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Depositional sequence mapping is a powerful technique to assist basin evaluation on a passive continental margin. The study method described in this paper helps to establish the tectonic and stratigraphic framework and provides a rapid assessment of hydrocarbon potential. The technique produces more realistic stratigraphic predictions in areas of sparse well control than can be achieved with isolated studies of geophysical structural mapping and well log lithostratigraphic correlation.

The basis of the study method is to use an integrated geological/geophysical database to define the sedimentary record in terms of depositional sequences. Sequences are separated by widespread unconformity surfaces and form the fundamental stratigraphic building blocks of basin analysis. In effect, each depositional sequence is a single basin entity possessing a basin margin, a depocenter and an internal facies pattern.

Sequences can be stacked together into groups of related sequences, a megasequence, or subdivided into subsequences and component gross depositional environments. It is of prime importance in effective basin analysis to recognize the correct level of sequence mapping which is required to determine the prospectivity of an area. Megasequence mapping is usually sufficient in areas of low prospectivity whereas sequence, or even subsequence, mapping may be necessary for specific prospect evaluation.

Depositional sequence boundaries are identified on seismic data as primary seismic reflections of widespread or regional extent. They are not generated by specific lithologic boundaries but rather by regional chronostratigraphic strata! surfaces which pass through all depositional environments from nonmarine to deep marine and cross all lithotypes from sandstone to limestone to shale. The modern day seabed is a good example of a single stratal surface.

Basin analysis, using the depositional sequence mapping technique, is a four-step procedure: step one, sequence boundary identification; s tep two, interpretation of sequence external form and internal attributes; s tep three, building a geologic model to determine resource potential, and; step four, developing play concepts, in conjunction with a structural consideration of the basin. It is stressed that direct well control is always required to add lithologic calibration to seismically derived geologic models.

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