RESERVOIR GEOLOGY OF THE TAR SAND DEPOSITS OF THE EASTERN DAHOMEY BASIN, NIGERIA: INSIGHT FROM FIELD AND PETROGRAPHIC STUDIES.

Abstract

This research deciphers the reservoir quality of the tar sand deposits occurring in the Ondo State, Nigeria sector of the eastern Dahomey Basin using field and petrographic studies approach. Twenty representative rock samples were collected across eight locations within the study area and subjected to extensive visual and petrographic studies in addition to on-field observation. Two distinct lithostratigraphic unit were identified within the study area. The fissile, fine grain basal shale sequence and the overlying sandstones sequence. The shale is believed to be the source of the tar sand that migrated upward into the overlying sand while the sand is divided into two lithofacies. The basal sandstone facies lie conformably on the shale. It is coarse grain, poorly sorted with few clay matrix and it is tar-saturated. This is overlain by fine grain, well sorted sand with good reservoir qualities. Outcrop analogues indicate that heterogeneities occur at multiple scales within the formation where geological structure such as cross bedding is visible. Mineralogical characterization indicate detrital composition composed largely of quartz, with subordinate contents of feldspars, rock fragments and minor amount of clays.