Dataset Information

Molecular Fractionation in the Organic Materials of Source Rocks.

ABSTRACT: Hydrocarbons that are transported in a hierarchal path from the nanoporous constituents of a shale matrix to natural and then hydraulic fractures are subject to continuous fractionation during the journey. The organic nanopores of a source rock matrix known as kerogen have pore sizes on the angstrom scale. At that degree of confinement, pores can act as a selective membrane, preferentially maintaining some components over the others in a continuous fractionation phenomenon that alters the adsorption/desorption isotherm. Several studies have considered the adsorption/desorption behavior of kerogen on the basis of a single component. In reality, methane is associated with other hydrocarbons, making that assumption questionable. The present work investigates the multicomponent gas sorption of kerogen structures via a molecular computational approach. The continuous fractionation results in the accumulation of heavier components. The compositional changes alter the phase behavior, enlarging the anticipated two-phase regime. Additionally, the ability of molecules to diffuse from kerogen was also found to be affected by the fractionation effect. These microscale effects provide some insights into the potential factors that influence the productivity at the reservoir scale.

SUBMITTER: Alafnan S

PROVIDER: S-EPMC7408196 | biostudies-literature | 2020 Aug

REPOSITORIES: biostudies-literature

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Molecular Fractionation in the Organic Materials of Source Rocks.

Alafnan Saad S Sultan Abdullah S AS Aljaberi Jaber J

ACS omega 20200724 30

Hydrocarbons that are transported in a hierarchal path from the nanoporous constituents of a shale matrix to natural and then hydraulic fractures are subject to continuous fractionation during the journey. The organic nanopores of a source rock matrix known as kerogen have pore sizes on the angstrom scale. At that degree of confinement, pores can act as a selective membrane, preferentially maintaining some components over the others in a continuous fractionation phenomenon that alters the adsorp ...[more]

PMID: 32775898

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Project description:The subduction system in southern Patagonia provides direct evidence for the variability of the position of an active continental arc with respect to the subducting plate through time, but the consequences on the arc magmatic record are less well studied. Here we present a geochemical and geochronological study on small plutons and dykes from the upper crust of the southern Patagonian Andes at ~?51°S, which formed as a result of the subduction of the Nazca and Antarctic plates beneath the South American continent. In situ U-Pb geochronology on zircons and bulk rock geochemical data of plutonic and dyke rocks are used to constrain the magmatic evolution of the retro-arc over the last 30 Ma. We demonstrate that these combined U-Pb and geochemical data for magmatic rocks track the temporal and spatial migration of the active arc, and associated retro-arc magmatism. Our dataset indicates that the rear-arc area is characterized by small volumes of alkaline basaltic magmas at 29-30 Ma that are characterized by low La/Nb and Th/Nb ratios with negligible arc signatures. Subsequent progressive eastward migration of the active arc culminated with the emplacement of calc-alkaline plutons and dikes?~?17-16 Ma with elevated La/Nb and Th/Nb ratios and typical subduction signatures constraining the easternmost position of the southern Patagonian batholith at that time. Geochemical data on the post-16 Ma igneous rocks including the Torres del Paine laccolith indicate an evolution to transitional K-rich calc-alkaline magmatism at 12.5?±?0.2 Ma. We show that trace element ratios such as Nb/Ta and Dy/Yb systematically decrease with increasing SiO2, for both the 17-16 Ma calc-alkaline and the 12-13 Ma K-rich transitional magmatism. In contrast, Th/Nb and La/Nb monitor the changes in the source composition of these magmas. We suggest that the transition from the common calc-alkaline to K-rich transitional magmatism involves a change in the source component, while the trace element ratios, such as Nb/Ta and Dy/Yb, of derivative higher silica content liquids are controlled by similar fractionating mineral assemblages. Analysis of a global compilation of Nb/Ta ratios of arc magmatic rocks and simple geochemical models indicate that amphibole and variable amounts of biotite exert a major control on the low Dy/Yb and Nb/Ta of derivative granitic liquids. Lastly, we suggest that the low Nb/Ta ratio of silica-rich magmas is a natural consequence of biotite fractionation and that alternative models such as amphibolite melting in subduction zones and diffusive fractionation are not required to explain the Nb/Ta ratio of the upper continental crust.

Dataset Information

Molecular Fractionation in the Organic Materials of Source Rocks.

Publications

Molecular Fractionation in the Organic Materials of Source Rocks.

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