Abstract
The sedimentation of Eocene age Dammam Formation started as a result of sea transgression above the evaporitic sequence of the underlying Rus Formation. The transgressing sea water filled the low land in coastal areas where muddy limestone was deposited and overlying brown dolomitized limestone was deposited in shallow peritdal environments. Due to continued transgression, a stratified intrashelf basin was developed where semi-anoxic to anoxic conditions prevailed and which resulted into the deposition of dark grey to black mudstone facies. Maximum flooding surface Pg20 of the Arabian Plate is correlated to lie within this zone. As the transgression subsided, shallow open marine conditions were developed where foraminifera nummulites thrived. These nummulites made shoals and banks behind which due to low energy protected setting in place deposition of calcareous sponge, Lockhartia, and nummulites occurred. Upwards, the nummulites gradual decrease and environments further shallow up resulting into the deposition of parallel laminated bioclastic limestone in inner ramp to intertidal environments. A second transgression took place in the middle of the Dammam Formation due to which a large lagoon type set up was developed. The chalky micritic facies was deposited in this quite water lagoonal setting with intertidal influence in its upper part. The proceeding yellowish brown dolomite facies has common fenestrae structures and rubbled zones along with chert nodules and stringers. This facies is very porous and was deposited in intertidal to supratidal environments. The uppermost dolostone has common chert stringers and bands and common rubbled zones developed by collapsed dissolution cavities likely related to exposure surfaces.
The Dammam Formation displays two transgressive-regressive depositional sequences. The formation has excellent reservoir characteristics in the form of overall high porosity which exceeds 40% in some zones. Hydrocarbon shows in the form of bitumen remains are observed within the formation.
