Quantifying Total, Mobile, and Non-Productive Oil: A Geochemical Method for Improved Perforation Selection and EOR Optimization

Abstract

High hydrocarbon saturation zones identified from petrophysical data are commonly perforated and tested; however, they often produce little or no oil, presenting a persistent challenge in reservoir evaluation. Original Oil in Place (OOIP) is typically estimated using conventional core analysis and geochemical methods, including programmed non-isothermal pyrolysis. These estimates are frequently revised throughout field development as actual production deviates from predictions. Low-temperature hydrous pyrolysis (LTHP), introduced by Lewan and Sonnenfeld (2017), offers an improved approach for quantifying mobile oil saturation, thereby enhancing reserve estimation and production forecasting. In this technique, crushed mature source-rock samples are heated isothermally in the presence of liquid water at 300°C for 12–24 hours. Under these conditions, retained oil is released through thermal expansion and water-driven displacement, aided by the immiscibility between oil and polar-rich bitumen. Importantly, the process does not generate new hydrocarbons from kerogen or bitumen. Originally developed for mature Niobrara cores, LTHP has demonstrated that the released hydrocarbons closely resemble produced oil. The method has since been extended to tight-oil reservoirs, including non-source rocks, to evaluate both the quantity and quality of retained oil. The recovered oil volumes are generally sufficient for detailed characterization, including determination of mobile oil saturation, API gravity, and compositional analysis using gas chromatography and gas chromatography–mass spectrometry. The evaluated well shows high hydrocarbon saturations within predominantly carbonate reservoir intervals based on log interpretations. However, routine core analyses indicate that these hydrocarbons are largely present as tar or bitumen, with a possible mobile oil fraction suggested by elevated S1 values from programmed pyrolysis. Despite these indications of significant hydrocarbon saturation, stimulation of these intervals resulted in poor oil recovery. The LTHP workflow was applied to core samples to quantify: (1) vaporizable oil, (2) mobile and immobile oil, (3) total oil, and (4) Enhanced Oil Recovery (EOR) oil. In addition, High-Resolution Gas Chromatography (HRGC) fingerprints of the LTHP-derived oils were analyzed to determine key geochemical characteristics of the mobile oil and to evaluate potential secondary alteration processes, such as water washing and biodegradation. Results indicate that mobile oil constitutes only a minor portion of the in-situ total oil (approximately 28% in high-saturation zones). Vaporizable oil, however, significantly overestimates the mobile fraction, likely due to the vaporization of organic-bound immobile oil associated with tar/bitumen. Intervals with higher total and mobile oil content are generally associated with non-dolomitic zones. HRGC analysis shows depletion of light hydrocarbons but preservation of well-defined n-alkane distributions and diagnostic molecular ratios, consistent with peak to late oil window maturity and a marine carbonate or shale source. While LTHP generates a recoverable petroleum phase, it is not intended to replace conventional core analysis or programmed pyrolysis. Instead, it serves as a complementary method that enhances the reliability of these established, cost-effective techniques.