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

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According to field studies of the Pugu Hill kaolin deposit and mineralogical investigations (X-ray, SEM), two main types of Miocene sandstones are distinguished: (i) clayey-silty kaolinitic sandstones with stacks of idiomorphic pseudohexagonal kaolinite crystals; (ii) massive clayey-silty kaolinitic sandstones with irregular fabric of isolated kaolinite plates. The Precambrian metamorphic rocks of the Uluguru Mountains are a probable source for the kaolinitic sediments of the Pugu Hill Formation. Decomposition products of weathering profiles on meta-anorthosites are mainly 7A-and 10A-halloysite and gibbsite. It is suggested that the kaolinitic sandstones were deposited in a former delta. The first sandstone type is considered to have developed in situ from weathered arkosic sandstone, and the second represents channel deposits in an old river system (probably a Miocene predecessor of the Ruvu River) with kaolin transported from a hinterland and redeposited. The variable formation and fabric of the kaolinites in the Pugu Hill Formation are due to in situ weathering of transported primary minerals (fddspar), and to transported weatheredmaterial (with secondary halloysite).

We investigated the basic geological and petrophysical properties of the multimodal pore systems in the Arab D limestone facies in Ghawar field, Saudi Arabia. The study used more than 500 mercury injection capillary pressure (MICP) data, which were type-curve matched using Thomeer Hyperbolas. The new MICP sample data were drawn from 10 cored wells that transect the Ghawar field from north to south and from a previous fieldwide study with 125 MICP samples. These 500 samples have a very rich statistical foundation in that they were selected using only random decimation within each of the facies from more than 3,500 core plugs all with assigned facies. In addition to MICP data and facies, a former, smaller sample set had both facies and Dunham texture codes. A new view of these pore systems emerged, that is built upon the intrinsic, fundamental and separate maximum pore-throat diameter modal elements named porositons. Porositons are stable, recurring and intrinsic modes in the maximum pore-throat diameter of the carbonate pore systems. Analytical results derived from the MICP data showed that the pore systems of the Arab D limestones can be classified based on porositons. The benefits of this new classification are demonstrated by considering in detail the relationships to geological facies, well-log responses, permeability modeling and simple nuclear magnetic resonance (NMR) well-log response. By analogy to the decoding of the Egyptian hieroglyphics using the Rosetta Stone, the use of porositons enables strong connections to be made between the geological facies, petrophysical and reservoir-flow properties of these complex carbonate rocks. The relationships between the new pore systems categories and the facies were thoroughly tested using north-south field trends.

This is • preprint of a piper intended for publication in a journal or proceedings. Since changes ma> be made before publication, this preprint is made available with the un derstanding that it will not he cited or reproduced without the permission of the author Mtttf* DISCLAIMER This document was prepared as an account of work sponsored by an agency of the L'nited Stales Government.