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Pay Allocation and Reservoir Depletion Analysis Through Geochemical Technologies


Understanding the relationship between source rocks and reservoirs in petroleum systems is a longstanding need for those in the oil and gas industry. As industry professionals continue to expand drilling in systems with stacked play, the need to understand the origin of the produced hydrocarbons increases, especially in unconventional systems. Geochemical tools offer needed perspective into this through many methods including play allocation and depletion studies. Pay allocation uses specified ratios of uncommon GC peaks in conjunction with common peaks (e.g., alkanes, toluene, etc.) to determine what percentage of a mixed origin sample comes from each of two or more end member oils. In essence, end members have chemical signatures that are unique to them, providing a “fingerprint” which can be quantified using GC peak ratios. By comparing the chemical fingerprints from mixed origin samples to pristine end members, we can determine the percentage of a given sample originating from each source. Depletion evaluation allows for the determination of how hydrocarbon within a given petroleum system will alter over space and time. The principles of depletion monitoring are based on the gas and volatile fractions responding to fractionation. The depletion trend will respond to the relative mobility of hydrocarbon molecular fractions and isotopes, as well as inorganic components. As a result, there will be a more mobile (volatile) phase that is preferentially produced up the well bore, leaving a less mobile (residual) phase in the reservoir. In the depleted reservoir, the gas phase will be compositionally wetter and isotopically heavier. Depletion and pay allocation parameters can be introduced into 3D space and interpreted with other geochemical, structural, and stratigraphic parameters to bring new perspective to the overall petroleum system. This can be utilized to optimize existing production as well as provide insight for future development of assets. By utilizing tools to understand depletion and pay allocation, companies will be better equipped to target future wells in more advantageous formations, understand the evolution of wells within fields, and avoid placing wells in depleted compartments.