Formation of Early Drift Continental Fragments: Global Examples and Petroleum Systems

Henry, Steven G.¹; Danforth, Al ²; Nuttall, Peter ³; Venkatraman, Sujata ³
(1)RIFTT, Las Cruces, NM. (2) Consultant to ION/GX Technology, Houston, TX. (3) ION-GX Technology, Houston, TX.

Deep imaging (18 sec., 25-40 Km), long offset (10 Km), regional (200-600 Km dip direction) 2D reflection seismic data (SPANsTM) have been acquired from continental margins around the world. Examples from Brazil, East Africa, and India illustrate continental fragments and their potential petroleum systems. The base of the crust (Moho), has been imaged as it rises from below 40 Km under the cratons (continental crust), to the continental oceanic boundary (COB), and then out under oceanic crust at depths of 7-15 Km beneath the seafloor. Detached continental fragments surrounded by oceanic crust and having a crustal thickness of 20-30 Km, have been interpreted based on the seismically imaged Moho with independent support from gravity modeling.

Continental fragments appear to be generated in areas where extensional faulting has failed to break through thick (>40 Km) continental crust and the displacement is taken up along continental strike slip faulting. Crustal scale pull-apart basins develop at these transitions, providing easy access for mantle derived volcanics to reach the surface. Continental fragments are typically covered by large volumes (2-5 Km thick) of rapidly deposited (1-5 Ma) basic volcanics. Some of this volcanic activity continues to develop and can form seaward dipping reflectors.

Support that these fragments are made of continental crust comes from mapping the COB from the fragment and matching the shape to gaps in the COB on the adjacent continental margin. Additional support is provided by observed gravity lows which overlie bathymetric highs, implying the deep root that is observed (Moho) on the seismic data. These fragments subside at a much lower rate (30-50 Ma), as determined by the age of the onlapping sediments, than a fragment of only oceanic volcanic material.

The extended period of time that these continental fragments are exposed above sea level and their positions in the proximal offshore, provide an opportunity to develop all the components for an active petroleum system. Circulation between the fragment and the continent is likely to be restricted, which would promote source rock development. Carbonate platforms (source, reservoir) are observed to form and grow on the isolated slowly subsiding fragment. The platform is eventually drowned and capped (seal) with deepwater shale. If the fragments are large enough to avoid becoming over mature during the rifting, they may also preserve a complete pre-drift petroleum system.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.