Source rock characterization and maturity modeling of the coastal and deep offshore basins of Tanzania

Cope, M., Western Geco, London, United Kingdom, and S. Massay, Tanzanian Petroleum Development Corporation, Dar Es Salam, Tanzania

Abstract

A study of some 1700 source characterization analyses from wells in the coastal basin of Tanzania has recognized the occurrence of four oil-prone source formations:

• Middle Eocene (mixed Type II/III kerogen)
• Upper Cretaceous (mixed Type II/III kerogen)
• Middle Jurassic (mixed Type II/III kerogen)
• Lower Jurassic (Younger Karroo, predominantly Type I/II kerogen)

In addition, Lower Cretaceous and Triassic (older Karroo) gas-prone source formations are developed in parts of the basin. Data are presented to indicate the source richness and petroleum potential of the various formations.

Seismic data indicate that Upper Cretaceous shales are regionally developed throughout the deep water area and that younger Karroo likely source-bearing half-grabens are present in the subcrop over much of the area.

Maturity modeling and calibration has been undertaken for the following key source-indicative surfaces over the deep offshore area:

• Base middle Eocene (for middle Eocene sources)
• Mid-Cretaceous unconformity (for Upper Cretaceous sources)
• Top Karroo (for Middle and Lower Jurassic sources)

The results of the modeling study demonstrate that the middle Eocene and mid-Cretaceous surfaces are presently at oil window levels of maturity, the mid-Cretaceous surface having entered the oil window in the early Miocene. The top Karroo surface is currently at gas generative or spent levels of maturity. Biomarker and isotopic evidence relates the maturation history of the principal source formations in the deep offshore area to the pattern of hydrocarbon occurrence in the coastal basin. The implications for hydrocarbon prospectivity in the deep-water areas are reviewed.

Rock-Eval kerogen type data for each of these formations is shown on Fig. 8. Eocene and Cretaceous formations are Type II to Type III kerogens and Middle Jurassic sediments shown a spectrum from Type I to Type II characteristics. Karroo formations appear to be dominantly Type II to III.

Given the likely depth of burial of these potential sources in the Mafia deep basin, the middle Eocene is expected to be immature whilst the Upper Cretaceous is expected to be mature for oil generation at the present-day. Jurassic formations are probably in the gas window or spent.