Compaction Controls on Porosity in Thamama Group. Carbonate Reservoirs in Abu Dhabi, U.A.E.
Erik J. Oswald, D. F. Goff, H. W. Mueller, H. Al-Habshi,
and S. Al Matroushi
Thamama Group carbonate reservoirs display a pronounced decrease in porosity and reservoir thickness moving away from the crests of producing anticlines. Chemical compaction is the major process by which reservoir quality has been degraded, reducing porosities from 25% in crestal areas to 10% off structure. Lateral variations in reservoir quality are largely controlled by stylolite frequency and intensity, with no significant accompanying variations in depositional facies.
The lateral homogeneity of the pre-stylolitized reservoirs and lack of significant post-stylolitization modification of porosity leads to a strong correlation between porosity and thickness. This relationship can be accounted for with a "closed-system" chemical compaction model wherein all material dissolved at the stylolite is locally reprecipitated adjacent to stylolites.
Porosity and reservoir thickness exhibit a non-linear decrease when plotted against subsea depth. The marked discontinuity of these plots suggests that differences in pore fluids during burial controlled the degree of chemical compaction, with stylolites preferentially forming in the water leg and inhibited in the oil leg. This discontinuity is more pronounced when subsea sections are re-datummed on the Halul Fm., and is interpreted to reflect a Santonian-age "paleo" oil-water contact during the major period of chemical compaction. Redatumming seismic lines to reflect this Santonian structure demonstrates that stylolite-degraded porosity in the oil leg of the northern part of the field was actually in the water leg during compaction, and was only moved into the oil leg by later southw rd tilting of the reservoirs during the Campanian. Migration of hydrocarbons prior to stylolitization is consistent with calculated thermal histories for the field, and suggests the bulk of stylolitization commenced after 2000 ft of burial at temperatures of approximately 150°F. Delineating the "paleo" OWC allows for improved prediction of porosity and permeability, and accounts for almost all of the observed variation in reservoir quality.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California