Onarep Presentation- Morocco's Geology

- Morocco's Geology -

MAIN GEOLOGICAL DOMAINS.

Five major geological domains are distinguished, these are :

the Anti Atlas and Saharian domain comprised of gently deformed Paleozoic sealed to the Northwest African Craton during the Hercynian orogeny.

the Mesetian domain characterized by deformed Paleozoic unconformably overlain by gently folded Meso-Cenosoic deposits.

the Atlasic domain corresponding to the inversion of the Atlasic through which was filled by thick Mesozoic sediments.

the Atlantic passive margin consisting of thick Mesozoic sediments locally deformed by salt tectonics and gravity induced imbrications with important turbidite sheet deposits basinwards.

the Rif domain a thrust fold belt where the deformation of the Meso-Cenozoic series decreases southward.

TECTONIC SETTING OF MOROCCO.

The post Precambrian structural evolution of Morocco was marked by two major compressional events (Hercynian and Alpine orogenies) separated by an extensional period related to the Atlantic opening.

HERCYNIAN OROGENY : The Upper Devonian-Carboniferous compression resulted in folding and faulting of the Paleozoic series. The major remnants of the hercynian fold belt crop out in the Anti Atlas and the Moroccan meseta.

TRIASSIC-LIASSIC EXTENSION : The opening of the Atlantic was preceded by Triassic-Lower Jurassic rifting which was followed by massive regional subsidence during the Jurassic and much of the Lower Cretaceous in the Atlantic passive margin. This extension was also responsible of the opening of the Atlas troughs.

ALPINE OROGENY : During the Upper Eocene-Oligocene, the Atlas Mesozoic troughs were inverted to form the High and Middle Atlas. The late collision of Africa with Europe during the Neogene resulted in the formation of the Rif which is a segment of the Western Mediterranean Alpine fold belt.

PETROLEUM GEOLOGY.

SOURCE ROCKS : The data base so far available has permitted to delineate the following facies as potential source rocks.

Paleozoic : The Ordovician, Silurian and Devonian organic facies are predominantly sapropelic (type II kerogen) although, in places, this facies is becoming lipidic (type I kerogen). These Paleozoic organic facies are widespread over the hole moroccan territory and constitute the continuity of those encountered in Algeria and north Africa in general. The Carboniferous organic facies, however, is predominantly humic (type III kerogen).
The Silurian source rocks with TOC value of up to 12 % (Tadla Basin) are the most important. The Ordovician and Devonian sequences exhibit interesting source rock intervals with TOC values in the range of 4 % in the same basin. Visean and Namurian sequences contain shally intervals with humic organic facies (type III kerogen) having TOC values around 1.5 % in the High Plateaux. Similar facies are also encountered in the Tadla Basin. The Westphalian and the Stephano-Autunian organic facies with frequent coaly and lignitic intervals recorded TOC values in excess of 30 % in the High Plateaux and Argana valley.

Some oils sampled from reservoirs or well shows in the Prerif (Tselfate field), Doukkala, Tadla, Essaouira basins are interpreted to be generated from Paleozoïc source rocks.

Triassic : The synrift sequence infilling Triassic graben and half graben along the atlantic coastal basins may yield excellent lacustrine (type I kerogen) source rock. The only place where these intervals were tested is in the Doukkala Basin. TOC values recorded are in the range of 2.5 %

Jurassic : Recent geochemical survey shows that the Rif, Middle and High Atlas basins contain rich Lower Jurassic (Liassic) source rock. The organic facies is predominantly amorphous type II kerogen with TOC values up to 10 % and within the oil window in most places. These facies are producing oil in the Prerif ridges and are the sources for many seeps distributed throughout the Prerif and Middle Atlas areas.
The proliferous character of this Jurassic organic facies may have a widespread distribution. The oil in Sidi Rhalem field in Essaouira Basin is produced from Oxfordian shale ( TOC up to 4 %). The oils produced in the Cap-Juby structure in the Tarfaya Basin are probably sourced from Jurassic marly facies. In the Tarfaya-Layoune-Dakhla Basin Lower and Middle Jurassic organic facies have TOC value ranging from 1.47 to 2.49 %.

Cretaceous : Marine organic facies (Aptian-Albian and Cenomano-Turonian) are, by far, the richest organic matter with TOC value up to 20 %. These facies are widespread over most of moroccan sedimentary basins. Under adequate burial, the Cretaceous marine organic facies may constitute an excellent effective source rock. Recent synthesis studies show that this could be the case in the Rif, Tadla and, basinward, in the offshore atlantic basins.

Neogene : The Neogene (Miocene and Oligocene) marls and shale have TOC value of up to 7 % in the atlantic basins and up to 2 % in the Mediterranean offshore area. This source rock is producing biogenic gas but can produce oil where sufficiently buried. This is seen particularly in the Rharb Basin where we have either biogenic gas or oil production depending on the depth of burial of this source rock.

RESERVOIR ROCKS : A variety of reservoir intervals have been evidenced by wells throughout the sedimentary sequence. Results are summarized as follows :

Paleozoic : Within the Carboniferous succession, most reservoir intervals are encountered in the Westphalian, Namurian and Visean of the High Plateaux. These intervals are mostly deltaic to turbiditic siliciclastics with porosity values around 11 % and reach 15 % in the Missour Basin. Similar intervals may exist in the Tindouf Draa area. Visean carbonate reservoir intervals are also identified in the High Plateaux and in the Tadla Basins.
Devonian reef carbonate reservoirs are expected in the Doukkala, Boudenib and Tindouf-Draa Basins. Cambro-Ordovician sandstone and conglomerate reservoir intervals are interpreted to have enhanced porosity by fracturation particularly in the Tadla and Tindouf-Draa areas.

Triassic : Triassic sands and conglomerates are widespread over most of moroccan sedimentary basins. In most cases these sands and conglomerates are deposited in a fluvial and deltaïc environments. Porosity values of these intervals are averaging 10 % in the Tadla, and High Plateaux and may reach 15 % in the Doukkala and 22 % in Essaouira Basin.

Jurassic : Numerous reservoir intervals are also encountered in the Jurassic succession; these are : (1) Reef carbonate in the Missour Basin with porosity value up to 10 %, (2) the Tarfaya Laayoune carbonate, (3) Sandy dolomite and carbonate of the Callovo-Oxfordian in Essaouira Basin where the porosity values range from 5 to 20 %, (4) the Middle Jurassic arkosic sand of the Haricha with porosity values up to 30 % , (5) the Middle jurassic sands in the Tadla Basin (porosity values up to 10 %), (6) the Lower Jurassic Carbonate in the Guercif Basin, Reef carbonates in the High Plateaux, the oolithic Limestone in the Tselfat and coarse sandstone in the Sidi Fili trend. Porosity values of the Lower Jurassic reservoirs are ranging from 10 to 30 %.

Cretaceous : Upper Cretaceous reservoirs are encountered in the Tadla Basin. These are comprised of Cenomano-Turonian Carbonate and Senonian sands, with porosity values around 10 %. Much better intervals are encountered in the Cretaceous of Tarfaya-Laayoune Basin where the porosity is ranging from 20 to 30 %.

Neogene : In the onshore basins the Neogene sequence contains good to excellent reservoir intervals. These are: (1) Oligocene sands in the Rif domain with porosity values ranging from 15 to 20 %, (2) Miocene sand in the Rharb Basin with porosity values up to 30 %, (3) turbiditic sands in the Meso-Rif with porosity values up to 17 %, and (4) sands and conglomerates of the melange (Prérif nappe) and the sands underlying the melange. These reservoir intervals recorded porosity values ranging from 8 to 20 %.

SEAL : In most Moroccan Sedimentary basins and for most prospective reservoirs, there exist adequate seals. These are: (1) Tertiary marl and shale; (2) Cretaceous shale and marls and, in places, salt and anhydrite; (3) Jurassic shale, anhydrite and in places salt; (4) thick Triassic shale and salt is the most adequate seal for any sub-salt reservoir; (5) within the Paleozïc succession each shally interval may act as a seal for underlying reservoirs.

PROSPECTS AND PLAYS : A variety of exploration concepts have been developed and numerous attractive prospects and leads were defined
The most recent studies, based on substantial amount of seismic data, particularly in the offshore, and a recent regional geochemical survey, and integrating the existing regional synthesis studies, have permitted to generate new exploration concepts. These studies have also permitted to define 178 leads (or prospective structures that require additional seismic to refine these structures as prospects) and 102 prospects that await to be drilled

These leads and prospects distributed throughout numerous sedimentary basins have been evidenced by multidisciplinary approaches in the domain of petroleum exploration

OFFSHORE DOMAIN :

Mediterranean offshore: As shown by geochemical analysis of neighboring surface samples in the onshore, this domain may contain various source rock intervals especially in the Messinian marls (TOC up to 2 %).

The Jebha well encountered turbiditic intervals with porosity up to 20 %. Other sandy reservoir intervals exist within the Miocene succession.

In the western part of this domain, the most common traps are stratigraphic and consist of roll-over structures related to syn-sedimentary normal faulting and shaly diapirism movements.

Traps in the eastern part are formed of various hanging wall anticlines resulted from Alpine compressional system.

Atlantic offshore: Recent studies have developed many exploration concepts in the Atlantic continental margin of Morocco.

Rharb : The Paleozoïc system, producing in the onshore in the Sidi Fili trend, is expected to be present in the offshore part of the Rharb Basin. The main reservoir is composed of Paleozoïc terrane, metamorphics and granite wash-out. In this same area the Mesozoic objectives include Triassic siliciclastics, Jurassic carbonates and Cretaceous carbonates and sandstones.

Casablanca : Important anticline structures comprised of Cambro-Ordovician quartzitic sandstone could be laterally charged from rich, oil prone, Silurian and Devonian source rocks. These structures have been mapped in the offshore Casablanca area. With respect to their age and petroleum system these structures exhibit many analogies with the producing ones in Algeria (Hassi Messaoud).

Hercynian structure in Casablanca

Ifni-Tarfaya and Safi : The synrift sequence infilling Triassic and infra-Liassic Newark-type half graben comprises alluvial fan and fluvio-deltaïc deposits providing excellent reservoir intervals, and lacustrine deposits that may act as an effective source rock. Several prospects and leads were defined in these areas.

Agadir-Essaouira : The most recent studies show that the Atlantic Jurassic carbonate platform is much wider than what has been considered so far and includes more traps than just those related to carbonate bank edge. In fact, in the light of the newly acquired seismic data, many other new exploration concepts have been developed. Such as traps related to a karstified and faulted carbonate shelf edge. These are laterally charged from mainly basinward Cretaceous source rocks.

The new exploration concepts developed in the Tarfaya offshore area consist of a prograding Lower and Middle Jurassic reef build ups distributed around a depocenter of a mature source rock deposited as a first post-rift sequence.

Various traps in the offshore Atlantic passive margin.

In this area various prospects and leads were defined within the Cretaceous succession. These include,roll- over anticlines, deep sea fan, canyon and sub-marine fan deposits...

Tarfaya Dakhla : The Tarfaya Dakhla area is the only zone, in this part of the offshore, where the Cretaceous source rock is put under adequate burial to generate oil. It is unfortunate , however, that the available seismic coverage has only permitted to define leads. Additional seismic is required to refine those leads as prospects.

ONSHORE DOMAIN :

Gharb, Rif-Prerif : In the Rifain domain the prolific Rharb Basin contains turbiditic sand sheets producing both oil and biogenic gas from combined stratigraphic and structural traps. Recent studies have developed two exploration play concepts for 3 distinct objectives in this basin. A play concept is developed in the central part of the basin where the dry gas (methane) is accumulated in Miocene turbiditic sand sheets overlying the melange (Prérif nappe). Depth to target is fairly shallow (1000 to 1500 m). Gas effect is confirmed in these prospects through AVO reprocessing. Another play concept is developed in the north-eastern and southern part of the basin. Objectives, producing both oil and gas, are comprised of Supra-nappe Miocene sands and Carbonates and sandstones at the top of the nappe. Here again, depth to target is fairly shallow

Section through the thrust of the "Prerifain Rides".

It is worth mentioning that oil production is also possible under the Prerif-nappe as suggested by a fairly high geothermal gradient which may bring any Miocene or Cretaceous source rock into maturation at that depth ( below the Prerif-nappe).

In the western Prerif, the proximity to the oil province of the Prérif-Ridges and recent geochemical studies underlaying the existence of mature source rock intervals within the Prerif-nappe, have stimulated a play concept with 3 distinct objectives. These are (1) Middle Jurassic arkosic sandstone; (2) Lower Jurassic sandstone and Carbonate, and (3) Lower Miocene calcarenite.

In the eastern Prerif, based on a recent geochemical survey and various oil seeps distributed in the area, a play concept, where oil and/or gas are produced from various objectives both in the nappe complex (Prerif, Ouezzane, Tsoul) and in the autochthonous (Tertiary, Mesozoïc and Paleozoïc), has been developed.

The Prerif-Ridges and the Sidi-Fili fault trend constitute the foothills of the Rifain chain. Various traps were defined in this oil province. They range from simple anticline and salt-cored thrusted anticline to erosional pinch out under the Upper Miocene unconformity and sand lenses within the Upper Miocene suppra-nappe sequence.

The seismic data acquired in 1990 and 1993 have evidenced an early salt tectonics which started since Early Jurassic time. Important salt related structures were formed prior to Tortonian source rock maturation. These observations lead to consider the oil pools along the Sidi-Fili trend, Boudraa, and Tselfat as being "mega shows" resulted from Neogene dismigration from deeper and more important reservoirs. More important traps are then to be identified and tested in the vicinity of these pools.

Similar play concepts may apply in the Saïss Basin particularily sub-salt structural traps (faulted blocks, anticline...) but also stratigraphic traps (Triassic clastic fan deposits, possible Lower Jurassic reef developments, Miocene channel fill deposits...)

INTERATLASIC BASINS :

High Plateaux and Missour Basin : Two petroleum systems were defined. A Paleozoïc system where Carboniferous and possibly Silurian source rock are charging Westphalian and base Triassic reservoirs, and a Jurassic system where Lower Jurassic source rocks are charging Lower to Mid-Jurassic reservoirs.

Paleozoïc traps, both structural and stratigraphic, are of various types. Triassic traps are mainly comprized of subsalt uplifted blocks and paleomorphologic structures. The Jurassic traps are mainly stratigraphic and consist of bank edge reef build-ups and in places salt-cored anticlines.

The timing of maturation and migration is a crucial factor especially for the Paleozoïc system. If maturation and expulsion occurred during Carbonifeous time then the Pre-hercynian traps would be the most important to look at. However, if maturation did not start until the Jurassic time, as suggested by recent geochemical study, the Pre-Jurassic traps would become the most important target to explore.

Guercif Basin : The Tertiary structural evolution initiated both atlasic and alpine structurations in the Guercif Basin. Any hydrocarbon generated during Jurassic and Cretaceous time has a chance to be trapped in atlasic structures.

Two main exploration concepts were developed. A Tertiary play concept with exploration objectives in Tortonian marls and at the base of Tertiary sequence. The second play concerns the Jurassic succession with two main objectives in Upper Jurassic clastics and Lower to Mid Liassic carbonate. It is worth mentioning that, in Guercif Basin, exist two different environments of deposition during the Jurassic time. One, to the east, characterized by platform shallow marine deposits; the other, to the west, characterized by open marine deposits. Transitional facies, between the two domains may, constitute hydrocarbon stratigraphic traps

In the Beni-Znassen - Beni Bou Yahi area possible traps may exist within the Jurassic succession. Lower Jurassic (Domerian) source rocks may charge karstified dolomite and dolomitic carbonate reservoirs.

MESETIAN AND COASTAL BASINS :

In the Mesetian Domain, two exploration concepts have been developped in Essaouira Basin. One concerns Paleozoïc and Triassic formations, the other have objectives in the Jurassic succession.

Essaouira Basin : is a Mesozoïc and Cenozoic passive margin coastal basin developed on a block faulted Paleozoïc basement. Silurian source rocks are charging, with wet gas, Triassic-Lower Liassic siliciclastic reservoirs (Meskala-Zelten), possibly Paleozoïc clastic and carbonate reservoirs (shows encountered by some Meskala wells), and Jurassic reservoirs both clastics and carbonates (Jeer, Kechoula gas fields).

The second play concept concerns the Jurassic petroleum system where Jurassic source rocks (Domerian and/or Oxfordian) are charging Argovian sandy dolomitic reservoirs (Sidi Rhalem oil field). Numerous prospects and leads were mapped in this basin.

Doukkala Basin : The play concept developed in the Doukkala Basin consists of Silurian and Devonian source rocks charging either possible folded Middle Devonian reef complexes or Permo-Triassic sandstones and conglomerates. Traps are generally of structural type. In the case of Middle Devonian traps, paleogeographic aspects should be considered in order to localize the most favorable reefs within a given structure.

A stratigraphic trap corresponding to a huge reef complex was defined within the Middle Devonian sequence. The reservoir is laterally charged from Silurian and Devonian source rock depocenter located to the east of the reef complex. A source-reservoir mass balance approach has shown that these source rocks yield sufficient charge to fill in this reef complex. The areal size of the reef complex range from 60 to 100 sqKm. The net pay of this reservoir is estimated to average 100 m.

Tadla Basin : Two petroleum systems are defined in the Tadla Basin. One Paleozoïc petroleum system where. Silurian source rocks, still in the generating stage in uplifted blocks, are charging Carboniferous, Triassic and Middle Jurassic reservoirs. The other system is Mesozoïc where Cretaceous source rock are brought into generation stage under the atlas foldbelt thrust and charging Cretacous reservoirs with proven excellent petrophysical characteristics.

The investigation of a limited zone, due to data control, has permitted to define several prospects and leads.

Souss Basin : is only partially covered by seismic. The only prospective zone defined with available data base is located to the west of the Klea fault. This zone corresponds to a depocenter of the synrift sequence where Triassic-Lower Liassic lacustrine source rock is expected to charge fluvio-deltaïc clastic reservoirs deposited within the same sequence.

SOUTH ATLAS BASINS :

Ouarzazate Basin : In the Ouarzazate Basin, recent basin analysis studies show the existence of rich Silurian source rocks and a serie of clastic and carbonate reservoir intervals ranging in age from Ordovician to Cenomanmo-Turonian. Despite a poor data control, some stratigraphic traps have been defined in the northern partof the basin.

Boudenib-Tafilalet : The most recent exploration studies carried out in the Boudenib-Tafilalet basin have evidenced a good source rock potential in the Silurian and Carboniferous successions. The main Paleozoïc reservoirs are the Ashgilian Carbonate and sandstones, the Devonian reef build-ups, and the Carboniferous turbiditic sands. No data is available on the western part of this basin where the Mesozoïc is expected to yield the primary exploration objectives.

Tindouf Basin : The scarse geochemical data available shows that the Silurian source rock is still within the oil window in the southern flanc, and may have reached gas window in the northern flanc of the moroccan part of the Tindouf Basin.

However, in the absence of any seismic data it is, at present, difficult to delineate any prospective zone in this basin.

Tarfaya-Laayoune-Dakhla basins : In the onshore part of the Tarfaya-Laayoune-Dakhla basin, numerous exploration concepts have been developed. A Casablanca type structure is expected to be present in the eastern-most part of the basin underlying the Precambrian overthrust. In the eastern part of the basin, the Triassic synrift sequence, infilling the half graben, is the main exploration target. The western part of the basin contains the same objectives as those described in the offshore Tarfaya Basin. That is Lower and Middle Jurassic reef developments surrounding a good source rock depocenter proven to be mature at the base of the first post-rift sequence . In this same part of the basin dry gas accumulation may be found in the Neogene where reservoir intervals are developed in the Miocene and Oligocene channel fill deposits.

Prospects & leads defined in some sedimentary basins.

MOROCCO'S ATLANTIC DEEPWATER FRONTIER : Morocco contains the North African largest offshore exploration area of about 300 000 sqKm . Despite all encouraging existing potential, Operators have just scratched the surface of this vast frontier.
In the offshore area, exploration perspectives have recently changed, and are still changing with the integration of the deep water frontier area. Exploration emphasis has shifted from the classic play of the Upper Jurassic shelf edge buildups to deeper buildups in Lower to Middle Jurassic in addition to a variety of traps including large Lower Paleozoic antiform structures (Casablanca), Triassic buried half grabens (Safi, Ifni) and Cretacous and Tertiary structural and stratigraphic traps (roll-over structures, deep sea fan, channel fill complexes) in addition to turbiditic reservoirs in a variety of structural styles such as salt overhang and traps against salt walls in the deepwater frontiers.

Geological cross section.

Various basin analogs are producing in the west African and Brazilian Atlantic margins. Prospects in the Moroccan Atlantic deepwater exploration area are expected to be, by all accounts, large and numerous and the production from any development would be large and profitable.

Important reserves are anticipated to be found in the Atlantic offshore area. The future of Morocco's petroleum resources will depend heavily on an integrated exploration of the offshore along the coastal margins and in deepwater frontiers basins.