Burial Metamorphism (Thermal Maturation) in Cretaceous Sediments of the Southern Benue Trough and Anambra Basin, Nigeria

Abstract

Organic matter reflectance, illite crystallinity, and fluid-inclusion techniques have been used to evaluate burial metamorphic conditions for the lithostratigraphic successions that accumulated during the Cretaceous in the southern Benue trough and Anambra basin of southern Nigeria. These successions were invaded by intrusives, volcanic rocks, and vein-type lead-zinc minerals, especially in the Albian-Cenomanian section of the Abakaliki anticline. The sequence includes lower Maastrichtian subbituminous coals in the Anambra basin. In exposed Cretaceous deposits on a northwest-southeast section from Enugu to Abakaliki, mean random vitrinite reflectance (Rom) in oil ranges from 0.55 to 0.67% in the lower Maastrichtian coals and shales, approximately 0.91% in the Coniacian shales, 0.97% in the Turonian shales, and up to 4.31% in the Albian shales. A corresponding increase in the illite content of the illite-smectite mixed-layer clay fractions is reflected in the low values of illite crystallinity indices coupled with a decrease from 32 to 0% in the percentage of smectite from the northwest-southeast Maastrichtian-Albian section. Fluid-inclusion pressure-corrected temperatures from vein quartz in the Albian shales range from 170 to 250°C. The results of this work show that thermal maturation in the Cretaceous successions increases from the post-Santonian (Campanian-Maastrichtian) Anambra basin into the older Benue trough where strong diagenetic to anchimetamorphic (i.e., very low grade metamorphism) conditions were reached. The data suggest that these sediments at the present outcrop levels originally were buried at higher maturity levels. Maximum erosion appears to have taken place on the axis of the Abakaliki anticline. The presence of bitumen in fractures and pores of the exposed Maastrichtian units in the Anambra basin suggests that the matured sediments generated some unknown quantity of petroleum. This finding, coupled with reported gas finds and some oil in previous exploration wells of the Anambra basin, enhances the possibilities of Cretaceous targets in the downdip regions.