North Oman Research Articles

Static Geological Modelling with Knowledge Driven Methodology

Geological modelling is an important topic of oil and gas exploration and production. A new knowledge driven methodology of geological modelling is proposed to address the problem of “hard data” limitation and modelling efficiency of the conventional data driven methodology. Accordingly, a new geological modelling software (DMatlas) (V1.0, Dimue, Wuhan, China) has been developed adopting a grid-free, object-based methodology. Conceptual facies models can be created for various depositional environments (such as fluvial, delta and carbonates). The models can be built largely based on geologists’ understandings with “soft data” such as outcrops analysis and geological maps from public literatures. Basic structures (fault, folds, and discrete fracture network) can be easily constructed according to their main features. In this methodology, models can be shared and re-used by other modelers or projects. Large number of model templates help to improve the modelling work efficiency. To demonstrate the tool, two case studies of geological modelling with knowledge driven methodology are introduced: (1) Suizhong 36-1 field which is a delta depositional environment in Bohai basin, China; (2) a site of the north Oman fracture system. The case studies show the efficiency and reliability within the new methodology.

Aquatic Coleoptera of North Oman, with description of new species of Hydraenidae and Hydrophilidae

Abstract We report the aquatic Coleoptera (families Dryopidae, Dytiscidae, Georissidae, Gyrinidae, Heteroceridae, Hydraenidae, Hydrophilidae and Limnichidae) from North Oman, mostly based on the captures of fourteen localities sampled by the authors in 2010. Four species are described as new, all from the Al Hajar mountains, three in family Hydraenidae, Hydraena (Hydraena) naja sp. nov., Ochthebius (Ochthebius) alhajarensis sp. nov. (O. punctatus species group) and O. (O.) bernard sp. nov. (O. metallescens species group); and one in family Hydrophilidae, Agraphydrus elongatus sp. nov. Three of the recorded species are new to the Arabian Peninsula, Hydroglyphus farquharensis (Scott, 1912) (Dytiscidae), Hydraena (Hydraenopsis) quadricollis Wollaston, 1864 (Hydraenidae) and Enochrus (Lumetus) cf. quadrinotatus (Guillebeau, 1896) (Hydrophilidae). Ten species already known from the Arabian Peninsula are newly recorded from Oman: Cybister tripunctatus lateralis (Fabricius, 1798) (Dytiscidae), Hydraena (Hydraena) gattolliati Jäch & Delgado, 2010, Ochthebius (Ochthebius) monseti Jä ch & Delgado 2010, Ochthebius (Ochthebius) wurayah Jäch & Delgado, 2010 (all Hydraenidae), Georissus (Neogeorissus) chameleo Fiká č ek & Trá vní č ek, 2009 (Georissidae), Enochrus (Methydrus) cf. tetraspilus Régimbart, 1903, Laccobius (Hydroxenus) leucaspis Kiesenwetter, 1870, Paracymus relaxus Rey, 1884 (all Hydrophilidae), Dryops lutulentus (Erichson, 1947) (Dryopidae), and Augyles flavidus (Rossi, 1794) (Heteroceridae). A total of 73 species of aquatic Coleoptera are presently known from Oman, although this number is expected to increase substantially with further sampling. A checklist of the species of aquatic Coleoptera from the Arabian Peninsula is provided.

Diving beetles of the genus Yola Gozis, 1886 from Oman, with description of a new species (Coleoptera: Dytiscidae)

Two species of the genus Yola Gozis, 1886 were recently collected in Al Dakhiliyah Province, north Oman, both members of the Yola bicarinata species group: Yola porcata (Klug, 1834), previously known in the Arabian Peninsula only from Saudi Arabia, is recorded from Oman for the first time; Yola unguicularis sp. n. is described and illustrated. The new species is unique within the members of the genus with modified protarsal claws. The number of known Yola species hereby increased to 48; six species are presently known from the Arabian Peninsula. The total number of Dytiscidae species occurring in Oman increased to 30.http://www.zoobank.org/urn:lsid:zoobank.org:pub:1528E569-B970-4612-A572-13B00D2992FD

Calcareous nannofossil assemblages of the Late Cretaceous Fiqa Formation, north Oman

Abstract. This study presents the first detailed calcareous nannofossil assemblage data from the Late Cretaceous succession of the subsurface Aruma Basin, north Oman. The taxonomic description and documentation of assemblage composition are based on extensive quantitative analysis of ditch cuttings and side-wall samples from eight hydrocarbon exploration wells across north Oman. The samples studied from those wells cover the Coniacian to earliest Maastrichtian deep marine shales and marls of the subsurface Fiqa Formation. These fine-grained siliciclastic deposits often yield moderately to well-preserved nannofossil assemblages, especially in the Campanian intervals. Consequently, diverse assemblages have been recorded from the Fiqa Formation, with a total diversity of ∼200 species, including two new species, Staurolithites ormae sp. nov. and Chiastozygus fahudensis sp. nov., which are illustrated and described. Extensive imaging of this diversity is provided here, as are stratigraphic distributions of the main components from a key reference well in north Oman, W-4. Poorly described groups such as Staurolithites are closely investigated and their utility for stratigraphic applications is highlighted. Relative abundances of nannofossil taxa with strong palaeoenvironmental preferences have revealed new insights into the palaeo-productivity of the basin. High-fertility species like Discorhabdus ignotus, Biscutum constans and Zeugrhabdotus erectus show substantial variations in abundance throughout the Fiqa Formation, reflecting long-term shifts in the productivity conditions of the basin. This is supported by abundance patterns of Watznaueria barnesiae and Micula staurophora that show a broadly inverse correlation with the high-fertility species. The Fiqa Formation represents a key regional seal unit for the Cretaceous hydrocarbon reservoirs of Oman, as well as being a productive unit elsewhere in the Arabian Peninsula. Beyond the Aruma Basin of Oman, this study will provide a key reference point for future biostratigraphic or palaeoenvironmental analyses of the Late Cretaceous calcareous nannofossil assemblages across the Middle East and other southern Tethyan areas.

Tectonic control on vein attributes and deformation intensity in fault damage zones affecting Natih platform carbonates, Jabal Qusaybah, North Oman

Understanding the factors that control fracture patterns in fault damage zones is fundamental to predicting fault zone permeability in the subsurface. In this contribution, we present outcrop data on vein attributes collected from 26 fault zones (10 strike-slip and 16 normal dip-slip) that cut Cretaceous Natih Formation platform carbonates exposed in the Jabal Qusaybah anticline, North Oman. Faulting occurred during the growth of the salt-cored anticline and progressed initially from dominant strike-slip faults (burial depth ∼3–4 km) to late normal dip-slip faults (burial depth <1–2 km). The displacements accommodated by both kinematic fault types range similarly from 0.1 to 100 m, and damage-zone width increases with displacement at the same rate for both types. Vein aperture (A), height (H), and spacing (S) were measured in vertical cross-sections (n = 10839 data) along fault-perpendicular, linear scanlines across fault damage zones. Data analyses indicate that, as the master slip surface is approached in each fault zone: (1) vein aperture and height generally increase; (2) vein spacing systematically decreases; and (3) deformation intensity, calculated as vein H/S ratio, increases. However, median H/S values calculated in each damage zone do not show a robust correlation with fault displacement. When analyzed collectively across-fault distributions, H/S ratios indicate that deformation intensity (i) in normal dip-slip fault damage zones is greater than in strike-slip fault damage zones; (ii) in strike-slip fault damage zones is symmetrical with respect to the master slip surface; and (iii) is locally asymmetrically distributed with greater deformation intensity in footwall blocks for normal dip-slip faults. Greater deformation in normal dip-slip fault zones is expressed by greater vein height, rather than smaller vein spacing. The main conclusion is that deformation intensity and vein attributes in fault damage zones are primarily controlled by burial stress and fault kinematics, and less importantly by rheological contrasts between layers, rather than local stress induced by fault displacement.

Lexical Resemblance among Modern South Arabian Languages in Oman

Modern South Arabian (MSA) languages make one central group of three distinct language groups that comprise minority languages in Oman. Contrary to their counterparts spoken in the north of Oman, MSA languages are spoken in the southern part of the country with some spoken in neighboring Yemen. Due to both geographical and linguistic proximity among these languages, they are often viewed even by some of their speakers as dialects of one another rather than languages of their own. Accordingly, the improper term 'dialects' is often used to refer to these languages in reference to other languages within the group. Chiefly based on common lexical items, this view, however, is unsubstantiated on research basis. This paper, hence, is an attempt to vindicate such view by measuring the extent of lexical resemblance among these languages using the Swadesh's one hundred word list as its framework. To this effect, speakers were asked to report word recognition of lexical items under investigation as well as mutual intelligibility to sentences in which recognized lexical items were used. Findings show that although there is a huge lexical resemblance among these languages exhibited by the fact that speakers could recognize numerous words from these languages, native speakers reported minimal mutual intelligibility to these languages.

Modern Recharge in a Transboundary Groundwater Basin Deduced from Hydrochemical and Isotopic Investigations: Al Buraimi, Oman

Groundwater samples (54) collected from different geological units (alluvium, Tertiary, ophiolite, and Hawasina) located in the transboundary groundwater basin in north Oman at the United Arab Emirates (UAE) borders were analyzed for general hydrochemistry and water isotopes, and subsets thereof were analyzed for 14C and 3H and 87Sr/86Sr. The chemical composition, percentage of modern carbon (pmc), δ2H, δ18O, and 87Sr/86Sr of the groundwater in the study area progressively change from the recharge zone in the elevated area of the North Oman Mountains (NOM) to the flat plains at the UAE borders. While the water-rock interaction is the dominant process controlling the groundwater chemistry, evaporation and groundwater mixing affect the hydrochemistry at the UAE borders. Therefore, groundwater evolves from carbonate-dominant in the NOM into sodium chloride-dominant close to the UAE borders. It is also evident that groundwater lateral recharge from the ophiolites into the alluvium retains the chemical affinity of the ophiolites. Groundwater dating (high pmc), homogeneous 87Sr/86Sr ratios, and enriched δ2H and δ18O demonstrate the presence of modern recharge in the shallow zones of the ophiolites and alluvium. However, deep zones and areas at the UAE border contain older groundwater form during cooler and wetter climatic conditions as supported by the depleted δ2H and δ18O and lower 87Sr/86Sr ratios and pmc. Furthermore, the data clearly showed that modern groundwater mixes with older groundwater along the flow path from the NOM into the UAE border. Modern recharge occurs as lateral recharge from NOM and direct recharge in the plain area. The current findings support future development of aflaj system along NOM slopes and shallow wells in the plain areas.

Does Solar Radiation Affect the Distribution of Dubas Bug (Ommatissus lybicus de Bergevin) Infestation

The Dubas bug Ommatissus lybicus is a serious pest of date palms. The infestation level of the Dubas bug varies from location to location, as well as from one season to the next. Climate factors are considered to be the main drivers for fluctuations in infestation levels. Few studies have examined the effects of solar radiation on O. lybicus infestation. This study was undertaken to examine the effect of solar radiation on O. lybicus infestation levels in Oman. Infestation data were collected during the spring infestation seasons of 2009 and 2016 from 49 and 69 locations, respectively, from seven governorates of North Oman. The monthly clear-sky potential solar radiation was calculated from a digital elevation model (DEM) with 20-m resolution in the ArcGIS environment, and the average daily solar radiation was calculated for each month. Ordinary least square regression (OLS) and geographic weight regression (GWR) models were run to find the relationship between infestation levels and solar radiation. The infestation level ranged from 0.02 insect/leaflet to 32.98 insects/leaflet, with an average of 7.50 insects/leaflet in 2009 and 0.17 insect/leaflet to 17.52 insects/leaflet, with an average of 4.38 insects/leaflet in 2016. The highest solar radiation was recorded in June, with an average of 27.7 MJ/m2/day, and the minimum was in December, with an average of 14.1 MJ/m2/day. The higher infestation rate showed a weak correlation with solar radiation.

New or little known Sphodrina from Iran (Coleoptera: Carabidae: Sphodrini)

SummaryNotes are provided on some new, unexpected Sphodrina species from Iran, a country apparently well known from the entomological point of view. Notes on the species of Taphoxenus (Lychnifugus) of the Sphodrus phyletic lineage are added, with descriptions of Taphoxenus (Lychnifugus) elburzenis n. sp. (type locality: Iran, Elburz: Damavand Mt, 10 km W of Reine, 52°02ʹ24ʺE, 35°53ʹ22ʺN, 2200–2650 m) and Taphoxenus (Lychnifugus) murzini n. sp. (type locality: Iran, Fars: NW Sepidan, Poolad Kaf, 30°22ʹ01ʺN, 51°54ʹ00ʺE, 3100 m). Laemostenus (Arabosphodrus) laserae n. sp. (type locality: S Iran: Hormozgan, Genu Mts, N Bandar Abbas, 27°24ʹ04.5ʺN, 56°10ʹ49.6ʺE, 1680 m) is described. This species is particularly interesting, because it is the first representative of subgenus Arabosphodrus besides the type species of the subgenus, endemic to the mountains of North Oman. New records and illustrations of Laemostenus (Laemostenus) luristanus are provided, a little-known species from central Iran described from only two specimens. The diagnostic features of all species newly described are illustrated and their possible relationships are discussed.

Coupling isotopic and piezometric data to infer groundwater recharge mechanisms in arid areas: example of Samail Catchment, Oman

Hydrochemistry and well hydrographs are coupled to assess groundwater recharge in the regional catchment of Samail, Oman. The complex geology comprises three aquifers: limestones of the Hajar Supergroup (HSG) at the highlands of North Oman Mountains (NOM); fractured/weathered ophiolites; and Quaternary alluvium. Groundwater flows south–north from the NOM to the coast. Samples from groundwater wells and springs (38) were analyzed for isotopes and major ions. Corrected 14C dating reveals modern groundwater across the entire catchment, while 87Sr/86Sr (0.70810–0.70895) shows greater homogeneity. Groundwater in the upper catchment is depleted in 2H and 18O, indicating a high-altitude recharge source (NOM), and becomes enriched downstream, with a slope indicating an evaporation effect. The hydrographs of nested piezometers located in the upper, middle and lower catchment show different recharge responses between deep and shallower depths. Head difference in response to recharge is observed upstream, suggesting a lateral recharge mechanism, contrary to vertical recharge downstream reflected in identical recharge responses. The homogeneous 87Sr/86Sr ratio, head changes, downstream enrichment of 2H and 18O, and the presence of modern groundwater throughout the catchment suggest that groundwater recharge takes place across the entire catchment and that the three aquifers are hydraulically connected. The recharge estimated using the chloride mass balance method is in the range of 0–43% of the mean annual rainfall.

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

This paper highlights the role of Triassic-Jurassic extension and late Cretaceous compression in the Mesozoic-Cenozoic (Alpine) structuring of North Oman. The syn/post-Mesozoic regional structural evolution is usually documented as a succession of two stages of deformation. The Alpine 1 phase, late Cretaceous in age, occurred in association with two ophiolite obduction stages (Semail and Masirah ophiolites). It was characterised by strike slip to extensional deformation in the North Oman foreland basin sub-surface. The Alpine 2 phase, Miocene in age, was related to the continental collision responsible for both the Zagros orogen and the uplift of the Oman Mountains. The Alpine 2 deformation was transpressional to compressional. Observation and interpretation of good quality 3D seismic in the Lekhwair High area enabled the distinction of two earlier phases. Early Mesozoic extension occurred concomitantly with the regional Triassic to Jurassic rifting, developing Jurassic-age normal faults. Late Cretaceous compression occurred prior to the main Alpine 1 phase and triggered the inversion of Jurassic-seated normal faults as well as the initiation of compressional folds in the Cretaceous overburden. These early phases have been ignored or overlooked as part of the North Oman history although they are at the origin of structures hosting major local and regional hydrocarbon accumulations.

Late Eocene Uplift of the Al Hajar Mountains, Oman, Supported by Stratigraphy and Low‐Temperature Thermochronology

AbstractUplift of the Al Hajar Mountains in Oman has been related to either Late Cretaceous ophiolite obduction or the Neogene Zagros collision. To test these hypotheses, the cooling of the central Al Hajar Mountains is constrained by 10 apatite (U‐Th)/He (AHe), 15 fission track (AFT), and four zircon (U‐Th)/He (ZHe) sample ages. These data show differential cooling between the two major structural culminations of the mountains. In the 3 km high Jabal Akhdar culmination AHe single‐grain ages range between 39 ± 2 Ma and 10 ± 1 Ma (2σ errors), AFT ages range from 51 ± 8 Ma to 32 ± 4 Ma, and ZHe single‐grain ages range from 62 ± 3 Ma to 39 ± 2 Ma. In the 2 km high Saih Hatat culmination AHe ages range from 26 ± 4 to 12 ± 4 Ma, AFT ages from 73 ± 19 Ma to 57 ± 8 Ma, and ZHe single‐grain ages from 81 ± 4 Ma to 58 ± 3 Ma. Thermal modeling demonstrates that cooling associated with uplift and erosion initiated at 40 Ma, indicating that uplift occurred 30 Myr after ophiolite obduction and at least 10 Myr before the Zagros collision. Therefore, this uplift cannot be related to either event. We propose that crustal thickening supporting the topography of the Al Hajar Mountains was caused by a slowdown of Makran subduction and that north Oman took up the residual fraction of N‐S convergence between Arabia and Eurasia.

Large Kombewa flake production in north Oman

AbstractIn this paper we present the results of surveys carried out in February 2016 along a palaeo‐drainage system near the village of Bisyah. We report first on the geological prospection, which was our main goal, and then present our discovery of the only known localities in north Oman of large Kombewa flake production (&gt;10 cm up to 20 cm). Among the scatter of artefacts, we found cores and large Kombewa flakes with two opposed bulbs of percussion, a technology that had not been known in this region until now. In spite of the local features, the typo‐technological traits of the artefacts suggest that at least part of them resemble in some way the Acheulean techno‐complex, but there are still many questions remaining on the age of these artefacts.

Origin of the saline paleofluids in fault-damage zones of the Jabal Qusaybah Anticline (Adam Foothills, Oman): Constraints from fluid inclusions geochemistry

The Jabal Qusaybah Anticline, in north Oman, is affected by syn-folding strike-slip and extensional fault zones developed during foreland deformation ahead of the Northern Oman Mountains thrust wedge, in Cenozoic times. Migration of fluids in fault-damage zones is recorded in complex calcite vein networks. By employing the microthermometric and compositional microanalysis of the fluid inclusions (crush-leach), two distinct generations of veins have been studied. The aim was to determine the source of elevated salinity in fluids involved in their cementation and explain their compositional evolution through fluid-rock interactions. The ionic ratios (Na/Br and Cl/Br) obtained from crush-leach analysis give supporting evidence that the elevated salinity of fluid inclusions in both vein groups originated from an evaporated seawater beyond the onset of halite precipitation (residual brines). The results reveal a gradual increase in salinity of the fluids, F/Cl molar ratios, as well as Li/Cl molar ratios. These results imply the progressively increasing contribution of evaporitic residual brines and fluids that interacted with, or were derived from siliciclastic rocks. We suggest that the most likely origin of the former fluids is provided by residual brines associated with precipitation of the Ara evaporites (Cambrian). The regional driving mechanism for such a significant fluid migration is believed to be compaction-driven upward flow that was channeled into faults and fractures during major deformational events.

Groundwater Modeling and Sustainability of a Transboundary Hardrock–Alluvium Aquifer in North Oman Mountains

This study aims at modeling groundwater flow using MODFLOW in a transboundary hardrock–alluvium aquifer, located in northwestern Oman. A three-dimensional stratigraphic model of the study area representing the vertical and spatial extent of four principal hydro-geologic units (specifically, the Hawasina, ophiolite, Tertiary and alluvium) was generated using data collected from hundreds drilled borehole logs. Layer elevations and materials for four layers grid cells were taken from the generated stratigraphic model in which the materials and elevations were inherited from the stratigraphic model that encompasses the cell. This process led to accurate grid so that the developed groundwater conceptual model was mapped to simulate the groundwater flow and to estimate groundwater balance components and sustainable groundwater extraction for the October 1996 to September 2013 period. Results show that the long-term lateral groundwater flux ranging from 4.23 to 11.69 Mm3/year, with an average of 5.67 Mm3/year, drains from the fractured eastern ophiolite mountains into the alluvial zone. Moreover, the long-term regional groundwater sustainable groundwater extraction is 18.09 Mm3/year for 17 years, while it is, respectively, estimated as 14.51, 16.31, and 36.00 Mm3/year for dry, normal, and wet climate periods based on standardized precipitation index (SPI) climate condition. Considering a total difference in groundwater levels between eastern and western points of the study area on the order of 228 m and a 12-year monthly calibration period (October 1996 to September 2008), a root mean squared error (RMSE) in predicted groundwater elevation of 2.71 m is considered reasonable for the study area characterized by remarkable geological and hydrogeological diversity. A quantitative assessment of the groundwater balance components and particularly sustainable groundwater extraction for the different hydrological period would help decision makers to better understand the water resources in the Al-Buraimi region. In addition, it would assist decision makers to improve existing strategies to enhance the decision making for future developments.

Hydro-chemical evolution of groundwater in a sequence of Tertiary Formations in Northwest Oman

Tertiary Formations, which are widely spread in the Arab Peninsula, are fundamental to the oil industry in the world’s largest oil producing area. These formations host major aquifers that satisfy water demands for oil production such as steam injection and reservoir pressure maintenance . The current study investigates the hydro-chemical evolution of groundwater in a sequence of Tertiary Formations in Northwest Oman in the Kingdom of Saudi Arabia and the United Arab Emirates borders. Stable isotopes (18O and 2H) show three different groundwater types: (1) groundwater formed from precipitation originating from the Mediterranean Sea, (2) groundwater formed from mixing of northern (Mediterranean) and southern (Indian Ocean) moisture sources, and (3) groundwater affected by evaporation. Radiogenic isotopes 14C and 3H indicate younger groundwater close to the recharge source in North Oman Mountains and older age in the plain desert area that constitutes the discharge zone. Groundwater chemistry notably evolves from Ca–Mg–HCO3 dominant in the recharge area to Na–Cl–SO4 dominant type in the discharge zone. Water–rock interaction is the main process controlling groundwater chemistry in the recharge zone while evaporation becomes progressively effective down the gradient. The geochemistry and stable and radiogenic isotopes of the Tertiary Formations indicate that the study area has witnessed wet periods during the Early Holocene with surface water bodies formed by groundwater discharge at distal parts. The sustainability of such old groundwater that is being exploited for oil production should be thoroughly investigated to secure future oil production.

Groundwater recharge to ophiolite aquifer in North Oman: constrained by stable isotopes and geochemistry

The current study employs geochemical and isotopic tools to understand hydrochemical and recharge processes characterizing ophiolite aquifer in North Oman in conjunction with the Hajar Super Group (HSG) aquifer. A total of 57 samples were analyzed for major ions and stable isotopes 2H and 18O. The geochemical composition of groundwater indicates that water–rock interaction and mixing are the main processes controlling groundwater chemistry. Groundwater in the HSG is characterized by carbonate minerals dissolution contrary to the groundwater in the ophiolites where silicates dissolution dominates. This results in differences in the groundwater chemical composition in the two systems. Isotopic characteristics of precipitation collected during the study period indicate two main moisture sources from the Indian Ocean and the Mediterranean Sea. Groundwater δ2H and δ18O values suggest two recharge sources to the ophiolite aquifer: lateral flow from the HSG and direct infiltration. Recharge from direct infiltration in the highlands, which is depleted in δ2H and δ18O, retains the same isotopic signature of precipitation, whereas that in the low land substantially reflects an evaporation effect.

Paleofluid Evolution In Fault-Damage Zones: Evidence From Fault–Fold Interaction Events In the Jabal Qusaybah Anticline (Adam Foothills, North Oman)

Abstract Fault zones are known to play a key role in controlling fluid migration, including hydrocarbons, at local and regional scales. In this contribution physicochemical data from veins are used to document the extent and maintenance of fluid conductivity associated with fault-damage zones in an anticlinal structure occurring in the frontal part of a foreland fold and thrust belt. Migration of fluids during major deformational events has been recorded by several generations of calcite and localized dolomite cementation in veins in marine Cretaceous carbonates of the Jabal Qusaybah Anticline (North Oman). Applying detailed petrographic and paragenetic relations, geochemical analysis (stable carbon and oxygen isotopes, strontium isotopes, and trace elements), and microthermometry, coupled with structural studies of fault zones and associated calcite veins, made it possible to link fluid circulation to the structural evolution of the Jabal Qusaybah Anticline. Based on the structural framework and fault systems, three groups of veins were distinguished. Group 1 veins are interpreted to be linked to burial and early-tectonic deformation in association with E–W extensional fault zones. Calcite cementation in this group occurred in a closed to semiclosed fluid system, which originated as a result of intraformational fluid movement along a low-connectivity fracture network. The δ13C values of these veins range from 3‰ to 3.5‰, and δ18O values vary from –10.3 to –8.1‰ V-PDB. Group 2 veins are related to syntectonic deformation along NE–SW strike–slip and incipient N–S extensional faults linked to culmination of the Jabal Qusaybah Anticline. From this stage onward, fluids correspond to progressive fault connectivity and dilation related to interaction of fault–folding systems. The cements precipitated during this stage have more variable δ13C values ranging from 0.9 to 3.2‰ and possess a wide range of δ18O values varying between –11.2 and 0.3‰ V-PDB. The cements in Group 2 veins originated from fluids characterized by a relatively broad range of precipitation temperature (75–107°C) and salinities evolving from seawater to moderately saline fluids (3.4–12.8 eq. wt% NaCl), with H2O–NaCl composition. The source of salinity is inferred to be evaporitic brines that originated from the Ara Group, and the large range of salinity is interpreted as the result of mixing between those brines and modified seawater. Group 3 veins are considered to be related to late–tectonic deformation corresponding to late fold amplification of the anticline hinge, transition from NE–SW left-lateral strike–slip to N–S extensional faulting, progressive regional and local uplift, and exhumation. Stable-isotope data support the occurrence of bacterial methanogenesis or CO2 degassing at the end of this stage. Their associated cements are characterized by higher δ13C and δ18O values compared to other groups of veins, ranging from 0.6 to 8.0‰ and –3.6 to 1.5‰ V-PDB, respectively. Fluids responsible for calcite precipitation in this group evolved from a moderate saline to a more saline (8.5–16 eq. wt. % NaCl) H2O–NaCl–CaCl2 brine. The progressive 18O enrichment relative to seawater and preceding cements and its covariation with fluid salinity in the later stage suggest the higher contribution of the evaporitic brines. This hypothesis is further supported by the increase in their 87Sr/86Sr ratios compared to both middle to Upper Cretaceous marine carbonates and other groups of veins. Although during this episode calcite cementation occurred at temperatures below 50°C, their geochemical signatures reveal that the faults were sufficiently conductive to transfer extraformational fluids late in the deformation history of this structure. The interference between strike-slip and extensional fault systems and their interaction with fold amplification is invoked to be responsible for enhanced conductivity in fault-damage zones.

Early Eocene orthophragminids and alveolinids from the Jafnayn Formation, N Oman: significance of Nemkovella stockari Less & Özcan, 2007 in Tethys

The upper member of the Jafnayn Formation in Wadi Rusayl and Al Khoud, Seeb Area in north Oman contains Nemkovella stockari Less & Özcan, 2007, an Early Eocene orthophragminid recorded here for the first time from the Arabian Peninsula. N. stockari, the only orthophragminid identified in Jafnayn Formation, is quite distinct from any other species from Tethys in having spiral and orbitoidiform chambers around the isolepidine embryon developed before the onset of annular chambers. The Oman specimens exhibit equal-sized principal auxiliary chambers and symmetrical spirals and are assigned to N. stockari bejaensis Özcan, Boukhalfa & Scheibner, 2014, an advanced form of the N. stockari lineage. The associated alveolinids, revised in this study, rotaliids and other age-diagnostic foraminiferal taxa in the transgressive basal part of the upper member have enabled us to revise the age as middle Ilerdian (Early Eocene), assignable to SBZ 7/8 and 8; OZ 3/4. Integrating a new record of this species from Arabian Plate margin in Belen, S Turkey, and previous records from north Africa, N. stockari appears to be a diagnostic marker for the Early Eocene along the southern peri-Tethys platforms, facilitating the Tethyan correlation by orthophragminids. The palaeobiogeographic distribution of N. stockari is discussed.

Facies, sequence stratigraphy, reservoir and seal potential of the Mafraq Formation, Sultanate of Oman: An integrated outcrop analogue study

ABSTRACT The mixed carbonate-siliciclastic Lower to Middle Jurassic Mafraq Formation unconformably overlies the Triassic Mahil Formation in outcrops of the Oman Mountains (pre-Mafraq Unconformity, known as pre-Marrat unconformity in other regions of Arabia). Together with the overlying Dhruma Formation, it is part of the Sahtan Group. This study provides: (1) a detailed facies analysis based on sedimentological logging of 12 outcrops. Twenty-four facies types were established and grouped into five facies associations, which can also be recognized in subsurface core intervals; (2) a detailed sequence-stratigraphic framework of the Mafraq Formation. Facies stacking and log patterns reveal cycle hierarchies on four scales from m-scale cycles, to several m-thick cycle sets, to tens of m-thick, high-frequency sequences, to 100 m-thick composite sequences; and (3) a documentation of potential reservoir and seal units. The study follows an approach from 1-D (outcrop sections) to 2-D (correlations and potential reservoir dimensions). The Mafraq outcrop type section, located in Wadi Sahtan is documented in an integrated way (facies, litho-, bio-, chemo- and sequence stratigraphy), together with additional outcrops of the Mafraq Formation throughout North Oman. 2-D correlation of the Mafraq Formation throughout North Oman is essentially based on cycle sets and provides key information about the lateral paleogeographic development of the formation. A general proximal-distal trend, from south to north, has been proposed by Ziegler (2001); outcrop data from the Oman Mountains confirms this trend and adds an EW-deepening component. The mixed carbonate-clastic Lower Mafraq Member (Sequence) with a coastal/estuarine to shallow-marine environment forms onlaps onto the pre-Mafraq Unconformity below, and thins out completely after some 10s of kilometers towards the southeast. The Upper Mafraq Member (Sequence) seems to be continuous over 10s of kilometers with less thickness decrease. Instead, a transition from a more distal carbonate shoal - backshoal environment in the northwest to a proximal clastic coastal/estuarine/terrestrial environment in the southeast can be observed. On a 100s km-scale significant thinning and a change towards terrestrial clastic facies can be observed southeast of the Oman Mountains area. Combined results from lateral/vertical logging, paleoenvironmental interpretations and correlation provided 3-D information about the dimensions of potential reservoir and seal units. Several potential reservoir/seal intervals and their dimensions in dip direction could be identified: (1) Lower Mafraq Sequence: various types of sandbodies, most of them with a lateral extent ca. 5 km, sealed by shales. (2) Upper Mafraq Sequence, northwestern part: oolitic grainstones, laterally correlative over 10–20 km, sealed by shales. (3) Upper Mafraq Sequence, southeastern part: channelized sandstones units, lateral extent up to km, sealed by shales.