Geological and Geophysical Evaluation of the Offshore Morondava Frontier Basin Based from Satellite Gravity, Well and Regional 2-D Seismic Data Interpretation
Madagascar is situated in the South West of the Indian Ocean about 400 km off the East coast of Africa. This study, drawing upon results from available wells, satellite gravity and seismic data, aims to evaluate the regional subsurface geology, structure and hydrocarbon potential of the offshore part of the Morondava Basin between the west coast of Madagascar and the Davie Ridge, covering an area of about 400,000 km2.
In 2003, TGS acquired, processed and interpreted 2,158 line km of good quality seismic data from offshore Morondava Basin. Once the presence of the deep water basin had been confirmed, 10,300 line km of new regional 2D seismic data were acquired in 2005-2006 to better define basin details.
Only six wells have been drilled within the offshore part of Morondava Basin; all are close to the coast in less than 40m water depth. Satellite gravity data interpretation suggests that western coastal basins of Madagascar can be split into two tectonic provinces: a western passive margin and an eastern failed rift. The structure, depositional environments, subsidence histories and petroleum systems of these provinces are quite distinct. Time thickness maps were generated to analyze the sediment thickness changes and to understand the processes responsible for such variations.
The offshore Morondava Basin is filled with over 10 km of sediment from Mesozoic to Recent age with minor volcanics, but no evidence of salt. The deepwater part of the basin is unexplored but considered to be highly prospective. Seismic interpretation established many types of structure with hydrocarbon potential. They include anticlinal features (up to 15 km span), sand pinch-outs, large scale channel features and stratigraphic traps. Most of these structures occur within the Cretaceous and may be overlain by mudstones. Sediments with good reservoir quality potential, including predominantly sandstone and subordinate limestone, are present. Burial, thermal and hydrocarbon expulsion histories for source rocks of Permo-Triassic and Mid-Jurassic ages were modelled, concluding that oil expulsion postdates trap formation. High seismic amplitudes (probable DHI's) exist within many of the identified closures and apparent gas chimneys have been identified.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009