plucky underdog:

In the late 1950's the world began to really use a lot more oil and gas. The oil finding rate was just about at its maximum, and nationalisation was a storm cloud over the horizon. OPEC hadn't been started It was the good old days of the "free market capitalism" approach to the oil business.

As the world changed from coal to oil for its fossil fuel BTUs, the amount in export trade went up a huge amount. At the beginning of the Second World War, Texas produced 60% of the oil and petrochemicals in world trade, although Texas was producing about 5 million barrels a day.
The rest of the oil in world trade came from Iran and the beginnings of the mid east fields from Shell and Anglo-Iranian Oil, now BP.

When the market was smaller and thinly traded, the decline rate was more closely tied to the performance of individual fields or groups of fields. But as the markets grew, the decline rate had a much larger group of producing field to average it over, so the curve got flattened. At any rate, thats my hypothesis and Im stticking to it. Bob Ebersole