Abstract: Effects Of Cyclical Water Level Shifts and Tectonic Variability on Deltaic Reservoir Facies in Lacustrine Rifts: An Example from Lake Malawi, Africa

Christopher A. Scholz

Recent high amplitude (greater than, or equal to 100-200 m) and high frequency (less than, or equal to 100 kyr) lake level changes in Lake Malawi are primarily a consequence of the delicate balance between evaporation and precipitation, a common condition in tropical lake systems. When superimposed on the variable tectonic activity observed on different rift margins, these dramatic shifts in base level result in complex stratal architectures on the lacustrine deltas.

During a recent study of five deltas on Lake Malawi, Duke University acquired 2200 km of small airgun single-channel digital seismic data, comparable amounts of side scan sonar and high-resolution seismic data, and a suite of 90 sediment cores from both the onshore and offshore regions. The deltas studied were located on different margins of the half-graben basins which make up the Malawi rift including (1) a flexural margin (Dwangwa and Linthipe Rivers), (2) axial margin (Songwe and Kiwira Rivers), (3) accommodation zone margin (Ruhuhu River), and (4) border fault margin (South Rukuru River).

The best sites for preservation of reservoir facies are within the lowstand systems tracts on all the margins. Of these, the best potential reservoir facies were observed on the flexural margin deltas. These delta lobes are 20-50 m thick, are composed primarily of sand, cover the large areas (up to 100 sq km), and are situated close to excellent potential hydrocarbon source facies. Axial margin deltas ranked second, but are smaller and thinner than flexural margin systems in part due to compartmentalization by intrabasinal faults. Border fault deltas ranked third and accommodation zone deltas fourth in terms of delta facies prospectivity, but show significant reservoir potential in their associated deep-water fan systems.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994