A select causing "blocking locks"?

 I want to know whyt this select is blocking an update session?
From user1:

SELECT   NVL (supplement_type, 0)
  FROM   serv_req
 WHERE   document_number = '710539';

From user2:

UPDATE   "ASAP"."SERV_REQ"
   SET   "SERVICE_REQUEST_STATUS" = '901',
         "ORDER_COMPL_DT" = SYSDATE,
         "LAST_MODIFIED_USERID" = :b2,
         "LAST_MODIFIED_DATE" = SYSDATE
 WHERE   "DOCUMENT_NUMBER" = :b1