Shedding Further Light on Upper Cretaceous – Neogene Subsurface Lithostratigraphy of Southwestern Iraq

Abstract

Exploratory activities carried out by oil companies in the latter half of the past
century proved the existence of voluminous reserve of oil and gas in the
southwestern area of Iraq. In view of this, it seemed more than prudent to add a new
knowledge to that currently existing about the subsurface lithostratigraphy of this
area. As a first step in fulfilling this mission, this paper will attempt to do so by
covering the time interval from the Upper Cretaceous to the Neogene. In turn, this
effort had entailed both the description of about 4707 metres of fully recovered
cores, plus the subjection of more than 4000 samples to existing petrologic analyses.
Findings worth mentioning does include the observation that with the exception of
the Paleogene sequence, the Upper Cretaceous and Neogene sequences are no
different from those found in other parts of southern Iraq. For instance, the Upper
Cretaceous sequence is found to be consisted of the common Hartha, Qurna, and
Tayarat Formations. The same could be said about the Neogene sequence which is
found to be again made of the common Ghar, Euphrates, Fatha, and Zahra
Formations. In contrast, the Paleocene – Lower Eocene sequence lithology is found
a bit different from those commonly encountered in other parts of southern Iraq.
This difference emanates from the intense sulphidzation as manifested in the
common occurrence of anhydrite, gypsum, and vanished evaporite being observed
through the lower Paleogene sediments of south west, Iraq. Because this
phenomena exhibits strong vertical as well as lateral variation, this sequence is
divided into an intensely evaporitic portion, which is introduced in this article under
the name of the Jerishan Group (Paleocene - Lower Eocene), inclusive of Umm Er
Radhuma, Rus, Jil/Rus, and Jil Formations. The latter formation is recently
introduced stratigraphic rock unit representing the non-evaporitic equivalent for the
Rus Formation. The less evaporitic portion (the Dammam Formation) of the
Paleogene was found to be the same as the ones found in other parts of the south
Iraq, Kuwait and Saudi Arabia.