Saudi-led OPEC and Russia and its allies, 24 nations in all, agreed to implement an additional collective oil production curb of 500,000 barrels a day, pushing the Brent crude price up by 1.2 percent. The original pact was to hold back about 1.2 million barrels per day. Some analysts express doubt about the new pact’s effect because based on some independent estimates, Saudi Arabia’s production...