Modeling The Effects Of Glauconite On Some Openhole Logs From The Lower Senonian In Egypt

Glauconite is commonly found in the Lower Senonian of the Gulf of Suez (Nezzazat Group Matulla Formation) and the Egyptian Western Desert (Upper Bahariya Formation) in sufficient quantities to have an impact on density, neutron, Pe, and resistivity log response. Glauconite is a complex mineral composed mainly of iron, potassium, and magnesium hydrated aluminum silicates. Due to its complex mineralogy in particular variations in iron content, it has a wide range of grain densities. Modeling the effects of glauconite on the responses of density, Pe and neutron porosity logs has been used to improve the determination of porosity and lithology from well log data. Glauconites are iron-rich minerals often found in sandstones, limestones, and siltstones. Sandstones rich in glauconite are commonly known as "greensands." Because of its complex composition, in particular ferric and ferrous iron, the grain density of glauconite ranges between 2.4 g/cm3 to 2.95 gm/cm3. The presence of iron also significantly affects neutron porosity response due to its large thermal neutron absorption cross section. These density and elemental effects have been observed on logs from wells in the October and Razzak Fields. The effect of glauconites on resistivity can be appreciable since they can have significant cation exchange capacity (CEC). This work evaluated potential problems associated with interpreting logs from glauconite-rich formations and proposes log interpretation procedures to determine porosity and water saturation in these lithologies. This procedure is based on petrophysical measurements from a well in the Gulf of Suez, October Field, and a well from the Razzak Field in the Western Desert. The effect of glauconite on the actual glauconite-rich rocks on density/neutron porosity response was investigated in detail. Core measurements from samples from these two wells included grain density determination, X-ray diffraction analysis, elemental analysis by X-ray fluorescence, and qualitative elemental analysis by scanning electron microscopy. Mineralogy was based on these analyses. Density, neutron and Pe models developed for various concentrations of glauconite helped quantify the effect of glauconite on their response. A regression analysis based on this model data included the effects of glauconite on predicting porosity from density/neutron porosity and Pe logging measurements with acceptable error. The possible effect of glauconite on formation resistivity was investigated by determining the CEC of samples and modeling its effect on electrical properties.