JARVIS, JERRY, TOBY GARWOOD, and JOHN GOVER, Enterprise Oil plc.

Abstract: The Evolution and Hydrocarbon Potential of the Deep-water Tarfaya Basin, Morocco

Introduction

The Tarfaya Basin lies on Morocco's Atlantic seaboard, to the east of the Canary Islands and south of the Atlas Mountains. This area has been subjected to a series of complex tectonic episodes throughout the Phanerozoic, which combine to form a potentially viable location for the accumulation of hydrocarbons.

In December 1997, Enterprise Oil were awarded the first deep water reconnaissance license. The work presented here is a synthesis of our studies since then.

Geological History

The Tarfaya Basin lies on the NW margin of the West African Craton. Following accumulation of a substantial thickness of Ordovician/Silurian sediments in the Lower Paleozoic, relative northward drift in the Hercynian resulted in collision with Amorica, causing shortening and the development of a major set of conjugate shear zones. Sinistral movement on the Bani Shear Zone produced NNE oriented en-echelon strike-slip faults throughout the region which have controlled basin morphology into the present. Atlantic rifting during the Triassic reactivated the faults in a dip-slip sense, forming half grabens segmented by ENE oriented Atlantic transform systems. Syn-rift sediments infilling the half-grabens were a combination of locally derived continental clastics and evaporites that formed following limited ingress of marine waters.

Following a major transgression in the Lower Jurassic, a thick transgressive carbonate platform wedge prograded across the area, overstepping the Triassic sequence. Towards the end of the Lower Jurassic, platform growth became more aggradational, marking the transition from Atlantic rift to drift. Reefs developed along the seaward margin of the platform. In the Lower Cretaceous a major delta system prograded across the shelf from the south terminating carbonate deposition. A system of deep water turbidites were shed off the front of the delta system northward into deeper water environments. During the late Cretaceous sediment supply to the basin was greatly reduced, resulting in a condensed sequence being deposited in the deep water. The start of the Tertiary is marked by a major basinal unconformity, which displays extensive erosion of the shelf break and scouring of the slope. Lack of Tertiary subsidence resulted in bypassing of the shelf and deposition of a thick terrigenous sequence in the deep water environment.

Exploration History

Eight wells were drilled between 1968 and 1972 by Esso, targeting Jurassic reefal build-ups and anticlines. The second well (MO-2) encountered the Cap Juby accumulation. Two appraisal wells were drilled on this structure and proved the existence of c. 500 MMBO STOIIP. In 1975 Esso acquired seismic lines beyond the shelf break, and in the same year drilled the unsuccessful Haute Mer-1 well in 520 m water depth. Subsequent exploration (and eight wells) has focused on the continental shelf area, with no commercial success. However, out of 18 offshore wells drilled in the immediate vicinity, 11 have encountered oil and/or gas shows.

To date, only the Jurassic carbonate and Cretaceous clastic plays have been tested on the shelf. Tertiary clastics, Cretaceous fans and Jurassic off-shelf facies plays remain untested in the deep water area.

Source Rocks

Liassic transgressive mudstones are expected to be overmature in basinal areas. Sea level rise in the mid-Cretaceous resulted in extensive flooding of the shelf and development of a widespread shallow sea with associated upwelling zone. Rapid increase in biogenic activity on the newly flooded shelf resulted in expansion of the oxygen minimum zone. This permitted the preservation of organic matter in both basinal locations (as encountered in DSDP and exploration wells) and in shelfal environments as seen in the onshore Tarfaya oil shale deposits.

Hydrocarbon Charge

Preliminary analysis suggests that maturity was attained during the early Tertiary with expulsion during the Miocene. The presence of the Canary Island volcanic centre to the west of the region, however, adds a degree of uncertainty to basin modelling, since geothermal gradients are poorly constrained.

Reservoirs/Seals

Two prospective reservoir sequences are present. In the early Cretaceous, Tan Tan delta sands were reworked and deposited as turbidite fans in the deep basinal areas.Tertiary sands, however, were fed directly into the deeper water. With less potential for sediment re-working, the risk on Tertiary reservoir quality is considered greater.

Interbedded pelagic shales and mudstones act as seals to the reservoir units.

Traps

Halokinesis has occurred in the basin since the Jurassic, creating salt diapirs and swells. In a similar fashion, shale diapirs are thought to exist in the southern part of the area, having developed following the mobilisation of Cretaceous pro-delta shales. Drape anticlines are common overlying these features, while on their flanks are potential pinch-out and truncation traps. Stratigraphic trapping mechanisms are also evident on seismic; slope apron and basin floor fan morphologies have been recognised, while slope talus units are thought to exist immediately below the shelf break. Trap timing ranges from the Jurassic to the present day which is favourable for the development of hydrocarbon accumulations.

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