Abstract
This study was conducted on black shale in Sargelu Formation
(Mid-Jurassic) in three selected areas in Kurdistan Region, N and NE Iraq,
namely Barsarin, Sargelu sections and Sheikhan-4 well. Samples of black
shale were collected from the above locations (23 from Barsarin, 26 from
Sargelu and 15 from Sheikhan-4) and were analyzed using X-Ray
Fluorescence (XRF) at Oldenburg University, Germany. The geochemical
analyses showed that black shale is distributed in these three locations in
different intensities. Black shale in this study is mainly composed of
silicates, clay minerals, carbonates and organic matter. For this reason here
black shale is defined as dark-grey and black coloured, laminated, organiccarbon rich calcareous sediment with TOC 0.4 wt%.
Ternary diagrams showed intensive weathering in the study area with
the presence of biogenic silica which was evident in Barsarin section. The
following elements Si,Ti, Al, Fe, Mn, Mg, K, As, Ce, Co, Cr, Cu, Rb and Th
showed their association with terrigneous fraction, Ca, Mg, Mn, Sr with
carbonates, Ba, Sr, U, Y with organic matter, As, Mo, Ni, Sr, U, Ce, P, Th,
Yand Zn, with apatite and Co, Cr, Cu, Mo, V, Ni, Sr, U, Y, Pb, Y, Zn with
sulfide phases. Element/Al ratios of some redox-sensitive elements such as
As, Co, Cr, Cu, Mo, Pb, U, V and Zn showed the following order of
enrichment Barsarin> Sargelu> Sheikhan-4.
Factor analysis showed that there are six factors which have influenced
the black shale geochemistry. The first two factors (terrigneous 28.39%) and
(redox-sensitive elements 20.47%) of the total variance 84.135% were the
most effective.
The depositional environment of black shale in the study area was
deduced using elemental indicators like Ni/Co, V/Cr, V/(V+Ni) and Th/U,
which denoted oxic-dysoxic to suboxic-anoxic in moving from Sheikhan-4,
to Sargelu and to Barsarin. Also, the study has concluded that the type of
black shale is marine depending on the deep seated and reduced environment
of Sargelu Formation. In addition, the study showed that the basin of
deposition of black shale was the deepest in Barsarin section, less deep in
Sargelu section and lastly shallowest in Sheikhan-4 drill site. This study
showed that there were periods of sea level changes which were greater in
Sheikhan-4 followed by Sargelu and lastly Barsarin. Black shales are
metalliferous in Cu, Mo, Ni, Pb, Sr, V and Zn in Barsarin, in Cr, Cu, Sr, Th
and Zn in Sargelu and in Cr, Sr, Th and Zr in Sheikhan-4.