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NIGER DELTA PETROLEUM SYSTEM 

The hydrocarbon habitat for the total Niger delta is classified as the Akata-Agbada (!) petroleum system (Ekweozor and Daukoru, 1994).  A petroleum system forms when hydrocarbon fluid moves outward from a pod of active source rocks to any nearby trap.  The name for the petroleum system is derived from the source rock being represented the upper portion of the Akata Formation (as well as the lower portion of the Agbada), whereas; the Agbada Formation represents the major reservoir system.  Since most oils of the Niger delta have a similar composition (generated by source rocks with a high terrigenous input), they are presumed to originate from the similar pods of active source rocks that are deposited in the seven depo-belts.  Ekweozor and Daukoru (1994) states that because each of the depo-belts is partitioned with respect to generation, migration, and accumulation, each cell or depo-belt can be studied independ-ently of the others.  With respect to the some of the essential elements/processes of the petroleum system, the depo-belts are independent but with respect to other essential ele-ments/processes, characteristics of the depo-belts are similar.  The essential elements for the Akata-Agbada (!) petroleum system are discussed and the similarities between the depo-belts (or portions of depo-belts) are listed.  To establish these essential elements/ processes for the depo-belts, the general habitat for Tertiary hydrocarbons in the Niger delta are cited. 

A short description and discussion of the interactive elements/processes characterized in each of the project areas follows: 
 

• Source and maturity of hydrocarbon generative organics: Source rocks are those sediments that can generate and supply hydrocarbons to the reservoirs.  These source rocks may be near or distal to the reservoirs.  The source rocks for oils reservoired in the project area apparently not been sampled, since the identify, type, and match of the specific generative source rocks to the reservoired oils are not available (probably since the rocks were not penetrated by the drill).  The identity of the type of organic matter that generated the oils has been successful but oil to source rock matching has not occurred.  Oil maturity work has been done and we feel we know the depths of the oil-generative window (as well as the phases thereof).  This is extremely important in relationship to timing of structural development and migration patterns.

• Migration (Timing): This is one of the essential elements in understanding the interactive relationships of the petroleum system and the reasons for having oil or gas in the reservoirs.  It probably varies between mega-structures, depo-belts, and between proximal (inner) to distal (outer) parts of the deltaic system tracts.  It must be considered with the structural timing of: 1) the large growth-fault systems (both structure building and counter regional); and 2) the syndepositional faults that are antithetic and/or synthetic to them.  This may vary from west to east across the delta and within depo-belts.  In addition, rejuvenation of some of the faults may have caused remigration that possibly caused biodegradation of some oils.

• Traps and Seals: Trap and seal formation and/or presence must be considered and integrated with: 1) the structural timing (more than one period); 2) sand and shale (clay) unit relationships; 3) closure against sealing (or non-sealing faults); 4) rejuvenation of faults causing traps to lose integrity; and 5) the resultant change of oil chemistry.

• Reservoirs: Reservoir mineralogy, provenance, depositional environments, sand/shale (clay) ratios, clay types, and petrophysics have been worked on and addressed by many workers.  The unknown elements here seem to be tied to the subsequent events that affected the reservoirs and their contained fluids.  Early migration of hydrocarbons is important because of its effect on delaying or chemically stabilizing the diagenetic process.  In the areas worked, it would seem that early migration has occurred often and, therefore, most reservoirs have retained their integrity, even with subsequent faulting events.

• Play Concept/Petroleum System: In this project and predicated on those elements noted above, we seem to continually come back to the question concerning oil and source rock chemistry and the effects of migration as related to timing of structure.  Answering this question will allow us to prioritize areas and better explain oil occurrence.  As with other major deltaic systems in the world, we change our ideas and interpretations, as new data becomes available.  The Niger delta is no exception.  Certainly, the Gulf Coast of the US is a prime example of this evolution of concepts.  Often play concepts within the petroleum system evolve or change as data sets increase or become more complete.

This reports speaks primarily to the above factors and their interactive relationships since these elements appear to be most essential in answering the question(s) posed for this study.  Often these factors are least known, and in some instances, it may seem that syntheses are “backing into” or that analogs are used to analyze questions posed for the study areas in this deltaic basin.