UPDATE 2-Operations at two California refineries hit by earthquake

(Adds details on Shell Martinez refinery)

Oct 15 (Reuters) - A small earthquake on Monday night hit operations at two oil refineries in Martinez, California, the companies said on Tuesday.

Marathon Petroleum Corp said it was working to return its 161,500-barrel-per-day refinery to normal operations following a quake centered in nearby Pleasant Hill, with a magnitude of 4.5.

The company confirmed an upset, or malfunction, resulting from the quake but said it did not result in any "known spills or releases" or injuries. "Marathon Petroleum personnel are working to assess integrity of equipment to ensure a safe restart and return to normal operations," the company said in a statement. Marathon had earlier reported the shutdown of multiple units because of the earthquake in a filing with the state pollution regulator. "No off-site consequences are anticipated," the filing said. Energy intelligence service Genscape earlier reported that a 37,000-bpd hydrocracker, 26,000-bpd catalytic reformer, 40,000-bpd hydrotreater, 14,000-bpd alkylation unit and a hydrogen plant were shut at the plant because of the incident.

Nearby, Royal Dutch Shell Plc said some equipment had been temporarily affected at its 156,400-barrel-per-day (bpd) refinery in Martinez, but that operations were now normal.

There were no major damages or injuries due to the quake in the Bay Area, local media reported.

A magnitude 4.5 quake is relatively small for earthquake-prone California, which lies on the San Andreas Fault. (Reporting by Sumita Layek and K. Sathya Narayanan in Bengaluru; Editing by Alex Richardson, Steve Orlofsky and Sandra Maler)