MS SQL: get permissions of a fixed server role
Maybe this is what you are looking for:
SELECT SYS.SERVER_ROLE_MEMBERS.ROLE_PRINCIPAL_ID, 
       ROLE.NAME AS ROLENAME, 
       SYS.SERVER_ROLE_MEMBERS.MEMBER_PRINCIPAL_ID, 
       MEMBER.NAME AS MEMBERNAME, 
       MEMBER.TYPE_DESC, 
       PERMISSION_NAME, 
       STATE_DESC, 
       CLASS_DESC
FROM SYS.SERVER_ROLE_MEMBERS
     FULL JOIN SYS.SERVER_PRINCIPALS AS ROLE ON SYS.SERVER_ROLE_MEMBERS.ROLE_PRINCIPAL_ID = ROLE.PRINCIPAL_ID
     FULL JOIN SYS.SERVER_PRINCIPALS AS MEMBER ON SYS.SERVER_ROLE_MEMBERS.MEMBER_PRINCIPAL_ID = MEMBER.PRINCIPAL_ID
     FULL JOIN SYS.SERVER_PERMISSIONS PERMISSIONS ON PERMISSIONS.GRANTEE_PRINCIPAL_ID = MEMBER.PRINCIPAL_ID