The effects of salt evolution, structural development, and fault propagation on Late Mesozoic-Cenozoic oil migration: A two-dimensional fluid-flow study along a megaregional profile in the northern Gulf of Mexico Basin

Sequential two-dimensional (2-D) forward modeling of fluid flow along a nor th-south, 600 km megaregional cross section across the northern Gulf of Mex ico Basin illustrates the influence of structural, stratigraphic, and therm al evolution on oil generation patterns and migration paths. Twelve megareg ional fluid flow models, which span from the Late Cretaceous to the Holocen e, were constructed for this study. Each model uses a sequential structural restoration and proprietary well data to constrain the structural and stra tigraphic development of the study area and to calibrate production depths along the megaregional profile. These fluid-flow models specifically addres s the levels of influence that salt evolution, sedimentation, thermal histo ry, and fault development induce on temporal oil migration patterns. Results from the sequential 2-D fluid-flow models across the northern Gulf of Mexico Basin indicate that allochthonous salt evolution and excess-press ure development from differential sedimentation strongly influenced Late Me sozoic-Cenozoic oil migration patterns along the entire megaregional profil e. Within the lower slope part of the profile, early and fairly rapid matur ation of source rocks was accompanied by slow elevation of excess pressures . As a result, oil migration in these regions was minimal, and the impact o f salt evolution on the fluid flow was restricted to circulatory patterns a t the base of salt stocks. Within the center of the profile (offshore Louis iana shelf), however, the evolution of allochthonous salt and the formation of high excess pressures, coeval with the development of listric and norma l faults, strongly impacted the oil migration patterns. Penetration of high excess-pressure regimes by both listric and normal faults directed fluid f low vertically upward along the fault systems. Upon encountering salt sheet s, oil migration in these regions exhibited both divergent and convergent f low patterns, flowing laterally along the base of the salt. A similar scena rio was observed in the northern part of the profile (southern onshore Loui siana), reflected by oil migration beneath the Terrebonne salt sheet. Upon evacuation of allochthonous salt in the central and northern regions of the profile, migration patterns were primarily lateral. When excess pressures in these regions exceeded 50 MPa, however, oil flowed vertically through th e salt welds and along suprasalt faults. A more localized and well-constrai ned study of fault migration in the Oligocene-Miocene detachment province f urther suggests that faults are important factors as migration pathways, wi th episodic flow directing oil migration into observed shallow reservoirs.