METHODS AND APPLICATIONS IN PETROLEUM AND MINERAL EXPLORATION AND ENGINEERING GEOLOGY

A fundamental challenge that operators in shale-gas industry face is the influence of stress and preexisting fractures on seismic velocities. This problem is generally responsible for the change in seismic velocity in all directions—horizontal, vertical, and azimuthal. 

Understanding these anisotropies is important in predicting the location of preexisting natural fractures in the rock and how induced fractures might grow. Favorable parameters in a shale-gas reservoir are mid-to-high kerogen content, low clay volumes, high brittleness, high effective porosity, and good permeability, along with well-developed microfractures. Of those parameters, mid-to-high total organic carbon (TOC) is favorable for shale-gas accumulation, and high brittleness is beneficial to hydraulic-fracture propagation.

 Hence, the aim is to find sweet spots with high TOC and high brittleness. In addition, experience showed that the organic content in shales that are measured by their TOC ratings also influences their compressional and shear velocities as well as their density and thus impacts their seismic responses (Harilal and Tandon, 2012). Consequently, with this in mind, seismic data are carefully acquired, processed, and analyzed in such a way that seismic response from shale formation can be associated with changes in their TOC.

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Seismic method plays a key role in identifying shale formation structures, their extension, discontinuities, and other properties that may control their quality. To economically develop shale-gas reservoirs, operators strive to maximize borehole exposure to the reservoir and create a complex fracture network to increase permeability. Moreover, most shale operators are using seismic data to map the thickness of the shale reservoir unit as well as to identify possible hazards, such as faults, karst zones, and collapsed chimneys, helping drilling engineers stay in zone and better geosteer long laterals.

Thus seismic surveys are preferably shot with wideazimuth angles, small trace intervals, high folds, uniform vertical and horizontal coverage, and long spread to meet the needs of the shale-gas exploration in terms of structural and lithological interpretations as well as fracture prediction. In fact, seismic data with wide-azimuth angles and long offsets contain relative complete information of lithological and physical properties of shale layers, providing data basis for lithological exploration. The seismic data with high folds, small trace intervals, and long offsets contain relative complete
features of dynamic and kinematic characteristics of effective reflected waves and interference waves, which is favorable for suppressing interference waves and utilizing effective waves and can increase the signal/noise ratio, the fidelity of the effective waves, and the imaging resolution of seismic data. With the improvement in the understanding of shale plays, in the processing stage of seismic data, the acquired data are subject to thorough processing operations to remove noise and improve signal from shales.