Evaluation Procedures in the Carboniferous of Northern Europe

Abstract

Formation evaluation in the Carboniferous has always been an extremely difficult task due to complex formation properties coupled with adverse logging conditions. A study was undertaken in co-operation with oil companies to propose optimum data acquisition techniques and formation evaluation procedures.The study covers a 300 kilometre-wide area of North Continental Europe. It includes data from 12 wells with different logging conditions and depths ranging from 2200 to 6700 meters.Cores were analysed for porosity, permeability and formation factor under reservoir pressure conditions. Thin sections, SEM photographs and X-ray diffraction results, have been utilised for clay typing and to establish the lithological model best suited to Carboniferous evaluation. This model includes: quartz, ferroan dolomite, haematite and mica. Illite and kaolinite are the two main clays.Formation water salinity was found to decrease from 230 Kppm in the uppermost formations, close to the Zechstein cap rock, to 26 Kppm in the oldest Westphalian horizons. Determination of the true formation resistivity takes account of the influence of the so-called Groningen effect.Core and log observations pointed to the presence of a dual porosity system with variations of cementation exponent with overburden pressure. Rock mechanical property computations, estimation of in-situ stress directions and fracture detection completed the study.