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Rharb Basin
Rharb Basin Located to north of the Moroccan capital of Rabat and south of Tangier and the Rif Mountains, the Rharb Basin contains a thick succession of Late Tertiary sediments. Exploration based on field geology and on the presence of surface oil seeps started in 1890 (Figure 1a–b). The Ain Hamra field was discovered in 1923. Field mapping of surface structures and later magneto-telluric surveys continued through to the 1950s with the discovery of some small shallow oil fields (Tselfat, Haricha and Bou Draa) and a series of small gas fields on the Sidi Fili trend associated with thrust folds in the Rif Mountain front (Figure 1c). The Mesozoic rocks involved in this thrusting as the African plate collided with Europe in the mid-Tertiary, are collectively known as the ‘Nappe’. The later Miocene to recent sediments of the foreland basin are locally termed the ‘Supra-Nappe’
Figure 1: Rharb Basin: a) Location map showing some early discovered oi and gas fields. b) Schematic of the exploration history of the Rharb Basin. c) Schematic structural section through the Bou Draa and Tselfat oil fields.
Petrofina started using multitrace seismic in the 1960s leading to several small gas discoveries. In the 1970s with the advent of ‘bright spot’ technology, additional gas discoveries were made, though several dry holes were also drilled. 2D seismic was the ‘norm’ in the Rharb until the winter of 2007–2008, when Circle Oil acquired a 3D seismic survey over the Sebou concession, which had been granted in 2006. The initial drilling programme commenced in September 2008 following interpretation of the 3D seismic and the first well was successfully tested as a gas discovery which was tied back into the existing small capacity pipeline to the local industry in the coastal town of Kenitra. First gas was produced in November 2008 and during the author’s time at Circle Oil, further drilling campaigns achieved an 85% success ratio to mid-2015. The succeeding operator has been Sea Dragon Energy who have continued with the play concept and have had further success.
A sequence stratigraphic subdivision of the mudrock-dominated Mio-Pliocene ‘Productive Series’ was used by Circle Oil to distinguish and map eight units for the central part of the Rharb Basin (Figure 1a–c). The gas-bearing sands are of good reservoir quality, but usually thinly bedded (1–15m). The sands appear to have been sourced from erosion of the uplifted Rif Mountains and transported into the Miocene foreland basin to the southwest into the opening eastern margins of the Atlantic Ocean. There are no Late Miocene evaporites in the Rharb Basin, as the Rif Mountains formed a barrier to marine incursions from the Mediterranean to the north. A recent exploration venture on the eastern margin of Morocco’s Mediterranean coast in the Guercif Basin is underway by Predator Oil and Gas holdings. The block lies 180 km north of the Triassic Tendrara discovery, but there is a different objective. Predator are targeting the same Miocene sands interval as the Rharb gas fields for shallow gas.
Modern 3D Seismic Application Where gas bearing, the seismic response of the sands shows up as bright negative amplitudes, due to the lower density and slower seismic velocity compared to the water-bearing sands and the encasing mudrocks (Figure 2). Modern 3D seismic makes the location and geometry of these anomalies stand out as clear drillable targets. The gas is contained in combination structural-stratigraphic pinchout traps, with top, bottom and lateral seals provided by coeval marls and shales. The reserves of the small onshore gas fields discovered to date, range from 1 to 20 Bcf. Circle Oil built a new gas pipeline to Kenitra on the coast for export and sale to local industries in 2011. Much larger structures are present in the offshore extension of the Rharb Basin, where ENI discovered the Anchois gas field in 2009. At that time the discovery was considered too small for development; however, Chariot Transitional Energy have taken over the blocks including Anchois, and the company has reprocessed and interpreted the 3D seismic and is now proceeding with development plans.
Figure 2: Rharb Basin: a) Seismic amplitude extraction map of +/- 40ms around top of bright amplitude anomaly. b) Seismic line through the bright amplitude anomaly. b) Schematic of Rharb productive series stratigraphy and gas trapping styles.
The example shown in Figure 5a–b illustrates the accurate placement of a gas discovery only 800m away from an abandoned well, based on amplitude extraction and geological understanding of the trapping mechanism. The faulting as shown is currently normal over the crest of a thrust anticline in the Nappe. The effective keystone stretching of the overlying younger rocks has created the structural trap and provided a migration pathway for hydrocarbons generated from the Jurassic/Cretaceous source rocks. Sourcing from the coeval, organically lean Miocene mudrocks is unlikely to provide local biogenic gas and the more likely source is considered to be the biogenic degradation by bacteria of an original thermogenic liquid phase hydrocarbon. This has been also reported by Sea Dragon Energy for their recent discovery in the Lalla Mimouna Block located just to the north of the previously successful Sebou Block in the Rharb Basin. The apparent lack of reservoir quality rocks in the few wells drilled to date into the ‘Nappe’ succession may locally downgrade the prospectivity of those strata.
Unfulfilled Potential
Morocco’s productive petroleum basins appear to suffer from a mixture of underexploration and underperformance. This may be due to the failure of early wells and then poor follow-through exploration ideas. Deeper drilling and alternative target horizons and traps have often led to success in other petroleum basins such as the Alberta Basin in Western Canada, as well as the North Sea. New exploration concepts and technology improvements are essential to the rejuvenation and success of exploration ventures. Just as in some of my old school end-of-year reports, perhaps explorers in Morocco – ‘could try harder and do better’.
Data source:Productive Petroleum Basins of Morocco. STUART HARKER. GEOSCIENCE & TECHNOLOGY EXPLAINED VOL. 18, NO. 5 – 2021
Следующий Бассейн: Liberia