AAPG Middle East Region GTW, Regional Variations in Charge Systems and the Impact on Hydrocarbon Fluid Properties in Exploration

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Reservoir Overprinting (Leaching): An Overlooked Factor Controlling the Composition of Reservoired Oil


As oil migrates and accumulates in a reservoir it interacts with the rock components that it encounters. One widely overlooked effect of this is the dissolution by oil of molecular components present in syn-depositional sedimentary organic matter present on the migration path or in the reservoir. This process has been called migration contamination, reservoir overprinting, solvent stripping and solubilization, but here will be termed “leaching”. Reports of this process in the literature date back to at least the early 1980’s, and it has been documented in very many basins around the world. Despite this, many workers are unaware of how common leaching actually is, and the potential impact on their interpretations. Leaching results in a reservoired oil containing additional molecular components that are not inherited from the source rock and are thus potentially misleading when making interpretations of the source rock identity and maturity. It can be recognized by the presence of molecular components that are inconsistent with the original oil composition and is often revealed by maturity imbalances between different compound types. On examination of such an oil the apparent maturity of the biomarkers is often significantly lower than the aromatic hydrocarbons or the gasolines, due to the leached organic matter being generally less mature than the source rock of the oil, and typically containing biomarkers in high concentration relative to the oil. Leaching is much less easily recognized when the overprinting organic matter is mature, and it is these cases, or where leaching is relatively minor, that are the most dangerous for potentially misleading the interpreter. Leaching is particularly commonly recognized in Tertiary systems, where stacked interbedded reservoirs result in migrating and reservoired oil coming into contact with sedimentary organic matter in interbedded shales and/or coals. Nevertheless, the process is generic to all petroleum systems, and an example will be shown from the Chalk-reservoired fields of the Norwegian Central Graben. Leaching is also more common and/or apparent when dealing with high maturity oils or condensates that have much lower concentrations of biomarkers, which are consequently easily overprinted. The process must happen to some degree for most oils but is usually so minor as to be insignificant. However, very many basins seem to contain some oils where leaching is a more likely explanation for their apparent low maturity, and failure to recognize this factor risks significant mis-interpretations potentially feeding erroneous information into petroleum systems models. Under these circumstances it is important to identify characteristics of the oil composition that are unaltered by leaching; such characteristics often include the bulk fraction carbon isotopes and molecular composition of components that are naturally abundant in the oil, such as aromatic hydrocarbons and gasolines.