Continental Facies Architecture and Hydrocarbon-Stained Shale Intervals of the Bima Formation at Yarandua, Upper Benue Trough, Northeastern Nigeria

Abstract

The Bima Formation forms the basal Cretaceous siliciclastic succession of the Upper Benue 
Trough and records early continental sedimentation associated with rift initiation. This 
study documents the facies architecture and depositional significance of Bima Formation 
outcrops at Yarandua, northeastern Nigeria, with emphasis on continental facies 
relationships and hydrocarbon-stained shale intervals. Three outcrop sections were logged 
and interpreted using lithology, texture, bed geometry, sedimentary structures, weathering 
character, nature of contacts, fracture patterns, and bituminous staining. Five recurring 
facies were identified: weathered basement, conglomerate, massive to cross-bedded 
sandstone, siltstone/mudstone, and shale. These facies are grouped into three facies 
associations: proximal braided fluvial-alluvial fan deposits, fluvial channel-bar to overbank 
deposits, and floodplain-ponded overbank/lacustrine shale deposits. The conglomeratic and 
poorly sorted sandstone units indicate high-energy traction deposition close to uplifted 
basement source areas, whereas interbedded sandstone, siltstone, mudstone, and shale 
reflect fluctuating discharge, channel migration, floodplain aggradation, and localized 
ponding. Oil-wet and dark bituminous shale surfaces in section OJ-003 suggest hydrocarbon 
impregnation, organic enrichment, or migration staining along fractures and weathered 
bedding surfaces. These features indicate that fine-grained intervals within the Bima 
Formation should not be treated only as passive floodplain deposits; locally, they may 
represent source-prone, seal, or migration-related facies within a simple continental 
petroleum-system framework. The studied successions therefore record deposition in a 
tectonically active continental rift setting dominated by braided fluvial, alluvial-fan, 
overbank, and shallow lacustrine processes. The results refine the sedimentological 
interpretation of the Bima Formation at Yarandua and highlight its relevance for reservoir 
heterogeneity and hydrocarbon prospectivity