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Abstract

Four types of stratigraphic cycles with a time duration greater than 10,000 years are recognized in the sedimentary record. In order of decreasing time duration and scale they are: (i) continental encroachment cycles, (ii) regression-transgression cycles, (iii) sequence cycles and (iv) parasequence cycles.

Continental encroachment cycles are defined on the basis of their onlapping against cratons and are bounded by the unconformities associated with the formation of supercontinents. They are reflected by long-term shoreline displacements induced by first-order eustasy. Continental encroachment cycles can be subdivided into subcycles which are defined using the same criteria at smaller scales. Regression-transgression cycles are defined on the basis of short-term shoreline displacements induced by second-order eustacy and are bounded by major downlap surfaces. Sequence cycles are defined on the basis of shelfal accommodation changes and are bounded by unconformities induced by relative sea-level falls associated with third-order eustasy. Sequence cycles are complete when all systems tracts are present. These complete sequence cycles occur in areas with high rates of sedimentation where all available shelfal space is filled. Incomplete sequence cycles do not have all the systems tracts and occur in areas with low rates of sedimentation where only part of the available shelfal space is filled. Parasequence cycles are intervals bounded by flooding surfaces or their correlative conformities.

The recognition and understanding of the architecture of the continental encroachment cycles, subcycles and/or the regression-transgression cycles and the location of the major downlap surfaces are important steps in the study of petroleum systems. They allow explorationists to locate the most likely marine source rocks. On seismic data, continental encroachment cycle and subcycle interpretations are used, particularly in the proximal part of sedimentary basins, where the encroachment is relatively easy to recognize. As an alternative, in the intermediate parts of the basins, where the offlap-breaks are usually identifiable, regression-transgression cycle interpretations can also be used to locate potential marine source rocks. In this paper, applications of the continental encroachment cycle and subcycle concept in locating potential marine source rocks using seismic data are presented, together with comments on the stratigraphic distribution of major potential marine source rocks.

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