Summary

Cementation factor "m" in Archie's law has been found to be related to carbonate pore structure and explains the electrical resistivity variations at a given porosity. In highly heterogeneous carbonates, understanding the variation of the cementation factor in different pore systems is important for accurate estimation of both hydrocarbon saturation and permeability. In this report, cementation factor derived from resistivity and density logs acquired in a carbonate formation onshore Alabama is investigated to relate its large variations to diagenetically generated high permeable zones. Detailed petrophysical analysis of over 8 wells reveals that "m" is a good indicator of pore structure. From log analysis, we find that the dominant pore type is dominated by micropores, where 1<m<1.9; the dominant pore types are intercrystalline and intergranular where 1.9<m<2.1; and the major pore type is vugs where 2.1<m<3. It is also found that m deviation log relative to a value of 2 agrees well with the velocity deviation log relative to the velocity calculated using time-average equation. Both deviations have similar trends with permeability measured on cores. Especially, high permeability zones can be correlated with positive m and Vp deviation logs. In the studied field, the two high fluid flow zones can have m deviation as high as 0.9 with an average m deviation of 0.4 for the two zones.

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