Capuano reported in a dissertation for the University of Arizona in 1988 that chemical and hydrogeologic conditions present within kerogen-rich geopressured sediments during diagenesis are uniquely different from those present within normally-pressured or kerogen-poor sediments. In geopressured sediments compaction-driven flow dominates such that fluids are transported upward to regions of lower temperature and pressure.
Overpressure development associated with hydrate decomposition due to tectonic
uplift/Sea level fall, continuous sedimentation, hydrothermal fluid rise (modified
from Foucher et al. [29]). T, temperature; P, pressure; SF, seafloor; BSR, bottom-
simulating reflector. After Li (2022)
In contrast flow in normally-pressured sediments is generally gravity-driven and therefore downward to regions of higher temperature and pressure. Calculation of the magnitude of fluid flow in geopressured sediments resulting from burial compaction and fluid thermal expansion indicates that the greatest magnitude of upward flow is at the top of the geopressured zone decreasing rapidly with increase depth.
Kerogen present within geopressured sediments decomposes releasing CO2 and CH4 at temperatures over 90° to 100°C, temperatures generally present in the upper portions of the geopressured zone. Calculation of the opposing rates of sediment burial with subsequent CO2 and CH4 release, and absorption of these gases by the upward flowing fluids indicates that this process can result in gas enrichment to or near to saturation in geopressured fluids within peLroleum-source rock.
Gas saturation is further enhanced by the temperature and pressure decreases and subsequent decreases in gas solubilities experienced by the fluids as they move upward. In contrast, in kerogen-rich normally-pressured sediments fluids flow downward, thereby diluting gases released from the sediments into the fluid. In addition, downward fluid flow results in temperature and pressure increases and subsequent increases in gas solubilities. In kerogen-poor geopressured sediments pore fluids generally lack a source of CO2 and CH4 and, therefore, do not become enriched in these gases.
The unique chemical and hydrologic conditions present during diagenesis of kerogen-rich geopressured sediments could produce alteration mineral assemblages having both distinctive mineralogies and depositional patterns. Authigenic quartz deposition should occur at the top of the geopressured zone and decrease rapidly with increased depth as a result of the decreased flux of upward fluid flow with increased depth. Carbonate minerals should deposit primarily above the zone of maximum CO2 generation from kerogen decomposition. Carbonate mineral deposition results from the upward flux of CO2 saturated fluids and subsequent decreases in fluid temperature and pressure and CO2 solubility. Feldspar dissolution and kaolinite-carbonate deposition occur within and above this zone of maximum CO2 generation.
Albite-carbonate precipitation could occur below the zone of CO2 generation. These predictions are in part borne out by observation as compared with authigenic minerai deposition within the Frio sandstone of the Texas Gulf Coast. These mineral assemblages and their patterns of occurrence could differ from mineral alteration produced in kerogen-poor geopressured sediments of normally pressured sediments. The minerals laumontite and anhydrite could deposit from pore fluids in these sediments, but should not deposit from fluid in kerogenrich geopressured sediments. In addition, quartz deposition by upwardly mobile fluids would not be expected in normally-pressured sediments in which gravitydriven fluids dominate the flow regime.
The history of mature sedimentary basins is complex and during that time the sediments undergo a number of changing conditions. The predicted
mineralogic relationships presented in this text, therefore. should prove useful in reconstructing geologic conditions in older basins, and ultimately assist in the prediction of petroleum formation, migration and entrapment.
Dr. Capuano also published in 1990 a paper based on her dissertation in GeoChimCosmoActa (here).
For related papers, see (here).
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