Controls on Porosity Evolution in Thamama Group Carbonate Reservoirs in Abu Dhabi, U.A.E.

Abstract

ABSTRACT The multiple Lower Cretaceous carbonate reservoirs found throughout Abu Dhabi display pronounced lateral heterogeneities in porosity, permeability, and reservoir thickness. Generally, these variables decrease away from the crests of producing anticlines. Detailed study of the Kharaib Fm. in one of these fields has found that pressure solution along stylolites, with concurrent local reprecipitation of calcite cements, is the major process by which reservoir quality has been degraded. Lateral variations in reservoir quality are largely controlled by stylolite frequency and intensity. No significant variations in depositional facies accompany this change. The lateral homogeneity of the pre-stylolitized reservoirs and lack of significant post-stylolitization modification of porosity leads to a striking correlation between porosity and reservoir thickness. These relationships can be accounted for using a "closed-system" chemical compaction model wherein all material dissolved at the stylolite is locally reprecipitated. Marked discontinuities in plots of subsea depth versus porosity and reservoir thickness 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 (OWC). Stylolite-degraded porosity in the oil leg of the northern part of the field can be explained by later southward 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.