AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Anhydrite Distribution Typing from Resistivity-Image and Open-Hole Logs in the 1st Eocene Reservoir, Wafra Field
(1) Chevron Energy Technology Company, San Ramon, CA.
(2) Chevron Energy Technology Company, Houston, CA.
Abundance and geometric configuration of replacive evaporite have been studied from high-resolution resistivity image logs in the 1st Eocene Reservoir of the Wafra field in the Partitioned Zone (PZ), Saudi Arabia and Kuwait. Pattern steamflooding of this reservoir is currently being implemented in a staged development approach. One of the main challenges associated with steamflooding in this anhydrite and gypsum rich carbonate reservoir is its substantial heterogeneity, exhibiting complex mineralogy, variable amounts of bedded, nodular and pore-filling evaporites, and a possible dual porosity system. The most commonly occurring facies, dolopackstone, frequently contains anhydrite nodules of variable size and concentration. The nodular anhydrites were formed in subtidal peloid packstone sediment shortly after burial, probably due to the downward seepage reflux of dense brines from salinas or evaporative ponds.
While mineral modeling from open-hole logs can generally estimate the proportion of anhydrite very well, coarser resolution of open-hole logs and lack of probing the heterogeneity in direction perpendicular to the borehole does not allow them to distinguish nodular forms by nodule number and size. In this integrated study using resistivity images, open-hole logs and core data, several logs have been derived from the resistivity image log to qualify nodular anhydrite, in good agreement with its classification from core.
On comparison with thin section description, samples of nodular anhydrite were exhibiting moderate to abundant fine-scale cementation as well. Their permeability was suppressed compared to samples with rare or no fine-scale cementation in thin sections, indicating that the origin of replacive nodules has affected the rock matrix as well.