Geology, basin analysis, and hydrocarbon potential of Mozambique and the Mozambique Channel

Abstract

The Phanerozoic structural and stratigraphic history of Mozambique can be separated into a “Karoo” and a post-Karoo phase. Outcrops of Karoo rocks, limited to the Zambezi valley and small outliers in the Mozambique belt to the north show an eastward facies change marked by an increased argillaceous content and the appearance of carbonates. The wide coastal plain and immediate offshore of southern Mozambique is marked by extensional horsts and grabens with a Mesozoic fill covered by Cenozoic sediments. Little is known of lower stratigraphic horizons. In this coastal province there is a facies transition from continental clastics in the northwest and west to more lacustrine and marine deposits to the southeast and east. Upwards the succession passes into the Cenozoic without a noticeable break. In the offshore in the north seismic lines help define two basins, the Zambezi Delta basin and the Mozambique Channel basin separated by the Beira High. In the south several offshore grabens have been defined, one of which, the South Mozambique graben, has been examined here by an integrated basin analysis technique. The technique, which uses available depositional, structural and thermal data, provides information on maturation and the timing of hydrocarbon maturation. The analysis showed variation in the extension rate with time, and two phases of rifting. The greatest rate of extension, accumulation and subsidence, between 96 and 76 Ma, depends upon interpretational assumptions. During this time interval fault blocks were systematically tilted “domino” fashion. This event broadly coincides with the rifting between Madagascar and Antarctica, a period of plate reorganization. The earlier rifting phase was probably related to the strike-slip separation of Madagascar from Africa. Modelling heat flow suggests that the later event is the more important from the maturation-hydrocarbon standpoint. The offshore, and potentially the deeper, onshore post-Karoo basins have traps, reservoirs and seals with the presumed source rock within Lower Cretaceous or Jurassic strata. There is, at the present time, no means to evaluate the potential of the Karoo rocks.