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. 2024 Feb 12;9(8):9728–9734. doi: 10.1021/acsomega.3c09928

Characterization of Lower Paleozoic Water and Gas Distribution in Yan’an Gas Field, Ordos Basin

Xiaoping Gao 1, Shulong Yuan 1, Yunlong Tan 1, Mengyi Wang 1, Yong Yang 1,*
PMCID: PMC10905687  PMID: 38434882

Abstract

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The distribution of gas and water in the tight gas reservoirs is complex. This limits exploration and development compared to conventional resources. Elucidating the characteristics that control fluid distribution is critical to unlocking the tight gas potential. This study combines geologic analysis with production data to reveal the water chemistry, gas–water distribution, and factors controlling zoning in the study area. The results show that (a) extracted water includes formation brines, condensate, and residual drilling fluids; (b) formation water dominates production, and the salinity of Lower Paleozoic brines is as high as 169,689 mg/L; (c) low NaCl and high metamorphic coefficients indicate that the water bodies are disconnected and the hydrocarbons are well-preserved in several gas–water systems; and (d) paleomorphological features, tectonics, and lithologies control the distribution of gas water. Discrete water bodies are widely distributed.

1. Introduction

Although we have seen a dramatic increase in tight gas, there are still many obstacles preventing further exploration and development of tight gas.1,2 Water extraction from gas wells in Yan’an gas field severely restricts the development of the gas reservoir.3 It is of great significance to reveal the distribution characteristics of water gas.4 Previous studies have shown that the gas–water relationship in tight reservoirs is diverse.5 It is pointed out that there are conventional gas–water distribution and gas–water countercurrent body distribution relationships in tight oil and gas basins.6,7 It is difficult to explain this complex gas–water distribution relationship with a single genesis mechanism.8−10

According to the nature of the basement, geological evolution history, and tectonic features, the Ordos Basin can be divided into six tectonic units: the Yimeng Uplift, the Yishan Slope, the Tianhuan ore-seeking depressions, the Jinshi Belt, the Western Marginal Fold Belt, and the Weibei Uplift.11−14 The Yishan Slope was formed at the end of the early Bailian period.15−17 It is the largest primary tectonic unit in the basin, 250 km wide from east to west and 400 km long from north to south.18−20 The current tectonic feature is a large, gently dipping monocline that dips to the west, with an average dip of about 1° and a dip of less than 1°.21,22 The Ordos Basin was uplifted as a whole at the end of the Ordovician in the Early Paleozoic and underwent hundreds of millions of years of weathering stripping and candle stripping without Silurian, Devonian, and Lower Carboniferous.23,24 During this period, the weathering crustal solution candle stripping zones formed by exposed weathering and buried karst are of great significance for the formation of ancient weathered crustal gas reservoirs in the Lower Paleoproterozoic.25−27 Feng et al.28 studied the geochemical characteristics of natural gas in the Paleozoic era. Zhang et al.29 conducted a systematic study on pore structures such as primary pores, secondary pores, and microcracks. Li et al.30 conducted a study on the genesis of hydrogen sulfide in the Jurassic Yan’an Formation in the Pengyang area of the Ordos Basin. Li et al.31 explored the main sources of CO2 and N2 in the Lower Paleozoic. Feng et al.32 analyzed the composition and isotopic characteristics of natural gas in the Yan’an area of the Ordos Basin. Han et al.33 analyzed the gas in the package and compared it with the produced gas. Wang et al.34 analyzed the geochemical characteristics of Paleozoic natural gas in the southern part of Jingbian gas field. Sun et al.35−38 analyzed the fluid distribution characteristics and evolution patterns in unconventional reservoirs. Qin et al.39 analyzed the fracture morphology characteristics and engineering mechanical strength within the core particles. Hou et al.40 used outcrop, drilling, logging, thin section, and geochemical data to study the sedimentary sequence patterns near the Carboniferous Permian boundary in the Ordos Basin.

In summary, currently, systematic research has been conducted on the distribution characteristics of gas and water, but there is relatively little research on the distribution patterns of gas and water based on the geochemical characteristics of formation water, and limited systematic studies have been conducted on the hierarchical water distribution pattern. Therefore, this paper takes Lower Paleozoic as the research object. First, the chemical properties of formation water are analyzed, and the coupling control mechanism of structural, paleomorphic, and lithological gas water distribution is elucidated. On this basis, the main controlling factors on the basis of geological analysis and production data are analyzed, revealing the water retention properties in dense dolomite isolation and the geological model of lens shaped water formation in isolated dolomite.

2. Geological Background

The carbonate reservoirs of the Majiagou Formation are widely distributed. The Majiagou Formation is a set of carbonatite and evaporite interbedded rocks consisting of six sections. The first, third, and fifth sections mainly consist of dolomite and mud crystal crushed dolomite and gypsum dolomite, which were formed in dolomitic ping, gypsum ping, and dolomitic gypsum ping, respectively. Sections 2, 4, and 6 consist mainly of limestone. Specifically, the fifth section has 10 subsections. Among them, the fifth subsection is the main reservoir, which is widely distributed in the east-central part of the basin (Figure 1).

Figure 1.

Figure 1

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Full range of core permeability testing techniques. Natural gas/oil is injected through the inlet, and water is drained from the core, which simulates the migration of hydrocarbons through the dense core.

3. Stratigraphic Water Chemistry and Types of Extracted Water

The Majiagou Formation has two main producing sections, namely, section 5-1+2 and section 5-4. In order to accurately understand the formation water characteristics of the Lower Paleozoic 5-1+2 section and 5-4 section reservoirs in the southern area of Yan’an gas field, in this paper, on the basis of the existing water analysis data of production wells, the formation water characteristics of the 5-1+2 section and 5-4 section of the reservoir are derived through collation, statistics, and analysis.

Table 1 shows the chemical characteristics of the formation water in the two members. As shown in Table 1, the cations in the formation water are mainly K+, Na+, Ca2+, and Mg2+. Among them, the concentrations of the alkaline cations K+, Na+, Ca2+, and Mg2+ varied widely. Stratigraphic water was dominated by alkaline cations, and the anions were mainly Cl–, SO42–, and HCO3–. The concentration of Cl– was the highest, accounting for 93% of the total anions. The salinity of the formation water ranged from 48.18 to 196.70 mg/L, with an average of 112.23 mg/L, which was much higher than the salinity of seawater of 35 mg/L, the saline water level, and the salinity of CaCl2 type water. The stratigraphy is mainly developed in a marine carbonate environment with a deeper burial and better closure conditions. Deep circulation of the water body occurs, and the original characteristics of more residual seawater are preserved. In the long process of geological development, the stratum water is continuously enriched and metamorphosed, forming the present stratum water with high salinity characteristics.

Table 1. Stratigraphic Water Chemistry.

  well PH density K+ Na+ Ca+ Mg+ Cl–
5-1+2 member Majiagou Formation A 5.44 1.08 676.47 9234.20 17317.20 3748.28 84,640.54
B 5.53 1.03 1014.07 819.61 1901.07 6134.42 65,780.94
C 5.46 1.16 654.97 642.49 16,266.76 4691.20 14,742.51
D 5.33 1.16 851.07 159.68 18,331.62 7752.22 79,434.29
E 5.36 1.09 659.00 621.71 16,002.70 11,454.66 72,537.09
F 5.41 1.19 964.83 97.75 4658.06 676.57 19,408.12
G 5.56 1.03 342.07 1032.96 13,986.03 11,198.94 77,629.18
H 5.55 1.05 108.40 781.46 8367.38 2862.53 19,536.54
range 5.31–5.92 1.02–1.19 73.5–1035 29.5–16,600 1200–27,815 262–12,775 9907–98,100
average 5.623 1.08 501 4308 16,189 4006 58,808
5-4 member Majiagou Formation I 6.18 1.12 617.63 17,267.32 26,250.76 6778.94 94,404.91
J 5.79 1.09 1000.36 7456.79 14,565.79 5432.03 93,309.42
K 6.30 1.09 449.79 10,353.56 13,342.02 5880.84 118,984.19
L 5.23 1.11 1078.02 20,195.23 39,399.24 5812.25 72,307.66
M 6.14 1.08 1320.28 18,756.54 14,147.63 7932.49 94,716.62
N 5.71 1.09 1073.44 19,720.42 41,819.87 1552.40 72,395.14
O 5.40 1.11 1314.16 4133.22 29,160.12 4327.28 111,960.16
P 5.64 1.10 482.78 18,356.33 29,357.67 8236.75 122,085.00
range 5.06–6.69 1.07–1.14 370–1510 3210–20,900 3930–44,500 428–12,100 60,466–124,000
average 5.639 1.12 836 8399 24828 3666 91,977
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The NaCl factor reflects the degree of metamorphism of the formation water and the degree of closure of the formation. In terrestrial sedimentary strata, if the NaCl factor of the formation water is greater than 0.87 and the salinity is high, it can be sedimentary or metamorphosed infiltration water. If the coefficient is less than 0.87, then it may be metamorphic sedimentary water or highly metamorphic permeable water. In general, the smaller the NaCl coefficient, the more reducing the environment is and the less it is affected by permeable water, which is favorable to the preservation of hydrocarbons. The NaCl coefficient in the study area ranges from 0.07 to 0.45, with an average value of 0.26. It indicates that the stratigraphic water in the study area is characterized by closed stagnant water with good closure and little exchange with external water bodies (Figure 2). The mineral ion content of formation water in section 5-1+2 of Majiagou Formation is the highest, followed by mixed formation water, and condensate oil is the lowest. Compared with the 5-1+2 section of Majiagou Formation, the 5-4 section has the highest mineral ion content in formation water, followed by mixed formation water, and the lowest mineral ion content in condensate.

Figure 2.

Figure 2

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(a–d) Relationships between Cl–, Ca+ , and Na+, K+ in 5-1+2 and 5-4 sections.

4. Results and Discussion

4.1. Gas–Water Distribution Characteristics

4.1.1. Structural Control on Gas–Water Partitioning

The Ordos Basin has been a west-high-east-low basin in the Ordovician-Jurassic. It was later modified by the Indo-Chinese and Yanshan movements to form the present west-dipping monoclinic tectonic environment. Seismic and drilling data indicate that the entire structure of the study area is relatively flat, with a high east and low west (Figure 3a). From the macroscopic point of view, tectonics has an important influence on the distribution of gas and water. The transition between the Indo-Chinese and Yanshan movements adjusted the gas transportation in the carbonate reservoirs of the Majiagou Formation. Initially, the gas was stored in the high part of the structure, but after the adjustment, the gas was stored in the favorable area. The aquifer is concentrated in the western part of the study area (Figure 3b). The present tectonics of the study area is dominated by nasal structures, with multiple rows of axes extending to the northeast in the context of large west-dipping monoclinic tectonics. Nose-like structures have an influence on the distribution of gas and water. In localized locations, small amplitude structures control the distribution of water bodies, which are mainly distributed in the lower part of the structure.

Figure 3.

Figure 3

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Water distribution and tectonic relationships in the Majiagou Formation (a) section 5-1+2 and (b) section 5-4.

Local tectonic height difference promotes gas–water differentiation, and natural gas is relatively enriched at the tectonic high points, resulting in the residual water in the reservoir being transported downward along the tectonic structure, forming a “relatively water-rich zone.”

4.1.2. Control of Gas–Water Generation by Paleomorphology

In the southern part of the study area, Ordovician paleomorphology controls the role and extent of paleokarst and paleokarst processes control the physical properties and lithology of paleokarst reservoirs and paleokarst crust. In the early depositional process, the reservoirs are dense carbonates with small space for gas and water storage and limited reservoir capacity. Later, the paleokarst action better improved the reservoir physical properties, which belonged to the tectonic diagenesis, enlarged the reservoir space, and greatly improved the pore throat connectivity. It is found that the strong karst action is mainly due to the high hydrokinetic energy. Good reservoirs with a high degree of karst are easily formed on both sides of the ancient trough. Karst slopes have low topography, and groundwater movement is dominated by horizontal movement. Air–water movement in the karst uplands is dominated by vertical seepage. Vertical fractures and solution holes are developed and mostly filled with mud, leading to the deterioration of physical properties. Therefore, it can be inferred that the gently sloping mounds near the paleoflush and karst slopes have better physical properties, followed by the remnant mounds in the karst uplands. It is found that paleomorphology controls the gas and water distribution mainly by influencing the physical properties and lithology of the reservoir. Gas wells in the study area are mainly distributed around ancient trenches and gentle slopes, and paleomorphology has a guiding effect on the gas–water distribution.

4.1.3. Control of Gas–Water Generation by Lithology

The Majiagou Formation is controlled by sedimentary phases, and carbonate reservoirs are developed. The mineral composition is mainly dolomite with an average content of more than 75%. It is followed by calcite, with an average content of 10–15%. Other components are mud, pyrite, hard gypsum, Illite, etc., with less content. 5-1+2 group reservoirs are mainly weathered crust karst reservoirs, and the dissolution holes (holes), intergranular holes, paste mold holes, and microscale cracks are more developed. Section 5-4 is mainly composed of various dissolution pores, dissolution fractures, and crystallization pores formed by dissolution transformation (Figure 4). The change of reservoir lithology controls the transportation and aggregation of natural gas to a certain extent, which leads to the obstruction of the venting process and the formation of a “relatively water-rich zone.” As shown in Figure 4, water bodies are primarily isolated in the northern ditch area. The distribution of water bodies in the reservoir is relatively decentralized, dominated by isolated water bodies with relatively water-rich areas of varying sizes. The water bodies are mainly distributed in the calcareous dolomite area.

Figure 4.

Figure 4

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Distribution of water associated with sedimentary microphases: Majiagou Formation (a) section 5-1+2 and (b) section 5-4.

4.2. Stratified Water Distribution Patterns

Based on the results of the above analysis, the distribution pattern of stratum water in the study area can be categorized into three types.

4.2.1. Patterns of Bottom Water

4.2.1.1. Tectonically Controlled Edge (Bottom) Water

The tectonically controlled lateral (bottom) water is mainly distributed in the western part of the study area (Figure 5a), with less distribution and is only locally visible. Due to the large tectonic height difference and strong exhaust capacity, the water distribution is controlled by the tectonic height, and the gas–water anisotropy is obvious in the reservoirs with good local physical properties. The results show that the upper part of the structure contains gas and the lower part is rich in water. The water is distributed in the lower part of the nasal depression. For example, well A4 is located in the raised part of the nasal structure and the gas is distributed in the higher part. The gas displaces the water body so that the water body is distributed on both sides of the protrusion in the form of lateral water.

Figure 5.

Figure 5

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(a–d) Stratified water distribution pattern.

4.2.1.2. Water Retention at the Bottom of the Structure

The water holding capacity at the base of the structure is predominantly located in the western part of the study area. Due to the small height difference of the structure, the venting capacity is insufficient. Unvented water is enriched in the lower part of the reservoir through the microstructure (Figure 5b). In addition, the reservoir conditions deteriorate in the direction of decreasing the tectonic dip. Natural gas is not sufficient to vent water from the reservoir, and localized lateral bottom water will also form. The down-dip direction of wells B3 and B4 in this section has insufficient gas venting capacity, and the reservoir physical properties are poor, forming stagnant water at the bottom of the formation.

4.2.2. Water Retention in Dense Dolomite Isolation (Gas–Water Zone)

The stagnant water in the dense dolomite isolate is influenced by local densification of the reservoir. As a result, water is not completely discharged in the aggregation process, but retained in the reservoir, forming a gas–water symbiotic relationship (Figure 5c). Influenced by the relative densification of the pore throat and reservoir, the vertical differentiation of gas and water is not obvious, forming a gas–water symbiotic relationship. The dolomite reservoirs in wells C2 and C4 of the profile are locally dense, and the water outflow is incomplete, forming a gas–water symbiotic relationship.

4.2.3. Isolated Dolomite Forming Lenticular Water (Lenticular Water)

Lenticular water bodies (Figure 5d) are located mainly in the northeastern part of the study area. Dolomite is an isolated lenticular body that is obscured by other rocks and is not connected by channels. As a result, natural gas cannot enter and water cannot drain, forming a lenticular water body. Independent water bodies are distributed in the stratigraphy of wells D2, D3, and D4 in the profile, and the lithology is dolomite. The upper and lower layers are relatively dense, isolating the reservoir and forming a lenticular water body.

5. Conclusions

On the basis of geological analysis and in combination with production data, this paper analyzes the chemical properties of formation water, reveals the water–gas distribution, and analyzes the main controlling factors. The main conclusions are summarized as follows:

  • (a)

    The gas field water in the study area can be categorized into three types: formation water, condensate water, and residual working fluid.

  • (b)

    At present, the gas field water in the study area is dominated by formation water, and the salinity of the Lower Paleozoic formation water is significantly higher, reaching 169,689 mg/L. The salinity of the gas field water is also higher than that of the Lower Paleozoic formation water.

  • (c)

    The NaCl coefficient of formation water in the well area is low, with an average value of 0.0856, and the metamorphic coefficient is 66.7564, which is much higher than 1. This suggests that the formation in this area is closed; the conditions for oil and gas preservation are better; the water body is disconnected, and there are multiple gas–water systems.

  • (d)

    The distribution of gas and water is controlled by many factors such as paleomorphology, tectonics, and lithology, and the water bodies are small and wide.

  • (e)

    According to the different types of water production and the characteristics of gas and water distribution of gas wells, specific drainage and gas extraction measures should be formulated. For unproductive wells, the production area should be avoided as much as possible.

Acknowledgments

This work was not supported by any funds.

The authors declare no competing financial interest.

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