List user who have access to schema

Hi All:

I'm looking for query which can list all users who have access to a
particular schema.
The user may be granted role, which is turn may have access to the schema.
If the schema name is sch1,

grant select on table sch1.tab1 to role_ro;
grant ALL on table sch1.tab1 to role_rw;

grant  role_ro to user1;
grant  role_rw to user2;

I'm looking for report like


Schema          |     Role             |    Username
------------------------------------------------------------------------
sch1                role_ro                  user1, user3
                       role_rw                  user2, user4


Thanks in advance,
-SR