Lithologic Reservoirs ›› 2026, Vol. 38 ›› Issue (1): 1-12.doi: 10.12108/yxyqc.20260101

• PETROLEUM EXPLORATION •    

Geological characteristics and genesis of trona deposit in Cretaceous Yixian Formation of Naiman Sag, Kailu Basin

HU Changhao1,2(), PEI Jiaxue2, YANG Xue2, CAI Guogang2, FAN Jiaming2, LI Li2   

  1. 1 School of Geosciences and Info-physics, Central South University, Changsha 410083, China
    2 Research Institute of Exploration and Development， PetroChina Liaohe Oilfield Company, Panjin 124010, Liaoning, China
  • Received:2025-07-27 Revised:2025-08-22 Online:2026-01-01 Published:2026-01-23

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Abstract:

Naiman Sag of Kailu Basin has developed unique trona deposit, which is paragenetically associated with salt and crude oil. Based on the data of seismic, logging data, drilling core laboratory analysis, the lithology assemblage, ore characteristics, deposit features and distribution patterns of the trona deposit in Cretaceous Yixian Formation were clarified. Meanwhile, the genesis and patterns of the deposit were investigated in terms of paleostructure, paleoclimate, sources of ore-forming materials, and their coupling relationships. The research results indicate that: (1) The Yixian Formation deposit of Cretaceous in Naiman Sag is characterized by deep burial, steep dip, fault development, thin individual layers and numerous interbeds. Ore body shape is approximately elliptical, with a north-south distribution direction and a lenticular distribution pattern featuring thickened centers and thinning edges. Two sedimentary sequences are developed: A soda-lake type sequence (mudstone-dolomite → trona → mudstone) dominates the deposit margins, while a saline-lake type sequence (mudstone-dolomite → trona → halite → trona → mudstone) prevails in the central zone. On the plane, trona-saline trona-salt distribute in an annular pattern. (2) The deposits are generally oil-bearing. Within mudstone layers, crude oil occurs in intergranular pores of fine clastics, inter-crystalline pores of clay minerals, fractures, and kerogen. In trona and salt layers, hydrocarbons primarily reside in mineral inter-crystalline pores and fractures, cleavage fractures, inclusions, and lattice defects. (3) The trona deposit in the study area is a ternary coupled mineral model of “paleogeomorphology-paleoclimate-volcanic activity”, the deposition of Yixian Formation constituted a closed lacustrine basin. Sodium ions (Na⁺) were continuously supplied to the basin by sodium-rich andesitic rocks in the periphery. The decomposition of organic-rich mudstone produces considerable CO2, which increases the concentration of CO2 in brine. The frequent volcanic activities during the Yanshan period led to the upwelling of deep-seated hydrothermal fluids and mantle-derived gas through faults, providing a large amount of CO2 and Na+ -rich hydrothermal fluid. A persistent arid climate towards the end of Yixian Formation resulted in extreme concentration of the lake water, reaching saturation and precipitating trona or halite. Subsequent diagenesis formed the present trona deposit.

Key words: trona deposit, paleostructure, paleoclimate, carbon source, sodium source, co-occurrence of oil, trona and salt, Yixian Formation, Cretaceous, Naiman Sag, Kailu Basin

CLC Number: 

  • TE121

Fig. 1

Regional location and structural outline (a), and Cretaceous lithostratigraphic column （b） in Naiman Sag, Kailu Basin"

Fig. 2

Distribution characteristics of trona deposit within Cretaceous Yixian Formation of Naiman Sag, Kailu Basin"

Fig. 3

Hydrocarbon evidences and occurrence modes within Cretaceous Yixian Formation trona deposit of Naiman Sag, Kailu Basin"

Table 1

Comparison of geological characteristics of trona deposits in domestic and abroad"

矿床名称 位置 面积/km2 埋深/m 成矿地质年代 品位/% 储量/108 t
安棚 河南南阳桐柏县 10.47 1 288~2 418 古近纪 44.00 1.500 2
吴城 河南南阳桐柏县 4.66 643~974 古近纪 41.68 0.326 7
查干淖尔 内蒙古锡林郭勒盟 21.00 20~30 第四纪 27.60 5.379 1
鄂尔多斯 内蒙古鄂尔多斯市 4.00~40.00 3~8 第四纪 11.40 0.113 7
塔木素 阿拉善右旗 10.91（2区） 360~630 白垩纪 69.45 7.090 0
29.08（5区） 380~650 白垩纪
绿河 怀俄明州绿河地区 3 370.00 198~914 古近纪 75.00 560.000 0
贝帕扎里 土耳其安卡拉 8.00 250~430 古近纪 66.47~90.22 2.350 0
奈曼 内蒙古通辽市 24.00 800~3 000 白垩纪 45.94~60.93 20.770 0

Fig. 4

Core and thin section photographs of primary minerals of trona deposit in Cretaceous Yixian Formation in Naiman Sag, Kailu Basin"

Fig. 5

Core and thin section photographs of secondary minerals in Cretaceous Yixian Formation trona deposit in Naiman Sag, Kailu Basin"

Fig. 6

Paleogeomorphology during the Cretaceous Yixian Formation depositional period in Naiman Sag, Kailu Basin"

Table 2

Analysis of chemical elements of rock samples of Cretaceous Yixian-Jufotang formations in Naiman Sag, Kailu Basin"

样
号		 深度/m Sr/Cu MgO/
CaO		 Fe2O3/
MnO		 δ13CPDB/
‰		 δ18OPDB/
‰		 沉积
时期
1 854.1 4.83 3.49 27.57 2.33 -9.21 九佛
堂组
沉积
时期
2 854.5 5.13 1.61 27.23 2.61 -8.51
3 855.0 4.99 1.39 28.79 3.12 -7.65
4 855.5 5.25 1.55 24.81 1.94 -8.30
5 856.0 4.54 2.83 31.20 2.11 -8.14
6 856.5 2.56 1.87 48.30 2.07 -8.97
平均值 4.55 2.12 31.32 2.36 -8.46
7 1 486.5 84.54 7.64 1.00 1.92 -4.00 义县组沉积
晚期（末期
沉积蒸发岩）
8 1 487.1 47.84 5.35 1.55 2.48 -4.16
10 1 488.0 71.06 4.86 0.37 2.05 -3.83
11 1 488.5 40.24 1.46 3.43 4.37 -5.61
12 1 489.0 40.28 9.31 0.56 2.15 -5.12
平均值 56.79 5.72 1.38 2.59 -4.54
13 1 881.1 5.36 0.92 31.69 2.15 -15.01 义县组沉积
早—
中期
14 1 882.3 1.56 1.03 35.73 0.41 -12.94
15 1 883.6 2.86 2.00 22.77 3.59 -12.52
16 1 884.5 3.40 0.57 15.05 3.19 -13.78
17 2 049.5 4.40 1.52 30.13 1.72 -17.69
18 2 050.0 3.49 2.08 20.06 1.62 -16.31
19 2 050.6 4.29 1.05 21.29 2.41 -16.50
20 2 051.2 5.92 1.15 50.44 2.51 -18.16
平均值 3.91 1.29 28.39 2.20 -15.36

Fig. 7

Sporopollen assemblages from Cretaceous late Yixian Formation mudstone in well N10 of Naiman Sag, Kailu Basin"

Fig. 8

Distribution characteristics of CO2 content in Yixian Formation of Cretaceous, Block N1, Naiman Sag, Kailu Basin"

Fig. 9

Characteristic minerals of volcanic hydrothermal origin within the Cretaceous Yixian Formation trona layer,well N33, Naiman Sag, Kailu Basin"

Fig. 10

Metallogenic model of the trona deposit of present （a） and the end deposition of Cretaceous Yixian Formation （b） in Naiman Sag， Kailu Basin"

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