Khalid Ahmed

Reservoir Quality Assessment from Clay Mineral Distribution in the Neogene of North Kuwait

The Neogene clastics illustrate wide variations in reservoir quality in the North Kuwait fields .There are two main controls on reservoir characteristics within the sedimentary succession namely matrix content and carbonate cement abundance. The detrital clay and dolomitic matrix content controls reservoir quality, with the poorest permeability and porosities associated with deposits containing abundant detrital matrix. Pervasively cemented sandstones will result in relatively low porosities (c.20-25%). However, subsequent partial dissolution allowing tortuously connected macro pores has resulted in the retention of higher than expected permeability (c.1000-10000mD). Samples that have undergone localized replacement of detrital clay by calcite generally have lower porosities (c.10-20%) and permeability (c.4-30mD ;). Coarser grained and well sorted (generally clean) samples typically having the best reservoir quality. The various pore types typically occur across all reservoir layers and facies associations, with the notable exception of primary interparticle macropores, which have not been identified within the interdistributary bay/lagoon facies association. However, intra-clay and intercrystalline pores generally occur as the dominant pore type within the non-channelised deposits (eg. Lower shoreface), while primary interparticle macro pores are the dominant pore type within channel sandstones. In terms of reservoir quality, the best quality is generally found in clean channel sands. As expected the poorest reservoir quality is found in deposits with no or limited macropores, which typically have intercrystalline and intra-clay pore types. The controls on reservoir quality also vary between facies association. The facies associations with high matrix abundance, such as floodplain, interdistributary bay/lagoon and lower shore face, are associated with the poorest reservoir qualities. In contrast, the reduction in reservoir quality associated with calcite cementation only occurs within clean Channel sandstones .The distribution is considered primarily by reservoir layer and then in terms by the facies associations within those layers. The total smectite, illite-smectite and palygorskite content varies across the field , with a low content in the north and centre (2-4.6%), a moderate content in the east (6.7-7.3%) and a high content in the south (11.2-14.4%). The Horizontal permeability ranges from 0.44mD to 10000mD with a geometric mean of 2026mD, whilst helium porosity ranges from 2.8% to 42.5% with a mean of 33.1%. There is no linear relationship between permeability and porosity, with a wide range of porosities for samples with similar permeabilities. This poorly defined porosity-permeability relationship is probably the result of authigenic carbonate and detrital feldspar dissolution, which has locally enhanced secondary macro porosity network. Often some of these secondary macropores are localized and may not be connected to the rest of the pore system; hence helium porosity increases relative to permeability. Dissolution, while common, varies in intensity which gives rise to the wide variety of porosities identified within the deposits.