Pre-tax profits plunged at oil group Tullow in the first half of the year from the same period the previous year as the firm felt the pinch of the slide in oil prices between the two periods.The firm posted a pre-tax profit of £34.8m in the six months to June 30, down by 83% from the £126m reported the previous year. Revenues slid 23% to £291.3m from £378m.Africa accounted for £188.4m of revenues, with £102.9m coming from the rest of the world.The average oil price fell by 34% from the first half of 2008 to $53 a barrel, Tullow said.It said its half-year results were in line with expectations and aside from the lower oil pricereflect a 16% decrease in production, following a decision to focus capital investment on Ghana and Uganda.