Depositional facies analysis using core samples from greater Ughelli Depobelt, Niger Delta Basin Nigeria

Abstract

Sedimentological analysis of core samples from Well XY-62 located within the greater Ughelli depobelt of the Niger Delta basin was carried out. Some physical deterioration had set in and some gaps had reduced the total core length available to 316 .90ft. The cores contain three sandstone and one mudrock lithofacies: Fine-grained mmscale laminated sandstone (Sl.f) 40.83%, medium to coarse-grained cm-scale cross-bedded sandstone (Sx.m) 35.34%, pebbly Sandstone (PS) 4.39%, bedded Mudstone (Mb) 19.44%. The sandstone lithofacies differ from one another mainly in grain size, as appears obvious, otherwise they are largely texturally, compositionally, diagenetically and structurally broadly similar. They are generally clean (zero to 20% clay content), moderately well to well sorted, pebbly, and texturally mature. Quartz appears to be the sole framework component, making the sandstones mineralogically mature i.e. quartz arenites and quartz wackes. Being generally weakly consolidated and scantily cemented, the sandstones are all in the early stages of diagenesis, an attribute that has had a desirable effect on their reservoir quality. The bedded mudstone lithofacies contains laminated claystones and fissile shales which are all thought to have compartmentalized the sand reservoirs rather than sealed them. Depositional environments have been inferred to be the foreshore setting, with lagoon subenvironments that favoured mudrock accumulation landward of a barrier bar complex. The overall motif read from both the strip log and the wireline logs indicate neither clear progradation nor unequivocal retrogradation of the lithofacies. It is therefore concluded that the XY- 62 cores were cut from an aggradational depositional setting.

Keywords: Cores, Sediment analysis, Depositional environments, Niger Delta