Iranian Journal of Oil and Gas Science and Technology

Abstract The petrophysical parameters of the Ghar Formation are characterized in this study. A combination of pre-stack seismic data gathers and well-log data is used to estimate water saturation and shale volume in the Ghar reservoir. First, the highest possible correlation between the well logs and the seismic inversion data was established for this purpose. After extracting the optimal wavelet, an accurate relationship between the estimated values and the core data was obtained. Next, using data from another well, the validity of the constructed model was examined. The results showed that the combination of three attributes—instantaneous cosine of phase, , and —can accurately estimate the shale volume of the reservoir, with a correlation coefficient of approximately 70%. Although the two layers in the Ghar section have a shale volume of about 10%, the overall shale volume in the reservoir area is negligible. The logarithm of the ratio of compressional wave velocity to shear wave velocity attribute exhibits the highest correlation, approximately 62%. Finally, validation using previously unutilized well-log data demonstrated an accuracy of about 90% in predicting these properties.

Abstract Water saturation and cation exchange capacity are the most significant parameters used to calculate a hydrocarbon zone potential. In clean formations, by applying the famous Archie model, which assumes that in the formation the only electric conductor is the formation water, the water saturation can be calculated. Additionally, in shaly sand formations this assumption may not be true as the ions are associated with clay minerals and transport electricity.
We attempt to utilize some logs, as well as experimental data, to improve the water saturation calculations and the cation exchange capacity within shaly sand layers for an Iranian oil field. Therefore, several shaly sand models were considered for the calculations of these values for three wells of the Ahvaz oil field. The validity of the different models was controlled through the measured values of cation exchange capacity and water saturation on core samples. The comparative results show that cation exchange capacity and water saturation calculated using Ipek-Bassiouni model are more indicative of zone hydrocarbon potential with correlation coefficients of 0.91 and 0.95 respectively. The results of this study show the requirement for the I-B model in studied reservoir and its superiority compared to other models because it is on the basis of principles reflecting the formation water and clay counter ions conductive behavior. 
Concerning to this model, two cement exponents are incorporated to illustrate the electric current path tortuosity in clay bound water and free water. In the current work, cementation exponent in free water (m_f) and clay bound water (m_c) were estimated from pure shale and clean sand respectively. The results of this study are promising and can be simply extended in other similar neighboring shaly sand reservoirs.

Abstract Petrophysical parameters such as porosity, water and oil saturations, formation resistivity factor, etc. describe the storage capability of the porous media or the capacity of rocks to hold fluids. The modified Archie’s equation
   . / . , also called the saturation equation, is used to determine the water saturation. Archie’s parameters, namely , , and , 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 () such as porosity, pore throat size, type of rock grains, type and distribution of clay content, degree of cementation, and overburden pressure. In the present paper, the results of electrical resistivity experiments are used to derive a new cementation factor correlation which can be applied to carbonate parts of Asmari and Sarvak formations located in south-west Iran. In Iran, the cementation factor is traditionally measured by Shell formula or is assumed equal to 2 to avoid difficulty. In the new formula,  increases with increasing porosity; however, in the Shell formula,  decreases with increasing porosity especially in the low porosity ranges, which is in disagreement with the current paper results. In addition, the results demonstrate that it is not possible to introduce constant  values or separate cementation factor correlations versus porosity for different petrofacies and rock types. Petrophysical evaluations are done to quantify hydrocarbon resources in formations under study. Then, the water saturation is calculated with different calculation methods of cementation factor, . The calculated water saturations are compared with the measured water saturations of preserved cores.