Introduction

The Qiongdongnan basin lies between Hainan Island and the Xisha Islands in the northern South China Sea. It is a Cenozoic continental margin extensional rift basin. This basin ranks among China’s key offshore trillion-cubic-meter gas provinces. The basin exhibits a “two depressions and two uplifts” structural pattern. Four first-order units are recognized the northern depression zone, central uplift zone, central depression zone, and southern uplift zone1,2,3. During the early exploration, the Ying9 oil-bearing structure in ‌Songtao uplift‌ and the Ya13 Gas Field in ‌Yanan Sag‌ discovered in shallow-water areas4. Later discoveries (‌L17 large gasfield, L25 gasfield and L18 gasfield) in the central depression-Lingshui sag include large-to-medium gas fields5. Recently, the BD21-1 gasfield was discovered in the Songnan-Baodao sag6. Crude oil in the Ying9 structure originates from Eocene lacustrine source rocks. Natural gas in the Ya13 field derives from marine-terrestrial transitional source rocks of the Yacheng formation. Gas in the Ledong-Lingshui and Songnan-Baodao sags were sourced from terrestrial-marine transitional facies source rocks of the Yacheng formation. The Yacheng formation source rocks in the Songxi sag are dominated by type III kerogen7. Their hydrogen index (HI) ranges from 50 to 200 mg/g. Exploration in the northern depression zone has shown limited success8. An inadequate understanding of source rock development patterns and distribution remains a critical exploration constraint9.

Rare earth element (REE) composition and distribution patterns are minimally affected by post-depositional processes10,11. They effectively preserve source characteristics and paleoenvironmental information of sediments12,13,14,15,16,17,18,19. Certain trace elements also reflect salinity and redox conditions of water bodies. This study systematically collected mudstone samples from Yacheng formation in Well ST31X. REE and trace element data were analyzed to trace sediment provenance, sedimentation rates, and depositional environments. These results provide scientific insights into Yacheng formation source rocks. This work focuses on trace and REE geochemical signatures compared to previous studies20,21. A novel perspective is introduced to evaluate organic matter characteristics and preservation conditions. This approach offers new strategies for assessing source rock potential and exploration targets in other regions of the Qiongdongnan Basin.

Regional geological overview

The Songxi Sag is situated in the central part of the northern depression zone in the Qiongdongnan Basin. It is bordered by the Hainan uplift to the north, adjacent to the Songtao uplift to the south, and in proximity to Fault No. 5. This fault governs the Sag’s structural framework and sedimentary fill patterns14 (Fig. 1). Four wells drilled around Songxi Sag have all failed. Well ST31X, located in the western sub-sag near the sag center, primarily targeted the Lingshui and Yacheng formations. It also aimed to explore Eocene source rocks. The well was designed assuming a dual hydrocarbon supply from the western sub-sag of Songxi sag and the eastern sub-sag of Yabei Sag. This setting was considered favorable for hydrocarbon generation and migration. The lower Lingshui formation (Sequence T62A) and the first member of the Yacheng Formation (Sequence T70) exhibited intact trap geometries with high exploration potential. The potential resources of predicted structures are estimated to be considerably substantial. But, no coal-bearing strata were encountered in Yacheng formation of Well ST31X. Cuttings logging showed no hydrocarbon indications, and gas logging revealed no anomalies. Well logging interpretations confirmed no hydrocarbon-bearing zones, it as a dry hole. Insufficient hydrocarbon supply capacity was identified as the primary cause of the well’s failure.

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Schematic diagram of the sampling wells location of Songxi sag in Qiongdongnan basin 3 Samples and experiments.

The Yacheng formation in Well ST31X is divided into three members, from top to bottom, the First Member (Y1), Second Member (Y2), and Third Member (Y3). Ya1 Member was dominated by gray pebbly coarse sandstone interbedded with gray mudstone.‌Y2 Member composed of yellowish-gray pebbly coarse sandstone interbedded with dark gray mudstone.‌Y3 member‌ were variable-thickness interbeds of gray pebbly coarse sandstone, brown–red argillaceous coarse sandstone, and brown–red mudstone. The Yacheng formation in Songxi Sag is primarily sourced from the Hainan uplift and surrounding low uplifts.‌ The western sub-sag develops a large delta system that delivers coarse-grained sandstones with high sedimentation rates and strong hydrodynamic energy.‌ Meanwhile, the eastern sub-sag‌ has a limited sediment supply, resulting in fine-grained suspended deposits with low energy levels. The eastern sub-sag is far from major sediment sources and is dominated by mid-deep lacustrine facies with significant water depth. Low-frequency, continuous high-amplitude seismic reflections indicate low-velocity anomalous strata in sag centers and gentle slopes. A chemocline is inferred at the sag center, creating an oxygen-depleted lower water column conducive to organic matter preservation. While western sub-sag adjacent to major sediment input zones, dominated by sand-rich deposits. Shallow water depth, open oxidizing conditions, and absence of chemocline. Low paleoproductivity and poor organic matter accumulation result in inferior source rock quality. Eastern sub-sag‌ was a semi-restricted water body with oxygen-depleted conditions below the chemocline and surface photosynthesis. It supports high paleoproductivity and the development of high-quality mid-deep lacustrine source rocks. ‌Western sub-sag‌ was an open oxidizing environment with limit organic sedimentation, leading to suboptimal hydrocarbon source potential.

Unlike other lithologies, mudstone exhibits the strongest adsorption capacity for rare earth elements (REEs), making it an accurate indicator of depositional environments. Therefore, mudstone samples from the Yacheng Formation (3058–3424 m) of Well ST31X were selected for this study. Testing of trace elements and REEs was completed at the CNOOC Zhanjiang laboratory. A Thermo Fisher X Series2 inductively coupled plasma mass spectrometer (ICP-MS) was utilized, with sensitivity of 50 Mcps/ppm and detection limit of 0.1 ppt. Samples were collected as ‌sidewall cores‌ at 2-m intervals, representing the complete lithological profile of the Yacheng Formation. Stored in sealed sample boxes labeled with depth and lithology details. Detailed preparation and analysis procedures include instrument calibration, sample pretreatment, ICP-MS operation, and data processing, samples were digested using HNO3, HClO4, and HF in Teflon crucibles22. Standard reference materials (GSR-4, SCo-1, UMT-1) and blanks showed linear calibration curves. The relative deviation between measured and certified values was lower than 5%, and the repeatability of test results for identical samples‌. REE and trace element data were standardized using Z-scores (mean = 0, variance = 1) to eliminate dimensional disparities and enable multivariate comparisons.

Element geochemical characteristics‌

Major element content and compositional features

Major element composition of Yacheng formation mudstone in Songxi sag include dominant components‌, they are SiO2 range from 31.56% to 90.21% (average 62.35%), Al2O3 range from 1.56% to 32.02% (average 10.01%), ‌secondary components‌ include CaO, Fe2O3, K2O, MgO, FeO, ‌Trace components include TiO2, Na2O, P2O5, MnO. Al2O3 and TiO2 show the strongest positive correlation (Pearson’s r ≈ 0.91). Al and Ti exhibit high stability in fine-grained sedimentary rocks, with minimal weathering impact, indicating abundant terrigenous siliciclastic material (fluvial/aeolian origins). The strong Al2O3–TiO2 correlation suggests stable parent rock compositions in the Hainan uplift provenance area during Yacheng formation deposition, with no significant tectonic or weathering alterations.

Trace element content and characteristics

Based on the trace element analysis of Yacheng formation mudstone from Well ST31X, a chondrite-normalized spider diagram (Fig. 2) was constructed using the McDonough W.F. (1989) chondrite standard. The results reveal a positive anomaly‌ including ‌Ba‌ range from 449 μg/g to 5425 μg/g (avg. 1868.83), ‌Nb range from 9.78 μg/g to 23.5 μg/g (avg. 16.09), ‌La‌ range from 19.6 μg/g to 76.7 μg/g (avg. 45.75), ‌W‌ range from 1.21 μg/g to 4.08 μg/g (avg. 2.60). ‌Negative Anomalies‌ include ‌Pb range from 14 μg/g to 51 μg/g (avg. 33), ‌Ti‌ range from 1651 μg/g to 5916 μg/g (avg. 4118), ‌P range from 268 μg/g to 2598 μg/g (avg. 783), ‌Sn‌ range from 2.31 μg/g to 5.8 μg/g (avg. 3.86), The three intervals exhibit consistent trace element trends. Exceptions include anomalously high ‌Mn‌ and ‌P‌ in some Member Y3 samples, and unusually low ‌Co‌, ‌Cr‌, and ‌Ni‌ in Member Y2 samples.

Fig. 2
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Mudstone trace element chondrite standardization spider web map in Yacheng formation of well ST31X.

Rare earth element content and characteristics

The total amount of rare earth elements W (∑REE) is distributed between 92.72 × 10–6 and 364.90 × 10–6, with an average value of 217.63 × 10–6. The distribution of light rare earth ∑LREE values is between 83.09 × 10–6 and 324.44 × 10–6, with an average value of 194.48 × 10–6. The distribution of heavy rare earth ∑ HREE value is between 9.63 × 10–6 and 46.29 × 10–6, with an average value of 23.18 × 10–6. The ratio of light to heavy elements (∑ LREE/∑ HREE) is distributed between 3.50 and 11.68, with an average value of 8.58. All four parameters above are higher than that in North America shale23. The mudstone of the Yacheng Formation shows a significant variation in ∑ REE, with relatively abundant light rare earth elements and depleted heavy rare earth elements. The W (∑ REE), ∑ LREE, and ∑ HREE values gradually increase from the first member to the third member of the Yacheng Formation, and the light/heavy rare earth element ratio is low from the first member to the third member.

The chondrite standardization24 are used to analyze the selected samples to eliminate the odd–even effect. The meteorite standard distribution pattern graph (Fig. 3) shows that the slope of the curve at the light rare earth section of the Yacheng Formation mudstone is significantly higher than that of the heavy rare earth section. This indicates a high fractionation between light rare earth elements and a low fractionation between heavy rare earth elements. There is a clear “valley” shape at Eu, with significant negative anomalies, only some samples of Ce show slight negative anomalies. The calculated (La/Yb)N in the Yacheng Formation mudstone of well ST13X is between 6.52 and 14.64, with an average value of 9.63. The range of (La/Sm)N values is between 2.81 and 4.69, with an average value of 3.86. The range of (Gd/Yb)N values is between 1.12 and 2.28, with an average value of 1.66. Indicating a moderate degree of light rare earth fractionation and a weak degree of heavy rare earth fractionation. Generally speaking, values of δEu and δCe greater than 1.05 are positive anomalies, while values less than 0.95 are negative anomalies. The analyzed samples have δEu values ranging from 0.58 to 0.98, with an average value of 0.71, indicating a significant negative Eu anomalies. The δCe values range from 0.80 to 1.04, with an average of 0.95, indicating a slight negative or normal anomalies. In terms of different members, there are not many differences in (La/Sm)N values among three members, indicating that there are not many differences in the degree of differentiation of light and heavy rare earths. The (La/Yb)N and (Gd/Yb)N values both show that the degree of differentiation of light and heavy rare earth elements in the third member is significantly higher than that in the first and second member. The δEu value of the first member is 0.78, the highest, while the second member have a value of 0.63, the lowest. The δCe value in the first member is the lowest one, with an average value of 0.93, while the average values for the second and third member are 0.96.

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Standardized REE distribution pattern of chondrites in mudstone samples of Yacheng formation in well ST31X.

Geological implications of elemental characteristics

Provenance and sedimentation rate analysis

The ‌∑REE-La/Yb diagram‌ can be used to determine the genetic characteristics of certain rock types24. Most mudstone samples from ‌Member Y3 and Y2‌ of ‌Yacheng formation‌ in Well ST31X plot near the intersection of ‌alkali basalt‌, ‌granite‌, and ‌sedimentary rock fields. Samples from ‌Member Y1‌ predominantly fall within the ‌sedimentary rock–calcareous mudstone domain‌, while some samples from ‌Y2 and Y3‌ indicate provenance contributions from ‌alkali basalt‌, ‌sedimentary rocks‌, and ‌granite. The ‌Yacheng formation‌ source rock exhibits ‌diverse parent rock sources‌ (Fig. 4), which consistent with its ‌REE distribution patterns‌. All mudstone samples from Yacheng formation in Well ST31X show ‌negative Eu anomalies‌, suggesting that their protoliths were derived from ‌granitic sources‌. This aligns with the ‌Eu-negative characteristics‌ of widespread granites and sedimentary rocks on ‌Hainan island‌. Combined with Shao Lei’s (2010) study on REE patterns in Well ST2425 (Lingshui to Sanya formations), the REE fractionation patterns of Well ST31X Yacheng mudstones closely resemble those of ‌Hainan island source rocks‌, confirming its role as the ‌dominant provenance‌.

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(Modified by ALLÈGRE et al. 1987).

La/Yb and ∑ REE diagrams of mudstone samples of Yacheng formation in well ST31

The ‌rare earth element (REE) fractionation‌ degree can indirectly indicate sedimentation rates. ‌The short residence time‌ of suspended particles in the water column corresponds to ‌flat normalized distribution patterns‌, with ‌La/Yb ratios‌ near 1.‌ Prolonged residence time‌ leads to significant deviations in La/Yb ratios from 126,27. For ‌Yacheng formation mudstones‌ in Well ST31X, ‌La/Yb ratios‌ range from ‌0.91 to 2.04‌ (average: 1.37), reflecting ‌relatively rapid sedimentation rates‌ in this interval.

Identification of marine versus continental depositional environments

Previous studies indicate that marine sedimentation of Yacheng formation‌ in the Qiongdongnan Basin was initiated during ‌Member Y3‌, followed by progressive water deepening and transgression expansion28. By ‌Member Y1‌, shallow marine deposits had covered the entire basin, reflecting an evolutionary sequence from ‌lacustrine → enclosed bay → coastal-shallow marine environments‌. Influenced by the ‌multi-depression/uplift structural framework‌ and seawater ingress directions, the ‌Changchang and Beijiao sags‌ (eastern basin) experienced earlier marine transgression than that in ‌Songxi and Yanan Sags‌29,30 (western basin). Analysis by Cai Guofu (2013) suggested that ‌Yabei sag‌ transitioned to marine deposition during ‌Member Y2‌, and ‌Yanan sag‌ evolved into ‌transitional coastal-shallow marine facies‌ during ‌Member Y331. However, whether marine or transitional facies developed in ‌Songxi sag‌ during Yacheng deposition remains uncertain due to dual controls, the ‌barrier effect of the Songtao uplift‌ and activity of the ‌No. 5 major fault‌.

This study integrates the ‌major elements‌ (CaO, P2O5, MgO, TiO2) closely linked to marine biota and seawater composition, and ‌trace elements32,33,34 (Li, Sr, Ni, Ga) with Sr/Ba ratios sensitive to salinity variations, to analyze the ‌marine-continental depositional environments‌ of Yacheng formation in Songxi sag. Geochemical data from Well ST31X show that ‌Member Y3‌ CaO range from 0.2% to 2.18% (avg. 0.77%), MgO range from 0.93% to 1.81% (avg. 1.45%), P2O5 range from 0.1% to 0.59% (avg. 0.24%), TiO2 range from 0.52% to 0.98% (avg. 0.79%). ‌Member Y2‌ CaO range from 0.19% to 0.46% (avg. 0.32%), MgO range from 0.78% to 2.17% (avg. 1.69%), P2O5 range from 0.06% to 0.19% (avg. 0.12%), TiO2 range from 0.28% to 0.78% (avg. 0.61%). ‌Member Y1 CaO range from 0.36% to 1.25% (avg. 0.75%), MgO range from 1.23% to 3.09% (avg. 2.14%), P2O5 range from 0.07% to 0.14% (avg. 0.09%), TiO2 range from 0.35% to 0.68% (avg. 0.52%). CaO shows a slightly increases during ‌Member Y3‌ sedimentation, while other parameters (P2O5, TiO2) exhibit minimal fluctuations. MgO gradually rises from ‌Y3 to Y1‌, whereas other elements decrease over time. Despite the largest sea-level rise, no significant variations in major elements were observed during ‌Member Y2. Geochemical values in Songxi sag are markedly lower than those in ‌Yabei Sag‌ during ‌Y2‌, which experienced marine transgression (Fig. 5).

Fig. 5
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Marine and continental sedimentary identification of mudstone samples of Yacheng formation in well ST31X.

Paleosalinity analysis

Ca/(Ca + Fe) ratio analysis‌ indicates a dominance of iron oxides in Yacheng formation (Fig. 6), pointing to deposition in ‌freshwater or brackish environments‌. Li, Sr, Ni contents‌ and ‌Sr/Ba ratios‌ exhibit ‌positive correlations‌ with water salinity, ‌Ga‌ shows a ‌negative correlation‌ with salinity35. Based on criteria proposed by previous studies (Table 1), the mudstone of Yacheng formation in ‌Well ST31X‌ is interpreted as ‌freshwater-dominated deposition‌. Th‌ is preferentially adsorbed and retained in clay minerals. Scholars propose the ‌U/Th ratio‌ as a key indicator, ‌continental freshwater deposition with an U/Th > 28, ‌marine saline deposition‌ with an U/Th < 28. In Well ST31X, Yacheng formation mudstones yield ‌U/Th ratios‌ range from ‌2.65 to 5.68‌ (average: 3.73), further confirming a ‌continental freshwater environment36,37‌.

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Ca/(Ca + Fe) ratio correlation diagram of mudstone samples from the Yacheng formation in Well ST31X‌

Table 1 Statistical table for distinguishing parameters of paleosalinity and trace elements in Yacheng formation of well ST31X.
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Redox conditions

Hatch et al. (1992), Jones et al. (1994), and Wignall et al. (1996) proposed geochemical proxies (e.g., ‌V/(V + Ni)‌, ‌V/Cr‌, ‌Ni/Co‌, and ‌U/Th‌) and quantitative criteria to evaluate redox conditions in depositional environments through trace metal analysis18,19,38. Calculations of these parameters yielded the following results (Table 2). ‌V/(V + Ni)‌ range from 0.73 to 0.90 (avg. 0.84), ‌V/Cr‌ range from 0.38 to 3.25 (avg. 1.95), ‌Ni/Co range from 0.75 to 2.93 (avg. 1.51), ‌U/Th range from 0.18 to 0.38 (avg. 0.27). The ‌V/(V + Ni)‌ ratio, which is sensitive to ‌dysoxic-anoxic transitional zones‌, indicates a ‌dysoxic-anoxic depositional environment‌ for Yacheng formation. Other parameters (‌V/Cr‌, ‌Ni/Co‌, and ‌U/Th‌) consistently suggest ‌oxic to suboxic water conditions‌ for Well ST31X. The weakly negative or near-normal ‌δCe values‌ further support ‌suboxic conditions‌, unfavorable for organic matter preservation and hydrocarbon generation.

Table 2 Statistical table of trace and rare earth element discrimination parameters for the redox environment of mudstone sedimentation of Yacheng formation in well ST31X.
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Paleoproductivity conditions

The ‌Ba/TOC ratios‌ in Well ST31X range from 414.91 to 7486.95 (average 1885.11), exhibiting high values that suggest enhanced paleoproductivity driven by ‌allochthonous organic matter input‌. Concurrently, ‌Ni/TOC ratios‌ range from 12.61 to 29.43 (average: 21.31), further supporting ‌microbial activity-enhanced organic matter preservation‌. The eastern sub-sag, which is‌ dominated by ‌low terrigenous input‌, ‌stable reducing conditions‌, and ‌transgressive-phase organic matter supply, represents a ‌high-productivity zone‌ favorable for organic enrichment. ‌Meanwhile, the western sub-sag‌ limited by ‌sand-rich sedimentation‌ and ‌open-water hydrodynamic conditions‌, which are characterized by ‌low productivity levels‌.

Enlightenments on petroleum exploration

The coal-forming depositional systems in the Yacheng Formation of the Qiongdongnan Basin are dominated by ‌braided river–braided delta–subaqueous fan systems‌ and ‌tidal flat–lagoon systems39‌. During the deposition of Yacheng formation in Songxi sag, large-scale marine transgression generally did not occur, with ‌continental alluvial fan–fan delta environments‌ prevailing40,41. Steep-slope fan delta coarse clastic rocks‌ were extensively developed in the western sub-sag near Well ST31X, adjacent to a major sediment injection point. These coarse-grained deposits extended long distances, leading to a spatial mismatch between the ‌depocenter‌ and ‌sedimentary center and limited water coverage concentrated toward the gentle-slope zone. In contrast, the ‌eastern sub-sag‌, which is far from large sediment sources, exhibited reduced terrigenous input, resulting in ‌greater water depth and coverage‌ compared to western sub-sag. Tidal flat zones around lagoons were also concentrated on the gentle-slope side of the eastern sub-sag, where peat accumulation initiated from the gentle slope and extended toward the sag interior. In the eastern sub-sag, Yacheng formation hydrocarbon source rock ‌with a thickness of 30 m to 100 m thickness, ‌TOC average 1.37%, exceeding values from Meishan (0.78%) and Sanya (0.99%) formations. ‌ Ro ranges from 0.7% to 1.2%, with a moderate hydrocarbon generation potential (estimated 3 × 108 m3/km2 to 5 × 108 m3/km2), with the gentle-slope tidal flat zone adjacent to the Songtao uplift identified as the most favorable area for source rock sedimentation (Fig. 6). In ‌ST24 area‌ of Songdong sag (east of Songxi sag), natural gas exhibits low dryness coefficient (0.88), with light methane carbon isotope (− 47.06‰) and light ethane carbon isotope (− 30.92‰). These isotopic signatures differ markedly from that in Yacheng coal-derived gas in Ya13 gasfield, it is speculated that ‌medium-deep lacustrine sapropelic-type source rocks‌ may developed in eastern ‌Songxi sag‌ (Fig. 7).

Fig. 7
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Sedimentary model diagram of Yacheng formation in Songxi sag (Revised according to references35,36).

Conclusions

  1. (1)

    The Yacheng formation mudstones exhibit relative enrichment of light rare earth elements (LREEs) and depletion of heavy rare earth elements (HREEs), with similar REE distribution patterns‌ across different stratigraphic intervals. A pronounced negative Eu anomaly‌ is observed, while no significant Ce anomaly is evident. REE distribution patterns indicate that the mudstone material was primarily derived from the upper crust, with characteristics highly similar to those of the Hainan island source area. ∑REE vs. La/Yb diagrams further reveal that the provenance included contributions from alkali basalts, sedimentary rocks, and granites.

  2. (2)

    The depositional environment of Yacheng formation mudstones is identified as a ‌terrestrial freshwater setting‌, based on comprehensive analysis of ‌major elements‌ closely associated with marine biota and seawater composition, combined with ‌trace elements‌ exhibiting high sensitivity to salinity variations.

  3. (3)

    ‌The depositional environment of Yacheng formation mudstones in Well ST31X is interpreted as ‌oxidizing to weakly reducing‌, based on integrated redox-sensitive indices including δCe and trace metal ratios (V/(V + Ni), V/Cr, Ni/Co, and U/Th), these conditions were ‌unfavorable for organic matter preservatio

  4. (4)

    The gentle slope zone of the tidal flat near ‌Songtao uplift‌ in the eastern sub-sag is a favorable area for the development of ‌Yacheng formation coal-bearing source rocks‌. Additionally, medium-deep lacustrine ‌sapropelic-type source rocks‌ may develop in eastern ‌Songxi sag‌, representing a promising target for enhancing hydrocarbon exploration efficienc