Chemical Characteristics of Ordovician Formation Water and Its Relationship with Hydrocarbon Distribution in Shunbei and Adjacent Regions, Tarim Basin, NW China

Abstract

The Ordovician system in the Shunbei area of the Tarim Basin hosts typical ultra-deep, fault-controlled fracture–vuggy hydrocarbon reservoirs. Compared with the surrounding Tabei and Tazhong areas, the genetic types of Ordovician formation water in Shunbei are more complex, and the relationships and spatial distribution of oil, gas, and water exhibit strong heterogeneity and pronounced fault control. This study systematically collected formation water geochemical and pressure data to clarify the geochemical characteristics and origin of Ordovician formation water in the Shunbei area and to investigate the indicative relationships between ion concentrations and ionic ratios of formation water and reservoir dolomitization, hydrocarbon migration, and accumulation. Research shows that, in contrast to the Tabei and Tazhong areas, the Ordovician formation water in Shunbei is predominantly of the calcium chloride (CaCl₂) type, with enrichment in Ca²⁺ and depletion in Mg²⁺, which may be related to euhedral dolomitization. In fault-controlled fracture–vuggy reservoirs, a high desulfurization coefficient—contrary to its interpretation in conventional reservoirs—corresponds to favorable zones for hydrocarbon accumulation. The rare earth element (REE) composition of the formation water is characterized by heavy REE enrichment, a distinct negative cerium (Ce) anomaly, and a positive europium (Eu) anomaly. Combined with hydrogen–oxygen and strontium isotopic data, these features indicate that the Ordovician formation water in Shunbei represents original depositional paleoseawater that has undergone cross-formational flow and concentration. The water bodies are divided into two distinct formation water systems bounded by the Shunbei No. 5 fault zone. Favorable zones for hydrocarbon enrichment are controlled by source rock distribution, and hydrocarbons migrate together with formation water along strike-slip faults within the Shunbei area, showing a northwest-to-southeast trend. The region between the middle segments of the Shunbei No. 4 and No. 8 fault zones is identified as a favorable area for hydrocarbon accumulation.

1. Introduction

Formation water, as a fundamental component of geological fluids, participates in diverse physicochemical reactions and mass–energy exchange during basin evolution, serving as a reaction medium that preserves abundant geochemical information [1]. Therefore, analyzing the hydrochemical and hydrodynamic characteristics of formation water provides key insights into fluid-evolution processes and the distribution of favorable hydrocarbon accumulation zones, offering essential fluid-dynamic constraints on hydrocarbon migration and accumulation. Global studies of formation-water chemistry across various sedimentary settings consistently highlight that fluid activity spans the entire hydrocarbon lifecycle—from generation and reservoir development to migration and preservation [2]. Integrating the chemical–isotopic signatures of present formation water and paleo-fluids yields reliable constraints on hydrocarbon migration pathways, accumulation processes, and reservoir destruction mechanisms [3,4,5]. Coupled with basin evolutionary history, investigations into the hydrochemical characteristics and spatial distribution patterns of formation water are thus crucial for elucidating reservoir formation and evolution mechanisms [1,6]. For example, formation water in the Shaximiao and Xujiahe formations of the Sichuan Basin exhibits fault-related upwelling, where overpressure facilitates vertical natural-gas migration in both free-phase and water-dissolved forms [7,8,9]. In the eastern Paris Basin (France), fluid inclusion data from Jurassic limestones indicate a transition from Late Jurassic–Early Cretaceous basinal brines to low-Cl⁻ Cenozoic calcite-vein fluids, reflecting breached formation-water sealing and subsequent dilution [10].

China’s superimposed basins contain abundant ultra-deep hydrocarbon resources [11]. Recent exploration breakthroughs in the Shunbei and Fuman oil–gas fields of the Tarim Basin further demonstrate that marine carbonate successions constitute one of the most critical domains for reserve growth and production enhancement in onshore China [12]. With improved understanding of industrial oil flows discovered in the Ordovician of the Shuntuoguole Low Uplift and of subsalt Cambrian dolomite reservoirs and strike-slip fault systems [13], Shunbei reservoirs have been widely recognized as fault-controlled fractured–vuggy traps, wherein the development of high-quality reservoirs is dominantly governed by fault zones [14]. Tectonic fracturing, followed by extensive fluid modification, produced these fault-controlled fractured–vuggy reservoirs, which are characterized by fault-aligned distribution, vertical cross-stratal connectivity, pronounced lateral heterogeneity, and the absence of a unified oil–water contact [15].

The Shunbei reservoirs occupy a structurally low position on the Tarim Basin platform. Compared with structurally higher reservoirs such as Tahe and Tazhong, multi-stage fault activity has exerted a more pronounced influence on fluid origins and spatiotemporal evolution in Shunbei. Variations in stratal openness and the intensity of water–rock interaction [16,17] further enhance the complexity of hydrochemical characteristics and the spatial distribution of oil–gas–water systems in this area [6]. This study established a methodological workflow for screening reliable formation-water data, specifying that authentic formation-water samples in the Shunbei area should be characterized by a flowback volume greater than 200%, a water cut exceeding 50%, and a pH value ranging from 5 to 8.3. Furthermore, precise identification was conducted by integrating ion coefficients tailored to different formation-water types. Based on this rigorous screening framework, the geochemical data of formation waters across the Shunbei and adjacent regions were systematically compiled. By analyzing the hydrochemical characteristics and genetic evolution of formation waters from the Ordovician Yingshan and Yijianfang formations, and integrating formation pressure data, this study elucidated the fluid attributes and their indicative significance for hydrocarbon distribution. These results provide important guidance for exploration and development of Ordovician marine carbonate reservoirs in the Shunbei area.

2. Geological Setting

The Shunbei Oilfield is located within the Shuntuoguole Low Uplift in the central Tarim Basin. Structurally, it occupies a saddle-like transition zone between two uplifts and two sags (Figure 1), bounded by the Shaya Uplift to the north and the Tazhong Uplift to the south, and connected laterally with the Awati Sag to the west and the Manjiaer Sag to the east [18,19]. This region represents a structurally low position characterized by relatively stable stratigraphic subsidence. The area is marked by complex geomorphic conditions and strong tectonic activity [20], hosting medium- to small-scale intraplate strike-slip faults that were active from the Middle Caledonian to Late Hercynian and exhibit pronounced inherited deformation [12].

According to previous studies, the structural evolution of the Shuntuoguole Low Uplift can be divided into four stages. Early Caledonian: a weak intracratonic extensional stage. Middle Caledonian to Early Hercynian: a phase of multidirectional compression with increasing stress, establishing the low-uplift framework and generating the intraplate strike-slip fault systems. Late Hercynian to Indosinian: continued compression accompanied by the sustained uplift of the Tabei High. Yanshanian to Himalayan: a stabilization stage characterized by adjustments of the paleo-uplift and maturation of the fault systems [15,21,22,23,24]. Consequently, the strike-slip fault systems in the Shunbei area exhibit a “large-scale spatial distribution with multiple coexisting fault systems” [25].

As shown in Figure 2, the study area has experienced stable stratigraphic subsidence and is dominated by marine carbonate deposits of the Ordovician System. The Ordovician sequence can be subdivided from bottom to top into the Penglaiba Formation (O₁p), Yingshan Formation (O_1–2y), Yijianfang Formation (O₂yj), Qia’erbake Formation (O₃q), Lianglitag Formation (O₃l), and Sangtamu Formation (O₃s). Among these, the Yijianfang and Yingshan formations are the primary hydrocarbon reservoir units [26].

Strike-slip faults play a crucial role in providing vertical connectivity between source rocks and reservoirs. The Lower Cambrian Yuertusi Formation (Ꞓ₁y), deposited in a slope–shelf environment, acts as a high-quality source rock, continuously supplying hydrocarbons upward. Regionally, the Upper Ordovician mudstones and marls serve as the regional seal, while tight limestones form local seals and lateral barriers for the fault-controlled fractured–vuggy traps. Together, these geological elements form an excellent source–reservoir–seal assemblage. This configuration highlights the critical role of strike-slip fault zones in the formation of deep marine carbonate reservoirs, with distinct control over reservoir development, trap formation, hydrocarbon migration, accumulation, and enrichment [20,27,28].

3. Materials and Methods

Screening reliable formation-water data is fundamental for interpreting the hydrochemical and geochemical characteristics of subsurface fluids. Traditional approaches have commonly relied on single diagnostic criteria—such as the presence of key trace ions (e.g., Sr, Ba, B, I, Br), ion-balance assessments, anion-evaluation methods, or characteristic ionic concentration ratios—to distinguish primary formation water from fluids affected by external disturbance. Building upon these earlier methods, this study places particular emphasis on the provenance and acquisition context of formation-water samples. Through field investigations at oil-production facilities and a thorough understanding of real-time production operations and reservoir-development dynamics, we developed an integrated screening workflow tailored to the multiple reservoir types present in the Shunbei area. This workflow provides a more robust basis for identifying formation-water compositions that represent true subsurface conditions.

The Shunbei Oil and Gas Field is a typical ultra-deep marine carbonate reservoir system, structurally controlled by strike-slip fault networks and characterized by fault-controlled fractured–vuggy architectures. Hydrocarbon distribution exhibits a distinct spatial pattern “oil in the north and west, gas in the south and east”, reflecting significant reservoir-phase heterogeneity and the absence of a unified oil–water contact. Crude oil is predominantly medium to light in quality, and condensate-gas reservoirs locally occur. Under these unique geological and reservoir-engineering conditions, production operations in Shunbei are typified by initially high reservoir pressures, rapid pressure depletion, and substantial volumes of produced water (Figure 3). Furthermore, development practices such as water or gas injection, acidizing, and the use of drilling fluids are routine and often essential. These operational and geological complexities collectively pose considerable challenges to the accurate identification and screening of representative formation-water data.

The selection of formation-water samples was initially restricted to wells without any history of water injection. Wells displaying stable production, flowback volumes exceeding 200%, and water cuts greater than 50% were prioritized as primary sampling targets. After confirming the absence of potential external contamination sources, the wellhead separator was identified as the preferred sampling location, with single-well oil–water storage tanks near the wellhead serving as secondary options. These measures ensured that the collected samples represented authentic and uncontaminated formation water. In accordance with the geological characteristics of the Shunbei reservoirs, a screening workflow tailored to the Shunbei Oil and Gas Field was established as follows:

(1)

Elimination of acid-fluid contamination. Acid treatments produce used in reservoir stimulation (pH: 1–3). A pH threshold of 5 was adopted. Samples with pH > 5 were retained to avoid the influence of acidizing fluids introduced during reservoir stimulation.

(2)

Elimination of fracturing-fluid contamination. Given the typically alkaline nature of hydraulic-fracturing fluids, and considering both the pH-dependent behavior of carbonate species and the chemical composition of fracturing fluids used in Shunbei drilling operations, samples were retained only when CO₃²⁻ was absent and pH < 8.3 (Figure 4). This criterion effectively removes samples affected by fracturing-fluid contamination.

(3)

Formation-water type classification. Following the classification scheme of Sulin (1956) [29], three water types are present in the study area: calcium chloride (CaCl₂), sodium sulfate (Na₂SO₄), and sodium bicarbonate (NaHCO₃). Consistency between water types, ionic-coefficient indicators, and the inferred degree of hydrodynamic closure was assessed. These criteria were integrated with regional geological context and the spatial distribution of total dissolved solids (TDS) to perform a comprehensive evaluation.

(4)

Screening of CaCl₂-type waters. Samples were retained when the sodium chlorine coefficient (γNa⁺/γCl⁻) < 0.9 and the desulfidation coefficient (γSO₄²⁻/γ(Cl⁻ + SO₄²⁻) × 100) < 3.

(5)

Screening of NaHCO₃-type waters. Samples were retained when the sodium chlorine coefficient was <5 and the desulfidation coefficient was <40.

(6)

Screening of Na₂SO₄-type waters. Samples were evaluated based on (i) the presence of gypsum–salt formations in the corresponding stratigraphic interval (samples retained where such formations are developed), (ii) reservoir fluid phase (samples from condensate- or gas-dominated reservoirs were removed), and (iii) whether TDS values fall within the higher range characteristic of the study area.

(7)

Removal of extreme values. Following the integrated screening process, samples were retained only when the discrepancy between measured TDS and the summed ionic concentration was less than 10 g/L.

4. Results

4.1. Chemical Characteristics of Formation Water

4.1.1. General Hydrochemistry

The dataset used in this study consists of formation-water geochemical analyses from oil, gas, and water wells in the Shunbei Oilfield, complemented by a comprehensive compilation of Ordovician formation-water data from 229 wells across the Shunbei region and adjacent areas. Following a rigorous multi-step screening workflow, more than 280 samples were ultimately identified as reliable. Due to space constraints,

Table 1. Statistical data of major ion concentrations and water types of Ordovician formation waters in the Shunbei and adjacent regions.

Area	Well	Epoch	TDS (g/L)	Na+-K+ (mg/L)	Ca2+ (mg/L)	Mg2+ (mg/L)	Cl− (mg/L)	CO32− (mg/L)	HCO3− (mg/L)	SO42− (mg/L)	pH	Water Type
ShunBei	SHB1	O2yj + O1–2y	30.30776	10,561.22	958.42	155.47	18,163.07	0	266.3	157.67	7.38	CaCl2
	SHB5	O2yj + O1–2y	102.8299	54,200.41	10,600.91	654.28	15,809.21	0	131.19	163.65	6.78	CaCl2
	SHB4	O2yj + O1–2y	106.47946	45,442.59	9972.71	772.63	89,752.1	0	208.67	126.87	6.72	CaCl2
	SHB8	O2yj + O1–2y	72.17896	21,215.56	870.99	56.73	45,191.34	785.76	5292.68	345.9	7.27	NaHCO3
	SHB10	O2yj + O1–2y	64.38257	11,321.49	535.27	21.1	8166.57	0	2682.81	211.97	7.14	CaCl2
	SHB12	O2yj + O1–2y	72.95248	27,477.92	436.17	67.49	41,120.23	0	3387.8	339.2	7.23	Na2SO4
TaBei	YueJin2	O	104.40876	9447.43	27,972.29	485.42	64,703.34	0	664.18	510.4	6.87	CaCl2
	S111	O	99.740	25,110	5337	203.6	148,300	486.1	964.18	309	6.38	NaHCO3
	TP35	O	189.04915	62,607.86	9484.73	122.35	11,3005.74	0	728.27	333.33	6.48	NaHCO3
TaZhong	Zhong1	O3s + O3l	51.33245	18,026.52	1801.8	118.1	30,794.71	0	632.66	225	6.9	Na2SO4
	Zhong7	O3s + O3l	58.07099	4547.14	12,022.39	758.2	30,155.19	769.58	420.67	123.75	7.04	NaHCO3
	Zhong9	O3s + O3l	93.54234	26,944.05	8252.08	416.64	56,531.05	0	811.69	484.6	7.24	CaCl2
Unreliable data	SHB4–5	O2yj + O1–2y	649.54	2189.53	238.49	13.09	3207.17	19.36	78.72	759.40	8.98	Na2SO4
	SHB71	O2yj + O1–2y	184.1	59.71	4.17	2.81	79.84	0	0	37.64	4.16	Na2SO4

shows only a representative subset of the analyzed formation water samples.

Formation waters from the Yijianfang and Yingshan formations of the Ordovician System exhibit pH values ranging from 5.4 to 8.0, with a mean of 6.98, median of 6.56, mode of 6.43, variance of 1.098, and standard deviation of 1.048. These statistical results indicate that Ordovician formation waters in the study area are generally weakly acidic. As illustrated in Figure 5 and following the classical hydrochemical classification of Sulin (1956) [29], calcium chloride (CaCl₂) waters overwhelmingly dominate the dataset, accounting for 89% of all samples. The remaining formation-water samples primarily fall into the sodium bicarbonate (NaHCO₃) and sodium sulfate (Na₂SO₄) types. Magnesium chloride (MgCl₂)-type water was detected in only two wells in the central Tarim (Tazhong) region; however, these samples were excluded from further analysis as they were deemed unreliable.

4.1.2. Ionic Characteristics

The major ions in formation water include potassium (K⁺), sodium (Na⁺), calcium (Ca²⁺), magnesium (Mg²⁺), chloride (Cl⁻), bicarbonate (HCO₃⁻), carbonate (CO₃²⁻), and sulfate (SO₄²⁻). Among cations, K⁺ and Na⁺ are dominant, whereas Cl⁻ accounts for more than 85% of total anions.

Ion-evaporation (ion-convergence) diagrams were used to plot ion concentrations against total dissolved solids (TDS). As illustrated in Figure 6A, Cl⁻ exhibits no systematic enrichment or depletion with increasing salinity. In contrast, Na⁺ and K⁺ show a consistent depletion trend relative to the theoretical seawater evaporation curve as salinity increases. To further analyze these trends, major cations (Na⁺ + K⁺, Ca²⁺, and Mg²⁺) are cross-plotted against Cl⁻ (Figure 6B). With increasing Cl⁻ concentrations, formation waters from both the Shunbei and Tazhong regions display pronounced depletion of Na⁺ and Mg²⁺, accompanied by relative enrichment of Ca²⁺. In contrast, the northern Tarim (Tabei) region shows limited Na⁺ + K⁺ variation, notable Mg²⁺ depletion, and consistent Ca²⁺ enrichment.

4.1.3. Planar Distribution of Salinity and Desulfurization Coefficient

In the plan view, the salinity of Ordovician formation water across the Shunbei area and its surrounding regions ranges from 5 to 260 g/L (Figure 7). In the Tabei and Tazhong regions, salinity exhibits a relatively simple spatial pattern, characterized by progressive concentration from structural depressions toward structural highs, with values typically exceeding 120 g/L (

Table 2. Characteristics of Total Dissolved Solids (TDS) and ionic ratio parameters of Ordovician formation waters in the Shunbei and adjacent regions.

Area	TDS (g/L)	Sodium Chlorine Coefficient	Desulfidation Coefficient	Metamorphism Coefficient
ShunBei	${\displaystyle \frac{2.05~259.75}{97.84/91.45}}(108)$	${\displaystyle \frac{0.35~60.08}{7.56/4.38}}(108)$	${\displaystyle \frac{0.43~96.02}{19.67/13.35}}(108)$	${\displaystyle \frac{-14.34~91.18}{37.16/31.07}}(108)$
TaBei	${\displaystyle \frac{32.35~205.98}{137.57/141.95}}(89)$	${\displaystyle \frac{0.55~1.98}{1.56/1.45}}(89)$	${\displaystyle \frac{0.35~21.81}{5.72/3.31}}(89)$	${\displaystyle \frac{1.15~63.08}{29.56/32.35}}(89)$
TaZhong	${\displaystyle \frac{4.85~219.08}{107.56/131.35}}(83)$	${\displaystyle \frac{0.33~4.56}{1.68/1.69}}(83)$	${\displaystyle \frac{0.75~49.51}{13.36/9.38}}(83)$	${\displaystyle \frac{-4.57~79.48}{37.56/35.95}}(83)$

Notes: T Note: ${\displaystyle \frac{Min~Max}{Average/Median}}(Number of Samples)$; Sodium chlorine Coefficient = γNa⁺/γCl⁻; Desulfidation Coefficient = γSO₄²⁻/γ(Cl⁻ + SO₄²⁻) × 100; Metamorphism Coefficient = γ(Cl⁻ + SO₄²⁻)/γMg²⁺.

).

In contrast, the Shunbei area displays a complex, fault-controlled salinity architecture. Formation-water salinity follows a first-order spatial trend of “low in the east–west depressions and high in the northern and southern structural uplifts.” Low-salinity zones (<30 g/L) primarily occur in three areas: (1) the Awati Depression to the west; (2) the region west of Well Shunbei-82 along the No. 8 Fault Zone; and (3) the segment between Wells Shunbei-46 and Shunbei-4-5 along the No. 4 Fault Zone to the east. Conversely, high-salinity zones are concentrated in the southern segments of the No. 5 (Well Shunbei-53-5), No. 4 (Well Shunbei-4-13H), and No. 8 (Well Shunbei-81) fault zones, as well as in the northern segments of the No. 1 (Well Shunbei-1-6) and No. 4 (Well Shunbei-4-10) fault zones. Within individual fault belts, systematic variations are observed: in the central segments of the No. 4 and No. 6 fault belts and the northern segment of the No. 8 Fault Zone, salinity increases outward from the fault core, whereas in the southern segments of the No. 4, No. 5, and No. 8 fault belts, salinity decreases away from the fault core.

Regarding redox conditions, the plan-view distribution of desulfurization coefficients in the Shunbei area and adjacent regions (Figure 8) reveals that values are consistently high and tend to increase outward from the fault cores toward the surrounding wall rocks.

4.2. Source and Evolution of the Ordovician Formation Water

4.2.1. Rare Earth Elements (REEs)

Inductively coupled plasma mass spectrometry (ICP-MS) provided concentrations for 41 trace elements. Rare earth elements (REEs) were normalized to Post-Archean Australian Shale (PAAS). The Ordovician formation waters in Shunbei are generally enriched in heavy REEs (HREEs) and exhibit a well-defined negative cerium (Ce) anomaly accompanied by a weak positive europium (Eu) anomaly (Figure 9). One sample (Well SHB12) displays an REE pattern markedly different from all other samples in the dataset.

4.2.2. Stable Isotopes

In the Shunbei area, δD values of Ordovician formation waters range from −32‰ to −10‰, and δ¹⁸O values range from +3‰ to +6‰, with respective averages of −21.7‰ and +5.1‰. All Shunbei data plot to the right of the Global Meteoric Water Line (GMWL) [30,31] (Figure 10). Compared with the Tabei and Tazhong regions, the Shunbei samples show a substantially tighter clustering and relatively heavier isotopic signatures. Two samples (SHB4-13 and SHBL2) exhibit anomalously enriched δD values, while the Well SHB12 sample plots directly on the meteoric water line.

Strontium isotope analyses reveal ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.709417 to 0.716454 (Figure 11). These values fall within the compositional range characteristic of marine carbonate systems but are consistently higher than the estimated Sr isotopic composition of global Ordovician seawater (approximately 0.7080–0.7090).

4.3. Formation Pressure Characteristics

Formation-pressure coefficients in the Shunbei region and adjacent areas are generally high (Figure 12). The Tabei area falls within the normal-pressure range (0.9–1.1). Most of the Tazhong area shows abnormally high pressure (1.1–1.4), with the northwestern part characterized by ultra-high pressure (>1.4). The Shunbei area is dominated by ultra-high-pressure systems (0.9–2.1). Specifically, pressure coefficients in compartments between major fault belts are consistently higher than those measured within the fault cores. The inter-fault regions between Faults No. 5 and No. 4 and between Faults No. 6 and No. 8 constitute the highest-pressure domains, with coefficients commonly exceeding 1.6 [32,33,34].

5. Discussion

5.1. Water–Rock Interactions and Fluid Evolution

5.1.1. Significance of Major Ion Patterns

The dominance of CaCl₂-type waters (89%) in the Shunbei area reflects a relatively closed hydrodynamic and hydrogeological system, which is generally favorable for hydrocarbon preservation. While NaHCO₃-type waters are typically associated with shallow intervals in conventional systems, their presence in the Shunbei region likely indicates that large strike-slip fault systems allow fluids to retain signatures of deeply buried Ordovician connate water [35,36,37].

The conservativeness of Cl⁻ (Figure 6A) confirms its stability during subsurface fluid evolution. However, the observed depletion of Na⁺ and K⁺ with increasing salinity suggests progressive consumption during water–rock reactions. Specifically, in the Shunbei and Tazhong areas, deep strike-slip faults likely enhance vertical fluid communication between Ordovician carbonates and overlying clastic reservoirs, promoting Na⁺ consumption through feldspar alteration (plagioclase/K-feldspar to albite transformation). In contrast, areas with weaker fault development exhibit limited cation variation.

The widespread Ca²⁺ enrichment and Mg²⁺ depletion (Figure 6B) indicate extensive calcite dissolution and dolomitization during burial diagenesis trace-element [36]. The precipitation of Ca-bearing minerals or dolomitization typically removes Ca²⁺, whereas dissolution releases it. The behavior of Mg²⁺ implies varying degrees of dolomitization within the Ordovician succession, which not only modifies hydrochemistry but contributes to the development of favorable dolomite reservoirs (Figure 13).

5.1.2. Interpretation of Isotopic and Trace Element Signatures

The REE signatures (negative Ce, positive Eu) collectively indicate that the Ordovician formation waters were derived primarily from paleo-seawater trapped during early burial [38], subsequently modified by varying degrees of high-temperature hydrothermal input. The anomaly in Well SHB-12 is attributed to drilling-fluid contamination, a conclusion supported by its position on the meteoric water line in the H-O isotope plot (Figure 10).

The muted δ¹⁸O variability and distinct rightward shift from the GMWL reflect extensive late-stage water–rock interactions (temperature-driven ¹⁸O enrichment) and potential hydrocarbon expulsion effects. The systematic enrichment in radiogenic ⁸⁷Sr (Figure 11) further indicates that, after initial deposition, the formation water experienced lateral migration and evaporative concentration, enhancing the contribution of crustal-derived ⁸⁷Sr. Taken together, the geochemical (CaCl₂ type, REE patterns) and petrophysical constraints (tight limestone host rock) demonstrate that the Ordovician formation water has remained in a deeply buried, hydrologically closed system since deposition, with no substantial influence from post-depositional meteoric recharge.

5.2. Implications for Hydrocarbon Accumulation

5.2.1. Re-Evaluating the Desulfurization Coefficient

Under a traditional reservoir paradigm, high desulfurization coefficients (Figure 8) indicate an open system and poor preservation. However, this interpretation is not applicable to the Shunbei fault-controlled fractured–vuggy system. Here, hydrocarbon charge occurs via long-distance migration along deep strike-slip faults [39]. During this process, formation water co-migrates with externally sourced fluids, undergoing extensive hydrochemical overprinting (sulfate enrichment). Paradoxically, in this context, elevated desulfurization coefficients serve as geochemical signatures of active fluid communication and long-distance migration, corresponding to favorable domains for hydrocarbon accumulation.

5.2.2. Pressure Seals and Fault-Controlled Accumulation

The anomalous ultra-overpressure observed in inter-fault compartments (Figure 12) is interpreted as the combined effect of lithofacies-controlled karst reservoirs encased in extremely low-permeability host rocks [40,41]. This demonstrates that formation water becomes effectively sealed within these systems under high-burial conditions. The high-yield breakthrough in Well SHBL2 confirms that these ultra-overpressured zones represent prime targets for hydrocarbon enrichment.

5.2.3. Genetic Models of Dolomitization and Reservoir Quality

The relative enrichment of Ca²⁺ and depletion of Mg²⁺ in Shunbei formation waters supports the occurrence of euhedral dolomitization in a closed system (isochemical replacement: 2CaCO₃ + Mg²⁺ + CO₃²⁻ → CaMg(CO₃)₂ + Ca²⁺). Unlike open-system dolomitization, this process releases Ca²⁺ and is considered constructive for reservoir development, potentially increasing porosity by up to 13% [42,43].

5.3. Regional Migration and Accumulation Model

The integrated framework (Figure 14) illustrates a distinct contrast between the Tabei/Tazhong uplifts and the Shunbei depression. Tabei and Tazhong are characterized by meteoric-modified NaHCO₃/Na₂SO₄ waters and low desulfurization coefficients. The transitional Tahenan zone represents a mixing interface.

In sharp contrast, the Shunbei area is governed by a fault-controlled circulation model. Formation water migrates toward the central Shunbei area from both the Awati (west) and Manjiaer (east) depressions in an irregular annular pattern. The No. 5 fault belt acts as a first-order hydrochemical boundary. In the eastern sector, fluids migrate predominantly north-to-south along fault conduits. The consistency among chemical characteristics, REE signatures, and isotopic compositions between the central segments of the No. 4 and No. 8 fault belts demonstrates that these fluids are connate sedimentary waters that have undergone cross-formational concentration. This corridor retains excellent preservation conditions and constitutes one of the most favorable targets for hydrocarbon enrichment in the Shunbei fault-controlled system.

6. Conclusions

(1) The Ordovician formation waters in the Shunbei region and adjacent areas are dominated by CaCl₂-type waters, with subordinate NaHCO₃-type and Na₂SO₄-type signatures. In Shunbei, Ca²⁺ is consistently enriched whereas Na⁺, K⁺, and Mg²⁺ are depleted—patterns likely related to euhedral dolomitization and fluid invasion associated with strike-slip faulting, which disrupts stratigraphic continuity and facilitates mixing with non-connate fluids. Compared with neighboring areas such as Tahe and Tazhong, Shunbei exhibits a distinct fault-controlled salinity architecture. Importantly, unlike in conventional hydrocarbon reservoirs—where low desulfurization coefficients typically indicate favorable preservation—Shunbei’s fault-controlled fracture–vuggy systems display the opposite trend. Here, high desulfurization coefficients delineate zones of enhanced hydrocarbon enrichment, reflecting the fundamentally different fluid-flow and preservation mechanisms operating within these deep, fault-governed carbonate reservoirs.

(2) Rare earth element (REE) geochemistry demonstrates that the Ordovician formation waters in Shunbei originated from paleo-seawater trapped during early burial, with localized modification by high-temperature hydrothermal fluids. Integration of hydrogen–oxygen and strontium isotope compositions confirms that these waters have undergone cross-formational concentration with minimal meteoric-water recharge. The persistence of connate-water signatures across multiple geochemical proxies indicates that the reservoirs maintain excellent sealing capacity and have remained hydrologically closed throughout their burial history—conditions highly conducive to hydrocarbon preservation.

(3) Formation-water geochemical characteristics define two distinct hydrochemical systems within the Shunbei region, separated by the No. 5 fault belt. At the regional scale, Ordovician formation water migrates from the Awati and Manjiaer depressions toward central Shunbei in an irregular annular flow pattern. Hydrocarbon accumulation is further governed by the spatial distribution and maturity of source rocks, with hydrocarbons migrating together with formation water along major strike-slip faults in a northwest–southeast direction. Notably, the corridor between the central segments of the No. 4 and No. 8 fault belts—defined by CaCl₂-type formation waters and REE–isotope signatures reflecting progressive seawater concentration—stands out as one of the most favorable zones for hydrocarbon accumulation.