68th EAGE Conference and Exhibition incorporating SPE EUROPEC 2006

There are several published papers on Vertical Seismic Profile (VSP) First-Break Amplitude (FBA) studies that attempt to relate seismic attenuation to depth. In this study, the basic principle is to normalize FBAs and then to transform them into the decibel (dB) domain. Thereafter, dB values are plotted in the depth-decibel domain rather than in the conventional linear depth-amplitude domain. The depth-decibel curve shows very good correlation with sonic logs. The similarity between a sonic log and a dB curve depends on the type of geophone used for VSP recording. A correlation can be achieved only when the accelerometer is sensitive to particle velocity acceleration. Acceleration and amplitude are high in unconsolidated lithology. Conversely, acceleration and amplitude are low in consolidated layers. Thus, the dB plot mimics a sonic log. Using this method, individual and cumulative attenuation gradients for varying lithological units can be defined and formation boundaries identified. Formation densities and interval velocities can also be computed using the FBA technique. It would appear that this methodology is not widely employed because most amplitude studies do not take into consideration the type of geophone used for VSP recording.

Statoil and Hydro, with READ Well Services as contractor, have jointly acquired 3 walkaway VSP lines from offshore Norway. With a total of 24 geophones settings and around 200 shot points we were able to image below, and estimate anisotropy parameters from, the dipping beds in the overburden. Utilizing migration algorithms that take into account a transversely isotropic medium with a tilted symmetry axis, had a significant impact on the interpretation of the target structures.

The present-day most efficient acquisition method in geophysics is 3D seismic survey. However, it usually requires application of regular dense network of seismic source and receiver points. If the area under survey comprises living facilities, forests, agricultural lands, it results in significant economic and environmental losses. In Western Siberia application of 3D survey is complicated by difficult natural and climatic conditions (non-frozen swamps, lakes, rivers, deep snow, heavily forested areas). In this case the problem cannot be completely solved even with application of modern equipment. In respect to HSE requirements irregular systems are more acceptable. They enable to lay the lines where it is possible notwithstanding the template designed.

PP/PS event registration (also called "event matching" or "event pairing") is an essential element in multicomponent data analysis. It aligns the related events in P-wave and C-wave (converted wave, also called "PS-wave") sections. Among other things, this enables the Vp/Vs ratio (also called "Gamma") to be calculated. It can be used for updating the migration velocity field, and under favorable conditions, it can be applied directly in lithology/fluid property estimation. In this paper, we discuss the problems of PP/PS event registration, and describe a new target-based, rock physics-guided, iterative workflow for implementing it. A Gulf of Mexico example using the methodology is presented.

The paper presents effects of shale content and porosity on elastic parameters of rocks. The analysis was based on theoretical relationships for porous media, known as the Biot-Gassmann’s and Kuster and Toksöz’s models, and on Raymer-Hunt-Gardner formulas. Well-logging data and results of the quantitative interpretation of well logs were also analysed. Data were acquired by the Geofizyka Krakow SA under the project no 6T12 2003 C/06006 realized in 2003 (Pietsch et al. 2003). The relationships between P-wave and S-wave velocities and reservoir parameters may contribute to solving some problems associated with seismic interpretation of wave forms in autochthonous Miocene deposits.

This paper contains description of calculation of a radiogenic heat production rate in the sedimentary rocks: the sandstones and the mudstones. The radiogenic heat is one of the elements of the surface heat flow. This heat represents the important component in modelling of the oil basins, especially in thermal calculating connected with the source rocks. It is claimed that the production of the radiogenic heat causes increasing of the temperature. That process leads to quick maturation of a kerogen, so the oils production is faster. Radiogenic heat production rate maight be computed on the samples or in the boreholes with using SGR and RHOB logs, according to the suitable equations.

Devonian fully dolomitized sedimentary succession occurring in the southern Estonia is represented by carbonate, siliciclastic and mixed-carbonate siliciclastic rock types. These three lithologic rock types were subdivided using 50 and 70% borders of insoluble residue (IR). Carbonate rock type includes dolostones (IR<25%) and dolomitic marlstones (IR>25% and IR<50%). Mixed rock type is represented by primary dolomitic marlstones and siltstones and diagenetic sandstones and sandy siltstones. Siliciclstic rock type is represented mainly by sandstones, siltstones and silty claystones. Porosity-density crossplots together with gamma-ray logs permit determination of clean lithologies, while discrimination of mixed rocks usually causes difficulties. The aim of the work is lithologic determination of mixed rocks. Density, porosity and magnetic susceptibility measured on 150 core samples from 8 boreholes accompanied with gamma-ray logs were analysed together with thin-sections and geochemical data. As a result mixed rocks were distinguished from carbonate and siliciclastic. Primary mixed rocks were discriminated from diagenetic. Density, porosity, magnetic susceptibility and gamma-ray spectrometry logging tools are recommended for lithologic determination of the rocks.

Electrical borehole wall images are widely used for borehole inspection and reservoir characterization. So far, this data is mostly qualitatively used to investigate structure and lithology mapping. We present a method for image characterization which is based on the application of texture analysis in order to transform image data into quantitative log curves. We derive so-called Haralick texture features from borehole wall images. Based on a supervised classification technique, we train texture features within assigned rock classes, determine their classifiers, and apply classification function on the entire data set. This enable automatic rock determination based on quantitative image data.

Novel approach is based on the method of reduction of gamma-ray activity and adsorptive diffusive activity core data towards relative parameters of well log methods and plotting relationships on their basis.
On the basis of core and geophysical data, relationships of dual gamma-ray index of gamma log with median diameter of grain fraction; gamma-ray activity; sum of close-grained fractions; sand fraction content; dominated fraction content; median pore diameter; correlation of Asp with content of clay minerals were plotted.
According to combination of comprehensive geological and physical information, definite number of reservoir types present in section is singled out (sandstones - medium-grained, fine-grained, close-grained, and siltstones). Reservoir type is defined by variation range of dominated filtrating fraction diameter and its proportion in the rock volume.
High concentration of siltstone fraction leads to significant distortion of parameters defined via well log.
Suggested approach allows evaluating content of sand and siltstone fractions, their proportion, average diameter of dominated fraction, and its volume. Together with clay fraction evaluation (volume of clay minerals) it is possible to perform a "simplified" lithologic analysis using well log, i.e. divide rocks into groups and perform a more sound evaluation of permeability and other volumetric data.

Ordovician carbonate succession from southern Estonia is mainly represented by primary limestones and calcitic marlstones and by dolomitized carbonate rocks (dolostones and dolomitic marlstones). For case study 110 core samples from two boreholes were analysed by geochemical and petrophysical methods. Rocks were initially subdivided into groups using data of carbonate chemical analysis. It is shown that primary and dolomitized rocks could be lithologically discriminated using porosity-density cross-plot. Gamma-ray log permits to determine thin reference layers of metabentonites and claystones, and to determine clayey rocks (marlstones) from more pure carbonates. Ordovician dolostones are characterized by increase in magnetic susceptibility the most significant in the late diagenetic dolostones. Their magnetic susceptibility may be higher than in marlstones. Magnetic susceptibility is offered to apply for dolostone genesis determination together with density, porosity and gamma-ray logs, and for express determination of dolostones in drill cores and outcrops.