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DOLBIER, RACHEL A., and REGINA N. TEMPEL, Department of Geological Sciences, Mackay School of Mines, University of Nevada, Reno, NV

ABSTRACT: Clay Mineral Distribution In The Albian Pinda Formation, Offshore Cabinda, Angola

Data from CABGOC 67-4ST and 79-1 wells, offshore Cabinda, Angola, reveal significant and pervasive clay mineral concentrations throughout the mixed siliciclastic/carbonate Pinda Formation. XRD analyses show that the main clay minerals present are chlorite and illite, commonly occurring as cements in the sandstone units although there are minor discrete grains in all units. Detailed petrographic and SEM analyses show both detrital and authigenic components to the clay mineral fraction. The authigenic clay minerals appear in two stages, an early diagenetic grain rimming phase, and a later, post-quartz cementation pore filling phase, The sheer volume of clay minerals, specifically chlorite, in these wells substantially reduces permeability and effects hydrocarbon recovery.

The sedimentary units comprising the Albian Pinda Formation were deposited in environments ranging from upper shoreface to shallow marine shelf. The clastic units contain abundant quartz and feldspars with subordinate amounts of lithic grains. The carbonate units range from calcitic siltstone to coral boundstone. All units have undergone significant diagenesis with several successive cementation events.

Although chloritization of smectite in response to increasing temperatures and pressures during burial has probably occurred, depth does not appear to be the controlling factor in authigenic clay mineral precipitation. Dissolution of feldspars and the flushing of carbonate rich waters through clastic units during diagenesis may have contributed to authigenic clay mineral growth. Stable isotope and microprobe analyses are being used to determine the geochemical conditions that resulted in the precipitation of authigenic chlorite from pore fluids.

AAPG Search and Discovery Article #90915©2000 AAPG Rocky Mountain Section, Albuquerque, New Mexico