former executive at the Mobil Oil Company pleaded guilty yesterday to evading taxes on more than $7 million in unreported income, including a $2 million kickback in connection with Mobil's oil business in Kazakhstan. J. Bryan Williams, 63, was charged in April with failing to report the $2 million kickback he received for negotiating Mobil's purchase in 1996 of a 25 percent stake in the Tengiz oil field in Kazakhstan for $1.05 billion. The field, which has estimated reserves of 9 billion barrels, now produces 261,000 barrels a day. Advertisement The guilty plea in United States District Court in Manhattan...