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Abstract

The Pompano Field, offshore Gulf of Mexico Basin, presents an opportunity to test migration mechanisms involved in delivering undersaturated oil from a deeply buried source bed to a series of relatively shallow upper Miocene to middle Pliocene reservoirs. An inclined salt sheet, developed in association with a counter-regional growth fault, separates the upthrown pools (mainly Miocene, Mississippi Canyon) from the downthrown pools (Pliocene, Viosca Knoll). In this situation, many geoscientists consider salt-sediment interfaces and other macroscopic planes of tectonic disruption (fault zones) to be essential petroleum migration conduits. We used the thermal and fluid flow code TEMISPACK to study fluid (water and oil-phase petroleum) flow in a representative 2-D section.

An important effect of the salt sheet is its localized disruptive effect on the local shallow thermal field (i.e., that penetrated by the wells). Failure to recognize and account for this effect could lead to over-estimation of the heat flow used to model maturation of the deeply buried source beds.

The main effect of the salt–which we assumed to form as an extrusive glacier–on fluid flow was to prevent de-watering of the muddy substrate, preserving its very high (near-initial) porosity and permeability. Under such conditions these silty mudstones comprise a conduit for rapid capillary flow of fluids; tectonic fracturing need not be invoked. Geochemical analysis confirms that oil-phase petroleum also exploited this pathway.

However, in Pompano, migration along the identified subsalt pathway seems not to be the volumetrically significant charging mechanism. Rather, most petroleum delivery occurs by vertical migration, provided that the appropriate relative permeability behavior is assigned to the basin’s fine grained rocks.

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