Sweet spot development in heterogeneous shales of lower Silurian
Abstract
The identification of sweet spots in shale is crucial for ensuring energy security and the economic development of gas resources. However, the pronounced heterogeneity of laminae and lithofacies presents substantial challenges in the accurate identification of sweet spots, particularly in shale formations deposited across the Early Silurian, which was significantly affected by episodic volcanism. To achieve a high-resolution understanding of sweet spot development and accurately identify the economic intervals, this study conducted an integrated analysis of continuous borehole core samples mostly from the Longmaxi Formation. This involved high-resolution lithological, mineralogical, petrophysical, and geochemical characterizations, combined with in-situ gas content measurements. The results establish lamina-scale criteria for sweet spot identification, emphasizing the critical role of fine-scale heterogeneity. Our findings demonstrate that sweet spot development during the Early Silurian is strongly governed by the architecture of laminae and lithofacies, which significantly influence reservoir quality. Based on the proposed criteria, the optimal sweet spot interval (Class I) was identified in the lower Longmaxi Formation, specifically within the integrated sublayer of ‘upper Long-112 + Long-113’. This interval is characterized by interbedded thick silty-thin muddy laminae and organic matter (OM)-rich mixed shale (ORMS). It exhibits the highest total organic carbon (TOC), excellent petrophysical and mechanical properties, well-developed nanopore systems, and the highest in-situ gas content. Additionally, Class II and III sweet spots were identified in the Long-114 and the integrated ‘Long-111 + upper Long-112’ sublayers, respectively. These intervals experienced a less intensive volcanism, exhibiting comparable laminae configurations and dominated by OM-moderate siliceous and mixed shale lithofacies, resulting in favorable reservoir quality and gas content, albeit relatively inferior compared to Class I. In contrast, no sweet spots are observed in the Long-12 and Long-2 intervals of the upper Longmaxi Formation, with trace volcanism evidence. These intervals are dominated by thick massive, muddy laminae and OM-poor argillaceous shale lithofacies and exhibit poor petrophysical and mechanical properties with low gas content. This study underscores the importance of laminae and lithofacies in heterogeneous shales and the controlling mechanisms of sweet spot development. The findings provide a scientific basis for more accurate resource evaluation and exploitation of shale gas resources.
Document Type
Journal Article
Date of Publication
1-13-2026
Volume
313
Publication Title
International Journal of Coal Geology
Publisher
Elsevier
School
School of Engineering
Funders
China National Petroleum Corporation CNPC (2021DQ-0405) / CNPC Scientific Project (2024ZZ-01)
Copyright
subscription content
Comments
Yuan, Y., Wu, S., Al-Khdheeawi, E., Lin, Z., Wang, J., Wang, M., Zhou, Y., Zhang, L., & Feng, Z. (2025). Sweet spot development in heterogeneous shales of lower Silurian. International Journal of Coal Geology, 313, 104902. https://doi.org/10.1016/j.coal.2025.104902