Cerro Negro production, upgrading and sales will not be halted, Seniat said in a statement, which said the closure started on Thursday.
Seniat took the move in response to the company's alleged failure to meet procedures related to VAT.
Exxon is one of several major oil companies negotiating plans with the Venezuelan government to increase state oil firm PDVSA's stake in the Orinoco projects to 60%.
Cerro Negro, where ExxonMobil and PDVSA hold 41%-plus and the UK's BP (NYSE: BP) the balance, takes extra-heavy crude of API grade 8 or heavier and transforms it into light sweet syncrude of API 30 or more.
The syncrude is then sent to Chalmette, a refinery 50:50-owned by PDVSA and ExxonMobil in Louisiana, for further processing.
Exxon officials in Caracas did not return calls seeking comment.
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