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Mid-Permian Khuff Sequence KS6: Paleorelief-Influenced Facies and Sequence Patterns in the Lower Khuff Time-Equivalent Strata, Oman Mountains, Sultanate of Oman

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Mid-Permian Khuff Sequence KS6: Paleorelief-Influenced Facies and Sequence Patterns in the Lower Khuff Time-Equivalent Strata, Oman Mountains, Sultanate of Oman GeoArabia, 2013, v. 18, no. 3, p. 135-178 Gulf PetroLink, Bahrain Mid-Permian Khuff Sequence KS6: Paleorelief-influenced facies and sequence patterns in the Lower Khuff time-equivalent strata, Oman Mountains, Sultanate of Oman Daniel Bendias, Bastian Koehrer, Michael Obermaier and Thomas Aigner ABSTRACT Khuff Sequence KS6 was studied in the Al Jabal al-Akhdar, Oman Mountains, in an area of 30 x 50 square kilometers by means of detailed sedimentological logging of five time-equivalent outcrop sections of the Saiq Formation. KS6 represents one transgressive-regressive, third-order sequence, and is composed of four facies associations each representing particular environments of deposition (backshoal, shoal, foreshoal and offshoal) with distinct sedimentological characteristics. Facies stack to form cycles and cycle sets that were used for correlation at a subregional scale and to reveal the KS6 stratigraphic architecture. During the initial phase of basin-fill, clastic sediments (“Basal Saiq Clastics”) were deposited in paleolows above the “Sub-Saiq Unconformity”. In contrast to younger Upper Khuff sequences KS4 to KS1, the underlying paleorelief strongly affects the thickness and facies composition of KS6. The correlation strategy to follow paleolandscape surfaces using all available sedimentological, biostratigraphic and lithostratigraphic data resulted in a stratigraphic architecture with subtle shingle geometries. Sequence KS6 shows a strong facies partitioning resulting in the necessity of two separate facies models for the transgressive (crinoidal ramp) versus regressive hemisequence (oolitic/peloidal carbonate ramp). This study revealed potential reservoir units in KS6, commonly regarded as non-reservoir in the subsurface of Oman and other parts of the Gulf region. The abundance, nature and lateral extent of reservoir facies strongly varies with stratigraphic position. In the transgressive part of KS6, crinoidal grainstones are concentrated around the margin of a gentle paleohigh. They might have the best reservoir potential, although early diagenetic cementation is common in most settings. Oolitic/peloidal grainstones in the upper regressive part have a much higher diagenetic reservoir potential and are laterally much more widespread. Thus, Khuff Sequence KS6 differs profoundly in its stratigraphic architecture from the more “layer-cake”-like KS4 to KS1 sequences. Facies and thickness patterns are controlled by a marked paleohigh to paleolow configuration, resulting from the antecedent uneven topography during the Neo-Tethyan syn-rift setting, in contrast to the post-rift setting with low tectonic activity during KS4 to KS1. INTRODUCTION The Khuff Formation represents one of the most important carbonate reservoir units across the Middle East (Figure 1). Six sequences, KS1 to KS6 in descending order, have been identified in Khuff- equivalent strata in the Oman Mountains (Figures 2 and 3; Koehrer et al., 2010). The Upper Khuff sequences KS1 and KS4 contain several reservoir zones and are well explored across the Gulf region. The lower sequences KS5 and KS6 are so far only very sparsely investigated, and fewer penetrations leave these units relatively underexplored. This study on Sequence KS6 is part of a larger research project on Khuff time-equivalent strata in the Al Jabal al-Akhdar area (Oman Mountains) in the Sultanate of Oman (Figures 1 and 2). Its aim is to unravel the geometries and distribution of Khuff grainstones as potential reservoir bodies. Initially a one-dimensional (1-D) facies and sequence-stratigraphic framework was proposed by Koehrer et al. (2010) for the Khuff time-equivalent strata at a type locality on the Saiq Plateau. Subsequently, 135 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/18/3/135/4567666/bendias.pdf by guest on 25 September 2021 Bendias et al. LATE PERMIAN (CHANGHSINGIAN) PALEOGEOGRAPHY 45°E 50° Inner shelf 55° carbonates IRAQ 30° 30°N N KUWAIT Middle and outer 0 250 shelf carbonates Kuh-i-Mand km Dalan North Pars Kangan SAUDI ARABIA Karan IRAN Khursaniyah Abu Sa'fah Zagros South Berri Pars High Abqaiq BAHRAIN North Field Ghawar AbuAl Bukhoosh Nasr 25° Khurais Awali Zakum 25° QATAR Gulf of Oman Riyadh Hail Study Area Harmaliyah UAE Muscat Arzanah Umm Arabian Shaif Shield Yibal Marginal-marine/ OMAN deltaic deposits Erosional limit 20° 20° 45° 50° 55° Figure 1: Paleogeographic map of the Arabian Gulf area during the Late Permian showing location of the study area and major hydrocarbon fields within the Khuff reservoir (red) (modified from Maurer et al., 2009, after Ziegler, 2001). The study area (Al Jabal al-Akhdar) is located about 130 km west of Muscat. Khuff grainstone geometries were documented on the Saiq Plateau from near-well-scale (< 2 km) by Zeller et al. (2011) to field-scale (< 10 km) by Koehrer et al. (2011), and to subregional scale (< 60 km) by Koehrer et al. (2012). These studies revealed an overall “layer-cake”-type geometry of shoal grainstone bodies in Upper Khuff sequences KS4 to KS1. Only subtle lateral heterogeneity of facies associations is observed in Khuff sequences KS1 to KS4 on a subregional-scale (60 x 40 km) in the Al Jabal al-Akhdar by Koehrer et al. (2012). Cycle sets and sequences were found to be highly correlatable, pointing towards a rather uniform gross depositional environment and absence of significant tectonic activity in this area during the post-rift phase of the Neo-Tethys Ocean. Post-depositional sediment deformations and structurally-related breccias and discontinuity surfaces are however described in sequences KS1 to KS4 from the Saih Hatat Window (Figure 2), paleogeographically located more proximally to the Arabian platform margin (e.g. Chauvet et al., 2009; Weidlich and Bernecker, 2011; Weidlich and Bernecker, 2012). However, heterogeneities markedly increase in KS5 and KS6, possibly due to deposition during a syn-rift phase. Walz and Aigner (2012) and Walz et al. (2013) report significant thickness variations and apparent downlaps on cycle and cycle set scale, which are interpreted to result either from differential subsidence or initial topography. This paper focuses on the architecture of the grainstones in Khuff Sequence KS6 on a scale of 30 x 50 sq km. It aims at assessing the possible influence of paleotopography on potential reservoir facies distribution and continuity. It follows a systematic 1-D/2-D/3-D approach: after a detailed documentation of 1-D outcrop sections, 2-D stratigraphic cross-sections are constructed, leading to 3-D geocellular models. 136 136 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/18/3/135/4567666/bendias.pdf by guest on 25 September 2021 Mid-Permian Khuff Sequence KS6, Oman OUTCROP MAP, OMAN MOUNTAINS 57°E 57°30' 58°558°30' 9° Gulf of Oman Q Jabal N Bawshar 0 25 Muscat km Semail Wadi Ophiolite S Tt Aday 23°30'N J Wadi Hulw 23°30' Wadi Bani Awf PTr Mayh PTr Wadi Sahtan Jabal Jabal Abu Da’vd Tayin Pc Wadi Mistal J Wadi CO CO Mijlas Tt P Al Jabal al-Akhdar JK Semail Ja ba Jabal Pc Ophiolite l J Ab Aswad Wadi Bani Hajir ya J d 23° JK 23° Saiq Plateau Semail Ophiolite Tt Tt Jabal S Hamrat ad Duru H Range am m S ah Hawasina Q Jabal Nappes Safra 57° 57°30' 58°558°30' Q 9° BAHRAIN 54° 58° (Q) Quaternary Kawr Group (Triassic – Cretaceous) 26° QATAR Gulf of Oman (Tt) Tertiary Al Aridh Group Abu (Triassic – Late Cretaceous) Dhabi Location UAE (J) Sahtan Group (Jurassic) Umar Group (Triassic – Cretaceous) Muscat (JK) Kahmah Group Hamrat Duru Group 22° OMAN 22° (end Jurassic – mid-Cretaceous) (Late Permian – Late Cretaceous) SAUDI Baid Formation Wasia Group (mid-Cretaceous) N ARABIA (Late Permian – Jurassic) 0 200 (PTr) Akhdar Group Muti Formation (mid-Late Cretaceous) km (Late Permian – Triassic) Arabian Sea (CO) Haima Group Aruma Group (end Cretaceous) (Cambrian – Ordovician) 18° 18° (S) Semail Ophiolite (P) upper Huqf Group YEMEN (Proterozoic – Cambrian) (mid-Late Cretaceous) 54° 58° Metamorphic sole (Pc) lower Huqf Group (Proterozoic) (mid-Late Cretaceous) Correlation lines (Figs. 28 to 30) Figure 2: Geological map of the Oman Mountains showing location of the studied sections. Note that the Saiq and Mahil formations of the Akhdar Group are shown together (PTr) (after Béchennec et al., 1993). GEOLOGICAL SETTING The study area is located in the Oman Mountains, about 130 km west of Muscat (Figures 1 and 2). The northern Oman region experienced several phases of long-term subaerial exposure between the Proterozoic and Early Permian, which resulted in times of non-deposition and erosion (e.g. Forbes et al., 2010). In the Oman Mountains, a distinctive angular unconformity between Neo-Proterozoic and Middle Permian strata is preserved, which is here termed as the “Sub-Saiq Unconformity” (Figures 3 and 4). The initial deposition of Khuff sediments started as a result of the drift of Cimmerian terranes away from Gondwana and the accompanying opening of the Neo-Tethys Ocean (e.g. Pillevuit, 1993; Stampfli and Borel, 2002). Subsequent flooding of the Arabian Plate resulted in the development of an epeiric carbonate ramp and the unconformable deposition of Khuff and equivalent sediments on folded Proterozoic strata (e.g. Glennie et al., 1974; Rabu et al., 1993; Searle, 2007). 137 137 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/18/3/135/4567666/bendias.pdf by guest on 25 September 2021 Bendias et al. According to the paleogeographic reconstructions of the Permian–Triassic by Konert et al. (2001a, b) the Arabian Plate drifted northwards, from 30° to 15° south. Sea-level oscillations of moderate amplitude and wavelength, and a transitional icehouse to greenhouse climate (Al-Jallal, 1995), created conditions very similar to those observed along the present-day Arabian Gulf coast (Strohmenger et al., 2006). According to paleogeographic reconstructions from Ziegler (2001), the deposition of Khuff strata took place on a shallow-marine to open-marine epeiric carbonate ramp (Figure 1). Stratigraphic Framework Permian–Triassic strata in the Oman Mountains (Al Jabal al-Akhdar) are subdivided into two formations by Glennie et al.
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