Abstract

The fractured carbonate reservoir of the Oseil Oil Field was discovered in 1993 and successfully appraised in 1998. This paper describes the results of the Kuwait Foreign Petroleum Exploration Company's (KUFPEC's) effort to describe and quantify the characteristics of this unconventional reservoir.

The Oseil reservoir is an Early to Middle Jurassic Manusela Carbonate. It is an extensively recrystallised mixture of oolitic grainstone, dolomites and wackestones. Matrix porosity is generally very low due to complex diagenesis and occlusion of the intergranular porosity by cementation, dolomitisation and recrystallisation. The reservoir is extensively fractured with all three wells intersecting a network of open, near vertical, natural fractures. This network is made up of fractures at a variety of scales from grain-scale to core-scale micro-fractures, to macro-fractures visible on borehole image logs with apertures on the order of several centimeters right through to the seismically mapped faults or mega-fractures that compartmentalize the field.

Quantitative and qualitative studies to characterize the Oseil reservoir have examined the nature of the fracture network at various scales, as a function of depth, lithofacies and spatial position. Results show that macro-fractures are more commonly developed in the coarser grained oolitic and dolomite litho-facies than the finer grained units. Fractures preferentially strike in a NNE-SSW orientation though local variations, both in terms of position and scale, do occur. Fracture geometry at the scale of core, logs and seismic is consistent. The fractures are the result of regional as opposed to local tectonics and should be widespread. The pervasiveness of the fracture network is confirmed by analysis of drill stem tests. Dual porosity log analysis techniques indicate total average fracture porosity in the oil column of 2.2%.

The Oseil field reserves are now estimated to exceed 40MMBO. The studies undertaken have encouraged KUFPEC and its partners to commence development of the field with first oil production due in 2002.