Deposition, diagenesis, and reservoir development of the Cretaceous Lidam formation, SE Sirt Basin, Libya.

Abstract

This study concentrates on the deposition, diagenesis and reservoir characteristics of the Upper Cretaceous (Cenomanian) Lidam Formation. Core, petrography and geochemical data were studied from five wells (P3-97, N6-97, R1-97， 3V1-59E, and 3V3-59E) in the SE Sirt Basin, Libya. Although the Lidam Formation is one of the main producing subsurface carbonate reservoirs in the Sirt Basin, this work is one of the first detailed studies of the formation. During this study, ten main facies and fourteen microfacies have been identified. These facies are; (1) anhydrite, (2) dolomite (3) sandstone (4) shale, (5) stromatolitic mudstone, (6) ostracod wackestone, (7) mollusc (8) bioclastic (9) peloidal, and (10) Algal facies. These deposits are inferred to have formed in supratidal sabkhas, intertidal, restricted marine shelf lagoon and higher energy shallow shoal environments. The overall depositional setting is interpreted as inner carbonate ramp deposits. Facies variations appear to be related to depth and energy changes and proximity to areas of clastic deposition. The whole stacking pattern of these facies suggests that the Lidam Formation consists of two large-scale cycles. The first cycle began with the laminated stromatolite and anhydrite facies which overlie the Nubian Sandstone and pass upward into lagoonal ostracod facies, and then the peloidal shoal deposits and are indicative of a transgression. The second cycle occurred during a relative fall in sea level at the end of the Cenomanian with a return to evaporitic sabkha deposits. The small scale cycles trends formed under transgressive and regressive conditions. These changes may have been controlled by tectonic events and changes in the rate of basin subsidence as well as a eustatic sea level rise during Cenomanian times.