Petroleum transformations in reservoirs

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Abstract The influence of post-pooling transformation processes on the composition of crude oil is often more important than that of source facies and transformations resulting from migration. The principal processes that alter hydrocarbons in the reservoir are maturation (thermal-time alteration), gas deasphalting, biodegradation and water-washing. Oils located at or near the surface may also be affected by inspissation (evaporation) and chemical oxidation. In the interest of predicting at an early stage in a new exploration play the quantity and quality of petroleum to be found, the exploration geochemist should be able to interpret correctly the nature of crude oil and natural gas composition in accordance with all these geochemical controls. The most important is thermal alteration, and a temperature envelope for oil generation of approximately 60–175°C is often quoted although, strictly speaking, according to chemical reaction kinetics theory generation and phase-out thresholds solely related to temperature do not exist. Still the most difficult geochemical subject to rationalize with respect to the nature of petroleum is the method and staging of migration, but uncertainties can be avoided to some extent by going back to the source rocks themselves and using the composition of the organic matter that may be extracted from them to make predictions about hydrocarbons in the reservoir. As more examples of compositional changes in petroleum are documented, it becomes increasingly easy to define reliable parameters that describe the petroleum evolutionary process. The most rarely seen and least understood composition with this process is that for pooled oils that remain relatively unaltered; in other words, that have retained most of their “original” or “immature” characteristics. Several examples are described that may improve our understanding of such oils, especially where the problem is one of distinguishing between “immaturity” and degradation. Natural gases behave differently from oils because they are far more mobile in the subsurface, and may be generated from a wider range of sources over a wide range of thermal conditions. There may be a phase-out depth for methane in the deeper parts of sedimentary basins where organic matter is low in hydrogen, and the presence of oxidizers such as sulphur could destroy light hydrocarbons. Natural gases may form independently of oils, are among the end products of the alteration of oils, and can themselves be alteration agents. The “wetness” of hydrocarbon gases can be used as a maturation index.