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Petroleum Geoscience

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Effects of hydrocarbon generation, basal heat flow and sediment compaction on overpressure development: a numerical study

Department of Geology and Geography, Auburn University, Auburn, Alabama 36849, USA(e-mail: leeming@auburn.edu)

This study outlines numerical experiments to investigate the effects of hydrocarbon generation, basal heat flow and sediment compaction on overpressure development in evolving sedimentary basins. The model integrates predicted groundwater flow and temperature and pressure distribution with thermal maturation simulations. The programme uses the Arrhenius kinetic model to simulate the kerogen–oil or oil–gas conversion processes. Such conversion processes result in an increase in fluid volume and overpressure development since oil and gas generated are less dense than their precursors. The model integrates an equation of state to calculate gas densities for the CH₄–CO₂–H₂O system over a wide temperature–pressure (T–P) range expected in sedimentary basins; this approach allows for prediction of the rate of pore volume increases and fluid pressure changes due to gas generation. Sample calculations of compaction of kerogen-rich shales in the Delaware Basin shed light on the magnitudes of overpressures created by hydrocarbon generation from the Late Pennsylvanian to Middle Permian. Oil generation can cause excess pore pressure (c. 425 atm) up to c. 40% of that generated by compaction only (c. 300 atm). Oil and CH₄ gas generation together yield the maximum excess pressure (c. 750 atm) up to about 150% of that generated by compaction only. There is much greater pore pressure build-up from oil to CH₄ conversion (c. 325 atm) than oil to CO₂ conversion (c. 75 atm) because density of CH₄ gas is less than that of CO₂ under the same P and T conditions. Sensitivity analyses also show that lower activation energy and higher pre-exponential factor lead to faster thermal cracking that allows oil or gas to reach peak generation earlier. Moreover, a basin experiencing a high heat flow throughout the burial history reaches hydrocarbon generation and overpressure development earlier. Calculation results also show that the oil and gas windows become deeper as the sedimentation rate increases. Thus, a basin experiencing high sedimentation rates would exhibit higher levels of thermal maturity and excess pore pressure over the deeper section. This also implies that greater overpressure may be expected at shallower depths in a basin with relatively low sedimentation rates. The modelling results demonstrate that kinetic parameters, basal heat flow and sedimentation rates all influence the timing, duration and depth of oil and gas generation, which in turn, profoundly affects the spatial and temporal distribution of overpressure.

KEYWORDS: hydrocarbon generation, overpressure, fluid migration, compaction