Hydrocarbon Generation, Migration, and Accumulation Related to Igneous Intrusions: An Atypical Petroleum System From the Neuquen Basin of Argentina

Abstract

Abstract The stratigraphic record of the Altiplanicie del Payún area, in the Northern Platform of the Neuquén Basin, Argentina, includes Tertiary sills and laccolites that reach thicknesses of up to 600 m. Occurrence of thermally immature oil-prone shales of the Late Jurassic Vaca Muerta and the Hauterivian Agrio Formations is profusely documented in the area. However, when sills have intruded the Vaca Muerta shales, source rock sections become mature, displaying a wide-ranging maturity spectrum spanning thicknesses that exceed 400 m. Commercial oil accumulations (20–33°API) and oil shows are found along the entire column, both in sandstone/carbonate reservoirs and fractured intrusives reservoirs. Oil-oil and oil-source rock correlations suggest a local generation related to the thermal effect of the intrusive bodies. Concurrently, diamondoid analyses point to mixtures of high-mature (cracked) with low-mature hydrocarbons: oils generated and cracked close to sills are considered to be mixed with oils generated from more distant source rock sections less affected by the thermal effect of the igneous bodies, as they migrate towards the sills and shallower reservoirs. A 2D model that included the thermal effect of the three main igneous bodies of the area was satisfactory achieved, accounting for thermal maturation, oil and gas generation, migration and accumulation. From the modeling, hydrocarbons migration is favored by elevated generation pressures, source rock fracturing and convective water flows. The integration of geochemical data and 2D-modeling of petroleum generation and migration related to the thermal effect of the igneous bodies led to the better understanding of this atypical petroleum system. Introduction In spite of the overall source rock immaturity, the lack of good-quality carriers that could favor mobilization of hydrocarbons from deep kitchens and the fairly recent trap formation related to sills emplacement, numerous commercial oil accumulations occur in Altiplanicie del Payún Area, in Northern Platform of the Neuquén Basin, Argentina. Tertiary sills and laccolites that reach thicknesses of up to 600 m and area extensions averaging 3.5 km2 intrude the stratigraphic. Most of the present-day structures in the area are the response to the deformation caused by the intrusion of these igneous bodies. Oil accumulations (20–33°API) are located along the entire column, both in fractured intrusive and sandstone/carbonate reservoirs. From these facts, the existence of a restricted generation process controlled by the thermal effect of intrusive bodies on close source sections becomes a feasible scenario. The integration of biomarker and bulk isotopic data on rock extracts and crude oils, supported by diamondoid analyses on selected oil samples, and 2D-modeling of petroleum generation and migration related to the thermal effect of the igneous bodies, led to the better understanding of this non conventional petroleum system. Geochemistry of oils and source rocks Occurrence of organic-rich shales of the Vaca Muerta and Agrio Fm. has been profusely documented in the area, with TOC values in the range 1–6 % and an average HI value of 500 mg HC/g COT. Both are largely immature to marginally mature in the ADP area (Ro 0.4–0.55%). However, when sills have intruded Vaca Muerta Fm., source rock sections show a full spectrum maturity range over thicknesses of more than 400 m in the surroundings of the igneous rocks (figure 2).