Tor Arne Johansen

Anisotropic approximations for mudrocks: A seismic laboratory study

An experimental technique for determining the in‐situ elastic properties of mudrocks with (horizontal) alignments in the microstructure is used to study the accuracy of a set of three nested scalar anisotropic approximations for transversely isotropic (TI) media. Each subsequent approximation adds one more velocity parameter and includes the previous as a special case. These approximations are convenient and robust because of their close relationship to standard geophysical measurements. There exists no good theory to predict the effects of an imposed stress on the elasticity of mudrocks. In this study, the tensor of elastic moduli of a single test specimen of mudrock subjected to an anisotropic stress field is determined from ultrasonic group velocity measurements involving pointlike transducers. The mechanical characterization (performed at constant pore pressure) is accompanied by detailed microscopic observations and analysis. The method was used to obtain accurate elastic constants for five well‐defi...

Rock physics modelling of shale diagenesis

A model for estimating the effective anisotropic properties of cemented shales is presented. The model is based on two mathematical methods for estimation of effective properties of a composite medium; a self-consistent approximation and a differential effective medium model. In combination these theories allow approximation of a shale with connected clay minerals and cement, and disconnected pores and quartz grains, which can be compared with the conditions in a real cemented shale. A strategy is also presented for estimation of stiffnesses in the transition zone from mechanical compaction to chemical compaction dominated diagenesis. Combining these theories with a shale compaction theory, enables modelling of the effective elastic stiffnesses for shales from deposition and mechanical compaction to deep burial and chemical compaction/cementing. Results from the model were compared with velocity data from three wells, showing good fit for velocity predictions, following the main velocity trends with increased temperature and depth.

Shale rock physics and implications for AVO analysis : A North Sea demonstration

Shales normally constitute more than 80% of sediments and sedimentary rocks in siliciclastic environments. Shales are important both in controlling the overburden seismic wave propagation as well as the reflectivity contrast between cap rocks and reservoir rocks in prestack seismic data. Therefore, during AVO analysis it is crucial to understand the seismic properties of shales as a function of mineralogy and compaction. In this study, we derive the local shale trend for a North Sea gas-and-oil field by integrating rock physics modeling with well-log and seismic data analysis.

Seismic mapping and modeling of near‐surface sediments in polar areas

A knowledge of permafrost conditions is important for planning the foundation of buildings and engineering activities at high latitudes and for geological mapping of sediment thicknesses and architecture. The freezing of sediments is known to greatly affect their seismic velocities. In polar regions the actual velocities of the upper sediments may therefore potentially reveal water saturation and extent of freezing. We apply various strategies for modeling seismic velocities and reflectivity properties of unconsolidated granular materials as a function of water saturation and freezing conditions. The modeling results are used to interpret a set of high‐resolution seismic data collected from a glaciomarine delta at Spitsbergen, the Norwegian Arctic, where the upper subsurface sediments are assumed to be in transition from unfrozen to frozen along a transect landward from the delta front. To our knowledge, this is the first attempt to study pore‐fluid freezing from such data.Our study indicates that the P‐ ...

Effects of fluids and dual-pore systems on pressure-dependent velocities and attenuations in carbonates

TheeffectsoffluidsubstitutiononP-andS-wavevelocitiesin carbonates of complex texture are still not understood fully. The often-used Gassmann equation gives ambiguous results when compared with ultrasonic velocity data. We present theoretical modelingofvelocityandattenuationmeasurementsobtainedata frequency of 750 kHz for six carbonate samples composed of calcite and saturated with air, brine, and kerosene.Although porosities 2%‐14% and permeabilities 0‐74 mD are relatively low, velocity variations are large. Differences between the highest and lowest P- and S-wave velocities are about 18% and 27% for brine-saturated samples at 60 and 10 MPa effective pressure, respectively. S-wave velocities are measured for two orthogonal polarizations;forfourofsixsamples,anisotropyisrevealed.The Gassmann model underpredicts fluid-substitution effects by 2% for three samples and by as much as 5% for the rest of the six samples. Moreover, when dried, they also show decreasing attenuationwithincreasingconfiningpressure.Tomodelthisbehavior, we examine a pore model made of two pore systems: one constitutesthemainanddrainableporosity,andtheotherismade ofundrainedcracklikeporesthatcanbeassociatedwithgrain-tograincontacts.Inaddition,thesedriedrocksamplesaremodeled to contain a fluid-filled-pore system of grain-to-grain contacts, potentiallycausinglocalfluidflowandattenuation.Forthetheoretical model, we use an inclusion model based on the T-matrix approach,whichalsoconsiderseffectsofporetextureandgeometry, and porefluid, global- and local-fluidflow. By using a dualpore system, we establish a realistic physical model consistently describingthemeasureddata.

Palaeogene clinoform succession on Svalbard expressed in outcrops, seismic data, logs and cores

The mountainsides along Van Keulenfjorden on Svalbard show excellent exposure of the Palaeogene succession. We report the results of an integrated scientific programme of well drilling and seismic acquisition that was initiated to supplement outcrop studies of clinoform development for educational and research purposes. The well, drilled in 2008, is 1085 m deep and a complete clinoform section of Eocene age has been cored. In 2009, 48 km of 2D seismic lines were acquired on the glaciers in the area using snowstreamer technology, with two of the seismic lines crossing the well location. The energy in the shallow window is dominated by direct waves, airwaves, and surface waves. Careful processing integrated with geological interpretation was necessary to image the Palaeogene clinoforms. Based on well and outcrop information and seismic facies, the Palaeogene clinoform succession may be divided into four main units: basin floor shales, sandy bottomsets, slope to basin floor clinoforms, and topsets. We now have a complete dataset containing outcrops, cores, wireline logs, and seismic data for research, education, and to build geological models in a basin floor to shelf setting. It can be used as an analogue for exploration worldwide.

Seismic Properties of Shales During Compaction

Shales, mudstones and their unconsolidated equivalents, constitute the vast majority of all sediments on Earth. The composition of such rocks may be roughly characterized (ignoring variation in clay mineralogy) by only two parameters: the water content (porosity) and the fraction of silt relative to total solid fraction (hereafter called the solid fraction of silt). The water content will decrease during compaction of the rock while the solid fraction of silt is unchanged. For shales and mudrocks the silt grains are mainly unconnected and the load-bearing component is the clay. Furthermore, it is reasonable to assume that the porosity of the uncompacted rock will decrease with increasing solid fraction of silt. The clay consists of small flakes (platelets) which after deposition have a random orientation, but which during compaction will become gradually more horizontally aligned. The degree of alignment will be reduced when silt is present since the platelets will drape around the much larger and rounder silt grains. Any alignment will result in an elastically anisotropic rock. For shales and mudstones without high carbonate content the compaction in the upper 2 km of the sediment column is mainly a mechanical process and it is then possible to calculate the orientation distribution function of the platelets as a function of the compaction (or porosity) of the rock. Given the composition (porosity and silt fraction) and the orientation distribution of the platelets one can use a variety of existing methods to model the elastic properties of the rock. All seismically observable properties of a shale (density, vertical Pand S-wave velocities, and the three anisotropy parameters of a transversally isotropic medium) are thus given by only two parameters, the porosity and the silt fraction. This reduction in the degrees of freedom would be very appealing for seismic inversion where the problem is seriously underdetermined.

A strategy for modelling the diagenetic evolution of seismic properties in sandstones

The geometrical distribution of various components in a composite sandstone is decisive for its overall stiffness and seismic velocities. Information about which constituents, for example, are load bearing, dispersed in pore fluid or present as contact cement is, therefore, necessary if the seismic properties are to be modelled reliably. A distribution scheme for quartz cement, K-feldspar and some of the most common clay minerals in sandstones (illite, kaolinite, smectite and chlorite) is suggested on the basis of thin-section observations made by a number of authors. This classification scheme facilitates rock physics modelling as a function of mineral concentrations. A composite rock physics model has also been developed to account for simultaneous combinations of mineral distributions. Well-known mineral reactions are used to make simple models of mineralogy versus temperature (depth) from different starting scenarios, as various minerals tend to follow different and predictable paths during burial and increasing temperature. The mineralogical trends are then entered into the composite rock physics model to produce the diagenetic evolution of seismic rock properties, and the procedure is used to estimate the effective rock properties of sandstones in a well log. The modelling allows deductions to be made about possible mineralogies and their distributions from seismic parameters. Finally, reflection coefficients resulting from sandstones subjected to various diagenetic processes are modelled and analysed. The results show that it is possible to discriminate between reflections emanating from the interfaces of a selection of common diagenetic scenarios.

Efficient and flexible seismic modeling of reservoirs: A hybrid approach

Model-based analysis of seismic data, now recognized as a key to a better understanding of images of subsurface structure, began in the 1960s. But these models were inflexible, primarily due to hardware and software limitations, and generally were simple homogeneous isotropic horizontal layers—i.e., with only vertical velocity variations.

Estimation of elastic moduli of mixed porous clay composites

We have developed a procedure for estimating the effective elastic properties of various mixtures of smectite and kaolinite over a range of confining pressures, based on the individual effective elastic properties of pure porous smectite and kaolinite. Experimental data for the pure samples are used as input to various rock physics models, and the predictions are compared with experimental data for the mixed samples. We have evaluated three strategies for choosing the initial properties in various rock physics models: (1) input values have the same porosity, (2) input values have the same pressure, and (3) an average of (1) and (2). The best results are obtained when the elastic moduli of the two porous constituents are defined at the same pressure and when their volumetric fractions are adjusted based on different compaction rates with pressure. Furthermore, our strategy makes the modeling results less sensitive to the actual rock physics model. The method can help obtain the elastic properties of mixed unconsolidated clays as a function of mechanical compaction. The more common procedure for estimating effective elastic properties requires knowledge about volume fractions, elastic properties of individual constituents, and geometric details of the composition. However, these data are often uncertain, e.g., large variations in the mineral elastic properties of clays have been reported in the literature, which makes our procedure a viable alternative.