Abstract
The Paleogene braided river delta deposits in the Lenghu tectonic belt of the northern Qaidam Basin document an environmental transition critical for understanding regional petroleum systems. Through integration of petrographic (thin-section and heavy mineral analyses), geochemical (trace and rare earth element analyses), and geophysical (well logging and 2D seismic interpretation) methods, this study investigates paleoclimate, paleosalinity, paleoredox conditions, provenance, and paleosedimentary evolution. A transition from humid, freshwater, oxic to semiarid-induced brackish, suboxic conditions is revealed, characterized by increasing salinity, depth, and reducibility, particularly during the deposition of the Xiaganchaigou Formation. The Saishiteng Shan is identified as the primary sediment source, reflecting a continental island-arc setting and supplying intermediate basement igneous rocks that influence reservoir quality. Tectonoclimatically controlled depositional facies shift from braided rivers to lacustrine deltas, characterized by thin-bedded discrete sand bodies and low sand-to-mud ratios. These integrated factors highlight the critical role of tectonoclimatic coupling, aridification-enhanced evaporite seals, and intermediate igneous provenance-driven secondary porosity development in controlling reservoir architecture, source rock potential, and petroleum system evolution within the Qaidam Basin and analogous lacustrine settings.
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Introduction
Hydrocarbon exploration in the Qaidam Basin of China has traditionally focused on its western and eastern regions, leaving the northern Lenghu tectonic belt—a critical transitional zone at the northeastern margin of the Tibetan Plateau—relatively underexplored. The Paleogene, marked by global greenhouse conditions and intensified tectonic activity1,2,3, exhibits high potential for deciphering lacustrine basin evolution and petroleum system development in high-altitude, tectonically active settings. While previous studies have documented the basin’s structural framework and sedimentary architecture4,5, persistent knowledge gaps exist regarding the synergistic controls of tectonics, climate, and provenance on Paleogene depositional systems. For instance, the northwestern Junggar Basin demonstrates how aridification (low precipitation) enhances evaporite seals6, and the southeastern Ordos Basin reveals linkages between humid freshwater environments and coal-bearing source rocks7. However, the Qaidam Basin’s unique position as a tectonically confined intermontane basin, influenced by the Altyn Tagh Fault and Qilian Mountain uplift, drives unresolved sedimentary and geochemical dynamics7,8 distinct from these analogues.
Current understanding of Paleogene environments in the northern Qaidam Basin remains fragmented. Provenance studies in the western basin8 and paleoclimate reconstructions in the eastern basin9 lack integration with sedimentological and diagenetic data, hindering predictive models of reservoir heterogeneity. Notably, the role of intermediate igneous provenance—a hallmark of the Saishiteng Shan source terrain10—in driving secondary porosity development remains unquantified, despite its implications for hydrocarbon storage5. These limitations constrain assessments of hydrocarbon migration and accumulation, particularly in understudied high-altitude lacustrine basins globally.
This study addresses these gaps through an integrated analysis of petrographic, geochemical, and geophysical datasets from the Lenghu tectonic belt. We aim to reconstruct Paleogene paleoenvironmental evolution (climate, salinity, redox) and its tectonic drivers, decipher sediment provenance and its diagenetic implications for reservoir quality, and establish a tectonoclimatic model for high-altitude rift basin evolution that explains compartmentalized petroleum systems via evaporite seals and provenance-controlled porosity. Our multidisciplinary approach reveals how the Altyn Tagh Fault’s tectonic isolation and Tibetan Plateau uplift-driven semiaridification11 synergistically control organic matter preservation, seal formation, and reservoir heterogeneity. By contrasting the Qaidam Basin with low-elevation analogues (e.g., Songliao Basin), we provide a mechanistic framework for hydrocarbon exploration in high-altitude lacustrine systems, advancing both regional geology and global sedimentary basin analysis.
Study area
The Qaidam Basin (Fig. 1a), a Mesozoic–Cenozoic fault-bounded depression in Northwest China with a Precambrian metamorphic basement12, functions as an intermontane lacustrine system surrounded by the East Kunlun, Altyn Tagh, and South Qilian orogenic belts13,14 (Fig. 1b). Structurally, it comprises four subunits: the northern, western, and eastern depressions, and the Altyn Tagh piedmont slope13,14 (Fig. 1b). The northern depression (3.4 × 104 km2) hosts nine secondary tectonic units, including the Lenghu tectonic belt in its western sector, which encompasses the Lenghu No. 4–7 structural belts5,15 (Fig. 1b).
(a) Location of the Qaidam Basin in NW China [Map generated using CorelDRAW 2020 (https://www.coreldraw.com/en/pages/coreldraw-2020/)]; (b) Position of the Lenghu tectonic belt within the northern Qaidam Basin [Map generated using CorelDRAW 2020 (https://www.coreldraw.com/en/pages/coreldraw-2020/)]; (c) SWW–NEE geological section illustrating structural architecture and stratigraphic thickness variations. Formation/Member abbreviations: MLJ, Middle/Lower Jurassic; LLH, Paleogene Lulehe Formation (Fm.); LXG, Lower Xiaganchaigou Fm.; UXG, Upper Xiaganchaigou Fm.; SGC, Shangganchaigou Fm.; XYS, Neogene Xiayoushashan Fm.; SYS, Shangyoushashan Fm.; SZG, Shizigou Fm.; QGQ, Quaternary Qigequan Fm.
Seismic data delineate two fault systems (Fig. 1c): (1) a shallow system governing Neogene–Quaternary anticlines, and (2) a deep system controlling Paleogene deposition16. The northern basin underwent progressive northeast-to-southwest deformation due to the Qilian Mountain fold-thrust belt and the Altyn Tagh strike-slip fault17, generating two NW–SE trending fault-fold zones with thinner northeastern (~ 1200 m) and thicker southwestern (> 2500 m) stratigraphic sequences18 (Fig. 1c).
The Qaidam block experienced Late Jurassic–Late Cretaceous uplift and denudation, producing a regional unconformity below the Paleogene strata19 (Fig. 1c). Paleogene units—comprising the Lulehe, Xiaganchaigou (lower and upper Members), and Shangganchaigou Formations—attain thicknesses of 2500–4000 m and accumulated in an early Eocene–Oligocene fan–fluvial–delta–lacustrine system20. During this period, the lacustrine basin expanded significantly, with water depth increasing initially before slight retreat21, while the paleoclimate transitioned from humid and warm to cooler and semiarid22.
Results
Sedimentary facies
Facies analysis of the Lenghu tectonic belt revealed a Paleogene transition from braided river to marginal lacustrine environments (Figs. 2, 3). The Lulehe Fm. was deposited in a large braided river delta plain with a small delta front. The lower Xiaganchaigou Fm. exhibited an increase in the delta front and a decrease in riverine extent compared to the Lulehe Fm. The upper Xiaganchaigou Fm. was dominated by braided river delta front and plain settings. The Shangganchaigou Fm. represented a delta front and shore-shallow lake association.
Lithofacies, logging characteristics, sedimentary structures of wells within different sedimentary facies. (a) Well L95 (braided river); (b) Well XX1 (braided river delta plain); (c) Well Y2 (braided river delta front); (d) Well L9-2 (braided river delta front/shore-shallow lake). Abbreviation: GSCPD, Grain size cumulative probability distribution.
Spatial distribution of sedimentary facies for various Paleogene Fms. in the Lenghu tectonic belt. (a) Late Lulehe Fm.; (b) Early lower Xiaganchaigou Fm.; (c) Late upper Xiaganchaigou Fm.; (d) Late Shangganchaigou Fm. The maps were generated using CorelDRAW 2020 (https://www.coreldraw.com/en/pages/coreldraw-2020/).
The lower Xiaganchaigou Fm. (braided river delta plain) displays: Interbedded argillaceous siltstone, fine and pebbly sandstone, and mudstone with low sand-to-mud ratios (0.4–0.6), indicating shallow-water conditions with intermittent subaerial exposure; Channel erosion evidenced by brownish-grey, grey-green fine and silty sandstone with basal scours and low-angle cross-bedding; Gamma-ray (GR) logs showing bell-shaped patterns (thin sand bodies), box-shaped low-amplitude variations (major sand bodies), and linear high-amplitude variations (interbedded siltstones); Grain size cumulative probability distribution curves indicating dominant saltation and suspension transport, consistent with distributary channels. Notably, interbedded argillaceous siltstones acted as barriers, compartmentalizing reservoir units.
Petrology
Lenghu tectonic belt sandstones are primarily feldspathic litharenite and lithic arkose, with a mean composition of Q35.5F34.3R30.25. Moderately to well sorted, subangular to subrounded grains with fine-, medium- and silty-grained textures dominate (Fig. 4a–h). Calcite, ferrocalcite, and anhydrite constitute the main cementing minerals (Fig. 4i,j). The clastic component comprises highly weathered feldspar (K-feldspar and albite), with lithic grains (30.2%) primarily of metamorphic and volcanic origin5 (Fig. 4k–p).
Petrographic characteristics of Paleogene sandstones. (a) Core photograph (CP) of coarse sandstone, Well L95, 3064.60 m, Lulehe Fm.; (b) CP of medium-fine sandstone with massive structure, Well XX1, 4847.05 m, lower Xiaganchaigou Fm.; (c) CP of fine sandstone interbedded with argillaceous siltstone, Well LL1, 3685.16 m, upper Xiaganchaigou Fm.; (d) CP of siltstone with oblique bedding, Well XX1, 4215.22 m, upper Xiaganchaigou Fm.; (e) Thin-section micrograph (TSM) of medium-coarse feldspathic litharenite, Well L95, 3062.30 m, Lulehe Fm.; (f) TSM of medium-fine feldspathic litharenite, Well XX1, 4852.45 m, lower Xiaganchaigou Fm.; (g) TSM of fine lithic arkose, Well LL1, 3681.38 m, upper Xiaganchaigou Fm.; (h) TSM of silty lithic arkose, Well XX1, 4215.22 m, upper Xiaganchaigou Fm.; (i) TSM revealing later calcite (red arrow) characterized by porous cementation, Well L90, 2928.31 m, upper Xiaganchaigou Fm.; (j) TSM revealing pore-filling anhydrite (cyan arrow) and replacing the early micrite calcite, Well L96, 2696.59 m, Lulehe Fm.; (k) TSM revealing metamorphic rock fragment (MF) (quartzite, yellow arrow), quartz overgrowth (red arrow) and feldspar, Well XX1, 4852.45 m, lower Xiaganchaigou Fm.; (l) TSM revealing MF (quartzite, phyllite and schist, yellow arrow), volcanic rock fragment (VF) (andesite, basalt and tuff, cyan arrow) and mica (pink arrow), Well L92, 2732.78 m, Shangganchaigou Fm.; (m) TSM revealing calcite cement, high-grade (orange arrow) and low-grade (green arrow) MF, and VF (cyan arrow), Well LQ1, 2968.23 m, Shangganchaigou Fm.; (n) TSM revealing quartz, feldspar, MF (quartzite, yellow arrow), calcite and anhydrite cements, Well LL1, 3680.58 m, upper Xiaganchaigou Fm.; (o) TSM revealing MF (quartzite and schist, yellow arrow), quartz and feldspar, Well XX1, 4852.45 m, lower Xiaganchaigou Fm.; (p) TSM revealing quartz with wavy extinction, feldspar with kaolinization or sericitization, MF (quartzite and phyllite, yellow arrow), VF (arkosite, cyan arrow), calcite and ferrocalcite cements, Well XX1, 4115.06 m, upper Xiaganchaigou Fm. Abbreviations: DQ, quartz; FS, feldspar.
Heavy minerals
Heavy mineral analysis revealed a dominant composition of magnetite (mean 29.1%), leucoxene (mean 19.5%), zircon (mean 17.2%), garnet (mean 16.1%), epidote (mean 8.2%), and sphene (mean 2.4%) (Table S1). The Lenghu No. 4–5 tectonic belts are characterized by magnetite–garnet–zircon–epidote assemblages (Fig. 5a), while the Lenghu No. 6–7 exhibit magnetite–leucoxene–garnet–zircon assemblages (Fig. 5b). Zircon–tourmaline–rutile (ZTR) values varied significantly across the Paleogene strata and tectonic subbelts: Lulehe (mean 15.5), lower Xiaganchaigou (mean 16.0), upper Xiaganchaigou (mean 22.5), and Shangganchaigou (mean 19.3) (Table S1). These variations in heavy mineral assemblages and ZTR values suggest provenance changes through time (Fig. 6).
Percentage distribution of heavy minerals in Paleogene sandstones of Well L95 (a) and Well LQ2 (b).
Element geochemistry
Major and trace element content and ratios
Mudstone samples exhibited elevated concentrations of V (mean 114.80 μg/g), Co (mean 18.98 μg/g), and Cr (mean 63.05 μg/g) compared to upper crustal values (Table 1). Key element ratios showed distinct trends (Table 2): Sr/Cu, Sr/Ba, V/(V + Ni), and V/Sc increased from 4.79, 0.45, 0.60, and 5.82 in the Lulehe Fm. to 9.53, 0.73, 0.70, and 6.98 in the Shangganchaigou Fm., respectively; Conversely, Rb/Sr, Th/U, and V/Cr decreased from 0.37, 3.96, and 1.82 in the Lulehe Fm. to 0.32, 3.84, and 1.64 in the Shangganchaigou Fm., respectively.
REE content and ratios
Total concentrations of 14 REEs in mudstones range from 27.72 to 220.07 μg/g (mean 129.55 μg/g) (Tables 3, 4). Chondrite-normalized REE patterns show light REE enrichment, heavy REE depletion, and negative Eu and Ce anomalies. (La/Yb)C values range from 5.73 to 30.54 (mean 9.11), and (Ce/Yb)C ratios range from 3.84 to 21.39 (mean 6.52) (Fig. 7a–d). The Lenghu No. 4–5 exhibited more pronounced negative Eu anomalies than the Lenghu No. 6–7. Three samples (L90-1, L92-1, XX1-4) showed positive δCeC anomalies (Tables 3, 4). REE concentrations generally increased from the Lower Paleogene (Lulehe and lower Xiaganchaigou Fms.) to the Upper Paleogene (upper Xiaganchaigou and Shangganchaigou Fms.) (Fig. 7e).
Discussion
Paleoenvironment
Paleoclimate: semiaridification driven by Tibetan Plateau uplift
The upward enrichment of Sr (Fig. 8a) and depletion of Cu (Fig. 8b) and V (Fig. 8c) suggest increasing water depth from the Lulehe Fm. to the Shangganchaigou Fm., reflecting climate-driven fluctuations in terrigenous influx and lacustrine sedimentation27. Rb/Sr ratios (0.26–0.37; Fig. 8d) indicate predominantly humid conditions during the early Paleogene, consistent with chlorite content and sporopollen records from the Xining Basin28,29. Notably, the gradual rise in Sr/Cu ratios (4.79 to 9.53; Fig. 8e) signals a pronounced shift toward semiaridity (low precipitation regime)30, contrasting with stable humid conditions in the Paleogene Ordos Basin31 but aligning with aridification trends documented in the Tarim Basin32. This divergence underscores the Qaidam Basin’s transitional position between monsoonal and semiarid regimes, modulated by the Tibetan Plateau uplift and its influence on regional atmospheric circulation. Lower Fe/Mn ratios during the Xiaganchaigou Fm. (mean 30.73–35.46 vs. 53.60 in the Shangganchaigou Fm.; Fig. 8f) further corroborate intensified evaporation under increasingly semiarid conditions. The global Eocene–Oligocene cooling patterns recorded in deep-sea foraminifera oxygen isotopes33,34 and the Paratethys Sea retreat35,36 likely amplified this regional semiaridification through teleconnections with plateau uplift.
Vertical variations of major and trace element contents, and relevant ratios, in Paleogene mudstone of the study area. Formation/Member abbreviations: Fig. 7.
Paleosalinity: tectonic isolation and evaporative enrichment
Sr/Ba37 and Th/U38 ratios (0.45–0.73; Fig. 8 g and 3.84–5.52; Fig. 8h) document a transition from freshwater to brackish conditions driven by evaporative concentration39,40. The elevated salinity in the Shangganchaigou Fm. (Sr/Ba = 0.73) exceeds values in the Ordos Basin (Sr/Ba < 0.3)22,31 and parallels hypersaline phases in the Eocene Green River Fm., USA39. This trend primarily reflects tectonic isolation by the Altyn Tagh Fault, which restricted freshwater influx through basin compartmentalization40,41—though episodic freshwater inputs during the Xiaganchaigou Fm., inferred from sporopollen records11, temporarily disrupted this trajectory. Such salinity increases enhanced organic matter preservation, evidenced by laminated shales with TOC ≤ 3.2 wt%42, mirroring hydrocarbon-rich facies in the Permian Lucaogou Fm. of the Junggar/Santanghu Basin43.
Paleoredox conditions: oxygenation dynamics in a high-altitude lake
V/Cr44,45 (1.64–1.95; Fig. 8i), V/Sc46 (5.82–6.98; Fig. 8j) and V/(V + Ni)47 (0.60–0.70; Fig. 8k) ratios collectively indicate predominantly oxic to suboxic conditions in shallow, oxygenated waters. These findings contrast with anoxic environments in the Meihe Basin48 but align with redox trends in the Xining Basin29. The prevalent negative Ce and Eu anomalies (Table 4) further corroborate oxidizing conditions49,50, though localized suboxia (positive Ce anomalies in three samples) likely resulted from episodic organic matter accumulation, akin to processes in the Songliao Basin51. While the oxic-suboxic regime limited bulk organic carbon burial, elevated salinity during the Shangganchaigou Fm. enhanced preservation via water-column stratification52, suppressing vertical mixing and creating anoxic microenvironments analogous to Lake Tanganyika53. This tectonoclimatically controlled redox equilibrium (regional oxidative weathering vs. transient suboxia) ultimately modulated hydrocarbon source rock potential22.
Provenance direction and attributes
Provenance analysis confirms that the Paleogene sediment in the Lenghu tectonic belt was primarily sourced from the Saishiteng Shan, with secondary contributions from the Xiaosaishiteng Shan4. Heavy mineral assemblages and ZTR indices demonstrate consistent provenance inheritance, showing increased sediment flux from the Saishiteng Shan (NEE) during the Paleogene (Fig. 9). Sediment supply from the Xiaosaishiteng Shan (NNE) peaked in the upper Xiaganchaigou Fm. before declining toward the Shangganchaigou Fm., supported by higher epidote content in the Lulehe and Xiaganchaigou Fms. indicating proximal input (Fig. 9). REE patterns corroborate this shift: pronounced negative Eu anomalies in early formations implicate Xiaosaishiteng Shan sources, while moderately negative anomalies in the Shangganchaigou Fm. reflect dominant Saishiteng Shan and Altyn Tagh Mountains contributions (Fig. 7).
Spatial distribution, ZTR isoline and provenance analysis of Paleogene heavy minerals in the western segment of the northern Qaidam Basin. (a) Lulehe Fm.; (b) Lower Xiaganchaigou Fm.; (c) Upper Xiaganchaigou Fm.; (d) Shangganchaigou Fm. The maps were generated using CorelDRAW 2020 (https://www.coreldraw.com/en/pages/coreldraw-2020/).
Regional studies indicate south-directed sourcing from the South Qilian Mountains54,55,56, though within the Lenghu belt, the Saishiteng Shan dominated while the Xiaosaishiteng Shan acted as a localized source (Figs. 4a–j, 9). Late Paleogene Qilian Mountain fold-thrust activity amplified sediment supply from Qaidam BeiShan and Lvliang Shan, whereas Altyn Tagh Fault sinistral strike-slip movement restricted northern input57 (Fig. 9). Concurrent semiaridification reduced both weathering rates and sediment yield, further limiting material availability58,59,60.
Tectonic setting discrimination diagrams and REE signatures confirm a continental island-arc setting61,62,63 (Fig. 10a–c). Geochemical proxies (La/Yb–ΣREE and Co/Th–La/Sc) indicate derivation from highly weathered intermediate igneous rocks of the Tanjianshan Group64 (Fig. 10d,e), formed during early Paleozoic magmatism61,62. This intermediate provenance notably controlled reservoir quality: elevated feldspar content (mean 34.3%) facilitated secondary porosity development during mesodiagenesis5, though concomitant clay growth reduced permeability (Fig. 4i,j,p). Unlike southern Qaidam (Qilian-dominated sourcing), Saishiteng Shan-derived sediment reflected differential uplift along the Tibetan Plateau margins54,56, directly driving reservoir heterogeneity through feldspar dissolution5,64.
Tectonic setting discrimination diagrams of Paleogene parent rocks. (a) La–Th–Sc tectonic discrimination diagram61; (b) Ti/Zr–La/Sc tectonic discrimination diagram61; (c) Comparison of Chondrite-normalized REE pattern with typical tectonic setting sandstones62; (d) Co/Th–La/Sc parent rock discrimination diagram63; (e) La/Yb–ΣREE parent rock discrimination diagram62. Tectonic setting: A, Oceanic island arc; B, Continental island arc; C, Active continental margin; D, Passive continental margin.
Sedimentary evolution and model
The Paleogene sedimentary sequence in the Lenghu tectonic belt exhibits a vertical succession from braided river and deltaic deposits to shore-shallow lake facies20 (Fig. 2). Unlike typical deltas21, these braided river delta systems feature thin-bedded siltstone and fine sandstone (typically 2–5 m thick) interbedded with oxidized mudstones, reflecting shallow-water conditions and intermittent subaerial exposure due to distant sediment sources20,22. Characterized by low sand-to-mud ratios (0.4–0.6) and progradational-retrogradational cyclicity, these deposits record tectonoclimatically controlled sediment supply variations65 (Fig. 3). The Lulehe to Shangganchaigou Fms. document a systematic transition: from alluvial–braided river systems during humid periods to fluctuating deltaic environments under semiarid conditions, with tectonic pulses modulating accommodation space and sediment dispersal66.
Based on stable basement subsidence, gentle paleotopography, and climatic controls, an “alluvial fan–braided river–braided river delta–shore-shallow lake” sedimentary model was established (Fig. 11). This high-altitude lacustrine basin model differs fundamentally from low-elevation analogues (e.g., Songliao Basin51) through three diagnostic characteristics: (1) Thin, discrete sand bodies (< 10 m thick); (2) Persistently low sand-to-mud ratios (0.4–0.6); (3) Evaporite-enhanced sealing capacity during semiarid phases6,39. These features directly derive from the basin’s unique tectonoclimatic setting: distal sediment sources, semiaridification trend, and structural confinement by the Altyn Tagh Fault22. Ultimately, the model explains compartmentalized reservoirs with limited lateral connectivity—a critical control on petroleum system architecture5,18.
Sedimentary model evolution and sequence assemblages of the Paleogene braided river delta sedimentary system during different climatic conditions and dry/flood periods66. (a) Alluvial fan–braided river (semihumid); (b) Delta (humid); (c) Delta (semiarid); (d) Alluvial fan–braided river (lowstand); (e) Delta (early transgressive); (f) Delta (late transgressive). Formation/Member abbreviations: Fig. 1c.
Coupling of climatic and tectonic controls
The interplay between climate and tectonics modulated Paleogene depositional systems in the Lenghu tectonic belt22,66,67. Initially, the sedimentary setting was characterized by a humid climate and freshwater with high oxygen content68. During the early Paleogene, the collision between the Asian and Indian plates drove intense Tibetan Plateau uplift, triggering northward displacement of the Qaidam Basin69 (Fig. 12). Coupled with warm airflows, this resulted in sustained warm-humid conditions and abundant precipitation. Subsequent climatic transition to a semiarid regime increased paleowater salinity, depth, and reducibility22,68. During the Lulehe Fm. deposition, fault activity and crustal uplift12 generated steep terrains promoting alluvial fans and braided rivers, with highly oxygenated meteoric water maintaining freshwater conditions (Figs. 3, 12). Subsequent tectonic reorganization during the Xiaganchaigou Fm. deposition reduced topographic gradients and enhanced lake transgression, expanding the lacustrine areas70 (Fig. 12). Concurrently, climatic transition from humid to semiarid conditions drove a systematic facies shift: early humid-phase “large delta plain, small delta front” systems evolved into late semiarid-phase “large front, small plain” deltas71 (Figs. 3, 12).
Progressive lake deepening under fluctuating climate increased paleosalinity (Sr/Ba: 0.45 to 0.73) and diminished oxygenation (V/Cr: 1.82 to 1.64) (Figs. 8, 12). Post-Xiaganchaigou, Tibetan Plateau uplift intensified through Qilian Mountain lateral extrusion and fault-fold activity, elevating the basin altitude, reducing the terrain slopes, and amplifying semiaridification72. This tectonoclimatic synergy culminated in brackish, suboxic conditions during the Shangganchaigou Fm. deposition, shifting the lake center northeastward and forming the saline lacustrine environments.
The interplay of tectonics and climate modulates petroleum systems in the Qaidam Basin: Tectonic processes govern basin subsidence, sediment supply, and structural architecture; Climatic regimes regulate weathering intensity, organic matter preservation, and depositional patterns68. This coupling dictates hydrocarbon habitat elements: (1) Source rock distribution (e.g., saline lacustrine organofacies39,52); (2) Reservoir geometry (compartmentalized sand bodies5,18); (3) Seal integrity (evaporite enhancement during semiarid phases6,16). Ultimately, it constrains hydrocarbon generation, migration, and accumulation efficiency. The Songliao Basin analogue73 validates this model’s universality but highlights the Qaidam’s uniqueness: whereas both record river-to-lake transitions, the Qaidam’s high-altitude and semiarid setting produced thinner sand bodies (< 10 m), lower sand-to-mud ratios (0.4–0.6 vs. > 0.8), and evaporite-enhanced seals6,39. These differences directly stem from tectonoclimatically controlled sediment supply and preservation dynamics, ultimately generating compartmentalized reservoirs critical for hydrocarbon entrapment5,18.
Conclusion
The Paleogene sedimentary evolution of the Lenghu tectonic belt records a tectonically and climatically driven transition from braided river delta to lacustrine depositional systems. Provenance analysis identifies the Saishiteng Shan as the dominant sediment source, supplying intermediate igneous detritus that enhanced secondary porosity development during mesodiagenesis. Concurrently, paleoenvironmental shifts—from humid, oxic, freshwater to semiarid, suboxic, brackish conditions—were controlled by tectonic isolation, evaporative concentration, and fluctuating lake levels. These processes notably influenced organic matter preservation in lacustrine mudstones, promoting source rock development while generating compartmentalized reservoirs through: (1) thin, discrete sand bodies (< 10 m), (2) low sand-to-mud ratios (0.4–0.6), (3) evaporite-enhanced seals. This integrated framework explains petroleum system development in high-altitude rift basins. Unlike low-elevation analogues, the Qaidam Basin’s tectonoclimatic coupling—specifically Tibetan Plateau uplift-driven semiaridification and intermediate igneous provenance—produces diagnostic reservoir heterogeneity and seal efficiency. These insights enable predictive models for reservoir architecture and source rock distribution in underexplored high-altitude lacustrine settings globally.
Materials and methods
Core, log, and seismic integration
This study integrated core, well log, and seismic data. Core and log data from 21 Paleogene wells in the Lenghu tectonic belt were used to analyze sandy sediments and interpret the sedimentary system, using indicators including rock color, lithology, texture, and structure. Facies distribution was delineated through single-well core columns, with deltaic and lacustrine facies mapped systematically. Sampling intervals were selected based on GR log trends to capture subfacies transitions. Seismic interpretation utilized two 3D blocks and 15 2D lines to resolve structural controls on deposition16,18.
Sandstone petrography and grain size
Petrographic analysis was performed on 24 blue epoxy-impregnated thin sections from 39 sandstone samples (Lulehe Fm.: 9; Xiaganchaigou Fm.: 22; Shangganchaigou Fm.: 8; total coverage: ~ 149 m from 11 key wells). Point counting (300 points per section) used a Zeiss Axio Imager A1 polarizing microscope following the Gazzi-Dickinson method74, with data reported to one decimal place. Grain size analysis of 18 sandstone samples employed an MS-2000 laser analyzer, adhering to the Chinese oil and gas industry standard SY/T 5434–200975. Calibration was verified using NIST-traceable glass beads (10–2000 μm range; ± 1% precision).
Heavy minerals and mudstone geochemistry
Heavy mineral analysis of 56 samples used a Leica DMLP polarizing microscope. Separation followed standard sodium polytungstate heavy liquid procedures (ρ = 2.85 g/cm3)76, with reproducibility confirmed via duplicate samples (relative standard deviation (RSD) < 5%). An additional 159 datasets from the Qinghai Oilfield archives and published sources77 supplemented provenance analysis. For geochemistry, 13 mudstone samples from ten wells (Lulehe Fm.: 4; Xiaganchaigou Fm.: 6; Shangganchaigou Fm.: 3; total depth span: 1678.3–5187.9 m) were analyzed by ICP-MS (PerkinElmer NexION 300D)78: (1) Calibration: USGS SCo-1 shale and NIST SRM 1648a standards analyzed in triplicate; (2) Precision: RSD < 5% for trace elements (e.g., V, Cr, Co); (3) Detection limits: 0.01 μg/g (REE) to 0.1 μg/g (major elements). Paleoenvironmental conditions and tectonic settings were interpreted using element ratios and REE patterns23,61,79. Sample size constraints are acknowledged; future work will expand spatial coverage to enhance statistical robustness.
Data availability
All data are provided within the manuscript or supplementary information files.
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Acknowledgements
We thank the Exploration & Development Research Institute, Qinghai Oilfield Company, PetroChina, for providing core samples and geological data. This work was jointly supported by the National Science and Technology Major Projects of China (Grant No. 2016ZX05033-001-002) and the Special Scientific Research Fund of Yunnan Communications Vocational and Technical College (Grant No. 202501JTBS001). We also acknowledge Qi Wang, Puyu Liu, and Wenwen Wu for technical assistance with core description, geochemical analysis, and seismic processing.
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L.G. conceptualized the study, conducted petrographic and geochemical analyses, and wrote the original manuscript. W.G. prepared Figs. 1, 23, 4, 5, 6, 7, 8 and performed sedimentary facies interpretation. X.G. prepared Figs. 9, 10, 11, 12 and conducted provenance analysis. Z.H. supervised sample collection, coordinated geochemical analysis, and validated data interpretation. All authors reviewed and edited the manuscript.
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Gong, L., Gai, W., Gao, X. et al. Tectonoclimatic controls on Paleogene braided river delta evolution and petroleum systems in the Lenghu tectonic belt, northern Qaidam Basin. Sci Rep 15, 31230 (2025). https://doi.org/10.1038/s41598-025-15392-z
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DOI: https://doi.org/10.1038/s41598-025-15392-z
