Oligocene/Miocene Depositional System: Volta Fan Fold Belt, Ghana

K. A. Nibbelink and J. D. Huggard: Devon Energy

Abstract

The Volta Fan Fold Belt inversion structures developed during the Upper Cretaceous in response to right lateral strike slip movement along a restraining bend in the Romanche fault zone across the Keta Arch. The depositional architecture of the Oligocene to Miocene deep-water sandstone reservoirs was controlled by topography, created by the inversion structures and subsequent erosion during the upper Oligocene, 30 million year, sea level low stand. Oligocene erosion cut numerous canyons across the shelf, which are 200 to 500 m deep, 1 to 2 km wide, and provide a critical path for sediment from the Volta River to the deep water.

Amplitude patterns from 3D seismic define deep water sequences that systematically fill the topographic relief created by the Upper Cretaceous structural and Oligocene erosional events. Upper Oligocene to lower Miocene sedimentation consists of (1) base of shelf fans at the mouth of the shelf canyons, (2) ponded fans behind the inversion structures, and (3) basin floor fans in front of these structures. Since much of the local topographic relief is already filled, the middle Miocene fan systems are deposited in a back stepping sequence, the larger fans being on the flanks of the Keta Arch. A major progradation of the shelf occurs during the upper Miocene and the deep-water fans are deposited across the entire arch. The Oligocene/Miocene deep-water sandstones as well as the deeper Upper Cretaceous sandstones should provide excellent reservoirs for the developing hydrocarbon system in the Volta Fan Fold Belt in Ghana.