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Hydrocarbon generation and migration under a large overthrust: The carbon-ate platform under the Semail Ophiolite, Jebel Akhdar, Oman

A. Grobe, J.L. Urai, R. Littke, N.K. Lunsdorf

PII: S0166-5162(16)30030-1DOI: doi: 10.1016/j.coal.2016.02.007Reference: COGEL 2595

To appear in: International Journal of Coal Geology

Received date: 2 December 2015Revised date: 24 February 2016Accepted date: 25 February 2016

Please cite this article as: Grobe, A., Urai, J.L., Littke, R., Lunsdorf, N.K., Hydrocarbongeneration and migration under a large overthrust: The carbonate platform under theSemail Ophiolite, Jebel Akhdar, Oman, International Journal of Coal Geology (2016), doi:10.1016/j.coal.2016.02.007

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Hydrocarbon generation and migration under a large overthrust: The carbonate

platform under the Semail Ophiolite, Jebel Akhdar, Oman.

Grobe, A., Urai, J.L., Littke, R., Lünsdorf, N.K.

Keywords: burial history, Oman Mountains, solid bitumen reflectance, temperature, Raman

spectroscopy, FT-IR

Highlights

- Thermal and burial history of Natih B in Wadi Nakhr;

- Natih B reached maximum of 240 °C at a maximum burial depth of 7.4-8 km;

- Paragenetic sequence of Natih and Muti Fm.;

- Proof that Raman spectroscopy of organic rich matrix reveals valid maturity results;

- Modelling of hydrocarbon generation and migration events.

Abstract

The Natih B source rock of the Jebel Akhdar dome in the Oman Mountains hosts at least

three different generations of solid bitumen. This study presents detailed analyses of

thermal maturity based on solid bitumen reflectance, Rock-Eval pyrolysis, Fourier

transform infrared spectroscopy and Raman spectroscopy of matrix and solid bitumen. The

Natih B source rock was sampled in different valleys (Wadis) around the Jebel Akhdar

Dome. Detailed petrography of organic material was conducted by optical and scanning

electron microscopy (SEM). The petrographic data was used to calibrate an integrated

thermal basin model of the Wadi Nakhr area. Due to ophiolite obduction the Natih B source

rock was buried to 7.4 to 8 km and a maximum temperature of c. 240 °C. Hydrocarbon

generation and associated primary migration was burial induced and took place from 78 to

66 Ma with solid bitumen precipitation in the source rock. A resulting high-reflective solid

bitumen generation reflects temperatures of deepest burial in the order of 225 °C to 240 °C.

Based on an elaborated paragenetic sequence two other solid bitumen generations were

linked to two oil migration events taking place during uplift (55-50 Ma and 48-45 Ma), after

deepest burial (around 65 Ma).

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1. Introduction

The Oman Mountains are unique as natural analogue to study exposed hydrocarbon

systems on the Arabian Peninsula, hosting the most prolific ones on earth. Source and

reservoir rocks crop out at the surface of the Jebel Akhdar dome at reservoir scale. Studies

in the past focused on the facies distribution and sequence stratigraphy of source rocks

(Grelaud et al., 2006; Homewood et al., 2008; Rousseau et al., 2006; Van Buchem et al.,

2002, 1996), reservoir rocks (Immenhauser et al., 2004, 1999), the origin of fractured

carbonate reservoirs in a high pressure cell (Bertotti et al., 2005; Gomez-Rivas et al., 2014;

Hilgers et al., 2006; Virgo et al., 2013) and associated fluid flow events (Arndt et al., 2014;

Breesch et al., 2009; Stenhouse, 2014). Solid bitumen in veins and host rocks provide

evidence on different hydrocarbon migration events (Fink et al., 2015). However, a

systematic analysis of their paragenesis is lacking and an absolute timing of the migration

events not known. Solid bitumen precipitation results from thermal cracking,

biodegradation or gas deasphalting of oil accumulations (Blanc and Connan, 1994; Rogers

et al., 1974). At temperatures above 150 °C oil becomes unstable and thermal cracking is

mainly responsible for solid bitumen precipitation (Dahl et al., 1999), whereas

biodegradation takes places at lower temperatures (<80 °C, Larter et al., 2006). In some

cases, multiple migration events can be deduced based on solid bitumen generations of

different reflectance (Gentzis and Goodarzi, 1990).

In this paper the data presented include two different paleo-thermometers, namely solid

bitumen reflectance and Raman spectroscopy of source rocks sampled at outcrops around

the Jebel Akhdar dome. Solid bitumen of different generations and their thermal maturities

were geochemically and petrographically analyzed. Combining an elaborated paragenetic

sequence with laboratory results and a well calibrated basin model enables to link thermal

maturity, deepest burial and burial history. Based on this approach we present a model of

the relative and absolute timing of oil generation and migration.

1.1. Geologic and tectonic evolution of the Oman Mountains

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The Oman Mountains expose mainly carbonate rocks deposited on a late Proterozoic

basement of the passive continental margin of the Neo-Tethyan Ocean (Figure 1, e.g. Breton

et al., 2004; Loosveld et al., 1996; Rabu et al., 1990; Roger et al., 1991). The succession of

the Oman Mountains is subdivided into the sedimentary units Autochthon A (late-

Proterozoic to Devonian age, i.a. Le Metour et al., 1990; Rabu et al., 1990) and B (mid

Permian to mid-Upper Cretaceous, i.a. Breton et al., 2004), the allochthonous units of Semail

ophiolite and sedimentary Hawasina nappes, and the Neo-Autochthon deposited on top of it

(Breton et al., 2004). The Autochthon B, also named Hajar Supergroup, is impressively

forming the flanks of the Jebel Akhdar dome and contains a formerly active hydrocarbon

system: The Natih formation (late Albian-early Turonian,

Figure 1). It acts as source and reservoir rock and is, from top to bottom, subdivided into

the members A to G (e.g. Philip et al., 1995). Natih B and E represent source rocks and the

Natih A acts as reservoir rock (Homewood et al., 2008; Van Buchem et al., 2002). The

overlying carbonates and conglomerates of the Muti Fm. are interpreted as northern

continuation of the Fiqa Fm. seal (Terken et al., 2001). The Muti Fm. was deposited after a

moving, flexural forebulge, formed in front of the ophiolite nappe, has uplifted the

carbonate platform and partly eroded the topmost Natih layers (Robertson, 1987). Directly

below the Natih the Nahr Umr shale is situated and represents a bottom seal.

Figure 1: Overview of sampled Wadis and locations (X) in the Oman Mountains (a) with

highlighted lithologies (WGS 84 UTM Zone 40N, Sources: Esri, DigitalGlobe, GeoEye, i-cubed,

USGS, Aerogrid, swisstopo, and the GIS user Community). Color legend is given by the

stratigraphic chart (b) and a cross section of the structure of the Jebel Akhdar anticline at

present day (c, compiled from Al-lazki et al., 2002; Filbrandt et al., 2006; Searle, 2007).

The Natih A is known as a large scale fractured carbonate reservoir in north Oman (Terken,

1999; Van Buchem et al., 1996). The evolution of the stress field is recorded by at least 7

vein generations, stylolites, faults and fractures. Briefly summarized in their genetic order,

earliest structures are related to burial and represented by layer confined veins (Virgo,

2015) and burial stylolites (Breesch et al., 2011; Gomez-Rivas et al., 2014). Large scale

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overthrusts occur and their top-to-S movement is linked to a compressive N-S stress regime

that was initiated by first subsea ophiolite obduction and generated N-S striking veins

(Grobe et al., 2015; Holland et al., 2009; Virgo, 2015). N-S oriented veins are overprinted by

NW-SE oriented ones and interpreted as onset of an overpressure cell at the top Natih Fm.

Possible causes are a rapid burial linked to the ophiolite obduction or hydrocarbon

generation within the formation (Hilgers et al., 2006; Holland et al., 2009). E-W and NE-SW

striking vein sets overprint these early structures and are associated to the obduction-

related overpressure cell and an anticlockwise rotating stress field during burial (Holland

et al., 2009). During deepest burial higher temperatures are preserved in ductile boudinage,

representing a large scale top-to-NE shear zone that occurs in the deeper Cretaceous

carbonates (Grobe et al., 2015; Holland et al., 2009). Holland et al. (2009) found bedding

parallel structures, e.g. veins and sigmoidal clasts, with movement indicating top-to-N to

top-to-E and also linked these features to the above described large scale shear zone. This

shear zone is also present in the Saih Hatat window in the East (Fournier et al., 2006). All

these structures are overprinted by E-W trending oblique to normal faults which are

reactivated as strike-slip faults (Grobe et al., 2015; Virgo, 2015). These stress field changed

were linked to the movement of India, bypassing Arabia in the East (Filbrandt et al., 2006).

Gomez-Rivas et al. (2014) related tectonic stylolites to this oblique collision with India. At a

later stage, the collision of Arabia and Europe resulted in the updoming of the Oman

Mountains (Figure 1c, Glennie et al., 1973; Hanna, 1990; Searle, 1985, 2007). Still unclear

are the movements of the faults and their exact timing: Some authors interpret at least

some of the faults as normal faults and date them prior to doming (Holland et al., 2009;

Virgo, 2015). Opposed, Gomez-Rivas et al. (2014) postulated a dominant strike-slip to

oblique character of the faults and date them prior to the top-to-NE shearing.

2. Methods

A total of 63 source rock samples were taken of the Natih B Fm. in different valleys/Wadis

around the Jebel Akhdar dome (Figure 1). Visibly unweathered material was dug out and

sampled. In addition, 16 samples were taken in neighboring formations directly above and

below the Natih formation.

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48 dark-colored samples were pulverized and used for the analysis of total organic (TOC)

and inorganic carbon (TIC). Elemental analysis was conducted on 100 mg of each sample by

combustion in a liquiTOC II analyzer under constant oxygen supply. The instrument is using

a temperature ramp method (300 °C/min to 550 °C, held for 600 s, quickly raised to

1000 °C and held for 400 s), which enables a direct determination of TOC and TIC without

the need of prior acidification. Generated CO2 is analyzed by a non-dispersed infrared

detector (NDIR: detection limit 10 ppm, rel. TOC error 0.6 %, TIC error 1.7 %).

To characterize the quality of organic matter, the 4 samples with highest TOC content were

selected for Rock-Eval VI pyrolysis according to Espitalié et al. (1977).

Fourier transform infrared spectroscopy (FT-IR) was used to determine the proportion of

aliphatic and aromatic compounds (cp. Ganz and Kalkreuth, 1991; Robin and Rouxhet,

1978). After HCl acidification, the carbonate free samples were analyzed using a Perkin

Elmer Spotlight FT-IR at attenuated total reflection mode (ATR) at wavenumbers of 4000 to

750 cm-1.

Based on their TOC content samples were selected for thermal maturity analyzes by solid

bitumen reflectance (BRr). Samples were cut perpendicular to bedding and embedded as

whole rock samples in an epoxy-resin-mix. During exothermal hardening temperatures of

36 °C were not exceeded. The embedded samples were polished with successively finer

abrasives (320, 400, 600 mm). Details of the preparation procedure are described in Littke

et. al. (2012). Reflectance measurements were conducted using a Zeiss Axioplan microscope

with an oil immersion lens of 50-times magnification and monochromatic light with a

wavelength of 546 nm, calibrated by standards of Gadolinium Garnet (1.714 %) and Cubic

Zirconia (3.125 %). Solid bitumen particles were only selected for measurement if any sign

of oxidation was missing. The gathered BRr data was used to calculate vitrinite reflectance

and maximum temperatures. According to Ferreiro Mählmann and Frey (2012) BRr values

below 1.5 % were transformed to random vitrinite reflectance (VRr) by the formula of Jacob

(Eq. 1, 1989).

[%] (Eq. 1)

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Within the range of 1.5 to 3.0 % BRr the equation of Landis and Castano (1995) was used

(Eq. 2).

[%] (Eq. 2)

For values above 3.0 % we assume BRr ≈ VRr. Maximum burial temperatures were

calculated based on the approach of Barker and Pawlewicz (Eq. 3, 1994). To estimate values

for faster burial hydrothermal temperatures were calculated as well (Eq. 4).

[°C] (Eq. 3)

[°C] (Eq. 4)

Moreover, Raman spectroscopy was carried out to investigate the maturity range of the

analyzed samples. In addition to local measurements of solid bitumen particles, this

technique enables the analysis of dispersed solid bitumen that is too small or too uneven to

be measured by reflectance techniques. The method is based on the reorganization of

amorphous carbonaceous material (kerogen) to graphite, which is caused by increased

temperatures (Aoya et al., 2010; Kouketsu et al., 2014; Lahfid et al., 2010; Wopenka and

Pasteris, 1993; Yui et al., 1996).

For this we used a Horiba Jobin Yvon HR800 UV spectrometer attached to an Olympus BX-

41 microscope with a 100x objective at the Geoscience Center, Department of Experimental

and Applied Mineralogy, University of Göttingen. A high-power diode laser with a

wavelength of 488 nm and an output power of 50 mW was installed. Sample alteration by

heating was avoided by applying a D1 density filter. With a Peltier CCD detector, each

spectral window (center at 1399.82 cm-1, grid of 600 lines/mm) was measured 5 to 10

times for 2 to 20 seconds. To transform the measured data into VRr values the scaled total

area (STA) approach of Lünsdorf (2016) was used and temperatures were calculated using

the equation (Eq. 5, based on data of Lünsdorf, 2016).

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[%] (Eq. 5)

Petrographic interpretation was supplemented by quantitative scanning electron

microscopy (SEM) with a Zeiss Supra 55 SEM, equipped with secondary electron detectors

(SE) comprising INLENS and SE2 (5 kV at 4-6 mm distance). An integrated energy-

dispersive X-ray spectroscope (EDX: Apollo 10 SDD) enabled semi-quantitative mineral

identification (20 kV, 8 mm).

To quantify maximum burial temperatures and reconstruct the burial history of the

analyzed Natih B samples, numerical basin modelling was conducted with the PetroMod1D

2014.1 software package (Schlumberger Technology Center Aachen, Germany). The

incorporated simulation uses forward modeling based on an event-stepping approach. A

combination of a conceptional model of the structural history of the area, the given

lithologies and outcrop geometries with defined thermal boundary conditions enables to

reconstruct a burial curve (Hantschel and Kauerauf, 2009; Nöth et al., 2001). By calibrating

this model with maturity data the thermal history of the Wadi Nakhr area was elaborated.

3. Results

3.1. Organic Geochemistry

Results of TOC and TIC measurements are summarized in Table 1. TOC values range from

0.20 (AG15SR44, 58, 63) to 1.06 wt.-% (AG15SR01) and TIC from 11.78 (AG15SR48) to

13.80 wt.-% (AG15SR47), respectively. The Wadis on the south-western flank of the Jebel

Akhdar (Nakhr, W of Nakhr and N of Al Hamra) showed generally higher TOC values than

the ones in the south-east (Wadis Tanuf and Muaydin) and in the north (Wadis Mistal and

Sahtan).

Table 1: Sample list showing sampling locations, formations and results of the elemental

analysis.

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Rock-Eval pyrolysis was performed on the Natih B source rock samples of highest TOC

contents (Table 2). S1 peaks are generally below 0.1 mg HC/g rock and S2 peaks below

0.2 mg HC/g rock. Calculated hydrogen indices (HI) are between 12–36 mg HC/g TOC,

oxygen indices (OI) between 122-298 mg CO2/g TOC, respectively (Table 2).

Table 2: Results of Rock-Eval VI pyrolysis.

Fourier transform infrared spectroscopy (FT-IR) shows hydrocarbon related peaks at 2912,

2842 (aliphatic components), 1629 and 1614 cm-1 (aromatic components, Figure 2).

Figure 2: Representative spectra of the FT-IR measurements, all measured at sample

AG15SR01. Labeled peaks represent aliphatic (2912, 2842 cm-1) and aromatic (1629, 1614 cm-

1) components.

3.2. Petrology and BRr of the Natih B

The amount of TOC was used to identify samples containing solid bitumen. However, if the

solid bitumen occurs finely dispersed within the matrix, high TOC content not always

represents a high number of measureable solid bitumen particles. Best results were

achieved for samples > 0.25 wt.-% TOC.

First, freshness of pyrites was checked for all polished sections, which is an excellent

indicator for degree of weathering that might have affected the rocks and the organic

matter (Littke et al., 1991). All investigated samples show fresh, unweathered pyrite.

Organic petrology showed that solid bitumen is present in the Natih B source rock in

different shapes: It was always present as finely dispersed, granular particles within the

matrix (Figure 3a-e) containing small micrinitized particles (white spots in matrix,

Figure 3b-e). In addition, solid bitumen was found in matrix pores (Figure 3a). Veins of

solid bitumen exist either as small, randomly oriented veins within the matrix (Figure 3b),

or within calcite veins (Figure 3f). Some veins are oriented parallel to weak zones next to

stylolites (Figure 3c). Likewise, solid bitumen filled former fossils (Figure 3d, e+g) or

showed flow-indicating structures (Figure 3b+h) or droplets (Figure 3i) and rhombohedral

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shaped bodies which are interpreted to be siderite pseudomorphs (Figure 3j+k). In the

surroundings of some veins red colored host rocks appeared (Figure 3g+i); these latter

invasion rims are characterized by light fluorescence colors.

Figure 3: Photographs of different solid bitumen appearances within the Natih B source rock:

Bitumen as particle in the matrix (a), in veins (b,f), stylolites (c) or former fossil shells (d+e). It

shows flow structures (b) or hydrothermal overprinting (h). It represents red host rock

overprinting (g+i) and replaces rhombohedral siderite pseudomorphoses (j, k). (All with oil

immersion at 50x magnification)

Reflectance measurements indicate that at least three different solid bitumen generations

are present in the Natih B source rock (Table 3). The first, high-reflective generation of 2.95

to 3.59 % BRr is represented by particles and filled open voids within the matrix

(Figure 3a); sometimes they appear in rhombohedral shaped bodies (Figure 3h, j, k). They

are not always homogenous and their surfaces are partly fractured and often micrinitized

(Figure 3j). The finely dispersed solid bitumen within the matrix also belongs to this

generation, as well as solid bitumen veins within carbonate veins (Figure 3f).

Measurements were only conducted on homogeneous, unaltered solid bitumen particles. To

exclude that the results were influenced by bireflectance, random reflectance and

bireflectance were measured at two samples. As no differences occur, an influence of

bireflectance is excluded.

Values between 2.26 and 2.56 % BRr characterize a second, medium-reflective solid

bitumen generation. It formed homogenous surfaces, filled voids within the matrix or is

oriented at already existing structures, e.g. stylolites (Figure 3b+c).

A third generation of low reflective solid bitumen (mainly 1.04 – 1.45 % BRr, and two

outliers of 1.64 and 1.90 % BRr) was present as fine crack-fillings within the matrix, as solid

bitumen veins and as fossil-fillings (Figure 3d+e). The veins are mainly oriented parallel to

bedding and associated with fluorescent host rocks in the surroundings (red at incident

light, Figure 3g, i, k). The generation is characterized by homogenous surfaces and pore-

shape related boundaries (Figure 3i).

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Table 3: Results of solid bitumen reflectance measurements of the Natih B source rock.

Calculated burial temperatures are in the order of 226-232 °C for the high reflecting

generation, 208-216 °C and 139-172 °C for the others, respectively (Table 4). Hydrothermal

peak temperatures, representing a higher heating rate that could appear at rapid burial,

range between 296-316 °C, 268-275 °C and 157-210 °C.

Table 4: Temperatures calculated based on solid bitumen reflectance measurements.

Karstification surfaces host organic particles which were accumulated while carbonate was

dissolved (Figure 4). Thereby, they often enrich all present kinds of solid bitumen as

residua and can be used as a quick-look method to unravel the included generations.

Reflectance measurements of these particles were avoided, as an influence of the

karstification process on their reflectance cannot be excluded. This enrichment process is

not observable at the filled stylolites; there the solid bitumen showed flat homogeneous

surfaces. Therefore, stylolite-associated solid bitumen is interpreted to fill cracks that were

generated close to the weak zones oriented parallel to stylolites and therefore appear after

the dissolution event of stylolitization.

Figure 4: Photograph of a karstification surface within the Natih B accumulating different

generations of solid bitumen (oil immersion, 50x magnification).

3.3. Raman Spectroscopy of the Natih B

In addition to the BRr measurements, Raman spectroscopy of carbonaceous material

(RSCM) was carried out on Natih B samples.

As typical for spectra measured at organic material a peak in the range of c. 1250-1450 cm-1

was observed (Figure 5). It represents defects in the carbon atom’s lattice structure and

vacancies in the aromatic ring structures (Pimenta et al., 2007) and is therefore named

defect band, D-band. The width of this peak can also give an idea of the disorder of C-atoms

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within a sample (Beyssac et al., 2002). The second peak appears around 1500-1605 cm-1

and evokes out of longitudinal stretching vibrations within C=C double bonds (Beyssac and

Lazzeri, 2012). As this effect is most pronounced in pure graphite the band is named G-

band. As a consequence of the different origin of peaks, information of the aromatic ring

structure and its relationship to sample maturation can be elaborated by correlating peaks

within a Raman spectrum and their shifts between spectra of different sample maturation

(Figure 5: matrix characterized by carbonate peak at around 1200 cm-1).

Figure 5: Representative spectra showing the consistency of measurements at solid bitumen

particles and of the matrix for the Natih B and the Nahr Umr formation. Matrix spectra not

only show D- and G-band of organic material but also a carbonate peak at c. 1200 cm-1.

This leads to calculated temperatures of 225 to 261 °C for the Natih B matrix, varying

slightly more than the 227 to 231 °C of the high-reflective solid bitumen particles (Table 5).

This proofs that RSCM of the matrix can be used to analyze thermal maturity for rocks of

high-grade diagenesis to anchimetamorphism, when kerogen is much more homogeneous

than at early stages of diagenesis. It represents a method independent of measurable,

homogeneous solid bitumen particles.

Table 5: Results of the Raman spectra analyses with used filters and measuring times.

3.4. Surrounding lithologies

3.4.1. Petrology, BR, RSCM

Samples of Muti and Nahr Umr Fm. contain the same three solid bitumen generations as in

the Natih B source rock. Their reflectance is shown in Table 6. In the Nahr Umr Fm. the 3rd

generation is missing (in the discussion we will argue that this generation represents

remnants of hydrocarbons produced in the Natih Fm., see 4.4).

Table 6: Results of the solid bitumen reflectance measurements directly above (Muti Fm.) and

below (Nahr Umr Fm.) the Natih formation.

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RSCM of Nahr Umr samples of the Wadi Nakhr gives temperatures of 211-280 °C, Muti Fm.

of the Jebel Shams 172-208 °C, the Hawasina nappes at Sint 193-209 °C and the top of the

Kahmah Group (Shuaiba Fm.) 251-269 °C, respectively (Table 5).

3.4.2. SEM and EDX

For EDX analyses of the composition of the rhombohedral particles we selected analogue

samples of Muti radiolarites from the Gorge pavement. In contrast to the Natih B, matrix

and veins are SiO2 dominated (Figure 6). We observe optically black-impregnated veins

with inclusions of rhombohedral grains with rims (Figure 7). The cores of the rhombohedra

are rich in C, representing the precipitated solid bitumen, and the rims contain Mn, Fe, Ca

and O associated with traces of Mg, K and F.

The shape of the rhombohedra is characteristic for (former) Fe-carbonate (siderite), which

also explains the presence of fluorine as indicated by EDX-SEM. Siderite is interpreted to

have been overgrown by Mn, Fe and Mg oxides before it was dissolved and their

pseudomorphoses replaced by a carbon rich fill, i.e. solid bitumen.

Figure 6: SEM results of SiO2 veins in a radiolarian Muti sample. Si and O are equally

distributed within matrix and vein and particles within the veins are showing C and Mn

enrichments.

Figure 7: SEM results of rhombohedral shaped bodies in SiO2 rich veins of Muti radiolarites.

Overgrowth rims are characterized by F, Fe and Mn enrichments, fillings show strong C

enrichments.

3.5. Numerical basin modelling

A 1D basin model of the Wadi Nakhr area (southern flank of the anticline) was compiled

with input parameters summarized in Table 7. Thicknesses are based on our field

measurements, complemented by literature data. The minimum thickness of the Hawasina

nappes was reconstructed to a value before the thinning by orogenic collapse (Al-Wardi and

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Butler, 2007; Beurrier et al., 1986). Petrophysical parameters of the lithologies are listed in

the Appendix 1. Large scale overthrusted ophiolite was 1-dimensionally realized as rapid

sedimentation.

Table 7: Input parameters for the Wadi Nakhr basin model: Thicknesses and lithologies are

compiled from field data in combination with literature data (Beurrier et al., 1986; Philip et

al., 1995; Pratt et al., 1990). Ages were compiled from Vahrenkamp (2013), Grélaud et al.

(2006), Razin et al. (2010), Rabu et al. (1990), Pöppelreiter et al. (2011) and Masse et al.

(1997). Eroded thicknesses of Semail Crust and Mantle are subject to the presented sensitivity

analysis.

According maximum burial depth was calibrated by thermal maturities presented above

(BRr, Raman spectroscopy) and the uplift path by fission track data (Saddiqi et al., 2006;

Wübbeler et al., 2015). Thermal boundary conditions are shown in Figure 8. The lower

thermal boundary, i.e. basal heat flow, at present day was set to 52 mW/m² (Rolandone et

al., 2013; Visser, 1991). Paleo heat flows were mainly influenced by intracratonic Cambrian

rifting leading to peak values, and passive margin development (Paleozoic to Jurassic)

representing a time of tectonic quietness and associated normal to low basal heat flows

(Loosveld et al., 1996; Terken et al., 2001). In general radiogenic decay and associated loss

of radiogenic minerals leads to a slight decrease of heat flow values over time. The paleo

water depth was reconstructed based on sequence stratigraphic results (Hillgärtner et al.,

2003; Immenhauser and Matthews, 2004; Immenhauser and Scott, 2002; Immenhauser et

al., 1999; Van Buchem et al., 2002). Paleo mean surface temperatures were reconstructed

by a latitude based approach of Wygrala (1989). Corrected by the influence of the paleo

water depth it defines the sediment water interface temperature (SWIT), representing the

upper thermal boundary. Base temperatures of the ophiolite are set to 120 °C by increasing

SWIT at times of obduction. Conducted sensitivity analyses of different ophiolite

temperatures showed no influence on Natih B maturity. Debated in the past, the original

ophiolite thickness is still unknown. Sensitivity analyses of different overburden

thicknesses (eroded Muti Fm., Hawasina nappes and Ophiolite) show that eroded

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overburden of c. 7400-8000 m including 5900-6500 m of ophiolite (Figure 9a) matches our

observations. Normal sedimentation at the Mesozoic carbonate platform was interrupted

by rapid burial induced by the ophiolite emplacement 78 Ma (Figure 9b). This results in a

strong temperature increase within the first million years of burial, e.g. from 40 to 120 °C in

the Natih B source rock, followed by an increase to maximum reached temperatures of c.

240 °C at 65 Ma (Natih B, Figure 9c). According to these temperatures the transformation

ratio vs. time plot (Figure 9d) represents times of oil generation from 78 Ma to 66 Ma.

Figure 8: Boundary conditions used for the thermal basin modelling. Peak of sediment water

interface temperature is representing temperature influence of the warm ophiolite

emplacement.

Figure 9: Results of the Wadi Nakhr area basin model: Sensitivity analysis of overburden

thicknesses (a), burial history (b), temperature evolution of the Natih B source rock over time

(c) and the corresponding transformation ratio (d). b) – d) for an ophiolite thickness of

6300 m.

4. Interpretation and Discussion

4.1. Paragenetic sequence

By combining petrographic observations and results of burial and thermal history

modeling, a paragenetic sequence is proposed: Siderite is interpreted to form during early

burial of the carbonate platform (parasequence 1). Iron, calcium and manganese rich oxides

overgrew the siderites and represent oxidizing conditions (parasequence 2, cf. Johnson et

al., 2005; Xiao-Fei et al., 2011).

After the formation of these overgrowths, siderite dissolved (parasequence 3). Rapid burial

during ophiolite obduction initiated hydrocarbon generation, which filled the former

siderite as pseudomorphoses (parasequence 4). Siderite dissolution under reducing

conditions may have been contemporaneous with hydrocarbon generation and primary

migration.

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By further temperature increase the hydrocarbons were cracked to pyrobitumen. Their

reflectance is interpreted to represent maximum burial temperatures (first solid bitumen

generation, parasequence 5).

The fact that the second generation solid bitumen particles are not as mature as the first

(high-reflecting) generation implies that this second migration event occurred after peak

burial temperatures, associated with uplift. These hydrocarbons are proposed to originate

from an external source (cf. 4.4) and have migrated through the Natih formation to form

medium-reflecting (second generation) solid bitumen in veins and stylolites

(parasequence 6).

The low-reflecting solid bitumen of the third generation is the only solid bitumen present in

dissolved fossil shells (parasequence 7). Therefore these shells were dissolved after

emplacement of high- and medium-reflecting solid bitumen, before the third hydrocarbon

migration event filled the fossils and the hydrocarbon was transformed into low-reflecting

pyrobitumen (parasequence 8).

This sequence is in agreement with burial curves elaborated for the foreland of the Oman

Mountains (Terken et al., 2001; Warburton et al., 1990). Fink et al. (2015) suggested a

similar burial history, with a focus on solid bitumen in calcite veins.

4.2. Depositional environment and maturity distribution in Natih B

TOC values of samples collected around the Jebel Akhdar Dome decrease from S to N and

from W to E. This pattern can be explained by facies changes to the center of the Natih B

intra-shelf basin located in the area of the Wadi Nakhr (SW Oman Mountains) or even

further to the southwest. It could also be related to the thickness of the Natih B, which also

increases slightly to the SW (Beurrier et al., 1986; Philip et al., 1995). Sequence

stratigraphic analyses also placed the main Natih B depocenter in the S of the Oman

Mountains (Grelaud et al., 2006; Homewood et al., 2008), although it remains to be shown

that part of these changes are not caused by tectonic thinning during the top to the NE

shearing event (e.g. Holland et al., 2009).

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Rock-Eval results classify Natih B as an overmature source rock. This is supported by the

high amount of aromatic C=C and C=O bonds and the low intensity of aliphatic compounds

(cp. Figure 2).

While measured maturities, representing maximum temperatures, and deepest burial are

now very well constrained (which is also supported by Rock-Eval and FT-IR data) and

fission track data allow quantification of uplift (Saddiqi et al., 2006; Wübbeler et al., 2015),

the timing of maximum burial is less certain. Ophiolite emplacement onto the continent

took place at 78 Ma and probably led to a fast burial (Hacker and Mosenfelder, 1996).

The first generation solid bitumen particles show only negligible bireflectance. This

suggests that no strain occurred during bitumen formation (Bruns and Littke, 2015), in

agreement with the absence of indicators for bedding parallel shear in all outcrops sampled

for this analysis (Holland et al., 2009, has shown that this bedding parallel shear is

heterogonous).

4.3. Maximum temperatures during burial

Paleo temperatures of the carbonate platform in the Oman Mountains are poorly known.

Our data on BRr of Natih B suggest maximum temperatures of 226-239 °C using the burial

equation of Barker and Pawlewicz (1994). Based on RSCM similar values of 226 to 261 °C

were calculated. BRr, as well as RSCM, is related to the transformation kinetics of organic

material. Therefore, factors affecting this transformation will effect both measurements, e.g.

duration at maximum temperature, but also pressure and strain (Khorasani and Michelsen,

1993). Comparing RSCM results of organic rich matrix and solid bitumen reveals that this

method enables thermal maturity measurements in the absence of optically visible solid

bitumen particles. (For higher heating rates during rapid burial, the hydrothermal equation

of Barker and Pawlewicz (1994) would suggest 296-316 °C but these are very unlikely due

to the metamorphic grade of the rocks as discussed below).

These temperatures are in agreement with Fink et al. (2015): They analyzed bitumen-

impregnated veins (containing the here measured generations 1 and 3) in the Natih A of the

Wadi Nakhr area and inferred maximum burial temperatures of around 225 °C. Quartz-

Calcite thermometry for the uppermost Natih A layers indicate temperatures of 166-255 °C

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(Stenhouse, 2014), also in agreement with our data, and with the lower anchizone

metamorphic grade of the rocks (Breton et al., 2004; Searle, 1985) and the fact that zircon

ages are not reset (Saddiqi et al., 2006) in the Jebel Akhdar, indicating that peak

temperatures were below c. 260 °C.

The results of numerical basin modeling presented above, calibrated by our maturity data,

imply a maximum paleo- depth of 7600-8200 m for the Natih B. An eroded overburden of c.

7400-8000 m including 5900-6500 m of ophiolite (Figure 9a) is in agreement with most

other estimates. A lower ophiolite thickness would correspond to thicker Hawasina nappes

if tectonic thinning as suggested by Al-Wardi and Butler (2007) is less strong.

4.4. Oil generation and migration

The three generations of solid bitumen within the Natih B source rock are interpreted to

reflect three different hydrocarbon migration events (Figure 10). Based on the observation

that only the high-reflecting generation is omnipresent in the matrix, we infer that it was

formed from oil generated by the Natih B with which it underwent deepest burial and

experienced peak temperatures (Figure 10c). We also note that this high-reflecting solid

bitumen was not found in the Nahr Umr Fm., below the Natih, in agreement with this

interpretation. Basin modeling shows that hydrocarbon generation is induced by burial

under the ophiolite and took place between 78 and 66 Ma, prior to deepest burial.

Figure 10: Sketched evolution and related hydrocarbon generation and migration: Burial by

sedimentation (1) and under the ophiolite (2, 3) induced hydrocarbon generation and solid

bitumen formation within the Natih B and led finally to deepest burial (3). Erosion and

doming tilted the layers (4) and two lateral hydrocarbon migration events from the foreland

basins occurred, where the Natih oil kitchen was still active (5,6).

The two lower-reflecting generations must represent two later hydrocarbon migration

events that took place after deepest burial (Figure 10e,f). In our basin model, the

temperatures inferred for the second, medium-reflecting generation are reached at 55-

50 Ma, and for the third, low-reflecting solid bitumen at 48 to 45 Ma.

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As the Natih B source rock in the Jebel Akhdar area became overmature at peak

temperature, the two later migration events have to be derived from an external source as a

consequence of lateral, intraformational petroleum migration. The most likely source for

this is a lower mature producing source somewhere in the SW and a possible migration

initiated due to tilting of the area by updoming of the Jebel Akhdar anticline (cp. Figure 10).

Both ideas are in agreement with interpretations by Fink et al. (2015). It should be noted

that, in most petroleum source rocks permeability parallel to bedding is one or two orders

of magnitude higher than bedding-perpendicular (Ghanizadeh et al., 2014), favoring lateral

migration.

According to Terken (1999) a presently active oil kitchen of the Natih is located 50 km SW

of Wadi Nakhr in the foreland. Here, oil production started slightly later at 70 Ma and

ophiolite obduction did not affect this area directly. This kitchen could represent the source

of our later migration events.

5. Conclusion

The Natih B source rock in the Jebel Akhdar shows thermal maturities that reflect peak

temperatures of 226-239 °C with a corresponding maximum burial depth of 7.6-8.2 km,

including approximately 6 km of obducted ophiolite.

Three generations of solid bitumen in the Natih B are linked to hydrocarbon generation

(78-66 Ma) and primary migration prior to deepest burial and two later, secondary

hydrocarbon migration events (55-50 Ma and 48-45 Ma) after peak burial. The latter events

can only be explained by long distance lateral migration of fluids, probably from the SW due

to tilting of the area by updoming of the Jebel Akhdar anticline.

Maturity data obtained by Raman spectroscopy provides the same results for organic rich

matrix and solid bitumen particles, and correlates well with measured BRr. In the absence

of solid bitumen particles it is regarded as an excellent paleothermometer at the stage of

high-grade diagenesis to anchimetamorphism.

Our data form the basis for a detailed understanding of ophiolite and nappe obduction on

underlying sedimentary sequences with special emphasis on temperature and pressure

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evolution. Integration of field data and numerical modelling is regarded as key to further

develop this approach.

Acknowledgements

The authors would like to express gratitude for fruitful discussions to Christoph von Hagke,

Simon Virgo, Reinhard Fink and Alexander Stock. Donka Macherey is thanked for preparing

and polishing the whole rock samples and Jop Klaver and Joyce Schmatz for conducting the

SEM measurements (all RWTH Aachen University). Reviewers Wiekert Visser and an

anonymous reviewer are thanked for constructive, helpful comments on an earlier draft of

this paper. Funding was provided by a RWTH Aachen University scholarship for doctoral

students. Maps in this publication were created using ArcGIS® software by Esri. ArcGIS®

and ArcMap™ are the intellectual property of Esri and are used herein under license

(Copyright © Esri. All rights reserved).

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Appendix 1: Petrophysical parameters for the used lithologies in the numerical basin

model

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Figure 1a

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Figure 1b

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Figure 1c

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Table 1

sample

location form

ation

longitude / latitude, UTM 40Q

TOC [wt.-%]

TIC [wt.-%]

TC [wt.-%]

AG15SR00

Entrance Wadi Nakhr

Natih B 520786 m 2561505 m 0.44 13.36 13.79

AG15SR01

Entrance Wadi Nakhr

Natih B 520786 m 2561505 m 1.06 12.42 13.48

AG15SR08

Entrance Wadi Nakhr

Natih B 520786 m 2561505 m 0.53 13.04 13.58

AG15SR09

Entrance Wadi Nakhr

Natih B 520786 m 2561505 m 0.41 13.40 13.80

AG15SR10

Entrance Wadi Nakhr

Natih B 520786 m 2561505 m 0.82 12.55 13.37

AG15SR11

Wadi Nakhr KH 3 517751 m 2561591 m 0.21 12.55 12.77

AG15SR13

Wadi W of Nakhr Natih 530217 m 2557793 m 0.43 13.41 13.84

AG15SR14

Wadi W of Nakhr Natih 530217 m 2557793 m 0.36 12.91 13.27

AG15SR15

Wadi W of Nakhr Natih 530217 m 2557793 m 0.37 13.28 13.65

AG15SR17

Wadi W of Nakhr Natih 530217 m 2557793 m 0.59 12.94 13.52

AG15SR22

Wadi N of Al Hamra Natih 531024 m 2557020 m 0.42 13.67 14.09

AG15SR23

Wadi N of Al Hamra Natih 531024 m 2557020 m 0.86 13.51 14.37

AG15SR29

Wadi NW of Al Hamra

Natih B 547871 m 2549637 m 0.31 13.18 13.48

AG15SR30

Wadi NW of Al Hamra

Natih B 547871 m 2549637 m 0.36 12.77 13.13

AG15SR32

Wadi NW of Al Hamra

Natih B 547871 m 2549637 m 0.46 13.21 13.66

AG15SR34

Wadi NW of Al Hamra

Natih B 547871 m 2549637 m 0.37 13.40 13.77

AG15SR35

Wadi NW of Al Hamra

Natih B 547871 m 2549637 m 0.26 13.19 13.45

AG15SR37

Wadi Tanuf Natih

B 547844 m 2549686 m 0.23 13.70 13.93

AG15 Wadi Tanuf Natih 547871 m 2549636 m 0.23 13.05 13.28

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SR39 B

AG15SR40

Wadi Tanuf Natih

B 568306 m 2539861 m 0.25 13.53 13.78

AG15SR42

Wadi Tanuf Natih

B 568306 m 2539861 m 0.21 13.10 13.32

AG15SR44

Wadi Muaydin KH 3 568371 m 2538788 m 0.20 12.92 13.12

AG15SR45

Wadi Muaydin

Natih B 568364 m 2538808 m 0.21 13.76 13.96

AG15SR46

Wadi Muaydin

Natih B 521644 m 2564675 m 0.22 13.39 13.61

AG15SR47

Wadi Muaydin

Natih B 521644 m 2564675 m 0.21 13.80 14.01

AG15SR48

Wadi Nakhr KH 2 521227 m 2564015 m 0.28 11.78 12.07

AG15SR50

Wadi Nakhr KH 2 521156 m 2563928 m - - -

AG15SR56

Wadi Sahtan Natih

B 530902 m 2584644 m 0.21 13.46 13.67

AG15SR57

Wadi Sahtan Natih

B 530900 m 2584640 m 0.23 13.25 13.48

AG15SR58

Wadi Mistal Natih

B 570959 m 2577241 m 0.20 12.49 12.69

AG15SR59

Wadi Mistal Natih

B 570960 m 2577241 m 0.22 13.36 13.58

AG15SR60

Wadi Mistal Natih

B 570961 m 2577241 m 0.21 13.20 13.41

AG15SR63

Wadi Mistal Natih

B 571042 m 2577142 m 0.20 12.98 13.17

AG 02

Chessboard Pvmt

Muti A 512673 m 2570368 m 0.22 11.74 11.97

AG 04

Gorge Pvmt Muti

A 514069 m 2572850 m 0.21 0.23 0.44

AG 05

Gorge Pvmt Muti

A 513794 m 2572960 m 0.00 0.22 0.22

AG 06

Gorge Pvmt Muti

A 513771 m 2573124 m 0.00 0.25 0.25

AG 07

Gorge Pvmt Muti

A 513794 m 2573214 m 0.00 0.23 0.23

AG 23

Wadi Ghul Muti

A 515139 m 2562679 m 0.21 0.27 0.48

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AG 25

Balcony Walk Nakhr

Nahr Umr 520913 m 2565658 m 0.23 12.45 12.67

AG 26

Balcony Walk Nakhr

Nahr Umr 521046 m 2565576 m 0.25 8.99 9.24

AG 27

Balcony Walk Nakhr

Nahr Umr 520879 m 2565342 m 0.24 11.60 11.84

AG 28

Balcony Walk Nakhr

Nahr Umr 520855 m 2565307 m 0.23 11.31 11.53

AG 29

Balcony Walk Nakhr

Nahr Umr 520866 m 2565186 m 0.24 10.09 10.33

AG 30

Balcony Walk Nakhr

Nahr Umr 520755 m 2565030 m 0.25 11.80 12.05

AG 31

Jebel Shams Muti

A 520186 m 2569814 m 0.25 7.15 7.40

AG 35

Jebel Shams Muti

A 515084 m 2568916 m 0.22 0.23 0.45

AG 37

Jebel Shams Muti

A 514820 m 2568047 m 0.34 1.25 1.59

AG 38

Jebel Shams Muti

A 514930 m 2567334 m 0.28 3.51 3.79

Table 2

sample location S1 S2 S3 TOC [wt.%]

HI [mg HC/g

TOC]

OI [mg

CO2/gTOC]

AG15SR01 Entrance

Wadi Nakhr

0.09 0.13 1.29 1.06 12 122

AG15SR10 Entrance

Wadi Nakhr

0.03 0.13 1.30 0.82 15 160

AG15SR13 Wadi W of Nakhr

0.05 0.16 1.29 0.43 36 298

AG15SR17 Wadi W of Nakhr

0.04 0.16 1.28 0.59 27 220

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Table 3

sample No. location formation TOC [%]

measured BRr [%]

1st gen (n)

std. 2nd gen (n)

std. 3rd gen (n)

std.

AG15SR00 Entrance Wadi Nakhr Natih B 0.44 3.08 (58) 0.19 2.28 (43) 0.24 1.42 (65) 0.24

AG15SR01 Entrance Wadi Nakhr Natih B 1.06 3.36 (57) 0.19 2.28 (79) 0.18 1.42 (8) 0.14

AG15SR08 Entrance Wadi Nakhr Natih B 0.53 3.59 (38) 0.25 2.40 (86) 0.20 - -

AG15SR10 Entrance Wadi Nakhr Natih B 0.82 3.30 (100) 0.20 - - - -

AG 21 Entrance Wadi Nakhr Natih B 0.29 - - - - 1.04 (15) 0.08

AG15SR13 Wadi W of Wadi Nakhr Natih B 0.43 - - 2.28 (13) 0.37 1.43 (92) 0.26

AG15SR15 Wadi W of Wadi Nakhr Natih B 0.37 - - 2.39 (14) 0.14 1.90 (24) 0.10

AG15SR17 Wadi W of Wadi Nakhr Natih B 0.59 - - 2.45 (24) 0.33 1.27 (54) 0.40

AG15SR21 Wadi N of Al Hamra Natih B - - - - - 1.45 (27) 0.14

AG15SR22 Wadi N of Al Hamra Natih B 0.42 3.34 (3) 0.08 - - 1.31 (84) 0.26

AG15SR23 Wadi N of Al Hamra Natih B 0.86 2.95 (17) 0.28 2.34 (66) 0.38 1.64 (23) 0.23

AG15SR30 Wadi NW of Al Hamra Natih B 0.36 - - - - 1.38 (26) 0.19

AG15SR32 Wadi NW of Al Hamra Natih B 0.46 - - - - 1.23 (44) 0.20

AG15SR34 Wadi NW of Al Hamra Natih B 0.37 - - - - 1.10 (30) 0.10

AG15SR35 Wadi NW of Al Hamra Natih B 0.26 - - 2.26 (9) 0.34 1.46 (94) 0.24

AG15SR40 Wadi Tanuf Natih B 0.25 - - - - 1.08 (25) 0.15

AG15SR46 Wadi Muaydin Natih B 0.22 - - - - 1.34 (33) 0.10

AG15SR57 Wadi Sahtan Natih B 0.23 3.32 (6) 0.20 2.56 (25) 0.56 1.27 (17) 0.12

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Table 4

location format

ion

measured BRr [%] calculated VRr [%]

calculated T burial (Barker&Pawlewicz)

calculated Thydro (Barker&Pawlewicz)

1st gen

2nd gen

3rd gen

1st gen

(VR ≈ BR)

2nd gen

(Landis &

Castano)

3rd gen

(Jacob ´89)

1st gen 2nd gen

3rd gen 1st gen 2nd gen

3rd gen

Wadi Nakhr area Natih B

3.08-3.59

2.28-2.45

1.04-1.90

3.08-3.59

2.47-2.62

1.04-1.57

226-239 °C

208-213 °C

139-172 °C

296-316 °C

268-275 °C

157-210 °C

Al Hamra area Natih B

2.95-3.34

2.26-2.34

1.10-1.64

2.95-3.34

2.45-2.53

1.08-1.41

223-233 °C

208-210 °C

142-163 °C

291-306 °C

267-271 °C

162-196 °C

Wadi Tanuf Natih B - - 1.08 - - 1.07 - - 141 °C - - 160.83

Wadi Muaydin Natih B - - 1.34 - - 1.23 - - 152 °C - - 178.65

Wadi Sahtan Natih B 3.32 2.56 1.27 3.32 2.73 1.18 232 °C 216 °C 149 °C 305.62 280.60 173.34

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Table 5

sample no. lithology location

measured at filter scan T

SV 10-1 Natih Wadi Nakhr

bitumen D1 10x2s 227 °C

SV 10-2 Natih Wadi Nakhr

bitumen D1 10x2s 227 °C

SV 10-3 Natih Wadi Nakhr

bitumen D1 10x2s 228 °C

SV 10-4 Natih Wadi Nakhr

bitumen D1 10x2s 226 °C

SV 10-6 Natih Wadi Nakhr

bitumen D2 10x5s 231 °C

AG 22-1 Natih Wadi Nakhr matrix D1 6x10s 261 °C

AG 22-2 Natih Wadi Nakhr matrix D1 6x10s 243 °C

AG 22-3 Natih Wadi Nakhr matrix D1 10x5s 225 °C

AG 22-4 Natih Wadi Nakhr matrix D2 10x5s 233 °C

AG 01-1

Shuaiba (Kh. Gp.) Wadi Nakhr

bitumen D1 10x5s 252 °C

AG 01-2

Shuaiba (Kh. Gp.) Wadi Nakhr matrix D1 10x5s 269 °C

AG 01-3

Shuaiba (Kh. Gp.) Wadi Nakhr matrix D1 10x5s 253 °C

AG 01-4

Shuaiba (Kh. Gp.) Wadi Nakhr matrix D1 10x5s 251 °C

AG 11-1 Hawasina Sint

bitumen D1 10x2s 209 °C

AG 11-2 Hawasina Sint

bitumen D1 6x10s 215 °C

AG 11-3 Hawasina Sint

bitumen D1 6x10s 193 °C

AG 11-4 Hawasina Sint

bitumen D1 10x2s 207 °C

AG 11-5 Hawasina Sint

bitumen D1 10x2s 213 °C

AG 25-1 Nahr Umr Balcony Walk

Nakhr matrix D1 5x15s 246 °C

AG 25-2 Nahr Umr Balcony Walk

Nakhr matrix D1 5x15s 226 °C

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AG 25-3 Nahr Umr Balcony Walk

Nakhr matrix D1 5x20s 267 °C

AG 25-4 Nahr Umr Balcony Walk

Nakhr matrix D1 5x20s 262 °C

AG 26-1 Nahr Umr Balcony Walk

Nakhr

bitumen D1 10x3s 213 °C

AG 26-2 Nahr Umr Balcony Walk

Nakhr

bitumen D1 10x3s 211 °C

AG 26-3 Nahr Umr Balcony Walk

Nakhr matrix D1 10x3s 275 °C

AG 26-4 Nahr Umr Balcony Walk

Nakhr matrix D1 8x20s 280 °C

AG 27-1 Nahr Umr Balcony Walk

Nakhr matrix D1 5x15s 261 °C

AG 27-2 Nahr Umr Balcony Walk

Nakhr matrix D1 5x20s 248 °C

AG 27-3 Nahr Umr Balcony Walk

Nakhr matrix D1 5x20s 266 °C

AG 30-1 Nahr Umr Balcony Walk

Nakhr matrix D1 5x10s 251 °C

AG 30-2 Nahr Umr Balcony Walk

Nakhr matrix D1 5x20s 257 °C

AG 30-3 Nahr Umr Balcony Walk

Nakhr matrix D1 5x20s 248 °C

AG 37-1 Muti Jebel Shams

bitumen D1 5x5s 193 °C

AG 37-2 Muti Jebel Shams

bitumen D1 5x5s 208 °C

AG 37-3 Muti Jebel Shams

bitumen D1 5x5s 191 °C

AG 38-2 Muti Jebel Shams

bitumen D1 10x10s 172 °C

AG 38-4 Muti Jebel Shams

bitumen D1 10x5s 206 °C

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Table 6

sample No.

location formation TOC [%]

measured BRr [%]

1st gen (n)

std. 2nd gen (n)

std. 3rd gen (n)

std.

AG 02

Chessboard Pvmt. Muti A 0.22

3.04 (9) 0.35 - - - -

AG 04

Road to Gorge Pvmt. Muti A 0.00

3.08 (6) 0.35 2.31 (9) 0.56 - -

AG 05

Way to Gorge Pvmt. Muti A 0.00

3.71 (8) 0.15 - - 1.75 (5) 0.05

AG 06

Way to Gorge Pvmt. Muti A 0.00

3.18 (9) 0.32 - - - -

AG 07

Way to Gorge Pvmt. Muti A 0.00

- - - - 1.55 (5) 0.11

AG 23 Road to Sint Muti A 0.21 3.63 (27) 0.18 2.88 (8) 0.10 - -

AG 31 Jebel Shams Muti A 0.25 - - 1.95 (19) 0.10 1.73 (18) 0.05

AG 35 Jebel Shams Muti A 0.22 - - 2.03 (2) 0.02 - -

AG 37 Jebel Shams Muti A 0.34 - - - - 1.41 (16) 0.08

AG 38 Jebel Shams Muti A 0.28 3.30 (34) 0.17 2.77 (34) 0.14 - -

AG15SR50 Wadi Nakhr Kahmah 2 - 3.47 (6) 0.14 - - - -

AG 25 Balcony Nahr Umr 0.23 - - - - 1.42 (24) 0.09

AG 26 Balcony Nahr Umr 0.25 - - 2.42 (39) 0.23 - -

AG 27 Balcony Nahr Umr 0.24 - - 2.50 (2) 0.06 - -

AG 29 Balcony Nahr Umr 0.24 - - 2.16 (16) 0.05 - -

AG 30 Balcony Nahr Umr 0.25 - - 2.39 (33) 0.19 1.33 (13) 0.07

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Table 7

layer

top [m

b.s.l.]

base [m

b.s.l.]

thickness [m]

eroded [m]

depo.

start [Ma]

depo. end [Ma]

erosion start [Ma]

erosion end [Ma]

assigned lithology PSE

Semail_Crust

-2033

-2033 0 3150 77.6 65 65 45 Upper crust (oceanic, basalt)

Semail_Mantle

-2033

-2033 0 3150 77.7 77.6 45 40 Peridotite (serpentinized)

Hawasina

-2033

-2033 0 1400 78 77.7 40 25 Limestone (micrite)

Muti -

2033

-1999 34 150 93.9 78 25 20 Conglomerate (typical) Seal Rock

Natih_A

-1999

-1957 42 95 93.9 Limestone (micrite)

Reservoir Rock

Natih_B

-1957

-1917 40 96 95

Limestone (organic rich - 10% TOC) Source Rock

Natih_C_D

-1917

-1847 70 98 96 Limestone (micrite)

Natih_E

-1847

-1836 11 100 98

Limestone (organic rich - 1-2% TOC) Source Rock

Natih_F_G

-1836

-1706 130 101 100 Limestone (ooid grainstone)

Nahr_Umr

-1706

-1706 0 112 101 Limestone (shaly) Seal Rock

Hiatus

-1706

-1666 40 113 112 Limestone (micrite)

Shuaiba

-1666

-1491 175

125.2 113 Limestone (micrite)

Kharaib_Lekha - - 70 129 125. Limestone (shaly)

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ir 1491 1421 2

Habshan

-1421

-1126 295

132.9 129 Limestone (micrite)

Salil -

1126

-1089 37

139.8

132.9 Limestone (micrite)

Rayda

-1089

-1089 0 147

139.8 Limestone (micrite)

Hiatus

-1089 -719 370

154.5 147 Limestone (micrite)

Sahtan -719 -719 0 190.

8

154.5 Sandstone (arkose, quartz rich)

Hiatus -719 -119 600 228.

7 201 Sandstone (arkose, quartz rich)

Mahil -119 -119 0 250

228.7 Limestone (micrite)

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Appendix 1

lithology

density

max. compressibility thermal conductivity heat capacity

[kg/m

3] [Gpa-1]

at 20 °C [W/m/K]

at 100 °C [W/m/K]

at 20 °C [kcal/kg/K]

at 100 °C [kcal/kg/K]

Upper crust (oceanic, basalt) 2870 - 1.80 1.81 0.19 0.22

Peridotite (serpentinized) 3100 - 2.20 2.10 0.17 0.19

Limestone (micrite) 2740 85.00 3.00 2.69 0.20 0.23

Limestone (shaly) 2730 68.65 2.30 2.18 0.20 0.23

Limestone (organic rich - 1-2% TOC)

2710 86.51 2.63 2.42 0.20 0.23

Limestone (organic rich - 10% TOC)

2550 95.68 1.45 1.55 0.20 0.23

Limestone (ooid grainstone) 2740 0.20 3.00 2.69 0.20 0.23

Sandstone (arkose, quartz rich)

2690 26.71 4.05 3.46 0.21 0.24

Conglomerate (typical) 2700 14.21 2.30 2.18 0.20 0.23

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Graphical abstract