Abstract

This work is to investigate facies geometries, quality, and connectivity in the Cyrenaican Upper Cretaceous carbonate slope deposits of the Al-Athrun Formation as an analogue for turbidite reservoirs. Seven closely spaced detailed stratigraphic sections were measured along a 400 m south-north oriented outcrop in the Cyrenaican Upper Cretaceous, NE Libya. Facies have been sampled and examined microscopically to identify texture, grain types, mud content, and diagenetic features. Field observations and lab analysis indicate one fining-upward turbidite sequence comprising two parasequences that consist of repeated centimeter-scale Bouma sequences made of four texturally distinctive depositional facies.

The Al-Athrun Formation is a fining-upward turbidite sequence of 8.7 m maximum thickness and consists of two parasequences separated by a sharp surface. The lower parasequence is dominated by a grainy facies (packstone-grainstone) that is dissected by small disconnected encased channels which contain a muddy facies (packstone-wackestone). The upper parasequence is made up of muddy facies (wackestone-packstone) that is dissected by larger and laterally connected channels which contain a grainy facies (grainstone-packstone). Furthermore, these turbidite parasequences are made up of smaller and incomplete Bouma sequences. The Bouma sequences are dominated by (Ta and Tb) units updip to the south and by (Ta, Tb, Tc, and Td) units down-dip to the north.

This fining-upward turbidite sequence contains intact and fragmented planktonic forams and carbonate lithoclastics that decrease in quantity and size upward. Also, the sequence shows rhythmic lamination, cross-lamination, convoluted laminae, chert nodules, large-scale syndepositional folded beds, and channels.

The Al-Athrun Formation is a fining-upward turbidite sequence of homogeneous carbonate lithology and variable textures. The depositional facies are amalgamated and bench-out suddenly in time and space. It includes two parasequences that were built by many repeated incomplete centimeter-scale Bouma sequences. Facies quality of this turbidite sequence decreases upward as the mud amount increases, changing upward from grain-supported to mud-supported facies. However, channels facies quality increases upward, changing upward from mud-supported disconnected small channels in the grainy facies to grain-supported and relatively laterally continuous larger channels sealed by muddy facies.