Impacts of the pore- and petro-fabrics on porosity exponent and lithology factor of Archie’s equation for carbonate rocks

What is it about?

Archie’s parameters, including the tortuosity factor ‘a’ and the porosity exponent ‘m’, are important parameters controlling the formation resistivity factor–porosity relationship. Though, they are among the most important parameters, they are still a matter of controversy due to their dependence on many geological factors. They are not constant values, but vary greatly from facies to facies and from sub-facies to another. So, a considerable error would be assigned when the Archie’s parameters are assumed to be constant values. The present study is a trial to assign the identity of Archie’s parameters and their controlling factors. Based on the petrographical and petrophysical study for 261 carbonate core samples (representing Risan Aneiza, Halal, Bir Maghara, Rajabia and Masajid Formations, in Sinai Peninsula in Egypt), the studied samples can be grouped into three major facies types which are further discriminated into ten sub-facies; each one has its diagnostic Archie’s parameters (1.06 6 ‘a’ 6 8.76, and 1.19 6 ‘m’ 6 2.67). Petrographically, the present samples are mostly characterized by intercrystalline pore spaces, and sometimes other pore types as vuggy and vuggy channels are assigned. The closed pore spaces are assigned also, but with no effect on the conductivity properties of the studied rocks. From the present, it is achieved that, the lithology factor ‘a’ for the carbonate rocks is dependent mostly on the permeability and formation factor, as an indication for the pore channels tortuosity, as well as the porosity and mineralogy; whereas the porosity exponent ‘m’ is dependent mostly on the porosity types and values, as well as permeability and formation factor. The pore and grain elongations are other factors controlling the Archie’s parameters. In other words, the porosity exponent ‘m’, is dependent mostly on the pore fabric, whereas the tortuosity factor ‘lithology factor’ is dependent mostly on the petro-fabric, as well as the pore fabric. Dependence of Archie’s parameters on the same factors, but with varying behaviors, can be explained by an inverse proportional relationship assigned between ‘a’ and ‘m’ (r = 0.93).

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