Børge Arntsen

Imaging through gas‐filled sediments using marine shear‐wave data

Marine multicomponent sea-floor data of excellent quality have been acquired over the Tommeliten Alpha field. The most dominating wave modes are interpreted to be conventional compressional PP-waves and converted PS-waves. The most important geophysical problem associated with the Tommeliten Alpha field is the presence of a gas chimney obscuring the conventional 3-D seismic image of the reservoir zone. The converted PS-waves effectively undershoot the gas chimney, leading to substantially improved images of the reservoir. Subsequent interpretation indicates the Tommeliten Alpha structure is a faulted dome.

Gas and fluid injection triggering shallow mud mobilization in the Hordaland Group, North Sea

Abstract During a regional seismic interpretation study of leakage anomalies in the northern North Sea, mounds and zones with a highly chaotic seismic reflection pattern in the Tertiary Hordaland Group were repeatedly observed located above gas chimneys in the Cretaceous succession. The chaotic seismic reflection pattern was interpreted as mobilized sediments. These mud diapirs are large and massive, the largest being 100 km long and 40 km wide. Vertical injections of gas, oil and formation water are interpreted to have triggered the diapirs. On the eastern side of the Viking Graben, another much smaller type of mud diapir was observed. These near-circular mud diapirs are typically 1–3 km in diameter in the horizontal plane. Limited fluid injection from intra-Hordaland Group sands, through sand injection zones, into the upper Hordaland Group is interpreted to have triggered the near-circular diapirs. This observed ‘external’ type of mobilization was generated at shallow burial (<1000 m) and should be discriminated from the more common ‘internal’ type of mud diapirism that is generated in deep basins (>3000 m). The suggested model has implications for the understanding of the palaeofluid system, sand distribution, stratigraphic prediction within the chaotic zone, seismic imaging, and seismic interpretation of the hydrocarbon ‘plumbing’ system.

Seismic modeling of gas chimneys

We propose a simple acoustic model explaining the main features of gas chimneys. The main elements of the model consist of gas diffusing from a connected fracture network and into the surrounding shale creating an inhomogeneous gas saturation. The gas saturation results in an inhomogeneous fluctuating compressional velocity field that distorts seismic waves. We model the fracture network by a randomwalk process constrained by maximum fracture length and angleofthefracturewithrespecttothevertical.Thegassaturation is computed from a simple analytical solution of the diffusion equation, and pressure-wave velocities are locally obtained assuming that mixing of shale and gas occurs on a scalemuchsmallerthanseismicwavelengths.Syntheticseismic sections are then computed using the resulting inhomogeneous velocity model and shown to give rise to similar deterioration in data quality as that found in data from real gas chimneys.Also, synthetic common-midpointCMPgathers show the same distorted and attenuated traveltime curves as those obtained from a real data set. The model shows clearly thatthefeaturesofgaschimneyschangewithgeologicaltime a model parameter in our approach, the deterioration of seismicwavesbeingsmallestjustafterthecreationofthegas chimney. It seems likely that at least some of the features of gas chimneys can be explained by a simple elastic model in combinationwithgasdiffusionfromafracturenetwork.

Numerical simulation of the Biot slow wave in water-saturated Nivelsteiner sandstone

The development of rock acoustic models is important for interpreting seismic data. These models should relate seismic properties such as wave velocity and attenuation to the production and lithological properties of reservoir rocks, such as porosity, permeability, fluid type, and fluid saturation.

Vertical propagation of low-frequency waves in finely layered media

Multiple scattering in finely layered sediments is important for interpreting stratigraphic data, matching well-log data with seismic data, and seismic modeling. Two methods have been used to treat this problem in seismic applications: the O’Doherty-Anstey approximation and Backus averaging. The O’Doherty-Anstey approximation describes the stratigraphic-filtering effects, while Backus averaging defines the elastic properties for an effective medium from the stack of the layers. It is very important to know when the layered medium can be considered as an effective medium. In this paper, we only investigate vertical propagation. Therefore, no anisotropy effect is taken into consideration. Using the matrix-propagator method, we derive equations for transmission and reflection responses from the stack of horizontal layers. From the transmission response, we compute the phase velocity and compare the zero-frequency limit with the effective-medium velocity from Backus averaging. We also investigate how the transition from timeaverage medium to effective medium depends on contrast; i.e., strength of the reflection-coefficient series. Using numerical examples, we show that a transition zone exists between the effective medium low-frequency limit and the time-average medium high-frequency limit, and that the width of this zone depends on the strength of the reflectioncoefficient series.

A new insight into the reciprocity principle

Reciprocity is usually applied to wavefields associated with concentrated point forces and point receivers, but, reciprocity has a much wider application potential. In many cases, however, it is not used at its full potential because (1) a variety of source and receiver types are not considered or (2) its implementation is not well understood. We obtain reciprocity relations for inhomogeneous, anisotropic, viscoelastic solids and for distributed sources and receivers, and test these relations with a full-wave numerical modeling algorithm. The theory and the numerical experiments show that, in addition to the usual relations involving directional forces, (1) the diagonal components of the strain tensor are reciprocal for dipole sources (single couple without moment), (2) the off-diagonal components of the stress tensor are reciprocal for double couples with moments, (3) the dilatation due to a directional force is reciprocal to the particle velocity due to a dilatational source, and (4) some combinations of the off-diagonal strains are reciprocal for single couples with moments.

Anisotropy in the salt outcrop at Cardona, Catalonia - implications for seismic imaging

The world’s largest oil discoveries in recent years have been pre-salt, in reservoirs located below or close to salt bodies. It is often assumed that the salt is homogeneous and that a near-constant isotropic velocity in the salt body can be used for seismic imaging. Here we show that this simplification may be too simple. Based on field observations from an old salt mine in Spain, we estimate anisotropic parameters describing the exploited salt diapir. In its outcrop we identify a regular pattern of alternating halite and clay layers, where the thickness of each layer is close to periodic. From this observation, we estimate corresponding anisotropic parameters for this salt outcrop and find that the degree of anisotropy is moderate, of the order of 5% difference between horizontal and vertical velocities. Furthermore, we identify potential shear zones within the salt outcrop that can be mapped over distances of several hundred metres. The thicknesses of these shear zones are of the order of metres. Based on these observations, it is a huge simplification to treat salt bodies as homogeneous and isotropic for seismic imaging purposes.

Fast finite‐difference modeling of 3‐D elastic wave propagation

We present an efficient method for wavefield modeling in arhitrarily inhomogeneous 3-D elastic media, based on optimized difference operators for spatial differentiation (Holberg, 1987). This technique offers accurate results on coarse spatial grids, ie. of the order of 3 gridpoints per shortest wavelength. In 3-D applications, this fast finite difference technique is at least two orders of magnitude more efficient than conventional finite difference schemes and typically 2.5 to 6 times more efficient than the pseudospectral method, depending on the sampling of the model. This makes it practicable to run realistic 3-D simulations on existing vector computers. In t,he present paper we discuss the practical implementation of this method and give timings for various model sizes and computer configurations. We also give several numerical examples, ranging from simulat~ions in simple geometries with well known solutions to geometries of realistic complexity.

Decoupling of seismic reflectors and stratigraphic timelines: A modeling study of Tertiary strata from Svalbard

In sequence stratigraphic interpretations, the key premise is that stratal surfaces effectively represent geologic timelines. When applied to seismic sections, the fundamental assumption is that primary reflections generally mimic stratigraphic timelines. The main objective of this study was to test how well key reflectors in a seismic section couple to timelines. To achieve the high level of ground control needed for such testing, we combined photogrammetry and traditional sedimentologic fieldwork to optimize the geologic model. We relied further on petrophysical analysis to derive a numerical model suitable for the simulation of seismic data. In spite of laterally discontinuous vertical-impedance contrasts (VICs), false seismic continuity was created, and we observed frequent decoupling of seismic reflectors and stratigraphic timelines. These observations demonstrate how the low-frequency seismic method fails to image normal complexity in a stratigraphic unit. A seismic correlation test showed that the ...

Acquisition Geometry Versus 4C Image Quality. A Study From Gullfaks South.

An evaluation into the effects of different acquisition geometries on image quality, for multi-component seismic data was carried out for both pressure wave (P-wave) and converted wave (PS-wave) data in both time and depth. This was achieved through decimation of source and receivers of existing seismic data, and through modelling of P-wave and PS-wave data for the Gullfaks South 3D pilot Ocean Bottom Seismic (OBS) data set acquired in 2000.