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Hydrocarbon traps are customarily subdivided into two main classes: structural and stratigraphic. A third important class, hitherto not considered separately, includes hydrocarbons trapped in buried hills, ancient valleys, fossil reefs, and other primarily geomorphological phenomena. These are referred to as paleogeomorphic traps. The analysis of and prospecting for this type of trap must proceed along purely geomorphological lines of reasoning.
Paleogeomorphology covers all geomorphic phenomena recognized in subsurface geology; i.e., all
buried relief features, whether formed on land or under water. Geomorphic processes may be divided into constructive and destructive. Constructive forms of interest to petroleum geologists are dunes, barrier beaches, organic reefs, etc. Destructive processes create hills and valleys, underground drainage in carbonates, submarine canyons, etc., and create or destroy porosity by weathering.
The interpretation of buried landscapes presents many problems still unresolved among geomorphologists and also highlights several lesser known geomorphological phenomena. Subsurface data reveal that many landscapes exposed for millions of years were not peneplains but still showed considerable relief. The presence of well-developed slopes favors the theory of scarp retreat. Summit levels may also be related to contemporaneous erosional processes.
The solution of paleogeomorphological problems is greatly aided by applying quantitative geomorphological principles. The geological aspects of paleogeomorphology concern primarily the identification of erosion-resistant and less resistant horizons and the influence of structure (folding and faulting) on ancient drainage systems.
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