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Late Cenomanian-Early Turonian Black Shales (Source rocks) in North Africa and Arabia: Regional Distribution, Structural Evolution and Depositional Model

Soua, Mohamed *1
(1) Studies & Exploration, Entreprise Tunisienne d'Activités Pétrolières (ETAP), Charguia 2, Tunisia.

1. Introduction

The Late Cenomanian-Early Turonian time is an important point for the geological history of the Tethyan margin as well as Arabia. At that time the whole study area (Figure 1) has been covered by an important transgression leading to a very large marine incursion.

This period is characterized by the development of extensional structures with tilted blocks and grabens, bounded listric normal faults (Figure 2). Subsidence during C/T time led to the accumulation of a thick sequence of laminated black shales and marls. Sharp variations in thickness and facies indicates that basin subsidence was driven by regional tectonic movements along NW-SE, NE-SW and East-West trending normal faults issued from Tethyan rifting of the Upper Jurassic-Lower Cretaceous as well as the Early Sirte Rifting for the North African and western Arabian sides. The reduction of emerged areas since the Cenomanian inferred from the generated palaeogeographic map is demonstrated. The overall evolution is also maintained by halokinesis of Triassic salt creating subsident rim-synclines, where organic-rich facies prevail.

This facies has covered also the northern platforms of the Sahara Domain. At the same time a subsident and deep marine basin developed along the Northeastern and in the Eastern side of the Arabian Domain allowing the deposition of deep marine black shale or restricted shallow carbonate, which is one of the most important hydrocarbon source rock of the North Africa including Libya & Egypt and the Arabian Domain including Lebanon, Jordan, Palestine, Southeastern Turkey, UAE, Kuait and Qatar.

2. Regional Setting

In the North African Domain, this period spanned the deposition of the Bahloul (Tunisia and Algeria), Alalgah, Yifran, Malagh (Libya) and Abu Roach (Eastern Egypt) and their equivalents. In the Arabian side, this time period spanned the deposition of the Mishrif, Ahmadi, and Rumaila (Arabian Peninsula), Natih (Oman), and Derdere (SE Turkey) formations, Negba (Palestine), Deir Hanna (Southern Lebanon) and their regional equivalents (see Figure 1). The sediments were deposited on platforms and within intrashelf basins on the passive margin of the Arabian Plate.

3. Paleofacies

Haq et al. (1988) suggested a maximum global coastal onlap for this time interval and, in Near the Saharan domain of the North African margin as well as in Arabia, the sedimentary record indicates the presence of widespread shallow-marine platform carbonates (Zebag and Kerker Formations) sometimes with repetitive organic-rich silty shales and shales interbedding within limestones sequences (Rumaila, Ahmadi, Natih and Derdere formations) indicate a reducing conditions while deposition and the record of the Oceanic Anoxic Event (OAE-2) in the Arabia domain. Sharland et al. (2001) defined a huge hiatus at the end of Cenomanian in the Arabia, the mid-Turonian unconformity. For this reason, in this study Paleofacies map of the whole area has been generated.

Figure 1. Cenomanian-Turonian Source rock and black shales distribution through the North African and Arabian domains


4. Cenomanian-Turonian Paleogeographic map of the North Africa and Arabia domains

Combination of C/T data identified a number of major features characterizing a key period of geological history of North Africa and Arabia domains. During the C/T times, Tunisia, Algeria and Libya and eastern Egypt situated at the southern Tethyan margin, were characterized by a general slope almost dipping to the north, promoting variable facies ranging from littoral, shelfal and lagoonal to bathyal. The general facies trend was related to a complex tectonic and halokinetic paleo-elevation resulting in variations of the C/T facies.

Organic-rich C/T sediments occur in many places in northern African margin and Western, North western and Eastern Arabia. The regional distribution of the C/T facies including the distinction of an organic-rich and an organic-lean anoxic facies is shown. Well and outcrop data confirm these organic-rich provinces, and the exact boundaries have been clearly traced through this study.

5. Structural cross-sections

Correlations (Figure 2) and structural cross-sections have been generated for this study (Oman - Algeria, Turkey-Algeria; Saudia Arabia-Algeria,..) showing the regional tectonic evolution of the Arabia and North African margins, and showing the structural styles within the panafrican shields. These regional tectonic evolution have been used in order to define the structural context of the paleogeography of the organic-rich facies distribution and deposition within the North Africa and Arabia domains using back-strepping analysis.

Figure 2. Correlationsof the main Cenomanian-Turonian stratigraphic units in North African (a) and Arabian (b) domains.



AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain