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 environments make boron an attractive monitor of fluidrrock interactions during diagenesis. Studies of B in marine sediments have shown that preferential adsorption of B on clay minerals leaves pore waters enriched in B. During diagenesis, clay minerals recrystallize and incorporate B into Ž 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 11 Ž . in the d B. In the Gulf Coast sedimentary basin USA , however, there is a general increase in B-content of formation Ž 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, . 11 10 864–894. and a decrease in d B with depth. This suggests that another source of B exists in deep basinal environments. Ž 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 Ž . 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 dB values of pore filling clays in sandstone reservoirs is y2"2‰. The B-content of the clay averages 144 ppm. Oil field waters show a range in B-content from 8–85 ppm and dB values from q28 to q37‰, increasing from the lowest to the uppermost reservoir. There is an apparent B-enrichment of fluids with progressive migration through Ž . Ž 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 ) Corresponding author. Fax: q1-602-965-8102. Ž . Ž . E-mail addresses: [email protected] L.B. Williams , [email protected] R.L. Hervig , [email protected] Ž . Ž . M.E. Wieser , [email protected] I. Hutcheon . 1 Fax: q1-602-965-9004. 0009-2541r01r$ see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2541 00 00289-8 ( ) L.B. Williams et al.rChemical Geology 174 2001 445–461 446 . Ž . 11 oil. AAPG Bull. 74, 857–878. contains significant B 140 ppm with a d B of y2"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 dB is released over a specific temperature interval during thermal maturation. The release of B from organic compounds could cause the observed regional B enrichment in waters deep in the Gulf Coast basin. q 2001 Elsevier Science B.V. All rights reserved.