Rikke Weibel

DIAGENESIS IN OXIDISING AND LOCALLY REDUCING CONDITIONS : AN EXAMPLE FROM THE TRIASSIC SKAGERRAK FORMATION, DENMARK

Abstract Diagenetic changes in red and white parts of the Skagerrak Formation (Triassic) from onshore wells in Denmark were analysed by scanning electron microscope, electron microprobe, and petrographic microscope in reflected and transmitted light. The diagenetic sequences of the red and white parts of the Skagerrak Formation are in many ways the same: early infiltration clays, followed by precipitation of caliche calcite, dolomite, mixed-layer illite/smectite, quartz, feldspar, kaolin minerals, illite, chlorite and anhydrite with increasing burial. Differences between the red and white parts of the Skagerrak Formation comprise oxidation stage in detrital minerals (oxidation of glauconite in the red part and reduction of biotite in the white part), the presence of eogenetic authigenic minerals (pyrite and poikilotopic calcite in the white part, hematite and red coatings in the red part), the amount of mesogenetic authigenic kaolin minerals (larger in the white part), the chemistry of mesogenetic authigenic chlorite (Mg-rich in the red part and Fe-rich in the white part), and the preservation stage of detrital opaque minerals (magnetite, hematite, chromite, ilmenite show none or minor alteration in the red part, whereas they have been almost completely altered in the white part). As the differences between red and white areas are found both in early eogenetic and in mesogenetic authigenic minerals, different oxidising conditions must have prevailed immediately after the deposition and continued into burial. Oxidising conditions were dominant in the arid Triassic climate and the widespread red colour of the Skagerrak Formation was probably formed during early diagenesis due to precipitation of iron oxides or hydroxides. Locally reducing areas were formed, probably by oxidation of organic material. Precipitation of oxidised iron minerals was thus prevented, and the original white colour was preserved. Reducing conditions prevailed until the reducing agent was exhausted, then an outer rim of the reduction spot might locally have been secondarily oxidised. These changing redox conditions explain the occurrence of hematized authigenic pyrite and crandallite group minerals in red areas.

Petrified wood from an unconsolidated sediment, Voervadsbro, Denmark

Abstract Field investigations of Middle Miocene fluviatile sediments in an open sand pit at Voervadsbro, Denmark suggest that the transformation processes of wood (lignification and petrifaction) depend on the permeability of the sediment. The petrifying agent in the wood is silica, which must have originated from weathering (and/or soil formation) of silicate minerals as the sediments have seen no history of volcanism. Petrographic investigations (by transmitted light microscopy and SEM) and X-ray diffraction have revealed the resultant of the petrifying agent to be quartz with a remarkably high crystallinity considering its age. The radial structures of the cell walls and the rim around the cell lumina-filling indicate that the first precipitation has been on the cell walls. The zonation of the quartz shows that its precipitation proceeded with interruptions, due to local differences in silica supply, seasonal changes and/or ground-water fluctuations.

Intense and widespread seismicity during the end-Triassic mass extinction due to emplacement of a large igneous province

Multiple levels of earthquake-induced soft-sediment deformations (seismites) are concentrated in the end-Triassic mass extinction interval across Europe. The repetitive nature of the seismites rules out an origin by an extraterrestrial impact. Instead, this intense seismic activity is linked to the formation of the Central Atlantic magmatic province (CAMP). By the earliest Jurassic the seismic activity had ceased, while extrusive volcanism still continued and biotic recovery was on its way. This suggests that magmatic intrusions into sedimentary strata during early stages of CAMP formation caused emission of gases (SO 2 , halocarbons, polycyclic aromatic hydrocarbons) that may have played a major part in the biotic crisis.

Chapter 10 Alteration of Opaque Heavy Minerals as a Reflection of the Geochemical Conditions in Depositional and Diagenetic Environments

Abstract Detrital opaque heavy minerals are minor but common constituents of most detrital sediments. It is sometimes possible to map the diagenetic paths of those opaques that are readily altered through changing oxidising and reducing geochemical environments, both depositional, post-depositional and diagenetic. The alteration of opaque heavy minerals from different depositional and diagenetic environments, reflecting various geochemical regimes, is reviewed and evaluated. The investigated deposits span from Triassic red beds (Skagerrak and Bunter Sandstone Formations) with early oxidising conditions (including local reduction spots representing early reducing conditions) through weakly reducing conditions of the Miocene Odderup Formation to the strongly reducing environment of the Gassum Formation, where abundant iron and organic matter, and thus related sulphate reduction, have influenced the alteration products. Extreme sulphur-dominated local environments are represented by Holocene carbonate-cemented sandstone pillars. When successive phases of oxidation and reduction occur, the resulting assemblage of primary detrital opaque heavy minerals and their alteration products converge; a detailed study can sometimes reveal the sequence of events. A comparison of the alteration sequence of detrital Fe–Ti oxides, together with identification of authigenic opaques, from red and drab parts of the succession in Triassic red beds (Skagerrak and Bunter Sandstone Formations), has enabled us to distinguish primarily oxidised horizons, primarily reduced areas, secondarily reduced originally oxidised areas, and secondarily oxidised originally reduced areas.

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 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 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).