The French Guyana Margin and the Demerara Plateau: Geological History and Petroleum Plays

Abstract

The French Guyana margin is located between 4 and 6°Non the western side of the Equatorial Atlantic Ocean. The geologic history of the margin can be divided into two main stages. In Jurassic and early Cretaceous times, the northwestern part of the margin (Demerara Rise) is believed to have represented the southern extremity of the Central Atlantic Rift and Ocean. Sedimentation at that time on the Demerara rise occurred mainly in an inner shelf environment, with significant continental influxes. As early as middle Albian times, shallow marine to open marine environments of deposition prevailed in response to the opening of the E-W-trending Equatorial Ocean and margins along a dextral shear zone between South America and Africa.

Exploration on the Demerara Rise was active in the 1960 s and 1970s, with only two wells drilled in French waters (Sinna Mary 1 and FG2-1). The first well reached the basement at a depth of 2104 m below early Cretaceous sandstones and younger sediments. The second one bottomed out in basaltic lava flows of Barremianearly Aptian age. A still untested play may be present in the Jurassic (?) section clearly depicted on the seismic below the Early Cretaceous interval. The southeastern side of the French Guyana margin is still very little explored, with no well ever having been drilled. The margin is topped off by a continental shelf about 120 km wide, but the slope is narrow with water depths rapidly deepening to 3000 m. A few seismic lines shot 15 years ago show, below the outer shelf and slope, a 3 s-thick sedimentary wedge of presumed early Cretaceous to Present age. The presence or absence of older Mesozoic troughs below the shelf cannot be inferred owing to the poor quality of seismic data at depth.