Molecular diamonds, known as diamondoids are naturally occurring hydrocarbon compounds found in crude-oil and rock extracts. Diamondoids feature unique thermal stability owing to the carbon cage-like backbone structures. Therefore, unlike classical geochemical compounds, diamondoids provide a robust molecular-marker for fingerprinting hydrocarbon fluids and assessing thermal maturity. In this study, we present the application of quantitative extended diamondoid analysis (QEDA), together with compound-specific isotope analysis of diamondoid hydrocarbons (CSIA-D) recovered from the major USA unconventional reservoirs.

The application of diamondoids is demonstrated for three major unconventional reservoirs of North America. A collection of crude oils, condensates, and rock-extracts were examined from Shale formations including the Eagle Ford and Woodford Shales. Core and fluid samples are located within three major US basins including Maverick, Permian and Anadarko basins. Hydrocarbons were analyzed using gas chromatography-mass spectrometry analytical method for measuring the absolute concentrations of diamondoid and biomarker compounds. Further, diamondoid-isolates were analyzed using isotope ratio mass spectrometry equipped with gas chromatography for CSIA-D.

Diamondoids composition provides a unique geochemical tool for the unconventional reservoir, in detecting petroleum mixing and fingerprinting fluids of various maturities. In the Anadarko Basin, the Woodford Shale produced oils in central Oklahoma show a dual signature of elevated diamondoid abundance together with the biomarkers, suggesting hydrocarbon mixing of black-oils and cracked dry and wet gas. Moreover, Eagle Ford fluids produced from the Maverick Basin are mature hydrocarbons lacking biomarkers to allocate their zone of production. Diamondoid fingerprints allowed for tracking their production through time and identifying the sweet-spot depth in which most hydrocarbons were contributing. In the Permian Basin, diamondoid fingerprints were used to establish unmixed end-members from oils produced from the Woodford Shale, Eilenberg Carbonate and Pennsylvanian shale, and identify highly mature mixed oil commingled between the Woodford Shale and Ellenburger carbonate source rocks.

Identification of the sweet-spots and allocating produced hydrocarbons are crucial factors in the successes of unconventional plays. Most produced fluids in major US unconventional reservoirs are at high levels of thermal maturity, at which classical biomarkers are either absent or in very low concentrations; whereas, diamondoids are abundant, providing a novel technique to fingerprint fluids throughout the range of different maturity stages. In unconventional reservoir operation, diamondoids can be used in time-lapse geochemistry as a cost-effective tool along with other seismic-based techniques.

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