Brent has lost a quarter of its value from its peak in June. That’s bad news for PetroChina Co. (857) and Cnooc Ltd. (883) , which made most of their profit this year from oil and gas production. The shares dropped in today’s trading. PetroChina, the nation’s biggest explorer, posted its lowest profit in eight quarters yesterday, while Cnooc Ltd., which only reports quarterly sales, saw a decline of 4.6 percent. The plunge in crude may cause PetroChina’s profit margin to shrink and lead Cnooc, the country’s biggest offshore oil and gas producer, to stop some high-cost projects. “The negative impact from lower crude prices hasn’t fully materialized yet as the prices in July and August were still OK,” said Laban Yu, a Hong Kong-based analyst at Jefferies Group LLC. “The fourth quarter will be really challenging for all upstream players if crude prices don’t quickly rebound toward $100.” […]