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

Year 2017, Volume: 6 Issue: 3, 259 - 274, 01.07.2017

Samuel Oni Akinade Olatunji

https://doi.org/10.18393/ejss.297245

Cited By: 5

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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