read/write/execute to pl/sql grants
Read: READ. 

Not SELECT. Forget SELECT. SELECT privilege allows users to run SELECT .. FOR UPDATE, which modifies the metadata in the database blocks (not the user data) and locks the affected rows.

READ privilege is available starting with 12c.

Write: INSERT, UPDATE, DELETE.

Execute: EXECUTE

For the rest (Object Privileges), most people do not even know about them and can happily live without them.