Thai upstream company PTT Exploration and Production Plc (PTTEP) said it plans to invest 2.8 billion baht to increase petroleum output at its Sirikit oilfield, an onshore petroleum block in Kamphaeng Phet, to take advantage of high oil prices, according to a Bangkok Post report. (US$1=40 baht.   The newspaper quoted President Maroot Marigadat saying…