Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Diagenesis and Porosity Evolution of a Dolomitized Paleocene-Eocene Carbonate Ramp; 1st Eocene Reservoir, Wafra Field, Partitioned Zone, Kuwait
(1) Energy Technology Company, Chevron, Houston, TX.
Discovered in 1953, Wafra is a super-giant oil field with production from multiple reservoirs and cumulative production of 2.5billion BOE (from ~25billion STOOIP). The 1st Eocene reservoir (actually Paleocene-Eocene) is the shallowest reservoir and was originally composed of aragonite and high-magnesium calcite. The present mineralogy is dominantly dolomite with minor, but locally abundant, evaporites (current porosity ranges from 35-45% and permeability from 10-1000mD). Diagenesis has therefore completely changed the depositional mineralogy.
The aim of this study is to characterize diagenesis and understand how the diagenetic evolution has affected reservoir properties. This study is based on the analysis of well logs, cores, biostratigraphy, microfacies analysis of 754 thin-sections, and bulk stable isotope analyses.
These strata have a complex paragenetic history beginning with early dolomitization and dissolution. Late-stage diagenesis includes anhydrite dissolution and calcite cementation. Dolomitization is interpreted as occurring by the evaporation of seawater during shallow burial.
Biostratigraphic analysis has significantly improved the dating of the reservoir, including identification of the Paleocene-Eocene boundary event: the Paleocene-Eocene Thermal Maximum (PETM). The interpreted position of the PETM is supported by isotope data and corresponds with a change in both style of sedimentation and diagenesis in the reservoir, apparently related to a global rise in temperature during the Late Paleocene-Early Eocene. The Late Paleocene succession is characterized by fabric-destructive replacement of thick, original ‘catch-up’ sedimentary cycles by sucrosic dolomite; interpreted to be a product of dolomitizing fluids percolating rapidly through extremely permeable original sediment.
Above the PETM, thinner ‘keep-up’ cycles with common intertidal strata are consistent with limited accommodation. These strata have been replaced by fabric-preserving, microcrystalline dolomites. The relatively poor permeability of this interval is interpreted to have slowed the flow of dolomitizing waters leading to differences in the style of dolomitization.
This work has implications for understanding the sedimentary and diagenetic response of epeiric carbonate ramp systems to both accommodation and global climate change. In addition it emphasizes the importance of diagenesis as a controlling parameter of reservoir quality in major hydrocarbon reservoirs.