Introduction

Recently, unconventional resources have attracted high attention worldwide. As one of the important unconventional resources, shale gas has gained significantly breakthroughs and commercial development of Paleozoic marine shale gas has been achieved in the Sichuan Basin. However, China’s marine shale gas exploration and development within the North China Platform remains in the exploratory phase, constrained by complex geological conditions. The Wulalike Formation, deposited as a marine hydrocarbon source rock within the Upper Ordovician of the Lower Paleozoic succession in the Ordos Basin, is primarily distributed across the mid-northern segment of the basin’s western margin. In recent years, high-yield commercial gas flows have been achieved from the Wulalike shale reservoirs, marking breakthrough advancements in shale gas exploration.

Compared to conventional petroleum deposits, shale gas reservoirs are generally characterized by self-generation and self-accumulation with ultra-low porosity and permeability1. Within a petroliferous basin, organic-rich shale is considered as the source rock of the conventional petroleum systems as well as the prominent target for unconventional hydrocarbon deposits2. Hydrocarbons within shale exist in varying states, mainly including adsorbed and free hydrocarbons. These hydrocarbons are mostly generated by thermal cracking of organic matter within sediments3. Therefore, The quantity and composition of organic matter govern the volume and type of hydrocarbons generated in source rocks (e.g. mudstone, shale and carbonate rocks). It has been previously confirmed that the contents of organic matter within sediments can be influenced by various depositional conditions4. In general, primary productivity decides the organic matter quantity generated per unit of time and area5. However, the generated organic matter can be affected by different depositional conditions and only part of the organic matter generated by primary productivity can be preserved. Hence, input and preservation conditions of organic matter determine the precursors of hydrocarbons in shale deposits. Based on this, different proxies, including biomarkers and elemental proxies, were applied to reconstruct the paleo-environments of ancient sediments5,6,7,8. These proxies have been widely applied in investigating the organic matter enrichment within shale deposits of different petroliferous basins worldwide and therefore implying the hydrocarbon generation potential of organic matter within shale matrix.

Depositional environment identification constitutes a fundamental component of sedimentological research and serves as a critical element in petroleum resource evaluation. The utilization of element migration, enrichment, and distribution patterns within sediments during depositional and diagenetic processes has increasingly emerged as a pivotal methodology for determining and reconstructing depositional environments. Currently, applications of elemental logging data are primarily focused on lithology identification and stratigraphic subdivision. These datasets demonstrate robust lithology discrimination capabilities, enabling the establishment of geochemical logging-based interpretation models and stratigraphic profiles. However, their implementation in depositional facies analysis remains limited and currently resides in its nascent stage. In this study, elemental logging data from well MJ1 in the western Ordos Basin were applied in paleoenvironmental reconstruction of the Ordovician sediments. This study primarily employs elemental logging data to conduct paleoenvironmental analysis of the Ordovician strata along the western margin, aiming to delineate the evolutionary characteristics of paleoenvironmental conditions in this region. The results of this study can provide fundamental data to support unconventional hydrocarbon exploration targeting the Wulalike Formation.

Geological setting

The Ordos Basin, situated in the southwestern part of the North China Platform, constitutes an integral component of the North China Craton9,10,11. It is tectonically bounded by fault systems: the Helanshan Fault-Fold Belt and Liupanshan Thrust Belt to the west, the southern marginal fault of the Hetao Graben to the north, the Lishi Fault to the east, the Weihe Graben to the south, and the Qin-Qi Orogenic Belt to the southwest (Fig. 1a)12. The basin’s present architecture exhibits north-south uplifting with eastern tilting and western thrusting, internally forming a monoclinal structure dipping westward13. This tectonic configuration initiated during the Mid-Yanshanian Orogeny and matured in the Himalayan Phase14. Six structural units are defined at present, including Yishan Slope in the central, Yimeng Uplift in the norrth, Jinxi Flexure-Fold Belt in the east, Tianhuan Depression in the west, and Weibei Uplift in the south15.

The study area encompasses the western Ordos Basin, where marginal fault zones demarcate adjacent tectonic domains. During the Paleozoic, intense N-S compression between the Yangtze and Siberian plates shaped the Ordos Block, forming paleotopographic highs to its north and south with a central depression16. By the Mesozoic, a major thrust system had developed along the western margin due to compression from the Alxa Block, while the eastern margin experienced uplift associated with paleo-Pacific subduction. These processes created a flexural fold geometry and collectively shaped a basin pattern characterized by a north-south orientation, with westward tilting along the eastern margin and east-verging thrusting along the western margin17.

The basin evolved into a stable cratonic basin from the Early Paleozoic, depositing regionally correlative Cambrian-Ordovician marine carbonates interbedded with clastics18. The Ordovician succession in the western margin is subdivided ascendingly as the Sandaokan Formation, Zhuozishan Formation, Kelimoli Formation and Wulalike Formation (Fig. 1b)12. The Sandaokan Formation predominantly comprises interbedded light gray dolomitic limestone, dolostone, light gray quartz sandstone, and white-to-gray limestone, with intercalated bioclastic limestone locally present. It exhibits a thickness ranging from 50 to 140 m (typically ~ 80 m), thickening westward. Dolostones are largely non-calcareous. The base is demarcated by dolomitic sandstone with local quartz conglomerate, distinguishing it from the underlying limestone of the Abuqiehai Formation via disconformable contact. Zhuozishan Formation is characterized by thick-bedded limestone and dolostone, with argillaceous or siliceous concretions in certain intervals and localized nodular structures. Regionally, it forms an almost exclusively carbonate-dominated succession. The lower boundary is defined by thick-bedded limestone contrasting with the dolomitic limestone and quartz sandstone of the underlying Sandaokan Formation, while the upper contact conformably transitions into the thin-bedded limestone and dark calcareous mudstone at the base of the overlying Kelimoli Formation, indicating conformable contacts both above and below12. Kelimoli Formation is dominated by thin-to-medium-bedded limestone, black shale, and calcareous mudstone. Wulalike Formation comprises primarily dark gray to grayish-black calcareous shale and argillaceous shale, including lithologies such as shale, dolomitic mudstone, calcareous mudstone, argillaceous limestone, and argillaceous dolostone12.

Fig. 1
figure 1

(a) Regional tectonic setting of the Ordos Basin within the North China Platform. (b) Stratigraphic framework of the Ordovician strata in the western margin of the Ordos Basin

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Methods and data processing

Methods

Elemental Logging is a geophysical technique that characterizes formation properties by real-time measurement of chemical element composition in drill cuttings or core samples. Its fundamental principle involves in-situ elemental scanning of cuttings (or sidewall cores) retrieved during drilling operations using Energy-Dispersive X-ray Fluorescence (ED-XRF)​or Laser-Induced Breakdown Spectroscopy (LIBS). This process yields quantitative datasets​for both major elements (e.g., Si, Al, Ca, Fe, K) and trace elements (e.g., V, Ni, Mo, U, Th).

Data processing

Undiagenetically altered marine carbonates faithfully preserve primary seawater signatures. Hence, for elemental distributions of sediments obtained from logging data, assessment of diagenetic alteration intensity must precede geochemical analysis to ensure data validity. In general, diagenetic fluids and meteoric waters exhibit significantly lower Sr concentrations than the Sr concentrations in seawater. Carbonates lose Sr during diagenesis; consequently, higher Sr concentrations better reflect primary seawater composition. Prior studies establish that Sr > 200 ppm is required for reliable seawater depositional environment. In addition, kinetic fractionation effects yield lower Mn concentrations in seawater relative to freshwater. Thus, minimally altered marine carbonates display low Mn abundances. Previously, sediments with Mn/Sr ratios between 2 and 3 retain primary geochemical signatures or exhibit only weak diagenetic alteration19. Given that certain diagenetic processes may elevate Sr concentrations, the Mn/Sr ratio provides a more robust discriminant.

Results

Major element distribution

In this study, the major elements of the samples were presented as their chemical element in percent (%). The major element distributions of the collected samples in this study were presented in Fig. 2. Major element distributions exhibit marked heterogeneity across sedimentary rocks from distinct formations, reflecting lithological and diagenetic disparities. In general, Si, Ca and Mg act as the dominant components of the element in the samples from the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations. The average contents of Si in above formations are 2.98%, 4.46%, 8.16%, 23.92% and 22.20%, respectively, and the average contents of Ca in above formations are 20.00%, 20.18%, 25.75%, 9.88% and 11.22%, respectively. The average contents of Mg in the sediments from the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 11.34%, 9.78%, 3.13%, 1.77% and 1.66%, respectively Apart from elements Si, Ca and Mg, the samples from the Ordovician sediments also have relatively high contents of Al, K and Fe. The average contents of Al in the sediments from the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 0.25%, 0.50%, 0.64%, 5.42% and 5.09%, respectively; the average contents of K in the sediments from the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 1.72%, 1.63%, 1.32%, 3.05% and 2.68%, respectively; the average contents of Fe in the sediments from the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 0.58%, 0.69%, 0.59%, 2.68% and 2.68%, respectively. Other major elements, such as Na, P, Mn and Ti, have relative low contents in the Ordovician sediments. The average contents of Na in the sediments from the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 0.80%, 0.80%, 0.38%, 0.28% and 0.35%, respectively; the average contents of P in the sediments from the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 0.19%, 0.19%, 0.28%, 0.09% and 0.12%, respectively; the average contents of Mn in the sediments from the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 0.01%, 0.01%, 0.01%, 0.04% and 0.03%, respectively; the average contents of Ti in the sediments from the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 0.03%, 0.04%, 0.03%, 0.25% and 0.26%, respectively.

Fig. 2
figure 2

Vertical distribution of major elements across the Ordovician formations in the western margin of the Ordos Basin.

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Trace element distribution

The selected trace element distributions of the collected samples in this study were shown in Fig. 3. In general, the selected elements, including Ni, Cu, Rb, Sr, Zr, Mo, Pb, Th, U, and Ba, show different distribution characteristics among the different formations in the Ordovician strata (Fig. 3). The distribution of trace elements in sedimentary rocks results from the complex interplay of multiple geological factors. Controlling influences can be traced throughout the entire sequence, from provenance, through deposition, to post-depositional diagenesis. Different trace elemental related proxies for reflecting paleoenvironments were deeply investigated in the discussion part.

Fig. 3
figure 3

Vertical distribution of selected trace elements across the Ordovician formations in the western margin of the Ordos Basin.

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Discussion

Input and preservation conditions of organic matter

Paleo-productivity

Paleo-productivity refers to the amount of organic carbon fixed by ancient marine or lacustrine ecosystems per unit of time and area5typically associated with the photosynthetic activity of planktonic organisms. During photosynthesis, phytoplankton can convert atmospheric CO2 into organic matter20and eventually settles and forms organic-rich sediments. High paleo-productivity often implies significant input of organic matter into the depositional environment. Previously, different proxies have been developed to indicate the paleo-productivity in ancient environments21,22,23. P is an important limiting nutrient for primary producers in aquatic ecosystems and it acts as an essential element for the skeletal system of organisms24. In general, the proliferation of primary producers leads to increased consumption of P, which consequently enhances the deposition of P in the sediments. However, Ti is a common element in the Earth’s crust, typically unaffected by biological processes. Hence, P/Ti ratio increases when higher paleo-productivity of the ancient marine or lacustrine ecosystem25. Cu and Ni can also been obviously influenced by organism activities and Cu/Al (ppm/%) and Ni/Al (ppm/%) ratios can serve as indicators of paleo-productivity22,24,26,27,28. In addition, the Al/Ti ratio minimizes the influence of variations in absolute terrigenous flux and instead acts as an indicator of the relative enrichment of nutrient-rich, clay-sized aluminosilicates within the total detrital sediment fraction. A higher ratio signifies a greater potential supply of crucial micronutrients and macronutrients associated with these clays, pointing to conditions conducive to enhanced biological productivity in the overlying water column during deposition.

In general, Ordovician sediments commonly experienced repeated redox oscillations driven by multiple sea-level fluctuations, leading to far more intense phosphorus remobilization than often appreciated. A critical complication arises from the thermal evolution of Ordovician source rocks. Samples having reached overmature stages may have experienced substantial Cu and Ni leaching during late-stage hydrocarbon generation and expulsion processes. Hence, as a baseline indicator of terrigenous flux and nutrient efficiency, Al/Ti is broadly applicable and reliable in Ordovician successions. In this study, Al/Ti ratios in the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 11.38, 13.18, 17.58, 22.33, and 19.41, respectively. Data from sedimentary proxies may suggest the Wulalike Formation had the highest paleoproductivity among the Ordovician strata.​.

Redox condition

Redox condition acts as an important factor influencing the preservation of the organic matters in sediments29. In general, oxygen is a primary factor driving the oxidation of organic matter, promoting its decomposition. In a reducing environment, the absence of oxygen limits oxidative processes, thereby slowing the breakdown of organic matter30,31. Apart from this, most microorganisms responsible for organic matter decomposition rely on oxygen for aerobic respiration. In anaerobic conditions, the activity of these microbes is significantly reduced, or the microbial community shifts to anaerobic organisms, which decompose organic matter at a slower rate, thus favoring its preservation32,33. Trace elements are commonly used as indicators of redox conditions in aquatic environments during sedimentary processes, owing to their sensitivity to changes in oxidation-reduction states24. This sensitivity arises from the distinct chemical behavior of trace elements under different redox conditions, which significantly alters their oxidation states, speciation, and concentration profiles.

U/Th ratio is a useful geochemical proxy for assessing the redox conditions of sedimentary environments and it has been widely applied in previous studies7,34,35. Generally, U is immobile and tends to precipitate out as uranium minerals such as uraninite (UO₂) at reducing conditions, while it is more mobile in its oxidized form at oxidizing conditions24. Th always exists as highly insoluble state in both oxidizing and reducing conditions36. Hence, U/Th ratio of < 0.75 and > 1.25 indicates oxic and anoxic environments, respectively 36.U/Th ratios provide a ​quantitative, diagenetically robust, and widely applicable​ tool for paleoredox reconstructions. When combined with complementary proxies (e.g., Mo, V, Fe speciation), they offer one of the most reliable frameworks for discriminating marine oxygenation states in ancient sedimentary systems. However, lithological differences across formations can also exert a significant influence on Al/Ti ratios. Carbonate-dominated intervals (e.g., Sandaokan and Zhuozishan formations) generally contain lower detrital clay fractions, which may dilute aluminosilicate-associated Al, resulting in lower Al/Ti values. In contrast, shale- or marl-rich intervals (e.g., Wulalike Formation) typically have higher proportions of fine-grained aluminosilicates, increasing the Al content relative to Ti and thereby elevating the Al/Ti ratio. Therefore, part of the observed stratigraphic variation in Al/Ti may reflect primary lithological shifts rather than solely changes in nutrient availability or productivity. Accounting for this lithological control is essential for a more robust interpretation of paleo-productivity trends. In this study, the average U/Th ratios in the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 1.02, 0.92, 0.71, 0.92, and 0.73, respectively. Data from sedimentary proxies suggest the Ordovician strata is generally characterized by a dysoxic condition.

Paleo-climate

Paleo-climate can influence temperature, precipitation, sea-level fluctuations, ocean circulation, and nutrient supply, thereby impacting the accumulation and preservation of organic matter. Sr/Cu ratio is always used as an effective proxy to reflect paleo-climate37,38. It is generally believed that Sr/Cu > 10 indicates a dry climate, 5 < Sr/Cu < 10 suggests a semi-humid to semi-arid climate, and 1 < Sr/Cu < 5 indicates a warm and humid climate39. In this study, Sr/Cu ratios in the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 15.95, 18.49, 32.22, 13.50, and 17.83, respectively.

Rb/Sr ratio also serves as an indicator of paleo-climate40,41. Rb is relatively stable, while Sr is more susceptible to leaching and loss in humid environments42. Hence, the Rb/Sr ratio is generally higher in humid environments, and arid climate conditions are typically associated with a lower Rb/Sr ratio. In this study, Rb/Sr ratios in the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 0.87, 0.82, 0.54, 0.71, and 0.50, respectively.

The classification of paleo-climate by Sr/Cu and Rb/Sr ratios is shown in Fig. 6. The results generally reveal a dry climate of the Ordovician strata. Meanwhile, among the different formations in the Ordovician, the Kelimoli Formation is generally characterized by the driest condition. Paleomagnetic constraints indicate that the western margin of the Ordovician Ordos Basin resided in a low-latitude position proximal to the paleoequator. This latitudinal setting aligns precisely with the basin’s diagnostic paleoclimatic signatures—specifically, the dominance of evaporitic facies and climatically sensitive geochemical proxies—consistent with the persistent influence of tropical atmospheric circulation patterns that govern global climate through latitudinal and altitudinal controls.

Fig. 4
figure 4

Paleoclimate proxies (Sr/Cu and Rb/Sr ratios) across the Ordovician strata in the western Ordos Basin.

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Paleo-salinity

Optimal conditions for organic matter accumulation and preservation include anoxia, reducing sedimentary environments, rapid burial, and the presence of silica-rich lithologies that limit microbial degradation. Among these conditions, paleo-salinity could influence the input and preservation of organic matter by controlling the growth of organisms43. In general, Sr/Ba is a reliable proxy in revealing characteristics of aqueous medium and the ratio increases from freshwater to saline water44. Sr/Ba ratios of < 0.5, 0.5-1, and > 1 reveal fresh water, brackish water and salt water, respectively6,42. In this study, ratios in the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 3.02, 3.21, 6.46, 0.77, and 0.85, respectively. The results indicate a salt water environment during the deposition of the Sandaokan, Zhuozishan, Kelimoli formaitons, and a brackish water environment during the deposition of Wulalike and Lashizhong formations.

Paleo-water depth

Paleo-water depth acts as an important factor influencing the organic matter enrichment and decomposition. Different elemental proxies were developed to reveal the paleo-water depth due to their different behavior in accumulation and dispersion at varying water depths during deposition45. As a stable element in sediments, K primarily resides in coarse detrital minerals (e.g.-feldspar, muscovite). In high-energy hydrodynamical settings (shallow water), coarse-grained minerals (sand fraction) are preferentially enriched, elevating bulk sediment K content. However, Rb is more reactive and prone to being adsorbed and its content tends to increase in deep lake sediments46. Therefore, Rb/K ratio was widely applied in determining paleo-water depth and the ratio increases with increasing water depth. In this study, Rb/K ratios of the Sandaokan, Zhuozishan, Kelimoli, Wulalike and Lashizhong formations are 0.005, 0.006, 0.009, 0.004, and 0.004, respectively. Previous studies demonstrate distinct Rb/K ratio trends across bathymetric gradients in both modern and ancient sedimentary systems. Analysis of shelf-to-basin transects in the Atlantic Ocean reveals Rb/K values of 0.004–0.006 in shallow-marine sandy sediments, increasing to 0.008–0.015 in deep-marine muddy deposits (Tribovillard et al., 2006). In addition, in Ordovician strata of the Appalachian Basin, deep-water graptolitic shale facies exhibit elevated Rb/K ratios (0.012–0.018), contrasting with lower values (0.003–0.005) in coeval shallow-water shelly limestone facies (Rimmer et al., 2004). Collectively, paleobathymetric proxies record significant shifts in water depth during Ordovician deposition in the Ordos Basin. Analysis of sedimentary facies and geochemical indices demonstrates a shallowing-upward mega sequence across key formations: Deposition of the Sandaokan​and Zhuozishan formations​occurred under relatively shallow-water conditions. A pronounced deepening event is registered during sedimentation of the Kelimoli Formation. Subsequent shallowing is documented through the Wulalike Formation to the Lashizhong Formation, reflected by transitional facies associations culminating in peritidal carbonates and evaporites characteristic of very shallow subtidal to supratidal environments.

Variation of paleo-environments across the ordovician

The variation of different paleo-environmental proxies across the Ordovician was presented in Fig. 5. Paleosalinity reconstructions based on Sr/Ba ratios reveal a progressive increase from the Sandaokan to Kelimoli Age, followed by a decline through the Kelimoli to Wulalike Age, indicating an initial salinity rise and subsequent freshening of paleoseawater. Concurrently, Sr/Cu and Rb/Sr ratios remained stable from the Sandaokan to Zhuozishan Age, reflecting persistently arid and hot climatic conditions, but showed a gradual decrease from the Kelimoli to Wulalike Age, signaling a transition toward warmer and more humid conditions. Synchronous trends are observed in redox-sensitive, bathymetric, and productivity proxies: U/Th ratios, Rb/K ratios, and Al/Ti ratios collectively indicate dominantly suboxic conditions, slow water deepening, and progressively enhanced paleoproductivity during the Sandaokan-Zhuozishan interval. This shifted markedly to anoxic/euxinic conditions, rapid deepening, and intensified productivity from the Kelimoli to Wulalike Age. These integrated paleoenvironmental trends align with global Ordovician eustatic records, demonstrating an oscillatory transgressive phase (overall sea-level rise) spanning the Sandaokan through Wulalike stratigraphic succession.

Fig. 5
figure 5

Variations of paleoenvironmental proxies across the Ordovician strata in the western Ordos Basin.

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Under the constraints of the reconstructed paleoenvironmental evolution and integrating previous sedimentological and petrological investigations, the Ordovician sedimentary facies in the western Ordos Basin have been classified47. The stratigraphic succession from the complete transgressive-regressive cycle dominated by an overall sea-level rise. This trend was punctuated by high-frequency sea-level oscillations, particularly pronounced during Zhuozishan Formation deposition. Seven principal sedimentary facies types have been identified as platform margin shoal facies, platform margin reefal facies, fore-slope facies, basin slope (or basin margin) facies, open marine shelf facies, and basin facies.

The depositional systems within key Ordovician successions of the western Ordos Basin—spanning the Sandaokan, Zhuozishan, Kelimoli, and Wulalike formations—exhibit complex lateral transitions between multiple facies belts. Despite these variations, the overall stratigraphic trend registers progressive water deepening, punctuated by numerous relative sea-level fluctuations. Spatially, the Sandaokan Formation demonstrates a westward-to-eastward progression from limestone-dominated basin margin facies through dolomitic limestone fore-slope deposits to platform interior and marginal facies (including shoal complexes and bioconstructed reefal banks). The overlying Zhuozishan Formation transitions similarly from basin margin limestones and dolomitic fore-slope sequences to dolomitized ridge-like paleohighs (interpreted as dolomitized platform margin buildups). In the Kelimoli Formation, facies evolve westward-to-eastward from thin-bedded open-marine shelf limestones to basin margin limestones, dolomitic fore-slope deposits, and peritidal dolomitic flats with circum-continental affinities. Finally, the Wulalike Formation is dominated by carbonaceous shale basin facies, with subordinate argillaceous dolomitic limestones of the open marine shelf and localized marl-rich shelf sequences.​.

Fig. 6
figure 6

Depositional model of the Ordovician strata in the western margin of the Ordos Basin.

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The paleogeographic evolution of the Ordos Basin can be reconstructed. In general, the basin commences with the well-consolidated North China Platform (NCP) in the Precambrian, which formed the unified Paleo-North China Continent. Through the Early-Middle Cambrian, the Yimeng Paleocontinent (north) and Qingyang Paleocontinent (south) experienced successive uplift, establishing the tectonic framework of the Central Paleo-High that persisted into the Late Cambrian-Ordovician. During the Sandaokan Age (Early Ordovician), marine waters gradually transgressed the Ordos Basin, initiating the first Ordovician-wide transgression characterized by shallow depths. The western basin margin developed predominantly platform margin facies. By the Zhuozishan-Kelimoli Ages (Middle Ordovician), progressive deepening facilitated a depositional transition along the western margin to​fore-slope, basin margin, and open marine shelf facies. This evolution culminated in the Wulalike Age with accelerated deepening, establishing a carbonaceous shale-dominated basin fill facies as the principal paleogeographic environment.

Conclusion

Elemental logging data from MJ1 well in the western Ordos Basin were employed to reconstruct the paleoenvironmental evolution of the Ordovician strata and assess its impact on organic matter enrichment. Geochemical proxies reveal a clear transgressive trend from the Sandaokan to the Wulalike formations, accompanied by progressive deepening, increased productivity, suboxic to dysoxic redox conditions, and semi-arid climatic settings. The Wulalike Formation exhibits the most favorable conditions for organic matter accumulation, as indicated by elevated Al/Ti and Sr/Ba ratios and moderately reducing environments. A refined depositional model delineates the lateral transition from platform margin to deep-basin facies, consistent with regional tectonic evolution and global sea-level rise. These results demonstrate the efficacy of elemental logging in paleoenvironmental reconstruction and provide a geochemical framework for evaluating shale gas potential in the western margin of the Ordos Basin.