Seismic attenuation in Jati area,Lower Indus Basin,Pakistan |
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| Authors: | Ehtisham Abdul Rehman |
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| Institution: | 1.Kuwait Foreign Petroleum Exploration Company (KUFPEC),Islamabad,Pakistan |
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| Abstract: | Reduced amplitude and distorted dispersion of seismic waves caused by attenuation, especially strong attenuation, always degrade the resolution of migrated images. To improve seismic imaging, attenuation must be compensated. This study addresses the factors causing seismic attenuation in Jati Block. Jati Block lies in Lower Indus Basin, Pakistan, approximately 25 mi north of the offshore Indus. Method used for Q factor is empirical equation method. Q factor correlation indicates that there are three major zones of attenuation in Jati block, i.e., zone I (surface to top Khadro), zone II (Upper Goru Formation), and zone III (Lower Goru Formation). Lowest Q value is in zone I, followed by zone II and zone III, respectively. Commonly, Khadro Formation (Basalt) of Paleocene and saucer-shaped igneous intrusion is considered as sources of attenuation. However, surprisingly, Khadro Formation of Paleocene and saucer-shaped igneous intrusion is zone of minimum attenuation and causes minimal transmission loss. Anisotropy analysis is performed to determine cause of attenuation within these zones. Thomson anisotropic parameters are computed for vertical wells using Backus averaging algorithm. These parameters are calibrated using sonic scanner data available for one well. Correlation of Q factor curve with mud log suggests that loose, unconsolidated sands and sand-shale layering are sources of attenuation within zone I. Attenuation in this zone is mostly due to fluid motion relative to the framework of loosely packed grains. Major lithology of Upper Goru Formation is marl. It is a slow formation, and in this formation, P wave loses energy to the formation in what is known as leaky P mode (sonic logging) and is dispersive (seismic). Epsilon (?) value is greater than almost 0.02 throughout Lower Goru Formations, indicating that this formation is strongly anisotropic. Fluctuation of epsilon (?) in Lower Goru Formation also suggests that this formation is causing layer-induced anisotropy. This layer-induced anisotropy in Lower Goru Formation added by dispersive and slow nature of Upper Goru Formation causes rapid attenuation. |
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