GeoArabia, v. 14, no. 3, 2009, p. 199-228 Gulf PetroLink, Bahrain New insights into the Hercynian Orogeny, and their implications for the Paleozoic Hydrocarbon System in the Arabian Plate Mohammad Faqira, Martin Rademakers and AbdulKader M. Afifi ABSTRACT During the past decade, considerable improvements in the seismic imaging of the deeper Paleozoic section, along with data from new well penetrations, have significantly improved our understanding of the mid-Carboniferous deformational event. Because it occurred at the same time as the Hercynian Orogeny in Europe, North Africa and North America it has been commonly referred to by the same name in the Middle East. This was the main tectonic event during the late Paleozoic, which initiated or reactivated many of the N-trending block uplifts that underlie the major hydrocarbon accumulations in eastern Arabia. The nature of the Hercynian deformation away from these structural features was poorly understood due to inadequate seismic imaging and insufficient well control, along with the tectonic overprint of subsequent deformation events. Three Hercynian NE-trending arches are recognized in the Arabian Plate (1) the Levant Arch, which extended from Egypt to Turkey along the coast of the Mediterranean Sea, (2) the Al-Batin Arch, which extended from the Arabian Shield through Kuwait to Iran, and (3) the Oman-Hadhramaut Arch, which extended along the southeast coast of Oman and Yemen. These arches were initiated during the mid-Carboniferous Hercynian Orogeny, and persisted until they were covered unconformably by the Khuff Formation during the Late Permian. Two Hercynian basins separate these arches: the Nafud-Ma’aniya Basin in the north and Faydah- Jafurah Basin in the south. The pre-Hercynian Paleozoic section was extensively eroded over the arches, resulting in a major angular unconformity, but generally preserved within the basins. Our interpretation suggests that most of the Arabian Shield, except the western highlands along the Red Sea, is the exhumed part of the Al-Batin Arch. The Hercynian structural fabric of regional arches and basins continue in northern Africa, and in general appear to be oriented orthogonal to the old margin of the Gondwana continent. The Hercynian structure of arches and basins was partly obliterated by subsequent Mesozoic and Cenozoic tectonic events. In eastern Saudi Arabia, Qatar, and Kuwait, regional extension during the Triassic formed N-trending horsts and graben that cut across the NE-trending Hercynian mega-structures, which locally inverted them. Subsequent reactivation during the Cretaceous and Neogene resulted in additional growth of the N-trending structures. The Hercynian Arches had major impact on the Paleozoic hydrocarbon accumulations. The Silurian source rocks are generally preserved in the basins and eroded over the arches, which generally confined Silurian-sourced hydrocarbons either within the basins or along their flanks. Furthermore, the relict Hercynian paleo-topography generally confined the post-Hercynian continental clastics of the Unayzah Formation and equivalents to the Hercynian basins. These clastics contain the main Paleozoic oil and gas reservoirs, particularly along the basin margins where they overlie the sub-crop of the Silurian section with angular unconformity, thus juxtaposing reservoir and source rock. 199 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/14/3/199/4567101/faqira_199-213.pdf by guest on 27 September 2021 Faqira et al. INTRODUCTION The Hercynian Orogeny refers to the tectonic deformation that affected parts of western Europe, northwestern Africa, and eastern North America during the Carboniferous, and is attributed to the collision of Gondwana and Laurasia, which together formed the Pangea supercontinent (e.g. Ruban et al., 2007). The term “Hercynian Orogeny” was extended to northwestern Arabia by Gvirtzman and Weissbrod (1984) to describe the regional pre-Carboniferous and pre-Permian uplift and erosion, which they mapped in Palestine and adjacent areas. We use the term “Hercynian” here for the timing of widespread deformation in the mid-Carboniferous, but not to imply that the Hercynian deformation in Arabia was caused by the Hercynian collision in the north Atlantic region. In Saudi Arabia, Powers et al. (1966) recognized an angular unconformity beneath the Permian Khuff Formation across the Central Arabian Arch (Figure 1). Al-Laboun (1988) recognized that the unconformity occurs below the Unayzah Formation and extends regionally throughout the Arabian Plate based on correlation of late Carboniferous – Early Permian clastic sequences. He further attributed the large regional thickness variations in these sequences to their onlap around several arches that were uplifted and eroded during the Hercynian Orogeny. McGillivray and Husseini (1992) further established that, in addition to uplift of the Central Arabian Arch, the Hercynian Orogeny resulted in the uplift of several N-trending, fault-bounded horst blocks in eastern and central Saudi Arabia, which include the major structural trends of Ghawar, Khurais, and Hawtah (Figure 1). They recognized that this uplift was responsible for erosion of a significant amount, up to 1,100 m, of Paleozoic section along the crests of these structures. However the effects of Hercynian deformation outside these structures remained poorly understood due to sparse well control and difficulty in seismic imaging of the deep Paleozoic section. The initial model of Wender et al. (1998) therefore assumed that the Hercynian truncation of the Paleozoic section was mainly localized along the major N-trending block uplifts and more or less symmetrical across them, and was adopted by other workers (e.g. Strohmenger et al., 2003). Subsequently, Konert et al. (2001) extended the Hercynian subcrop map throughout the Arabian Plate, which revealed the major Hercynian Arches and basins described in this paper. However, it was difficult to separate the effects of Hercynian structural development from subsequent structural growth, particularly during the Triassic and Cretaceous periods. This paper provides a revised interpretation of the Hercynian deformation in Saudi Arabia based on new seismic and well data, and extends this interpretation throughout the Arabian Plate based on published data. It shows that the Hercynian Orogeny was manifested regionally by the formation of three regional NE-trending swells (arches) that are separated by large basins (Figure 2). This paper reinterprets the N-trending fault-bounded uplifts in Saudi Arabia as secondary features that were initiated or reactivated during the Hercynian Orogeny , but their main growth occurred during Triassic extension and Late Cretaceous and Neogene compression. This paper also discusses the major impact of the Hercynian deformation on hydrocarbon presence in the Paleozoic section. It demonstrates that Silurian source rocks and Permian – Carboniferous reservoirs are present only within the Hercynian basins, and largely absent over the Hercynian Arches. Since the inception of its Paleozoic non-associated gas exploration program in 1994, Saudi Aramco has acquired thousands of kilometers of high-fold (240–480) and long-offset (7.2 kilometers) 2-D seismic data, along with sparse and full 3-D seismic surveys that cover a substantial part of eastern Saudi Arabia. This new generation of seismic surveys also benefited from improvements in acquisition and processing technologies, particularly pre-stack time and depth migration, and proprietary multiple- suppression techniques, which provided much-improved imaging of the deep Paleozoic section. In addition, the drilling of numerous deep gas exploration wells provided the necessary geological control points to support the seismic interpretation (Figure 1). This data forms the basis for a new interpretation of the Hercynian deformation described in this paper. Although the Paleozoic section is relatively seismically transparent and contaminated with multiples, there are several key Paleozoic reflectors that are essential for interpretation. The base of the transgressive Permian carbonates of the Khuff Formation is a prominent reflector due to the 200 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/14/3/199/4567101/faqira_199-213.pdf by guest on 27 September 2021 New interpretation for the Hercynian arches in the Arabian Plate 35°E 40° 45° 50° 55° 60° 38°N Caspian 38° TURKEY Sea Bitlis Suture N 0 400 CYPRUS km 34° SYRIA EURASIAN 34° PLATE Hercynian well Mediteranian penetration Sea IRAQ Hail Arch JORDAN 30° 30° Dead Sea transform ARABIAN PLATE KUWAIT IRAN Dibdibah Summan Trough Platform Arabian Gulf 26° 26° BAHRAIN QATAR En Nala Anticline Gulf of Oman EGYPT SAUDI ARABIA Central Arabian Arch UAE 22° 22° OMAN Arabian Shield Qatar Arch Arch Huquf - Haushi Red Sea Rub’ Al-Khali Basin 18° SUDAN 18° Hadhramout Arch AFRICAN PLATE ERITREA YEMEN 14° Arabian Sea 14° Gulf of Aden ETHIOPIA 35° 40° 45° 50° 55° 60° Figure 1: Map of the Arabian Plate showing plate boundaries, major structural elements and well data used in the Hercynian sub-crop revision. sharp transition from carbonates to clastics. This reflector further approximates the location of the Hercynian unconformity, particularly where post-Hercynian clastics of the Unayzah Formation are thin or absent. The Hercynian unconformity is difficult to map as it is sandwiched between seismically transparent sandstones, and its recognition in wells is mostly dependent upon palynological data. The