The 2,358-hour commercial pilot and his passenger were in cruise flight approximately seven nautical miles north of the airport when the engine of their helicopter lost power. The pilot reported that he entered an autorotation and the helicopter impacted a petroleum coke mound. The helicopter subsequently tumbled down the face of the mound and came to rest on its left side. An FAA inspector performed an on-scene examination of the helicopter. According to the inspector, the helicopter's 46-gallon fuel tank was found to be empty at the accident site. Following the recovery of the wreckage, the inspector tested the fuel tank for leaks and none were found. The inspector stated that the last entry in the maintenance logbooks revealed the Hobbs meter was reading 366.1 hours, and the Hobbs meter at the accident site was reading 369.1 hours. During an interview conducted by the FAA inspector, the mechanic, who performed the last maintenance action prior to the accident flight, revealed that he had removed 44 gallons of fuel from the fuel tank and then placed the same fuel back into the tank after the required maintenance was accomplished. On the day before the accident, the pilot flew to an air show at another nearby airport, from where he flew an unknown number of short flights. Prior to departing the air show for his home base, the pilot purchased an additional 15 gallons of fuel. On the day of the accident, the pilot flew from DWH to IWS, and then towards his final destination of EFD, when the helicopter lost power. Numerous attempts, albeit unsuccessful, were made by the investigator-in-charge to obtain a completed Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2) from the pilot.