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 spill points 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 occur 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. INTRODUCTION Throughout the Niger Delta we find a sequence of under compacted marine clays (Akata Formation), overlain by paralic deposits (Agbada Form.), in turn covered by continental sands (Benin Form.). In the paralic interval growth fault associated rollover structures trapped hydrocarbons. The petroleum geology of the Niger Delta1,2 is characterised by the occurrence of multiple reservoirs in the imbricated and superimposed off flap cycles of the paralic interval. Typically, in a rollover structure virtually all available sands contain at least some hydrocarbons over an interval ranging from 1 500 to 6 000 ft, provided the caprock shales overlying the sands are sufficiently thick. The hydrocarbons are not of local origin because the shales in the paralic interval, although often having source-rock characteristics, have generally been found to be immature. The main source rock is thought to be the shale of the Akata Formation. Over most of the delta the upper part of the Akata shales can probably be considered mature source rock. A remarkable feature, common to all fields is, that nowhere do we observe trapping of hydrocarbons against the downthrown side of a growth fault, unless they are juxtaposed against an overpressured zone across the fault. In many reservoirs the fault intersection with the top of the reservoir appears to be the spill point of the accumulation. This seems to indicate that the growth faults form vertical conduits connecting successive reservoirs. Several studies have been carried out in Nigeria to learn more about the composition and structure of fault zones. A series of 1:20 scale diameter logs have been examined, which show that the widths of shear zones of normal faults in Nigerian oilfields generally range from less than a foot to about two feet.