The next declaration of a sale would take Berkshire's interest below 5%, the threshold above which the company is required to declare any shareholding changes that cross a full percentage point.
However, analysts suspect Buffett is about to leave the stock entirely, if he hasn't already done so.
"Considering the price level and volumes traded in PetroChina since 25 September, we would not be surprised if Buffett has already sold off his entire stake," JP Morgan analyst Brynjar Eirik Bustnes said in a note.
CLSA
