AAPG Geoscience Technology Workshop

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Oil Below “Oil Window”: Deep Mechanism of Hydrocarbon Field Formation

Abstract

Large technological progress enabled recent deep and ultra-deep discoveries. The new subsurface experience goes beyond the conventional concepts of petroleum geology. So, the underlying principles should be revised to cover deep and ultra-deep reservoirs. This research is a product of critical analysis of the petroleum geology issues addressing the hydrocarbon generation, migration and accumulation. It integrates new conceptual ideas with the advanced phase behaviour simulation of hydrocarbon fluids at large depths. The mechanism suggested explains how oil can be accumulated in deep and ultra-deep reservoirs below the commonly known “oil window”. During the burial of sediments, secondary destruction occurs at large depths, that is liquid hydrocarbons generated from kerogen at the first stage of its destruction are converted into gas. A build-up of gas causes abnormally high pore pressure, which leads to dissolution of the heavy petroleum hydrocarbons in gas and microfracturing of rock. Under these conditions, a given mass of high-pressure gas can evaporate and hold about an equal mass of heavy hydrocarbons typical of oil. This gas saturated with vaporized “oil” has a high mobility and escapes from the source rock through a system of microfractures driven by its own pressure to further migrate upwards. Our main statement is that “oil” being captured from a deep source rock by high-pressure gas migrates with it in a vaporized form. This fluid can be considered as single-phase rich gas or supercritical fluid. When the fluid flow reaches shallower reservoirs, at lower pressure and temperature it becomes over-saturated with heavy hydrocarbons, which condense into a liquid phase to form oil. Once condensed, oil accumulates and forms an oil rim below a gas cap. The suggested mechanism of hydrocarbon field formation has been successfully tested on the Urengoy field in West Siberia. Core samples from deep wells SG-6 and SG-7 showed the evidence of microfracturing in the depth interval of 5100-5685 m. Composition of the deepest fluid sample used in our PVT calculation was sufficient to simulate the composition and properties of the fluids accumulated at shallower depths and explain the depth distribution of oil, gas condensate and gas within the depth range from 4 km to 1 km. There are reasons to believe that capturing of heavy hydrocarbons from a source rock by high-pressure gas and subsequent condensation are the most important processes behind the migration and accumulation of oil in a sedimentary basin. The condensation mechanism gives a better understanding of the conditions required for the formation of oil fields. The first prerequisite is a source rock buried deep enough to enable generation of the secondary destruction gas (~5 -7 km or more). The second prerequisite is a build-up of sufficiently high pressure to cause rock microfracturing and enable vaporization of large amounts of heavy hydrocarbons. Thus, the research suggests an alternative natural explanation of the hydrocarbon migration and accumulation at both large and shallower depths. It helps better understand the distribution of deep and ultra-deep oil and gas resources and improves the theoretical basis for future discoveries.