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Article

Evaluation of Geo-Engineering Sweet Spots in Deep Shale Gas Reservoirs of the Northern Luzhou Block

by
Shaojun Liu
1,2,*,
Xuefeng Yang
1,2,
Shengxian Zhao
1,2,
Hao Xu
3,
Yue Lei
1,2,
Yongyang Liu
1,2,
Lieyan Cao
1,2,
Bo Li
1,2,
Yuanhan He
1,2,
Ziqiang Xia
1,2 and
Meixuan Yin
1,2
1
Shale Gas Research Institute, PetroChina Southwest Oil & Gas Field Company, Chengdu 610051, China
2
Key Laboratory of Sichuan Province for Shale Gas Evaluation and Production, Chengdu 610051, China
3
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, Chengdu 610059, China
*
Author to whom correspondence should be addressed.
Processes 2026, 14(4), 594; https://doi.org/10.3390/pr14040594
Submission received: 30 October 2025 / Revised: 29 January 2026 / Accepted: 4 February 2026 / Published: 9 February 2026
(This article belongs to the Topic Global Unconventional Shale Oil and Gas: Geological Mechanisms, Technological Innovations, and Economic–Environmental Sustainability Assessment)
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Abstract

The deep formations (burial depth: 3500–4000 m) in the northern Luzhou Block boast favorable geological conditions for shale gas accumulation. However, field development is hindered by the frequent casing deformation of shale gas wells and significant variations in single-well productivity. These issues severely restrict the efficient development of shale gas resources. Existing studies mainly focus on the identification and optimization of geo-engineering dual sweet spots, but few have established a systematic and comprehensive evaluation system from the perspective of engineering risk prevention and control. Based on traditional research on geo-engineering dual sweet spots, this study integrates engineering risk factors. It innovatively establishes a geo-engineering dual sweet spot evaluation system that incorporates engineering risks. Four key evaluation indicators for shale matrix geo-engineering sweet spots are selected: the continuous thickness of a Class I reservoir, the structural location, the fault scale, and natural fracture characteristics. Accordingly, shale matrix geo-engineering sweet spots are classified into three categories: Class I-A Area, Class I-B Area, and Class II Area. Meanwhile, three key indicators affecting fault slip—the angle between fractures and the maximum horizontal in situ stress direction, fracture dip angle, and friction coefficient—are optimized to establish the fault slip risk evaluation criteria. Combined with the distribution characteristics of slip faults, the engineering risks are divided into three levels: high, medium, and low. Finally, by coupling the geo-engineering sweet spots of a shale matrix with engineering risk zones, the geo-engineering sweet spots of shale reservoirs in the study area are classified into four categories (I, II, III, IV).

1. Introduction

After more than a decade of theoretical exploration and technical practice, remarkable achievements have been made in the exploration and development of shale gas in the Wufeng–Longmaxi Formation of the Sichuan Basin. A shale gas geological theory with Chinese characteristics has been formed along with targeted exploration and development technologies [1,2,3,4,5,6]. However, compared with shallow shale gas wells, deep shale gas (burial depth > 3500 m) faces persistent challenges such as casing deformation, which significantly impacts gas field development [7,8,9,10,11,12,13,14,15,16,17]. Therefore, accelerating the analysis of geo-engineering risk conditions in deep shale and optimizing the evaluation technology of geo-engineering sweet spots are crucial for promoting the development of deep shale gas in China.
Shale gas sweet spots refer to the optimal areas or intervals for exploration and development under current economic and technological conditions [18,19]. In recent years, scholars at home and abroad have focused on the evaluation of geo-engineering dual sweet spots using comprehensive multi-parameter analysis, achieving good results [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34]. Ma Xinhua (2018) and Zhang Shaolong (2023) pointed out that the “sweet spot layers” in the Longmaxi Formation of southern Sichuan are characterized by low density, a high uranium–thorium ratio, a high quartz content, a high total organic carbon (TOC) content, and a high free gas content [21,22]. Xu Chunbi (2017) established formulas for a geological sweet spot index, engineering sweet spot index, and comprehensive sweet spot index using TOC, porosity, Young’s modulus, and Poisson’s ratio as indicators [23]. Liao Dongliang (2020) proposed geological sweet spot parameters (TOC content, kerogen content, porosity, gas saturation, pore pressure) and engineering sweet spot parameters (mud content, brittleness index, carbonaceous content, stress difference coefficient) [24]. Jiang Tingxue (2016) developed a calculation method for geological and engineering “sweetness” and evaluated their correlation with productivity [29]. Zhu Douxing (2018) classified and evaluated geological sweet spots (structure, faults, TOC, total gas content, porosity, reservoir thickness, etc.) and engineering sweet spots (burial depth, dip angle, brittleness index, fractures, formation pressure, in situ stress, etc.) based on 12 indicators, realizing the combination of dual sweet spots for shale gas evaluation [30].
Although these studies have made progress, most focus on identifying favorable geological and engineering conditions. Few have integrated engineering risk prevention and control into a systematic evaluation system. Engineering risks (such as casing deformation caused by fault slip) have a significant impact on well productivity and service life, making risk-integrated evaluation essential for practical application. The evaluation system for geological engineering dual sweet spots of shale oil shares the same framework and evaluation criteria as that for its shale gas counterparts [35].
To address this limitation, this study takes the Northern Luzhou Block as the research area. It analyzes the main controlling factors of shale matrix geo-engineering sweet spots, integrates engineering risk assessment, and establishes a graded evaluation system for shale reservoir geo-engineering sweet spots. The system has been applied in well location deployment and development plan optimization, providing guidance for the efficient development of deep shale gas in southern Sichuan.

2. Overview of Exploration and Development

Vertically, the Sichuan Basin develops six sets of hydrocarbon source rock strata. Among them, the Ordovician Wufeng Formation–Silurian Longmaxi Formation has achieved large-scale economic development in shallow areas (burial depth < 3500 m) [10,36], and national shale gas demonstration areas such as Changning–Weiyuan and Zhaotong have been established. With the intensification of shale gas exploration and development, the deep areas (burial depth > 3500 m) of the northern Luzhou Block have become a key strategic zone for the sustained increase in reserves and production of shale gas in the Wufeng–Longmaxi Formation [37].
The northern Luzhou Block is located at the junction of the low-steep structural belt of the southern Sichuan Basin and the low-fold structural belt of the southwestern Sichuan Basin (Figure 1). The structure is stable, gradually widening and flattening from north to south with decreasing folding intensity. The Wufeng–Longmaxi Formation in this area is deposited in the central part of a deep-water shelf. The continuous thickness of Class I reservoirs (the criteria for Class I reservoirs are as follows: TOC ≥ 3%, porosity ≥ 5%, gas content ≥ 3 m3/t, and brittle minerals ≥ 55%) ranges from 7 to 18 m, increasing gradually from north to south, indicating good potential for large-scale development.
However, casing deformation occurs frequently in the block: the casing deformation rate reaches 50% in the W06 and W14 well zones. Affected by casing deformation, gas production from these wells fails to meet expectations. This severely restricts the construction of deep shale gas production capacity. Therefore, it is necessary to comprehensively evaluate the distribution of geo-engineering sweet spots and engineering risk conditions to guide subsequent development plans and support reserve and production growth.

3. Research Methods

The northern Luzhou Block has complex geological and engineering conditions. The accurate graded evaluation of geo-engineering sweet spots requires in-depth geological understanding and precise engineering risk assessment. This study systematically analyzes geological and engineering conditions, establishes evaluation indicators for shale matrix geo-engineering sweet spots and engineering risks, and realizes the coupling classification of dual sweet spots.
Note: All data used in this study are owned by the authors’ affiliated institution. The planar mapping software Geomap 4.0 was also purchased by the institution, and there are no legal disputes arising therefrom.

3.1. Geological Conditions

3.1.1. Stratigraphic Characteristics

The Longmaxi Formation in the study area has a thickness of 400–600 m. Based on sedimentary cycles, lithological assemblages, logging responses, paleontological features, and geochemical characteristics, it is divided into two members (Long 2 and Long 1) from top to bottom. The Long 1 member is further subdivided into two submembers (Long 1-1 and Long 1-2) from bottom to top, and the Long 1-1 submember is divided into seven layers (Long 1-11 to Long 1-17) from bottom to top.
The Long 1-1 submember is dominated by organic-rich black carbonaceous shale with abundant graptolite fossils and a thickness of 55–65 m. It thins gradually from east to west. The Long 1-2 submember is characterized by sandy mudstone interbeds at the top and dark gray shale at the bottom (distinguished from the underlying gray–black shale of the Long 1-1 submember) with a thickness of 130–170 m.

3.1.2. Structural and Fault Characteristics

Affected by multiple tectonic movements, the northern Luzhou Block has complex folding and faulting characteristics [38]. The southern segment of the Huayingshan tectonic belt spreads in a broom-like pattern from northeast to southwest. It forms an overall “graben-horst alternation” structure. The structures in the block are classified into three types: synclines, slopes, and anticlines (Figure 2):
Synclines: Six synclines (e.g., Fuji and Desheng synclines) are identified with apparent stratum dip angles of 0–5°, poorly developed microstructures, and a total area of 1000.2 km2.
Slopes: Apparent stratum dip angles of 5–15° are identified with relatively well-developed microstructures, covering 1646.8 km2.
Anticlines: Nine anticlines (e.g., Luoguanshan and Yundingchang anticlines) are identified with apparent dip angles > 15°, well-developed microstructures, and a total area of 260.6 km2.
Faults in the study area show multi-period, multi-level, and multi-type characteristics due to the superposition of tectonic activities [18,19] (Figure 3). Since the Indosinian period, the target strata have been affected by tectonic stresses from three periods (Indosinian, late Yanshanian–early Himalayan, mid-late Himalayan), forming three sets of faults with orientations of nearly east–west (EW), northeast–southwest (NE-SW), and nearly north–south (NS) at the top of the Wufeng Formation [39]. Faults are classified into four levels based on displacement:
Level I faults: large-scale faults extending to the surface, controlling regional structures with clear seismic event axis disruption; these are rarely developed in the study area.
Level II faults: developed in anticline highs with planar extension > 3.5 km and vertical displacement of 100–300 m; dominated by plate-type thrusts, extending downward to the Cambrian salt layer and upward to the Silurian or Triassic.
Level III faults: distributed in wide and gentle synclines with EW orientation, planar extension > 1 km, and vertical displacement of 40–100 m; dominated by plate-type thrusts, extending downward to the Cambrian or basement and upward to the Silurian.
Level IV faults: small-scale faults mainly distributed in synclines with planar extension of 0.4–1 km and vertical displacement of 20–40 m; dominated by low-angle shovel-type thrusts, only disconnecting the Longmaxi Formation.
Large faults (Levels I and II) are mainly distributed in anticlines and slopes, while small faults (Levels III and IV) are concentrated in synclines.

3.1.3. Sedimentary Characteristics

The northern Luzhou Block is located in the depositional center of a deep-water shelf. Trace element data (U/Th > 1.25) indicate that the thickness of strongly reducing deep-water deposits is generally >4 m. Based on lithology, logging responses, sedimentary structures, paleontology, and redox conditions, the Wufeng Formation to Long 1-1 submember is classified as a deep-water shelf subfacies, which is further subdivided into four microfacies (Table 1). The organic-rich siliceous mud-shelf and organic-rich argillaceous mud-shelf microfacies are developed from the Wufeng Formation to Long 1-14. The water depth gradually shallows from Long 1-15 to Long 1-17, but the depositional center remains in the northern Luzhou Block throughout all strata.

3.1.4. Reservoir Characteristics

Based on reservoir quality and engineering conditions, the Wufeng Formation to the Long 1-1 submember is divided into two intervals: the lower intervals (Wufeng Formation to Long 1-14 layer) and the upper intervals (Long 1-15 to Long 1-17 layers). Based on the logging interpretation results of 24 evaluation wells in the northern Luzhou Block, the total organic carbon (TOC) content of the lower intervals ranges from 3.2% (σ = 0.25%) to 4.0% (σ = 0.30%), while the TOC of the upper intervals ranges from 2.0% (σ = 0.24%) to 2.6% (σ = 0.28%). The organic pores in the Wufeng Formation to the Long 1-1 submember are primarily circular or elliptical in shape, often developing in a networked, spongy, or beaded pattern. These organic pores are well preserved with pore sizes mainly distributed between 20 and 200 nm. Inorganic pores, on the other hand, tend to be nearly circular, square, diamond-shaped, or irregular, and they are predominantly composed of calcite intraparticle pores and quartz interparticle pores with pore sizes ranging from 30 to 1000 nm. The porosity of the lower intervals in this area ranges from 4.2% (σ = 0.3%) to 5.5% (σ = 0.4%), while the porosity of the upper intervals is between 4.4% (σ = 0.35%) and 5.7% (σ = 0.32%).
Based on the reservoir distribution characteristics of the upper and lower intervals in the northern Luzhou Block (Table 2), the cumulative thickness of Type I + II reservoirs (Type I reservoirs) in the lower interval ranges from 21 to 30 m, gradually thickening from the northeast to the southwest. For the upper interval, the thickness of Type I + II reservoirs ranges from 30 and 50 m, gradually thickening from west to east. In the lower intervals, the continuous thickness of Type I reservoirs ranges from 7 to 18 m. The TOC of these reservoirs primarily ranges from 3.2% and 4.0% with porosity between 4.2% and 5.5%, gas saturation between 50% and 65%, and gas content between 5.0 and 7.0 m3/t. The reservoir thickness gradually increases from northwest to southeast. In the upper intervals, the continuous thickness of Type I reservoirs ranges from 1 and 6 m. The TOC of these reservoirs ranges from 2.0% and 2.6% with porosity between 4.4% to 5.7%, gas saturation between 55% and 75%, and gas content between 4.6 and 6.2 m3/t. The reservoir thickness gradually increases from south to north (Figure 4).

3.1.5. Development Characteristics of Natural Fractures

Natural fractures are important reservoir spaces and flow channels for shale gas, significantly affecting gas productivity and fracturing effectiveness. Shale has extremely low permeability without fractures. Fracture formation is mainly related to rock brittleness, organic matter hydrocarbon generation, formation pore pressure, horizontal stress difference, faults, and folds. The northern Luzhou Block has a complex structure and has experienced multiple tectonic movements (Caledonian, Hercynian, Indosinian, Himalayan), leading to the widespread development of fractures associated with faults. Fractures are classified into three types (Figure 5):
Reticulate fractures: formed during sedimentation and diagenesis, mainly distributed in stable, gently dipping synclines (e.g., Fuji and Desheng synclines) and associated with Level III and IV faults.
Unidirectional fractures: tectonically induced, mainly occurring in narrow slopes and steep anticlines (e.g., northern Fuji syncline, Baozang syncline) and controlled by Level I and II faults.
Small faults: Small-scale extensional or shear fractures distributed in fault zones and structural transition zones.
Field data show that natural fractures can improve gas production, but large-scale natural fractures near wells may cause engineering problems such as reduced drilling accuracy and casing deformation.

3.2. Engineering Conditions

3.2.1. Rock Brittleness Characteristics

Rock minerals in the study area mainly include quartz, feldspar, carbonate minerals, clay minerals, and pyrite. The lower interval is dominated by siliceous shale with a brittle mineral content of 59–77.6% (silicon mainly from biological sources). The upper interval is dominated by argillaceous shale with a brittle mineral content of 47.9–65.3% (silicon partially from terrestrial inputs). The lower interval has higher brittleness. It is more prone to fracturing under external stress.

3.2.2. Rock Mechanics Characteristics

Shale strength affects wellbore stability and fracturing feasibility, while deformation characteristics affect wellbore integrity. Triaxial compression tests on single-well cores show that the Long 1-11 layer has a Young’s modulus of 3.685–5.004 × 104 MPa (average: 4.316 × 104 MPa) and a Poisson’s ratio of 0.195–0.315 (average: 0.234). The high Young’s modulus and low Poisson’s ratio indicate strong brittleness. This is conducive to the formation of complex fracture networks during fracturing.

3.2.3. Stress Characteristics

Affected by multiple tectonic activities and fault interactions, the in situ stress in the deep shale reservoir has complex characteristics: large triaxial stress differences, rapid directional changes, and a strike–slip stress state. In situ stress tests (acoustic emission Kaiser effect method) on typical wells show that the maximum horizontal principal stress (112.7 MPa) > vertical principal stress (106.6 MPa) > minimum horizontal principal stress (98.8 MPa), confirming a strike–slip stress regime (Table 3).
Faults significantly affect stress distribution: stress values in faults are lower than in the surrounding strata, and stress concentration occurs at fault branches and inflection points. The direction of maximum horizontal principal stress is nearly east–west (85–110°) in gentle synclines, and it is perpendicular to the long axis of the structure in narrow synclines and structural highs (Figure 6). The horizontal differential stress in the study area is 9.61–16.17 MPa, which is higher than that in the Changning and Weiyuan Blocks.

3.2.4. Fault Slip Risk

Research shows that fault slips are the main cause of casing deformation in the northern Luzhou Block. To evaluate fault slip severity, three core indicators are selected based on sensitivity analysis, expert scoring, and field verification (Table 4):
(1)
Angle between fault strike and maximum horizontal in situ stress direction (weight = 0.5): Smaller angles indicate higher fault activity.
(2)
Fault dip angle (weight = 0.3): Faults with dip angles of 0–90° are prone to instability, and higher angles increase instability. Most faults in the study area have dip angles > 60°.
(3)
Friction coefficient (weight = 0.2): Based on physical experiments and literature [40], the friction coefficient of open faults in the shale ranges from 0.5 to 0.7 with small variation.
Fault slip assessment was conducted in four production areas (W06, W02, W08, W14 well zones), identifying 11 strong slip faults, 28 medium slip faults, and 38 weak slip faults (Owing to confidentiality considerations, Figure 7 only presents the evaluation results for the slippage faults in the Well W06 zone):
Strong slip faults: mainly distributed in the W06 well zone (2), W02 well zone (5), and central–eastern W14 well zone (3).
Medium slip faults: predominantly located in the W06 well zone (18).
Weak slip faults: mainly distributed in the W14 well zone (12), northern W06 well zone (8), and W08 well zone (5).
Based on the classification of fault slip risk zones, differentiated well location deployment modes were formulated for the high-, medium- and low-risk zones in the Luzhou Block to mitigate the adverse impacts of casing deformation. The specific technical requirements are as follows. First, the horizontal section trajectory should avoid crossing high-risk slip faults, and the landing point (Point A) of the horizontal well should be at least 100 m away from high-risk slip faults, ensuring that the ratio of effective fracturing length to the total length of the horizontal section reaches 100%, as shown in Figure 8a. Second, if the horizontal section needs to cross 1 to 2 medium-risk slip faults, these faults should be arranged close to the termination point (Point B), and the ratio of the effective fracturing length from Point A to such faults to the total length should be no less than 70%, as shown in Figure 8b. Third, if the horizontal section is intended to cross more than two low-risk slip faults, the total length of the horizontal section should be appropriately shortened on the premise of ensuring an effective fracturing length of no less than 1500 m so that the actual number of crossed slip faults is controlled within 0–1, as shown in Figure 8c.

3.3. Evaluation Methods of Geo-Engineering Sweet Spots

3.3.1. Shale Matrix Geo-Engineering Sweet Spots

Field production data (from 15 typical wells) show a strong positive correlation between the estimated ultimate recovery (EUR) of shale gas wells—excluding those with complex engineering conditions—and the continuous thickness of Class I reservoirs (Figure 9, R2 = 0.9678). Combining geological and engineering analysis, four key evaluation indicators are selected: continuous thickness of Class I reservoirs, structural location, fault scale, and natural fracture characteristics. The northern Luzhou Block is classified into three sweet spot categories (Table 5, Figure 10):
Class I-A Area: continuous thickness of Class I reservoirs > 10 m; dominated by wide and gentle synclines; underdeveloped Level I–IV faults; reticulate fractures predominant. Area: ~1340 km2, distributed in the W01 well zone, W06–W04–W03 well zones, W15–W13–W11–W10 well cluster, and W23–W19–W18 well zones.
Class I-B Area: two scenarios: (1) continuous thickness of Class I reservoirs > 7 m, reticulate fractures predominant, underdeveloped Level I–IV faults; (2) continuous thickness of Class I reservoirs > 10 m, underdeveloped reticulate fractures, relatively developed Level III–IV faults. Area: ~1039 km2, mainly located in the W07-W08-W09 well zones, W20–W14–W12 well zones, and eastern W19–W18 well zones (Yunjin Syncline).
Class II Area: four scenarios: (1) continuous thickness of Class I reservoirs > 5 m, dominated by narrow slopes and faulted anticlines (structural highs), relatively developed Level III faults, underdeveloped reticulate fractures; (2) continuous thickness of Class I reservoirs > 5 m, underdeveloped Level I–III faults, well-developed reticulate fractures; (3) continuous thickness of Class I reservoirs > 7 m, developed Level III–IV faults, underdeveloped reticulate fractures; (4) continuous thickness of Class I reservoirs > 10 m, complex fault blocks, developed Level II–III faults, underdeveloped reticulate fractures. Area: ~938 km2, mainly distributed in northern regions and high structural positions of faulted anticlines.

3.3.2. Engineering Risk Assessment

Frequent casing failures in the study area are caused by multiple tectonic movements, multi-level faults, and complex in situ stress, which reduce the single-well EUR (Estimated Ultimate Recovery) and service life. The fault slip potential of the target formation is the core factor controlling engineering risks. Based on the distribution characteristics of slipping fractures, engineering risks are classified into three levels (Table 5, Figure 11):
High-risk areas: densely distributed strong slip faults; located in structural highs (micro-relief zones); developed Level II+ faults; dominated by unidirectional fractures. Area: 274.3 km2, distributed around wells W03, W06, W07, W09, W12, W14, and northern/southern W01 well zone.
Medium-risk areas: moderately/weakly developed slipping fractures (relatively dense); located in slope zones (moderately developed micro-reliefs); dominated by Level III faults and unidirectional fractures. Area: 381.9 km2, located in the slope zone of the W14 well zone and adjacent to Level II–III faults.
Low-risk areas: underdeveloped slipping fractures; located in synclinal zones; dominated by Level IV–V faults and reticulate fractures. Area: 294.6 km2, distributed in synclinal zones of the W01, W02, W14, and W18 well zones.

4. Results

Adopting the “multi-factor coupling control unit” concept [41], we couple shale matrix geo-engineering sweet spots with engineering risk zones. This classification divides the study area into four types of shale reservoir geo-engineering sweet spots (Figure 12):
Type I: Class I-A/B sweet spots + low-risk areas. Area: 365 km2, distributed in the central W01 well zone, southern W02 well zone, W10–W11–W13–W15 well cluster, W18 well zone, and eastern W12 well zone.
Type II: Class I-A sweet spots + medium-risk areas. Area: 244 km2, distributed in the northern/southern W01 well zone, central W02 well zone, southern W08 well zone, western W10–W11–W13–W15 well cluster, and southwestern W18 well zone.
Type III: Class I-A sweet spots + high-risk areas/Class I-B sweet spots + medium-risk areas. Area: 357 km2, distributed in the northern/southern W01 well zone, W03–W07 well zones, central W08 well zone, W12 well zone, and southern W14 well zone.
Type IV: Class I-B sweet spots + high-risk areas. Area: 174 km2, distributed in the eastern/western W08 well zone, W12 well zone, and W14 well zone.

5. Discussion

Shale gas exploitation is jointly controlled by geological and engineering factors. The accurate evaluation of geo-engineering dual sweet spots requires integrating favorable geological conditions and engineering feasibility. It also requires considering engineering risks. Previous studies have mostly focused on identifying geological and engineering sweet spots but lacked any systematic integration of risk prevention and control, leading to inconsistencies between evaluation results and field production.
This paper targets deep shale gas in the southern Sichuan Basin, adopts the dominant factor analysis method, and identifies key geological sweet spot indicators (continuous thickness of Class I reservoirs, fracture types, structural characteristics) and engineering risk indicators (structural setting, distribution of natural fractures and slipping fractures, micro-relief characteristics). A coupled evaluation method for dual sweet spots is established, which addresses the gap between traditional evaluations and practical engineering needs.
This method has strong applicability to deep shale gas blocks in the southern Sichuan Basin, especially for complex engineering conditions (e.g., high casing deformation rates). However, its application to areas with simpler geological and engineering conditions (e.g., Changning, Weiyuan Blocks) may be limited, as these regions have underdeveloped faults, simple in situ stress states, and low casing deformation rates.
This study has several limitations: (1) geological sweet spot evaluation does not include shale reservoir fluidity indicators (e.g., permeability); (2) engineering evaluation fails to consider the impact of interactions between natural weak planes (bedding planes, microfractures) and fracturing fluids on fracturability; (3) a single evaluation criterion is difficult to adapt to diverse geo-engineering environments. Future research should use big data and artificial intelligence technologies to develop tailored evaluation criteria for different reservoir models. This will improve the universality and accuracy of sweet spot evaluation.

6. Conclusions

(1)
The northern Luzhou Block has favorable geological conditions for shale gas accumulation but complex engineering conditions with frequent casing deformation severely restricting development efficiency. This study establishes a systematic evaluation framework for shale reservoir geo-engineering sweet spots by integrating geo-engineering conditions and engineering risk assessment.
(2)
The lower interval of the Wufeng–Longmaxi Formation in the study area is deposited in a strongly reducing environment with excellent reservoir quality: Class I reservoirs have a continuous thickness of 7–18 m, high brittle mineral content, high Young’s modulus (3.685 × 104–5.004 × 104 MPa), and low Poisson’s ratio (0.195–0.315), indicating strong fracturability. Affected by multiple tectonic movements, the block has well-developed multi-period, multi-level faults and natural fractures with a strike-slip stress regime, large horizontal stress differences, and rapid directional changes, which easily induce fault slip and casing damage.
(3)
Core evaluation indicators for fault slip intensity include the angles between the in situ stress direction and the fault strike, fault dip angle, and friction coefficient. A total of 11 strong-slip faults, 28 medium-slip faults, and 38 weak-slip faults are identified in the study area.
(4)
Shale matrix geo-engineering sweet spots are classified into three categories (Class I-A, I-B, II) based on four indicators: the continuous thickness of Class I reservoirs, the structural location, the fault scale, and the natural fracture characteristics. Engineering risks are graded as high, medium, or low based on slipping fracture distribution. Coupling these classifications results in four types of shale reservoir geo-engineering sweet spots (I, II, III, IV).
(5)
The sweet spot classification has been applied to optimize development plans for the W06, W02, W08, and W14 well zones: Type I/II sweet spots (W14, W01 well zones) prioritize low-risk area development; Type III sweet spots (W06 well zone) adopt phased deferred development and research trials due to high engineering risks; Type IV sweet spots (W08, W14 well zones) implement real-time dynamic optimization based on W06 well zone trial results. By virtue of the hierarchical classification evaluation technology for geological-engineering sweet spots, the casing deformation rate of shale gas wells in the study area has decreased from 53% in 2022 to 33% in 2025. This achievement has significantly reduced casing deformation risks.
(6)
This method exhibits good adaptability to geo-engineering sweet spots in complex structural belts. In its application, particular attention should be paid to the accurate characterization of fractures and in situ stress fields as well as the quantitative evaluation of fault slip risks.

Author Contributions

Conceptualization, S.L.; Methodology, S.L. and X.Y.; Validation, S.Z.; Formal analysis, H.X.; Investigation, S.L.; Resources, M.Y.; Data curation, Y.H. and M.Y.; Writing—original draft, Y.L. (Yue Lei); Writing—review & editing, L.C.; Visualization, Y.L. (Yongyang Liu) and Z.X.; Project administration, B.L.; Funding acquisition, S.L. and Z.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

Authors Shaojun Liu, Xuefeng Yang, Shengxian Zhao, Yue Lei, Yongyang Liu, Lieyan Cao, Bo Li, Yuanhan He, Ziqiang Xia and Meixuan Yin were employed by Shale Gas Research Institute, PetroChina Southwest Oil & Gas Field Company. The remaining author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Regional tectonic location of the northern Luzhou block.
Figure 1. Regional tectonic location of the northern Luzhou block.
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Figure 2. Fine division and distribution of structural types in the northern Luzhou Block.
Figure 2. Fine division and distribution of structural types in the northern Luzhou Block.
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Figure 3. Classification of faults at the bottom boundary of the Wufeng Formation in the northern Luzhou Block.
Figure 3. Classification of faults at the bottom boundary of the Wufeng Formation in the northern Luzhou Block.
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Figure 4. Isoline map of continuous thickness of Type I reservoir in the upper and lower intervals of the study area. (a) Isoline map of continuous thickness of Type I reservoirs in the lower intervals of the research area. (b) Isoline map of continuous thickness of Type I reservoirs in the upper intervals of the research area.
Figure 4. Isoline map of continuous thickness of Type I reservoir in the upper and lower intervals of the study area. (a) Isoline map of continuous thickness of Type I reservoirs in the lower intervals of the research area. (b) Isoline map of continuous thickness of Type I reservoirs in the upper intervals of the research area.
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Figure 5. Seismic prediction of natural fracture distribution in Wufeng Formation, northern Luzhou Block.
Figure 5. Seismic prediction of natural fracture distribution in Wufeng Formation, northern Luzhou Block.
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Figure 6. Distribution of current maximum horizontal principal stress direction in the northern Luzhou Block.
Figure 6. Distribution of current maximum horizontal principal stress direction in the northern Luzhou Block.
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Figure 7. Risk assessment and prediction of slip faults in Well W06 zone, northern Luzhou Block.
Figure 7. Risk assessment and prediction of slip faults in Well W06 zone, northern Luzhou Block.
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Figure 8. Optimization model for horizontal section crossing slip faults.
Figure 8. Optimization model for horizontal section crossing slip faults.
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Figure 9. Relationship between single-well EUR and continuous thickness of Type I reservoir.
Figure 9. Relationship between single-well EUR and continuous thickness of Type I reservoir.
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Figure 10. Hierarchical evaluation distribution of geological sweet spots in the lower intervals, northern Luzhou Block.
Figure 10. Hierarchical evaluation distribution of geological sweet spots in the lower intervals, northern Luzhou Block.
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Figure 11. Hierarchical evaluation distribution of engineering risks in the northern Luzhou Block.
Figure 11. Hierarchical evaluation distribution of engineering risks in the northern Luzhou Block.
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Figure 12. Distribution of geo-engineering sweet spots in the northern Luzhou block.
Figure 12. Distribution of geo-engineering sweet spots in the northern Luzhou block.
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Table 1. Sedimentary microfacies classification standards in the northern Luzhou Block.
Table 1. Sedimentary microfacies classification standards in the northern Luzhou Block.
MicrofaciesSedimentary EnvironmentGamma-Ray (GR)
(API)		LithofaciesSilica:Calcium:Clay (%)Layer
Limestone
mud-shelf		U/TH < 0.75128Carbonate-bearing siliceous-argillaceous mixed shale45:2:53The top of Long 1-16
Argillaceous
mud-shelf		1.25 > U/TH > 0.75177Carbonate-bearing argillaceous-siliceous shale50:15:35The top of Long 1-14 to Long 1-17 layer
Organic-rich argillaceous
mud-shelf		U/TH > 1.25230Carbonate-bearing argillaceous siliceous shale; siliceous-argillaceous mixed shale55:10:35Long 1-14 to Long 1-16 layer
Organic-rich siliceous mud-shelfU/TH > 1.5210Clay-bearing siliceous shale;
siliceous shale		65:25:10Wufeng Formation to Long 1-13 layer
Table 2. Reservoir characteristics of the Luzhou Block.
Table 2. Reservoir characteristics of the Luzhou Block.
StratumFormationType I Reservoir Thickness (m)Type I + II Reservoir Thickness (m)TOC
(%)		Porosity
(%)		Gas Saturation (%)Gas Content (m3/t)
Upper intervalsLong 1-15–7 layer1~630~502.0~2.6%4.4~5.7%55~754.6~6.2
Lower intervalsWufeng Formation to Long 1-13 layer7~1821~303.2~4.0%4.2~5.5%50~655.0~7.0
Note: The criteria for Class I reservoirs are as follows: TOC ≥ 3%, porosity ≥ 5%, gas content ≥ 3 m3/t, and brittle minerals ≥ 55%. The criteria for Class II reservoirs are as follows: TOC ranging from 2% to 3%, porosity ranging from 3% to 5%, gas content ranging from 2 to 3 m3/t, and brittle minerals ranging from 45% to 55%.
Table 3. Triaxial stress experimental data for the northern Luzhou Block (acoustic emission Kaiser effect method).
Table 3. Triaxial stress experimental data for the northern Luzhou Block (acoustic emission Kaiser effect method).
BlockWell NameLayerTriaxial Stress/MPaHorizontal Difference Stress/MPaStress Structure State
Horizontal MaximumHorizontal MinimumVertical
LuzhouWell W06
zone		W06Long 1-11109.693.6101.316strike-slip
W05Long 1-11102.689.795.512.9strike-slip
W04Long 1-11103.689.29814.4strike-slip
Well W02
zone		W03Long 1-11113.197.9106.915.2strike-slip
W24Long 1-11114.399.1108.215.2strike-slip
W02Long 1-11115.399.79105.0515.51strike-slip
Well W01
zone		W01Long 1-11103.7290.65101.1213.07strike-slip
Well W08
zone		W07Long 1-1194.583.689.810.9strike-slip
W08Long 1-11101.8292.2197.969.61strike-slip
W09Long 1-1196.586.093.210.5strike-slip
Well W14
zone		W13Long 1-11115.2699.09105.3116.17strike-slip
W12Long 1-1197.9287.390.7410.62strike-slip
W11Long 1-11113.5197.40103.6816.11strike-slip
W15Long 1-11109.2396.04105.1113.19strike-slip
YuxiW23Long 1-11108.9189.02102.319.89strike-slip
ChangningN1Long 1-1182.171.977.410.2strike-slip
WeiyuanY1Long 1-1197.283.391.113.9strike-slip
Table 4. Evaluation index for shale fracture slippage in the northern Luzhou Block, southern Sichuan.
Table 4. Evaluation index for shale fracture slippage in the northern Luzhou Block, southern Sichuan.
Classification of Fault SlippageAngle Between Fault and Maximum Principal Stress Direction (50%Weight)Fault Dip Angle
(30%Weight)		Fault Sliding Friction Coefficient and Activity Difficulty Index
(20%Weight)
Small AngleHigh AngleFault Length
(km)		Distance from Adjacent Faults (km)
Strong<30°>70°>4<2
Medium30°~45°45°~70°2~42~4
Weak45°~60°<45°<1>4
Table 5. Evaluation criteria for shale matrix geo-engineering sweet spots and engineering risks in the northern Luzhou Block.
Table 5. Evaluation criteria for shale matrix geo-engineering sweet spots and engineering risks in the northern Luzhou Block.
Evaluation of Geological Engineering Sweet Spots in Matrix-Dominated ShaleEngineering Risk Evaluation
Continuous Thickness of Class I ReservoirStructural LocationFault ScaleNatural Fracture Type CharacteristicsClassified EvaluationDistribution Characteristics of Slipping FracturesClassified Evaluation
>10 mmainly characterized by wide and gentle synclinesfaults of Grade IV, III, II, and I are underdevelopednetwork fractures are predominantClass I-A Areadensely distributedhigh-risk areas
7~10 mmainly characterized by wide and gentle synclinesfaults of Grade IV, III, II, and I are underdevelopednetwork fractures are predominantClass I-B Areamoderately densely distributedmedium-risk areas
>10 mfaults of Grade IV and III are underdevelopedunidirectional fractures are predominant
<7 mdominated by anticlines and slopesfaults of Grade IV and III are underdevelopedunidirectional fractures are predominantClass II Areaunderdevelopedlow-risk areas
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Liu, S.; Yang, X.; Zhao, S.; Xu, H.; Lei, Y.; Liu, Y.; Cao, L.; Li, B.; He, Y.; Xia, Z.; et al. Evaluation of Geo-Engineering Sweet Spots in Deep Shale Gas Reservoirs of the Northern Luzhou Block. Processes 2026, 14, 594. https://doi.org/10.3390/pr14040594

AMA Style

Liu S, Yang X, Zhao S, Xu H, Lei Y, Liu Y, Cao L, Li B, He Y, Xia Z, et al. Evaluation of Geo-Engineering Sweet Spots in Deep Shale Gas Reservoirs of the Northern Luzhou Block. Processes. 2026; 14(4):594. https://doi.org/10.3390/pr14040594

Chicago/Turabian Style

Liu, Shaojun, Xuefeng Yang, Shengxian Zhao, Hao Xu, Yue Lei, Yongyang Liu, Lieyan Cao, Bo Li, Yuanhan He, Ziqiang Xia, and et al. 2026. "Evaluation of Geo-Engineering Sweet Spots in Deep Shale Gas Reservoirs of the Northern Luzhou Block" Processes 14, no. 4: 594. https://doi.org/10.3390/pr14040594

APA Style

Liu, S., Yang, X., Zhao, S., Xu, H., Lei, Y., Liu, Y., Cao, L., Li, B., He, Y., Xia, Z., & Yin, M. (2026). Evaluation of Geo-Engineering Sweet Spots in Deep Shale Gas Reservoirs of the Northern Luzhou Block. Processes, 14(4), 594. https://doi.org/10.3390/pr14040594

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