LITHOSTRATIGRAPHY OF OIL‐BEARING TERTIARY BIOHERMS IN THE SIRTE BASIN, LIBYA

Abstract

In the early Palaeocene, a carbonate platform existed in the southern part of the Sirte Trough (a NW‐trending depression in the eastern Sirte Basin), and a shale basin in the north. A major transgression occurred during the early Palaeocene, and part of the platform subsided, resulting in the formation of a basinal re‐entrant within the carbonate province. A positively northerly‐trending ridge, which was active in the early Palaeocene, persisted within the re‐entrant during the late Palaeocene. This positive element provided a platform on which isolated bioherms grew, surrounded by shale deposits in the subsiding basin. The bioherms began as foraminiferal banks that built‐out on local mounds in shallow water. Sparse coral growth bound coralline detritus, until richer coral development resulted in a large coral reef.A barrier reef surrounding the re‐entrant developed during the late Palaeocene and grew basinwards, advancing over its own talus. A second major transgression closed the Palaeocene epoch, and clastic deposition over the Sirte Trough terminated reef growth and carbonate deposition.The reef, back‐reef and basinal environments are each characterized by distinctive lithologies and fauna. The bioherms form excellent hydrocarbon reservoirs in the Intisar A and D fields, which are probably sourced by the Sirte and Hagfa Shales. Six bioherms, some in excess of 300 m in thickness, have been discovered, and some contain commercial oil accumulations.