2D SEISMIC INTERPRETATION, BASIN AND PETROLEUM SYSTEM MODELLING OF THE BLOCK 5 OFFSHORE SOUTHERN TANZANIA

Abstract

2D seismic reflection data have been used to map the lithology horizons and the associated structures within Block 5. Two major unconformities are recognized from the interpreted seismic lines. These unconformity surfaces are related to the Mid Jurassic and Albian erosion events respectively. The Albian erosion event was followed by the deposition of Cenomanian shale, which is sealing the Albian sandstone reservoir.

The Block 5 area has all essential elements for hydrocarbon generation and accumulation. For the purpose of this study, the Campanian black shale and Jurassic source rocks are taken into consideration. Eocene, Paleocene, Santonian, Albian and Triassic sandstones have been identified as the potential reservoir units. An events chart has been constructed in order to summarize the petroleum system in the area.

1D and 2D petroleum system modeling of Zeta-1 well from Block 5 have been used to assess the source rock maturity, estimate timing of hydrocarbon generation and predict the accumulated volumes of oil and gas within Block 5. 1D modelling is done by reconstructing the burial and thermal histories by utilizing the age of horizons, layer thicknesses and other relevant parameters. Zeta-1 well geochemical data and bottom hole temperature (BHT) are used to calibrate the 1D model. Seismic line EO-0068 is used for 2D basin modelling.

It is found that main oil and gas generation for the Jurassic source rocks began in the Early Cretaceous and Late Cretaceous respectively which is compatible with the start of hydrocarbon generation according to the transformation ratio (TR). The Early Jurassic source rocks have been completely transformed. The Middle Jurassic (Bajocian) source rocks continue to generate dry gas to present day along with late oil. The Campanian black shales are immature to begin hydrocarbon generation.

About 401.62 MMbbls and 112818.76 Mm3 of oil and gas respectively are predicted to accumulate within Block 5. Large part of the gas is accumulated in the older Triassic sandstone and the remaining volume is accommodated in the Albian reservoir. The generated oil is accumulated in the Santonian sandstone reservoir.