Abstract
Repeated palynological analyses of samples collected from lacustrine black organic rich shales and carbonate rocks which are abound in the Mamfe Basin have consistently yielded extremely low count of palynomorphs. Geochemical analysis of the same rocks revealed very high Total Organic Carbon (TOC) content of up to 16.10wt%, whereas similar rocks elsewhere as expected, yielded high abundance and diversity of palynomorphs. Several geologic factors involved in the burial history of sedimentary rocks may account for this low sporomorphs count, however, microscopic analyses of the changes in their colour, provides a quick and relatively cheap approach by which the thermal alteration of the sediments can be reconstructed. Palynomorphs data from this and previous studies of the same sediments were compared and confirmed to be very poor in abundance however, the few recognizable species undoubtedly permitted the assignment of an Albian-Turonian age to these sediments under study. Spore-pollen colour variation (Munsell colour standards) has proven to have a positive correlation with thermal alteration hence, sediments and organic matter maturation. Using a three sporomorph group (SG) that include: (1) leiotrilete spores of the genera Cyathidites, Deltoidospora, Dictyophyllidites, Gleicheniidites, and Leiotriletes (SG-1) with sporoderm thicknesses <1μm, 1–1.5μm and >1.5μm, respectively); (2) trilete, regulate spores of the genus Lycopodiacidites (SG-2); (3) trilete, striate spores of the genus Cicatricosisporites (SG-3); and (4) the gymnosperm-pollen taxon Classopollis torosus (SG-4). Results show here that the colour index varied from 2.5 to 5.0, indicating low to high maturity with Kerogen types I, II and III corresponding to a paloetemparature range of 60 to 140oC for an estimated stratigraphic interval of 1000 to 3000m in the Mamfe Basin.
Highlights
The investigation of the extent of thermal maturation of an identified source rock unit in a prospective basin such as the Mamfe Basin is essential to the evaluation or prediction of its petroleum potentials
Repeated palynological analyses of samples collected from lacustrine black organic rich shales and carbonate rocks which are abound in the Mamfe Basin have consistently yielded extremely low count of palynomorphs
Estimates of sporomorph colours using Munsell colour standards under defined, constant optical conditions yielded a range of spore and pollen grain colours which include yellow orange, yellow brown, dark yellow brown, dark gray to black.These colours are dependent on taxonomically related long-ranging sporomorphs with similar sporoderm thicknessesand indicate that different sporomorph groups yield varying degrees of fidelity withrespect to deciphering thermal alteration
Summary
The investigation of the extent of thermal maturation of an identified source rock unit in a prospective basin such as the Mamfe Basin is essential to the evaluation or prediction of its petroleum potentials. Fossilized pollen and spores have and organic cell wall (organic composition) and Marshall (1990) noted that, following the increase in burial depth and temperature, the organic matter in the sediments alters progressively as hydrogen and oxygen are lost in excess of carbon (Al-Mashramah, 2011). It is on this basis that sporomorphs provide a reliable alternative method for establishing thermal maturation of sediments based on its temperature-dependent colour changes and is invaluable in the prediction of the hydrocarbon window. Sporomorph colours change from pale yellow to orange and brown to opaque with jgg.ccsenet.org
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