Case Study: LWD Lithology Determination Improves Pay Evaluation

Capture spectroscopy provides accurate calcite and dolomite volumes in complex carbonate formation

Challenge: Evaluate porosity and hydrocarbons in complex carbonate formation.

Solution: Use improved capture spectroscopy measurement from EcoScope multifunction LWD service to distinguish calcite from dolomite.

Result: Determined volumetric lithology accurately, independent of borehole condition. Enhanced formation porosity and hydrocarbon evaluation. Improved pay zone identification and evaluation.

Evaluate complex carbonates

Accurate determination of formation lithology and grain density was critical for porosity and hydrocarbon evaluation in a complex carbonate formation. Standard capture spectroscopy could not provide the desired accuracy because it was unable to distinguish calcite from dolomite. A photoelectric factor (PEF) measurement could distinguish calcite from dolomite, but the shallowness of the PEF measurement made it sensitive to borehole conditions, mud weighting material, and invasion. An inaccurate lithology determination due to this sensitivity would result in inaccurate calculation of porosity and hydrocarbons in place.

Accurately determine lithology

The EcoScope multifunction LWD service provided the accurate lithology determination needed—with an improved capture spectroscopy measurement that could distinguish calcite from dolomite. This measurement—unlike the standard capture spectroscopy measurement—was sensitive to the proportion of magnesium (Mg) in the formation. Because the replacement of calcium (Ca) by Mg converts calcite to dolomite, the quantification of the Mg in the formation permitted the two minerals to be distinguished.

Improve pay identification

Comparison of the EcoScope lithology determination with thin section analysis of a core taken across the complex carbonate interval showed good agreement. The EcoScope service was more accurate than standard capture spectroscopy because it could distinguish calcite from dolomite—and more accurate than PEF measurement because it was relatively insensitive to borehole conditions. As a result, the EcoScope service enhanced formation porosity and hydrocarbon evaluation, and improved pay zone identification and evaluation.

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