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Recently, Cantrell et al. (2014)<ref name=Cantrelletal2014 /> reviewed the [[Tethyan Petroleum System]]s of [[Saudi Arabia]] where they describe two major petroleum systems: the [[Paleozoic Petroleum System|PPS]] related to the Paleo-Tethys and a [[Mesozoic Petroleum System]] (MPS) associated with the Neo-Tethys ([[:file:M114CH01FG04.jpg|Figure 4]]). The elements of the two petroleum systems (e.g., [[source]], [[reservoir]], and [[seal]] characteristics) show significant differences. The [[Paleozoic Petroleum System|PPS]] is [[siliciclastic]]-dominated, whereas the [[Mesozoic Petroleum System|MPS]] is [[carbonate]]-dominated. These two petroleum systems are separated in geological time by the closure of the Paleo-Tethys and the amalgamation of [[Pangea]], followed by subsequent breakup of [[Pangea]] and opening of the Neo-Tethys (Cantrell et al., 2014).

Recently, Cantrell et al. (2014)<ref name=Cantrelletal2014 /> reviewed the [[Tethyan Petroleum System]]s of [[Saudi Arabia]] where they describe two major petroleum systems: the [[Paleozoic Petroleum System|PPS]] related to the Paleo-Tethys and a [[Mesozoic Petroleum System]] (MPS) associated with the Neo-Tethys ([[:file:M114CH01FG04.jpg|Figure 4]]). The elements of the two petroleum systems (e.g., [[source]], [[reservoir]], and [[seal]] characteristics) show significant differences. The [[Paleozoic Petroleum System|PPS]] is [[siliciclastic]]-dominated, whereas the [[Mesozoic Petroleum System|MPS]] is [[carbonate]]-dominated. These two petroleum systems are separated in geological time by the closure of the Paleo-Tethys and the amalgamation of [[Pangea]], followed by subsequent breakup of [[Pangea]] and opening of the Neo-Tethys (Cantrell et al., 2014).

The key elements of the [[Mesozoic Petroleum System|MPS]] are shown in [[:file:M114CH01FG04.jpg|Figure 4]]. The [[Mesozoic Petroleum System|MPS]] contains the [[Jurassic]] [[Hanifa Formation|Hanifa]] and [[Tuwaiq Mountain Formation|Tuwaiq Mountain]] Formations as the principal source rocks, with an average resident [[TOC]] content of about 3.5 wt.%, and sometimes as high as 14.3 wt.% (Cantrell et al., 2014<ref name=Cantrelletal2014 />). The reservoirs extend from the Middle [[Jurassic]] to Upper [[Cretaceous]] [[carbonate]]s (the major reservoir being the [[Arab Formation]]). Regional [[seal]]s are provided by Arab [[anhydrite]]s and the evaporitic [[Hith Formation]]. The [[Mesozoic Petroleum System|MPS]] has been previously discussed in detail (e.g., Carrigan et al., 1994<ref name-Carriganetal1994>Carrigan, W. J., G. A. Cole, E. L. Colling, and P. J. Jones, 1994, Geochemistry of the Upper Jurassic Tuwaiq Mountain and Hanifa Formation petroleum source rocks of eastern Saudi Arabia, in B. J. Katz, ed., Petroleum source rocks: Springer-Verlag, New York, p. 67–87.</ref>; Cole et al., 1994<ref name=Coleetal1994>Cole, G. A., M. A. AbuAli, S. M. Aoudeh, W. J. Carrigan, H. H. Chen, E. L. Colling, et al., 1994, Organic geochemistry of the Paleozoic petroleum system of Saudi Arabia: Energy & Fuels, v. 8, p. 1425–1442.</ref>; Cantrell et al., 2014<ref name=Cantrelletal2014 />).

The key elements of the [[Mesozoic Petroleum System|MPS]] are shown in [[:file:M114CH01FG04.jpg|Figure 4]]. The [[Mesozoic Petroleum System|MPS]] contains the [[Jurassic]] [[Hanifa Formation|Hanifa]] and [[Tuwaiq Mountain Formation|Tuwaiq Mountain]] Formations as the principal source rocks, with an average resident [[TOC]] content of about 3.5 wt.%, and sometimes as high as 14.3 wt.% (Cantrell et al., 2014<ref name=Cantrelletal2014 />). The reservoirs extend from the Middle [[Jurassic]] to Upper [[Cretaceous]] [[carbonate]]s (the major reservoir being the [[Arab Formation]]). Regional [[seal]]s are provided by Arab [[anhydrite]]s and the evaporitic [[Hith Formation]]. The [[Mesozoic Petroleum System|MPS]] has been previously discussed in detail (e.g., Carrigan et al., 1994<ref name=Carriganetal1994>Carrigan, W. J., G. A. Cole, E. L. Colling, and P. J. Jones, 1994, Geochemistry of the Upper Jurassic Tuwaiq Mountain and Hanifa Formation petroleum source rocks of eastern Saudi Arabia, in B. J. Katz, ed., Petroleum source rocks: Springer-Verlag, New York, p. 67–87.</ref>; Cole et al., 1994<ref name=Coleetal1994>Cole, G. A., M. A. AbuAli, S. M. Aoudeh, W. J. Carrigan, H. H. Chen, E. L. Colling, et al., 1994, Organic geochemistry of the Paleozoic petroleum system of Saudi Arabia: Energy & Fuels, v. 8, p. 1425–1442.</ref>; Cantrell et al., 2014<ref name=Cantrelletal2014 />).

[[Paleozoic Petroleum System]]s: A summary of the PPS is shown in [[:file:M114CH01FG04.jpg|Figure 4]]. This petroleum system contains the basal [[Silurian]] Qusaiba hot [[shale]], and to a lesser extent the Qusaiba warm [[shale]]s, as its principal source rocks, with reservoirs extending from the [[Ordovician]] to the early [[Triassic]]. [[Seal]]s occur at different stratigraphic levels, with the evaporitic [[Sudair Formation]] of the early [[Triassic]] age serving as the regional top seal of the PPS. Hanadir and Ra’an shales of the [[Ordovician]] [[Qasim Formation]] may also present some source potential.

[[Paleozoic Petroleum System]]s: A summary of the PPS is shown in [[:file:M114CH01FG04.jpg|Figure 4]]. This petroleum system contains the basal [[Silurian]] Qusaiba hot [[shale]], and to a lesser extent the Qusaiba warm [[shale]]s, as its principal source rocks, with reservoirs extending from the [[Ordovician]] to the early [[Triassic]]. [[Seal]]s occur at different stratigraphic levels, with the evaporitic [[Sudair Formation]] of the early [[Triassic]] age serving as the regional top seal of the PPS. Hanadir and Ra’an shales of the [[Ordovician]] [[Qasim Formation]] may also present some source potential.

Source Rocks: Although several intervals of fine [[clastic]]s (e.g., [[shale]]s and [[mudstone]]) are potential source rocks of various organic richness, the basal hot [[shale]] member of the [[Qusaiba Formation]] of the [[Qalibah Group]] ([[:file:M114CH01FG02.jpg|Figure 2]]) shows a basin-wide occurrence ([[:file:M114CH01FG01.jpg|Figure 1]]) and is organic rich (Cole et al., 1994<ref name=Coleetal1994 />). Termination of [[glaciation]] at the end of the [[Ordovician]] resulted in a major sea-level rise during the early [[Silurian]] time, leading to deposition of the upward-coarsening [[progradation]]al [[Qalibah Group]]. This rapid [[transgression]] caused displacement of earlier shallow marine [[siliciclastic]]s and resulted in the deposition of organic-rich sediments within [[anoxic]] intra-shelf depressions of the northern [[Gondwana]] (Jones and Stump, 1999<ref name=Jonesandstump1999>Jones, P. J., and T. Stump, 1999, Depositional and tectonic setting of the Lower Silurian hydrocarbon source rock facies, Central Saudi Arabia: AAPG Bulletin, v. 83, p. 314–332.</ref>). As these intra-shelf depressions were filled with [[anoxic]] [[sediment]]s, more [[oxic]] depositional environment led to a widespread deposition of warm, organic-lean [[shale]]s of the [[Qusaiba Formation]] (Lüning et al., 2000<ref name=Lüningetal2000 />). The organic-rich basal hot [[shale]] of the [[Qusaiba Formation]] is best developed in the subsurface of east-central [[Saudi Arabia]], as well as in the northwest Saudi Arabia, and has an average [[TOC]] content of about 5 wt.%, with maximum values as high as 20 wt.% (Cole et al., 1994<ref name=Coleetal1994 />). Several [[Paleozoic]] oil and gas fields in [[Saudi Arabia]] are known to have been sourced from the basal Qusaiba hot [[shale]] (AbuAli et al., 1991<ref name=Abualietal1991>AbuAli, M. A., U. A. Franz, J. Shen, F. Monnier, M. D. Mahmoud, and T. M. Chambers, 1991, Hydrocarbon generation and migration in the Paleozoic sequence of Saudi Arabia: Society of Petroleum Engineers, SPE 21376, p. 345–356.</ref>, 1999<ref name=AbuAlietal1999>AbuAli, M. A., J. L. Rudkiewicz, J. G. McGillivray, and F. Behar, 1999, Paleozoic petroleum system of Central Saudi Arabia: GeoArabia, v. 4, no. 3, p. 321–335.</ref>; Mahmoud et al., 1992<ref name=Mahmoudetal1992 />; McGillivray and Al-Husseini, 1992<ref name=Mcgillivrayandalhusseini1992 />; Cole et al., 1994<ref name=Coleetal1994 />; Jones and Stump, 1999<ref name=Jonesandstump1999 />). This hot shale unit contains type II amorphous organic matter, with [[graptolite]] and [[chitinozoan]]s, and ranges in thickness from 10 to 250 ft (3–70 m) as given by Mahmoud et al. (1992)<ref name=Mahmoudetal1992 />, Wender et al. (1998)<ref name=Wenderetal1998 />, AbuAli et al. (1999)<ref name=AbuAlietal1999 />, and AbuAli and Littke (2005)<ref name=Abualiandlittke2005 />. Within the [[Qusaiba Formation]], a thick sequence of nonradioactive, light to medium gray [[shale]] overlies the basal hot shale. This lean shale still contains poor to moderate [[organic]] richness (up to a few weight percent [[TOC]]) with mixed oil and gas potential (Cole et al., 1994)<ref name=Coleetal1994 />, and due to its thickness, it can also be a volumetrically important source rock for [[hydrocarbon]] resources in [[Saudi Arabia]].

Source Rocks: Although several intervals of fine [[clastic]]s (e.g., [[shale]]s and [[mudstone]]) are potential source rocks of various organic richness, the basal hot [[shale]] member of the [[Qusaiba Formation]] of the [[Qalibah Group]] ([[:file:M114CH01FG02.jpg|Figure 2]]) shows a basin-wide occurrence ([[:file:M114CH01FG01.jpg|Figure 1]]) and is organic rich (Cole et al., 1994<ref name=Coleetal1994 />). Termination of [[glaciation]] at the end of the [[Ordovician]] resulted in a major sea-level rise during the early [[Silurian]] time, leading to deposition of the upward-coarsening [[progradation]]al [[Qalibah Group]]. This rapid [[transgression]] caused displacement of earlier shallow marine [[siliciclastic]]s and resulted in the deposition of organic-rich sediments within [[anoxic]] intra-shelf depressions of the northern [[Gondwana]] (Jones and Stump, 1999<ref name=Jonesandstump1999>Jones, P. J., and T. Stump, 1999, Depositional and tectonic setting of the Lower Silurian hydrocarbon source rock facies, Central Saudi Arabia: AAPG Bulletin, v. 83, p. 314–332.</ref>). As these intra-shelf depressions were filled with [[anoxic]] [[sediment]]s, more [[oxic]] depositional environment led to a widespread deposition of warm, organic-lean [[shale]]s of the [[Qusaiba Formation]] (Lüning et al., 2000<ref name=Lüningetal2000>Lüning, S., J. Craig, D. K. Loydell, P. Storch, and B. Fitches, 2000, Lower Silurian “hot shales” in North Africa and Arabia: Regional distribution and depositional model: Earth Science Reviews, v. 49, p. 121–200.</ref>). The organic-rich basal hot [[shale]] of the [[Qusaiba Formation]] is best developed in the subsurface of east-central [[Saudi Arabia]], as well as in the northwest Saudi Arabia, and has an average [[TOC]] content of about 5 wt.%, with maximum values as high as 20 wt.% (Cole et al., 1994<ref name=Coleetal1994 />). Several [[Paleozoic]] oil and gas fields in [[Saudi Arabia]] are known to have been sourced from the basal Qusaiba hot [[shale]] (AbuAli et al., 1991<ref name=Abualietal1991>AbuAli, M. A., U. A. Franz, J. Shen, F. Monnier, M. D. Mahmoud, and T. M. Chambers, 1991, Hydrocarbon generation and migration in the Paleozoic sequence of Saudi Arabia: Society of Petroleum Engineers, SPE 21376, p. 345–356.</ref>, 1999<ref name=AbuAlietal1999>AbuAli, M. A., J. L. Rudkiewicz, J. G. McGillivray, and F. Behar, 1999, Paleozoic petroleum system of Central Saudi Arabia: GeoArabia, v. 4, no. 3, p. 321–335.</ref>; Mahmoud et al., 1992<ref name=Mahmoudetal1992>Mahmoud, M. D., D. Vaslet, and M. I. Al-Husseini, 1992, The Lower Silurian Qalibah Formation of Saudi Arabia: An important hydrocarbon source rock: AAPG Bulletin, v. 76, p. 1491–1506.</ref>; McGillivray and Al-Husseini, 1992<ref name=Mcgillivrayandalhusseini1992>McGillivray, J. G., and M. I. Al-Husseini, 1992, The Paleozoic petroleum geology of central Arabia: AAPG Bulletin, v. 76, p. 1473–1490.</ref>; Cole et al., 1994<ref name=Coleetal1994 />; Jones and Stump, 1999<ref name=Jonesandstump1999 />). This hot shale unit contains type II amorphous organic matter, with [[graptolite]] and [[chitinozoan]]s, and ranges in thickness from 10 to 250 ft (3–70 m) as given by Mahmoud et al. (1992)<ref name=Mahmoudetal1992 />, Wender et al. (1998)<ref name=Wenderetal1998 />, AbuAli et al. (1999)<ref name=AbuAlietal1999 />, and AbuAli and Littke (2005)<ref name=Abualiandlittke2005>AbuAli, M. A., and R. Littke, 2005, Paleozoic petroleum systems of Saudi Arabia: A basin modeling approach: Geo-Arabia, v. 10, no. 3, p. 131–168</ref>. Within the [[Qusaiba Formation]], a thick sequence of nonradioactive, light to medium gray [[shale]] overlies the basal hot shale. This lean shale still contains poor to moderate [[organic]] richness (up to a few weight percent [[TOC]]) with mixed oil and gas potential (Cole et al., 1994)<ref name=Coleetal1994 />, and due to its thickness, it can also be a volumetrically important source rock for [[hydrocarbon]] resources in [[Saudi Arabia]].

Reservoir Rocks: The main reservoirs of the PPS are the sandstones of the Devonian Jauf Formation, sandstones of the Permian Unayzah Formation, and carbonates of the Permian Khuff Formation (Figure 2). The Ordovician Sarah Formation, underlying the regional Qusaiba hot shale source rock, consists mainly of fine- to coarse-grained sandstone sequences of glacial and glacio-fluvial origin. The formation is widely distributed in central and northwestern Saudi Arabia. Cantrell et al. (2014) has noted that pre-Qusaiba clastics (e.g., Sarah sandstones) are generally considered to be tight due to advanced diagenesis, particularly cementation by quartz overgrowths, which has reduced reservoir quality.

Reservoir Rocks: The main reservoirs of the [[Paleozoic Petroleum System|PPS]] are the sandstones of the [[Devonian]] [[Jauf Formation]], sandstones of the [[Permian]] [[Unayzah Formation]], and [[carbonate]]s of the [[Permian]] [[Khuff Formation]] ([[:file:M114CH01FG02.jpg|Figure 2]]). The [[Ordovician]] [[Sarah Formation]], underlying the regional Qusaiba hot [[shale]] source rock, consists mainly of fine- to coarse-grained [[sandstone]] sequences of glacial and glacio-fluvial origin. The formation is widely distributed in central and northwestern [[Saudi Arabia]]. Cantrell et al. (2014<ref name=Cantrelletal2014 />) has noted that pre-Qusaiba clastics (e.g., [[Sarah sandstones]]) are generally considered to be tight due to advanced [[diagenesis]], particularly cementation by [[quartz]] overgrowths, which has reduced reservoir quality.

Seal Rocks: The PPS contains a number of regional seals. The pre-Qusaiba reservoirs (e.g., the late Ordovician Sarah sandstone Formation) are sealed by the overlying Lower Silurian Qusaiba hot shale. The major regional seals for the Unayzah reservoirs are the transgressive shales and carbonates of the basal Khuff Formation. The Khuff reservoirs are sealed by the evaporite members associated with each carbonate cycle, and the fine clastics of the Sudair Formation are a regional seal for the PPS (Figure 2).

Seal Rocks: The [[Paleozoic Petroleum System|PPS]] contains a number of regional [[seal]]s. The pre-Qusaiba reservoirs (e.g., the late [[Ordovician]] [[Sarah sandstone]] Formation) are sealed by the overlying Lower [[Silurian]] Qusaiba hot shale. The major regional seals for the [[Unayzah reservoir]]s are the transgressive shales and carbonates of the basal [[Khuff Formation]]. The Khuff reservoirs are sealed by the evaporite members associated with each carbonate cycle, and the fine clastics of the [[Sudair Formation]] are a regional seal for the PPS ([[:file:M114CH01FG02.jpg|Figure 2]]).

Petroleum Generation and Entrapment: Previous petroleum system and basin modeling studies of the PPS of the east-central part of the Arabian Basin (AbuAli et al., 1999; AbuAli and Littke, 2005) suggested that maturation and oil generation from the Qusaiba basal hot shales commenced as early as the Triassic. Accordingly, early oil expulsion began at about 210 Ma (Triassic), with peak oil expulsion occurring at about 152 Ma (Late Jurassic) and peak gas expulsion at about 140 Ma (Early Cretaceous) (Figure 4).

Petroleum Generation and Entrapment: Previous petroleum system and basin modeling studies of the [[Paleozoic Petroleum System|PPS]] of the east-central part of the [[Arabian Basin]] (AbuAli et al., 1999<ref name=Abualietal1999 />; AbuAli and Littke, 2005<ref name=Abualiandlittke2005 />) suggested that maturation and oil generation from the Qusaiba basal hot shales commenced as early as the [[Triassic]]. Accordingly, early oil expulsion began at about 210 Ma (Triassic), with peak oil expulsion occurring at about 152 Ma (Late [[Jurassic]]) and peak gas expulsion at about 140 Ma (Early [[Cretaceous]]) ([[:file:M114CHh01FG04.jpg|Figure 4]]).

Based on modeling results, hydrocarbon expulsion from the Silurian hot shale postdated the main phase of trap formation during mid-Carboniferous basin inversion in east-central Saudi Arabia (Figure 4).

Based on modeling results, hydrocarbon expulsion from the Silurian hot shale postdated the main phase of trap formation during mid-[[Carboniferous]] basin inversion in east-central [[Saudi Arabia]] ([[:file:M114CH01FG04.jpg|Figure 4]]).

The Silurian Qusaiba shales, mainly basal hot shales and possibly, in a limited manner, the overlying warm shales, owing to organic richness and favorable maturity both in the northwest and east-central Arabian Basin, have sourced Paleozoic oil and gas reservoirs and lately have been considered potential target for unconventional resources.

The Silurian Qusaiba shales, mainly basal hot shales and possibly, in a limited manner, the overlying warm shales, owing to organic richness and favorable maturity both in the northwest and east-central [[Arabian Basin]], have sourced [[Paleozoic]] oil and gas reservoirs and lately have been considered potential target for unconventional resources.