A decrease in the neutron log apparent porosity reading below that expected on the basis of the hydrogen indices of the formation component. Excavation effect results from the presence of a second formation fluid with a hydrogen index lower than that of the water. Thus, for example, in the presence of gas saturation:

;#; f_N = fS_gH_g + fS_wH_w – Df_Nex
;#;

where f_N is derived for the existing lithology type and where S_g and S_w are respectively gas and water saturation. H_g and H_w are respectively the hydrogen indices of gas and water, and Df<sub>Nex is the excavation effect.

The term “excavation effect” originates from the comparison of a fully water-saturated formation with another one containing the same water content, but having a larger porosity, the additional pore space being filled with zero hydrogen-index gas. On the basis of hydrogen index. both formations should give the same neutron porosity response. However, the second formation differs from the first in that the additional pore space occupied by the gas has been provided by “excavating” some of the rock framework. The two formations give neutron log apparent-porosity responses which differ by the amount of the excavation effect for this case.

Excavation effect is greater for larger contrasts between the hydrogen indices of the second fluid and the formation water, for higher formation porosities, and for intermediate water saturations.