Abstract

The logging suite for this well consists of the following logs: 1.) Array Laterolog – Rxo, 2.) Neutron-Lithodensity, 3.) Spectral Gamma Ray Log, and 4.) Geochem (ECS) Log. The Avalon Shale interval analyzed was 7500 ft to 8270 feet thick. In the middle of this interval is a thick limestone (200ft) that separates the Avalon into a lower and upper shale intervals.

First using GEOCHEM Log (ECS) data mineralogy was available. Total porosity (ɸtotal) was calculated using a variable matrix analysis, and volume of kerogen (Vke) was calculated using TOC determined from the Schmoker Equation. Next using ρb and ɸNls data and the Simultaneous Equation method developed by Rick Lewis with Schlumberger the volume of clay (Vclsim) was determined and substituted for Vclgeochem and the mineralogy percentages from the ECS Log were recalculated. As with the first example total porosity (ɸtotal) was calculated using a variable matrix analysis, and volume of kerogen (Vke) was calculated using TOC determined from the Schmoker Equation.

When the volume of clay (Vclsim) derived from the simultaneous equation method was compared to the volume of clay (Vclgeochem) derived from the ECS Log there was general agreement in the volume of clay from both methods. The exception was an interval in the upper Avalon shale (7627ft to 7743ft) were the Vclsim was much greater than Vclgeochem. An examination of the Neutron-Lithodensity Log reveals a large increase in neutron porosity (ɸNls > 30%) which is why the simultaneous equation method yields much higher values for Vclsim over the interval from 7627ft to 7743ft.

The petrophysical interpretation problem is what is causing the Vclsim to be much greater than the Vclgeochem over this interval. Let us examine four possibilities: 1.) a increase in clay content, 2.) the presence of smectite, 3.) the presence of chlorite, or 4.) a change in organic maturity. The high ɸNls values are not the result of an increase in clay content, because the ECS Log indicates uniform to slightly less clay over this interval (7627ft to 7743FT). The high ɸNls values are not due to the presence of smectite (high clay bound water high ɸNls) due to the high resistivity (HRLT > 100ohm-m). The high ɸNls values are not due to the presence of chlorite (chlorite has high ɸNls values), because there is no increase in the Pe value and the ECS Log shows slight an decrease in iron content not an increase. When a Maturity Index (MI: Zhao and others, 2007) from well log data was calculated for both the upper and lower Avalon shales the MI for the lower Avalon was 5.5, and for the upper Avalon shale excluding interval 7627ft to 7743ft was 5.9 indicating Wet Gas+Oil maturity. However, over the interval from 7627ft to 7743ft the maturity index (MI) was 3.9 indicating only Oil maturity. A lower thermal maturity would result in a higher ɸNls value, because ɸNls decreases with increasing thermal maturity.

The author does not have an answer, but would like to suggest the following. An examination of the Spectral Gamma Ray Log revealed and increase in uranium over the interval from 7627ft to 7743ft. Therefore the interval from 7627ft to 7743ft may rich in bitumen which commonly has concentrations of uranium. Bitumen also has soluble water that would increase ɸNls. However the resistivity would be high because the water in the bitumen is combined and non-conductive. The possible presence of bitumen in this interval with the high ɸNls values due to the combined water may cause the Maturity Index (MI) Method to be invalid. Or maybe the thermal maturity is lower due to a change in the type of original organic matter [i.e. Type III instead of Types I or II] which may mature slower.