Application of Raman Spectroscopy for Type II Kerogen Maturity Determination in Unconventional Reservoirs

Abstract

Abstract Oil and gas production from unconventional resources has become increasingly important in many locations worldwide and is seeing increased exploration and appraisal efforts in the Middle East. Complexity and high cost of unconventional operations necessitate robust characterization of potential prospects to facilitate profitable exploitation of the resources. Kerogen maturity and its lateral variations are fundamental reservoir quality (RQ) parameters to evaluate when appraising a new area in an unconventional project. Understanding the type and maturity of organic-rich rocks facilitates hydrocarbon typing (identifying oil, condensate or dry gas in the resource), thus allowing to estimate the profitability of the play. The novel application of Raman spectroscopy presented here allows rapid maturity determination and early hydrocarbon typing at the wellsite, shortening decision-making processes in the development of unconventional resources. Comparison with traditional maturity techniques revealed that the accuracy of the Raman method is congruent, while measurements are faster and easier to apply at the wellsite. Furthermore, the non-destructive character of the method makes it feasible to efficiently combine this technique with other workflows to increase effectiveness. Conventionally, thermal maturity is derived from direct, optical reflectance measurements on vitrinite or solid bitumen requiring a specialized microscope and an experienced organic petrologist. Other techniques, such as Rock-Eval pyrolysis, are destructive and based on correlations that can be unreliable in rocks with low organic carbon content or very mature formations. Thermal maturity, derived from Raman spectroscopy of unknown formation samples, can be expressed as vitrinite reflectance equivalent (VRE, %). In contrast to other techniques that require polishing of the sample surfaces or crushing of the samples in addition to excessive solvent cleaning, the technique developed here was applied directly on core chips or drill cuttings with minimal sample preparation. The method has shown significant improvements in operating time (maturity available in minutes, directly at the wellsite instead of days after sampling in a laboratory), complexity reduction (measurements executed directly on drill cuttings instead of pulverized and cleaned core material) and cost saving (quick and easy spectroscopic technique instead of complex pyrolytic setup or microscopy which requires multiple man hours for proper characterization). This novel methodology has been calibrated for different kerogen types against thermal maturity values carefully measured by classical organic petrography. Robust correlations developed allow for determination of vitrinite reflectance equivalent with average deviations below ± 0.2 %VRo. The methodology was applied in basin scale multi-well projects, executed in several countries, and tested in well-known formations such as Qusaibah, Vaca Muerta and the sulfur-rich Najmah and Makhul. Creating reliable knowledge of hydrocarbon typing in the resource supports faster decision making considering the sweet spot for shale gas production, field development strategy and definition of landing zones.