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The Campanian Quartz Claystone Conundrum of the African Transform Margin - A Re-Evaluation of the Possible Origins of this Quartz Rich, Silty Claystone

Brown, Allen; Birkhead, Scott; McLean, David; Towle, Philip; White, Howard; Wu, Yafei

Since 2007, Anadarko Petroleum Corp. has participated in drilling over 40 deepwater wells along the African Transform Margin between Sierra Leone and Benin, a distance of over 1800 km. Many of these wells encountered a 100m to 225m thick Mid to Lower Campanian section that is described in the field as medium to dark grey, non-calcareous "claystone". On wireline logs however, the sequence may be interpreted as porous sandstone with Gamma Ray averaging 30-40 API units and as low as 15 API. Neutron-Density responses are similar to relatively clean sandstones. Total porosities are 15% or higher yet permeabilities are extremely low. A similar, but much thinner, quartz-dominated "claystone" has also been observed in the Santonian and Turonian. Several avenues of investigation were undertaken to help explain the origin of this unusual facies.

This "quartz claystone" is common in the deepwater wells studied although it has not been observed in shallow water wells due to probable dilution from continental input of clays and sands. Also, examples in deeper water environments have been observed where this "conundrum" facies is absent due to removal by turbidite channel systems typically rich in sands.

X-ray diffraction (XRD) indicates the interval is composed of 60% to 80% quartz with varying amounts of clay minerals. Laser particle size analyses (LPSA) show that this "conundrum" section typically consists of 50-70% clay-sized particles with very fine- to medium-grained silt making up most of the remainder. Very fine- to fine-grained sand makes up less than 4% of the total volume. MICP analysis of several wells indicates a seal capacity potential of 370m to 805m of oil column.

The origin of this "conundrum" facies was proposed earlier as Aeolian with windblown, clay-size quartz originating from paleo-arid regions of central Africa. Further studies, particularly under high SEM magnification, indicate that a combination of processes may account for this unusual facies. At 40,000X magnification, euhedral quartz can be seen in the viewed samples. This suggests a possible ‘siliceous ooze' origin. However, upon further magnification up to 128,000X (FESEM) from Ion Milled samples, discreet aggregates of apparent detrital quartz were observed. This suggests that an aeolian source still could be a major contributor to this facies in addition to an authigenic origin from silicia enriched seas.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013