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

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 of labile grains, unstable/stable rock fragments and stable quartz arenites. The main aim of this study is to investigate maturity and area of deposition and attempt to shed light on source area paleo-weathering conditions. Twenty one samples of shales and sandstones units were collected from a depth precisely between 1160 to 11,480m at a well in western Niger Delta, grinded, pulverized and sieved with less than 75μm. About 10g was packed and sent to Acme analytical Laboratory LTD., Vancouver, Canada. From the results, various plots and indexes inferring maturity and area of deposition were utilized. Using the A-K-F ternary plots of Englund and Jørgensen (1973), the depositional environment is transition zone. The silicate weathering indexes CIA, CIW and PIA values ranges from 45-65, on average indicates low to moderate weathering in the source area with extreme weathering of some sand fraction. Various calculated values of the weathering indices: Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), Chemical Index of Weathering (CIW) and scatter plots of formulated ratios of Al/Na, K/Na, and Rb/K vs chemical index of alteration (CIA) were plotted. The moderate values below average suggest low to moderate weathering conditions in the source area or during transportation. This also inferred their recycling processes are insignificant. The clay content is low and feldspars are averagely high implying immaturity. The calculated ZTR index for the sand ranges from 36.4-75.0 from with an average mode of 55.5% implying almost all contain mineralogically 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.

Keywords

• PIA
• CIW
• CIA
• AKF
• ZTR index
• weathering index
• Niger delta
• immature sediments
• depositional environments

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