The research appears in the Journal of Palaeogeography (Chinese Edition) and targets the Lower Liang 2 Submember in the Fuxing area of the Sichuan Basin. Led by Professor Xian Benzhong of China University of Petroleum Beijing, the group applied cyclostratigraphic techniques to decode how a continental interior lake responded to astronomical forcing on Milankovitch timescales.
Previous work has documented the influence of orbital forcing on paleoclimate and sedimentation, but most of that record comes from marine or long term archives rather than lakes. The authors argue that lacustrine systems in continental interiors respond strongly to orbit driven climate variability, yet their higher frequency depositional patterns and their link to hydrocarbon potential are still under constrained.
To address that gap, the investigators used natural gamma ray logging curves as a proxy for changes in sediment composition and paleoclimate through the shale rich interval. They combined these logs with detailed core description, elemental geochemical data, and total organic carbon measurements to characterize facies and organic enrichment at high resolution.
Time series analysis using the Acycle software identified distinct Milankovitch scale signals in the gamma ray data. The spectrum includes long eccentricity around 405 thousand years, short eccentricity near 128 thousand years, obliquity at about 43 thousand years, and precession near 21 thousand years, all superimposed on the Jurassic lacustrine record.
By correlating those orbital periods with fourth and fifth order sequence stratigraphic units, the team built a high frequency chronostratigraphic framework for the Lower Liang 2 Submember. They calculate an average sedimentation rate of roughly 4.2 centimeters per thousand years, noting that this is among the first precise estimates of this kind for a Jurassic lake basin in the region.
The reconstruction shows clear astronomical pacing of paleoenvironmental conditions across the study interval. During high eccentricity phases, the climate became warmer and wetter with stronger seasonal contrast, which increased terrestrial input, expanded lake area, and boosted primary productivity in surface waters.
Under these high eccentricity conditions, the deep parts of the lake accumulated organic rich laminated mudstone, defined as Lithofacies Association A. These deposits show elevated preservation of organic carbon and form key components of the shale oil system in the Lianggaoshan Formation.
When eccentricity was low, the region experienced relatively drier conditions with reduced runoff, more intense weathering, and lower lake levels. In these intervals, deltaic sand bodies developed on upper slopes as Lithofacies Association B2, while hyperpycnal flows delivered sandier material downslope into deeper water as Lithofacies Association B1.
The authors conclude that the 405 thousand year eccentricity cycle exerts the primary control on climate evolution, organic matter accumulation, and basin scale lithofacies distribution in the studied interval. Short eccentricity at about 128 thousand years modulates these long period trends, dividing organic rich and sand rich packages into finer scale units that are also important for reservoir architecture.
A central aspect of the work is the integrated treatment of both background mudstone deposition and event sedimentation generated by hyperpycnal flows within the same astronomical framework. By tying geochemical indices such as Sr/Cu, C value, Sr/Ba, and Ti/Al, together with TOC trends, directly to eccentricity cycles, the study connects weathering intensity, nutrient supply, and water column structure to orbital forcing.
Correlation among three wells shows that even deep water lacustrine settings can contain stratigraphic gaps, challenging the assumption that lake centers always record continuous deposition. These discontinuities have implications for sequence stratigraphic interpretation and for the thickness and continuity of shale reservoirs derived from such successions.
For shale oil exploration, the research suggests that intervals formed during high eccentricity phases are more likely to host thicker and richer organic shale layers. Recognizing the timing and stacking of these orbital windows can improve predictions of the vertical and lateral distribution of shale oil sweet spots and help refine well placement and reservoir models in continental basins.
The chronostratigraphic framework produced for the Lianggaoshan Formation offers a template for regional correlation across the Sichuan Basin and for other lacustrine basins with comparable datasets. The authors emphasize that combining cyclostratigraphy, geochemistry, and sedimentology reduces uncertainty in mapping shale systems and assessing their resource potential at both local and basin scales.
The study illustrates how concepts from astronomical forcing and paleoclimate research can be applied directly to petroleum geology in continental lake environments. The authors note that as unconventional hydrocarbons remain part of the energy mix during ongoing transitions, such integrative approaches will be increasingly important for evaluating and managing these resources.
Research Report: Lithofacies development patterns of lacustrine shales under astronomical forcing: a case study of the Lianggaoshan Formation in Sichuan Basin
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