BURIAL METAMORPHISM (THERMAL MATURATION) IN CRETACEOUS SEDIMENTS OF THE SOUTHERN BENUE TROUGH AND ANAMBRA BASIN, NIGERIA

Organic matter reflectance, illite crystallinity, and fluid-inclusion techniques have been used to evaluate burial metamorphic conditions fo r the lithostratigraphic successions that accumulated during the Creta ceous in the southern Benue trough and Anambra basin of southern Niger ia. These successions were invaded by intrusives, volcanic rocks, and vein-type lead-zinc minerals, especially in the Albian-Cenomanian sect ion of the Abakaliki anticline. The sequence includes lower Maastricht ian subbituminous coals in the Anambra basin. In exposed Cretaceous de posits on a northwest-southeast section from Enugu to Abakaliki, mean random vitrinite reflectance (R(o)m) in oil ranges from 0.55 to 0.67% in the lower Maastrichtian coals and shales, approximately 0.91% in th e Coniacian shales, 0.97% in the Turonian shales, and up to 4.31% in t he Albian shales. A corresponding increase in the illite content of th e illite-smectite mixed-layer clay fractions is reflected in the low v alues of illite crystallinity indices coupled with a decrease from 32 to 0% in the percentage of smectite from the northwest-southeast Maast richtian-Albian section. Fluid-inclusion pressure-corrected temperatur es from vein quartz in the Albian shales range from 170 to 250 degrees C. The results of this work show that thermal maturation in the Creta ceous successions increases from the post-Santonian (Campanian-Maastri chtian) Anambra basin into the older Benue trough where strong diagene tic to ''anchimetamorphic'' (i.e., very low grade metamorphism) condit ions were reached. The data suggest that these sediments at the presen t outcrop levels originally were buried at higher maturity levels. Max imum erosion appears to have taken place on the axis of the Abakaliki anticline. The presence of bitumen in fractures and pores of the expos ed 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 w ells of the Anambra basin, enhances the possibilities of Cretaceous ta rgets in the downdip regions.