Characterization of Permo-Triassic Reservoirs through Thermal Maturity Assessment of Westphalian Source Rocks in the Cheshire Basin

Introduction

Hydrocarbon exploration plays a central role in global energy strategies [1, 2, 3]. Sedimentary basins serve as key repositories of organic-rich layers that facilitate the generation, migration, and trapping of petroleum [4, 5, 6]. A systematic and integrated characterization of these basins is crucial for improving exploration success rates and minimizing geological uncertainty, particularly in frontier regions. Within the United Kingdom, the Permo-Triassic basins exhibit a broad spectrum of petroleum systems, ranging from mature, high-yield provinces, such as the East Irish Sea Basin, to underexplored and geologically complex regions, like the Cheshire Basin.

The Cheshire Basin, situated in northwestern England, represents a Late Paleozoic to Mesozoic extensional basin characterized by a classic half-graben geometry (Figure 1A) [7, 8, 9, 10]. It is primarily composed of Triassic fluvial sandstones and mudstones, which unconformably overlie eroded Carboniferous strata [11]. Structurally, the basin is dissected by numerous normal faults, particularly concentrated along its eastern and southeastern margins, resulting in significant compartmentalization of the subsurface architecture. The basin is subdivided into two major structural domains: the Wem–Audlem sub-basin to the south and the Sandbach– Knutsford sub-basin to the north (Figure 1B) [7]. Despite its proximity to productive hydrocarbon provinces—notably the East Irish Sea and East Midlands basins—and the presence of potential reservoir-quality lithologies, the Cheshire Basin has yet to yield any commercial hydrocarbon discoveries [12, 13]. As a result, it remains classified as a non-productive or frontier exploration basin [14, 15].

Hydrocarbon exploration in the Cheshire Basin dates to the late 18th century, marked by early reports of natural gas seepages, most notably at the Coalport Tar Tunnel [14-

16]. However, the absence of advanced subsurface imaging and drilling technologies during this early period limited systematic exploration and geological characterization. Modern exploration efforts began in the 1970s, facilitated by the acquisition of 2D seismic data and the drilling of ten exploration wells [17]. Although minor hydrocarbon shows were encountered, none of the wells yielded commercially viable accumulations. This failure has been primarily attributed to poor reservoir quality and limited hydrocarbon saturation, leading to the subsequent plugging and abandonment of all wells [8, 9]. A major geological constraint on the basin’s petroleum system is the extensive pre-Permo- Triassic erosion of Carboniferous source rocks, which likely disrupted the essential source–reservoir–seal relationships required for effective hydrocarbon generation, migration, and entrapment [18, 19]. In addition, the identification of prospective structural traps has been hindered by the basin’s complex fault architecture and the limited resolution of early seismic datasets [20]. These geological and technical limitations have collectively contributed to the Cheshire Basin’s continued classification as an underexplored and non-productive frontier region [16, 18, 19].

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Nevertheless, the basin’s western margin, adjacent to the East Irish Sea Basin, remains of interest due to the inferred preservation of deeply buried Carboniferous source rocks and potential trap configurations. Comparative stratigraphic and structural parallels with neighboring productive basins further underscore the need to reassess the petroleum system elements of the Cheshire Basin, including source rock maturity, reservoir quality, and trap integrity [14, 15, 22].

Recent advances in subsurface reservoir characterization and modeling have demonstrated the value of integrated analytical and numerical approaches for understanding fluid flow behavior, storage efficiency, and uncertainty in complex geological systems. Sector-scale reservoir simulations have been widely used to evaluate multiphase flow dynamics under different injection strategies, including continuous and water-alternating-gas (WAG) schemes, providing insights into recovery mechanisms and subsurface performance [23]. Furthermore, hysteresis-aware modeling has been shown to play a critical role in accurately capturing gas–water flow behavior and storage efficiency in CO₂-based injection processes, particularly in heterogeneous reservoirs [24]. Complementary to numerical modeling, pressure transient analysis techniques have proven effective in diagnosing fluid soaking, fracture connectivity, and flow regimes in multi-fractured reservoirs, offering valuable constraints for reservoir-scale interpretation and model calibration [25]. Collectively, these studies highlight the importance of integrated geological, petrophysical, and modeling workflows, which form the methodological foundation for the basin-scale characterization undertaken in this study.

This study aims to provide a comprehensive re- evaluation of the Cheshire Basin’s hydrocarbon potential through a synthesis of existing seismic, stratigraphic, and well data. By revisiting its tectonostratigraphic framework and exploration history, the research seeks to elucidate the geological factors that have limited its exploration success, while identifying underexplored domains that may offer future prospectivity under updated models of basin evolution and petroleum systems analysis.

Geological and Tectonic Framework of the Cheshire Basin

The Cheshire Basin, located in the northwest Midlands of England, is a well-documented example of a fault- bounded, Permo-Triassic extensional rift system, covering an area of approximately 3,500 km² [26]. It forms part of a broader NNW–SSE trending rift corridor that extends from the Inner Hebrides in northwest Scotland to the Wessex Basin in southern England [8, 9]. The basin exhibits significant structural complexity, underlain by a deeply eroded Lower Palaeozoic basement considered overmature for hydrocarbon generation [27]. Overlying this basement are a series of Carboniferous sub-basins that formed within the Variscan foreland and are bounded by NE–SW trending faults (Figure 2) [26, 28]. These faults were later reactivated during Permian extension, serving as primary controls on syn-rift subsidence, accommodation space, and sediment dispersal [10, 11]. The basin fill comprises a thick sequence of Permian to Triassic siliciclastic sediments, notably the Collyhurst Sandstone, Sherwood Sandstone Group, and Mercia Mudstone Group, deposited during phases of active rifting and subsequent post-rift thermal subsidence [26].

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The post-rift tectonic evolution of the Cheshire Basin was characterized by ongoing thermal subsidence into the Jurassic, evidenced by the preservation of Lower Jurassic strata within structurally controlled depocenters. As illustrated in Figure 3, these deposits reflect early basin inversion processes, likely triggered by compressional reactivation of extensional faults [8, 9]. Additionally, the westward migration of depocenters during the Permian and Early Triassic, driven by differential subsidence and fault-controlled sedimentation, contributed to the basin’s complex internal geometry. Structurally, the basin has undergone multiphase tectonic overprinting, including later-stage inversion associated with the Alpine Orogeny [10, 26]. Seismic interpretation indicates that many of the basin’s faults are inherited from pre-existing Carboniferous structures, adding to the structural heterogeneity and complicating the development of effective hydrocarbon traps [20]. These geological complexities—combined with post- depositional uplift, erosion, and glacial modification during the Cenozoic and Quaternary—have significantly influenced the basin’s hydrocarbon prospectivity [29].

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Data Availability and Control

The availability of subsurface data in the Cheshire Basin is limited due to its underexplored nature. Four wells with usable datasets were identified: Knutsford 1, Winsford 1, Elworth 1, and Burford

The availability of subsurface data in the Cheshire Basin is limited due to its underexplored nature. Four wells with usable datasets were identified: Knutsford 1, Winsford 1, Elworth 1, and Burford 1. The Sherwood Sandstone was intersected in all wells, while the Collyhurst Sandstone was restricted to Knutsford 1. Data control challenges included: • Depth unit conversion (meters to feet) • Calibration of wireline logs relative to Kelly Bushing and Mean Sea Level • Manual alignment of log data due to spatial well separation

Core Analysis

Core analysis provides the most direct and detailed insights into subsurface lithology, porosity, permeability, and depositional characteristics. Cores represent physical samples of reservoir formations and are invaluable for validating interpretations derived from wireline data and models.

In this study, the Knutsford 1 well yielded a key core section from 7,501.23 ft to 7,532.65 ft, representing a crucial interval of the target reservoir. Cores were analyzed at the British Geological Survey (BGS) Core Store in Keyworth. Analytical procedures included:

• Cleaning and visual examination of core segments
• Conversion of depth units to match wireline log headers
• Photographic documentation and facies identification Similar integrated reservoir-scale modeling approaches have been widely applied to evaluate fluid flow behavior, recovery mechanisms, and uncertainty in subsurface systems, including CO₂-based injection and alternating gas– water processes [23, 24].

Discussion

The integration of seismic interpretation, facies distribution modelling, and thermal maturity analysis provides a coherent framework for evaluating the petroleum system of the Cheshire Basin. Although key petroleum elements—source rock, reservoir, and seal—are present [30, 32], the basin has yet to yield any significant hydrocarbon discoveries. This apparent contradiction is likely attributed to the complex structural framework and compromised seal integrity, both of which are strongly evidenced by seismic and stratigraphic data.

Seismic profiles across the basin reveal an extensive fault network that disrupts stratigraphic continuity and penetrates key seal units such as the Northwich Halite and Manchester Marl [28, 33]. These formations are typically considered effective seals; however, their breach by post- depositional faulting has likely provided vertical migration pathways for hydrocarbons. Mikkelsen and Floodpage [14, 15] identified that many faults extend to the surface, supporting the hypothesis that these structures have facilitated hydrocarbon leakage. In addition, a significant post-Triassic erosional event has led to the thinning of the Northwich Halite [34], further compromising seal integrity and increasing the risk of hydrocarbon escape.

Facies analysis across the basin confirms significant lateral and vertical heterogeneity, particularly within the Sherwood Sandstone Group [35, 36]. Knutsford 1, the deepest well in the basin, encountered a complete sequence of the Sherwood Sandstone, Manchester Marl, and Collyhurst Sandstone. Sedimentological analysis indicates that depositional environments varied from aeolian to fluvial and lacustrine, representing cyclic transgressive– regressive phases [21, 30]. Aeolian facies—characterized by well-sorted grains and minimal shale—are dominant in the northern and southern margins, while fluvial and channel- fill facies prevail in the central and eastern sectors. This spatial facies variability has direct implications for reservoir quality and connectivity, particularly regarding porosity and permeability heterogeneity.

Core data from Knutsford 1 further supports the interpretation of high-quality reservoir potential in the Collyhurst Sandstone. The presence of clean grain textures, moderate sorting, and low gamma-ray values indicates minimal clay content and favorable reservoir conditions [30], despite the lack of direct porosity measurements. However, the limited areal distribution of the Collyhurst Sandstone and its absence in shallower wells constrain its potential as a widespread reservoir target.

From a source rock perspective, the Westphalian Coal Measures represent a laterally extensive and regionally proven source, particularly in the adjacent East Irish Sea and Anglo-Dutch basins [37, 38]. Thermal maturity modelling suggests that within the Cheshire Basin, the Coal Measures reached early oil window conditions during the Jurassic and peaked in wet gas generation during the Late Cretaceous and Tertiary [39]. However, post-depositional uplift associated with the Hercynian Orogeny, followed by erosion, resulted in reduced TOC content and source rock degradation [40]. These geological processes significantly lowered the basin’s hydrocarbon generation capacity. Present TOC values (~50 mg/g TOC) indicate diminished generative potential, although the formation initially held promise for conventional dry gas production.

Structural compartmentalization further complicates the petroleum system. Multiphase faulting and tectonic inversion have segmented the basin, influenced both the migration and trapping of hydrocarbons [28, 41]. Seismic interpretation highlights abrupt facies transitions and variable reservoir geometries, suggesting that structural deformation has influenced both depositional architecture and post-depositional modification. Notably, the thinning of the Manchester Marl in the eastern and southern regions further undermines seal capacity in those areas [21, 33].

The Cheshire Basin exhibits several promising geological attributes required for hydrocarbon accumulation; however, the interplay of post-depositional tectonics, compromised seals, source rock degradation, and sedimentological heterogeneity collectively reduces its commercial viability. Future exploration should focus on detailed fault seal analysis [42], basin modelling to assess migration pathways, and geochemical correlation with adjacent productive basins to reevaluate its remaining potential.

Facies Correlation

Facies correlation across the study wells provides critical insights into the spatial variability of depositional environments and the stratigraphic architecture of the Cheshire Basin reservoirs. The correlation framework established in this study is based on sequence stratigraphic principles, which enable the recognition of key stratigraphic surfaces, parasequence stacking patterns, and lateral facies transitions [43, 44].

The four wells examined—Knutsford 1, Winsford 1, Elworth 1, and Burford 1—are aligned in a north–south transect, allowing for systematic evaluation of facies continuity and reservoir geometry. Two principal reservoir intervals were identified and correlated, although their distribution is not uniform. Knutsford 1, the deepest well in the dataset [21], penetrates both the Sherwood Sandstone Group and the underlying Collyhurst Sandstone Formation, separated by the Manchester Marl Formation. In contrast, the other three wells encounter only the Sherwood Sandstone Group, with the Collyhurst Sandstone absent due to shallower total depths.

Correlation of the Sherwood Sandstone Group reveals significant vertical and lateral facies variability. Four main depositional facies—aeolian, channel-fill, lacustrine, and fluvial—were recognized and correlated across the wells [30, 34]. Knutsford 1 preserves the thickest and most complete Sherwood Sandstone succession, whereas the shallower wells display partial sections, lacking the deeper stratigraphic members. Horizon flattening at the top of the Sherwood Sandstone enhances recognition of parasequences and reveals at least eight discrete parasequences within the combined Sherwood and Collyhurst intervals (Figure 13) [43].

The Collyhurst Sandstone Formation, present only in Knutsford 1, is dominated by a single aeolian facies. Gamma- ray log responses indicate this interval is a clean, high-quality reservoir with minimal shale content, suggesting favorable petrophysical properties [21, 30]. This lithological purity, coupled with its position beneath the Manchester Marl seal, highlights the Collyhurst as a potential secondary reservoir target in deeper parts of the basin.

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Overall, the facies correlation demonstrates that reservoir quality and continuity within the Cheshire Basin are strongly controlled by depositional environment, stratigraphic position, and structural depth. The greater completeness of the stratigraphic succession in Knutsford 1 compared to the other wells underscores the importance of depth penetration in fully characterizing the basin’s reservoir architecture. These correlations form the basis for subsequent reservoir modelling and resource evaluation.

Conclusion

This study re-evaluates the hydrocarbon potential of the Cheshire Basin by integrating seismic interpretation, facies analysis, and thermal maturity modeling. Although the basin contains all essential petroleum system elements—source, reservoir, and seal—commercial hydrocarbon accumulation has not been achieved. This is primarily due to post-depositional faulting and erosion, which have compromised seal integrity and reduced source rock effectiveness.

The Westphalian Coal Measures exhibit limited generative potential due to uplift-related TOC loss, while the Sherwood and Collyhurst Sandstones offer good local reservoir quality but poor regional continuity. Seismic evidence indicates widespread faulting that breaches key seals, increasing the likelihood of hydrocarbon leakage.

Despite these limitations, the basin retains potential for unconventional resources, particularly coalbed methane in structurally preserved zones. Future exploration should focus on fault seal analysis, geochemical assessment, and the evaluation of unconventional targets to reassess the basin’s residual prospectivity.

Author Contributions

Mr. Majid Hussain and Muhammad Paryal wrote the manuscript draft. Gulfam Hussain generated the idea, models, and maps. Muhammad Abdullah Khan checked the English writing and technical review, and Dr. Fathi Boukadi reviewed the paper and amended parts of the script write-up.

Conflicts of Interest

There are no conflicts of interest to declare.

Nomenclature

The following abbreviations are used in this manuscript