D / H Ratios and late-stage dehydration of shales during burial

Details D / H Ratios and late-stage dehydration of shales during burial D / H Ratios and late-stage dehydration of shales during burial

Feb 1980

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980gecoa..44..341y&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 44, Issue 2, pp.341-352

Physics

13

Scientific paper

D / H Ratios of OH hydrogen in clay minerals of three sequences of deeply buried Gulf Coast shales were measured 1. (1) to investigate the relationship between these ratios and particle size and mode of formation of clay minerals 2. (2) to search for evidence of isotopic fractionation between residual pore water and expelled pore waters, and 3. (3) to test the hypothesis that montmorillonite illite conversion is the most important mechanism of the late-stage dehydration of these shales. Samples were taken from subsurface to depths of 18.000 ft. D Values range from -33 to -73% The range in D values among different size fractions of a shale is about 15-20%. in the upper sections of the wells; the range decreases with depth of burial. The large range is mostly due to the variation in proportion of clays of different origins. Differences in D among different size fractions indicate isotopic disequilibrium between clays of different sizes. In CWRU Well No. 6, which penetrated to about 18,000 ft below surface, the < 0.1- m clays from depths down to 8300 ft are depleted by about 20%. relative to deeper levels, suggesting parallel difference in D between their co-existing pore waters. I conclude that: 1. (i) significant D-H fractionation occurred between residual and expelled pore water in these shales, and 2. (ii) the conversion of montmorillonite to illite during burial diagenesis of shales is the most important mechanism of late-stage dehydration. The isotope fractionation factor ( ) between the OH hydrogen in clay minerals and water was estimated to decrease by about 0.00016 ± 0.00003 per °C increase in temperature (in the range 29-120°C). Assuming that montmorillonite - water is 0.96 ± 0.02 at 25°C, the following equation may be obtained: 1000 In

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