GEO ExPro

Defining Shale Potential in The Netherlands.

A recent study assesses unconventional plays and bypassed conventional pay in the Netherlands.
This article appeared in Vol. 11, No. 3 - 2014

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Farm in northern Holland, with fields of flowers in bloom - windmill from 1774 in the background. Courtesy of Hans A. Rosbach

The Netherlands is a long-established petroleum economy, having successfully produced oil and gas both onshore and offshore for over sixty years. In common with other established hydrocarbon markets, the producing fields of The Netherlands are mature and many are in a state of declining production. The Dutch government, together with the state oil company, EBN, and the geological survey, TNO, have been charting the decline in hydrocarbon production in The Netherlands, as the many mature fields reach the end of their productive life, and the graph shows a worrying trend towards ever increasing reliance on imports.

EBN has therefore taken the decision to commission a series of studies to assess what measures can be taken to arrest this decline.

The Netherlands has always adopted a proactive approach to the domestic hydrocarbon industry, as witnessed by the involvement of the state oil company, EBN. It maintains a significant stakeholding in nearly every licensed field both onshore and offshore, acting for and on behalf of the Dutch government, in a non-operating capacity, but involved both technically and commercially in exploration and production requirements in the licence.

The Dutch geological survey, TNO, are also more actively involved in the hydrocarbon business than some of their other EU counterparts. The management of the country’s released data archive is central to the ease with which oil companies have been able to explore and license acreage. Fifteen years ago, TNO took the decision to design their own online repository for released seismic and well data, which it also maintains. NLOG contains a full digital archive of all available data associated with wells and seismic surveys, which is free for anyone to download and use – a factor that has greatly benefited all operators and contractors working in The Netherlands.

When considering what could be done to halt the decline in production, one of the first areas of assessment was the potential for unconventional resource play development. The Dutch government has neither permitted nor banned unconventional hydrocarbon exploration and production, or the controversial stimulation technique of hydraulic fracturing, or fracking. In an effort to understand better the potential for such play development, and whether the country actually had a shale play that might be a ‘game-changer’, EBN has commissioned various studies to be undertaken to determine this. These have looked at the geology of the source rocks in The Netherlands in comparison with other mature shale plays in the US, and have also assessed and addressed the environmental impacts as well as the economic benefits of developing such plays. To enhance their own investigations they engaged the services of the US Reservoir Intelligence specialists, NuTech.

Pioneer in Shales

NuTech well log analysis results showing petrophysical and geomechanical properties for the shale sequences. (Source: NuTech) NuTech has been in the shale business ever since there was a shale business. The company was founded by former executives of Numar, a specialist in Magnetic Resonance Imaging (MRI) tool design. NuTech took their understanding of how MRI tools read in various matrix conditions in relation to how other triple-combo logging tools responded in the same conditions, and applied this knowledge to the conventional log data in order to reproduce the response of an MRI tool.

The resultant effect provided NuTech with a unique petrophysical log analysis service capable of accurately measuring very fine porosity, thereby determining clays, silts, sands, and other mineralogy within the rock matrix, as well as determining free and bound fluids within the pore space. This ‘textural’ approach to petrophysics gave NuTech a unique advantage and the company was soon being sought out to provide their specialised analytical services for use in bypassed pay identification in the US Gulf of Mexico offshore market, which was at the same stage of maturity and declining production fifteen years ago as the North Sea is now.

NuTech’s ability to measure finegrained porosity and to accurately determine permeability in tight rocks and complex formations led to the company coming to the attention of the early pioneers in the unconventional market and over the last twelve years the company has been involved in the discovery and proving of every shale play in North America.

Their approach gave oil companies the ability to explore through the effective use of datasets pulled from the archive to analyse play potential, rather than the more expensive approach of exploring with the drill bit. They set about reassessing significant volumes of legacy wells, whose original targets had been conventional pay in sandstone reservoirs, but whose drilled depth and logged data penetrated the shale source rocks. Taking a representative sample of suitable wells over the entire extent of a basin, and assessing both the log data and rock material (drill core and cuttings), they were able to provide detailed reservoir and completion parameters from this combined approach of petrophysical and rock properties analysis, all before the drilling of a single new well. The resulting parameters for each analysed well – porosity, permeability, Vclay, TOC, Ro, OIIP / GIIP, etc. – were then able to be mapped over the extent of the basin, and so determine the ‘sweet spots’ of the shale play, which determined where best to lease or license, and where best to drill. 

This Reservoir Intelligence approach – the technical quantification and commercial qualification of unconventional plays – has been built into an impressive set of regional field studies covering the entire extent of North American shale plays, incorporating NuTech’s history of work in petrophysics, core analysis, basin-wide property mapping, reservoir modelling, completions design and production optimisation.

The Netherlands Study

With this background in providing technical appraisal solutions for both problematic conventional and unconventional play development, NuTech was the obvious choice for EBN when tasked with the job of defining the potential for shale play development in The Netherlands.

After an initial pilot project, the geographic and technical scope of work for The Netherlands study was agreed. Geographically it would look at a series of legacy well data sets over the West Netherlands Basin (both its onshore and offshore extents) and the Geverik Member onshore in the North. The two main technical objectives were, firstly, to identify and analyse the shale play opportunities in unconventional reservoirs of the Lower Jurassic Posidonia Formation and the Namurian Geverik Member as well as other shale and marl sections in the area; and secondly, to identify bypassed hydrocarbon opportunities in conventional reservoir sections throughout both the onshore and offshore sections of these geological basins. As such, a full Total Depth (TD) analysis was agreed for each selected well.

  • Posidonia Shale study of the West Netherlands Basin. (Source: EBN)

  • Net pay isopach (m) of the Posidonia Shale play. (Source: NuTech)

The decision to address both onshore and offshore wells for the study was an interesting one, and one of the first times such an assessment has been undertaken for a shale play. The geological complexity of the West Netherlands Basin and the limited pool of wells with sufficient data for analysis was one of the primary factors governing this decision. While actual development of shale plays from offshore locations is perhaps not yet a viable commercial proposition, the inclusion of bypassed pay analysis in the study, and the fact that many of the offshore fields are declining and facing decisions of abandonment and rig decommissioning, meant that more sound economic judgements could be made in this respect, weighing up the full picture of hydrocarbon potential left to be developed before approving a request to shut down a field.

Only eight wells address the Geverik Member, although there is some recent core material. More legacy data exists for the Posidonia Shale in vintage wells drilled from the 1960s–90s. A limited number of onshore wells contain core and cuttings but very few of the offshore wells do. Similarly, of the 160 or so wells for which the available logs were reviewed, a number did not penetrate the Posidonia shale, or were not logged over the shale, or did not contain a complete enough suite of logs for full analysis. However, in keeping with NuTech’s equivalent US studies, sufficient legacy rock and log data was available to determine petrophysical and rock mechanical properties, and to map these characteristics for the Posidonia shale on a regional basis.

What Next?

Conventional producing reservoirs in The Netherlands and those identified as showing potential for bypassed pay. (Souce: EBN) The next steps in proving economic viability of the Posidonia, in what is structurally a highly complex part of The Netherlands, will focus on improving the understanding of geographical variations in formation thickness, reservoir quality and spatial continuity. To achieve this aim, full reservoir modelling of the basin, incorporating geological structure and faulting from the available seismic data, will be undertaken to help develop an improved understanding of the petroleum systems and assist guidance towards formation sweet-spots. However, additional vertical wells will undoubtedly need to be drilled to capture more log and core data.

The study reviewed each selected well to TD, mapping the key shale characteristics on a regional basis, but taking account of all conventional reservoir opportunities – both those produced or still producing, and those deemed to have been missed. As a partner in every well reviewed, EBN were in a perfect position to determine which of the identified and viable conventional reservoir sections had been produced, and which had not. Of the latter category of so-called bypassed pay elements, three stood out – the Holland Greensand, the Delfland Sandstone, and the most prospective of all, the Brabant Limestone. Of significant interest is the fact that the Brabant Limestone extends into the UK Southern Gas Basin where it is known as the Cornbrash, and where too it is understood that completions and production have been limited to date.

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