HYDROCHEMICAL CONSTRAINTS ON FLUID-MINERAL EQUILIBRIA DURING COMPACTION DIAGENESIS OF KEROGEN-RICH GEOPRESSURED SEDIMENTS

Hydrochemical conditions characteristic of kerogen-rich geopressured sediments contribute to the development of a predictable alteration mineralogy. The following conditions result in the geopressured fluid becoming enriched, and in some cases, saturated with CO2 and CH4: 1. (1) the upward and generally restricted flow of fluids in geopressured sediments, in contrast to the generally more rapid flow of fluids in nongeopressured sediments; 2. (2) the coincidence of the depths of geopressuring with the geothermal temperatures necessary for CO2 and CH4 release; and 3. (3) the opposing rates of sediment subsidence and CO2 and CH4 transfer into the upward-flowing fluid. Calculation of the phase relations in the systemCaO-FeO-MgO-Na2O-Al2O3-CO2-H2S-H2 SO4-SiO2-H2 O-HCl, as constrained by the above hydrochemicalcriteria, allows prediction of three patterns of mineral deposition during diagenesis of kerogen-rich geopressured sediments. Quartz deposition is favored in the upper portions and margins of the geopressured section and at the contact between geopressures and normal pressures because of increased fluid flux atshallower depths within the geopressured section. Carbonate deposition could occur above the zone of CO2 release from kerogen degradation,as a result of the upward flux of CO2- and CH4-enriched (andpossibly saturated) fluids, and the subsequent decrease in fluid temperature, pressure, and CO2 solubility.Kaolinite-carbonate could deposit within and above the zone of CO2 release from kerogen as a result ofsilicate dissolution by CO2-rich acid pore fluids, followed by the potential for illite-carbonate and albitecarbonate deposition upon CO2 depletion. In contrast, laumontite and anhydrite should not depositduring diagenesis of kerogen-rich geopressured sediments but could deposit during diagenesis of nongeopressured or kerogen-poor geopressured sediments. These mineralogic relationships compare favorablywith observed relationships in the kerogen-rich geopressured sediments of the Frio Formation from theTexas Gulf Coast. © 1990.