The move was likely due to a rise in new production of heavier oil from fields such as West Qurna, they said.
The second largest OPEC producer plans to offer two grades of crude - light and heavy - for exports from its southern terminals, the sources said, citing an e-mail from SOMO.
The heavier grade with an API gravity of 24-25 degrees will be loaded from its Single Point Mooring (SPM) terminals while the light grade will have an API gravity of 29 degrees, one source said. SOMO will set monthly official selling prices (OSPs) for each grade, he said.
Buyers are to respond by the end of the week, the source said. SOMO could not be immediately reached for comment.