Oil prices were down in Asian trade on Thursday as a surge in US crude reserves and the dollar's strength against the euro curbed demand. New York's main contract, light sweet crude for delivery in September, shed 21 cents to 76.35 dollars a barrel, while London's Brent North Sea crude for September was down eight cents at 75.29 dollars.An unexpected increase in American crude oil reserves also impacted oil prices after the US Department of Energy said crude reserves had risen 400,000 barrels in the week ending July 16, indicating weak demand.