Mesozoic and Cenozoic structural evolution of North Oman: New insights from high-quality 3D seismic from the Lekhwair area

Accepted Manuscript Mesozoic and Cenozoic structural evolution of north Oman: New insights from highquality 3D seismic from the Lekhwair area Loïc Bazalgette, Hisham Salem PII:

S0191-8141(18)30099-3

DOI:

10.1016/j.jsg.2018.02.006

Reference:

SG 3600

To appear in:

Journal of Structural Geology

Received Date: 20 September 2017 Revised Date:

16 January 2018

Accepted Date: 20 February 2018

Please cite this article as: Bazalgette, Loï., Salem, H., Mesozoic and Cenozoic structural evolution of north Oman: New insights from high-quality 3D seismic from the Lekhwair area, Journal of Structural Geology (2018), doi: 10.1016/j.jsg.2018.02.006. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Mesozoic and Cenozoic structural evolution of North Oman: New insights from high-quality 3D seismic from the Lekhwair area

080, Muscat, Sultanate of Oman.

Corresponding author, E-mail: [email protected], Telephone: +968 24679063

Keywords:

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Oman, structural evolution, faults, seismic, Middle-East, Arabia.

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By Loïc BAZALGETTE* and Hisham SALEM, PDO, Field Development Centre, Mina al Fahal, PC100, PO-

Abstract

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This paper highlights the role of Triassic-Jurassic extension and late Cretaceous compression in

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the Mesozoic-Cenozoic (Alpine) structuring of North Oman. The syn/post-Mesozoic regional

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structural evolution is usually documented as a succession of two stages of deformation. The

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Alpine 1 phase, late Cretaceous in age, occurred in association with two ophiolite obduction

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stages (Semail and Masirah ophiolites). It was characterised by strike slip to extensional

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deformation in the North Oman foreland basin sub-surface. The Alpine 2 phase, Miocene in

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age, was related to the continental collision responsible of the Zagros orogen and of the uplift

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of the Oman Mountains. The Alpine 2 deformation was transpressional to compressional.

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Observation and interpretation of good quality 3D seismic in the Lekhwair High area enabled

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the distinction of two earlier phases. Early Mesozoic extension occurred concomitantly with the

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regional Triassic to Jurassic rifting, developing Jurassic-age normal faults. Late Cretaceous

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compression occurred prior to the main Alpine 1 phase and triggered the inversion of Jurassic-

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seated normal faults as well as the initiation of compressional folds in the Cretaceous

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overburden. These early phases have been ignored or overlooked as part of the North Oman

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history although they are at the origin of structures hosting major local and regional

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hydrocarbon accumulations.

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Introduction

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Located at the eastern edge of the Arabian Peninsula, the North Oman region has been the

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scene of a complex succession of tectonic events which can be traced back to Proterozoic

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times. This tectonic evolution took place in association with major regional geodynamic

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episodes dating from the late Precambrian (about -700 Ma) to the recent Plio-Quaternary

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period (Al Kindi and Richard, 2014, Al-Barwani, 2003, Sharland et al., 2001, , Loosveld et al.,

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1996). A wide variety of evidence showing the influence of these tectonic episodes can be

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observed from the present day structural styles of outcrops in the Oman Mountains and in their

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foreland, as well as from abundant sets of sub-surface data (Al Kindi and Richard, 2014, Richard

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et al., 2014, Filbrandt et al., 2006, Mount et al., 1998). In North Oman the complex structural

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history is recognized as one of the main factors which controlled the setting and evolution of

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local petroleum systems as well as their economic prospectivity (Terken et al., 2001, Terken,

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1999). Among the different stages forming this structural history, the latest Mesozoic and

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Cenozoic episodes have been identified as important due to their role on the present day

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structure of the region. They also had a crucial impact on the evolution of the major North

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Oman oil and gas fields. Hence, the Mesozoic to Cenozoic episodes have influenced the

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hydrocarbon migration chronology, the trapping mechanisms, the geometry of the fields, and

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their internal architecture through the creation of networks of fractures, fracture corridors and

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faults (Al Kindi and Richard, 2014, Richard et al., 2014, Filbrandt et al., 2006).

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A simple two-stage scenario based on sub-surface kinematic indicators identified in the

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foreland of the Oman Mountains was proposed by Filbrandt et al., 2006 to describe the

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Mesozoic to Cenozoic deformation chronology (also known regionally as Alpine deformation). It

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involves a Late Cretaceous (Maastrichtian, -70 Ma) episode often referred to as “Alpine 1”

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deformation stage. Alpine 1 stage is characterised by an overall strike slip to extension regime

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with a maximum horizontal compression oriented along the north-west /south-east direction.

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The transtensional character of the Late Cretaceous deformation is indicated by typical

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conjugate, en-échelon fault patterns observed quite pervasively in the region’s sub-surface.

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This episode persisted most likely until the Early Palaeocene (-65 Ma), based on an apparent

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lack of conjugate faulting extending into the Shammar units (Mid-Palaeocene age). It

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corresponds roughly to the timing of the obduction of the Masirah ophiolite in the central

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eastern part of Oman (Rollinson, H., 2017, Immenhauser et al., 2000, Shreurs and

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Immenhauser, 1999, Shackleton and Ries, 1990). The Masirah ophiolite obduction event

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followed the earlier Semail ophiolite obduction of late Campanian age (-78 Ma) (Nicolas, 1989).

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The emplacement of the ophiolite units corresponds to one or more deformation climaxes.

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However, the geodynamic system at the origin of the obductions has evolved for many tens of

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millions years before the obduction phases (Gaina et al, 2015; Richard et al 2017). The crustal

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deformation processes associated with the two ophiolite systems are interpreted as

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responsible for the observed strike slip to extensional strain patterns. In a regime driven by

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overall NW-SE plate convergence, the origin of the extensional component (involving a vertical

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maximum stress direction) could be related to a three dimensional outer-arc-of-hinge context.

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Such a context could have developed in a crescent-shaped forebulge formed at the front of the

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ophiolite thrust. This context would have developed due to the crustal-scale flexure of the

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autochthonous formations deformed at the front of the obducted units. The two obduction

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episodes were followed by a period of relative tectonic quiescence during the Eocene to

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Oligocene (Nolan et al., 1990). At the end of this period the area was affected by a second

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major tectonic episode characterised by a strong north-east / south-west compression. This

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compression episode often referred to as “Alpine 2” deformation stage is Oligocene to Miocene

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in age (Filbrandt et al., 2006). It occurred in association with the Red Sea early opening phase to

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the West and South West part of the region, and the continental collision in the Zagros fold and

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thrust belt (Loosveld et al. 1996). In North Oman, the most obvious expression of the Alpine 2,

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mid to late Miocene compression stage is the creation of the Oman Mountains with

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culminations in the Al Jabal Akhdar, Saih Hattat, or Eastern Hajar regions (Filbrandt et al., 2006).

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In association with the collision phase, gravitational collapse events might have locally occurred

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in the areas of topographic culmination as reported by Hanna, 1990. However if such events

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might have occurred in the region of the Oman Mountains, they do not seem to be relevant in

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the sub-surface in the area of interest of this paper. In the sub-surface, clear evidences for

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compressional inversion of earlier extensional or strike slip structures inherited from the late

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Cretaceous episode are classically found in the North Oman hydrocarbon fields of Fahud and

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Natih (Al Kindi and Richard, 2014, Filbrandt et al., 2006). For example, one of the most

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impressive evidence of inversion of strike slip motion during the second phase of Alpine

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deformation was observed on a WNW-ESE fault of the Haban structure (Al Kindi and Richard,

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2014).

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Recent high-resolution 3D seismic data acquired or re-processed by Petroleum Development

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Oman (PDO) for North Oman enabled the observation of subtle structures such as Jurassic-

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seated inverted faults and low amplitude compressional folds at Cretaceous level. These were

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previously undetectable or very challenging to identify and hence were not reported by

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Filbrandt et al. (2006). This paper presents detailed observations of such structures in the

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Lekhwair oil field region and suggests interpretations in the context of the North Oman

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structural evolution. The aim is to highlight sub-surface structures which can help to constrain

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early deformation episodes. They would have occurred respectively at late-Triassic to early

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Jurassic time and during the mid-cretaceous (Aptian to Turonian), prior to the first Alpine stage

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of deformation (Alpine 1). Structures of similar age and style were also observed in seismic data

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in Oman (Al Kindi and Richard, 2014) and in Kuwait (Richard et al, 2014). However, the

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corresponding deformation episodes were previously overlooked or ignored due to the small

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number of clear examples both at surface (outcrops) and in the sub-surface. Nevertheless their

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identification and their integration as part of the North Oman tectonic history are important.

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They would indeed allow a better understanding, characterisation and calibration of sub-

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surface structures at regional scale as well as at the scale of the hydrocarbon fields. They may

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also have important implications in terms of charge and trapping mechanisms in these fields,

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although this discussion is not part of this paper’s focus.

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1- Geological context: the Lekhwair High area

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This paper focuses on the Lekhwair field area (or Lekhwair High area), which is situated in North Oman, close to the border with the United Arab Emirates (Figure 1). Structurally, and at

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regional scale, it is an uplifted zone which has registered a vertical movement of more than

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1000m locally (Filbrandt et al., 2006). It is located in the foreland of the Oman Mountains,

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about 100 Km southwest of the present-day frontal thrust of the Semail Ophiolite and

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Hawasinah complex. Various structural origins have been suggested by different authors to

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describe the origins of the Lekhwair High. Terken et al (2001) propose that it forms part of a

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foreland bulge. The bulge would have initiated and amplified in front of the Semail ophiolite

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and Hawasinah thrust complex during the late Cretaceous obduction phase. The cross-sections

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presented in Figure 2 illustrate this interpretation. However, Filbrandt et al (2006) object that

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the curvilinear subcrop pattern present below the Late Cretaceous / Tertiary unconformity does

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not support the hypothesis of a peripheral bulge related to the ophiolite emplacement and to

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the thrusting of related allochtonous units. Importantly, these authors also argue that that the

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long axis of the structures is perpendicular to any forebulge. Alternatively, Romine et al., 2004

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and Al Kindi and Richard (2014) suggest that the Lekhwair High could be part of the

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continuation of the Oman Western Deformation Front structure (Loosveld et al., 1996) and

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would be caused by the successive late Cretaceous and Miocene reactivations of deep half

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grabens present at Abu Mahara level (Neoproterozoic formations).

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It can be noted that the main Lekhwair High area is also essentially located outside the eastern

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limit of the Fahud Salt Basin (Figure 1). Hence no (or locally very little) Ara salt is present in the

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area of interest. This can be a factor influencing the structural style observed in the overburden

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and especially the creation of the characteristic dense conjugate fault pattern (“fishnet” fault

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pattern). Figure 3 shows a sub-surface tectonostratigraphic framework of the Lekhwair area

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along a short northwest to southeast section (inspired by Al Kindi and Richard, 2014 and

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Romine et al., 2004). The main regional tectonic events are put in perspective with a sketch

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compiling the major stratigraphic units deposited in the region. The complete absence of late

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Precambrian to Cambrian Ara salt can be noted in the Lekhwair section while it appears toward

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the Southeast, at the neighbourhood of the Yibal field. The Cambrian Nimr, Amin and Lower

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Haima units are also missing, or are much thinner in the Lekhwair region than in the

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southeastern part of the basin.

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The two main Alpine tectonic episodes which have occurred in the region are reported

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respectively at the end of the Cretaceous and during the middle part of the Cenozoic. In the

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Middle-East region, the early Mesozoic period is associated with the opening of the Tethys

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Ocean. Al Kindi and Richard, 2014, also report a phase of subtle “Jurassic rifting” in East Oman,

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synchronous with a NW-SE extension in the northwestern Arabian plate (Richard et al., 2014).

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2- Structural and kinematic indicators of the Alpine tectonic episodes in the Lekhwair High area

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This section of the paper summarises observations made in a sub-sample of the high quality 3D

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seismic survey available in the Lekhwair High area. The sub-sample covers 3 main hydrocarbon

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fields which include reservoirs in the Upper and Lower Shuaiba carbonate formations (Aptian).

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These fields, named respectively Field 1, 2 and 3 for the purpose of this paper are highlighted

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on the maps of Figure 4. The observation of various 3D seismic attribute volumes and of

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horizons in this sector enables the identification of characteristic structures which can be used

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as kinematic indicators of distinct stages of deformation.

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2.1.

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Three maps of the area of interest are presented in Figure 4. They correspond to different

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seismic attributes draped on or computed from the Top Shuaiba horizon picked for the whole

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regional volume. -

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Observations of structures at Top Shuaiba level (top horizon maps)

Figure 4 (a) shows a map view of the Top Shuaiba horizon draped with a seismic spectral decomposition attribute. 3D spectral decomposition attributes are usually used

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to identify heterogeneities of lithological origins. They can also enhance structural

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interpretation thanks to the sharp imaging of fault systems as depicted in the map

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(Ahmad and Rowell, 2012, Partyka et al., 1999). Red lineaments drawn on the map

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represent a large number of faults which could be interpreted at Top Shuaiba level. The

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faults belong clearly to two sets oriented respectively along the NNW and the WNW

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directions. The two sets form an acute angle of about 60 degrees, which suggests that

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they originated as conjugate fault sets. Locally, a clear tendency to en-échelon geometry

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can be observed, forming respectively WNW alignments with a left-stepping arrangement (dextral strike slip tendency) and NNW alignments with a right-stepping

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arrangement (senestral stike-slip tendency).

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The map of Figure 4 (b) displays the horizon curvature attribute computed at Top

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Shuaiba level. The horizon curvature was calculated in Fault and Fracture Solutions,

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which is Shell’s proprietary software package for structural geology and fractured

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reservoir characterisation and modelling (previously known as SVS, Rawnsley et al.,

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2004). Horizon curvature can be calculated at different wavelength in order to highlight

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various types and scales of structures. On this map, a wavelength of 2500 m was used. It

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clearly shows alternating trends of high positive (red) and negative (blue) curvature

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elongated patterns striking toward the northeast. The zones of high positive curvature

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correspond in this case to low-amplitude anticlinal structures while the negative

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curvature zones underline synclines. An illustration is given by the AA’ schematic cross-

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section which cuts through the Field 2 structure. Generally, the anticlines overlap quite

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well with the outlines of the main hydrocarbon fields, at least in the western part of the

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area of interest.

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The map of Figure 4 (c) displays the horizon curvature attribute computed at Top Shuaiba level with a wavelength of 50 m. It highlights a set of NNW and WNW

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lineaments which differs drastically from the northeast trending structural grain

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depicted by the 2500 m curvature attribute as displayed in Figure 4 (b). The

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orientation, geometry and location of these short wavelength lineaments overlaps with

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the characteristics of the conjugate, en-échelon faults interpreted on the map of Figure

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4 (a). In vertical cross-section they appear as successions of small throw, high angle

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normal faults often forming narrow grabens, as illustrated in the sketch at the bottom

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right of the map (BB’ section).

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Figure 5 captures a sector of the 50 m wavelength curvature map presented in Figure

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4 (c). The selected sector is centred on Field 3. On the map, NE-SW lineaments forming a

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clear but much localised en-échelon pattern are highlighted in white. Lineaments similar

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in terms of orientation and geometry are visible further to the west of the area of

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interest, although not as clearly highlighted by the horizon curvature attribute (see

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Figure 4 (c), south of Field 1 structure).

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2.2.

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Kinematic interpretations of the above observations in the structural context of North

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The WNW-SE and NNW-SSE lineaments observed in the maps of Figure 4 (a) and Figure 4 (c)

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form a dense conjugate, locally en-échelon set of faults and small-scale grabens. Following

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Filbrandt et al., 2006, these fault sets are classically interpreted in the region as a transtensional

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fault pattern resulting from the first stage of Alpine deformation which occurred during the late

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Cretaceous (Al Kindi and Richard, 2014, Filbrandt et al., 2006). The large wavelength, low

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amplitude anticlines of NE-SW axis (Figure 4 (b)) are most likely the result of a compressional

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folding event (Al Kindi and Richard, 2014). The axis of the compression necessary to initiate and

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develop such anticlines was oriented in the NW-SE direction. The kinematics of these folds and

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the origin of the compression will be discussed in the following sections of this paper.

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The NE-SW lineaments observed in Figure 5 and to the southwest of Figure 4 (b) curvature

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map have a clear en-echelon geometry which points-out that their origin is most likely

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associated with a strike slip dominated regime. Their NE-SW orientation is compatible with an

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initiation and development during the second stage of Alpine deformation at Miocene age.

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2.3.

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Observations at Jurassic level and associated structures (data from vertical cross-

sections and horizons)

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Figure 6 shows a NW-SE seismic section through Field 1 and Field 2 areas. Vertically the

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stratigraphic intervals covered by the section range roughly from surface (Quaternary deposits)

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down to base Jurassic. The stratigraphic intervals were interpreted and confirmed thanks a

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significant number of well ties. More than 1200 wells were available in the area of interest at

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the time of the study, although only a small number them penetrated the lower stratigraphic

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units. From bottom to top the following structural observations can be made. -

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Three faults are present across the Jurassic units below Field 1 and Field 2, up to top Mafraq, which is the stratigraphic position of the Top Jurassic to Base Cretaceous

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unconformity in the Lekhwair High area. The largest of these is located below the

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Eastern part of Field 1 and shows a clear reverse offset of the top Mafraq reflector. The

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throw of the smaller faults is unclear. The detail view of Figure 6 (b) highlights the fact

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that the footwall compartment of the main (largest) fault is consistently located at a

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shallower structural position than the hanging wall. -

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The top Mafraq to top Shuaiba interval (Cretaceous units) present low amplitude folds overlying the faults observed in Jurassic intervals. These folds form structural highs

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which control the structure and closure of Field 1 and Field 2. They correspond to the high positive curvature lineaments illustrated in map view by Figure 4 (b).

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The local structural high formed by the folded Cretaceous units in Field 1 is clearly cut

across by the base Cenozoic reflector. Here the Cenozoic (Tertiary) unit forms a clear

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unconformity of erosional origin cutting across the top of the top Shuaiba. Cenozoic

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formations deposited above top Shuaiba show no obvious evidence of flexure.

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The fault pattern present at top Mafraq level can be observed on the map of Figure 7. The

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WNW-SE and NNW-SSE conjugate set described at top Shuaiba level (Figure 4(a) and Figure 4

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(c)) is present (red lineaments) but appears less dense and possibly less segmented. Their

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tendency to form en-échelon patterns seems less clear. A set of NE-SW oriented fault appears

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at top Mafraq level, essentially located in the areas of Field 1, 2 and 3. They are underlying the

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low amplitude folds observed in Figure 4 (b) and Figure 6 at Cretaceous level.

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The geometry of the NE-SW faults at Mafraq level and their interactions with the other

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structural elements present in the area of interest is illustrated by the map and section of

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Figure 8 for a sector centred on Field 3. Figure 8 (a) and (b) show that the NE-SW faults (in

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blue) at Mafraq level are cut-across and offset laterally by the NNW-SSE and WNW-SE

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conjugate faults (in red). The apparent horizontal offset along the NNW-SSE and WNW-SE faults

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is limited. It ranges typically between 50 and 150m. The cross section of Figure 8 (c) shows that

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low amplitude folds are also present in Field 3 in shallower units (Cretaceous). The folds are

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localised above the tip of the faults present at Jurassic/Mafraq level (as observed for Field 1 and

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2 in Figure 6). The relative position of the footwall and hanging wall compartments around the

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fault is similar to the configuration observed in the example of Figure 6 (b), with a footwall

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generally shallower than the hanging wall.

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2.4.

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Kinematic interpretations of the above observations in the structural context of North

Oman

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The relative structural position of the footwall and hanging wall compartments of the reverse

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faults observed in the Jurassic intervals below Fields 1, 2 and 3 tend to imply that these faults

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were not initiated as reverse faults. They would rather result from the compressional

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reactivation (inversion) of pre-existing normal faults forming series of horst and grabens. The

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most probable timing for the normal faulting of the Jurassic units would be the Tethys

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rifting/opening stage which occurred during the early Mesozoic (Figure 3). Part, if not all, of

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these fault development was therefore probably synchronous with the deposition of the

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Jurassic sediments. Although evidence is not easy to identify in the area of interest studied for

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this paper due to the insufficient depth range covered by the seismic data, a Triassic to Jurassic

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extensional origin of the deep NE-SW inverted fault system is most likely. Such origin can clearly

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be evidenced in the Yibal area (North Oman), with the observation of clear fault-growth

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structures (Bazalgette et al., in Preparation). The inversion of the Jurassic-seated NE-SW faults

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must have occurred under a compressional setting (reverse faulting conditions) characterised

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by a NW-SE oriented maximum compression. This is supported by the observation of the

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compressive folds affecting the Cretaceous units up to the Cenozoic unconformity. These folds

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are the expression of the inversion of the Jurassic-seated fault in their overburden. They are

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compatible with a NW-SE orientation of compression and provide an evidence that the

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compressional event occurred after the deposition of the latest folded units (Upper Cretaceous,

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Aptian-Albian to Cenomanian age) and before the deposition of the post-unconformity

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Cenozoic formations (Palaeocene age). This relative timing range is narrowed down thanks to

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the observed intersections and offset of the NE-SW Jurassic-seated reverse faults by the WNW-

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SE and NNW-SSE conjugated fault pattern of late Cretaceous (Maastrichtian) age. In the cross-

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section views of Figure 6 and Figure 8 (c), Jurassic-seated faults of smaller scale are

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interpreted as lower confidence faults (identified by dashed lines) and have an unclear throw,

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though they are still observable as relatively clear discontinuities of seismic reflectivity and of

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reflector dip. The unclear throw may be the result of a incomplete inversion of the early

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Jurassic normal fault during the later compressional stage. The geometrical relationships

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between the different fault sets present at Jurassic level are summarised in the block diagram

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of Figure 9. The implications of these geometrical relationships in terms of fault kinematics,

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deformation timing and evolution of the main stress component orientations are also captured

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in Figure 9.

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3- Reconstruction of the Mesozoic to Cenozoic structural evolution of the Lekhwair area

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The compilation and sorting of the observations and interpretations listed in the previous

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section of this paper allow to propose a multi-phased scenario for the Mesozoic to Cenozoic

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structural and kinematic evolution of the Lekhwair High area. A summary of the structures

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described above is given in

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Table 1 together with hypotheses to explain their presence in the Lekhwair High area.

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Following this, the multi-phased deformation scenario is illustrated by the block diagram

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compilation of Figure 10.

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The structural evolution starts at Early Jurassic time with the deposition of the corresponding

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stratigraphic units in a context of relative tectonic quiescence (Figure 10 (a)). Later during the

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early to mid Jurassic period, a stage of NW-SE extension is registered. During this period, a set

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of NE-SW normal faults is created locally. It is roughly restricted to the Jurassic (sub-Mafraq)

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formations (Figure 10 (b)). The Jurassic extension stage is followed by a period of tectonic

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quiescence, ranging roughly from the end of the deposition of the Mafraq formation up to the

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late Cretaceous time (Figure 10 (c)). Following this quiescent episode, the Lekhwair High area

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undergoes a NW-SE oriented compression (“Alpine 0” compression). The compression influence

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is clearly evidenced by the local inversion of the Jurassic-seated normal faults and by the

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compressional folding of the above-lying Cretaceous formations (Figure 10 (d)). The “Alpine 0”

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stage of deformation is at the origin of the structure initiation of hydrocarbon field 1, field 2

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and field 3 (Figure 4). Its expression is subtle and appears limited in the area of interest to

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zones where Jurassic normal faults were present. The Jurassic faults most likely provided

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“weakness zones”, which inversion enabled the formation of the local folds at Cretaceous

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levels. Without such weakness zones, the intensity of the “Alpine 0” compression phase seems

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to be insufficient to trigger the development of structures observable at the resolution and

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scale of the seismic dataset used in this paper. We propose a late Cretaceous age for the

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“Alpine 0” phase of deformation. The timing of this phase postdates the deposition of the

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Cretaceous carbonate reservoirs of North Oman (including the Shuaiba formation which

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produces oil in the area of interest) and could coincide with the early stages of regional

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shortening setting up the scene for the later obduction of the Oman ophiolites. At the end of

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the Cretaceous (Maastrichtian), the dominant NW-SE compression stops and the main

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compressional stress tilts to vertical (σ1= σV) while the main horizontal stress keeps its initial

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NW-SE direction (σ2= σH). At this time the regional context is dominated by the obduction

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phases of the Semail ophiolite (Cogniacian to Mid-Campanian age, Searle, 2007) followed by

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the Masirah ophiolite (Late Maastrichtian to Palaeocene, Gnos and Perrin, 1996, Marquer et al.,

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1998, Peters, 2000) obduction episodes. This regional deformation stage (“Alpine 1” phase)

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triggers the development of the conjugate NNW-SSE and WNW-SE fault pattern (Figure 10 (e)).

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The “Alpine 1” phase persists until the end of the Maastrichtian period and possibly until the

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early Palaeocene. It is followed by a tectonic quiescence period between the Palaeocene and

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the Oligocene to Miocene times (Figure 10(f)). During this time period, the reliefs created

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during the “Alpine 0” and “Alpine 1” phases get eroded and later early Cenozoic formations

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(essentially Palaeocene to Eocene in age) are deposited on top of the unconformity. At

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Oligocene to mid-Miocene time a strong NE-SW oriented compression occurs at regional scale

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(“Alpine 2” phase), in association with the continental collision in the Zagros region and the

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creation of the Oman Mountains. In North Oman, this phase is associated among others with

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the local inversion of earlier structures as observed in Natih and Fahud fields (Al Kindi and

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Richard, 2014). Transpressional or compressional fold and fault patterns also initiate within this

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context, for instance in the foothills of the Salakh Arch (Storti et al., 2016). In the Lekhwair High

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area this phase of deformation is extremely subtle. The only clear evidence highlighting it is the

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presence of the NE-SW en-échelon fault pattern in and close to the southern region of

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hydrocarbon Field 2. This deformation episode is illustrated by Figure 10 (g). The subtle

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character of the structures resulting from the “Alpine 2” deformation phase in the Lekhwair

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region may be interpreted as the result of local stress distribution heterogeneity. This could be

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due to the relative neighbourhood of the Salakh Arch foothills and of the Natih and Fahud

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structures. These large-scale structures have undergone much more intense deformation at

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that time and could have acted as a local stress shield.

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4- Discussion

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4.1.

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Traditionally, the first phase of Alpine deformation is described in North Oman, south of the

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Semail ophiolite front, as well as regionally as a single tectonic episode characterised by a

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strong regional strike slip to extension regime (Richard et al., 2017, Al Kindi and Richard, 2014,

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Filbrandt et al., 2006, Loosveld et al., 1996). However, structural observations documented in

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this paper show that a clear compressional tectonic pulse predated locally the regional strike

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slip to extension phase. This compressional pulse is evidenced in the Lekhwair High area by the

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presence of inverted faults affecting the Jurassic formations and by compressional folds

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Regional importance of the “Alpine 0” compression pulse

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developed in the Cretaceous overburden, prior to the Cenozoic unconformity (Figure 10 and

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Table 1). Similar observations have been documented recently in neighbouring fields in the

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UAE (Richard et al., 2017), as well as further away in the North Kuwait carbonate fields (Richard

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et al., 2014). Despite the subtle character of the structures created during this compressional

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pulse, examples described in this paper and the UAE and North Kuwait cases show that it can

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be at the origin of hydrocarbon traps of significant importance. This justifies a clear distinction

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from the “Alpine 1” deformation phase.

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4.2.

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Regional geodynamic context: origins of the Jurassic extension, “Alpine 0” compressional pulse and transition to “Alpine 1” strike slip to extension

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The local observations documented above and their implications for the North Oman structural

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evolution must be put in perspective with the large-scale geodynamic context and with the

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tectonic interactions at plate boundaries between India and Arabia. Gaina et al., 2015, Richard

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et al., 2014, Konert et al., 2001, Loosveld et al., 1996 used various sources of geological and

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geophysical date to constrain these context and interactions at the scale of plate tectonics. -

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Jurassic extension

From 170 to 150 Ma, Gaina et al., 2015 describe the southeast drifting of the Indian block

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towards the southeast, away from the Arabian plate (Figure 11 (a)). This kinematic setting

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occurs in association with the Neotethyan Ocean opening. During this period, oceanic spreading

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processes are active and oceanic crust is created at the ridge between the Southeastern margin

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of Arabia and the Indian block. This time corresponds to the phase of production at the ridge of

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oceanic lithosphere rocks, which will be later involved in the Masirah ophiolite units (Rollinson,

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2017). In the Arabian plate, this context is favourable to the occurrence of a local NW-SE

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extension regime and to the creation of the Jurassic-seated normal faults observed in North

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Oman as documented in this paper and by Al Kindi and Richard, 2014.

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“Alpine 0” compression

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During the late Cretaceous (about 87 Ma (Campanian)), the initial motion of India towards the

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southeast gets reversed. The Indian plate follows an anticlockwise rotation combined with a

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general motion toward the northwest. (Figure 11 (b)). During the earlier stage of this

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geodynamic phase, the northwest motion of the Indian plate induces a compressional regime in

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the Arabian plate. This earlier episode corresponds to the “Alpine 0” compressional pulse

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evidenced by the inverted normal faults and compressional folds described in this paper. We

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suggest that the “Alpine 0” episode may stop during the obduction of the Semail ophiolite (78

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Ma, Nicolas et al., 1989, Searle, 2007). At that time, the obduction of dense oceanic lithospheric

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material creates a significant overload on the northern edge of the Arabian plate and a low

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amplitude flexure at its front. This flexure can generate a certain amount outer arc of hinge

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extension responsible of the transition between the “Alpine 0” NW-SE compression to a strike

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slip to extensional regime (“Alpine 1”) characterised by a NW-SE orientation of the maximum

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horizontal stress axis. The “Alpine 1” deformation phase persisted until early Palaeocene (65

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Ma) and the episode of obduction of the Masirah Ophiolite.

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Similarly to those observed in the Lekhwair High, faults of Jurassic origin inverted during the

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Late Cretaceous have been found regionally. They have been documented in the UAE and in

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Kuwait (Richard et al., 2017, Richard et al., 2014). Such structures also exist in Oman at the

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close southeast of the area of interest (Bazalgette, in Prep). This tends to give a regional

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significance to the “Alpine 0” compressional pulse.

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Conclusion

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The observation and interpretation of high quality 3D seismic in the Lekhwair High area enabled

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the identification of features which can enhance the understanding of the structural evolution

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at local and regional scale. Jurassic-seated faults were documented. They represent the

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expression of a subtle extension phase in North Oman, which can be related to the major

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regional phase of Jurassic rifting. The local inversion of these Jurassic faults evidences a pulse of

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compression (“Alpine 0” compression) which occurred before, and needs to be distinguished

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from the regionally documented first phase of Alpine deformation (“Alpine 1” strike slip to

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extension). The integration of these observations in the regional tectonic context documented

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in the abundant bibliography enabled proposing both chronological and geodynamic

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interpretations. These interpretations take into account the main geological events which have

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occurred since the late Cretaceous, including the two stages of ophiolite obduction.

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The effect of the late Cretaceous compression pulse on the structuring of North Oman is subtle

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and its observation is typically associated with the presence of Jurassic-seated faults. However

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it is at the origin of the creation of structures hosting major hydrocarbon accumulation in North

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Oman (Lekhwair High area, Yibal area) and in other countries in the Middle East (Kuwait, UAE).

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Acknowledgements

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The authors would like to thank PDO and the Ministry of Oil and Gas (MOG) in the Sultanate of

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Oman for their support and for the permission to publish this paper. Many thanks to Pascal

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Richard, to Hendrick Vermont, Joachim Amthor and to Jurg Neidhardt for exciting and fruitful

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technical discussions about the geology of North Oman and of the Middle-East Region. Authors

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would also like to thank Pr. Bruce Levell (Oxford University) and an anonymous reviewer for

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their very careful and constructive work in reviewing and improving this paper.

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Figure 1: North Oman regional map (modified from Al Kindi and Richard, 2014, Al-Barwani & McClay, 2008, Al-Siyabi, 2005, Loosveld et al., 1996 and Droste, 1997). The area of interest (circled with blue dashed-line) is located in the Southern foreland of the Oman Mountains, in the northwestern part of Oman. The black dashed-line aims at roughly highlighting the North Oman region (coloured map). It is not a representation of political borders.

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Figure 2: Simplified regional NE-SW cross-section through the Oman Mountains, the Fahud Basin and the Lekhwair High (inspired by and modified from Terken et al., 2001, Terken, 1999 and Hanna, 1990). In this section, the Lekhwair High is interpreted as part of the “foreland bulge” which limits the southwest extent of the flexural basin and was developed at the front of the Oman Mountains during the late Cretaceous Semail Ophiolite obduction episode. (a) North Oman geographical map with cross-section location. (b) NE-SW cross-section.

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Figure 3: Simplified sub-surface tectonostratigraphic framework of the Lekhwair region (Inspired by and modified from Al Kindi and Richard, 2014, and Romine et al., 2004).

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Figure 4: Top Upper Shuaiba horizons showing (a) seismic spectral decomposition attribute, (b) seismic spectral decomposition attribute, major hydrocarbon field outlines and main interpreted fault lineaments, (c) large wavelength (λ λ=2500 m) horizon curvature attribute and (d) short wavelength (λ λ=50 m) horizon curvature. Positive curvature values (in red) indicate convex-upward structures (e.g., anticline folds). Negative curvature values indicate convex downward structures (e.g., syncline folds). Note that the geometric effect of fault throw (even subtle) on the topography of seismic horizons is similar to this of short wavelength folds.

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Figure 5: Detailed view of top Upper Shuaiba horizon curvature (l=50m) centred on Field 3. A set of NE-SW oriented lineaments are visible in the western part of the field structure (interpreted as plain white lines (high confidence lineaments) and as dashed white lines (lower confidence lineaments)).

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Figure 6: (a) Raw WNW-ESE seismic cross-section (amplitude attribute) through Field 1 and Field 2 structures (two way time, 5 x vertical exaggeration). (b) Interpreted NW-SE cross-section. The base Cenozoic reflector is highlighted in yellow. It forms an erosive contact through the underlying Upper Shuaiba unit (green reflector). The apparent on-lap patterns formed by the reflectors below this horizon and which intersect the upper Cretaceous units are artefacts (seismic multiples of the base Cenozoic reflector). The Cretaceous reflectors show a succession of low-amplitude folds (see Figure 4 (b) for map view) initiated above underlying reverse faults. These faults are observed at Mafraq level and below (Jurassic-age formations). (c)

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Zoom focusing on the Jurassic part of the section, around the main reverse fault below Field 1. As underlined by the black arrows, one can observe that the fault’s footwall compartment is located consistently at a shallower structural position than the hanging wall, while moving away from the fault zone. This shows that the fault at Jurassic level have most likely originated as normal faults and were reactivated (inverted) lately under a compressional deformation regime (horizontal shortening axis oriented NW-SE).

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Figure 7: Top Mafraq (Jurassic) seismic amplitude map with simplified structural interpretation. The map shows the interaction between a conjugate WNW-SE and NNW-SSW fault pattern (red lineaments) and a set of NE-SW oriented faults (highlighted in blue). The NE-SW faults observed at Jurassic level in Figure 6 are visible respectively within Field 1 and Field 2 outlines and in their alignment.

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Figure 8: (a) Detail map of Top Mafraq horizon centred on Field 3. (b) Interpretative lineament map showing in red the WNW-SE and NNW-SSE conjugate fault network (strike slip to extension origin). NE-SW Lineaments highlighted in light blue correspond to reverse faults observed on the large scale map of Figure 7 and on the cross section of Figure 6 (plain line: high confidence faults, dashed lines: low confidence faults). The NE-SW faults are cut-across and offset laterally by the WNW-SE and NNW-SSE conjugate faults. (c) NW-SE seismic cross-section through the above structure (see Figure 8 (a) for section trace).

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Figure 9: Simplified block diagrams (not to scale) summarising the geometrical and chronological relationships between the Jurassic-seated NE-SW reverse faults (in blue), the low amplitude NE-SW fold in the Cretaceous overburden and the NNWSSE and WNW-SE conjugate fault set (in red). The diagrams are inspired by the observations of seismic data in Field 3 (Figure 8). The NE reverse fault is cut-across and offset laterally by the cojugate set, which evidences an earlier origin for the former. The lateral offset along the WNW-SE fault segments is dominated by dextral strike slip movement while the offset along the NNW-SSE set is senestral. These cross-cutting and offset relationships imply that the reverse fault set occurred during a first compressional event (T1). The stress field at T1 was characterised by a horizontal, NW-SE oriented maximum stress (red σ3=σ σV). The conjugate fault pattern occurred arrows, σ1=σHmax) and a vertical direction of the minimum stress component (σ afterward, during a stage T2. In terms of stress, T2 was characterised by a maximum stress oriented vertically (σ σ1=σ σV) and a NW-SE maximum horizontal stress component (red arrow, σ2=σ σHmax). It can be noted that the direction of the maximum horizontal stress component did not change between T1 and T2, the main difference being the rotation of σ1 from horizontal (NW-SE) to vertical. (a) Block diagram focussing on fault cross-cutting relationships at Mafraq level. (b) Block diagram including low amplitude folds observed in the Cretaceous overburden. (c) Stress field conditions during the creation of the conjugate fault pattern. The magnitude of the vertical stress and maximum horizontal stress components were most likely similar to explain the co-existence of a combined vertical and lateral offset along the fault planes.

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Figure 10: Series of conceptual bloc diagrams summarising the Mesozoic to Cenozoic structural and kinematic evolution in Lekhwair area (diagrams compile structures observed in Field 1, Field 2 and field 3).

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Figure 11: Regional-scale geodynamic context and implications on local deformation context in North Oman (inspired and modified from Gaina et al., 2015 and Richard et al., 2017). (a) Context at Jurassic time: the Indian block moves toward the SE, away from the Arabian plate. This causes a local NW-SE extensional regime in the Arabian plate. (b) Context at late Cretaceous time: the earlier SE movement of India with regards o Arabia gets inverted. The Indian plate rotates anticlockwise towards the NW. Two main stress episodes are registered on the Arabian plate. (1) NW-SE compression (i.e., “Alpine 0”) during the early inversion phase, followed by (2) NE-SW strike slip to extension (“Alpine 1”, with maximum horizontal stress oriented NW-SE). Episode (2) would be roughly synchronous with the initiation of the obduction (78 Ma) and may be related to a local outer-arc of hinge extension occurring at the front of the Semail ophiolite obduction zone. The “Alpine 1” would end during the Early Palaeocene, during the obduction of the second Oman ophiolite (Masirah ophiolite).

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Table 1: Summary of structures observed in the Lekhwair High area, interpreted deformation and stress regime and proposed timing of occurrence or reactivation. Note that the latest NE-SW en echelon faults have only been clearly observed at the close neighbourhood of hydrocarbon field 2. Simplified regional-scale sections are proposed for the two Cretaceous episodes, highlighting the respective deformation and stress contexts.

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Deformation kinematics Faulting under strike-slip Tectonic quiescence

σH axis orientation NE-SW

Proposed age and context of occurrence or reactivation Oligocene to Miocene

Tectonic phase Alpine 2

N/A

Paleocene to Oligocene

N/A

Faulting under extension / strike-slip / transtension

NW-SE

Late Cretaceous (Maastrichtian)

NE-SW compressional folds in Cretaceous units

Folding under compression

NW-SE

Late Cretaceous (Post Albian-Cenomanian and Pre-Maastrichtian)

Alpine 0 (or early Alpine 1)

NE-SW Jurassic-seated faults

Fault reactivation under compression

NW-SE

Late Cretaceous (Post Albian-Cenomanian and Pre-Maastrichtian)

Alpine 0 (or early Alpine 1)

NE-SW Jurassic-seated faults

Faulting under extension

Late Triassic to Early Jurassic

Thetys rifting

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Observed structures NE-SW enéchelon faults CretaceousCenozoic unconformity NW-SE and NNW-SSE conjugate faults

NE-SW

Alpine 1 (or Late Alpine 1)

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Highlights

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A compilation of structural observations from good quality seismic data is presented Interpretations based on these observations are proposed, in the context of the North Oman structural evolution The role of the early Mesozoic rifting and late Cretaceous compression on the regional structure is highlighted

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