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The AAPG/Datapages Combined Publications Database
Organic matter in Tertiary strata of the Niger delta is mainly a mixture of types II and III kerogen as defined by pyrolysis, has a high pristane/phytane ratio (> 1%), and is composed of the macerals vitrinite and minor liptinite. The main palynofacies are structured woody material, opaque organic matter, cuticles, pollen and spores, and minor amorphous organic matter. The geochemistry, petrology, palynofacies, and distribution data indicate a predominantly terrestrial and detrital source of the organic matter irrespective of facies, formation, or age.
The distribution and abundance of the organic matter are related to both the age of the strata and depositional environment. The most significant variation is the decline in mean total organic carbon (TOC) content from late Eocene (2.2% TOC) to Pliocene strata (.90% TOC) and an associated general decline in pyrolysis-defined hydrogen index (HI) and pristane/phytane ratio. The decrease in TOC and HI in younger strata reflects increased dilution of a nearly constant supply of terrestrial organic matter with the generally higher sedimentation rates of younger strata. A simple linear dilution model accounts for about 50% of the variance in measured TOC. 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 foraminiferal assemblages) is significant, although less important than variation due to age. The highest TOC and HI values occur in nonmarine swamp, marsh, and floodplain deposits, reflecting proximity to the organic source. Beach, barrier and offshore bar facies have lower TOC and HI values due to selective winnowing of organic matter and greater oxidation. From continental to transition marine depth zones, a marked decline in TOC and HI occurs as a result of dilution, selective transport, and oxidation of the organic matter. From transitional to bathyal depth zones, little variation is found in TOC but a general increase is found in HI, which reflects a relative increase in more hydrogen-rich autochthonous organic matter. In all facies, autochthonous organic matter comprises less than 20% of the TOC. No interpretable variation appears in maceral or palynofacies abundance with sediment type, which probably reflects the homogeneity of the organic matter.
Paradoxically, no rich source rocks occur in the Tertiary succession 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 more than compensated for by their great volume, by the excellent migration routes provided by interbedded permeable sands, and possibly by the relatively rapid rate of maturation. 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.
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