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

Abstract

Sequential two-dimensional (2-D) forward modeling of fluid flow along a north-south, 600 km megaregional cross section across the northern Gulf of Mexico Basin illustrates the influence of structural, stratigraphic, and thermal evolution on oil generation patterns and migration paths. Twelve megaregional fluid-flow models, which span from the Late Cretaceous to the Holocene, were constructed for this study. Each model uses a sequential structural restoration and proprietary well data to constrain the structural and stratigraphic development of the study area and to calibrate production depths along the megaregional profile. These fluid-flow models specifically address the levels of influence that salt evolution, sedimentation, thermal history, 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-pressure development from differential sedimentation strongly influenced Late Mesozoic-Cenozoic oil migration patterns along the entire megaregional profile. Within the lower slope part of the profile, early and fairly rapid maturation of source rocks was accompanied by slow elevation of excess pressures. As a result, oil migration in these regions was minimal, and the impact of salt evolution on the fluid flow was restricted to circulatory patterns at the base of salt stocks. Within the center of the profile (Begin page 1946) (offshore Louisiana shelf), however, the evolution of allochthonous salt and the formation of high excess pressures, coeval with the development of listric and normal faults, strongly impacted the oil migration patterns. Penetration of high excess-pressure regimes by both listric and normal faults directed fluid flow vertically upward along the fault systems. Upon encountering salt sheets, oil migration in these regions exhibited both divergent and convergent flow patterns, flowing laterally along the base of the salt. A similar scenario was observed in the northern part of the profile (southern onshore Louisiana), 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 the salt welds and along suprasalt faults. A more localized and well-constrained study of fault migration in the Oligocene-Miocene detachment province further suggests that faults are important factors as migration pathways, with episodic flow directing oil migration into observed shallow reservoirs.