Migration of oil and its subsurface expressions in the deep-water Gulf of Mexico

Abstract

The most robust evidence indicating that petroleum migrates in the Gulf of Mexico (GoM) is the presence of oil slicks at the surface of the sea coming from naturally sourced oil at the sea bottom. The mechanisms for oil migration in the GoM involve the interplay of several variables (including faults, pressure, salt diapirism, etc.) These are not fully understood locally, let alone regionally, and are not well constrained to basin evolution. The use of numerical models helps to represent and visualize oil migration from the source to the traps and or seeps but it is limited by the lack of a complete geological picture. However, despite these limitations, numerical models are an important means of estimating the timing of oil migration. Salt movement through time has a huge influence on the patterns of oil migration and re-migration. The underlying geometries of Allochthonous Salt can help focus oil to an up dip trap or in turn result in the breaching of a trap. Subsequent salt withdrawal may result in a weld which may or may not longer seal the hydrocarbons in place. The characterization of successful plays in the GoM involves understanding seeps, oil fluxes, surface anomalies or hard crusts, along with plausible migration pathways of leakage and remigration of oil as related to salt tectonics. These elements are some of the building blocks to high-grade prospective areas for oil exploration. Introduction The first and most obvious observation is the presence of natural seepage occurring at the surface like sea-bottom pock marks and oil slicks (Fig. 1) The MacDonald Image Laboratory of Florida State University posted a video on the web (http://www.sarsea.org/natural_seapage/gallery.html) where it is possible to fully appreciate the most important characteristics of oil seepage: spatial variability of the point of origin, presence of carbonate hard crust, and domes and pock-marks. Secondly, the identification of possible migration pathways relies on seismic data. By looking at deep seismic reflection data we can interpret mechanisms likely influencing petroleum movement from the source rock to the intermediate traps, and eventually into seeps. Interpreted faults and fractures seem to be the main conduits for oil migration. The examples in this work show the dominant impact of salt movement on the migration. This salt was originally deposited over 160 mya. and has had a complex history of movement in resulting diapirs and salt canopies. Therefore, improving the combined knowledge of halokinesis, oil geochemistry and structural evolution of the basin it is possible to have a better picture of the migrating oil into traps, intermediate reservoirs and ultimately seeps.