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

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

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2009

Author dc:creator

• Deaf, Amr Said