资源环境科技发展态势分析平台

The basic reservoir flow unit in shallow-marine depositional systems is the parasequence since it is bounded top and base by regionally extensive shales that can form barriers to fluid flow. As a result, it is the key stratigraphic unit requiring parameterization for both exploration and development purposes. A database comprising 3385 measurement points from 385 parasequences from 28 locations, and 16 different passive margin and foreland basins was compiled from outcrop and subsurface data. Analysis of the data suggests that a mean value for parasequence thickness is 17 m (56 ft); mean parasequence sandstone fraction is 0.60, and mean shoreface sand-body thickness is 8 m (26 ft). The mean maximum-parasequence-progradation-distance is 23 km (14 mi). The results indicate that there are two critical controls that can significantly vary the parameter values stated above: (1) stacked-shoreline trajectory or systems tract and (2) shelf gradient. The analyses indicate that shelf gradient is the most important parameter that has a first-order control on parasequence properties. Shelf gradient in the ancient can be approximated to high-gradient or low-to-moderate-gradient categories via analysis of parasequence thickness change versus progradation distance from wells, mean parasequence thickness, or mean progradation distance from seismic attribute data. Parasequence progradation distance is to some degree predictable if stacked-shoreline trajectory, shelf-gradient category, and mean parasequence thickness are known. The data suggest that there are two types of parasequences: accommodation-dominated and supply-dominated parasequences. The progradation distance of these parasequences are controlled by delta autoretreat mechanisms modulated by the combination of fifth- and fourth-order Milankovitch-scale, orbitally forced, climate-change-driven, sediment-supply cycles interacting with eustatically and tectonically driven accommodation cycles.