Improving geologic understanding with gravity and magnetic data: Examples from Gabon, Nigeria and the Gulf of Mexico

Abstract

John M. Jacques, Marianne E. Parsons, Antony D. Price and David M. Schwartz draw upon recent work to provide evidence as to why gravity and magnetic survey data can still provide vital geological clues for oil and gas exploration. Better understanding of geology on regional and prospect levels using potential field data continues to encourage the melding of these fields to produce enhanced interpretations. Three case studies are presented in this article, all with different geologic settings and targets. All of the examples draw on databases from non-exclusive gravity and magnetic surveys. Two large databases and their interpretation in Gabon and Nigeria were the topics of recent poster papers at the African Symposium, sponsored jointly by the Houston Geophysical Society and the Petroleum Exploration Society of Great Britain (PESGB) in Houston in September, 2003. They are illustrative of the approach that utilizes regional data (in this case satellite-derived gravity) to develop the tectonic framework and more detailed shipborne or airborne gravity and magnetic data to focus on specific targets. The continued high level of activity off West Africa attests to the wide-ranging potential for further development in this region. Many areas, both onshore and offshore, have been extensively explored with 2D and 3D seismic surveys and many wells. Gravity and magnetic data provide a low cost way to screen large areas as well as construct important alternative models to delineate subsurface structures and reach a better understanding of the geology. The density contrasts presented by the juxtaposition of sediments with shales and salt make detailed gravity modelling in this region a valuable exercise. The magnetic data provide insight into mapping basement surfaces and delineating shallower volcanics and in some cases shale or salt diapers. In the Gulf of Mexico, the reconstruction of the paleo-tectonic history is also enhanced by incorporating the potential field data. New work is ongoing which blends the geologic information with the gravity and magnetic data using regional scale modelling techniques. Gravity and magnetic data have been traditionally thought of as regional screening tools capable of providing basin edges or basement mapping. In recent years, the application of these data has been greatly expanded to include modelling of prospect-level targets.