CLINOFORM GEOMETRY AND SEISMIC FACIES OF LOWER AND MIDDLE EOCENE CARBONATE SEQUENCES, SIRTE BASIN, NORTH CENTRAL LIBYA

Isolated carbonate platforms are common and contain important petroleum reservoirs. Internal geometry and stratigraphic architecture of well-developed prograding clinoforms in the lower and middle Eocene carbonate successions on an isolated platform in Sirte rift basin, Libya, were interpreted from 3-D seismic and well data. Two packages of clinoforms are defined based on their geometries, and form a seismic unit bounded by a toplap and truncational top and a downlap base. The top package shows high angle sigmoidal clinoforms, and the basal package is high angle oblique tangential. The slope angle of individual clinoforms and clinoform packages varies from updip to downdip. The maximum slope angle of the sigmoidal clinoforms packages is 23^o, and that of the oblique tangential basal package 30^o. The width of individual clinoforms varies from 1.7 km in top package to ~ 2.5 km in basal package; and the height ranges from 220-350 m. The overall progradation direction is N-NW. In well data, individual clinoforms consist of 5-100 m thick coarsening upward cycles of grainstone and packstone, possibly rudstone and breccia. Four major seismic facies (SF) show complex lithology and depositional systems. SF1 consists of high angle, sigmoidal, continuous reflections of high amplitude clinoforms in the top package. SF2 has high angle oblique tangential and semi-continuous reflections of moderate to high amplitude clinoforms in the basal package. SF1 and SF2 correlate with high resistivity and serrated log facies of possible coarse debris flow deposits. SF3 consists of subparallel and semi-continuous reflections of moderate amplitude located basinward of the clinoform packages. Its hummocky to chaotic internal configuration and serrated log pattern suggest debris flow deposits. SF4 consists of large mounded bodies with bi-directional downlap terminations and shows serrated log motif. It was interpreted as lagoonal patch-reef bodies deposited during sea-level transgression.