Development of Sediment Overpressure and Its Effect on Thermal Maturation: Application to the Gulf of Mexico Basin

Abstract

High sedimentation rates can potentially lead to overpressuring and sediment undercompaction within basins. Sediments with anomalously high porosity, in turn, induce low thermal conductivities and so tend to act as a thermal insulator to the flow of heat. In the Gulf of Mexico basin (Gulf basin), the generation of overpressure is caused mainly by the inability of pore pressure fluids to escape at a rate commensurate with sedimentation. We modeled the generation and dissipation of abnormal sediment pore pressure due to variations in sedimentation rate, facies, 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 hydrostatic. An important feature of our modeling approach is the assumption that sediments are incapable of significant expansion in response to increasing pore pressure. Sediments are assumed to hydrofracture when 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 been created (and dissipated) at various times within the Gulf basin and track, in general, the west to east migration of sediment loads deposited since the Cretaceous. Although predicted overpressures of more than 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 occured during the Miocene-Holocene. Maximum overpressures (~13.6 kpsi; excess hydraulic head of 9.4 km) are predicted for the present day. Overpressure development during the Miocene-Quaternary, a consequence of rapid sediment deposition associated with the Mississippi delta system, is also predicted to be associated with undercompaction. This 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 predict that by the end of the Neogene, temperatures would have been approaching s eady state over broad regions of the Gulf basin implying that the highest temperatures occur in the deepest parts of the basin. In contrast, during the Quaternary, the rapid progradation of overpressured and undercompacted sediments resulted in a thick section that has yet to reach thermal equilibrium and thus is anomalously cold with respect to its present depth. The predicted vitrinite reflectance indicates that for most of the Gulf basin history, the depth to the top of the 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 oil window defines a maturation front that propagated from the offshore into the End_Page 1367------------------------------ onshore regions of the northern Gulf basin as a function of time. As such, hydrocarbon generation is predicted to have occurred continuously within the Jurassic and Cretaceous sections of the onshore region during the entire Cenozoic. Prior to this, maturation fronts within each of the onshore basins resulted in maturation of Upper Jurassic source rocks during the Early Cretaceous. In the offshore Gulf Coast area, pre-Tertiary source rocks are predicted to be overmature for 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 subsidence.