High-resolution study of Jurassic mudstones in China’s Sichuan Basin reveals that astronomical cycles control climate, sedimentation, and organic matter accumulation—key insights for global shale oil exploration.
As global energy demand continues to rise, unconventional resources such as shale oil have become increasingly vital. However, predicting the distribution of high-quality organic-rich shale remains a significant challenge, largely due to incomplete understanding of the fine-scale depositional processes that govern their formation. A new study published in the Journal of Palaeogeography (Chinese Edition) (Vol. 27, No. 5, 2025) advances this field by demonstrating how Earth’s astronomical cycles—specifically variations in orbital eccentricity—exert a dominant control on lacustrine shale deposition, organic enrichment, and lithofacies architecture. The research, conducted on the Jurassic Lianggaoshan Formation in the Sichuan Basin, China, provides a high-resolution climatic and sedimentological framework that enhances the predictability of shale oil reservoirs.
Previous studies have recognized that astronomical forcing influences paleoclimate and sedimentation over geological timescales, yet most focus has been on marine or deep-time records. In contrast, lacustrine systems—especially those deposited in continental interiors—are highly sensitive to orbital-driven climate change, but their high-frequency depositional response has been less systematically decoded. The research team, led by Professor Xian Benzhong from China University of Petroleum (Beijing), addressed this gap by applying cyclostratigraphic analysis to the Lower Liang 2 Submember in the Fuxing area of the Sichuan Basin.
The investigators used natural gamma ray (GR) logging curves as a paleoclimate proxy, combined with detailed core observations, geochemical elemental analysis, and total organic carbon (TOC) measurements. Through advanced time-series analysis using the Acycle software, they detected multiple Milankovitch cycles within the shale-rich interval, including long eccentricity (405 ka), short eccentricity (128 ka), obliquity (43 ka), and precession (21 ka) signals. These cycles were correlated with fourth- and fifth-order sequence stratigraphic units, enabling the establishment of a high-frequency chronostratigraphic framework with an average sedimentation rate of approximately 4.2 cm/kyr—one of the first such precise reconstructions in a Jurassic lacustrine setting in this region.
The results reveal a clear astronomical pacing of paleoenvironmental conditions. During high eccentricity periods, enhanced seasonal contrast led to warmer, wetter climates, increased terrestrial input, lake expansion, and elevated primary productivity. These conditions favored the deposition of organic-rich laminated mudstone (Lithofacies Association A) in deep-water settings. Conversely, low eccentricity intervals corresponded to drier climates, reduced runoff, intensified weathering, and lower lake levels. Under these conditions, deltaic sand bodies developed on upper slopes (Lithofacies Association B2), while hyperpycnal flows delivered sandy sediments into deeper waters (Lithofacies Association B1). Importantly, the study establishes that long eccentricity (405 ka) exerts the primary control on climate evolution, organic matter accumulation, and basin-scale lithofacies distribution, whereas short eccentricity (128 ka) modulates these patterns at higher frequency.
One of the key innovations of this work is its integrated demonstration of orbital control on both normal mudstone deposition and event sedimentation (hyperpycnites) in a lake basin. By linking elemental geochemical proxies (e.g., Sr/Cu, C-value, Sr/Ba, Ti/Al) and TOC trends directly to eccentricity cycles, the researchers provide a mechanistic understanding of how astronomical forcing regulates weathering, nutrient supply, water column stratification, and ultimately, the preservation of organic carbon. The high-resolution correlation across three wells also confirms that even in deep-water settings, stratigraphic gaps can occur, challenging previous assumptions about continuous sedimentation in lake centers.
These findings carry important implications for shale oil exploration in continental basins worldwide. By recognizing that the best organic-rich shale intervals are preferentially deposited during specific orbital configurations—namely high eccentricity phases—explorationists can better predict the vertical and lateral distribution of “sweet spot” in subsurface shale formations. The high-resolution sequence framework also offers a powerful tool for regional correlation and reservoir modeling, reducing uncertainty in targeting producible zones.
This research not only advances the understanding of terrestrial sedimentology under astronomical forcing but also delivers a practical methodology for applying cyclostratigraphy in resource evaluation. As energy transitions continue to underscore the importance of unconventional hydrocarbons, such integrative studies that bridge paleoclimate science and petroleum geology will play an increasingly vital role in sustainable resource development.
DOI: 10.7605/gdlxb.2025.038