EXP-00091 Exporting questionable statistics
  
Cause: Export was able to export statistics, but the statistics may not be useable. The statistics are questionable because one or more of the following happened during export: a row error occurred, client character set or NCHARSET does not match with the server, a query clause was specified on export, only certain partitions or subpartitions were exported, or a fatal error occurred while processing a table.
  
Action: To export non-questionable statistics, change the client character set or NCHARSET to match the server, export with no query clause, or export complete tables. If desired, import parameters can be supplied so that only non-questionable statistics will be imported, and all questionable statistics will be recalculated.

解决：
linux下的oracle用户的环境变量语言集和oralce数据库中的环境变量语言集不相同。
查看oracle的环境变量语言集：
数据库服务器字符集：
select * from nls_database_parameters
客户端字符集：
select * from nls_instance_parameters

将oracle用户的环境变量语言集改成和数据库服务器字符集一样。
 export NLS_LANG=american_america.ZHS16GBK