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Effects of hydrocarbon generation, basal heat flow and sediment compaction on overpressure development: a numerical study
Jennifer Hansom and Ming-Kuo Lee
Department of Geology and Geography, Auburn University, Auburn, Alabama 36849, USA (e-mail: leeming@auburn.edu)
ABSTRACT: This study outlines numerical experiments to investigate the effects of

hydrocarbon generation, basal heat flow andsediment 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 overpressuredevelopment 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 compactionof 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% ofthat 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 reachpeak 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 porepressure 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 temporaldistribution of overpressure.
KEYWORDS: hydrocarbon generation, overpressure, fluid migration, compaction

INTRODUCTION Many low-permeability sedimentary strata are characterized by elevated pore fluid pressures that are much greater than the hydrostatic pressures. There is considerable interest in understanding the origin of overpressures in sedimentary basins because the presence of geopressured fluidsrepresents a major hazard for well blowout during drilling. Overpressures can influence basin fluid migration strongly and thus play an important role in creating ore deposits and localizing petroleum reservoirs (e.g. Bethke 1989; Lee & Williams 2000). Various dynamic mechanisms have been proposed for generating overpressures in sedimentary basins, including
Petroleum Geoscience, Vol. 11 2005, pp.353–360

disequilibrium compaction (Bethke 1986; Shi & Wang 1986), tectonic collision (Ge & Garven 1992), aquathermal expansion (Sharp 1983), clay dehydration (Burst 1969), gravity flow (Toth 1962; Lee & Bethke 1994; Wolf et al. 2005), gas capillary seals (Revil et al. 1998; Lee & Deming 2002) and hydrocarbon generation (Luo & Vasseur 1996; Lee & Williams 2000). In old, mature basins abnormal...