This project is carried out in the context of the Innovation Program Upstream Gas and is part of the Dutch Top Sector policy ‘Energy’. The project is titled ‘Improved sweet spot identification and smart development using integrated reservoir characterization (Phase 2)’. The total budget of the project is 200,000.- €, half of the budget is accounted for by the Dutch government. The other half is funded by the two industrial partners Energie Beheer Nederland (EBN) and Wintershall Noordzee B.V..
1.1.1 Background of study – project plan
Since well data from shale gas and coalbed methane (CBM) plays is limited and poorly understood, it is crucial to develop non-conventional methods to optimize gas recovery and mitigate production risks, thereby enabling economic development of these plays. Therefore, the full suite of available subsurface and field data and -methods needs to be integrated to successfully use well log responses for 1) identification, 2) characterization of sweet spots and 3) reservoir behaviour predictions. The first phase of the project (Phase 1: 2012-2013) focused on the integrated sedimentological and petrophysical reservoir characterization, leading to a better understanding of shale plays and insight into the ideal paleoenvironments that offer most favourable shale gas conditions. All data and interpretations were translated and upscaled to log scale to suit the tools of the operator. The second phase will build on the results so far to validate hypothesis, calibrate geochemical data and to investigate which concepts can be assumed to be generally favourable for shale gas plays.
The first stage Sweet Spot project Phase 2 is focussed on the calibration and further interpretation of geochemical data. For the calibration of the present geochemical composition the collected samples will be analysed for clay-fraction XRD. The mineral composition, especially the clay fraction and the clay composition, will give an indication of the hydraulic fracturing potential of the identified zones, especially when the results of this study are linked to the results of the hydraulic fracturing and well placement study of TNO and the fracture network study of Utrecht University According to Gasparik et al. (2012) the clay composition is furthermore the main factor influencing the amount of adsorbed gas in the Posidonia Shale Formation. Extending upon the XRD results, Rock-Eval analysis will be performed to gain insight into the differences in the type of the organic fraction of the samples will be assessed for the different zones and linked to the identified biofacies. The organic material gives an indication of the total hydrocarbon generation potential of the individual zones. According to current knowledge (e.g. Passey et al., 2010) overmature oilprone type II source rocks appear to have the most potential. Most Posidonia shale data indicate Kerogen Type 2 or 3 but more detailed knowledge on the vertical and lateral variability in potential kerogen type will serves economic purposes. Based on the results of the palynological biofacies study from the first phase of the project (Phase1) the organic matter type can be linked to specific depositional environments, helping the conditions and high-potential zones to be better identified (e.g. for biosteering).
The second stage of the Sweet Spot project Phase 2 is based on the hypothesis formulated in the project’s Phase 1. It is designed to both test the hypothesis and gain insight into sweet spots in the Dutch on- and offshore. Comparison of the geochemical data from the Whitby Shale (UK) to the Posidonia shale in the Netherlands will give insights into the correlation and extrapolation potential of sweetspots basin-wide. In this respect it is important to understand the regional versus basin-wide paleoenvironmental setting. Extending the dataset towards the deeper zones of the NWGB using offshore wells from the Netherlands, gives us more control in the lateral extent of the prolific shales and also further increases our understanding on the play. This project integrates and discusses large scale controlling parameters such as the influence of the Boreal Realm, the change to multi-basinal patterns and terrestrial inputs from structural highs.
Using all data, interpretations, correlations and hypotheses from both the first and second phase of the Sweet Spot project, we aim to translate the Posidonia Shale Formation’s specific characteristics and properties to more generalized concepts concerning shale plays. For this analogy, several proven shale gas plays are cross referenced based on published data and data provided by the Sweet Spot’s project industrial partners. Not only is this translation of concepts of importance for shale occurrences from different geographical locations, but ii can also act as an introduction for future research on under explored shale play intervals in the Dutch subsurface.
1.1.2 Geological description (modified from the SSI1 report)
The Toarcian Posidonia Shale Formation is part of a very distinctive global stratigraphic interval with a present-day distribution from central to northwestern Europe, comprising the surface and subsurface of the U.K. (Mulgrave Shale Member), Germany (Posidonienschiefer, or Ölschiefer, Figure 3) and France (Schistes Carton). Given the relatively uniform lithological characters (dark-grey to brownish-black, bituminous, fissile claystones and siltstones) and thickness (mostly around 30-60 m) across these basins, it is commonly suggested that the deposition of the Posidonia Shale took place over a large oceanic domain during a period of high eustatic level, restricted circulation in the water column and relative tectonic quiescence. The present-day distribution of these stratigraphic units was probably controlled by erosion at basin margins and non-deposition over bounding paleotopographic highs (Pletsch et al., 2010). The official Dutch nomenclature (Van Adrichem Boogaert and Kouwe, 1993-1997) describes the Posidonia Shale Formation as deposited in a low-energy pelagic environment under oxygen-deficient conditions, partly controlled by a eustatic phase of high sea level; however, recent research suggests that this simplistic process and environmental framework should be reconsidered (e.g. Ghadeer and Macquaker, 2011; Trabucho-Alexandre et al., 2012).
Figure 1-1 Paleogeographic setting during the Toarcian with location of the study areas (modified after Ruebsam et al., 2014). Y: Yorkshire/Whitby, UK, H: Hils syncline, Germany, L:
Lorraine Sub-Basin, Luxembourg, D: Dotternhausen, Germany, S: Sancerre, France, P: Peniche, Portugal, C: Colle di Sogno, Italy
In the Netherlands, the formation is restricted to the axes of Late Jurassic rift basins (West Netherlands Basin and its extension into the Roer Valley Graben, the Central Netherlands Basin, and isolated locations in the Lower Saxony Basin in the onshore and Broad Fourteens Basin and Dutch Central Graben in the offshore, Figure 1-2).
The common view is that the sediments deposited outside the basin centres were locally eroded in parts of the Netherlands due to inversion events (Wong et al.,
2007), although this hypothesis is debated following observations of syn-sedimentary tectonics in the Early Jurassic. The Posidonia Shale Formation conformably overlies the non-bituminous claystones of the Lower Jurassic Aalburg Formation, although bituminous intervals have been identified also in the Aalburg Formation (De Jager et al., 1996), and it is conformably overlain by non-bituminous clay- and siltstones of the Middle Jurassic Werkendam Formation (Van Adrichem Boogaert and Kouwe 1993-1997; TNO-NITG, 2004), although hiatuses and unconfomities were identified at several locations.
The Posidonia Shale Formation consists of dark-grey to brownish-black bituminous fissile claystones and forms a very distinctive interval throughout the subsurface of the Netherlands. It isrecognizable by its high gamma ray and resistivity readings on wire-line logs (Van Adrichem Boogaert and Kouwe, 1993-1997). Evaluation of wireline log responses showed that subdivisions can be made into distinct zones within the Posidonia Shale Formation, which are correlatable between wells throughout the basin (see chapter 2.6). A similar vertical zonation of the Posidonia Shale Formation is observed also in Germany (locally referred as the Posidonienschiefer Formation) on the basis of both geochemical and sedimentological parameters (e.g. Röhl et al., 2001; Frimmel et al., 2004; Schwark and Frimmel, 2004).
Figure 1-2 Source rock facies map of the Posidonia Shale Formation and its equivalents (Doornenbal and Stevenson, 2010)