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.

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 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 second reflector is at the base of transgressive Silurian shales of the Qusaiba Member of the Qalibah Formation, which disconformably overlie Ordovician clastics. The third reflector is the Middle Cambrian Burj Limestone, which is present in the northern and eastern part of the Arabian Peninsula (Al-Hajri and Owens, 2000), but was not deposited in central and southern Arabia (Konert et al., 2001). The fourth reflector is at the base of transgressive Cambrian sandstones above the dense crystalline basement and infra-Cambrian sediments, but its amplitude decreases with increasing depth due to diminishing impedance contrast.

GEOLOGIC SETTING

The Arabian Plate was part of the Proterozoic Gondwana Supercontinent, which was consolidated during the late Proterozoic into a crystalline basement by a process of terrane accretion (Stoesser and Camp, 1985). The Gondwana continental margin is thought to have extended during the Paleozoic from Central Iran into Turkey (Ruban et al., 2007). Localized Neoproterozoic (Ediacaran) and Lower Cambrian salt basins underlie the Paleozoic section within Interior Oman, the Arabian Gulf, Rub’ Al-Khali Desert and eastern Saudi Arabia, but elsewhere Lower Cambrian sandstones rest unconformably over the peneplaned igneous/metamorphic basement.

The Middle and Upper Cambrian to Devonian section consists mainly of marine sandstone, shale, and minor carbonates that were spread by marine transgressions over a broad stable platform exceeding 1,000 km in width. The stability of the platform was interrupted during the mid-Carboniferous by the Hercynian Orogeny, which caused major uplift and erosion, and was followed by continental environments of deposition during the late Carboniferous – Early Permian interval (Konert et al., 2001). This was followed during the mid-Permian by rifting and opening of the Neo-Tethys Ocean along the Zagros Suture, and a marine transgression that deposited a thick sequence of carbonates and evaporites of the Khuff Formation along the newly formed Neo-Tethyan margin.

Figure 3 summarizes the distribution of Paleozoic stratigraphy across the Arabian Plate from Syria to Oman based on published data (Best et al., 1993; Aqrawi, 1998; Wender et al., 1998; Brew et al., 1999; Al-Husseini, 2004; Svendsen, 2004; Al-Hadidy, 2007; Konert et al., 2001, and Mohammad, 2006). It also shows the extensive erosion of the Cambrian – Devonian rocks across the three Hercynian arches and their preservation within the intervening basins. This is somewhat mirrored by the distribution of syn-(?) and post-Hercynian upper Carboniferous to Lower Permian clastics, which were deposited within the basins and onlap the arches. Previous studies (McGillivray and Husseini, 1992; Konert et al., 2001) have shown that thickness and facies variations in the Cambrian – Devonian formations throughout most of the Arabian Plate do not indicate the existence of the arches prior to the Carboniferous.

Previous papers discussed the impact of the tectonic events throughout geologic history on the Paleozoic hydrocarbon potential in the Arabian Plate (Abu-Ali et al., 1991, 1999, 2001 and 2005; McGillivray and Al Husseini, 1992; Cole et al., 1994; Wender et al., 1998; Jones and Stump, 1999; Konert et al., 2001; Sharland et al., 2001; Boote et al 1998; and Traut et al., 1998). The Paleozoic oil and gas accumulations extend from Algeria to the west to Iran, and from Turkey to the north to Oman (Mahmoud et al., 1992). The common element between these accumulations is that they were mainly sourced from Lower Silurian hot shale, which extends throughout North Africa and the Middle East (Husseini, 1991) except Oman where most of the hydrocarbons were sourced from Neoproterozoic and Cambrian source rocks (Terken et al., 2001).

POST-HERCYNIAN STRUCTURES IN SAUDI ARABIA

Triassic Extension

It is important to isolate the effects of post-Hercynian deformation, which overprinted the earlier Hercynian structures. Figure 4 shows a regional isopach map of the Triassic interval extending from the middle of the Jilh Formation to the base of the Sudair Formation. Also shown are the inferred major Triassic faults, which cut the basement and lower Paleozoic section, but did not completely propagate upsection, and are usually reflected in the Permian – Lower Triassic succession (Khuff Formation) by draping over high and low blocks. It is evident that the Triassic was a period of major faulting along north-south trends, resulting in several horst trends, which include the Ghawar-Fadhili-Berri trend (El Nala Anticline), the Khurais-Burgan trend, the Summan Platform and the Qatar Arch. These trends are the foundations of the major anticlines that contain the major oil and gas accumulations. The Dibdibah Trough and the eastern Rub’ Al-Khali basin are sites of anomalously thick Triassic sediments, indicating that they underwent major subsidence. For example, deep well H-2, which was drilled in the southern part of Dibdibah Trough (Figure 4) encountered an anomalously thick section of Triassic carbonates and shales that were 410 m thicker than expected. Regionally, Sharland et al. (2001) calculated that as much as 3,000 m of sediments occurs within the Triassic troughs from data in van Bellen et al. (1959), Mitchell-Thome (1960), and Sadooni (1995).

The lack of any indication of Triassic erosion over the horst trends also suggests that they were not uplifted, but are mainly residual highs between the adjacent grabens. This regional pattern of enechelon N-trending horsts-and-grabens indicates diffuse extension in an east-west direction. The Triassic extension in the Arabian Plate following the opening of the Neo-Tethys Ocean, which started earlier in the Middle Permian along the Zagros Suture.

Deep wells drilled over highs within the Triassic horst blocks, such as Khurais and the Summan platform, generally penetrated basement or Cambrian – Ordovician formations unconformably beneath the Permian Khuff Formation. This indicates that they are reactivated Hercynian block uplifts, over which deep erosion had occurred during the Carboniferous. It was therefore assumed that the adjacent Triassic grabens would contain a more complete Paleozoic section. However, the recent well H-2 in the Dibdibah Trough (Figure 4) penetrated Proterozoic basement unconformably beneath the Khuff Formation, which indicates that the Triassic Dibdibah Trough is an inverted Hercynian high. In fact, most of the area of Figure 4 was part of the Hercynian Al-Batin Arch.

Cretaceous and Neogene Compression

The north-south horst trends were reactivated during the Late Cretaceous First Alpine Orogeny (Loosveld et al., 1996) and underwent major uplift, which is manifested by the substantial erosion beneath the pre-Aruma unconformity (Wender et al., 1998), and on the flanks by thickening of the Upper Cretaceous section (Figure 5). The First Alpine Orogeny in Arabia was due to regional compression, which resulted from the closing of the Neo-Tethys Ocean, and the obduction of ophiolites along the plate margin. Following relative quiescence during the Paleogene, the north-south trends were reactivated again during the Neogene Second Alpine Orogeny, resulting in additional anticlinal growth and erosion beneath the pre-Neogene unconformity (Wender et al., 1998).

THE HERCYNIAN ARCHES

The new seismic and well data were used to revise the regional sub-crop map beneath the Hercynian unconformity (Figure 2). The new interpretation suggests that the Hercynian was not dominated by faulting, as previously thought, but by the upwarping of regional NE-trending highlands that are several hundred kilometers in width. These highlands were areas where the pre-Carboniferous section was extensively eroded during the Early Carboniferous. The arches were also areas where syn- to post Hercynian clastic sediments of late Carboniferous – Early Permian age are thin or absent, indicating that they persisted as highlands until the Middle Permian, when they were unconformably covered by transgressive clastics, carbonates and evaporites of the Khuff Formation (Figure 3).

From the sub-crop pattern beneath the Hercynian unconformity (Figure 2), we interpret the presence of three major Hercynian arches in the Arabian Plate, which are here named the Levant Arch, the Al-Batin Arch and the Oman-Hadhramaut Arch. These arches are separated by two large basins in which the pre-Hercynian section (Cambrian – Devonian) is generally preserved, and further overlain by Carboniferous to Lower Permian clastics, which include the Unayzah in Saudi Arabia, the Al-Khlata and Gharif in Oman, the Markada in Syria and the Ga’ara in Iraq. These Hercynian basins are named the Nafud-Ma’aniya and Faydah-Jafurah basins.

Al-Batin Arch

Al-Batin Arch extends northeast from the Arabian Shield, and is bounded from the north and south respectively by the Nafud-Ma’aniya and Faydah-Jafurah basins. The presence of this Arch is illustrated by interpreted seismic sections oriented across its northern and southern flanks (Figures 6 to 12).

The seismic sections selected for illustration are generally well-constrained by well ties. The seismic lines in Figures 6 to 9 are located in various orientations across the southern flank of Al-Batin Arch, and they illustrate the progressive angular truncation of the Cambrian – Devonian section beneath the Hercynian unconformity towards the axis of the Arch. Similar truncation across the northern flank of the Arch is illustrated in Figures 10 and 11. To illustrate the pre-Hercynian growth, these seismic sections were flattened along the base of the Permian Khuff carbonates, which is approximately parallel to the Hercynian unconformity. The apparent lack of faults along both flanks of the Al-Batin Arch indicates that the deformation was due to regional upwarping of the basement. The timing of initiation of Al-Batin Arch is constrained by the erosional truncation of the Upper Devonian Jubah Formation. Knox et al. (2007, in their Figure 11) showed a northward transport direction for the Cambrian to Lower Carboniferous sandstones succession, whereas the Upper Carboniferous – Permian sandstones indicates northeastward direction. The northeastward sediment transport direction of the Carboniferous – Permian sandstones was initially documented by Evans et al. (1991) at Jabal Khurb al Ahmar. The change of the transport direction during the late Carboniferous may indicate that the Al-Batin Arch developed during the mid-Carboniferous time. The Arch persisted as a positive topographic feature throughout the Carboniferous and Early Permian, until it was covered by the Upper Permian transgression of the Khuff Formation.

The NE-trending Al-Batin Arch, as defined in this paper, is different from the E-trending Central Arabian Arch in both age and location (Figures 1 and 2). The Al-Batin Arch is a 200–400 km broad Carboniferous uplift that used to extend northeast from the Arabian Shield towards the northern part of the Arabian Gulf and beyond. The Central Arabian Arch is a Tertiary feature that extends from the eastern bulge of the Arabian Shield. This is illustrated in Figure 12, which is a north-south seismic traverse that shows how the inverted Carboniferous Al-Batin Arch is located north of the Tertiary Central Arabian Arch.

Figure 13 is a schematic cross section across Al-Batin Arch in central Arabia, restored to its configuration at the end of the Paleozoic. It illustrates the deep Hercynian erosion that removed between 1,500 to 2,500 m of Paleozoic section from its c. 200 km wide crest, and the onlap of syn- and post-Hercynian continental clastics of the Unayzah Formation along its flanks.

The Levant Arch

The Levant Arch is defined by the widespread erosion of pre-Hercynian section along a north-northeast trend that extends from eastern Egypt to Turkey along the Levant area (Figure 2). The Levant Arch is bounded from the east by the Nafud-Ma’aniya basin and from the west by the Hercynian Kufrah Basin in Libya (Boote et al., 1998). Figure 14 is a schematic cross section showing the inferred Hercynian truncation of the Cambrian – Devonian section across the eastern flank of the Levant Arch (Best et al., 1993; Konert et al., 2001; Gvirtzman and Weissbrod, 1984; Aqrawi, 1998; Al-Hadidy, 2007; and Mohammad, 2006).

Gvirtzman and Weissbrod (1984) initially recognized the existence of this arch based on published well data, and they called it the Hercynian geanticline of Helez. However, we prefer calling it the Levant Arch in order to reflect its greater geographic extent. Gvirtzman and Weissbrod (1984) also recognized that Levant Arch was more extensive; because Mesozoic rifting and seafloor spreading that created the eastern Mediterranean basin removed its western flank. They inferred two phases of uplift, pre-Carboniferous and pre-Permian. The pre-Carboniferous uplift resulted in the truncation of the Cambrian – Devonian beneath an angular unconformity, which was then on-lapped by the Carboniferous section. The angular nature of this unconformity suggests that it corresponds to the Hercynian unconformity as described in this paper. Best et al. (1993) recognized that the post-Hercynian Carboniferous section (Markada Formation) was confined to a NE-trending Carboniferous trough in Syria, where it locally exceeds 1,000 m in thickness. Thus, the absence of the Carboniferous section over the Levant Arch is probably due to its confinement within the flanking Hercynian basin, rather than removal by pre-Permian erosion. The age of the Markada Formation is not well known, and may be Late Carboniferous to Early Permian by analogy with syn? to post-Hercynian deposits elsewhere. Due to the general absence of a Devonian section in Syria (Konert at al., 2001), the uplift of the Levant Arch is poorly constrained as post-Silurian, and pre-Late Carboniferous.

The Oman-Hadhramaut Arch

The Oman-Hadhramaut Arch is defined by the progressive truncation of the Silurian and Ordovician section beneath the Hercynian unconformity towards the southeast (Svendsen, 2004). The widespread erosion of the Devonian and Silurian section over most of Oman and Yemen indicates that the entire region was uplifted during the Hercynian Orogeny (Figure 15). The axis of the Arch is not well defined because of the subsequent tectonic events along the Oman-Hadhramaut margin, but the sporadic outcrops of Proterozoic basement along the southeast coast of Oman and Yemen suggest a northeast trend. Furthermore, the Oman-Hadhramaut Arch was probably part of a more extensive arch that was later fragmented by the opening of the Tethyan and Indian Oceans.

The Al-Khlata Formation, which spans the late Carboniferous to Early Permian time, thickens from ~100 m in outcrops along the Huqf uplift to over 800 m in the Interior Oman basins (Osterloff et al., 2004), which suggests that it on-lapped the Oman-Hadhramaut Arch, and was confined within the flanking Hercynian basin and salt-withdrawal pods. The timing of Hercynian uplift and erosion in Oman is constrained as later than Devonian Misfar Formation, and earlier than the lower Al-Khlata Formation of Late Carboniferous (Westphalian) age (Stephenson et al., 2003).

The Hercynian regional uplift of the Interior Oman Ediacaran-Lower Cambrian salt basins and removal by erosion of a thick post-Ordovician section probably preserved source rocks in the Huqf Supergroup from deeper burial. In contrast, other Ediacaran-Lower Cambrian basins such as the South Gulf Salt basin are very deeply buried beneath a complete Paleozoic section in the Faydah-Jafurah basin, and their source rocks are probably overcooked.

THE HERCYNIAN BASINS

Two large basins developed during the Hercynian Orogeny, the Nafud-Ma’aniya Basin in the north and Faydah-Jafurah Basin in the south. The two basins trend generally northeast, parallel to the adjacent Hercynian arches (Figure 2). Figures 15 and 16 are schematic cross sections across the Faydah and Jafurah basins, restored to their configuration at the end of the Paleozoic. Figure 16 is based on our interpretation of deep well penetration in Iran published by Kashfi (1992), along with published data from adjacent countries. These sections illustrate the general preservation of pre-Hercynian section within the basins, which locally underwent Hercynian erosion that removed part or all of the Lower Carboniferous and Devonian section. These sections also illustrate the confined deposition of syn- and post-Hercynian predominantly continental clastics of the Unayzah, Al-Khlata, Gharif, Ga’ara, and Markada formations within these basins.

The Hercynian paleotopography also influenced the thickness of the syn?- and post-tectonic clastic deposits of the Unayzah, Al-Khlata and Gharif formations in the Faydah-Jafurah Basin (Figure 17). These Upper Carboniferous – Lower Permian sediments are thin or absent over the Hercynian arches, and this thinning is attributed to onlap of the arches rather than subsequent erosion. Furthermore, the presence of proximal glacial deposits near the arches in Oman (Osterloff et al., 2004) and eastern Saudi Arabia (Melvin and Sprague, 2006) may in part be due to alpine glaciation in the adjacent Hercynian highlands.

A second factor controlling the thickness of Permian – Carboniferous clastics is the relative resistance of underlying lithologies to erosion. In eastern Saudi Arabia, the post-Hercynian Unayzah Formation thickens from ~30 m to 300 m over the Silurian Qalibah Formation, because the shales were less resistant to erosion (Norton and Neville, 2006). Similar observations have been reported in Syria (Best et al., 1993; Brew et al., 1999) and Oman (Svendsen, 2004).

The lower part of the Unayzah Formation (Unayzah C) displays large thickness variations (0–500 m), which is attributed to the infilling of the relict Hercynian topography, whereas the upper part (Unayzah A and B) are more uniform in thickness. Similar variations were observed in Oman, where the Al-Khlata Formation varies in thickness between 100–800 m (Osterloff et al., 2004) due both to salt withdrawal and to relief on the Hercynian unconformity.

The Unayzah sandstones are mature quartz arenites, which suggests that they were derived from the erosion of the Cambrian – Devonian sandstones over the Al-Batin Arch. The rarity of igneous and metamorphic clasts further suggests that the crystalline basement was not extensively eroded over the Al-Batin Arch. The accumulation of the Unayzah Formation in the adjacent basins effectively leveled the post-Hercynian topography over the Arabian platform by the mid-Permian, when the Khuff carbonates transgressed over a peneplaned platform.

The Nafud-Ma’aniya Basin extended over northern Saudi Arabia, Iraq, Jordan and Syria. In some areas, this basin contains a nearly complete Carboniferous section with little evidence of any Hercynian erosion, which renders the identification of the Hercynian unconformity difficult. The Carboniferous section includes the Berwath Formation in Saudi Arabia, the Harur, Raha, and Ga’ara formations in Iraq, and the Markada Formation in Syria.

Al-Laboun (1988) mapped the thickness of Carboniferous – Permian clastics across the basin and showed that the section exceeds 500 m in central Iraq. In Syria, Best et al. (1993) recognized that the Carboniferous Markada Formation of sandstones, claystone and argillaceous carbonates was confined to a northeast trending trough in Syria, where it locally exceeds 1,000 m in thickness. They observed that this formation overlies the lower Paleozoic section with angular unconformity, and Brew et al. (2001) noted that it onlaps the Silurian on many seismic lines. These observations suggest that it is post-Hercynian, and is equivalent to other post-Hercynian clastics of the Unayzah, Ga’ara and Al-Khlata formations.

HERCYNIAN FAULTS IN EASTERN SAUDI ARABIA

It was previously thought that the Hercynian deformation in eastern Saudi Arabia was manifested mainly by fault-block uplift along several north-south trends (McGillivray and Husseini, 1992; Wender et al., 1998; Konert et al., 2001; Al-Husseini, 2000, 2004). These trends include from west to east the Hawtah, Nuayyim, Khurais-Bargan, El-Na’la, (Ghawar-Berri), Qatar Arch, and Kidan (Figure 18).

Based on limited well data, Wender et al. (1998) recognized the erosional truncation of the Devonian section along the eastern boundary fault of the Ghawar uplift beneath the Hercynian unconformity (Figure 2) and assumed that the truncation of the Paleozoic section was more or less symmetrical across the entire structure (see Figures 5 and 911 of Wender et al., 1998). This symmetrical model was also applied in Kuwait by Strohmenger et al. (2003).

Subsequently, additional seismic and well data have revealed that the regional Hercynian subcrop pattern is asymmetric both along and across the structure (Figure 2), being primarily controlled by the presence of Al-Batin Arch to the northwest. Furthermore, the absence of post-Hercynian clastics of the Unayzah Formation from the northern part of Ghawar (Figure 17) is attributed mainly to their on-lapping Al-Batin Arch from the south.

The structural growth of the Ghawar anticline is illustrated in Figure 19, which show a seismic cross section constrained by deep well penetrations, and flattened at different geologic times. Figure 19a was flattened at the base of the Khuff carbonates, which approximates the Hercynian unconformity, and illustrates the regional angular truncation of the pre-Hercynian section towards the Al-Batin Arch, and the relatively minor growth of the Ghawar structure during the Hercynian along its bounding faults. Figure 19b was flattened along the Triassic Minjur Formation, and illustrates the growth of the structure during Triassic extension, which was manifested by the upward propagation of the bounding faults. Figure 19c was flattened at the top of the Cretaceous, and illustrates the substantial renewed growth of Ghawar and adjacent Harmaliyah structures during Late Cretaceous. This growth resulted in erosion of the middle Cretaceous Wasia Group beneath the pre-Aruma unconformity, and onlap of a wedge of Aruma sediments along the flanks. Figure 19d shows the structure at present-day and the folding of the Cretaceous section due to renewed growth during the Tertiary.

Xiao et al. (2003) mapped the complex geometry of the N-trending deep faults under Ghawar field, and concluded that they combined dip slip with right-lateral slip components during the Hercynian. Figure 20 is a Hercynian subcrop map showing the extension of the west-bounding thrust south of Ghawar towards the Midrikah and Nujayman gas fields (Al-Shammery and Al-Hauwaj, 1995). This thrust attains a maximum throw of approximately 300 m near the southern end of Ghawar (Haradh), which is also indicated by the deep Hercynian erosion that exposed a window of Ordovician section in southern Ghawar. The dramatic change in strike of this thrust towards the southeast therefore coincides with maximum Hercynian throw and hanging wall erosion. This indicates maximum horizontal strain oriented northwest, and is consistent with a regional stress regime with maximum horizontal compressive stress oriented northeast.

Al-Husseini (2000) noted that the prevailing N-trend of the fault-bounded structures in eastern Saudi Arabia has the same orientation as terrane boundaries in the eastern Arabian Shield, such as the Amar-Idsas and Nabitah sutures. While we do not agree with some aspects of his tectonic model, recent unpublished aeromagnetic data substantiate the presence of N-trending Proterozoic terrane boundaries in the basement under eastern Saudi Arabia. This suggests that the master bounding faults are basement faults that were reactivated during the Hercynian Orogeny.

The postulated northeast compressive stress regime accounts for the initiation of the N-trending uplifts in eastern Arabia as transpressional thrusts with right lateral offset, but it does not account for the northeast orientation of the much larger Hercynian uplifts such as the Al-Batin Arch. Viewed from a regional Gondwana-wide perspective, the N-trending block uplifts in central and eastern Saudi Arabia are local intra-plate phenomena, and the amount of crustal shortening during the Hercynian is probably small.

ORIGIN OF THE HERCYNIAN STRUCTURES

The regional pattern of the Hercynian subcrop in the Arabian Plate indicates that it underwent broad regional uplift and subsidence along northeast trends during the mid-Carboniferous. The facies and thickness variations in the Cambrian to Devonian section indicate that these mega-structures did not form prior to Hercynian Orogeny (Konert et al., 2001). Comparison of the Hercynian arches and basins in Arabia with those in North Africa (Boote et al., 1998) reveals many similarities in size, shape, and orientation (Figure 21), which suggests a common origin.

Plate tectonic reconstructions indicate collision of the Gondwana with Laurasia supercontinents during the Hercynian along the western margin of North Africa. While this collision resulted in extensive deformation along the plate margin in Morocco, it is highly unlikely that the stress would extend about 5,000 kilometers across Gondwana to form the Al-Batin Arch.

This Hercynian warping resulted in broad flexure of the continental crust within the interior of the Gondwana craton, and hundreds to thousands of kilometers away from its plate margins. Viewed thus, it is similar to ‘epirogenic’ uplift and subsidence in other cratons, such as the subsidence that formed the Cretaceous seaway within North America. The causes of such vertical movements are poorly understood, and may be related to thermal expansion or contraction of the crust, coupled with sediment loading effects.

IMPACT OF HERCYNIAN STRUCTURES ON HYDROCARBON PLAYS

The formation of regional arches and basins during the Hercynian had a major impact on the Paleozoic hydrocarbon plays throughout the Arabian Plate because it affected the regional distribution and juxtaposition of source and reservoir rocks. In addition, reactivated Hercynian faults locally provided conduits for vertical hydrocarbon migration from source to reservoir.

Source Rock Distribution, Maturation, Generation and Migration

The main source rocks in the Paleozoic section are the regionally extensive organic-rich “hot” shales near the base of the Silurian section throughout Arabia and North Africa (Mahmoud et al., 1992; Balducci and Pommier 1970; Ala and Moss, 1979; Beydoun, 1988; Ala et al., 1980; Grantham et al., 1988; and Boote et al., 1998). These rocks are thought to have sourced over 80% of the Paleozoic oil and gas fields in Arabia and North Africa. Additional source rocks are known in the Upper Devonian Fransian shales of North Africa (Boote et al., 1998), and in the Neoproterozoic (Ediacaran) section in Oman (Konert et al., 2001).

The main effect of the Hercynian deformation has been the removal by erosion of the Silurian source rocks from large areas over the Levant, Al-Batin, and Oman-Hadhramaut arches (Figure 2). This is illustrated by the truncation of the base Qusaiba seismic reflector along the flanks of these arches (Figures 611). The widespread erosion of the Silurian source rocks over these arches limits the Paleozoic hydrocarbon potential to the Hercynian basins and their margins. On the other hand, the deep burial of the Silurian source rocks in the Hercynian basins, such as the Faydah-Jafurah basin, has extended maturation of these rocks into the wet and dry gas windows.

Basin modeling in the Faydah-Jafurah basin indicates that the Silurian source rocks started generating oil along the flanks and wet gas in the basin center during the Jurassic Period (Abu Ali and Littke, 2005). At present, the basin centers have became over-mature and the basin flanks are generating wet to dry gas, except where depth of the Silurian source rocks are shallower than 3,000 m, where they remain in the oil generation window.

Furthermore, it is noteworthy that the main Paleozoic oil and gas accumulations in the Permian – Carboniferous Unayzah Formation in central and eastern Saudi Arabia are distributed along the sub-crop of the Silurian Qusaiba Member (Figure 2). The majority of these fields occur in low-relief anticlinal closures that are not bounded by faults at the reservoir level. It is therefore inferred that the Hercynian unconformity provided a direct path for hydrocarbon migration from the Silurian source rocks to the overlying Unayzah reservoirs. This juxtaposition was critical for hydrocarbon charge, and explains why the majority of the Unayzah fields are located along the southern margin of the Al-Batin Arch (Figures 2 and 22a).

On the other hand, the vertical migration of hydrocarbons through the thick Silurian shales is impeded by their low vertical permeability, and requires faults to act as vertical conduits. The reactivation of N-trending Hercynian faults during the Triassic and Late Cretaceous is thought to have provided such vertical conduits, and accounts for hydrocarbon accumulations in the Devonian Jauf Formation and the basin-ward Permian Unayzah reservoirs (Figure 22b).

The Permian Khuff carbonate reservoirs (Khuff A, B, C and D reservoirs) are typically sealed within thick sections of impervious carbonate and anhydrite. Hydrocarbon charge into these reservoirs requires substantial faults or fracture swarms to breach the tight base of the Khuff Formation, even in areas where the Khuff overlies the Silurian source rocks directly (Figure 22c). Such vertical migration explains why some of these reservoirs, such as the giant North Field of Qatar, Awali Field of Bahrain and Abqaiq Field of Saudi Arabia are located well within the Hercynian basins.

Reservoir Rock Distribution

The main Paleozoic reservoirs in Arabia are the syn? to post-Hercynian sandstones of Upper Carboniferous – Lower Permian age. They include the Unayzah A, B and C reservoirs in Saudi Arabia and the Khlata and Gharif reservoirs in Oman. These formations are generally restricted within the Hercynian basins, and are thin or absent over the arches. This is attributed to their confined deposition within the basins, and onlap of the Hercynian arches, which formed highlands, along the basin margin. There is no evidence for extensive erosion of these deposits beneath the pre-Khuff unconformity (Figure 3). In general, the thickness of these deposits is partly controlled by the relative resistance of the underlying formations to erosion. They are generally thin over well indurated Ordovician sandstones, and thicken over the less resistant Silurian shales.

The pre-Hercynian reservoirs in Saudi Arabia include Ordovician and Devonian sandstones whose distribution is limited to the Hercynian basins due to their erosion over the arches (Figure 2).

Trap Formation

In central and eastern Saudi Arabia, transpressive reverse faulting during the Hercynian, mainly along north-south trends, set the stage for the subsequent growth of major structural traps (e.g. Figure 19). However, much of this early structural growth was leveled by concurrent Hercynian erosion and the subsequent deposition of syn? to post Hercynian clastics of the Unayzah Formation. The Late Permian Khuff carbonates do not display any significant facies or thickness variation across these structures, which attest to the complete leveling of the Hercynian topography. However, the subsequent reactivation of these Hercynian faults during the Triassic, Cretaceous and Neogene resulted in the growth of the structural traps that contain the world’s largest oil and gas fields (Figures 19a to 19d).

CONCLUSIONS

The mid-Carboniferous Hercynian deformation was the principal tectonic event that affected Arabia during the Paleozoic, resulting in a major angular unconformity. The main effect of the Hercynian deformation in the Arabian Plate was the formation of three broad NE-trending arches over which the Cambrian – Devonian section was deeply or entirely removed by erosion. These arches are: (1) Levant Arch extending from Egypt along the Levant coast to Turkey; (2) Al-Batin Arch extending from the Arabian Shield to Iran; and (3) Oman-Hadhramaut Arch located along the southeast coast of Oman. Hercynian deformation in central and eastern Saudi Arabia also included the initiation of north to northwest trending thrusts due to a NE-directed principle horizontal stress caused by an unknown tectonic event. These were reactivated during the Triassic, Late Cretaceous, and Neogene as fault-bounded block uplifts. We do not support the interpretation that these blocks and intervening basins (e.g. Dibdibah Trough) were originally formed in the Neoproterozoic and Early Cambrian as proposed by Al-Husseini (2000).

The three arches are separated by the Nafud-Ma’aniya and Faydah-Jafurah Basins, in which the effects of Hercynian erosion are subdued or nonexistent, and the Cambrian – Early Carboniferous section is generally preserved from erosion. The Hercynian basins were subsequently filled by predominantly continental clastics during Carboniferous to Early Permian time. The Hercynian arches and sags in Arabia are an extension of the same structural style in North Africa.

The Hercynian arches controlled the paleotopography and depositional limits of the syn? to post Hercynian clastics of late Carboniferous – Early Permian age. These clastics were deposited within the Hercynian basins and on-lap the arches along their flanks. Their source was probably from the erosion of the Cambrian – Devonian rocks over the arches. The thickness of these sediments was affected by the pre-existing Hercynian topography and they tend to thicken over Silurian shales that were less resistant to erosion.

The Hercynian Orogeny played an important role in the evolution of the Paleozoic petroleum systems in Arabia by controlling the distribution of the reservoirs and source rocks, and by initiating reverse faults in the basement. The prolific and extensive Silurian source rocks are preserved in the basins and eroded over the arches, which limits the hydrocarbon potential of arches. On the other hand, Neoproterozoic source rocks in Oman were preserved from over maturity by Hercynian uplift, and are too deeply buried in the basins for hydrocarbon generation. Consequently, the main pre-Khuff oil and gas accumulations are concentrated along the flanks of the Hercynian arches. Devonian and older reservoir rocks were eroded from the arches, whereas the deposition of Late Carboniferous – Early Permian reservoir rocks was confined to the Hercynian basins. However, the reservoir rocks of the Late Permian – Triassic Khuff Formation were unaffected by the preceding Hercynian deformation. The Hercynian Orogeny also resulted in reverse faults in the basement that provided channels for vertical hydrocarbon migration, and were reactivated during the Mesozoic and Cenozoic to form structural traps.

ACKNOWLEDGEMENTS

The authors thank the management of Saudi Aramco and the Ministry of Petroleum and Minerals of Saudi Arabia for granting permission to publish this paper. We also thank the three anonymous GeoArabia reviewers and Moujahed Al-Husseini for their useful comments and suggestions. Our thanks also go to Cartography Division of the Exploration Organization, Saudi Aramco for their help in preparing the Figures of this paper. The final design and drafting by GeoArabia’s Graphic Designer Nestor Niño Buhay is appreciated.

ABOUT THE AUTHORS

Mohammad Faqira is presently the Chief Geologist of the Pore Volume Assessment Division at Saudi Aramco. He joined Saudi Aramco in 1987 after completing his BSc in Petroleum Geology from King Abdulaziz University in Jeddah (1986), Saudi Arabia. In 1991, he obtained an MSc in Geology from the Colorado School of Mines, USA. Mohammad worked for Saudi Aramco’s Area Exploration Department from 1991 to 2005, and was involved in exploration programs in the Red Sea, central Saudi Arabia, western Rub’ Al Khali, Summan Platform, northern Ghawar and Arabian Gulf. He assumed several management positions: Chief Explorationist for Eastern Area Exploration (2004-2005), Chief Geophysicist for Geophysical Data Processing (2006-2007) and, since 2007, Chief Geologist for Reserves Assessment Division. Mohammad is a member of the AAPG, DGS and EAGE societies.

mohammad.faqira@aramco.com

Martin Rademakers has over 31 years of worldwide exploration experience. Martin has a BSc in Geology from University of New York and an MS in Geophysics from University of Tulsa, USA. He began his career as an Exploration Geophysicist with Borehole Exploration in Tulsa. Martin then worked at Templeton Energy, LASMO and Plains Resources prior to joining Saudi Aramco in 1996. He is highly interested in seismic stratigraphic analysis in carbonate reservoirs. Currently Martin is generating exploration opportunities for Saudi Aramco in the Gotnia Basin as part of the Summan Exploration Team.

rademamj@aramco.com

AbdulKader M. Afifi is Manager of Exploration Technical Services Department at Saudi Aramco. Abdulkader joined Saudi Aramco in 1991, and has held several technical and supervisory positions in the Exploration Organization including Chief Explorationist and Chief Geologist. He obtained his university degrees in Geology: BSc from the King Fahd University of Petroleum and Minerals (1977), MSc from the Colorado School of Mines (1981), and PhD in 1990 from the University of Michigan, USA. From 1980-1986 he worked for the U.S. Geological Survey in Saudi Arabia, and evaluated mineral resources including the Mahd Adh Dhahab gold district. Abdulkader is an Active Member of the AAPG, DGS and SPE. He was a member of the GEO Conference Program Committee (1998 and 2000), Vice President of DGS (1995), Middle East Region Representative for the AAPG Advisory Council (2002-2004), and is an AAPG Distinguished Speaker (2004). He has written and co-authored several papers including two on the Paleozoic Stratigraphy and Hydrocarbon Habitat of the Arabian Plate (GeoArabia, 2001, AAPG Memoir 74).

abdulkader.afifi@aramco.com