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The lacustrine Beibuwan Basin contained large petroleum reserves in the northern continental shelf of the South China Sea. Due to the great sedimentologic complexity of the lacustrine basins, a significant financial risk associated with exploration needed to be reduced by specific techniques or significantly increasing knowledge of the lake system. Sedimentary infill, sequence stratigraphy, and organic geochemistry of the Fushan Depression in the Beibuwan Basin were studied, and enhanced our comprehension of the genetic relations between the sediment + water supply rates (climate change) and potential accommodation (tectonic subsidence and inherited topography) change. Here, we report two different lake‐type evolution cycles developed in two separated sags in the Fushan Depression, which are characterized by: (a) overfilled, (b) balanced‐fill, (c) overfilled, (d) balanced‐fill, and (e) overfilled in the Huangtong Sag, and (a) overfilled, (b) balanced‐fill, and (c) overfilled in the Bailian Sag and the centre Huachang Transfer Zone. Climate change is the dominant factor in the first three phases of the lake‐type evolution in both two sags, and transtensional tectonic setting is the controlling factor in the fourth and fifth phases of the lake‐type evolution in the Huangtong Sag. The same lake‐type with similar source rock and reservoir properties and observations indicate that these associations of source rock and reservoir play elements occur in a wide variety of tectonic settings. Moreover, the discovery of the cyclical process of the lake‐type evolution in the Fushan Depression can be a reference in the study of other similar lacustrine basins under an analogous tectonic and climate background. [ABSTRACT FROM AUTHOR]

Early Carboniferous Luzhai Formation developed continuous and thick shales, which is a critical succession for hydrocarbon exploration and production in southern China. However, no high-resolution stratigraphic sequence framework or cyclostratigraphic studies on early Carboniferous (Tournaisian Stage) are available for this organic-rich shale formation. In this research, Milankovitch cycle evidence has been reported in two boreholes of Luzhai Formation in Guizhong Depression. Natural gamma-ray logging data allowed the establishment of a ∼5.9-Myr floating astronomical timescale (FATS) based on stable long eccentricity (405-kyr) cycles in Luzhai Formation. We anchored FATS to Devonian and Carboniferous boundary age (359.3 ± 0.4 Ma), and an absolute timescale for intervals of 359.3 to 353.4 Ma was developed. Correlation coefficient (COCO), evolutionary COCO, and evolutionary TimeOpt method (eTimeOpt) were employed for quantitatively fit observed stratigraphic cycles to astronomical cycles for providing possible range of sedimentation rates. Sedimentary noise model was adopted for detecting orbital forcing-controlled high-resolution sea-level changes which were correlated to ∼1.2-Myr obliquity amplitude modulation (AM) cycles. Sedimentary noise models (autocorrelation coefficient analysis, ρ1, and dynamic noise after orbital tuning analysis, DYNOT) revealed a correlation between obliquity AM cycle (∼1.2-Myr) and sea level changes in early Carboniferous Luzhai Formation. The ∼1.2-Myr obliquity AM was the main driver of third-order sequence development, the 3rd-order sequence boundaries were related to maximum values of DYNOT median curve and minima values of ∼1.2-Myr amplitude modulation (AM) curve. • A new Floating ATS for the first time to calibrate the Tournaisian depositional sequences. • The deposition of Luzhai Formation shale in Early Carboniferous lasted ∼5.9-Myr. • Third-order sequences and sea-level change can be generated by cyclostratigraphic research. • ∼1.2-Myr obliquity amplitude modulation dominated sea-level changes. [ABSTRACT FROM AUTHOR]

Early Cambrian Qiongzhusi Formation has attracted great attention due to shale gas finding. However, this organic-rich shale formation is yet no robust high-resolution stratigraphic sequence framework. Here the evidence on Milankovitch cycles in three wells drilled through Qiongzhusi (QZS) Formation in Sichuan Basin has been reported. The logging data made it possible to develop a floating timeline calibrated to stable eccentricity cycles of 405-kyr within ~10.92 and ~7.34 Myr intervals in intracratonic rift and low bulge of Sichuan Basin, respectively. In this research, astronomical time scale was anchored to the U Pb Zircon age of QZS Formation/Maidiping Formation boundary (526.86 ± 0.16 Ma), developing an absolute timescale for considered intervals extending from 526.86 to 515.94 Ma. Average spectral misfit and correlation coefficient analyses were applied to quantitatively measure the fitting of the witnessed sedimentary cycles to astronomical periods and provide a possible sedimentation rate range. A sedimentary noise model was chosen to detect high-resolution sea-level variations under the control of orbital forcing and variations of sea-level were linked to obliquity modulation cycles of about 1.2 Myr. The orbital obliquity modulation cycles witnessed for early Cambrian QZS Formation (~1.2 Myr) were correlated to eustatic variations of sea level generated based on sedimentary noise model (DYNOT and ρ1). Furthermore, correlations between sequence stratigraphy and cyclostratigraphy were discussed, and the third-order sequence boundaries well corresponded to minima ~1.2 Myr orbital obliquity modulation curve and high values of DYNOT. The previous sequences framework was demonstrated to be isochronous and credible. • Orbtial cyclicity has been detected by time series method using the K channel of GR logs in shale sequence. • Floating and absolute Astronomical Time Scales have been established for the Qiongzhusi Formation in the Sichuan Basin. • Reconstruct the sea-level changes by using DYNOT simulation approach. • ∼1.2-Myr amplitude modulation of obliquity dominated sea-level change. [ABSTRACT FROM AUTHOR]

It has been demonstrated that the sedimentary deposits of Eocene succession in Bohai Bay Basin in China responded to astronomically forced climate changes. Several sections have been evaluated for cyclostratigraphy and many astronomical time scales (ATSs) have been constructed in Paleogene Bohai Bay Basin. However, the calibrations of age model and the constructed ATSs failed to reach an agreement and many of them relied on totally different orbital interpretations within the same section/borehole. In this work, we generated a new ATS for the middle Eocene lacustrine strata of middle to lower Member 3 (Es3m-l) and upper Member 4 (Es4u) of Shahejie Formation in Bohai Bay Basin. In this research, high resolution natural gamma-ray (GR) data were used for the cyclostratigraphy analysis of Es3m-l and Es4u Members of Well Fanye-1. We filtered ~11.11 m cycles of t Es3m-l and ~9.26 m cycles of Es4u which were directly correlated to the short eccentricity cycles (~100-kyr) of La2004 solution. The geomagnetic polarity ages of C18r/C19n, C19n/C19r and C19r/C20n in Geological Time Scale 2020 were adopted as age markers for the tuning in this study. This astrochronology provided astronomical time scale (ATS) in the range of 39.268 ± 0.050 Ma to 43.621 ± 0.050 Ma in Es3m-l and Es4u Member. The astrochronology of Well Fanye-1 of the same interval had been studied before. However, the ATS built in this paper used correlation coefficient (COCO) analysis and average spectral misfit (ASM) approach to provide quantitative measurement for fitting the observed sedimentary cycles to astronomical periods and given a range of plausible sedimentation rate. In this research, a sedimentary noise model was applied for the detection of high-resolution lake-level variations controlled by orbital forcing. The lake-level variations generated from the sedimentary noise model were supported by previous sequence stratigraphic interpretations. Variable sedimentation could be affected by lake-level changes which have been demonstrated by the evolutionary correlation coefficient analysis of GR series. In addition, from the ATS and thin section observations, it was concluded that carbonate-clay laminae in lacustrine shale were generated annually. Seasonal carbonate-clay laminated shale formations are controlled by orbital scale variability of monsoon and are always developed in a relatively high eccentricity amplitude under deep water environment conditions. Finally, the lake-level rise at ~40.500 Ma to ~40.050 Ma was demonstrated to be the response to Middle Eocene Climate Optimal (MECO) event in lacustrine records. • We provide a new astrochronology for the Es3m-l andEs4u in Well Fanye-1 of middle Eocene in Bohai Bay Basin. • Comparisons of the ATSs of the same borehole and the same interval between this study and previous study have been made. • Sedimentary noise modeling of lacustrine strata is used for paleolake level reconstruction. • Astronomical forcing had a significant impact on carbonate-clay laminated shales development. [ABSTRACT FROM AUTHOR]