GRAIN SIZE DISTRIBUTIONS, POROSITIES, AND PERMEABILITIES OF A GRAVELLY POINT BAR DEPOSIT, MAD RIVER, OHIO, USA

Understanding the grain size distribution, porosity, and permeability of gravelly river deposits is critical to the effective development and management of hydrocarbon reservoirs and aquifers composed of such deposits. The purpose of this study was to determine the grain size distributions, porosities, and permeabilities of a gravelly point bar of the Mad River, Ohio. Two sites on the bar were sampled, one on the upstream part of the bar, and one on the downstream part. Armor layers were sampled separately from underlying subsurface deposits. Bulk grain size analyses (sample sizes 46-76 kg) were conducted to determine mean grain size (D_m) and sorting. D_m and sorting for upstream armor layer and subsurface samples are 22.63 mm and moderate (0.81 Φ) and 10.56 mm and poor (1.73 Φ), respectively. D_m and sorting for downstream armor layer and subsurface samples are 23.75 mm and poor (1.46 Φ) and 7.46 mm and very poor (2.20 Φ), respectively. The armor layers are coarser and better sorted than subsurface samples, consistent with the winnowing of fines. Subsurface samples indicate a slight fining from the upstream to the downstream part of the bar, reflecting flow around the bar whereby the locus of maximum velocity shifts from the inner bank upstream to the outer bank downstream. Porosities of subsurface samples were determined by digital image analysis using the computer software jPOR macro of ImageJ. Images were acquired by photographing slices of in situ cores impregnated with phosphorescent epoxy under black light. Porosities of 8 slices from the upstream core ranged from 26.6-34.6%, mean of 30.3%. Porosities of 20 slices from two downstream cores ranged from 14.8-33.0%, mean of 23.8%. Permeabilities were calculated using the Kozeny-Carmen equation, which requires harmonic means (calculated from the grain size distributions) and porosities. Permeabilities for the upstream and downstream subsurface samples are 560 and 260 darcys, respectively. The somewhat higher permeability of the upstream sample is attributable to its higher porosity because the harmonic means were similar. The higher porosity is, in turn, attributable to the somewhat better sorting of the upstream sample. These results contribute to our knowledge of the textural characteristics of gravelly river bar deposits and fluid flow through those deposits.