Sedimentology, palaeoenvironment and structural interpretation of the Cretaceous SW Anambra Basin, Nigeria

Abstract

The Inland Anambra Basin of Nigeria, formed on the western segment of the Lower Benue Trough, represents the sag phase of the Trough. Its basin infill comprises shallow to marginal marine and freshwater sedimentary clastic deposits of the Cretaceous to Paleogene age. The basin forms part of the larger rift systems of sedimentary basins of the West and Central African Rift Systems (WCARS), a geological chain of faulted and rifted structures that are genetically related. An important part of this study is to investigate the paleosedimentary history of the basin, paleoclimatic implications, paleowater depth, subsurface structures, formation of source beds, petroleum potentials, and evidence for the genetic link between the Anambra Basin and the Benue Trough and other WCARS basins. In order to address this scientific contribution, an integration of multi-proxy tools involving sedimentary geochemistry, mineralogy, organic petrography, rock evaluation, and satellite gravity survey was used in this thesis to provide robust data and new insights. Satellite-derived gravity data based on the total horizontal derivative technique records anomalous Bouguer values ranging from -58 and +28 mGal in generic directions of NE-SW and NW-SE, and the CRUST 1.0 model reports a Moho depth of 37 km. Seven high-anomaly zones (HR1-HR7) and four low-anomaly areas were identified in the research area (HL1-HL4). Additionally, the sedimentary basin depth ranged from 3.5 to 5.0 km, indicating adequate depths for source rock formation and hydrocarbon accumulation. Furthermore, all Cretaceous and Paleogene Formations were influenced by the primary structural trends that dominated the basin's formation history. The transgressive units of the Nkporo, Mamu and Imo Formations in the western segment of the basin from subsurface data revealed dominantly warm, humid tropical paleoclimates with high rainfall in the Late Cretaceous-Paleogene epoch, which corresponds to geologic events in WCARS Cretaceous basins in West Africa. Thus, the geologic record in West Africa indicates intense precipitation and high temperatures during the Cretaceous. The paleoenvironment was reconstructed as predominantly brackish to shallow-marine with some freshwater incursions. Deposition under an oxic environment is detected using a combination of paleoredox markers. On the other hand, anoxic conditions only occurred infrequently in the study area. The presence of wavy laminations in the mudrocks reveals receding seawater and strong paleohydrodynamic conditions. In addition, poor to medium primary productivity was recorded due to terrestrial clastic

influx and hydrodynamic settings. Ultimately, the southwestern Anambra Basin is characterized by shallow sea deposits influenced by strong hydroenergy circulation. Furthermore, a high-resolution petroleum investigation suggests that the source rocks have a high potential for conventional gaseous hydrocarbons. An integrated approach based on palynofacies, maceral data, and rock eval interpretations supports type III kerogen in a marginal thermal stress stage. The results of this study contrasted favorably with other Cretaceous WCARS basins and showed that type III kerogen predominated. The basin's northern portion shows good shale gas potential. Finally, shallow marine to marginal marine transgressive, maximum flooding surface, and highstand deposits were detected in this study. The Anambra Basin and the other WCARS basins are genetically linked, as evidenced by similarities in paleoweathering, paleoclimate, paleowater depth, paleosalinity, paleoredox conditions, palynofacies assemblage, and structural trends, despite differences in sediment origin, paleogeographic positions, and the exposure of some WCARS basins to thermal fracture and crustal upheaval.