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 hydrogen index (HI) and
pristane/phytane ratio. The decrease in TOC and HI 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 HI values occur in nonmarine swamp/marsh and flood-plain deposits. From continental to transitional depth zones, TOC and HI decline markedly; from transitional to bathyal depth zones, TOC varies little but HI increases. The variation in TOC and HI between environmental depth zones reflects the combined effects of selective transport, dilution, and oxidation of detrital organic matter and the rate of production of hydrogen-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.