Rhone Maritime Area

Abstract

This Passive margin in the Western Mediterranean formed by rifting of a back-arc basin

during the Early-Mid Miocene, subsidence has led to the deposition of between two and

ten kilometres of sediment of which 8 kilometres is believed to be prospective. It has two

prospective petroleum systems, Pre-Messinian sourced chiefly in Oligocene Shales and

Sapropels, these have not though been well explored and so there is a high degree of risk

Exploring the licence area. The 2nd is the Plio-Quaternary Upper Sequence, it is possible

that some charge occurs from Pre-Messinia layers but without this charge may be

accomplished by Biogenic processes releasing gas. The basin is a challenging exploration

target as a result of salt, but with improved 3D and Multi-azimuth seismic and with other

geophysical methods it should be possible to recognise the key geometries of the basin;

rifts in the basement, prograding mud fans, the canyons and steeper, coarser grained fans

of the Messinian Drawdown. In the upper sequence seismic resolution should be greatly

improved and sands and mud drapes should be distinguishable.

Immediate targets for exploration should be; undrilled Oligocene syn-rift sediment, drilling

technology and funds allowing; and studies to understand structures/chaotic facies

described in literature as possibe gas escape structures.

Introduction

The Rhone Maritime Area (RMA), offshore Southern France, lies in water depths of 2000

metres; a part of the submarine fan system originating on the Gulf of Lions platform it is a

underlain by a passive margin formed by Oligocene rifting, oriented NE-SW, and Miocene

accretion of oceanic crust. Since the end of active Mid Ocean Ridge volcanism the RMA

and the wider Provence Basin have undergone considerable thermal subsidence.

Subsidence has resulted in continuous sedimentation along the margin with inflow from

the Alps, the Pyrenees & the Massive Central. The Gulf of Lyons shelf margin lies at ~90

kilometres from the coast, the shelf slope lies at ~160 km and salt diapirism can be

observed at between ~250 km to ~330 km from the coast of France at Agde (Google n.d.).

The Rhone Maritime Area is licensed by the French government to Melrose Resources

Plc1 of Edinburgh.

1  [email protected] , Exchange Tower, 19 Canning Street, Edinburgh, EH3 8EG

1

Structural Evolution

Pangea broke up during the Triassic forming a wide ocean, Tethys, of which the

Mediterranean is a relict along with internal basins through the Middle East, Central Asia

and the Tarim Basin in China. During the Jurassic this rifting ceased and the Tethys ocean

started contracting by subduction, and by accretion of micro-continents to the European

margin.

Cretaceous opening of the Bay of Biscay combined with the Northward compression from

the African margin caused anticlockwise rotation of Iberia to form the Pyrenees, a doubly-

vergent orogen. Continuing subduction, and extension linked to the opening North Atlantic,

caused extension from the Early Eocene (Priabonian) along the European Cenozoic Rift

System(Dezes et al. 2004)(ECRIS), extension originated locally at Northerly latitudes and

evolved southward, joining depo-centres. Africa is not moving directly North but rotating

anticlockwise which results in North-Westward subduction in the Western Mediterranean.

By comparison, in the Eastern Mediterranean subduction is Northerly, which results in the

E-W orientation of Hellenic orogens.

From the Early Oligocene rifting intensified between Europe-Iberia and Sardinia-Corsica,

as subduction continued, rifting was particularly strong in the Gulf of Lions as a result of

crustal weakening caused by Pyrenean thrust faulting, (The Office of Oil & Gas Exploration

and Production (BEPH). DGEMP-DIREM, BEPH 2007), (Dezes et al. 2004). Rifting of the

crust underlying the Valencia Trough was arrested by Early Miocene (Burdigalian)(Grup de

Geodinimica y Analisi de Conques & Roca 2002) and is unlikely to have progressed as

rapidly as in the vicinity of the Gulf of Lions, the former having experienced less

deformation during the earlier Pyrenean Orogeny. By the Early Miocene the Northern

Balearic/Catalan Tranform Zones had formed and are observed across the width of the

Provence Basin, along with the Northern, Arlesian/Asinara Transform Zone(Maillard et al.

2003). Between these transfer zones oceanic accretion took place during the Mid Miocene

(Burdigalian-Langhian). Accretionary spreading was rapid, adding ~200km of oceanic

crust, by 30 degree rotation of Sardinia-Corsica, over less than 3 Ma(Speranza et al.

2002), timing is poorly constrained. Spreading of the basin was driven by subduction

rollback which caused Eastward movement of the Corso-Sardinian block until they

accreted to the Apulian during the Tortonian(Seranne 1999), at which point subduction

moved Eastwards to the Tyrrhenian Sea.

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Since the Langhian the basin has undergone stable subsidence, underlying structures

however strongly effect the overlying sediments. Tranfer zones provided an early control of

the deep basin in which salt was deposited during the Messinian. And NE-SW striking rift

structures provide the dominant control on sedimentation both on the Upper Platform of

the Gulf of Lions and in the basin (AUZENDE et al. 1973),(Mascle & Vially 1999).

Differential compaction of Miocene sediments allows Salt deformaion to mimic the

underlying Oligocene thrust geometry closely controlling the development of Salt tectonics,

NE-SW trends are observable on maps of sea floor bathymetry.

Stratigraphy

Largely described according to seismic facies with relatively few wells and no penetration

of the Pre-Messinian sedimentary sequence stratigraphic descriptions of the deep sections

are not available. There are however analogues in the Valencia Trough for Mesozoic

reservoir, at the Amposta Field Shell and Repsol have successfully exploited a karstified

Cretaceous carbonate charged by Jurassic rocks. This is the play that has, so far

unsuccessfully, been investigated in the Gulf of Lions, where 11 boreholes have been

drilled on structural highs. These Mesozoic units are for the purposes of this exercise

treated as basement, as Pyrenean thrusting and Oligocene rifting are likely to have

resulted in loss of any mature carbonates.

Oligocene synrift sediments are likely to be similarly variable to those found in the 3180m

section drilled in the Gallicia field (Camargue Basin) North of the Gulf and on shore, but

which is an extension of the NE-SW rift basins. At Gallicia lowest units are coals which

transition into Bituminous limestone, (Mascle & Vially 1999). Sapropels may also be

expected in areas of greatest subsidence. Elongate delta deposits are also likely, although

it is not explicitly mentioned in the literature, the presence of Coals and Carbonates in the

Gallicia Field reservoir section is indicative of a delta.

Post rift sediments of Early Miocene age are clastic slope and deep marine turbidite

deposits, though some basin edge carbonates may also be expected. Typically mixed

muds and sands are expected, along with coarser turbidites. Pre-Messinian draw down

events caused a well recorded erosion of the Upper Shelf, and a reworking of sediments

down submarine canyons(Lofi & Bern 2008)� , this is likely to have delivered a greater

volume of coarse sediments to the base of the Shelf Slope(Bache et al. 2009). Messinian

desiccation will have deposited a thick, and possibly repeated sequence of Evaporites into

3

the deep basin, whilst at shallower levels the erosion surface remains. (Mark Pawlewicz,

2004)

Transgressive episodes at the end of the Messinian are likely to have caused a return to

mud rich deposition above the evaporite sequence, later regressive episodes will repeat

the reworking of Upper Shelf deposites to the deep basin. There is a great deal of interest,

and literature discussing the Messinian and Post-Messinian of the Gulf of Lions margin.

Maillard suggests a sediment depth of 7760 metres from Sealevel to Acoustic basement.

(Maillard et al. 2003)

The Autun 1 well Quaternary section is formed of Silty Marls, Carbonated Clays and

Sandy/Clayey Canyon fill. Four seismic stratigraphies are described by (LOFI et al. 2003) ;

Parallel to divergent reflectors are interpreted as shallow marine sands, Parallel reflectors

mimicking lower units are interpreted as mud drapes, a transparent facies is interpreted as

a transitional zone from marine to slope deposits and a chaotic facies is interpreted as

a)Incised Valley Fill or b) gas chimney effect. It seems likely that gas escape and coarse

channel fill sediments may occur in the same positions. So this could be seen as a positive

indicator hydrocarbons in the shallow subsurface.

Play Concepts

As there have been no successful wells drilled in the Rhone Maritime Area, and the

sparsity of data the Play Concepts provide here are very speculative, and based on theory.

1. Oligocene – Oligocene

i. Oligocene source rocks are expected to be syn-rift sediments of multiple types;

coals and bituminous carbonates have been discussed by Mascle at Gallicia,

this appears to have been a deltaic succession forming on the young rifted

margin. Reservoir is suggested as Oligocene deltaic sands(Mark Pawlewicz,

2004)(Mascle & Vially 1999). Sealng is likely to be by Overlying Miocene

Shales, bounding fault blocks are likely to be involved.

Source rocks with high organic content have been found at locations throughout the

Mediterranean if not yet in the Provence Basin, but as Pre-Messinian is yet to be drilled

there is a good chance that sapropels will be found Sub-Salt.

4

2. Oligocene – Miocene

Three Miocene plays are suggested;

i. Canyon sands draped by shales and ultimately sealed by Messinian salts and

sourced directly from Miocene shales or from Oligocene organic rich sediments.

ii. Coeval carbonates draped in organic rich shales may provide a good reservoir,

but this is more speculative.

iii. Immediate subsalt plays and stranded turbidites within the Messinian salts may

also provide reservoir and source, though tight.

3. Oligocene – Neogene

i. Its is suggested that erosion of of Miocene sediment during the Messinian may

have exposed Oligocene units, or allowed uninterrupted transfer of

hydrocarbons from Oligocene sources previously discussed into Neogene

turbidites sands, or into Upper Shelf Gulf of Lions canyon sands where they may

be trapped beneath shallow mud drapes as described by Maillard.

4. Pliocene – Quaternary Biogenic Gas

i. Pliocene and Quaternary Biogenic Gas are suggested as a potential resource

on the basis of analogues in the Plio-Quaternary of Egypt(DOLSON et al.

2005) and Taiwan(Fuh et al. 2009).

5. Mesozoic – Paleozoic Gas

i. There is some disparity in the literature as to the prospectivity of the Pre-

Oligocene and Mesozoic basement section, (Mascle & Vially 1999) exploration

of the Pre-Oligocene is highly speculative. Having already seen to have failed in

the 11 wells drilled so far it is suggested that older sediments may be barren.

Pawlewicz meanwhile does not draw any clear division of Pre-Miocene rocks but

treats them as undifferentiated. This possibly due to a different quality of seismic

available at the time of his writing the USGS report. Pawlewicz categorically

states that basement rifted blocks should act as the chief trap type across the

basin. There is a good chance that sourcing could occur from Oligocene source

into older basement porous rocks.(Mark Pawlewicz, 2004)

These are the main play fairways discussed in the literature, it is worthwhile also

considering vertical escape structures of sand, or mud.

5

Trap Types

In the Pre-Messinian it is likely that faulting will provide a dominant trap type, (Mark

Pawlewicz, 2004) block faulted basement is likely to act as a reservoir for Oligocene

sourced hydrocarbons. In higher units the dominant trapping mechanism is likely to be

stratigraphic. Where salt is active this is also expected to act as a sealing unit.

Subsidence, Maturity &Migration

Sunsidence within the Provence Basin has been rapid, particularly in the Rhone Maritime

Area, USGS maximum estimates for sediment thickness is 12 km (Mark Pawlewicz,

2004), more conservative estimates put sediment thickness at between 2 km, on the Gulf

of Lions shelf, and 10 km in the deep basin(Seranne 1999). Pawlewicz suggests:

Km Erathem System Series Unit Lithology

1-2 Cenozoic Quaternar

y

Tertiary

Pleistocene/

Pliocene

Post-

Messinian

Shale & Turbidites

1-2 Cenozoic Tertiary Uppermost

Miocene

Messinian Evaporites

3-4 Cenozoic Tertiary Lower Miocene Pre-

Messinian

Turbidites & Pelagic

Sediments including

sapropels. Potential

source and reservoir

2-4 ? Cenozoic Tertiary Rifted Basement

(Mark Pawlewicz, 2004)

Pawlewicz quotes a figure of (Jean Burrus & Audebert 1990) for a regional Geothermal

gradient of 46C – 96C/km, allowing for minimum and maximum thickness and

temperatures according to the USGS:

Overburden: 2km => 92C / 4km => 184C @ 46C/km

6

2km => 180C / 4km => 360C @ 95C/km

Oil Window: 60 – 120C / 2 – 4km

Gas Window: 100 – 200C / 3 – 6km

N.B. - Access not available to (J. Burrus & Foucher 1986). The above seems to suggest

that Pre-Messinian should be beyond gas window if sediments are as thick as

expected. ??? Nowhere in the literature search has the author found evidence to suggest

that it is thought that hydrocarbons will be 'over-cooked', for the purpose of this piece of

work the above data is to be 'set aside'. As reported by Pawlewicz hydrocarbons in the

Provence Basin should be in the gas window, and in places locally within the oil window.

Migration is likely to be along faults vertically, and both laterally and vertically through

porous sediment. Also of importance for migration would be slumps and water escape

structure which may introduce hydrocarbons to otherwise clean rock bodies..

There is also likely to be Biogenic gas generation is shallow Supra-Salt sediments.

Particularly it would be of interest to have pressure data from the basin as the rapid

subsidence of the basin may provide forces for hydrodynamic flow pushing fluids toward

the continental margin. Where this is the case it is highly likely that the reserves will be

caught in stratigraphic traps.

It is possible that there may also be some hydro-dynamic involvement in this basement as

a result of the youth of the basin and the rapid burial by rifting and subsequent thermal

subsidence upon cessation mid-ocean ridge volcanism.

Industry Activity

The Provence Basin is under explored, the USGS quotes generous reserves the majority

of which are likely to be found in the Rhone Maritime Area. As the USGS numbers (Mark

Pawlewicz, 2004) for possible reserves of the Provence Basin are the only figures found

by the author in the literature for this basin and as the USGS number are generally a good

estimate they are accepted.

USGS Mean Undiscovered Resources

51 TCF (1.4 TCM) gas; 0.42 MMMBBL oil; 2.23 MMBBL condensate.

7

As no reserves are posted and no wells drilled it would be fair to use the USGS figure for

Resources, whilst qualifying it by saying that Pliocene-Quaternary Biogenic gas is

proposed which would add to reserves => 505 MMBOE {derived from Pawlewicz,

2004}. Melrose Resources have reported 10 Tcfe of reserves in their Rhone Maritime Area

License, it is hypothesised that these are the large fault bounded basement structures. As

yet it is not known when Melrose plan to drill these structures.

It is worthwhile examining the French government websites for information about the area,

where it is should be possible to access geographic information about the margin.

There is a very high degree of academic interest in the basin, and a large number of

publications relevant to the industry.

Conclusions

The Rhone Maritime Area is a good prospective basin, in a challenging environment and

would provide good opportunities for a farm-in. It might become a more strategically

important resource to the European Union if relations with Russia and the FSU worsen.

The most prospective reserves would be those in the Pre-Messinian sediments, it is

however the technical difficulties connected to exploiting these resources may make the

other less prospective resources more attainable. Particularly the resources Onshelf Gulf

of Lions, which lie on trend from the proven Camargue Basin. It would also be beneficial to

understand the wider petroleum system to drill through the Oligocene source section. The

targets are likely to be gravity and seismic anomalies, gravity data will prove particularly

useful to deal with the variations of acoustic properties through the Miocene evaporite

sequence.

Plio-Quaternary gas should be constrained with further geophysical/geochemical surveys

to investigate possible gas escape structures.

8

Acknowledgements

Dr Helen Lewis for reading lists.

References

AUZENDE, J.M., BONNIN, J. & OLIVET, J.L., 1973. The origin of the western Mediterranean basin. Journal of the Geological Society, 129(6), 607-620. Available at: http://jgs.lyellcollection.org/cgi/content/abstract/129/6/607.

Bache, F. et al., 2009. Messinian erosional and salinity crises: View from the Provence Basin (Gulf of Lions, Western Mediterranean). Earth and Planetary Science Letters, 286(1-2), 139-157. Available at: http://www.sciencedirect.com/science/article/B6V61-4WT39TY-3/2/0b17494de93fc495b3ed979e3a498388.

Burrus, J. & Foucher, J., 1986. Contribution to the thermal regime of the Proven al Basin �based on Flumed heat flow surveys and previous investigations. Tectonophysics, 128(3-4), 303-334. Available at: http://www.sciencedirect.com/science/article/B6V72-488941F-9/2/b555c8d1aa4ef7d05648d1e40e2572e9.

Burrus, J. & Audebert, F., 1990. Thermal and compaction processes in a young rifted basin containing evaporites; Gulf of Lions, France. AAPG Bulletin, 74(9), 1420-1440. Available at: http://aapgbull.geoscienceworld.org/cgi/content/abstract/74/9/1420.

Dezes, P., Schmid, S. & Ziegler, P., 2004. Evolution of the European Cenozoic Rift System: interaction of the Alpine and Pyrenean orogens with their foreland lithosphere. Tectonophysics, 389(1-2), 1-33. Available at: http://www.sciencedirect.com/science/article/B6V72-4D5KSBN-1/2/2c8bbf184b1c1d71813fb938de7d92c2.

DOLSON, J.C. et al., 2005. Key challenges to realizing full potential in an emerging giant gas province: Nile Delta/Mediterranean offshore, deep water, Egypt. Geological Society, London, Petroleum Geology Conferenceseries, 6, 607-624. Available at: http://pgc.lyellcollection.org/content/6/607.abstract.

Fuh, S. et al., 2009. The biogenic gas potential of the submarine canyon systems of Plio-Peistocene foreland Basin, southwestern Taiwan. Marine and Petroleum Geology, 26(7), 1087-1099. Available at: http://www.sciencedirect.com/science/article/B6V9Y-4TPPF5F-1/2/7d5902316ea409bae785fe4d045dd9d2.

Google, gulf of lion - Google Maps. Available at: http://maps.google.co.uk/maps?q=gulf%20of%20lion&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&um=1&ie=UTF-8&sa=N&hl=en&tab=wl [Accessed November 1, 2009].

Grup de Geodinimica y Analisi de Conques & Roca, E., 2002. GGAC Western Mediterranean. Available at: http://www.ub.es/ggac/research/medit/medit.htm [Accessed October 30, 2009].

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Lofi, J. & Bern , S., 2008. Evidence for pre-Messinian submarine canyons on the Gulf of �Lions slope (Western Mediterranean). Marine and Petroleum Geology, 25(8), 804-817. Available at: http://www.sciencedirect.com/science/article/B6V9Y-4SFXK9K-1/2/def454eeb805f3bf7c59ecd453852bbd.

LOFI, J. et al., 2003. Plio-Quaternary prograding clinoform wedges of the western Gulf of Lion continental margin (NW Mediterranean) after the Messinian Salinity Crisis. Marine geology, vol. 198(2003, vol. 198, no3-4, pp. 289-317 [29 page(s) (article)]).

Maillard, A. et al., 2003. Influence of differential compaction above basement steps on salt tectonics in the Ligurian-Proven al Basin, northwest Mediterranean. � Marine and Petroleum Geology, 20(1), 13-27. Available at: http://www.sciencedirect.com/science/article/B6V9Y-493GKBV-1/2/f6e7e9837a1ec3ecd63bf7342a7c5e75.

Mark Pawlewicz, , 2004. The Pre-Messinian Total Petroleum System of the Provence Basin, Western Mediterranean Sea.

Mascle, A. & Vially, R., 1999. The petroleum systems of the Southeast Basin and Gulf of Lion (France). Geological Society, London, Special Publications, 156(1), 121-140. Available at: http://sp.lyellcollection.org/cgi/content/abstract/156/1/121.

Seranne, M., 1999. The Gulf of Lion continental margin (NW Mediterranean) revisited by IBS: an overview. Geological Society, London, Special Publications, 156(1), 15-36. Available at: http://sp.lyellcollection.org/cgi/content/abstract/156/1/15.

Speranza, F. et al., 2002. Age of the Corsica-Sardinia rotation and Liguro-Proven al �Basin spreading: new paleomagnetic and Ar/Ar evidence. Tectonophysics, 347(4), 231-251. Available at: http://www.sciencedirect.com/science/article/B6V72-45214D8-1/2/3f0c034274d6c378e4743b3a0213d82d.

The Office of Oil & Gas Exploration and Production (BEPH). DGEMP-DIREM, BEPH, 2007. Highlights of Oil and Gas Exploration and Production in France. Available at: http://www.developpement-durable.gouv.fr/energie/petrole/beph-ang/beph-reperes-ang.htm [Accessed October 31, 2009].

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Appendices

Glossary of USGS

1 Barrel Oil Equivalent (BOE) (1*10E9) = 51 Trillion Cubic Feet (TCF) 1*10E12

(Klett et al. 2000)

Fig ?. Line Diagram of the Gulf of Lions Margin; red & then blue is the Messinian Erosional

Surface. Note ravinement during Messinian and transfer of sedimen to the deep basin.

(Bache et al. 2009)

(P. A

Ziegler

&

Dezes

2006)Ol

igocene

Rifts on

crustal

11

thickness, Red thin green/blue thick plate.

12

(Seranne 1999) Evolution of Paleogeography of Western Meditteranean. Accretion of

blocks pre Eocene overtaken by subduction of Tethyan crust. Subduction volcanism, heat

flow and stretching...rifting and ocean basin formation. Also subduction rollback.

13

(Seranne 1999) Stylized representation of different deformational styles...

(Seranne 1999)ser Rollback: Mechanism for extension in a compressional system.

(Seranne 1999) Rollback labelled with positions of continental blocks relevant to the West

14

Mediterranean.

From (LOFI et al. 2003) Profile through Gulf of Lions Basin and into Rhone Maritime Area.

GLP 1 & GLP 2 both lie just within the Rhone Maritime Area.

15

(Mascle & Vially 1999)Mascle's Tectonic Chart

16

(Maillard et al. 2003) Location Map of Rhone Maritime with major faults/transfer zones.

17

(Maillard et al. 2003) Extent of Salt and relationship to underlying Eocene faults

18

(Maillard et al. 2003)Composite section across centre of Rhone Maritime Area showing

effect of Messinian salt tectonics.

(Maillard et al. 2003) seismic character and velocity model provides depth estimate for

basin.

19

Map showing location of Seismic Lines A & B. These illustrate change of structure N & S of

North Balearic Transform Zone.

(Grup de Geodinimica y Analisi de Conques & Roca 2002)

20

(Grup de Geodinimica y Analisi de Conques & Roca 2002)

21

(Grup de Geodinimica y Analisi de Conques & Roca 2002)

22