Following recent federal decisions in terms of energy policy, the GEothermie 2020 program has been initiated by the State of Geneva and the Services Industriels de Genève (SIG), to assess, and ultimately develop geothermal resources of the trans-border Greater Geneva Basin (GGB). After a first evaluation of potential geothermal reservoirs in the area carried out in 2011, it has appeared that no consistent geological model was available to apprehend the deep subsurface and units of interest, nor reservoir models. Subsequently, two PhD research projects have been initiated in 2013, at the early stage of the GEothermie 2020 program. Their aims have focussed on building a solid background knowledge on the regional geology oriented towards reservoir issues, and providing decision-making tools in support of geothermal exploration.
Common themes have been defined as (1) collecting existing data of the deep subsurface and to constitute an homogenized, usable dataset of well and seismic data, (2) building 3D geological models based on seismic data investigations, focussing on structural peculiarities, (3) quantifying reservoir properties in units of interest, (4) understanding their distribution throughout the basin, in order to ultimately (5) populating 3D geological models with values of reservoir properties representative for the studied area. The present study has specifically focussed on the characterisation of reservoir properties and their distribution using well and outcrop data, hence the points 1, 3, 4 and 5. It has also provided minor contributions to build geological models.
Behind these applied objectives, several research questions have been formulated:
• What is the palaeogeographic facies distribution and how this affects reservoir occur-rence in the region of study?
• What are the primary (sedimentation) and post-depositional process (diagene-sis/fractures) controlling reservoir properties distribution?
• Can we define a predictive model for reservoir quality?
The approach used to answer these questions is developed in the following paragraphs, and main results are summarized. First, the dataset has been constituted. All the wells reaching at least the Mesozoic units in an extended studied area have been selected. Reports and logs have been collected, the latter having been digitized and converted to enable inter-well correlations. Available cores have been described and sampled for routine core analyses
189 (RCA, porosity, permeability, density measurements) and microfacies description, including details on diagenetic features and on the pore network.
In order to apprehend vertically the sedimentary succession of the GGB, ranging from the Carboniferous to the Quaternary, one specific well, Humilly-2, has been fully investigated with complementary measurements (QEMSCAN, MICP, Vp). This well is located in the centre of the basin, penetrated the entire sedimentary succession, was cored in several inter-vals and equipped with TD-pairs, which make it a reference to calibrate seismic data as well.
Results of "in-depth" investigations have confirmed the position of five potential geother-mal reservoirs (Lower Cretaceous, Kimmeridgian, Bajocian, Muschelkalk, Buntsandstein).
Among those, the Kimmeridgian Reef Complex unit presents the most promising reservoir quality. Stratigraphical sequences have been re-interpreted in a coherent framework, accord-ing to well log interpretation and facies description in cores, interpretation of depositional environments and their evolution through time. Coupled with the assessment of petrophys-ical properties, this full investigation of Humilly-2 has provided consistent knowledge to anchor subsequent global well analysis in the basin.
Once the vertical sedimentary succession has been recognised at one location in the GGB, the lateral evolution of sedimentary bodies has been investigated. This part is presented in Chapter 4, and has been co-authored with Maud Brentini, who is currently working on a parallel study. The microfacies analysis performed on subsurface data have been integrated with outcrop material, and a full review of existing literature on regional, sedimentological field studies. Because inconsistencies have been highlighted in the definition of stratigraph-ical intervals and their nomenclature, a review of stratigraphstratigraph-ical units has been realized to ensure correlations across the basin, and compatibility of the dataset with more global stud-ies (HARMOS). The evolution of the sedimentary and depositional environments through time has been established, which has provided constraints to lateral variability of distinct sedimentary bodies.
In Chapter 5, reservoir property measurements have been integrated into this sedimentolog-ical and stratigraphsedimentolog-ical analysis. Investigations on reservoir properties have been completed with quantitative petrophysical and fluid parameters found in well documents. Such data have been collected and organized in a database created in collaboration with the French geological survey (BRGM), in the framework of the European GeoMol project. Results of reservoir assessment have been presented as a practical document, summarizing sedimento-logical and petrophysical (range of) values for the main five potential reservoirs of the GGB.
According to this global reservoir analysis, which considers the entire well dataset, theReef Complex unit has been picked as the best potential reservoir.
In order to further investigate the distribution of reservoir properties in the Reef Complex unit at different scales, an integrative rock typing workflow has been defined. It explores independently in rock samples the different factors which can influence the pore network (de-positional environment, impact of diagenesis and fracturation). Rock types corresponding to specific porosity-permeability values are defined according to each factor and superimposed.
This approach has allowed to understand which factor provides the better prediction of the static, porosity and permeability properties, and which of them controls their distribution
190 Chapter 7. Conclusions and recommendations in space.
In the studied case, results show that diagenesis controls the development of pores in the rock matrix, but the distribution of the diagenetic impact follows depositional environments and trends. A large variability of co-existing depositional environments has been highlighted, among which patch reef-related deposits show the largest connected pore network. The latter consists mainly of intercrytalline micropores, which developed in the automicrite fraction, specifically below sequence boundaries. Porous sucrosic dolomite intervals have also been recognised within patch-reefs, and at the base of the reef sequence. The latter interval ap-pears more continuous than other microporous zones, and could play a key role in connecting the best reservoir bodies. Fractures seem also well developed in microporous, reef-related rock types, and meter-thick tight dolomite intervals. Different fracture patterns have been highlighted in theReef Complex and the overlying unit, which seem interconnected by larger faults. The mechanical stratigraphy has been pictured, mainly from outcrop observations, and seems to correspond to the subsurface according to well and seismic data.
Rock types defined from the micro to the macro-core scale have been upscaled to the log domain. Electrical rock types have been defined according to specific log signals. These rock types have been compared to the previous one, to verify their intercompatibility. Only few modifications have been applied to rock types defined on rock samples, in order to enable them to be properly represented by logs, intense fracturation being likely not identifiable in the latter. The resulting petrophysical rock types (PRTs) take into account the complex architecture of original depositional facies, the strong impact of diagenesis on matrix reser-voir properties and only the largest enhanced permeable features (faults). It represents a dual porosity-permeability system. The lateral distribution of PRTs have been constrained according to depositional trends, and properly defined stratigraphical sequences. Therefore, this detailed study on the Reef Complex unit has depicted a really heterogeneous reservoir, showing significant reservoir property anisotropy. It has provided necessary input to popu-late further reservoir models and a "training image" to propagate reservoir property in three dimensions.
Upscaling issues are commonly addressed in rock typing studies. In this case, the poor quality and representativity of the well dataset has restricted the well log interpretation, and thus mathematical verification of rock types compatibility between the core and log domains. This problem increases uncertainties related to the vertical distribution of PRTs along wells, but also their distribution in the inter-well space. Therefore, the rock typing workflow has been designed to enable further integration of new well data, which could confirm the definition of PRTs, and improve their prediction ability.
This study, which has integrated quantified petrophysical measurements with sedimentolog-ical data and concepts, stratigraphsedimentolog-ically well constrained, has yielded a fundamental knowl-edge to apprehend the different reservoir units in the GGB. The specific reservoir rock typing analysis performed in the best potential reservoir has provided keys to understand the reser-voir behaviour through the characterisation of the pore network, its interconnectivity, and clues to predict the distribution of conductive bodies. It has also highlighted that enhanced permeability structures are present in the basin, even at the sub-seismic scale. This reservoir
191 should therefore be considered as a promising but heterogeneous, dual porosity-permeability play. The characterisation and integration of specific fracure attributes into the rock type definition is recommended in the future, to properly address this duality.
The present investigations have demonstrated that a wide range of reservoir property values (from poor to promising) characterizes the different reservoir units of the GGB. This diversity in terms of reservoir quality, which has been fully developed for theReef Complex reservoir unit, highlights complex reservoir architectures and a heterogeneous distribution of promis-ing reservoir properties. In the shallowest Cretaceous unit, first exploration wells drilled recently for water resources have already proved the reservoir potential of this unit at the foothill of the Jura Mountains. Similarly to the Bavarian and Paris basins, where geother-mal energy has been successfully produced since decades in heterogeneous reservoirs, this complexity in reservoir units of the GGB is not thought to prevent sustainable, exploitable resources. However, according to the present study, it might be expressed by surprising -but not disappointing- results of first exploration drillings, because to date, the restricted subsur-face dataset has not enable the precise assessment of resource volumes available. Being at the beginning of exploration, new results should be considered as additional constraints on the subsurface vision and reservoir models. Finally, this study has provided one small step for the global comprehension and assessment of intricate carbonate reservoirs, but a substantial leap for the understanding of reservoirs in the GGB, towards the successful development of geothermal resources.
192
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