Compositional fractionation of shale oil during migration and production: Identifying primary reservoirs and quantifying their contributions in an intercalated unconventional shale play

Abstract

Chemometrics analysis integrated with biological markers and stable isotopic data offers an effective approach for unravelling crude oil mixing in complex multilayered unconventional plays. In this study, we investigated petroleum fractionation during primary migration and later production by comparing biomarker compositions and stable carbon isotope signatures of extractable organic matter (EOM) and produced crude oils from five shale–sandstone interbeds within a multilayered, organic-and liquid-rich shale system. Biomarkers and isotopic parameters showing minimal fractionation were selected for chemometrics analysis to explore genetic relationships between produced oils and EOM from individual reservoirs and to quantify their relative contributions over four months of production. The results show that hydrocarbon fractionation primarily occurs during oil migration and production from shale to sandstone, leading to an increase in the relative abundance of saturate and aromatic fractions compared to their relative proportions in the source material. Stable carbon isotopic ratios exhibit negligible variation during upward vertical migration but progressively become lighter during lateral migration in horizontal wells. Although the concentrations of tricyclic terpanes, hopanes and steranes were notably varied relative to the saturate fraction, the relative distributions of individual homologues remained largely unchanged, which justifies the applicability of ratio-based chemometric approaches. In contrast, n-alkanes, alkylcyclohexane and naphthalenes display clear fractionation, with enrichment of lighter-molecular-weight components. Further comparison of biomarkers revealed that long-distance lateral flow in horizontal wells enhances compositional fractionation compared to vertical wells. Chemometric results clearly indicated that the sandstone layers are the primary reservoir, which contributes more than 50% to the produced oil, with a gradual increase in quantities during the subsequent four months of production. Conversely, following fracturing of sandstone layers in horizontal wells, oil contributions from the adjacent shale layers are limited in volume and unsteady. The integrated chemometric framework presented here offers a reliable tool for effectively resolving oil mixing from multiple sources in hydraulically fractured complex unconventional shale plays. These insights can support more informed decisions related to future drilling strategies, fracturing design, and overall field development.