Porosity correction in the Najmah Kerogen Formation

Abstract

The Najmah Formation has recently been established as a major gas-condensate reservoir and source rock in northern Kuwait. Vast economic reserves have been estimated from this Middle Jurassic formation. The formation is mainly composed of inter-bedded cemented peloidal packstone, argillaceous and bituminous (kerogen-rich) limestone. The kerogen-rich intervals are characterized by high Uranium content and extremely high apparent porosity as observed from conventional porosity logs. A standard approach for determining the effective porosity from conventional porosity logs provided highly exaggerated porosity values, which are not acceptable when compared to the lithological composition of the formation and core-derived porosity. A study was performed to obtain more realistic values of effective porosity for the high-kerogen intervals. Three key wells were selected for the study, based on the available nuclear magnetic resonance logs and core porosity. A relationship of core porosity and/or NMR data with the effective porosity (PHIE) was found to be reasonably consistent for the three key wells and was considered to be related to the kerogen content. Another approach was carried-out by establishing a relationship between NMR, conventional porosity and uranium content. The output of this relationship is presented by corrected porosity (PHIK). Therefore, a fair estimate of derived PHIK, in the absence of core or direct NMR measurements, was achieved. However, such an approach is relatively lower in accuracy when compared with the core and NMR porosity benchmarks.