Palynology, palynofacies and hydrocarbon potential of the Cretaceous rocks of northern Egypt

Abstract

Recent hydrocarbon exploration in the Egyptian northern Western Desert
and the Gulf of Suez have revealed relatively rich hydrocarbon accumulations,
mainly of gas, and demonstrates promising future prospects. In order to improve
our understanding of these areas and to provide a biostratigraphic framework for the
poorly-dated Lower Cretaceous successions palynological analyses were carried
out on 134 ditch cutting samples from the Abu Tunis 1x drilled in the northern
Western Desert, and 78 samples from the BB80-1 borehole in the Gulf of Suez area.
Palynostratigraphic investigations focussed on the lower parts of the
borehole successions as earlier studies have largely ignored these Cretaceous
sediments. A central objective was therefore to construct a biostratigraphic scheme,
for both boreholes. Analysis of the Abu Tunis 1x samples enabled the identification
of eight palynozones largely defined by first occurrences of spores, gymnosperm
and angiosperm pollen and dinoflagellate cysts. Three new palynostratigraphically
defined age divisions are described for the lower part of the Abu Tunis 1x
succession, and a more refined biostratigraphy is made for the upper part of the
sequence. In contrast, the Gulf of Suez BB80-1 borehole samples proved
palynologically lean and provided less information for age dating. It was only
possible to define two palynozones of lower age-resolution than that for Abu Tunis
1x. Spore and pollen grains recovered from both boreholes show characteristics of
the Cretaceous Phytogeographic Provinces of northern Africa-northern South
America. Sporomorphs of the pre-Albian Dicheiropollis/Afropollis Province were
recognised from the lower part of the Abu Tunis 1x borehole and sporomorphs
characteristic of Albian-Cenomanian Elaterate Province identified from both. No
spore and pollen grains of the Senonian Palmae Province have been recognised
due to the complete marine nature of the early Santonian sediments of the Abu
Tunis 1x borehole.
In order to understand the palaeoenvironmental conditions prevailing in the
two boreholes during the deposition of the clastic and carbonate sediments,
quantitative palynological data was combined with geophysical wireline data and
cuttings lithologies. The quantitative distribution of certain terrestrial palynomorphs
with known botanical affinities and palaeoenvironmental significance have been
used as proxy indicators for identifying palaeoclimatic and palaeoceanographic
conditions in both borehole regions.
In general, the lower part of the Abu Tunis 1x succession (consisting of shale
and sandstones) was deposited in deltaic settings during a regressive cycle with
sediments of the upper Alam El Buieb Formation and the Alamein Formation
representing the late Barremian-Aptian transgression cycle, during which shallow
marine settings prevailed. Clastics of the Dahab and Kharita formations represent
another regression in marine sedimentation, where fine silts and a few shale
horizons of the latter formations were deposited in a delta channel system that
prograded through time over prodelta sediments as a response to sea level fall.
Mixed clastic and carbonate sediments of the upper Kharita and lower Bahariya
represent more distal marine deposition as a response to a second minor rise in sea
level, where a partially marine isolated, brackish lagoonal depositional system
developed that was subjected to occasional marine incursions. Integration of the
same datasets demonstrate that the upper carbonate-dominated part of the Abu
Tunis 1x succession (the upper Bahariya, Abu Roash, and Khoman B formations)
was deposited mainly in deeper marine settings interpreted as outer shallow marine,
during a major transgressive cycle. The upper part of the BB80-1 borehole also
shows this late Cretaceous marine transgression, represented by high
concentrations of phytoplankton-rich carbonate sequences. The lower part of this
Gulf of Suez sequence is of latest early Cretaceous age, and appears to have been
deposited in a continental basin, far from source vegetation, possibly in alluvial
settings, which witnessed occasional marine incursions represented by deposition of
a few organic-rich marine shale intercalations that are interpreted as shallow
marginal marine in origin. These environmental fluctuations are related to global
sea level fluctuations and global tectonic processes, such as the breakup of
Western Gondwana during the opening of the Southern Atlantic Ocean.
Investigation of the hydrocarbon potential of the Abu Tunis 1x and BB8-1
shows that the first borehole has source rock potential, with the second of no
potential due to its organic-poor lithology. The lower part of the Abu Tunis 1x
borehole represented by the Alam El Bueib Formation sediments is regarded as a
non commercial gas-prone source rock; this is indicated from visual kerogen study
and vitrinite reflectance investigations of its thermal maturation. A burial history
reconstruction for the Abu Tunis 1x borehole sequence indicates that the lower part
of the Alam El Bueib source rock entered the early stage of thermal maturation
during the Oligocene and is currently at the early mature stage. By investigating the
organic matter quality and conducting maturity analyses such as vitrinite reflectance
studies, the overlying clastics of the Dahab, Kharita and lower Bahariya, and the
carbonates of the upper Bahariya, Abu Roash and Khoman formations are shown
also to contain relatively high amounts of oil-prone organic matter, but it is immature,
and thus they are not active source rocks in the region of the Abu Tunis 1x borehole.
The BB80-1 borehole is made of a thick organic-poor, porous sandstone unit of the
Malha and lower Raha formations that are intercalated by a few organic-rich shale
horizons. This sandstone lithology is regarded as having no hydrocarbon potential.

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