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Sequence Stratigraphic Framework of the Second Eocene, Wafra Field, PZ

Frydl, Paul *1; Dvoretsky, Rachel 1; Bachtel, Steve 1; Saller, Art 1; Aconcha, Enzo 1; Pollitt, David A.1
(1) Chevron, Houston, TX.

The giant Wafra Field lies in the Partitioned Zone between Saudi Arabia and Kuwait. Dolomitized subtidal and peritidal sediments of The Second Eocene reservoir contain a large heavy oil accumulation. The deposits are interpreted to have formed in a low-energy, shallow ramp environment during the Paleocene. Since its discovery in 1954, multiple reservoir interpretations have been made; however, a robust sequence stratigraphic interpretation has never been completed on a field-wide scale. This study utilizes existing core, petrographic and wire line log data to build a sequence stratigraphic framework from cored wells and extend it across the field. Re-interpretation of the Second Eocene reservoir architecture will support up-coming modeling efforts and field management decisions.

The Second Eocene interval ranges in depth from 1800 to 2300 ft and is subdivided into two reservoirs based on field pressure data and the occurrence of water. The lower reservoir is approximately 250-300 feet thick and is characterized by a bioturbated shallow subtidal wackestone - packstone deposits. The upper reservoir is approximately 100-150 ft thick and is characterized by a heterogeneous accumulation of shallow subtidal wackestone - packstone and peritidal mudstone - packstone deposits. These deposits form stacked, cyclic packages that shoal upward into thick anhydrite beds that form the reservoir seal and separate the Second Eocene from the overlying First Eocene reservoir. Within the upper reservoir, sequence boundaries can be picked on the basis of vertical stacking patterns, where progressively thinning cycles of subtidal strata with peritidal caps are overlain by relatively thick intervals of subtidal deposits. Assignment of sequence boundaries was based on regional correlation of parasequence boundaries within cored wells, as well as sedimentary characteristics of capping peritidal deposits. Within the lower reservoir, selection of sequence boundaries from core based on cycle stacking is more challenging due to the more homogenous nature of the deposits compounded by pervasive, fabric-destructive dolomitization. As a result, identification of sequence stratigraphic boundaries in this reservoir relied more on characteristics of wire line log data.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California