The Azigan Formation and the Lower Member of the Tahaqi Formation, dating to the Late Pennsylvanian, are pivotal exploration targets within the Tarim Basin. This region exhibits extensive siliciclastic-carbonate mixed sediments. However, the lack of high-resolution sequence stratigraphic frameworks significantly limits advanced petroleum geological research. Using principles of sequence stratigraphy and cyclostratigraphy and leveraging outcrop and thin section data alongside GR series analysis, this study systematically investigates the lithological, cyclic, and sequence stratigraphic characteristics of these formations. A total of 12 different lithofacies were identified, and 3 third-order sequences, 15 fourth-order sequences, and 16 long eccentricity cycles were delineated. A 1.2 Ma long slope signal was also identified. An astronomical timescale was established with 298.9 Ma as the anchor, defining the boundary between the upper and lower members of the Tahaqi Formation, revealing a link between long eccentricity cycles and the formation of fourth-order sequences. Moreover, the relationship between the 1.2 Ma long obliquity cycle and third-order sequences, as well as its role in driving sea-level changes in southwestern Tarim, is explored. The interplay between long obliquity and eccentricity cycles influenced the region’s mixed siliceous clastic and carbonate deposition. Warm and humid climatic conditions coupled with sea-level rise enhanced the input and transport of clastic materials, facilitating large-scale mixed sedimentation.

Sub-sag 21 in the eastern Yangjiang Sag, Pearl River Mouth Basin, South China, contains a thick lacustrine source-rock interval within the lower Wenchang Formation and is a major exploration target on the northern margin of the South China Sea. However, the timing of deposition during the early to middle Eocene remains poorly constrained, and the applicability of quantitative palaeoclimate reconstruction methods in low-latitude lacustrine basins requires further evaluation. In this study, we analyzed mudstones from the lower Wenchang Formation in Well E1. Using cyclostratigraphic constraints, we applied AstroGeoFit to construct an astronomically tuned age model, and combined palynological coexistence analysis with geochemical weathering proxies and linear–regression calibration to quantitatively reconstruct and cross-validate mean annual temperature and mean annual precipitation. Within this time-calibrated framework, we further quantified organic-carbon burial to evaluate the relationship between palaeoclimate evolution and organic-matter enrichment. The AstroGeoFit results indicate that the top of the lower Wenchang Formation in Well E1 is constrained to 44.563 Ma, and that the studied succession spans 50.249–44.563 Ma. Palynological coexistence analysis identifies three palaeoclimate phases within this interval. Method evaluation shows that the temperature reconstruction based on major-element geochemistry agrees well with the pollen-based temperature record, whereas one precipitation reconstruction based on weathering proxies shows the most robust agreement and stability relative to the pollen-based precipitation record. Reconstructed mean annual temperature ranges from 10.77 to 22.20 °C, and reconstructed mean annual precipitation ranges from 1188.27 to 1871.89 mm. Correlation analyses on the tuned timescale show that precipitation is more strongly associated than temperature with organic-matter accumulation parameters, including total organic carbon and organic carbon accumulation rate, indicating that organic carbon burial in the eastern Yangjiang Sag lake basin was mainly controlled by hydrological forcing. During the Early Eocene Climatic Optimum, carbon burial in low-latitude lakes was, therefore, not a simple response to elevated temperature, but instead reflected the integrated effects of precipitation, runoff, stratification, material supply, transport, and preservation. The evolutionary sequence further suggests that early high productivity was diluted by rapid sedimentation, reducing total organic carbon; subsequent cooling, lake deepening, and strengthened stratification enhanced organic matter preservation; and finally, tectonic subsidence together with regional humidification promoted the development and long-term preservation of high-quality lacustrine source rocks.

The constituent elements of source-to-sink systems and their coupling relationships are key controls on the development of sedimentary systems and the spatial distribution of sand bodies. Taking the Paleogene strata in the southern Laizhouwan Sag of the Bohai Bay Basin as a case study, we integrate drilling, logging, core, thin-section, and high-resolution 3D seismic data to quantitatively characterize basement lithology and effective provenance area, drainage-unit subdivision, types and scales of sediment transport pathways, and geometric parameters of depositional fans, within a source-to-sink analytical framework. The results show that: (1) Two distinct provenance types are developed in the southern Laizhouwan Sag, including Proterozoic granitic–gneissic basement and Mesozoic volcanic–clastic basement. These provenance types exhibit pronounced differences in effective source area, vertical relief, and drainage-network configuration across different sequence stages. (2) Two main categories of sediment transport pathways are identified, namely paleo-valleys and fault-controlled troughs. V-shaped, U-shaped, and W-shaped paleo-valleys show systematic morphological transitions along topographic gradients. The width-to-depth ratio of transport channels exerts a significant control on depositional fan scale, with U-shaped valleys exhibiting the highest sediment transport efficiency. Finally, (3) the depositional domain is dominated by near-source fan-delta systems, whose scale shows a strong positive correlation with effective provenance area and transport-channel morphology. Overall, the southern Laizhouwan Sag is characterized by a typical fault-controlled, steep-slope source-to-sink system, in which sedimentary system distribution is jointly governed by effective provenance area, sediment transport pathway geometry, and fault-related slope-break zones. This study provides a quantitative example and methodological reference for source-to-sink system characterization and prediction of favorable sand body distribution in continental rift basins.

This study conducts a cyclostratigraphic analysis of the third member of the Eocene Shahejie Formation (Es3) in the Laizhou Bay Sag, Bohai Bay Basin, to investigate the influences of astronomically driven climate variations on sea-level changes, sedimentation rates, and depositional environments. We integrated high-resolution geophysical well logs, ostracod fossils, and palynological data from Well B-2 for cyclostratigraphic and paleoclimate analyses. Time series analysis identified orbital cyclicity in the natural gamma-ray (GR) log, with its significance confirmed by correlation coefficients and statistical significance tests. By tuning the GR log to the 405 kyr eccentricity cycle, we constructed a ~7.695 Myr floating astronomical timescale. Integrating the preliminary biostratigraphic framework (based on ostracods and palynology) with the La2010d astronomical solution yielded a high-resolution absolute astronomical timescale for the 1317–2594 m interval of Well B-2, spanning from 33.9 to 41.6 Ma. Sedimentary noise modeling reconstructed the Eocene sea-level curve in the study area, revealing that the 1.2 Myr obliquity modulation cycle was a key driver of sea-level fluctuations. The δ¹³C and δ¹⁸O records confirm the presence of the Middle Eocene Climatic Optimum (MECO), indicating that its stratigraphic signature constitutes a robust marker for regional stratigraphic subdivision in the southern Bohai Bay Basin. Our findings provide new insights into the climatic evolution of the Es3 member in the southern Bohai Bay Basin.

Quantitative reconstruction of source-to-sink (S2S) systems provides insights into sediment dispersal in continental rift basins. However, relationships among source-area geometry, sediment-routing architecture, and downstream depositional development remain difficult to constrain in structurally heterogeneous settings. This study integrates three-dimensional seismic data, well logs, core observations, and petrographic data to investigate Paleogene S2S systems in the southern Laizhouwan Sag, Bohai Bay Basin. Effective source areas range from 10.0 to 98.5 km² across the investigated stratigraphic intervals. Sixteen principal sediment-routing pathways are identified, with mapped lengths of approximately 4.6–18.4 km. Measurements from 34 depositional-area records indicate slope gradients of 0.96–4.76°. Two end-member S2S architectures are recognized. Fault-controlled steep margins are characterized by structurally confined pathways and proximal fan-delta deposition, whereas gentler margins contain more extensive routing networks associated with braided-river-delta development. Variations in depositional-body dimensions are examined in relation to source-area geometry, routing morphology, and basin-margin configuration. These dimensions record preserved sediment accumulation rather than directly measured sediment flux. The results provide a quantitative framework for evaluating S2S configurations in rift-lake basins while highlighting uncertainties associated with paleogeomorphic reconstruction, temporal matching, and sediment preservation.