Deriving a Correlation of Cementation Factor in Nubian Sandstone Formation (Upper and Lower Cretaceous), Sirt Basin

Abstract

The understanding of reservoir rock properties such as porosity, permeability, water saturation, and resistivity assists engineers to improve the characterization of the reservoir, and the cementation exponent m is an intrinsic property of the rock related to the geometry of the electrically-conductive water network imposed by the pore walls or surfaces of solid insulating materials. Archie’s parameters, namely m, n, and a, are sometimes assumed constant to simplify petrophysical measurements. But these parameters are not constant, particularly in heterogeneous reservoirs. Inaccurate estimates of these parameters can cause significant errors in the calculation of water saturation when using Archie’s equation and lead to discrepancies between log interpretation and production test results. There are many factors affecting cementation factor (m) such as porosity, pore throat size, type of rock grains, type and distribution of clay content, degree of cementation, and overburden pressure. This study has been undertaken in two oil fields: A-Libya, and B-Libya in the Sirt Basin located in Libya, the Nubian Sandstone formation is the main reservoir in these oil fields. Laboratory measurements such as porosity, and resistivity were conducted on core samples selected from two different fields. The results of electrical resistivity experiments are used to derive a new cementation factor correlation which can be applied to Nubian formations located in Sirt Basin. Crossplots of formation factor and porosity were created from measured data points for one hundred and ninty eight core samples. The analysis procedures for each field was made sequencely and finally, a good relation between formation resistivity factor and porosity for two fields and the correlations for calculation cementation factor were obtained for the Nubian sandstone formation, and the average Relative error of the data points when deriving these correlation was very small which indicates that these correlations are consistent.