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 C_org 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 φ_hc is calculated as

where ρ_ma is the matrix density measured by Litho Scanner service, φ_T is the total porosity from another log, X_hc is the hydrocarbon fraction of oil, and ρ_hc 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 C_org 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.