Canadian Natural Resources, the third biggest producer of natural gas in Canada, may have its credit ratings cut because of increased debt from its proposed C$2.4 billion ($1.5 billion) purchase of Rio Alto Exploration Ltd. Standard and Poor's put Canadian Natural's “BBB+” corporate and senior unsecured ratings on negative watch, which means they will be reviewed and possibly lowered, the ratings company said in a statement. The gas producer's “BBB-” preferred stock rating also was placed on watch. All the ratings are investment grade. Companies that are assuming or taking on new debt from acquisitions are usually reviewed by ratings companies. Standard and Poor's said that it expects Canadian Natural to retain its current ratings, pending a successful completion of the merger. Canadian Natural had C$2.66 billion in long-term debt at the end of the first quarter. The company plans to assume C$972 million in debt to buy rival gas-producer Rio Alto. The purchase will be financed partly with a C$500 million line of credit.