Dergiler / EURASIAN JOURNAL OF SOIL SCIENCE / 2017 / Cilt: 6 - Sayı: 3

Depositional environments signatures, maturity and source weathering of Niger Delta sediments from an oil well in southeastern Delta State, Nigeria

Depositional environments signatures, maturity and source weathering of Niger Delta sediments from an oil well in southeastern Delta State, Nigeria

Akinade Shadrach OLATUNJI, Samuel Oluwaseun ONI

Sayfa
259–274
DOI
—

Abstract

Attempts have been made to classify the sediment on their degree of maturity.Compositional maturity is a reflection of intensity of weathering and a function oflabile grains, unstable/stable rock fragments and stable quartz arenites. The main aimof this study is to investigate maturity and area of deposition and attempt to shed lighton source area paleo-weathering conditions. Twenty one samples of shales andsandstones units were collected from a depth precisely between 1160 to 11,480m at awell in western Niger Delta, grinded, pulverized and sieved with less than 75μm. About10g was packed and sent to Acme analytical Laboratory LTD., Vancouver, Canada.From the results, various plots and indexes inferring maturity and area of depositionwere utilized. Using the A-K-F ternary plots of Englund and Jørgensen (1973), thedepositional environment is transition zone. The silicate weathering indexes CIA, CIWand PIA values ranges from 45-65, on average indicates low to moderate weathering inthe source area with extreme weathering of some sand fraction. Various calculatedvalues of the weathering indices: Chemical Index of Alteration (CIA), Plagioclase Indexof Alteration (PIA), Chemical Index of Weathering (CIW) and scatter plots offormulated ratios of Al/Na, K/Na, and Rb/K vs chemical index of alteration (CIA) wereplotted. The moderate values below average suggest low to moderate weatheringconditions in the source area or during transportation. This also inferred theirrecycling processes are insignificant. The clay content is low and feldspars areaveragely high implying immaturity. The calculated ZTR index for the sand rangesfrom 36.4-75.0 from with an average mode of 55.5% implying almost all containmineralogically immature sediments. The calculated Zircon- Tourmaline-Rutile (ZTR)index shows that majority of the sample depths have >43% ZTR index but below 75%which corresponds to generally immature sediments.