The Texas-based land driller has begun a related investigation into unauthorised payments made for assets it never received.
AG Edwards analyst Poe Fratt said the fraud was difficult to detect because the person that appears to have been involved was a long-standing employee in a company where turnover within the executive ranks has been relatively low.
"In our judgment, the former officer was able to prey upon the trust of the rest of the management team and the company's accounting staff to misappropriate funds," Fratt said in a report.
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