Jurassic carbonate reservoir rocks of northeastern Saudi Arabia contain productive oil at several levels. For more than 20 years, samples and other data have been collected on the main oil reservoir--the Upper Jurassic Arab-D zone. Diamond cores from 12 wells with high Arab-D recovery were selected for detailed petrologic examination as part of the present study. Nearly 4,000 thin sections have been cut at closely spaced intervals and examined in detail; of these, 1,200 were analyzed by point counting. Particle size, particle type, authigenic constituents, and visual porosity were recorded. Calcareous algae, stromato-poroids, and Foraminifera proved to be the main skeletal elements; aggregate pellets, "algal" nodules, and "fecal" pellets are the most important nonskeletal particles.

Examination of Arab-D cores, equivalent rocks on outcrop, and modern calcareous Persian Gulf sediments has shown that these carbonates can best be understood by considering them to be products of mechanical deposition with certain environments being characterized by specific particle sizes and types. Arabian carbonates can be divided according to original particle size and sorting or obliteration of original texture into five groups--(1) aphanitic or fine-grained limestone, (2) calcarenitic limestone, (3) calcarenite, (4) coarse carbonate clastic, and (5) dolomite. Classification of Arab-D rocks according to this scheme has permitted (a) recognition of distinctive stratigraphic units for correlation and reservoir zonation, (b) delineation of original environment-sedimentation patterns and (c) relation of reservoir properties to original textures and secondary changes.

Arab-D rocks represent the transition from continuous carbonate deposition to precipitation of nearly pure anhydrite. The lower part of the reservoir consists of mixed mud and nonskeletal sand. Near the middle, a thin persistent unit of aphanitic limestone records an episode of muddy deposition over a wide area. Widespread shallow-water conditions during upper Arab-D time are suggested by a pronounced increase in skeletal sand and clean-washed calcarenite derived in large part from dasyclad algae and stromatoporoids. Many features known to be persistent on outcrop have proved equally continuous in the subsurface, indicating that changes which affected sedimentation must have operated on a large scale.

Regionally, reservoir units show gradual lateral change. The picture to emerge is that of a broad shelf with finer lagoonal sediments being deposited in the west, dominantly calcareous sand in the form of offshore bars accumulating near the present coast, and presumably deeper water mud with sparse sand being laid down in the east. The sediment patterns cut across and are apparently unrelated to modern structure.

Of the original textural elements, carbonate mud matrix exerts a dominant control on reservoir rock behavior. As mud content increases relative to sand content, porosity and permeability uniformly decrease. Original rock textures have been altered in at least six ways. Addition of dolomite (which exhibits strong affinity for mud-size particles, little for calcarenites) has the most pronounced effect on reservoir properties. In any textural group, porosity and permeability (a) progressively decrease as dolomite increases from 10 to 80 per cent, (b) increase where dolomite forms 80 to 90 per cent of the rock, and (c) again decrease as dolomite exceeds 90 per cent.