Microscopic mechanisms of intra-source micro-migration and enrichment of lacustrine shale oil: A case study of Chang73 in the Ordos Basin, NW China

WANG Yingzhu, HOU Yuting, YANG Jijin

doi:10.11698/PED.20240297

0 20251003 - 20251003

DOI: https://doi.org/10.11698/PED.20240297

Microscopic mechanisms of intra-source micro-migration and enrichment of lacustrine shale oil: A case study of Chang7₃ in the Ordos Basin, NW China

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1. Key Laboratory of Deep Petroleum Intelligent Exploration and Development, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China;
3. PetroChina Changqing Oilfield Company, Xi'an 710018

Received date: 2024-05-06

Revised date: 2025-07-18

Online published: 2025-08-26

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Abstract

Lacustrine shale oil in China exhibits a huge resource potential but a highly heterogeneous distribution. Deciphering its intra-source micro-migration and enrichment mechanisms is crucial for accurately predicting geological sweet spots. Taking the Chang7₃ submember of the Yanchang Formation in the Ordos Basin as an example, we integrated high-resolution scanning electron microscopy (SEM), optical microscopy, laser Raman spectroscopy, rock pyrolysis, and organic solvent extraction experiments to identify solid bitumen of varying origins, obtain direct evidence of intra-source micro-migration of shale oil, and establish the coupling between the shale nano/micro-fabric and the oil generation, micro-migration and accumulation. The results show that the Chang7₃ shale with rich alginite in laminae has the highest hydrocarbon generation potential but a low thermal transformation ratio. Frequent alternations of micron-scale argillaceous-felsic laminae enhance expulsion efficiency, yielding consistent aromaticity between in-situ and migrated solid bitumen. Argillaceous laminae rich in terrestrial organic matter (OM) and clay minerals exhibit lower hydrocarbon generation threshold but stronger hydrocarbon retention capacity, with a certain amount of light oil/bitumen preserved to differentiate the chemical structure of in-situ versus migrated bitumen. Tuffaceous and sandy laminae contain abundant felsic minerals and migrated solid bitumen. Tuffaceous laminae develop high-angle microfractures under shale overpressure, facilitating oil charging into rigid mineral intergranular pores of sandy laminae. Fractionation during micro-migration progressively decreases the aromaticity of solid bitumen from shale, through tuffaceous and argillaceous, to sandy laminae, while increasing light hydrocarbon components and enhancing OM-hosted pore development. The intra-source micro-migration and enrichment of the Chang7₃shale oil result from synergistic organic-inorganic diagenesis, with compositional fractionation being a key mechanism for forming laminated sweet spots.

Key words： lacustrine shale oil; solid bitumen; intra-source micro-migration; enrichment mechanism; Chang7₃; Ordos Basin

Cite this article

WANG Yingzhu, HOU Yuting, YANG Jijin . Microscopic mechanisms of intra-source micro-migration and enrichment of lacustrine shale oil: A case study of Chang7₃ in the Ordos Basin, NW China[J]. Petroleum Exploration and Development, 0 : 20251003 -20251003 . DOI: 10.11698/PED.20240297

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