As far as I know, Weblogic 6.x used to ship an RDBMS authentication provider, but versions 7.x and 8.x no longer do so.
Apparently you have to implement your own, or put your users and roles in Weblogic's internal LDAP tree. They do provide example code to implement a custom authenticator.
Here is a link that mentions this (while it mentions Weblogix _Express_ and version 7.0, I think it is true of the full version too):
Migration from Tomcat to Weblogic Express 8.1 Search for "org.apache.catalina.realm.JDBCRealm" to find the section discussing this.
Tim