Source Rocks and Maturation

Four potential source formations for oil are recognized in the Coastal Basin:

• Middle Eocen shales
• Upper Cretaceous (Maastricthian to Campanian) shales
• Middle Jurassic shales
• Lower Jurassic (Younger Karroo) shales

In addition Lower Cretaceous shales and Older Karroo shales and coals are potential sources for gas.

Eocene and Cretaceous sources are Type II to Type III kerogens with TOC contents up to 12.1% and 7.4% respectively (corresponding HI values up to 396 and 688). Middle and Lower Jurassic sediments show Type I to Type II characteristics (TOC up to 8.7%, HI up to 1000). Older Karroo formations appear to be dominantly Type II to III. Each of these sources is likely to be present in the MDOB. The Eocene and Upper Cretaceous open marine shales are thought to be developed throughout the basin within Layers 3 and 5 respectively. Lower and Middle Jurassic shales are probably more restricted in distribution, occurring in pre- or post- break-up isolated half-grabens.

Recorded geothermal gradients in Coastal Basin wells vary from 18 to 51°C/km (Fjaeran et al, 1987). The higher values of the range are thought anomalous due to the significant tectonic inversion of the area associated with local 'hot-spots' under Mafia and Pemba Islands. Away from these areas more 'normal' passive margin-type geothermal regimes are expected to be developed basinwards to the east.

Confirmation of this situation is given by the measured geothermal gradient of 27°C/km recorded at DSDP Site 242 (Marshall & Erickson, 1974).
In view of the extensive inversion of the coastal area it is difficult to assess true maturity-depth relationships in the well sections. However, it should be noted that in the least uplifted well (Tan Can-1) the oil window is recorded between depths of 3000m and 3900m based on vitrinite reflectance data.

Given the likely depth of burial of potential sources in the MDOB the Middle Eocene is expected to be mature for early to peak oil in the deepest parts of the basin at the present-day.

The Upper Cretaceous is expected to be mature for hydrocarbons throughout the area, mostly for oil with some gas generation in the deeper tracts. Jurassic and older formations are mostly in the gas window or spent.

Preliminary burial history and maturation modeling of the MDOB confirms that Maastrichtian/Campanian-aged potential source formations have been hydrocarbon generative since the early Miocene through to the present day age (Fig. 6)

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FIGURE 6. EARLY MIOCENE BASIN-FILL SEQUENCE:  CHANNEL DISTRIBUTION FROM RMS AMPLITUDE

FIGURE 7. BURIAL HISTORY AND MATURATION PLOT