I am currently working for Schlumberger Business Consulting, created by Schlumberger 2 years ago. If you want O&G consulting in the upstream segment, no doubt SBC is the best place to be. Booz Allen was good in the past, but recently some high ranking O&G partners left Booz. AT Kearney is not present in O&G upstream consulting, CERA is not a consulting firm but a market survey store, Woodmack and Celerant are very technical, far from strat & orga jobs.