Abstract

The vertical distribution of bedding-parallel stylolites in cores from the approximately 50-m-thick Thamama-B reservoir zone of the Lower Cretaceous Kharaib Formation is compared between two wells on the water-filled flanks and two wells on the crest of a giant oilfield where oil has previously been interpreted as having preserved higher porosity on the crest. Stylolite abundances indicate division of the reservoir zone into three intervals:

• The top 12 ft (3.7 m) of the zone contains abundant low-amplitude stylolites in both crest and flank locations.

• The main, central part of the zone has abundant high-amplitude stylolites in the flank wells but nearly no stylolites in the crest.

• The basal 17 ft (5.2 m) of the zone contains high-amplitude stylolites in all wells, but these are somewhat more abundant in the cores from the crest of the field.

The contrast in stylolite abundance in the center interval is consistent with oil having inhibited chemical compaction on the crest of the field. Minor dissolution along wispy seams in the center interval of the crest is interpreted as representing an incipient stage of stylolite development. The similarity in stylolite abundance in the top interval between crest and flank wells is interpreted as reflecting stylolite formation pre-dating oil emplacement because of greater depositional concentration of clay in thin laminations resembling flaser bedding. Stylolite growth in the basal interval may also partly predate oil filling, but the evidence for timing is unclear because stylolites in the heavily bitumen-stained basal interval of the crestal wells have thick bitumen coatings that may be residues from dissolution of the surrounding limestone.

Negative correlation between porosity and both amplitude and proximity of stylolites supports the model of porosity loss by calcite cementation derived from stylolites. The overall porosity difference between crest (22%) and flanks (12–16%) approximately matches the volume of cement that would have been supplied by the thickness of strata dissolved on the flanks. The match between porosity difference and thickness variation supports a diagenetic system closed to significant calcite import or export.

Profiles of bulk-chemical analyses reveal how clay (proportional to aluminum) and dolomite (proportional to magnesium) vary both vertically in the formation and laterally between the crest and flank locations. Higher clay values, suggested to facilitate stylolite development, occur mainly in the top and basal parts of the zone, reflecting episodic deposition of siliciclastic fines. Higher dolomite contents throughout the wackestone-dominated lower two thirds of the reservoir zone (around 5–10 wt. %) than in the grain-supported upper one third (little or no dolomite) are suggested to reflect differences in early dolomitization by seawater attending slower sedimentation rates in the lower part of the zone. Like the variations in chemical compaction, dolomitization is thus a manifestation of the layer-cake geometry of this reservoir. Detailed core descriptions from each well and tables of bulk-rock chemical analyses and stylolite data are available as supplemental material.