--> Abstract: New Exploration Concept and Petroleum Systems in the Tertiary Rharb Basin, Northern Morocco, by M. Dakki, M. Hssain, and R. El Abibi; #90923 (1999)

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M. DAKKI, M. HSSAIN and R. El ABIBI,- ONAREP

Abstract: New Exploration Concept and Petroleum Systems in the Tertiary Rharb Basin, Northern Morocco

The Tertiary Rharb basin in northern Morocco covering an area of about 20000 km 2 is a syn-orogenic to postorogenic basin developed within the south western edge of the Rifain thrust belt (fig. 1).

Petroleum Exploration was concentrated in the onshore part of the basin where several gas fields, were discovered and being produced. To date about 12000 km of seismic profiles have been acquired out of which less than 50% has been shot since 1980. Drilling activity has only concerned the same area where about 70 exploratory wells have been drilled on the supra-nappe sequence.The northern and the northeastern parts of this basin received little attention because of the lack of the seismic coverage. The offshore part was tested by only one well.

Sedimentary evolution started by a very thick series of Eocene and Oligocene turbidites in Acilah-Larache area. During the Early to Middle Miocene continental type, molassic sediments overlaying the Mesozoic carbonates and clastics were deposited.These are composed of sandy-conglomeratic shales and sandy-limy marls.The distribution of these facies depends on the paleogeographical conditions during the deposition. Direct marine influence took place in the Middle Miocene with a shaly to shaly-sandy sedimentation. During the Lower Tortonian time, a fall in sea level induced the deposit of marly series with turbiditic sandy lenses. During the same period an increase in sea level permitted the deposit of a very thick series of marls within the subsiding zones and carbonates along the borders of the basin. The same phenomenon happened during the Upper Tortonian, the Missinian and the Pliocene times. The thickness of the Upper Miocene series is highly variable depending upon the morphology of the complex of nappes and upon the importance of syn-sedimentary growth faults. It may reach more than 3000 in in the central part of the basin.

Oil and biogenic gas deposit in the Rharb basin indicate the presence of source-rock within the supra-nappe Miocene and the complex of nappes (Miocene and Cretaceous).

Recent geochemical studies have demonstrated the presence of a high geothermal gradient (5° to 6°C/100m) from modelling in onshore zone, which implies that the beginning of oil window may be at the depth of 1600m (fig. 2). The ALG-l well, located in the centre of the basin has showed that at depths greeter than 1600 in, type 11 and I source rocks would be mature and would generate oil.This increases also the probability of generation from the complex of nappes. The same studies combined with the seismic interpretation have permitted to define two petroleum systems within the Rharb basin.

- The first one (Cretaceous-Tertiary) lies to the northern part of the basin in which oil is interpreted to be sourced from the complex of nappes and trapped within Upper Miocene sand lenses.
- The second system (Miocene) occurs in the Central and Southern parts of the basin. In this areas biogenic gas is expelled from thermally immature Miocene source-rock and reservoired within adjoining Upper Miocene sand reservoirs.

Good quality reservoirs exist within the Upper Miocene sequence. Data from drilled wells show sandy reservoirs with good porosity reaching 33% and good permeability of about 400 mD.

Eocene and Oligocene turbiditic sandy formations of the Habt unit, in the North present also good porosity. Carbonate and sandstones deposited within neretic bioclastic platforms may constitute potential reservoirs.These sediments are defined in the northern part of the basin.

Marls deposited during the increase of sea level provide good seals for turbiditic clastics and carbonates reservoirs. Sealing characteristics of these marls are very good that a thin layer would be sufficient to completely isolate the reservoirs.

Integrated studies based on seismic reflection acquisition and processing have permitted to define various prospects within the supra-nappe sequence. AVO reprocessing increases greatly the rate of success (fig. 3).

In the central and the southern parts of the basin, many amplitude anomalies have been identified within the supranappe sequence at shallow depths (700 to 1300 m). Seismic reprocessing such as AVO (Amplitude versus offset) is needed before drilling. In these zones, at depth greeter than 1600 m,Type 11 and I source rocks would be mature and would generate oil.This increases also the probability of generation from the complex of nappes. As far as liquid hydrocarbons are concerned, the deeper parts of the basin, which is practically unexplored, may present potential objectives.Wells drilled to date in this part of the basin have only tested biogenic gas objectives related to amplitude anomalies.

In the north and north eastern parts of the basin, the same studies have shown the presence of various zones of interest within the supra-nappe sequence. Objectives are shallow (between 100 and 700m) and are visible on seismic as an amplitude anomalies related to gas cap over oil accumulations.

In Acilah area the lack of seismic made very difficult the delineation of prospects, but encouraging geochemical results and geological evolution of the Tertiary sequence provide critical data for hydrocarbon potential evaluation. In this area oil is interpreted to be sourced from the Habt and the Tangier units and trapped within Oligocene and Eocene turbiditic sandstones.

Fig. 1: Rharb Basin Location Map and Its Petroleum Systems

Fig. 2: Maturation Profile in ALG-1, HR-17 & DOG-1.

Fig. 3: Example of Oil/ Gas Objective

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England