Seismic Expression of the Cenozoic Mass Transport Complexes, Deepwater Tarfaya-Agadir Basin, Offshore Morocco

Abstract

Abstract The Tarfaya-Agadir Basin in offshore Morocco is a frontier exploration area with no deepwater industry well penetrations and limited well control in the updip shelfal equivalent. Recent detailed 3D seismic mapping and study by Shell has identified the presence of two large mass transport complexes (MTCs) in the Tertiary interval, in addition to a number of smaller and younger MTCs. The large MTCs, named Tejas A and Tejas B, exhibit distinct seismic characteristics that may be significant to understanding the evolution of the basin during the Tertiary. The basal Tejas A is characterized by numerous kilometerscale, coherent transported blocks. The size and quantity of transported blocks observed in Tejas A is unique in Morocco and the Atlantic Margin. The MTC in the younger Tejas B is a thick chaotic unit with distinctive downdip erosional lobes. This deposit is overlain by high amplitude, parallel seismic facies. The chaotic seismic facies is often sharp-edged, with well-defined pressure ridges. Only a few small transported blocks and erosional remnants have been observed in Tejas B. Failure deposits account for a significant percentage of the Tertiary stratigraphic interval in the 3D survey area. Introduction Exploration for hydrocarbon in offshore Morocco Atlantic Margin began in the late 1960s and continued intermittently until 1990. This early phase of exploration focused exclusively on the shelf, relying on 2D seismic data, with mixed results and no commercial discoveries. Following a period of no activity, interest in the area increased again in the late 1990s, especially in the deepwater area. Shell is the operator of two deepwater concessions in the Tarfaya-Agadir Basin (Fig. 1) and has carried out a detailed subsurface evaluation based largely on recently acquired 3D seismic data. The availability of 3D seismic data has allowed for a detailed interpretation of the paleogeography and depositional systems of the study area in the Tarfaya- Agadir Basin (Fig. 1). The results of this study highlighted the presence and the significance of mass transport complexes (MTCs) in the southern part of the basin within the Tertiary stratigraphic interval. Two main Tertiary MTCs, named Tejas A and Tejas B, have been identified, in addition to several smaller and younger MTCs, including one at the present day seafloor. These different MTCs exhibit distinct seismic characteristics. 3D seismic semblance slices have been used to clearly illustrate the many interesting features observed, including kilometer-scale transported blocks, arcuate ridges near flow terminations, and possible basal glide tracks. The identification of MTCs in the Tertiary interval has important implications on its prospectivity in the southern part of the Tarfaya-Agadir Basin. On the basis of 2D seismic data, Tejas A was interpreted by earlier explorationists to be a deepwater turbidite fan with channel-levee complexes. The present study has conclusively shown that Tejas A is in fact a MTC, with very little likelihood of containing producible reservoirs. However, the extensive chaotic seismic facies within Tejas A may act as very effective regional seals.