The following steps need to be taken to be able to delete archived redo logs that are no longer required by the CDC capture process: 1. Determine the new first_scn for your capture process
FROM DBA_CAPTURE cap, CHANGE_SETS cset
WHERE cap.CAPTURE_NAME = cset.CAPTURE_NAME
As per Oracle documentation: This query "determines a new first_scn value that is greater than the original first_scn value and less than or equal to ...