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

Abstract

Large variations in the boron isotopic composition of sedimentary environments make boron an attractive monitor of fluid/rock interactions during diagenesis. Studies of B in marine sediments have shown that preferential adsorption of 10B on clay minerals leaves pore waters enriched in 11B. During diagenesis, clay minerals recrystallize and incorporate 10B into the mineral structure (Spivack, A.J., Palmer, M.R., Edmond, J.M., 1987. The sedimentary cycle of the boron isotopes. Geochim. Cosmochim. Acta 51, 1939–1949). This process should cause a depletion of B in the pore water with an increase in the δ 11B. In the Gulf Coast sedimentary basin (USA), however, there is a general increase in B-content of formation waters (Land, L.S., Macpherson, G.L., 1992. Origin of saline formation waters. Cenozoic Section, Gulf of Mexico Sedimentary Basin. Geochim. Cosmochim. Acta 76, 1344–1362; Moldovanyi, E.P., Walter, L.M., 1992. Regional trends in water chemistry, Smackover Formation, Southwest Arkansas: Geochemical and physical controls. AAPG Bull. 76, 864–894.) and a decrease in δ 11B with depth. This suggests that another source of 10B exists in deep basinal environments. We know that oil reservoir brines are commonly enriched in boron (Collins, A.G., 1975. Geochemistry of Oilfield Waters. Elsevier, New York, p. 496.), therefore 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 stacked hydrocarbon reservoirs in the Gulf of Mexico sedimentary basin at a depth of 3500–4350 m. Extraction of boron from organic matter (oil 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 and compared to in situ analyses using secondary ion mass spectrometry. The δ 11B values of pore filling clays in sandstone reservoirs is −2±2‰. The B-content of the clay averages 144 ppm. Oil field waters show a range in B-content from 8–85 ppm and δ 11B values from +28 to +37‰, increasing from the lowest to the uppermost reservoir. There is an apparent 11B-enrichment of fluids with progressive 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 Mississippi: implications to Gulf of Mexico crude oil. AAPG Bull. 74, 857–878.) contains significant B (140 ppm) with a δ 11B of −2±2‰, similar to the pore filling clay minerals in the sandstones. While kerogen comprises only ∼2% of the sedimentary basin, its influence can be significant if B with distinctly low δ 11B is released over a specific temperature interval during thermal maturation. The release of B from organic compounds could cause the observed regional 10B enrichment in waters deep in the Gulf Coast basin.