Cracking the Code: High Oil Saturation Without Oil Production? A Game-Changing Novel Geochemical Methodology for Assessing Total, Mobile/Immobile and EOR Oil; A Case Study from Abu Dhabi, UAE
Authors: Koksalan T. , Khan K. A. , Alhadhrami R. , Salahuddin A. , Warsito E. , Ruble T. 
Published: November 2024

It is a common yet perplexing phenomenon in the oil industry that high-saturation zones, as indicated by petrophysical measurements, are frequently perforated and tested, yet they often yield negligible or no oil flow, which can be quite puzzling and disappointing. Additionally, the original oil in place (OOIP) is typically estimated through conventional core analysis techniques and geochemical tests, such as programmed non-isothermal pyrolysis. However, these estimates are often revised over the lifecycle of the field as the actual OOIP values are found to deviate from the predicted production profile. A relatively new method, low-temperature hydrous pyrolysis (LTHP), was introduced by Lewan and Sonnenfeld (2017) to improve reserve estimates and production forecasts via more accurate quantification of mobile oil saturation. As they described for mature Niobrara cores, LTHP releases retained oil that is similar to produced oil. LTHP is a technique in which crushed mature source-rock core is isothermally heated in contact with liquid water at 300°C for 12 to 24 hours. These conditions, allow retained oil to be released from the mature rock by thermal expansion and water displacement of the oil and its dramatic difference in miscibility with polar-rich bitumen in the presence of water, but are not sufficient to generate oil from the bitumen or kerogen within mature source rocks. The method has also been extended to determine quality and quantities of retained oil in non-source rock tight-oil reservoirs. The quantities of released oil typically are sufficient to determine mobile oil, API gravity, gas chromatography, and gas chromatography/mass spectrometry signatures.

The examined well exhibits signs of substantial hydrocarbon saturations in primarily carbonate lithology reservoir sections, as determined by log evaluations. Routine core analyses show hydrocarbon saturations that appear to be tar/bitumen but with a potentially mobile oil component as indicated by elevated S1 yields from programmed pyrolysis results. Subsequent stimulation of these reservoir intervals resulted in low oil recovery despite the indications of good hydrocarbon saturation.

Consequently, the LTHP workflow was implemented on core samples to quantify (1) "vaporizable oil", (2) "mobile/immobile oil", (3) "total oil" and (4) "Enhanced Oil Recovery (EOR) oil". Additionally, the High-Resolution Gas Chromatography (HRGC) fingerprints of the LTHP oils from these core samples were evaluated for standard geochemical parameters to evaluate the probable characteristics of the mobile oil and assess any secondary alteration via processes like water washing and/or biodegradation.

The results show that mobile oil represents only a minor fraction (28% in zones of high saturation) of the in-situ total oil and that vaporizable oil significantly overestimates the mobile oil fraction likely due to vaporization of organic bound immobile oil associated with tar/bitumen. Zones of elevated total and mobile oil appear to be in intervals free of dolomite. HRGC fingerprints of mobile oil show loss of volatile compounds but retain intact n-alkane envelopes and molecular ratios that are consistent with a peak to late oil window thermal maturity and a likely marine carbonate or marine shale source. LTHP, which releases a true and collectable petroleum phase, is not intended to replace conventional core analyses or programmed pyrolysis, but rather supplement these better known, inexpensive and more rapid analyses.