AAPG Middle East Region Geoscience Technology Workshop

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Geological Techniques to Model Rock Conductivity


Understanding the conductive elements in the rock mass is the solution to understanding the causes of low resistivity pay. Logging tools measure the rock conductivity by applying electromagnetic induction principles. Accurate representations of the formation elements influencing logging tool response are characterized with geological observation. In the presentation accompanying this abstract, we will look at how geological observation set the framework for understanding the physics of the conductive element in the rock – that which causes the low resistivity response. We will review various historic petrophysical models: Archie’s equation, the Indonesian equation, Waxmin Smits and the Dual Water model. These saturation equations are re-written as equations of conductivity to demonstrate how the clay component creates a conductive path for the electromagnetically induced ground loop in the logging measurement. Thin section observations across the low resistivity pay intervals in a Permian clastic reveal additional conductive elements that are not captured in the historic models. The presence of bimodal grain size, with fine grain clay coatings, appears to be creating the conductive component in this formation. A new petrophysical model is introduced to quantify the grain size distribution and degree of clay coating. The total conductive signal is proportional to the combined surface area of the clay-coated grains and the capillary bound water. The new model development is highly dependent on geological observation techniques that frame the conductive rock features.