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