Estimating attenuation properties of bentonite layer in Cut Bank oil field, Glacier County, Montana

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Title: Estimating attenuation properties of bentonite layer in Cut Bank oil field, Glacier County, Montana
Author: Karakurt, Necdet
Abstract: Acquisition and interpretation of 3 -D seismic data led DeAngelo and Hardage (2001 ) to describe the channel system in the south central Cut Bank area in Glacier County , Montana . The presence of a low velocity layer called Bentonite was also discovered in the area with the help of well -logs . Bentonite is a volcanic ash , which lies on both sides of the channel system and is absent within the channel . DeAngelo and Hardage (2001 ) shot a vertical seismic profiling (VSP ) survey at well # 54 -8 to analyze the formation structure in depth , since seismic signals around the reservoir area were unclear in the 3 -D survey . This research attempts to estimate the attenuation properties of the Bentonite layer in the Cut Bank oil field . VSP data is processed for velocity information and estimation of seismic Q using the spectral ratios method (SRM ) . The SRM theoretically assumes that the propagating signal is a plane seismic wave traveling vertically from one point to another in a homogeneous model . The amplitudes at the start and end points are known and relate to each other with the attenuation coefficient in a frequency range . The relation between the seismic amplitudes at z distance from each other can be expressed as a linear function of frequency after a few modifications . SRM uses the linearity of the logarithmic ratio of the seismic amplitudes over a frequency range . In theory , ratios plotted against a frequency range must produce a flat line . However , in practice , the logarithmic ratios are expected to draw an approximate line (curve ) , where some of the data points deviate from the origin of the line . Thus fitting a line to the ratios curve and calculating the slope of this curve are necessary . Slope of the curve relates to the seismic attenuation coefficient and further to the seismic Q . The SRM results suggest that Bentonite may have a Q value as low as 5 . This highly attenuative and thin (20 to 40 feet throughout the south central Cut Bank Unit ) layer alters seismic signals propagating through it . A thorough analysis of the amplitude spectra suggests that seismic signals dramatically lose their energy when they pass through Bentonite . Low energy content of the signals below the Bentonite layer highlights that the recovery of the seismic energy is less likely despite the presence of multiples , which are known to affect the seismic signals constructively . Therefore , separation of reflected events is a greater challenge for the thin reservoir sand units lying underneath the Bentonite layer . Thus the Bentonite layer in the Cut Bank oil field has to be taken seriously and data processing should be done accordingly for better accuracy .
URI: http : / /hdl .handle .net /1969 .1 /3282
Date: 2006-04-12

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Estimating attenuation properties of bentonite layer in Cut Bank oil field, Glacier County, Montana. Available electronically from http : / /hdl .handle .net /1969 .1 /3282 .

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