We investigate the relationship between Underground Gas Storage (UGS) operations and ground deformation of three UGS fields in the Po Plain basin, Italy, hosted in Pliocene clastic deposits within anticline structural traps. Sentinel-1 InSAR data from 2015 to 2021 were analyzed to quantify seasonal uplift and subsidence patterns associated with the cyclic injection and withdrawal of gas. The methodology evaluates correlations between UGS activity and the seasonal amplitude of vertical displacement using cross-correlation parameters R and K, which measure the shape (R) and scale (K) similarity between vertical displacement time series and gas volume fluctuations. Results show that, with the UGS injection/withdrawal plan implemented until 2021, seasonal displacement peaks occur within gas field boundaries and diminish outward. Along the major axis of the anticline traps, UGS-related vertical displacements cease before reaching the field boundary, while transversally, they extend up to approximately 0.5 km beyond. Frequency distributions of seasonal amplitude, R and K values were used to define threshold values for R and K, enabling a quantitative identification of the effective UGS influence area, with GNSS data providing additional constraints. Our findings highlight the influence of structural trap geometry and bounding faults in shaping surface subsidence and uplift patterns. These findings underscore the need for advanced monitoring technologies and a comprehensive understanding of subsurface geology to an effective management of UGS operations. As global demand for gas storage increases, integrating geomechanical modeling with ground deformation monitoring will enhance risk assessment, ensure operational safety, and optimize gas storage strategies. The proposed methodology provides valuable insights for monitoring induced ground deformation, offering a framework for sustainable and effective UGS management.

Abstract Geosteering consists in the integration of different formation evaluation and drilling technologies, together with 3D and 2D reservoir modelling, to optimize the placement of highly deviated wells within a selected interval of the reservoir. During geosteering, the 2D structural model is constantly updated to fit the geological model to the observed data, in order to plan the required trajectory adjustments and keep the wellbore within the target interval. The introduction of deep reading measurements and inversion modelling allows consistently mapping and tracing one or multiple formation boundaries along the wellbore, without ever crossing these boundaries. Incorporating reservoir and near-reservoir data acquired during geosteering operations allows improving the 3D field models for future planning of new wells. The presented case study shows how, integrating high-resolution 2D structural mapping delineated during the geosteering of two wells drilled in a UGS field, can improve the quality of the field 3D structural model. The scalability of the process means that both the information from new wells drilled and from reprocessed legacy data can be incorporated, enhancing the structural understanding of the sub-surface. In the current application, two wells were imported and processed together, but there is no limitation to the number or sequence of well data imported and reprocessed: as new wells are drilled, these can be added to the 3D structural model for further processing and refinement. Alternatively, if suitable data were available form older wells that were not used to adjust the 3D structural model, they could be reanalyzed and reintegrated to update the structural model.

Abstract Underground Gas Storage (UGS) reservoirs are often a late development of hydrocarbon reservoirs converted to storage after reservoir depletion. This conversion commonly requires the drilling of new wells tailored to the needs of UGS and the deployment of services like geosteering to optimize the well placement in the target UGS reservoir. However, UGS is an application with specific technical challenges. Modelling of underground conditions can be difficult due to lack of suitable data, or due to the changing fluid properties connected to the cyclicity of charging and depletion of the reservoir, adding additional uncertainty to the challenges posed by geological variability. This case history details the integration of Logging While Drilling (LWD) data to land and geosteer two infilling wells for UGS activity optimization.The target for both wells was a sand layer, a few meters-thick, with optimal petrophysical properties at the top of the reservoir sequence. The results of the geosteering operations confirmed the uncertainties regarding the structural trend and formation properties in a peripheral area of the field. Unexpected variations in the reservoir architecture required rapid changes of wellbore trajectory to achieve the landing objectives. After landing, both wellbores were geosteered within the few meters-thick target interval, although lateral formation changes generated anomalous tool responses compared to the data measured in the offset wells. After achieving the operational target length, Well A exited the reservoir through the roof due to a rapid steepening of the formation dip, exceeding the project’s dogleg limitations. TD was called without further extension of the hole section. Well B, instead, was successfully geosteered beyond the planned TD.

Abstract. The Po Plain (Italy) is one of the most densely populated and productive regions of Europe, characterized by a flourishing economy (also linked to strategic subsurface resources) and several world cultural and natural heritage sites. The coupling of socio-economic interests with geological hazards (i.e. seismic, subsidence, and flooding hazards) in this area requires accurate knowledge of the subsurface geology, the active geological processes, and the impact of human activities on natural environments to mitigate the potential natural and anthropic risks. Most data unveiling the subsurface geology of this region were produced by the hydrocarbon exploration industry. Indeed, the Po Plain hosts many hydrocarbon fields that have been discovered since the early 1950s, giving rise to the subsurface exploration through extensive seismic reflection surveys and drilling of numerous deep wells. In this work, geological and geophysical data from 160 deep wells drilled for hydrocarbon exploration and/or exploitation purposes in the Po Plain and in the facing northern Adriatic Sea have been collected and digitized along with several published geological cross-sections and maps. These data have been used to reconstruct the overall subsurface 3D architecture and to extract the physical properties of the subsurface geological units. The digitized data are suitable to be imported into geo-software environments so as to derive the geophysical and mechanical properties of the geological units for a wealth of applied and scientific studies such as geomechanical, geophysical, and seismological studies. The integrated dataset may represent a useful tool in defining regional first-order strategies to ensure the safety of the urbanized areas and human activities and to reduce natural and anthropic risks that may affect this crucial region of Europe. In particular, the data collected would be useful to highlight sensible areas where data collection and more detailed studies are needed. Nowadays, such issues are particularly relevant for the underground industry development related to the increasing interest in possible CO2 and hydrogen underground storage, which can play a fundamental role in the energy transition process towards decarbonization goals. The full dataset is available at the following link: https://doi.org/10.5281/zenodo.8126519 (Livani et al., 2023).

Summary The scope of the research is to cast light on the deformation characteristics of underground formations involved in gas storage (UGS) activities in the Italian Po Plain area. The reliable prediction of ground movement caused by reservoir compaction/expansion is mandatory for the safety of both storage systems and urban settlements (especially in high urbanized area) and it is dependent on a solid knowledge of the soil/rock deformation behaviour under changing stress. Two case studies were presented in terms of subsidence analysis via a multi-disciplinary (static/dynamic/mechanical) 3D numerical simulation approach. A coherent and full dataset is available for each reservoir modelling, which includes also deformation/strength parameters from lab and logs and 10+ years of InSar surface movement data adopted for the geomechanical model calibration. The cases show strong similarities in terms of structural/geological contest, formation lithologies and UGS strategies, among others. Based on the subsidence analysis results and the analysis of InSar data, a common correlation between induced pressure variation in each reservoir and corresponding ground movement was inferred, resulting in an equivalent deformation behaviour. The paper represents the first step in defining a common deformation response, if any, for the numerous UGS systems present in the Po Plain basin.

The paper presents a multi-physics investigation of the ground movements related to the cyclical and seasonal injection and withdrawal of natural gas in/from a depleted reservoir located in the Po Plain area, Italy. Interferometric Synthetic Aperture Radar (InSAr) data (from 2003) and Global Navigation Satellite System (GNSS) data (from 2008) provided a full and coherent panorama of almost two decades of ground movement in the monitored area (more extended than the field boundary). The analysis of the acquired millimetric-scale movements together with the detailed geological analysis, both at reservoir and at regional scale, represents the focal point for understanding the investigated phenomena. Based on this information, a fully integrated and multidisciplinary geological, fluid-flow and geomechanical numerical modeling approach was developed to reproduce the main geometrical and structural features of the involved formations together with the poromechanics processes induced by the storage operations. The main achievement of the adopted methodology is a deep knowledge of the system and the involved processes, which is mandatory for the safety of the urbanized areas and the effective management of the underground resources.

The strong morphological similitude of the block-in-matrix fabric of chaotic rock units (mélanges and broken formations) makes problematic the recognition of their primary forming-processes. We present results of the comparison between magnetic fabric and mesoscale structural investigations of non-metamorphic tectonic, sedimentary, and polygenetic mélanges in the exhumed Late Cretaceous to early Eocene Ligurian accretionary complex and overlying wedge-top basin succession in the Northern Apennines (northwest Italy). Our findings show that the magnetic fabric reveals diagnostic configurations of principal anisotropy of magnetic susceptibility (AMS) axes orientation that are well comparable with the mesoscale block-in-matrix fabric of mélanges formed by different processes. Broken formations and tectonic mélanges show prolate and neutral-to-oblate ellipsoids, respectively, with magnetic fabric elements being consistent with those of the mesoscale anisotropic “structurally ordered” block-in-matrix fabric. Sedimentary mélanges show an oblate ellipsoid with a clear sedimentary magnetic fabric related to downslope gravitational emplacement. Polygenetic mélanges show the occurrence of a cumulative depositional and tectonic magnetic fabric. The comparison of field and laboratory investigations validate the analysis of magnetic features as a diagnostic tool suitable to analytically distinguish the contribution of different mélange forming-processes and their mutual superposition, and to better understand the geodynamic evolution of subduction-accretion complexes.

This study was carried out by the SEADOG Research Center at Politecnico di Torino (Italy). The purpose of this work was to evaluate which complexity degree would be required to reliably approach a subsidence study for different scenarios. The study was based on sensitivity analyses which were performed using a series of 3D synthetic numerical models of which the structural characteristics and geological and mechanical properties were based on available public data of onshore and offshore hydrocarbon fields in Italy. An array of simulations, both one-way and two-way coupled, were carried out to assess the magnitude and extension of subsidence potentially induced by hydrocarbon production. The results allowed the calculation of subsidence indices defined as the rate of compaction propagation (i.e., the ratio between the maximum surface displacement and the maximum reservoir compaction) and as the rate of volume loss (i.e. the ratio between the volume of the subsidence bowl or cone and the volume variation of the reservoir). These indices together with the degree of the underground systems' heterogeneity led to the definition of the Intact Rock Qualitative Subsidence Index (IRQSI), upon which the needed complexity degree of a subsidence study can be discerned.

La variazione della quota altimetrica del piano campagna e imputabile a cause sia naturali sia antropiche. Queste ultime dipendono principalmente dall'emungimento degli acquiferi e dalla coltivazione dei giacimenti di idrocarburi. La verifica che la subsidenza potenzialmente indotta dall'estrazione di fluidi dal sottosuolo non abbia impatto sugli edifici e sulle infrastrutture esistenti nell'area di interesse deve essere condotta valutando sia l'estensione areale del cono di subsidenza sia gli spostamenti massimi. Nel presente articolo viene proposto un approccio numerico 3D per l'analisi dei fenomeni accoppiati fluidodinamici e tenso-deformativi che governano l'evoluzione della subsidenza. Tale approccio si basa sull'integrazione di tre modelli di riferimento: il modello geologico, quello fluido-dinamico e quello geomeccanico. L'approccio di analisi descritto e stato efficacemente adottato per valutare le variazioni altimetriche indotte sul piano campagna dalle attivita di uno stoccaggio sotterraneo di gas naturale. L'abbondanza e la qualita delle informazioni disponibili ne facevano un caso studio ideale. L'analisi totalmente integrata ha consentito di ottenere un modello affidabile, come dimostrato dal fatto che il modello e in grado di riprodurre l'evoluzione delle variazioni altimetriche del piano campagna storicamente indotte dalle operazioni di stoccaggio, registrate nel tempo mediante acquisizioni satellitari (misure InSAR).

The multi-disciplinary work described in the paper was aimed at analyzing and predicting the cyclical ground surface movements induced by underground gas storage (UGS) activities in a depleted gas field located in the Po Plain (Italy). The field has been operated as a storage facility for nearly three decades. Currently, the possibility of delta-pressuring the reservoir (i.e. to increase the maximum operating pressure above the initial reservoir pressure) to enhance the storage performance is being considered. Significant information was collected over time: 2/3D seismic surveys, geological and sedimentological studies, 60+ logged wells, geotechnical lab tests and 50+ years of production history and monitoring were available for the development of a fully integrated static–dynamic–geomechanical analysis. The mechanical aspects of the study are the focus of this paper. The data coming from different sources at different scales were analysed and integrated to set up and characterize a 3D finite element method mechanical model to calculate the surface movements induced by UGS activity by adopting an elasto-plastic constitutive law. The model was then calibrated via a back analysis approach, i.e. the model parameters were fine-tuned so that the simulated subsidence/uplift would compare satisfactorily with the ground movements collected over nearly 10 years of monitoring via interferometric synthetic aperture radar analysis in the region under investigation. Eventually, the calibrated model was used as a forecasting tool for subsidence evaluation under different future storage strategies, including delta-pressuring conditions. Results proved that no significant subsidence is expected even if the maximum operating pressure reached 120 % of the initial formation pressure.

In the Northern Apennines of Italy, mud-rich olistostromes (sedimentary melanges) occur at different stratigraphic levels within the late Oligocene-early Miocene sedimentary record of episutural/wedge-top basins. They are widely distributed along the exhumed outer part of the Ligurian accretionary complex, atop the outer Apenninic prowedge, over an area about 300km long and 10-15km wide. Olistostromes represent excellent examples of ancient submarine mass-transport complexes (MTCs), consisting of stacked cohesive debris flows that can be directly compared to some of those observed in modern accretionary wedges. We describe the internal arrangement of olistostrome occurrences in the sector between Voghera and the Monferrato area, analysing their relationships with mesoscale liquefaction features, which are commonly difficult to observe in modern MTCs. Slope failures occurred in isolated sectors along the wedge front, where out-of-sequence thrusting, seismicity, and different pulses of overpressured tectonically induced fluid flows acted concomitantly. Referring to the Northern Apennines regional geology, we also point out a gradual lateral rejuvenation (from late Oligocene to early Miocene) toward the SE and an increasing size and thickness of the olistostromes along the strike of the frontal Apenninic prowedge. This suggests that morphological reshaping of the outer prowedge via mass-transport processes balanced, with different pulses over a short time span, the southeastward migration and segmentation of accretionary processes. The latter were probably favoured by the occurrence in the northwestern part of the Northern Apennines of major, inherited palaeogeographic features controlling the northward propagation of the prowedge. Detailed knowledge of olistostromes, as ancient examples of MTCs related to syn-sedimentary tectonics and shale diapirism, and of their lateral variations in term of age and size, provides useful information in regard to better understanding of both the tectono-stratigraphic evolution of the Apenninic prowedge and the submarine slope failures in modern accretionary wedges.

The External Ligurian Units in western Monferrato (NW-Italy) have been always described as an undifferentiated chaotic complex. This map, at 1:10,000 scale, describes in detail the tectono-stratigraphic setting of these Units in the sector of the Alps–Apennines junction. Here, the External Ligurian Units represent the northwestern prolongation of the Northern Apennines and consist of a Late Cretaceous chaotic succession represented by the Argille varicolori and the overlaying Monte Cassio Flysch. The late Eocene–Miocene episutural succession of the Tertiary Piedmont Basin rests unconformably on the External Ligurian Units. The mapped crosscutting relationships between stratigraphic unconformities and faults allow us to describe a complex tectono-stratigraphic setting that is the product of four tectonic stages. Layer-parallel extension related to Late Cretaceous–early Eocene deformation occurred in the internal sector of the Alpine accretionary wedge and is preserved within the External Ligurian Units which is sealed by the late Eocene deposits of the Tertiary Piedmont Basin. The unconformity at the base of the Oligocene succession records the drowning of shelf sediments controlled by NW-striking left-lateral transtensive faulting. A WNW-striking and NE-verging thrust superposes the External Ligurian Units onto the late Eocene–Oligocene deposits and it is sealed by the gravitational emplacement of late Oligocene Polygenetic argillaceous breccias. Both the WNW-striking thrust and the Polygenetic argillaceous breccias are cut by NW-striking right-lateral transpressive faults that are, in turn, sealed by the Tortonian unconformity.

We document in this study the internal structure of the Late Cretaceous-late Oligocene Ligurian accretionary wedge in northwestern Italy, and the occurrence in this exhumed wedge of broken formation and three different types of melanges that formed sequentially through time. The broken formation is the oldest unit in the accretionary wedge and shows bedding-parallel boudinage structures, which developed as a result of layer-parallel extension at the toe of the internal part of the Alpine wedge front during the Late Cretaceous-middle Eocene. This broken formation experienced an overprint of tectonic, diapiric, and sedimentary processes as a result of continental collision in the late Oligocene. The NE-vergent thrusting and associated shortening produced a structurally ordered block-in-matrix fabric through mixing of both native and exotic blocks, forming the tectonic melange. The concentration of overpressurized fluids along the thrust fault planes triggered the upward rise of shaly material, producing the diapiric melange, which in turn provided the source material for the downslope emplacement of the youngest, late Oligocene sedimentary melange. The sedimentary melange units unconformably cover the collisional thrust faults, constraining the timing of both this episode of contractional deformation related to continental collision and the combination and overlap of tectonic, diapiric, and sedimentary processes. Our multiscale structural analysis of the Ligurian accretionary wedge shows that tectonic, diapiric, and sedimentary processes played a significant role in its evolution, and that the interplay between and the superposition of these different processes strongly controlled the dynamic equilibrium of the accretionary wedge in the NW Apennines-western Alps. This kind of polygenetic melange development may be common in many modern and ancient accretionary complexes, and the processes involved in their formation are likely to be responsible for major tsunamic events in convergent margins.

Melange poligenici: interazione di processi tettonici, sedimentari e diapirici in cunei di accrezione esumati.L'interazione e sovrapposizione di processi tettonici, sedimentari e diapirici a diversi livelli strutturali di cunei di accrezione, puo portare alla formazione di melange poligenici. Il riconoscimento del ruolo avuto da ognuno di questi processi e spesso problematico a causa della forte convergenza del fabric dei loro prodotti, della successiva deformazione e metamorfismo che obliterano le tracce del prevalente processo di formazione, e per il fatto che questi processi possono interagire e sovrapporsi in modi complessi. In questo lavoro mostriamo come diversi melange formati in differenti posizioni strutturali di cunei di accrezione esumati e associati bacini wedge-top, non rappresentano il prodotto di un unico processo, come invece sono stati comunemente interpretati in letteratura. Al contrario, essi rappresentano dei melange poligenici formati dall'interazione e sovrapposizione di processi tettonici, sedimentari e diapirici. Gli esempi discussi riguardano il Taconic melange della Hudson River Valley (Appalachi settentrionali), di eta tardo ordoviciana, le Unita Liguri esterne di eta Cretacico superiore affioranti in Monferrato e la successione caotica messiniana del Bacino Terziario Piemontese.

Mélanges originated from sedimentary processes (sedimentary mélanges) and olistostromes are frequently present in mountain chains worldwide. They are excellent fossil examples of mass-transport complexes (MTC), often cropping out in well-preserved and laterally continuous exposures. In this article we will show the results of the integrated study of fossil MTCs, including sedimentary mélanges/olistostromes, with a focus on the Apennines of Italy. Fossil MTCs, especially the basin-wide ones, are composite and multi-event units involving the entire spectra of mass-transport processes. The down-slope motion of these bodies is enabled by the relative movement of discrete masses, with progressive stratal disruption of rocks/sediment involved and flow transformation. Three kinds of MTC are here distinguished, in which the movements are enabled by (1) shear-dominated viscous flows within a muddy matrix, (2) mud-silt-sandy matrix sustained by fluid overpressure, (3) concentrated shear zones/surfaces with advection of grains and fluid (overpressured basal carpets). These MTC types may represent end-members of a continuum of products and correspond to different kinematics of transport and emplacement and to different relationship with the substratum. These observations should result in a better knowledge of mass-transport processes and bodies, in relation with the basin floor geometries.

Evoluzione tettonica del cuneo di accrezione Ligure in Monferrato (NO-Italia): nuovi dati dall'analisi di melanges tettonici, sedimentari e diapirici.L'evoluzione tettonica del Monferrato e ben conosciuta a partire dagli episodi deformativi oligocenici. Al contrario, la sua evoluzione pre-oligocenica legata alle fasi di accrezione del Cretacico superiore-Eocene medio del cuneo di accrezione Ligure e ancora poco conosciuta.Le Unita Liguri Esterne che costituiscono il substrato della successione del Bacino Terziario Piemontese in Monferrato sono conosciute come un complesso caotico indifferenziato di eta Cretacico superiore-Eocene medio. Lo studio dettagliato di queste Unita ha permesso di distinguere al loro interno tre unita litostratigrafiche: le Argille varicolori (Santoniano-Campaniano), il Flysch di Monte Cassio (Campaniano superiore(?)-Maastrichtiano), e le Brecce argillose poligeniche (Oligocene superiore). Le Argille varicolori e le Brecce argillose poligeniche rappresentano il prodotto dell'interazione e sovrapposizione di processi tettonici, sedimentari e diapirici che hanno operato in momenti diversi dell'evoluzione del cuneo di accrezione Ligure formando melange poligenici. L'analisi della deformazione e la comprensione dei rapporti di sovrapposizione tra i diversi tipi di melanges poligenici individuati hanno permesso di: (i) distinguere piu fasi deformative (Cretacico superiore, Rupeliano e Oligocene superiore); (ii) comprendere i rapporti di interazione e sovrapposizione tra processi tettonici, sedimentari e diapirici che hanno portato alla formazione di diversi tipi di melanges poligenici; (iii) ridefinire piu in dettaglio, rispetto ai dati di letteratura, i tempi della deformazione dell'intervallo di tempo Chattiano–pre Burdigaliano.

The Hamburg Klippe of the Central Appalachian orogenic belt exposed in eastern Pennsylvania displays a complex record of poly-phase mélange and broken formation development in a convergent margin setting. It includes an imbricate stack of tectonic slices, which consist of upper Cambrian to Upper Ordovician deep-water and continental slope sedimentary rocks, emplaced by gravity sliding onto the Laurentian passive margin during deposition of the Upper Ordovician Martinsburg Formation. Based on their internal structure and stratigraphy, the block-matrix ratios and relations, and the inferred tectonic settings of origin, we have differentiated the following mélanges and broken formations, whose evolutionary stages coincide with specific deformational phases in a complete orogenic cycle from subduction to collision: (i) sedimentary broken formations without exotic blocks, formed by in situ and local down-slope remobilization during the early stages of closure of the Octoraro Sea basin in the Early Ordovician; (ii) two types of sedimentary mélanges with exotic blocks, formed at the front of the advancing accretionary wedge during the Middle Ordovician subduction of the Laurentian continental margin beneath a microcontinent–magmatic arc tectonic assembly; (iii) layer-parallel, extensional broken formation and a diapiric mélange, formed in the outer trench and at the toe of the accretionary wedge, respectively, during the early–Late Ordovician; (iv) precursory olistostromes, formed during the Late Ordovician collisional episodes as the Hamburg Klippe was emplaced in the Martinsburg Formation on the downgoing Laurentian continental margin; and, (v) contractional deformation-related broken formations, formed at the base of main thrust faults overprinting the previously formed mélanges and broken formations. This sequential development of different mélange types in the Central Appalachians was strongly controlled by the degree of consolidation of the layered strata and their rheological differences, the structural level of mélange formation within the accretionary wedge, and the kinematics of deformational processes.