Evidence for Expulsion of Hydrocarbons from Early Mesozoic Source Rocks: Deepwater, Tanzania

Connolly, David L.; Hemstra, Nanne

Many basins, worldwide, are dominated by vertical hydrocarbon migration. In seismic data these migration pathways are seen as vertically aligned zones of chaotic seismic reflectivity (gas chimneys). Through detection of these gas chimneys, we can determine from what stratigraphic interval the chimneys originate, and thus identify potential source rocks. We can also determine where the chimneys terminate, and thus identify which reservoir intervals are being charged. On regional 2-D seismic data, this information can be used as input to 2-D basin models, and to predict prospective play fairways. The process of chimney detection involves an interpreter selecting reliable examples of gas chimneys, and using a set of seismic attributes, run through a neural network to create chimney probability sections, which can be overlain on the seismic. These results are validated based on a set of criteria.

Gas chimneys have been highlighted on a set of regional 2-D seismic lines, which are located in deepwater, Tanzania. Chimney picks were made in areas with reliable chimneys, supported by DHI's. The processing results are validated by an association with DHI's, surface seep indications, piston core data, and basin modeling, and are generally reliable for the Jurassic and younger intervals.

Chimney processing results shown, indicate HC migration from the Lower Jurassic or possibly older Karoo interval. HC generation occurs in faulted half graben kitchens, with HC migration often focused on basement highs. The western sub-basins may be more gas-prone, based on the depth of the Jurassic interval today, while the outboard half grabens are currently in the oil window. Chimney results also provide evidence for hydrocarbon charge into the Upper Cretaceous reservoir interval, which is productive on trend to the south in Mozambique.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013