Secondary ferroan dolomite rhombs in oil reservoirs, Chadra sands, Gialo Field, Libya

Oil-productive, Oligocene Chadra sands in Gialo field, Libya, at depths of 600 to 750 m (2,000 to 2,500 ft), contain small dolomite rhombs. The rhombs are present as single crystals on detrital grains and as nonmosaic aggregates in pore space. The dolomite is calcium-rich and contains up to 10% iron but no measurable sodium or strontium. Total dissolved solids of produced interstitial water from the Chadra sand range from about 4,500 to 10,000 ppm. Introduction of fresh (meteoric) water into the Chadra sands, which were deposited in a shallow-marine (shelf) environment, was responsible for formation of the dolomite rhombs. Iron in the dolomite rhombs was derived from alteration of glauconite.