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
Highlights
The concept of interpreting rocks in terms of modern processes dates back to the 18th and 19th centuries
Assuming that the laws that govern physical and chemical processes have not changed through time, detailed measurements of sedimentary rocks can be used to make estimates of the physical, chemical and biological conditions that existed at the time of sedimentation
A depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and the type of rock that forms after lithification
Summary
Description of study area: Well XY-62 was drilled within OML 30 located in the Greater Ughelli depobelt of the Niger Delta. A third cycle of deposition began in the Eocene and continues to present day It is during this third cycle that the Modern Delta was formed (Short and Stauble, 1967) recognized three sub-surface stratigraphic units in the modern Niger Delta – Benin, Agbada and Akata formations. Akata Formation: This is at the base of the delta It is of marine origin and is composed of thick shale sequence (potential source rock), turbidite sands (potential reservoir in deep-water) and minor amounts of clay and silts. The approximate range of thickness is from 0-6000meters and the formation crops out subsea in the outer delta but is not seen onshore This formation formed during lowstands when terrestrial organic matter and clays were transported to deep water areas characterized by low energy conditions and oxygen deficiency (Michele et al, 1990). Data entry and storage was done using Apple Macintosh power book switched on to core description option
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