NORTHERN MOROCCO HAS INGREDIENTS FOR FRUITFUL EXPLORATION, DESERVES A MODERN LOOK

S. Kumar Bhattacharjee
Sita Oil Exploration House Inc.
Houston

Northern Morocco encompasses three different geological units--Rharb, Rif, and Prerif--and occupies more than 45,000 sq km of sedimentary basinal area.

There are two distinct characters of the sediments--allochthonous nappe and autochthonous ante-nappe (Fig. 1).

The area is a proven hydrocarbon producer. Early discoveries like Ain Hamra date to the 1930s through 1950s, when Morocco became almost self sufficient in its crude oil production.

EXPLORATION HISTORY

The exploration history of the area can at best be described as a see-saw, although the documented reference to oil goes back to the 14th century and the first oil well was drilled in 1890.

Moroccan oil potential was given even wider notoriety when a development well in Tselfat field blew out in 1934 and oil flowed at the rate of 1,500 b/d. Then during the 1950s a trend of small oil fields was discovered along a well known strikeslip fault zone of Sidi Fili.

Success waned, and combined with difficulty in understanding the pre-nappe section of Mesozoic because the seismic mapping tools were not sensitive enough at that time, most of the activities shifted to other countries in the region.

For 20 years or more hardly any serious exploration took place until during the 1970s a joint exploration program was initiated by BRPM, the government agency for natural resources, and Elf Aquitaine.

Later seismic and geological studies continued under Onarep, Morocco's national oil company, which drilled a few wells.

Probably 15 exploratory wells have been drilled in this vast area, and most did not reach the objective or were drilled on undefined structures.

GEOTECTONIC FRAMEWORK

Most of northern Morocco is covered by what is known in geological literature as nappe, the Rif-Betic belt.

There are more theories in literature on this belt than any other single geologic unit. In fact, the area was once described as "Mecca for geological researchers."

A simple explanation can be assumed as to the reason for this vast belt, i.e., that it was caused by convergence upon the Alboran plate by the southwesterly rotating Iberian plate and northerly moving North African plate, resulting in the formation of an accretionary wedge that was then obducted onto Morocco and southern Spain. This occurred perhaps during Mio-Pliocene time.

Northern Morocco's basement fabric is mostly related to Tethys and early Hercynian phase of orogeny, especially during Triassic when extensive rift systems developed and were filled with shale, sands, and salts.

In fact during Lias this area had hardly any clastic at all, and carbonate lithology dominated. During Dogger a great influx occurred of deltaic sands, turbidites, and thick flysch mostly due to erosion of highgrounds created by granite intrusions (see table).

HYDROCARBON SOURCING

Many oil companies worry about source rock distribution, quality, and quantity that could generate sufficient quantities of oil.

The answer to this critical query from U.S. and European independents is yes. There are plenty of proven source rocks that have generated accumulations of economic significance and future undiscovered traps.

The known and tested source rocks are:

• Cretaceous (Albian, Aptian, Cenomanian) shales

• Lower Jurassic (Toarcian) marl

• Upper Jurassic (Domerian, Infra-Lias) limestones and dolomite.

OIL FINGERPRINTING

Source studies are done by various companies for verification and confirmation of potential.

Several samples of oil went through modern fingerprinting process.

The results confirm that the source rocks identified above have generated commercial oil.

RESERVOIR ROCKS

There is no lack of quality reservoir rocks along the entire Mesozoic and Tertiary sections (Fig. 3).

Some of the commercially tested reservoirs are:

• Tertiary--Miocene sands and sandy limestones

• Cretaceous--sands

• Jurassic--oolites, sandy limestone, limestones and fractured shales

• Triassic--basal conglomerates

• Paleozoic--fractured basement and granite wash

The porosity, from lab work on samples, ranges from excellent to moderate.

SEALS

There is no shortage of excellent quality seals in northern Moroccan geology along the entire sedimentary section from Trias evaporate to Miocene nappe.

Seals are also provided in some areas by the thrusts as well as other types of faults, including halokinetic faults.

MIGRATION, DISTRIBUTION

The discovered fields provide a good microscopic view of the maturation and migration process in the region.

The Mesozoic sources reached maturity within the range of 2,800-4,200 m.

Pinpointing the migration date exactly is not easy at this time, but the author's general opinion is that it has commenced with late Alpine activities and with the arrival of the nappe complex.

There are instances where the migration, especially within the Triassic grabens, might have commenced during Lower Tertiary.

The main migration path was vertical, noticeably along the fault planes that have allowed oil seeps in some cases. Lateral migration may have occurred in areas where associated gas also has been generated.

EXPECTED TRAP STYLES

The most prevalent trapping mechanism would be structural, either fault closed (horsts) or thrust bound.

There should be plenty of stratigraphic plays, especially for pinch-outs, and halokinetics related traps. Combination plays could also exist.

CHALLENGES, REWARDS

Blanketing of the autochthonous section by a complex melange-nappe made it very difficult to image the subsurface geological picture in the past.

This resulted in lack of active interest from easy oil finders, and when oil discoveries proliferated in other basins of North Africa oil companies went on a massive exodus from Morocco.

They left behind the government operated agencies to carry on. The agencies lacked technical skills and sufficient funding at that time. The negative image of the area created by old literature must be reexamined now and smart exploration applied.

Rewards can be seen coming forth as new geophysical tools are available now and can be applied in prime exploration targets in the area to image pre-nappe section.

The rewards will also depend on new play concepts and geological understanding of the area.

CONCLUSION

This area should be a good target for the U.S. and European independents that can find commercial fields of 32-40 gravity oil.

These companies should keep in mind the proximity of a vast European market.

Morocco's government is progressive and the most stable in Africa. King Hassan II and the people of Morocco are friendly.

The country has an old infrastructure of western style economy. The area got a bad rap stemming from poor exploration efforts and not from the basin's hydrocarbon starvation. Excellent communication, low overhead, and inexpensive labor would make return on investment very good on any exploration investment.

The reserve estimate for the area stands at 800 million-1.2 billion bbl of oil and significant gas.

Copyright 1992 Oil & Gas Journal. All Rights Reserved.