About This Item
Share This Item
Recent offshore acquisition of 12,800 km (8,000 mi) of seismic reflection data, with gravity and magnetic profiles encompassing the southern half of the Mozambique basin, reveals new facets of the subsurface geology. Integrated interpretation of these new geophysical data with old well information results in the development of depositional and tectonic models that positively establish the hydrocarbon potential of the basin. Previous drilling was sparse and predated modern seismic technology and exploration philosophy, leaving the area classified as a frontier province.
Pessimistic views regarding the liquid hydrocarbon potential in the basin stem from the following common observations. (1) The only exploitable hydrocarbon discoveries to date have been gas. (2) The only known source rocks are of post-Jurassic age and predominantly contain land-derived organic carbon, and hence are considered as gas prone. (3) Today's inferred geothermal gradient is such that the oil window is at least 5,000 m (16,000 ft) deep, and below the "acoustic basement," implying that existing hydrocarbons result from biodegradational processes not conducive to oil generation. (4) Old seismic data did not reveal well-developed structural traps.
However, the recent comprehensive interpretation affords the following conclusions. (1) Significant oil shows accompanying wet gas discoveries suggest that the South Mozambique basin is a mature province, as the hydrocarbon associations imply thermogenic processes. Hence, the geothermal history must have been more favorable than is generally inferred from present-day gradients. (2) Super-Karroo marine Jurassic sequences have been encountered in the Nhamura-1 well onshore, and Triassic marine sequences have been interpreted offshore from the application of seismic stratigraphy and well correlation. Furthermore, extrapolation of the continental character of the older Karroo from intracratonic locations to paleocontinental margins may not be valid, as exemplified by the basinward increas in marine character of the Sakemena and Ecca formations in Madagascar and Natal, respectively. Accordingly, the local presence of oil-prone source rocks is likely. (3) Steeply dipping reflectors truncated by the pre-Cretaceous unconformity testify to significant tectonic activity preceding the breakup of Gondwanaland. Hence, preconceived ideas about the depth of the economic basement and the absence of mature source rocks of pre-Cretaceous age should be revised. (4) Wildcats in the vicinity of ample structural closures have not been, in retrospect, optimally positioned nor drilled to sufficient depth to test the viability of prospects mapped along a major offshore extension of the East African rift system delineated by this new survey.
End_of_Article - Last_Page 1201------------