--> ABSTRACT: Sedimentology and Origin of Source Rocks in the Tertiary Niger Delta, by R. Marc Bustin; #91032 (2010)
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Sedimentology and Origin of Source Rocks in the Tertiary Niger Delta

R. Marc Bustin

Organic matter in Tertiary strata of the Niger delta is mainly a mixture of types II and III, has a high pristane/phytane ration (> 1.0), and is composed of the macerals vitrinite and minor liptinite. The main palynofacies are structured woody material, cuticles, pollen, spores, and opaque and minor amorphous organic matter. The distribution and abundance of the organic matter reflects the age and sedimentology of the strata. There is a progressive decline in mean total organic carbon (TOC) content from upper Eocene (2.2% TOC) to Pliocene strata (0.90% TOC) and an associated general decrease in Previous HithydrogenNext Hit Previous HitindexNext Hit (Previous HitHINext Hit) and pristane/phytane ratio. The decrease in TOC and Previous HitHINext Hit in younger strata mainly reflects increased dilution of a nearly constant supply of terrestrial organi matter associated with the generally higher sedimentation rates of younger strata. The low pristane/phytane ratio of younger strata may reflect less oxidizing depositional conditions.

The variation in organic matter with depositional environments and environmental depth zones (defined by fossil assemblages) is significant although less important than that of age. The highest TOC and Previous HitHINext Hit values occur in nonmarine swamp/marsh and flood-plain deposits. From continental to transitional depth zones, TOC and Previous HitHINext Hit decline markedly; from transitional to bathyal depth zones, TOC varies little but Previous HitHINext Hit increases. The variation in TOC and Previous HitHINext Hit between environmental depth zones reflects the combined effects of selective transport, dilution, and oxidation of detrital organic matter and the rate of production of Previous HithydrogenTop-rich autochthonous organic matter.

No rich source rocks occur in the Niger delta and, as conventionally measured, the strata have little or no oil generating potential. The poor quality of the source rocks has been compensated for by their great volume and excellent migration routes. The Niger delta type of source rock--although an end member in terms of general source rock composition--appears to be relatively typical of Tertiary deltas.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.