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Hydrocarbon Distribution Patterns in Nigerian Growth-Fault Structures Controlled by Structural Style and Stratigraphy

K. J. Weber

Growth faults are considered to be major migration conduits in the Niger delta. However, the hydrocarbon distribution often shows such seemingly erratic patterns that doubt remains about the actual migration processes. After considering the sequential aspects of hydrocarbon generation and possible contemporaneous structural deformation, some of the apparent inconsistencies in the along-fault migration theory can be explained. When we study the relationship of fault-sealing capacity with the sand and shale thickness distribution, systematic patterns of hydrocarbon distribution are clearly revealed in many fields. The occurrence of several thick, somewhat undercompacted clay layers that locally form effective seals to vertical migration is also important.

Structures with a predominance of thick sands and thin shales can trap large volumes of hydrocarbons, but only if they are unfaulted. More complex growth-fault structures, cut by secondary faults, will only be prominent oil fields if the shales are sufficiently thick to cause widespread fault sealing and shale-to-shale juxtaposition along faults. In many cases the lateral distribution of hydrocarbons over a series of fault blocks can be predicted fairly accurately on the basis of these considerations.

An interesting phenomenon related to the proposed migration system is the occurrence of water trapped downdip from a hydrocarbon accumulation.

Differences as large as 1,000 ft can exist between the oil-water contacts on opposite flanks of a reservoir.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.