Abstract: Modeling Petroleum System Dynamics in the Deep Water Gulf of Mexico

Düppenbecker, S. J. and A.S. Pepper - BP; J. Wendebourg, J. M. Gaulier, F. Schneider - IFP

The deep water Gulf of Mexico contains a Mesozoic - Neogene/Quaternary petroleum system which, at an early stage of exploration maturity, has already yielded ca. 5 bn boe reserves. We infer that it can serve as an analogue for young, rapidly filling basins around the world. In our experience, an understanding of the flow dynamics of the system is critical in order to predict presence and distribution of petroleum within the many sedimentary play fairways. This paper demonstrates our methodology for evaluating charge risk in exploration by applying 2D and 3D integrated basin modeling for one deep water sub-basin. We will illustrate our assessment of petroleum presence, composition, phase and fluid properties in the context of available geological, geochemical and geophysical information.

In the deep water Gulf of Mexico the petroleum source rock has not been penetrated. Evidence for source presence and age has been derived from the geochemistry of discovered petroleum and seabed seepage, as well as from regional extrapolation of data from the onshore margins of the depositional system. It is now widely accepted that the thermogenic petroleum originates from Mesozoic condensed sections which have been buried to considerable depth by Neogene-Quaternary sedimentation. Average sedimentation rates from the Miocene - Present day are high by global standards and instantaneous rates locally exceed 4 km / Ma. Consequently transient effects dominate the temperature and pressure regime: formation pressures are higher while temperatures and heat flows are lower than their expected equilibrium values. For example, local geothermal gradients as low as 17 C/km indicate the transient state of the entire crustal-scale thermal regime, and without thermal modeling at the crustal scale, the thermal and expulsion history of the source rocks cannot be quantified reliably. Understanding the controls on the observed pressure regime proves critical to identifying driving and resistive forces for both water and petroleum migration. This also leads to better pressure prediction which helps in the planning and safe drilling of exploration wells

Delivering petroleum charge from the deeply buried source rocks to reservoirs in the Neogene/Quaternary play fairways requires vertical migration of many km in a relatively short time frame. The large petroleum volumes trapped in some deep water fields and the occurrence of seepage suggests an efficient petroleum delivery system from source to trap. Similarly, the pattern of petroleum distribution in stacked pool complexes implies limits to the effectiveness of seals and progressive leakage of fill. Basin modeling tools allow us to infer the evolution of migration pathways, rates of vertical migration and filling histories of traps. Migration patterns are strongly controlled by spatial variation of potential carrier beds and the structural evolution - in particular the complex salt withdrawal pattern - which set up deep focusing geometry. Timing of migration and trap fill imply a complex dependency on petroleum phase mobility and the subsurface pressure and temperature regime.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil