BIRMINGHAM, Ala. — The Laclede Group is spending about $1.28 billion in cash to acquire the natural gas utility business of Energen.

For Energen, the sale pushes it closer to becoming a pure exploration and production company. The Birmingham company said Monday that it will use the proceeds to reduce debt, allowing it to accelerate drilling and development of its Permian Basin assets starting next year.

The deal also comes with about $320 million of debt for Laclede. Energen’s after-tax proceeds are estimated at $1.1 billion.

The Laclede Group Inc. is a natural gas utility with about 1.13 million customers in Missouri. With the acquisition of Alabama Gas Corp., it picks up more than 422,000 customers.