I checked the alert log file after I stopping the services and noticed the following.

--Oracle does not log the shutdown operation in the alert log file if service is shutdown.

--Oracle does a instance recovery when you stop and start the services.

Sample of Alert Log File when stopping and starting the services
---------------------------
alter database open
Beginning crash recovery of 1 threads
Tue Jun 26 14:19:22 2001
Thread recovery: start rolling forward thread 1
Recovery of Online Redo Log: Thread 1 Group 1 Seq 12127 Reading mem 0
Mem# 0 errs 0: C:\ORACLE\ORADATA\ORCL8I\REDO01.LOG
Tue Jun 26 14:19:23 2001
Thread recovery: finish rolling forward thread 1
Thread recovery: 22 data blocks read, 22 data blocks written, 41 redo blocks read
Crash recovery completed successfully
Tue Jun 26 14:19:25 2001
Thread 1 advanced to log sequence 12128
Thread 1 opened at log sequence 12128
Current log# 2 seq# 12128 mem# 0: C:\ORACLE\ORADATA\ORCL8I\REDO02.LOG
Successful open of redo thread 1.
Tue Jun 26 14:19:25 2001
SMON: enabling cache recovery
Tue Jun 26 14:19:25 2001
ARC0: media recovery disabled
Tue Jun 26 14:19:26 2001
SMON: enabling tx recovery
Tue Jun 26 14:20:37 2001
Completed: alter database open


Sample of Alert Log File when shutting down immediate and starting up the database using OEM
--------------------------------------------------------
Tue Jun 26 14:17:35 2001
/* OracleOEM */ ALTER DATABASE OPEN
Tue Jun 26 14:17:36 2001
Thread 1 opened at log sequence 12127
Current log# 1 seq# 12127 mem# 0: C:\ORACLE\ORADATA\ORCL8I\REDO01.LOG
Successful open of redo thread 1.
Tue Jun 26 14:17:36 2001
SMON: enabling cache recovery
Tue Jun 26 14:17:37 2001
ARC0: media recovery disabled
Tue Jun 26 14:17:38 2001
SMON: enabling tx recovery
Tue Jun 26 14:18:15 2001
Completed: /* OracleOEM */ ALTER DATABASE OPEN


From this it seems to me that Oracle does a shutdown abort when you stop the services as it is not doing instance recovery when you do a shutdown immediate.

Please correct me if I am wrong.

Thanks
Anurag