Exploration methods – seismic analysis
Seismic surveys are essential for understanding the geology of sedimentary basins, including the shale formations. The tests involve generation of seismic (elastic) waves which propagate into the ground and recording their return to the surface.
Today, seismic waves are commonly produced by special mechanical truck-mounted devices (called seismic vibrators) that generate small vibrations in the ground. As seismic wave travels deep into the Earth, it is reflected by rock boundaries which display different density and elastic wave propagation velocity. Seismic wave is refracted and partly deflected at the boundary, and then travels back to the ground surface. Special sensors that are called geophones receive the signal and transmit it to recording units. Seismic tests allow for imaging the deep geologic structure and for correlation of individual boreholes. At the same time they are almost neutral to the environment.
2D and 3D seismic surveys are most often used in shale gas exploration. A 2D seismic profile represents a “slice” of the earth and provides an image of the structure solely along the seismic line, while 3D seismic provides a three-dimensional set of seismic data. The advent of 3D seismic transformed the exploration industry by enabling a much more detailed image of the geological structure comparing with 2D surveys. On the other hand, shooting and interpretation of 3D seismic are more costly and time consuming projects.
4D seismics involve a comparison of seismic data that have been acquired in the same area at specific time intervals. Usually, they cover the period that follows the commencement of production: by comparing seismic and borehole data collected before the commencement of production with those measured after a certain period of production, 4D models provide an insight into the changes that have occurred in the rock formation.
Integration of information provided by other methods is required in order to perform a detailed analysis of seismic data. This makes it possible to assess a number of critical formation parameters, such as the type and thickness of the rocks, depth to reservoir, presence of faults, etc. Moreover, it helps to assess rock fracturability, estimate the directions and extension of induced fractures, assess TOC (organic carbon) contents, etc.
Correlation of time-scale seismic data with depth-scale borehole data is the first step of the interpretation. This is done using a synthetic seismogram or vertical seismic profiling (VSP).
The next steps of interpretation include:
- structural interpretation: image analysis for the detection of structures such as anticlines or faults, based on the layout of seismic boundaries,
- seismostratigraphy: seismic data are used to assess interrelations between seismic image and lithology in the context of geometric layout so as to determine the depositional system,
- reservoir interpretation: intended to identify petroleum reservoirs.
Unconventional accumulations are identified and characterized using using various parameters and advanced methods, including:
- Integration of data derived from seismic attributes, borehole data and AVO (amplitude versus offset) analysis, which provides an insight into the physical parameters of the rock, such as Young's modulus, Poisson's coefficient, P and S wave impedance, etc. The values of Poisson's coefficient and of the Young's modulus help to assess brittleness of shale rocks.
- Amplitude versus angle and azimuth (AVA(Az)) analysis and velocity versus azimuth (VVAz) analysis enable an assessment of stresses in the rock mass and their orientation. Significant measured changes in the recorded 3D seismic data may reflect existing fractures in the rock medium. These changes are called seismic azimuthal anisotropy. Wide-angle and multicomponent seismic (3C) data are useful in the assessment of existing fractures and their orientation.
- Seismic inversion, which results in reestablishment of acoustic pseudo-impedance curves based on seismic data, following more processing allows for estimation of TOC %, which can be incorporated into basin modeling software in order to assess the hydrocarbon generative potential of the basin.
Seismic surveying is on the rise since the inception of unconventional hydrocarbon exploration in Poland. Concession areas are now covered with numerous new 2D and 3D seismic lines that provide a much better insight into the geology of these areas, so that new reservoirs, both conventional and unconventional ones, may be discovered.
authors: Andrzej Głuszyński, Sylwia Kijewska