Understanding the Internal Variability of Mudstones: Dark Mudstone of the Campano- Maastrichtian Mamu Formation as Case Study

Abstract

The objective of this research work was to investigate approximately 6m of Campano-Maastrichtian dark mudstone outcropping at Uzebba, Benin Flank, SW Anambra Basin using sedimentological, geochemical, mineralogical, and palynofacies techniques. Our findings show that the mudstone succession can be subdivided into 3-broad units from bottom to top. Unit 1 is characterized by weak to moderate bioturbation, high quartz content (≥50 %) as well as grained dominated microfabric with planar to wavy laminations. Unit 2 has the lowest particle size, zero to mild bioturbation, low quartz content as well as matrix dominated microfabric with thin to indistinct laminations, which are mostly planar. Unit 3 show intermediate particle size, weak to moderate bioturbation, moderate quartz content as well as grain dominated microfabric with much thicker laminations than units 1and 2 that vary from wavy to curved. Geochemical and palynofacies proxies for detrital influx and paleoproductivity are congruent. They show that units 1 and 3 received significant detrital contribution as well as high terrestrial paleoproductivity under low relative sea level, which promoted the development of good petrophysical and geomechanical properties suitable for gas shale development. Conversely, Unit 2 received less  significant detrital contribution as well as increased marine paleoproductivity under higher relative sea level.

Keywords: Detrital influx, paleoproductivity, unconventional reservoirs, Benin flank