Mapping Sag and Rift Basins from Inverted Gravity and Magnetic Compilations: Potiguar Basin, Brazil; the East African Rift System and Applications to Petroleum Systems Analysis

Abstract:

Summary
From basin to continental-scale mapping projects, the authors have employed methods to derive accurate depth to basement and sediment thickness values in a semi-automated fashion. Inputs comprise data, methods and constraints. The basic data are themselves compilations of topography/bathymetry, gravity and magnetics, depth and thickness mapping. The potential field data are inverted for basement depth using existing mapping as constraints; sediment thickness is derived by subtracting topographic elevation from basement depth. Results were vetted against a collection of published maps, profiles and point data until consistency was achieved. Most values were satisfactory except over known basins of less than 30-40 km width. Modified techniques were then applied to the Potiguar Basin of northeastern Brazil; and to the East African Rift and the resulting values were stitched into the regional outputs. General application of our method is quick and accurate but requires scanning input data for pitfalls such as evidence of narrow basins.

Our key internal application is integration with geochemical data and methods in a non-exclusive petroleum systems study from which selected Brazilian-margin examples are shown. Gravity imagery, supported by published maps and sections, provides an excellent picture both of source rock containers and structures that influence migration of generated hydrocarbons. Our sediment thickness values correlate across multiple basins with limits of thermal generation. Analysis of steadily increasing numbers of geochemical control points offers an understanding of what source layers may be generating and at what levels the expelled hydrocarbons have been detected. Combining these data types yields distribution and likelihood maps, particularly for Cretaceous syn- and post-rift hydrocarbon systems, of the Atlantic margin basins.

References
Odegard, Mark E. (2011) A Sediment Thickness Map of South America Using Automated Inversion of Magnetic and Gravity Data for Depth to Basement, Extended Abstract, 12th International Congress of The Brazilian Geophysical Society held in Rio de Janeiro, Brazil.