Prestack Depth Migration for 'Easy' Onshore Plays?

Morgan Brown

Traditionally, prestack depth migration (PSDM) was considered an exotic technology, applicable only for imaging 'complex' geology. Offshore subsalt imaging drives application of the most advanced wave equation PSDM technologies, like Reverse-Time depth Migration (RTM). However, given the recent resurgence of onshore exploration, we ask: can PSDM make an impact on 'easy' onshore plays? We list three key reasons for (re)considering PSDM: 1) Remove false time structures caused by lateral variations in velocity. The 'fault shadow' effect is a radical example of a false time structure. However, with horizontal shale play drillers trying to stay in thin zones, even mild variations in velocity can play havoc on wells drilled on time migrated data. While time images can be 'tied' to well control, consider that horizontal drilling generally drains a given reservoir with fewer boreholes, meaning fewer well tops per square mile. A lot (of structure) can happen in between two horizontal wells! 2) Improve focusing of dipping beds and faults. Time migration laterally mispositions dipping reflectors and blurs fault truncations. Drillers hate surprises in the form of 'hidden' faults, which can present a geohazard, or require geosteering (think a 20-foot fault cutting a 20-foot zone). The combination of modern wide-azimuth, high-fold seismic with PSDM presents a powerful tool for fault delineation. 3) Improve the accuracy of seismic attributes. Most prestack attributes, whether AVA elastic inversion or azimuthal anisotropy analysis, are computed with time migrated data. However, reflector dip and/or velocity variation render the conversion of surface offset or surface source/receiver azimuth to true reflection angle ambigious or impossible. In theory, PSDM, with true reflection angle gathers, is the ideal vehicle for attribute computation, and in the coming years, will become increasingly adopted. The talk, illustrated with a number of onshore US examples, seeks to highlight the following key points: 1) PSDM is a powerful tool for unconventional exploration. 2) PSDM matches or beats time migration in terms of vertical and spatial resolion. 3) PSDM isn't for every project, and expectations must be calibrad. 4) Dense, iterative velocity analysis is key to PSDM success. 5) Anisotropic PSDM is the only rigorous way to handle depth/seismic miies. 6) RTM is the 'gold standard' PSDM algorith. 7) PSDM angle gathers are the optimal input for attribute analysis.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012