Abstract: Major Marine Source Rocks and Stratigraphic Cycles

Bernard C. Duval

The identification of continental encroachment cycles and subcycles by using sequence stratigraphy can assist explorationists in locating source rocks. The continental encroachment cycles are associated with the breakup of the supercontinents and fit a smooth long-term eustatic curve. They are first order, with a duration greater than 50 m.y., and are composed of transgressive and regressive phases inducing major changes in shoreline.

The limit between the transgressive and regressive phases corresponds to a major downlap surface, and major marine source rocks are often found in association with this surface, particularly in the northern hemisphere. Potential "secondary" source rock intervals can also be sought by sequence stratigraphy because each continental encroachment cycle is composed of several subcycles, and the same configuration of a regressive forestepping phase overlying a transgressive backstepping phase also creates a downlap surface that may correspond with organic-rich intervals.

The stratigraphic distribution of source rocks and related reserves fits reasonably well with continental encroachment cycles and subcycles. For instance, source rocks of Silurian, Upper Jurassic, and Middle-Upper Cretaceous are associated with eustatic highs and bear witness to this relationship.

The recognition and mapping of such downlap surfaces is therefore a useful step to help map source rocks. The interpretation of sequence stratigraphy from regional seismic lines, properly calibrated with geochemical data whenever possible, can be of considerable help in the process. Several examples from around the world illustrate the power of the method: offshore of eastern Venezuela, coastal basin of Angola, western Africa, the North Sea, south Algeria, and the North Caucasian trough.

AAPG Search and Discovery Article #90953©1995-1996 AAPG Distinguished Lecturers