M.H. Kamel, M. H. El-Difrawy, A. Bayoumi, and H. Hosney
Geophysics Dept., Faculty of Science, Cairo University, Giza, Egypt
ABSTRACT
Standard formation evaluation techniques were applied to electrical, porosity, and radiation well log data of a sedimentary succession in seven boreholes scattered through the southern sector of the Gulf of Suez Basin of Egypt. During the deposition of the examined sedimentary interval, based on geological and drilling evidences, the area of concern may be tectonically differentiated into a quiescent western basin, a tectonically uplifted eastern sector, and a major fracture zone separating these two tectonic units.
The qualitative interpretation of the spatial and vertical variations in the determined total porosities and mechanical parameters (shear modulus, bulk modulus, Young’s modulus, and the compressional and shear wave velocities) gave supporting evidence to the geologically-based tectonic differentiation of the investigated area in the simple sense that average total porosity of the examined interval increased towards the major fracture zone and as the burial depth got shallower. Expectedly, the mechanical parameters behaved inversely to porosity. The observed relations between the extent of deformation, on the one hand, and porosity and mechanical parameters, on the other, enhanced the assumption that the investigated intervals were dominated by fracture porosity.
An innovative quantification for these relations was made possible through crossplotting, in each of the investigated wells, each of the mechanical parameters versus total porosity on a semi-log graph (with porosity on a linear abscissa and the mechanical parameter on a logarithmic ordinate). The plotted points fell extremely well on a straight line giving high correlation coefficients and low dispersion measures. The negative slope (B) of the line best fitting the data points testified to the definite exponential decay of mechanical parameters with increasing total porosity and is a measure of the rate of exponential decay with porosity. The exponent B was found to faithfully follow the spatial distribution of deformation and overburden pressure; i.e., B increased approaching fracture zones or approaching ground surface. This interdependence stimulated the assumption that this parameter was mainly a reflection of open—fracture density, hence a possible substitute for permeability. This parameter was, therefore, termed the “Pseudo-permeability index.”
The reported exponential dependence was found to be valid over a wide spectrum of porosity values despite the fact that the investigated intervals had complex and varying matrix lithology, shale content, and saturation conditions, and lie at varying depths of burial, and had different tectonic backgrounds. Nevertheless, the substantiation and generalization of our findings require more field data; if it is proven valid, standard values for B would be possible to obtain and a new simple measure of productivity would be within reach.