Abstract: Major Marine Source Rocks and Stratigraphic Cycles

Bernard Duval, Carlos Cramez, Peter Vail

The identification of continental encroachment cycles and subcycles from the four types of stratigraphic cycles assists explorationists in locating petroleum generating subsystems. The continental encroachment cycles are associated with the breakups of the supercontinents and fit a smooth long-term eustatic curve. They are first-order cycles with a duration of 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. Marine source rocks are most likely to be found in association with this surface. Potential secondary source rock intervals can also be identified by sequence stratigraphy. Actually, each continental encroachment cycle i composed of several encroachment subcycles (Sloss-type "sequences") characterized by major downward shifts in the continental encroachment and bounded by major unconformities. Within each encroachment subcycle a regressive forestepping phase overlies a transgressive backstepping phase emphasizing a downlap surface corresponding often with major organic-rich intervals. The stratigraphic distribution of oil and gas proposed by Ulmichek fits well with the continental encroachment cycles and subcycles (several examples from around the world are shown).

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994