SQL> create sequence s;

Sequence created.

SQL> select to_char(s.nextval,'XX') s from dual connect by level <= 16;
S
---
  1
  2
  3
  4
  5
  6
  7
  8
  9
  A
  B
  C
  D
  E
  F
 10

16 rows selected.

CREATE sequence mhe_seq START WITH 1 INCREMENT BY 1
/

SELECT to_char(mhe_seq.nextval, 'XXXX') base_16
FROM   dual
CONNECT BY LEVEL < 100
/


DROP sequence mhe_seq
/

SQL> create sequence s;

Sequence created.

SQL> select s.nextval, 
  2         UTL_RAW.CAST_TO_VARCHAR2(UTL_ENCODE.BASE64_ENCODE(UTL_RAW.CAST_TO_RAW(to_char(s.nextval)))) b64
  3  from dual;
   NEXTVAL B64
---------- ----------
         1 MQ==

1 row selected.