LONDON, Jan 17 : Oil prices edged up on Friday, heading for a fourth consecutive week of gains, as the latest US sanctions on Russian energy trade heightened expectations for oil supply disruptions. Brent crude futures were trading up 36 cents or 0.4 percent higher at $81.65 per barrel by 2:13 p.m. Saudi time, having gained 2.4 percent so far this week. US West Texas Intermediate crude futures were up 53 cents or 0.7 percent at $79.21 a barrel, having climbed 3.5 percent for the week.