AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
(1) Energy Technology Company, Chevron, Aberdeen, United Kingdom.
(2) Energy Technology Company, Chevron, Houston, TX.
(3) Chemostratigraphy Ltd., Welshpool, United Kingdom.
(4) University of Greenwich, Greenwich, United Kingdom.
(5) Fugro-Robertson, Llandudno, United Kingdom.
(6) Saudi Arabian Chevron, Houston, TX.
Stratigraphic correlation in carbonate successions can be problematic; age diagnostic faunas are often lacking and a strong facies control is exerted on faunal assemblages. However, a stratigraphic framework is a prerequisite for reservoir models and the identification of exploration targets. Whole rock geochemical data has been used to define a chemostratigraphic correlation framework for the First Eocene reservoir, a pervasively dolomitized carbonate of Palaeocene/Eocene age containing 14-22°API oil, from the Wafra Field in the Partitioned Zone. The stratigraphy is dominated by highly dolomitized, meter-scale depositional cycles comprised of basal peloid dolopackstones (subtidal) that are capped by peloid dolowackestones and dolomudstones (intertidal). Bedded/nodular evaporites and algal bindstones are present.
Firstly, approximately 450 samples from five wells were analyzed using ICP OES - MS yielding data for 10 major elements, 23 trace elements and 14 rare earth elements. A traditional chemostratigraphy, based on key element ratios derived from P2O5, MnO and U has defined 7 isochemical chemostratigraphic packages and 15 geochemical units within the reservoir that provide a means for high resolution correlation between closely spaced wells.
Secondly, a long range correlation methodology is proposed using key elements (Al2O3 + SiO2 + TiO2 + K2O) as a proxy for a pseudo-cyclic fluctuating terrigenous content. Each cycle starts with low terrigenous input, increasing to a maximum several tens of feet above, before sharply decreasing at the base of the next cycle. To better understand the origin of such cycles, chemostratigraphic and XRD data from example depositional cycles and across a sequence boundary have been collected. Magneto-and biostratigraphic data has been used to test the chronostratigraphic nature of these correlations. The interpreted terrigenous cycles offer the potential for longer range stratigraphic correlations that may have strong chronostratigraphic implications.