Abstract: The Pelagian Basin in North Africa is one of the important oil and gas basins. Based on the seismic，drilling，
analytic and testing data，the comprehensive analysis of the forming conditions of the Upper Cretaceous fractured limestone
reservoir reveals the following points：the formation of the oil and gas reservoir in this region is mainly controlled by oil resource
conditions，the strike and abundance of the fracture and fault，as well as the distribution of effective transport layer and thick cap
rock；the structural areas developed northwest faults and fractures，composite passage system and thick cap rock are favorable
areas of hydrocarbon migration and accumulation. The faults and fractures developing in the direction of northeast are unsealed，
which is beneficial for the lateral and vertical migration of the hydrocarbon and for the improvement of the limestone reservoir
property. The movement of salt in the late Palaeocene makes the mudstone of the Haria formation，which is the regional covering
strata，partially eroded，resulting in the maldistribution and the change of the thickness of the cap rock in the region，which
can greatly influence the height of hydrocarbon column of reservoir that can be sealed. As a result，it is pointed out that the
complex anticline belt in central NK and the nosing belt in Eastern NK are two favorable areas of oil and gas accumulation，
which provide guidelines for the exploration of carbonate reservoir in this region.

Key words: fractured limestone reservoir, composite passage system, reservoir-forming condition, Upper Cretaceous, PelagianBasin