DEVELOPMENT OF SEDIMENT OVERPRESSURE AND ITS EFFECT ON THERMAL MATURATION - APPLICATION TO THE GULF-OF-MEXICO BASIN

High sedimentation rates can potentially lead to overpressuring and se diment undercompaction within basins. Sediments with anomalously high porosity, in turn, induce low thermal conductivities and so tend to ac t as a thermal insulator to the flow of heat, In the Gulf of Mexico ba sin (Gulf basin), the generation of overpressure is caused mainly by t he inability of pore pressure fluids to escape at a rate commensurate with sedimentation. We modeled the generation and dissipation of abnor mal sediment pore pressure due to variations in sedimentation rate, fa cies, formation porosity, and permeability within the Gulf basin using finite-element techniques to solve the differential equations of both heat and fluid transport within compacting sediments. We assume that the porosity-effective stress relationship within the sediment follows a negative exponential steady-state form when the pore pressure is hy drostatic. An important feature of our modeling approach is the assump tion that sediments are incapable of significant expansion in response to increasing pore pressure, Sediments are assumed to hydrofracture w hen the pore pressure approaches the lithostatic pressure, rather than a common assumption of porosity expansion even in lithified sediments . From our modeling, we conclude that significant overpressures have b een created (and dissipated) at various times within the Gulf basin an d track, in general, the west to east migration of sediment loads depo sited since the Cretaceous. Although predicted overpressures of more t han 0.75 kpsi (i.e., an equivalent excess hydraulic head of 500 m) of Campanian-Maastrichtian age remain to the present day, the main phase of overpressure development in the Gulf basin is predicted to have occ ured during the Miocene-Holocene, Maximum overpressures (similar to 13 .6 kpsi; excess hydraulic head of 9.4 km) are predicted for the presen t day, Overpressure development during the Miocene-Quaternary, a conse quence of rapid sediment deposition associated with the Mississippi de lta system, is also predicted to be associated with undercompaction, T his undercompaction led to increased temperature gradients during the Miocene and Quaternary despite the fact that the anomalous basal heat flow engendered by extension had practically dissipated, We further pr edict that by the end of the Neogene, temperatures would have been app roaching steady state over broad regions of the Gulf basin implying th at the highest temperatures occur in the deepest parts of the basin. I n contrast, during the Quaternary, the rapid progradation of-overpress ured and undercompacted sediments resulted in a thick section that has yet to reach thermal equilibrium and thus is anomalously cold with re spect to its present depth. The predicted vitrinite reflectance indica tes that for most of the Gulf basin history, the depth to the top of t he oil window remained at approximately 2.5+/-0.5 km below sea floor ( bsf). Similarly, the depth to the base of the oil window ranged from 3 .5 to 6.5 km bsf This relatively constant position of the top of the o il window defines a maturation ''front'' that propagated from the offs hore into the onshore regions of the northern Gulf basin as a function of time. As such, hydrocarbon generation is predicted to have occurre d continuously within the Jurassic and Cretaceous sections of the onsh ore region during the entire Cenozoic. Prior to this, maturation front s within each of the onshore basins resulted in maturation of Upper Ju rassic source rocks during the Early Cretaceous. In the offshore Gulf Coast area, pre-Tertiary source rocks are predicted to be overmature f or liquid hydrocarbons at present. In the offshore regions affected by Quaternary sedimentation, the depth to the top of the oil window has been significantly depressed in response to sediment loading and subsi dence.