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AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Diamondoids as molecular markers for coupled oil cracking and expulsion evaluation in unconventional petroleum systems

Abstract

Diamondoids are a group of compounds with chemical structures similar to the diamond crystal lattice. Thanks to the recalcitrant nature, diamondoids are capable of withstanding thermal maturity to a significantly high degree compared to biomarkers. Concentration variation of diamondoids in oil may thus be used as a molecular marker for oil cracking for a series of natural samples with different maturity levels. However, the use of this marker in Shale oil reservoirs faces new challenges, as oil cracking and oil expulsion are closely coupled to each other. The diamondoids are also subject to generation and decomposition in the petroleum system through time. In this study, our major objective is to explore the use of diamondoids as molecular markers for oil expulsion and cracking in the unconventional petroleum systems. We propose a model to quantify diamondoids concentration variations due to the variation of oil compositions with maturity (for example, percentage of saturates and asphaltenes). We then collect natural samples through the vertical and horizontal drilling process to analyze the diamondoids concentrations. The major difference between modeled and measured diamondoids concentrations in oil should mainly be correlated to expulsion efficiency in natural systems. In addition to drilling samples, a set of production oil samples were analyzed to quantify the diamondoids. The oils represented a maturity gradient as expressed by their API gravity and colors. In addition to isomer ratios of diamondoids, the concentrations would be used to compare with those from drilling samples. The difference between the two types of samples may indicate the producibility of oil. The results may eventually be applied to real-world wells to guide the most economic zones.