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

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. Introduction A geologic understanding of variations in reservoir quality is essential to understand flow behavior and predict reservoir quality away from well control. These geologic models are either depositional facies models, relating reservoir quality to rock fabrics produced during deposition, or diagenetic models, relating reservoir quality to post-depositional processes that have altered these fabrics. P. 251

Comparison of the diagenetic and reservoir quality evolution between the anticline crest and flank of an Upper Jurassic carbonate gas reservoir, Abu Dhabi, United Arab Emirates

“…33 peloids and dissolution of aragontic skeletal fragments in the shallower burial regime, and stylolitization in the deeper burial regime (Oswald et al, 1995;Neilson et al, 1998;Fabricius and Borre, 2007;Vandeginste and John, 2013;Paganoni et al, 2016), or both. The mass released from stylolitization would presumably diffuse and reprecipitate within a few meters away from stylolites .…”

Dynamics of cementation in response to oil charge: Evidence from a Cretaceous carbonate field, U.A.E.

“…Sources for burial calcite, provided by chemical dissolution sites, are suggested by the presence of intense stylolitization leading to porositypermeability degradation (Harms and Choquette, 1965). Reservoir quality in Abu Dhabi 'Thamama' fields degrades from the crest to flank due to preferential stylolitization within the water leg and inhibition of more pervasive chemical compaction due to the early introduction of oil (Oswald et al, 1995;Grotsch et al, 1998). Compact anhedral textures as well as near total meso-and macropore occlusion by cements may therefore reflect abundant sources of solutes due to chemical compaction with accompanying solution-transfer cementation.…”

“…The field is faulted and fractured (Tamura et al, 2004;Edwards et al, 2006;Edwards, 2006). The main source of oil was the Shu'aiba basinal facies that is thought to have reached maturity during the late Cretaceous (Murris, 1980;Oswald et al, 1995;Milner, 1998) around or shortly after the onset of uplift and the start of trap formation (Gumati, 1993). The main phase of oil charge within the Kharaib Formation has been constrained to the late Eocene (Taher, 1997).…”

“…Welldeveloped stylolite levels show lower porosity and permeability due to enhanced chemical compaction with accompanying solutiontransfer cementation. Stylolites formed preferentially in the water leg (Oswald et al, 1995), but are also present in the oil leg reservoir facies and ubiquitous in muddy, dense facies in the lower section of reservoir cycles where alumino-silicate minerals probably catalysed pressure dissolution (Ehrenberg, 2003). Stylolitization is thought to have commenced after 600 m of burial at temperatures of approximately 65.6°C (Oswald et al, 1995).…”

Carbonate porosity creation by mesogenetic dissolution: Reality or illusion?: Reply

“…Stylolitic dissolution and associated calcite cementation then proceeded to reduce microporosity on the water-filled flanks of the same fields. Inhibition of porosity loss by early oil charge is a welldocumented phenomenon in this type of reservoir (Oswald et al, 1995;Neilson et al, 1998;Melville et al, 2004).…”