AAPG Bulletin, V. 82 (1998), No. 6 (June 1998), P. 1191-1206.

Burial Metamorphism (Thermal Maturation) in Cretaceous Sediments of the Southern Benue Trough and Anambra Basin, Nigeria¹

Samuel O. Akande² and Bernd D. Erdtmann³

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received July 15, 1996; revised manuscript received May 12, 1997; final acceptance January 9, 1998.
²Department of Geology, University of Ilorin, P.M.B. 1515, Ilorin, Nigeria.
³Institut für Geologie und Paläontologie, Technische Universität Berlin, Ernst Reuter Platz 1, 10587 Berlin, Federal Republic of Germany.

This paper is a contribution to an ongoing research project on the stratigraphy and burial history of the Nigerian Benue trough with financial support from the German Volkswagen Foundation and a University of Ilorin Senate Research grant. We thank Shell Petroleum Development Company and the NNPC for permission to use the samples from the oil exploratory wells. Chevron Nigeria Limited is thanked for the logistics support to Akande to present this paper at the special poster session on the petroleum geology of Nigeria at the 1995 AAPG International Conference in Nice, France. 

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 (R_om) 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.