Why abundant fractures do not necessarily lead to shale oil enrichment

Bedding-parallel fractures within shale form migration pathways and storage space for hydrocarbons. However, the hydrocarbon saturation in such fractures shows marked variability, for reasons that remain poorly understood. Numerous bedding-parallel fractures occur within shales of the Lucaogou Formation in the Jimusar Sag, Junggar Basin, northwestern China, which exhibit pronounced heterogeneity in hydrocarbon saturation. In this study, we investigated the factors causing this heterogeneity, based on fluid inclusion analyses, in situ U-Pb ages of fracture-filling calcite cements, and extractable organic matter, total organic C (TOC) contents, and petrophysical properties (porosity and permeability) of the host shales. The results suggest that the bedding-parallel fractures formed mainly during 58.5–11.6 Ma, coinciding with the period of hydrocarbon generation in the Lucaogou Formation, which enhanced the shale oil retention within these fractures. The hydrocarbon saturation in the bedding-parallel fractures was affected by the timing of hydrocarbon generation and the hydrocarbon generation potential and reservoir properties of the shale. The TOC contents and porosity of the host shales exhibit a positive correlation with the hydrocarbon saturation in the bedding-parallel fractures, which together contribute to shale oil enrichment, particularly with respect to TOC contents. In contrast, the low permeability of the Lucaogou shale limited the effects of the permeability and pore-throat characteristics on the hydrocarbon saturation in the bedding-parallel fractures. Coupled analysis of the bedding-parallel fracture line density and shale oil production suggests that shale oil enrichment may depend on the abundance of fractures and their hydrocarbon saturation quality. Our findings challenge the prevailing notions that fracture development uniformly enhances hydrocarbon enrichment, and that in shales with high TOC contents and porosity, fractures have a key role in facilitating hydrocarbon accumulation. Importantly, the hydrocarbon saturation in fractures is a key parameter in evaluating their effectiveness.