Case Study: Litho Scanner Spectroscopy Quantifies Oil Volume in Low-Resisitivity Chert

High-definition spectroscopy service accounts for inorganic carbon in determining organic carbon for the calculation of oil volume

Challenge: Accurately calculate the oil content for a low-resistivity chert reservoir where the conventional Archie parameters are not confidently known.

Solution: Directly measure the elemental concentration of total carbon with Litho Scanner high-definition spectroscopy service, from which the amount of inorganic carbon associated with carbonate minerals is subtracted to determine the organic carbon (Corg) and in turn the oil volume without having to rely on the subdued resistivity logging response from the chert reservoir.

Result: Quantified the oil content based on the oil volume from the Litho Scanner service’s Corg determination.

Oil content obscured by low-resistivity reservoir

An operator needed accurate, quantified measurements instead of estimated values to calculate the oil volume in a play characterized by complex and spatially variable chert reservoirs intercalated with limestone formations with a wide range of depositional, structural, and diagenetic histories. The limestones and particularly the cherts are highly varied in their alteration and petrophysical character, ranging from dense, tight, and highly fractured to high-porosity tripolitic cherts characterized by microporosity. The high porosity coupled with high water saturation depresses the resistivity response, so using the conventional Archie equation in determining the oil saturation leads to significant underestimation of the hydrocarbon content.

Elemental composition quantified with dual spectroscopy measurements

Litho Scanner high-definition spectroscopy service combines a high-output pulsed neutron generator and unique cerium-doped lanthanum bromide gamma ray detector to directly measure both inelastic and capture gamma ray spectra and quantify an expanded set of elemental weight fractions from previous-generation spectroscopy tools.

These direct measurements provide a new approach instead of the conventional carbon to oxygen ratio for estimating hydrocarbon saturation. The Corg  measurement from Litho Scanner service is combined with a porosity log to compute the oil volume without requiring input of the formation water resistivity or Archie saturation exponent or cementation factor. Thus, the resulting saturation is independent of clay or lithology effects and does not require a calibration database. The oil volume φhc is calculated as 

Litho Scanner oil volume

where ρma is the matrix density measured by Litho Scanner service, φT is the total porosity from another log, Xhc is the hydrocarbon fraction of oil, and ρhc is the hydrocarbon density.

Oil volume accurately calculated, independently of resistivity parameters

The dry-weight elemental composition measured by Litho Scanner service and the resulting determination of Corg were used to accurately calculate the oil volume. The resulting quantification of the oil saturation avoids the underestimation that would have resulted using a conventional resistivity-based calculation.

Download: Litho Scanner Spectroscopy Quantifies Oil Volume in Low-Resisitivity Chert (2.52 MB PDF)

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