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Diamondoids in Oil-Inclusions: A Novel Technique to Unravel Petroleum Mixing in the Anadarko Basin


The Anadarko Basin in Oklahoma is one of the most prolific petroleum provinces in the United States, with a number of organic-rich source rocks within the oil and gas window. Recent discoveries of unconventional liquid-rich targets such as STACK and SCOOP have sparked interest in understanding the hydrocarbon charge aspect in these tight reservoirs. While these emerging plays in Oklahoma have substantially increased proven-oil reserves, major questions remain with respect to variations in produced oil compositions and GOR together with unusual thermal maturity distributions. Until very recently it was impossible to confidently correlate high-maturity fluids to their source rocks or to other high-maturity fluids. However, it has been shown that these correlations can be accurately and confidently made using the distributions of large diamondoids, an analytical technique termed QEDA (Quantitative Extended Diamondoid Analysis). This is the first study which recognizes higher diamondoids in fluid inclusions. Oil-bearing fluid inclusions are observed to occur in STACK play cores located in Canadian, Blaine, and Kingfisher Counties. Oil-inclusions were extracted with organic solvents then analyzed along with crude-oil samples using Gas Chromatography-Flame Ionization Detector (GCFID), Gas Chromatography-Mass Spectrometry (GC-MS), and with Gas Chromatography-Triple Quadrupole Mass Spectrometry (GC-MSQQQ). All samples were spiked with deuterated higher diamondoid standards to arrive at precise concentration values. Results suggest that oil-inclusions obtained from Canadian County are the most mature. They contain abundant diamondoids and lack biomarkers. QEDA allowed for fingerprinting such mature oil-inclusions and correlation with black crude-oils. The results suggest that the oil-inclusions are a mixture containing both Mississippian and Woodford source-rock contributions. Moreover, other oil-inclusions located in Kingfisher and Blaine counties bear a strong sterane and terpane Woodford fingerprint. Maturity sensitive ratios such as triaromatic steroids, methyl phenanthrene index and microthermometry analysis indicates different episodes of petroleum charges, with low maturity 06-0.8% VRE, and low homogenization temperatures (78°C), and a high maturity hydrocarbon 1.2-1.4% VRE charge associated with high diamondoid concentrations and high homogenization temperatures (148 °C). This study introduces an entirely new geochemical field, i.e., the correlation of high-temperature fluid inclusions to their source rocks using QEDA. We believe this new methodology will have widespread applications.