Diagenesis and reservoir quality of cretaceous sandstones of Nkporo formation (Campanian) southeastern Benue trough, Nigeria

Abstract

The Afikpo Basin of southeastern Benue trough represents an elongate NE-SW oriented depocenter located north-west of the Oban Massif. During Campanian-Maastrichtain times, two parallel facies belts of basal sandstones of the Nkporo formation were developed in a wide range of alluvial fan complex, tidal channel and deltaic environments. Detailed petrological study of the unit by thin section, scanning electron microscopy and isotopic analysis reveals that early diagenetic effects include the precipitation of siderite, and illite-smectite rims. Deep burial effects include physico-chemical compaction and the formation of quartz overgrowths, siderite, illite/illite-smectite and ankerite. Involved fluids were in part connate meteoric water derived from compaction of the underlying freshwater beds. Important post burial effects, controlled by deep meteoric influx from the surface, were ankerite and labile grain dissolution and formation of kaolinite, calcite and dawsonite, the formation of dawsonite reflecting eventual stagnation of the aquifer. Precipitation of authigenic quartz overgrowth and crystallization of cementing material are some of the diagenetic changes responsible for porosity reduction. On the other hand, dissolution and replacement of framework minerals enhance the porosity as well as permeability. Reservoir quality varies from marginal to intermediate in the west to poor in the east, with predictable trends being directly linked to depositional environment and diagenesis. Key words: Benue trough, Nkporo formation, fan delta complex, fan delta fan, cretaceous, clay minerals, reservoir quality, diagenetic history and alteration.