Use of Elemental Capture Spectroscopy to Accurately Determine Reservoir Lithology and Porosity in High Pressure and High H₂S Environment

The lithology of a producing gas reservoir in an area of the Sichuan basin in southwest China is a complex carbonate. Core analysis has revealed a strong correlation between dolomitization and reservoir quality, the process of dolomitization increases reservoir porosity and permeability, and hence overall reservoir rock quality. As such dolomite content is a key parameter in the classification of reservoir rock quality.

Historically, in carbonate reservoirs the estimation of dolomite content from wellbore geophysical data has been via neutron-density cross-plots. The technique was successful when reservoir lithology was relatively simple, such as a combination of limestone and dolomite, and the reservoir fluid was either oil or water. The presence of gas can shift both the neutron porosity and formation density measurements such that a dolomite lithology could be misinterpreted as limestone. However, the photo-electric absorption measurement acquired with modern formation density logs is strongly dependent on lithology and only minimally affected by reservoir fluid. Hence from the combination of neutron porosity, formation density and photo-electric absorption one can resolve simple carbonate lithology even in the presence of gas.

The aforementioned gas reservoir in the Sichuan basin has both high reservoir pressures and high concentrations of H₂S, hence to maintain a safe drilling environment the drilling fluid is weighted with barite. The addition of barite makes photo-electric absorption measurements meaningless for petrophysical evaluation.

This paper describes a new technique to accurately describe complex reservoir lithology from elemental capture spectroscopy. The technique is independent of photo-electric absorption and includes elemental measurements of magnesium, calcite, quartz and sulfur. Additionally formation matrix density is derived from the measured elements, which dramatically improves the evaluation of formation porosity. A field example is shown where excellent agreement with core is achieved.