Sedimentology of a Basin-Margin Lacustrine Fan-Delta Depositional System in the Campano Maastritchian Bida Formation, Bida Basin, Nigeria

Abstract

Basin margin sedimentary deposits are increasingly recognized as holding significant potential 
for both hydrocarbon source and reservoir rocks, depending largely on the prevailing 
paleogeographic and depositional conditions. Despite their importance, the depositional 
environments of sedimentary successions along the eastern margin of the Bida Basin, 
particularly around Lapai, have not been systematically documented. This study addresses 
that gap by examining part of the eastern margin basin fill through detailed field 
investigations and facies analysis of relatively well-exposed outcrops. The analysis led to the 
recognition of seven lithofacies, which were further grouped into three genetically related 
facies associations: (i) offshore lacustrine, (ii) subaqueous fan-delta, and (iii) subaerial fan 
systems. The offshore lacustrine association is dominated by mudstone facies (Fm), massive 
sandstone (Sm), and normally graded gravelly sandstone (Sn). These facies are interpreted 
as the products of suspension settling, hyperconcentrated density flows, and turbidity 
currents, respectively. The subaqueous fan-delta association is characterized by the 
intercalation of debris-flow deposits (Facies Gmu, Sm, and Sn) with fine-grained mudstone 
and siltstone intervals (Fm), reflecting the interplay between mass-flow processes and 
background lacustrine sedimentation. In contrast, the subaerial fan association is 
represented predominantly by ungraded, matrix-supported conglomerates (Gmu) and pebbly 
to massive sandstones (Sm), with occasional trough cross-bedded conglomerates (Gt) and 
minor mudstone intercalations (Fm). The observed vertical stacking pattern of these 
associations indicates a progressive progradation of alluvial fan systems into the lacustrine 
environment, reflecting a dynamic interplay between tectonic activity, sediment supply, and 
lake-level fluctuations along the basin margin. From a petroleum system perspective, the 
subaqueous fan-delta association is interpreted as the most prospective target for 
conventional hydrocarbon reservoirs, owing to its textural heterogeneity and potential for 
sand-rich channelized deposits. Meanwhile, the offshore lacustrine facies association holds 
significant potential for unconventional hydrocarbon resources, especially as organic-rich 
fine-grained intervals could act as both source rocks and tight reservoirs. Overall, this work 
underscores the importance of integrating detailed outcrop-based facies analysis with 
paleoenvironmental reconstructions to generate robust conceptual paleogeographic models. 
Such models are invaluable in assessing the petroleum potential of basin-margin successions, 
particularly in underexplored frontier basins like the Bida Basin.