Marathon Petroleum will spend $1.2 billion upgrading its Galveston Bay refinery complex in the City of Texas City over the next three years as part of a program to integrate and modernize its Texas refineries.

For the past few years the Ohio-based refiner has been in the process of integrating its Galveston Bay refinery, now the second biggest in the nation, with the adjacent Texas City refinery, which it bought from BP for $2.4 billion in 2013.

Called South Texas Asset Re-positioning (STAR) program, the program was first announced in 2015 as a project with staged investments that would expand crude processing capacity and allow for more production of ultra-low sulfur diesel, distillates and gas oil. Integrating its Texas refineries also would allow Marathon to reduce production costs.

This week Marathon gave more specifics about the program in an investor presentation filed with the U.S. Securities and Exchange Commission April 1. The Ohio refiner said it would spend about $1.5 billion on STAR program, including $1.2 billion of investments between 2019 to 2022, when the project will be complete.