Abstract

Fault identification plays an important role in oil and gas exploration of the Ordovician reservoirs underlying the Permian igneous rocks in the fault-controlled Shuntuoguole uplift, Tarim Basin, China. Faults identified through conventional analysis of seismic coherence attributes, however, usually contain many artifacts or pseudofaults in which velocities of the igneous rocks tend to be selected incorrectly during migration imaging. We have evaluated a quantitative method to identify the pseudofaults by numerical simulation of depth migration on synthetic seismic records generated by finite-difference modeling of wave propagation. Detailed information on the spatial distribution and velocity range of the Permian igneous rocks is used to build a refined velocity model for the wave-equation-based forward modeling, calibrated with core, well log, and seismic data. It is found that the thickness and the migration velocity error used for the igneous rocks in depth migration are important factors affecting the pseudofault throws. There exists a strong quantitative relationship between igneous rock thickness (x) and pseudofault throw (y),   y=ax2+bx+c, where a, b, and c are constants for a given migration velocity error. For a maximum migration velocity error of 15% for the dacite bodies in the studied area, a, b, and c are 0.0002, 0.1864, and 3.7753, respectively. Using this relationship, the maximum fault throw (Hxmax) of a pseudofault caused by an arbitrary thickness of any dacite body can be calculated. If the actual fault throw (H) of an apparent fault measured on the migrated seismic profile is greater than the calculated maximum fault throw for the fault, i.e., H>Hxmax, then the apparent fault is a real geologic fault. Otherwise, it is a pseudofault. The method has been successfully applied to produce corrected fault distribution maps for geologic evaluation of the Ordovician reservoirs in the Shuntuoguole region.

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