The influence of organic matter on the boron isotope geochemistry of the gulf coast sedimentary basin, USA

Large variations in the boron isotopic composition of sedimentary environme nts make boron an attractive monitor of fluid/rock interactions during diag enesis. Studies of B in marine sediments have shown that preferential adsor ption of B-10 on clay minerals leaves pore waters enriched in B-11. During diagenesis, clay minerals recrystallize and incorporate B-10 into the miner al structure (Spivack, A.J., Palmer, M.R., Edmond, J.M., 1987. The sediment ary cycle of the boron isotopes. Geochim. Cosmochim. Acta 51, 1939-1949). T his process should cause a depletion of B in the pore water with an increas e in the delta B-11. In the Gulf Coast sedimentary basin (USA), however, th ere is a general increase in B-content of formation waters (Land, L.S., Mac pherson, G.L., 1992. Origin of saline formation waters. Cenozoic Section, G ulf of Mexico Sedimentary Basin. Geochim. Cosmochim. Acta 76, 1344-1362; Mo ldovanyi, E.P., Walter, L.M., 1992. Regional trends in water chemistry, Sma ckover Formation, Southwest Arkansas: Geochemical and physical controls. AA PG Bull. 76, 864-894.) and a decrease in delta B-11 with depth. This sugges ts that another source of B-10 exists in deep basinal environments. We know that oil reservoir brines are commonly enriched in boron (Collins, A.G., 1 975. Geochemistry of Oilfield Waters. Elsevier, New York, p. 496.), therefo re this study examines organic matter as a possible source of boron during thermal maturation. Samples of water, oil, and cored sediments were collected from three stacke d hydrocarbon reservoirs in the Gulf of Mexico sedimentary basin at a depth of 3500-4350 m. Extraction of boron from organic matter toil and kerogen) was done by Parr Bomb volatilization, with mannitol used as a B-complexing agent. The isotope ratios were measured using negative thermal ionization a nd compared to in situ analyses using secondary ion mass spectrometry. The delta B-11 values of pore filling clays in sandstone reservoirs is - 2 +/- 2 parts per thousand. The B-content of the clay averages 144 ppm. Oil f ield waters show a range in B-content from 8-85 ppm and delta B-11 values f rom +28 to +37 parts per thousand, increasing from the lowest to the upperm ost reservoir. There is an apparent B-11-enrichment of fluids with progress ive migration through clay-rich sediments. Very little B (ppb) was found in the oil, but kerogen extracted from the oil source rock (Sassen, R., 1990. Lower Tertiary and Upper Cretaceous source rocks in Lousiana and Mississip pi: implications to Gulf of Mexico crude oil. AAPG Bull. 74, 857-878.) cont ains significant B (140 ppm) with a delta B-11 of -2 +/- 2 parts per thousa nd, similar to the pore filling clay minerals in the sandstones. While kero gen comprises only similar to 2% of the sedimentary basin, its influence ca n be significant if B with distinctly low delta B-11 is released over a spe cific temperature interval during thermal maturation. The release of B from organic compounds could cause the observed regional B-10 enrichment in wat ers deep in the Gulf Coast basin. (C) 2001 Elsevier Science B.V. All rights reserved.