S. Nelskamp

Multidisciplinary Approach for Detailed Characterization of Shale Gas Reservoirs: A Netherlands Showcase

The success of shale gas exploration and production in the United States has triggered other countries around the world to look into possibilities of producing gas from different shales. As it turns out, one of the main difficulties when looking for shale gas is obtaining an in depth understanding of the shale formations and interpret where the sweet spots for gas production will be. This is mainly caused by the large differences in properties of shale formations. To improve our understanding of the vertical and lateral property changes that determine high and low productivity zones in shales, TNO has proposed a workflow including well log interpretation, mud log interpretation, depth modeling, organic geochemistry, basin modeling, palynology, electron microscope analyses, mineral typing and porosity identification and characterization. This method serves as a solid basis for sweet spot determination and proper resource estimates. The proposed method has been applied to the two main possible shale gas reservoirs in the Netherlands; the Lower Jurassic Posidonia Shale Formation (PSF) and the Carboniferous (Namurian) Epen Formation. The latter is of particular interest as it contains the highly organic basal Geverik Member (GM). For the two formations detailed and accurate depth maps have been produced. Research on the PSF shows that a number of areas are expected to have reached gas maturity, together around a fifth of the total area of the formation. Besides shale gas there appears to be potential for shale oil. High productivity zones within these shales are expected in the deepest part of formation and orientated along the distribution axis within the West Netherlands Basin. In the multidisciplinary workflow a 3D faciesmodel of the formation is created using palynology and organofacies to detect prospective layers within the formation. This faciesmodel is then used in combination with log-interpretaion, seismic interpretation and sedimentological interpretation to detect most prospective zones and areas. FIB-SEM microscopy shows that coccoliths present in the formation can have large favourable effects on the porosity and permeability providing for a micropermeability pathway for oil and gas when interconnected. When the proposed workflow is applied to the Geverik Member, a correlation between silica content and Total Organic Carbon (TOC) is found, i.e. log-derived TOC is in general close to 10% when the dominant mineral type is silica. Geomechanical analysis of rock properties shows that the lower zone of the Geverik Member is the favourable layer to target, i.e. the layer is dominantly build up of quartz and silica suggesting preferential conditions for hydraulic stimulation.

Shallow gas migration and trapping in the cenozoic eridanos delta deposits, Dutch offshore (Z045)

Shallow gas in the Dutch offshore predominantly occurs in shallow marine to continental deposits of the Plio-Pleistocene Eridanos delta (Fig. 1). This delta was fed by an ancient fluvio-deltaic system that prograded through Northwestern Europe due to simultaneous uplift of the Fennoscandinavian Shield and accelerated subsidence of the North Sea Basin. The dimensions of its drainage area and the thickness of the deltaic deposits make it one of the largest deltas and complex in the world. The origin of the shallow gas may be deep subsurface thermogenic sources or biogenic sources in shallower strata. It is either structurally trapped in anticlines associated with rising salt domes or occurs in stratigraphic or depositional traps.

Sweet Spots For Hydraulic Fracturing And Oil Or Gas Production In Underexplored Shales Using Key Performance Indicators: Example Of The Posidonia Shale Formation In The Netherlands

While extensive data and experiences are available for hydraulic fracturing and hydrocarbon production from shales in the U.S.A., such a record is lacking in many underexplored shale basins worldwide. As limited data is usually available in these basins, analysis of shale prospectivity and identification of sweet spots for oil & gas production needs to be based on upfront geological characterization and modelling before drilling, hydraulic fracturing and gas production has commenced. In this paper, different reservoir characterization techniques and modelling approaches are combined to quantify key performance indi-cators that describe potential shale productivity across the formation. The performance indicators describe the potential for hydrocarbon generation, storage, and flow stimulation based on a limited number of shale properties, and are benchmarked using data from the prospective Barnett Shale Formation in the U.S.A. The approach has been applied to the underexplored Posidonia Shale Formation in the South of the Netherlands (West Netherlands Basin). Local maxima of the mean performance indicator across the formation correlate with local maxima in vitrinite reflectance and depth. Average performance for the Posidonia is lower than for the Barnett shale, mainly due to lower maturity and brittleness. Besides higher clay content and associated lower brittleness, the potential for hydrocarbon generation and storage capacity of the Posidonia are comparable to oil mature Barnett shale from noncore areas. Predictions of gas production in the Posidonia based on simulations of hydraulic fracturing and gas flow using well-based models are in rough agreement with gas production observed in noncore Barnett areas, but low brittleness may hamper succesfull hydraulic fracturing. The example shows that the approach can be applied to underexplored shales worldwide to quantify prospectivity, map out sweet spots, point to preferred drilling sites, and optimize field development planning.