CARACAS (Reuters) - Venezuela has arrested the state oil company s boss for the western region and eight other executives at PDVSA, according to an internal company memo and a half-dozen sources in the OPEC member s oil industry.  It was not immediately clear why Gustavo Malave and the other employees were apprehended, though a series of corruption probes are under way at PDVSA and have entangled other employees.  The sources said Malave was arrested on Monday in Zulia state, Venezuela s traditional oil-producing region near Colombia, in what would be one of the highest-profile detentions of a PDVSA executive.  PDVSA, the prosecutor s office, and Malave did not immediately respond to a request for comment.  Separately, Venezuela s new chief prosecutor Tarek Saab on Thursday announced he was investigating  spectacular  overpricing in a dozen contracts in the nation s Orinoco oil belt, on the other side of the country.  The reputation of PDVSA - short for Petroleos de Venezuela SA - has been tarnished in recent years by graft investigations involving high-profile staff.  The company has blamed the problems on a small group of employees and executives, and promised a war on corruption. Last year, the opposition-led congress said $11 billion was lost at PDVSA between 2004 and 2014, when Rafael Ramirez was in charge of the company. He denied the allegations. The Caracas-based company is the financial motor of leftist President Nicolas Maduro s government, but is reeling from low oil prices, mismanagement, and lack of investments. 