The Campanian quartz ‘claystone’ conundrum of the African Transform Margin

Abstract

AbstractAn unusual deep-water lithofacies has been penetrated by numerous wells along the equatorial margin of West Africa by Anadarko Petroleum and other operators. An exploration well drilled in 2009 was initially thought to contain very thick, high-quality sands in the Lower Campanian section. Wireline log interpretations calculated the section to be a quartz-rich reservoir rock with 67–83% quartz based on X-ray diffraction analysis and effective porosity values as high as 25%. This petrophysical analysis appeared to differ from the well site geological description of 200 m of ‘claystone’. The conflict between field observations and the interpreted wireline data led to the re-evaluation of earlier wells and interest in gathering data from this unusual lithofacies from subsequent wells. Field observations (from wellsite geologists) derived from this Lower Campanian section have been consistent in describing this lithofacies as ‘claystone’. Sidewall cores from several of the deep-water wells that penetrated this lithofacies were also consistently described as claystones. These descriptions were from visual inspection and conclusions from laboratory analysis utilizing scanning electron microscopy, X-ray diffraction analysis, laser particle size analysis and thin section point counts as evaluation tools. The sidewall cores typically contain >90% silt- and clay-sized particles with extremely low measured permeability as a result of highly reduced pore throat diameters. With the presence of such small grain sizes, these samples have high measured seal capacities. It is important to recognize that this unusual lithofacies exists and can easily be mistaken for a high-quality, quartz-rich sand when, in fact, it is a very low permeability ‘claystone’ with a high percentage of quartz. Wireline data over this lithofacies alone can be very misleading and rock samples must be analysed to validate or invalidate the log interpretations. Misidentifying this lithofacies as an exploration target could have obvious financial ramifications. The challenge of this regional study was to formulate a model that could explain the presence of this deep-water lithofacies over an extensive area encompassing several basins with different sediment source provenances. Based on well sample analysis, palaeoclimatic research and the use of modern day analogue examples, the deposition of aeolian-derived clay-sized quartz particles is proposed as the most likely source of this lithofacies.