Lars Kristensen

Downflank hydrocarbon potential identified using seismic inversion and geostatistics: Upper Maastrichtian reservoir unit, Dan Field, Danish Central Graben

Porosities considerably higher than anticipated from porosity/depth trend models are encountered in a Maastrichtian reservoir unit on the western flank of the Dan Field. Because there is a good correlation between seismic impedance and well log porosity, inverted seismic data are used to infer that highly porous zones are widely distributed. The distribution of these high porosity zones is predicted using geostatistical methods based on the inverted seismic data. These predictions contradict the general assumption that porosity deteriorates with depth in the study area. Annealing cosimulation is applied using inversion-derived seismic impedances as soft data and well log porosities as hard data. The sensitivity of the porosity characterization to hydrocarbon in-place estimates is investigated through the calculation of water saturations using height above free water level and simulated porosity as input parameters. Multiple realizations show that calculated hydrocarbon in-place estimates are more sensitive to the location of the free water level than to uncertainties related to the geostatistical reservoir characterization.

Characterization and zonation of a marly chalk reservoir: the Lower Cretaceous Valdemar Field of the Danish Central Graben

In the Valdemar Field of the Danish Central Graben, production is from the Upper Hauterivian–Aptian succession (Tuxen and Sola formations) which comprises interlayered pelagic/hemipelagic chalks, marly chalks and marlstones. Based on core data, the reservoir chalks (6–10% insoluble residue (IR)) and marly chalks (10–30% IR) possess porosities in the 20–48% range and matrix permeabilities of 0.1–4 mD. The porosity (ϕ) of these chalks correlates negatively with the IR (particularly clay) content. A permeability cut-off of 0.1 mD was defined for reservoir studies, corresponding to a ϕ of 20–30% and an IR of c. 35%. A detailed reservoir zonation, based on integration of core and petrophysical data constrained within a sequence stratigraphic framework, illustrates the stratigraphical compartmentalization of the field. The distinctive nature of this heterogeneous argillaceous chalk reservoir is illustrated by comparison with Maastrichtian–Danian chalks of the Central Graben. For a given porosity, the matrix permeability of the Valdemar Field chalks is a factor of ten lower than that of an equivalent Maastrichtian chalk, the irreducible water saturation is a factor of ten higher and the capillary entry pressure of the Lower Cretaceous reservoir is 2–3 times that of the younger chalks.

Mapping and characterization of thin chalk reservoirs using data integration: the Kraka Field, Danish North Sea

The integration of 3D seismic data, well logs and synthetic seismic data has been used to identify an additional Intra Danian seismic horizon in the chalk reservoir of the Kraka Field, Danish North Sea. Mapping of this seismic horizon has allowed production of a separate thickness map for the main reservoir unit, the Danian Porous, in the greater Kraka area. The unit is less than 25 m thick in most areas and, to produce reliable reservoir maps, it has been necessary to use well data to guide the seismic interpretation. It is impossible, however, to resolve the reservoir stratigraphy properly in areas where the Danian Porous is thinner than c.15 m due to tuning effects. The lateral porosity distribution has been mapped using a combination of well log data and seismic data inverted for acoustic impedance. The Danian Porous unit is characterized by average porosities over 28% and shows no evidence of depth-related porosity reduction. Rather, the impedance data indicate the presence of positive porosity anomalies both over the crest and downflank towards the southeast. Comparison of impedance-derived porosities with those derived from well data indicates that the seismic-based data reflect the variations in porosity but underestimate the highest porosity by 3–4%. Faults and fractures are important for production of the Kraka Field. Detailed mapping of seismic horizons, supplemented with seismic attribute mapping, has proved useful for outlining areas with high fault intensity in the northwestern part of the field but has been unsuccessful in identifying individual faults as recognized from log data.

Pre-drilling assessments of average porosity and permeability in the geothermal reservoirs of the Danish area

Denmark constitutes a low-enthalpy geothermal area, and currently geothermal production takes places from two sandstone-rich formations: the Bunter Sandstone and the Gassum formations. These formations form major geothermal reservoirs in the Danish area, but exploration is associated with high geological uncertainty and information about reservoir permeability is difficult to obtain. Prediction of porosity and permeability prior to drilling is therefore essential in order to reduce risks. Geologically these two formations represent excellent examples of sandstone diversity, since they were deposited in a variety of environments during arid and humid climatic conditions. The study is based on geological and petrophysical data acquired in deep wells onshore Denmark, including conventional core analysis data and well-logs. A method for assessing and predicting the average porosity and permeability of geothermal prospects within the Danish area is presented. Firstly, a porosity-depth trend is established in order to predict porosity. Subsequently, in order to predict permeability, a porosity–permeability relation is established and then refined in steps. Both one basin-wide and one local permeability model are generated. Two porosity-depth models are established. It is shown that the average permeability of a geothermal prospect can be modelled (predicted) using a local permeability model, i.e. a model valid for a geological province including the prospect. The local permeability model is related to a general permeability model through a constant, and the general model thus acts as a template. The applied averaging technique reduces the scatter that is normally seen in a porosity–permeability plot including all raw core analysis measurements and thus narrows the uncertainty band attached to the average permeability estimate for a reservoir layer. A “best practice” technique for predicting average porosity and permeability of geothermal prospects on the basis of core analysis data and well-logs is suggested. The porosity is primarily related to depth, whereas the permeability also depends on porosity, mineralogy and grain size, which are controlled by the depositional environment. Our results indicate that porosity and permeability assessments should be based on averaged data and not raw conventional core analysis data. The uncertainty range of permeability values is significantly lower, when average values are used.

The Valdemar Field, Danish Central Graben: field compartmentalization and regional prospectivity of the Lower Cretaceous chalk play

Despite representing a widespread play in the Central North Sea, production from Lower Cretaceous chalks is currently confined to the Valdemar Field in the Danish Central Graben. The field comprises a heterogeneous reservoir succession, less than 100m thick, consisting of hemipelagic chalks, marly chalks, and marlstones of Late Hauterivian–Early Aptian age. Although the field has in-place reserves in the order of 115 × 10 6 m 3 (725 × 10 6 BBL), the recovery from this complex reservoir was initially estimated to be only approximately 1%, primarily due to low permeability. The argillaceous chalks of the Lower Cretaceous reservoir are highly faulted and fractured, overpressured and undercompacted, giving rise to a complex distribution of hydrocarbons. Due to the lithological heterogeneity of the succession, internal stratigraphic barriers are common and result in stratigraphic compartmentalization. In addition, clay smearing in fault zones has created structural barriers and the development of structural compartments, as reflected by spatial differences in oil saturations, oil types and maturity, formation pressure and porosity distribution. Analysis of the reservoir properties and structural development of the Valdemar Field has provided data that can be extrapolated to the remainder of the Danish Central Graben, indicating that the Lower Cretaceous is most prospective in the central and southern Danish Central Graben.

The influence of climate on early and burial diagenesis of Triassic and Jurassic sandstones from the Norwegian-Danish Basin

Climate changes preserved in sandstones are documented by comparing the sediment composition and early diagenetic changes in sandstones deposited during arid to semi‐arid conditions, the Skagerrak Formation, with sandstones of the Gassum Formation deposited in a humid well‐vegetated environment. The study area covers the easternmost part of the Norwegian–Danish Basin, for which the Fennoscandian Shield functioned as sediment source area. The depositional environments of the formations, their distribution and burial depths are well‐constrained, facilitating a comprehensive petrographical and geochemical study complemented by porosity and permeability measurements of cores widely distributed in the basin (1700 to 5900 m burial depth). The Skagerrak Formation had an immature composition with more abundant feldspar, rock fragments and a larger variability in the heavy mineral assemblage when compared to the Gassum Formation, which was characterized by quartz and more stable heavy minerals. The arid to semi‐arid climate led to early oxidizing conditions under which abundant iron‐oxide/hydroxide coatings formed, while the evaporative processes occasionally resulted in caliche and gypsum precipitation. Under the humid climate, kaolinite precipitated due to leaching of feldspar and mica, and the abundant organic matter caused reducing conditions, which led to other Fe‐rich phases, i.e. pyrite, Fe‐chlorite and siderite. The inherited early diagenetic pore fluids and mineral assemblage also affect the mineral changes occurring during deeper burial, so dolomite preferentially formed in the sandstones deposited in an arid environment, while ankerite characterizes sandstones deposited under humid conditions. In addition to climate‐induced burial diagenetic changes, there are also temperature‐dependent phases, such as illite and quartz cement. Despite the same sediment source area remaining active during the entire period, the sediments that reached the Norwegian–Danish Basin were immature during the arid interval, although mature during the humid period. This has implications for provenance investigations as well as diagenetic investigations of sandstone reservoir quality.

Porosity and Permeability Depth Trends, Examples from the Danish Upper Triassic−Upper Jurassic Gassum Formation

ty-depth and permeability-depth trends, established for all cored intervals of the Gassum Formation. The porosity-depth and permeability-depth trends represent sandstones alteration during burial due to mechanical compaction and diagenesis. The focus of this study is on the Gassum Formation, which has the largest potential and is the main target for planned geothermal wells in Denmark, as it is widely distributed and generally occurs within the depth interval of 800-3000 m, thereby reaching sufficient high temperatures and still assumed to maintain the required porosity and permeability. The Gassum Formation occurs with thicknesses of 50–150 m in central and distal areas of the Danish part of the Norwegian−Danish Basin, thickening locally in association with salt structures and major faults (up to 300 m in the Sorgenfrei−Tornquist Zone) and thinning or being absent on the structural highs, such as the Skagerrak−Kattegat Platform and the Ringkobing−Fyn High (Fig. 1). The Gassum Formation consists of shoreface, fluvial, estuarine, lacustrine, lagoonal and marine facies (Nielsen 2003).

Geothermal Energy Production Assessment from Premature Reservoir Evaluation

Assessment of the geothermal energy production potential, in an early decision stage of a development project, is subjected to great uncertainty due to the often sparse data coverage. By using all available data i.e. wireline logs, core analysis data and seismic interpretations to construct a preliminary geological model for the area of interest, the basis for decision is strengthen. The geological model forms the basis for subsequent reservoir simulation, where the uncertainty in water flow rates from the various input parameters can be quantified. Different production strategies can be examined and optimized. Furthermore, any identified faults or geological baffles potentially constraining fluid flow may be incorporated and evaluated. The present paper illustrates the construction and use of a subsurface model to address uncertainties in early decision making for geothermal energy production projects.

Geothermal Energy in Denmark – Potential, Policy and Progress

A new assessment of the geothermal resources in Denmark published by GEUS concludes that the Danish subsurface contains huge geothermal resources (Mathiesen et al. 2009). To rationalise administration the Danish Energy Agency (DEA) has established a new simple application procedure with a standard license period and work program. These initiatives and rising prizes on fossil fuels have together with public concerns related to climatic changes and increasing emission of CO2 to the atmosphere triggered the awareness of the large potential of the geothermal resources, which may contribute to a safe, sustainable, price stable and reliable supply of energy. It is thus expected that geothermal energy may play an important role in the future energy strategy in Denmark (Fenham et al. 2010; Nielsen et al. 2011).