The role of faults in hydrocarbon migration and trapping in Nigerian growth fault structures

Abstract

A fault zone model is presented which is capable of explaining the migration, distribution and trapping of hydrocarbons in Nigerian rollover structures. The main factor accounting for the occurrence of hydrocarbons in a rollover structure appears to be the presence of a large wedge of overpressured marine source-rock shale on the upthrown side of the structure bounding growth fault. The driving force for the migration is considered to be the fluid-pressure differential resulting from the juxtaposition of overpressured shale against initially hydropressured paralic sands across the fault. This mechanism can explain the presence of hydrocarbons in the lowermost series of paralic sands opposite overpressured shale but not that of shallower accumulations. Model experiments and outcrop studies in Germany have indicated that shear zones of normal faults in sand/shale sequences usually consist of smeared-in laminae of shale and wedges of sand. In Nigeria the spillpoints of accumulations in rollover structures often coincide with the highest point of contact of bounding growth fault and reservoir. These observations suggest that migration of hydrocarbons along major faults may oocur along sandy stringers and wedges in shear zones. In this way, superimposed reservoirs can be successively filled with hydrocarbons when connections between hydrocarbon-bearing sand in the shear zone and the reservoir sand are available.