Subtidal dolostones are most common in the lower to middle cycles. Subtidal facies vary from dolomudstones to coated dolograinstones, and locally contain sparse pelecypod, gatropod, and ostracod bioclasts, but no normal marine fauna (e.g., brachiopods, bryozoans, echinoderms) occur, which indicates predominantly restricted marine paleoenvironments. Peritidal dolostones predominate the upper parts of cycles. Peritidal facies vary from stromatolitic dolobindstones, and dolomudstones to dolopackstone-grainstones with vadose features and dessication cracks. Dolopackstones and dolograinstones are composed mostly of micritic (microbialite) intraclasts, peloidal grains, and sometimes ooids. Fenestral (keystone) fabrics are common, and locally there are solution cavities and breccias. Thin intraclastic tempestites occur. No direct evidence of evaporite deposition has been recognized. Dolostones vary from very finely crystalline dolomite with well-preserved depositional fabrics to more coarsely crystalline dolomite with unrecognizable depositional fabrics. The diagenetic history of the Bromide is complex and contains early to deep-burial diagenetic features.