B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 166; Flight Crew Total 1799; Flight Crew Type 1799
Situational Awareness
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown; Inflight Event / Encounter Fuel Issue
General Maintenance Action
Aircraft
Passing 10;000 ft the CA (Captain) and I noticed a fuel imbalance of approximately 15;000 lbs with the left side indicating low.  We monitored it until level off at FL360.  The fuel indication would indicate a higher fuel burn; and then increase in quantity; that is to say it was somewhat erratic.  It continued to be erratic and then settled down to indicate the 15;000 lbs imbalance. The CA (Captain) communicated with [Technical Support Maintenance Control] and Dispatch; and decided it was in the best interest of safety to request priority handling and recover the aircraft to ZZZ.  We coordinated with ATC and request priority handling.  We ran the Fuel Leak Checklist and determined a leak was not confirmed.  We flew a normal approach to Runway XXL and landed uneventfully.  We were me by airport rescue and firefighting on X7 short of X where they had us shutdown both engines due to some leak from the engine area.  After testing the liquid it was determined that it was not gas or hydraulic fluid.  They said it was safe to taxi in with 1 engine; but after starting number 2 the airport rescue and firefighting said there was a leak and we shut down the engine.  After that we coordinated with operations for a tug to tow us in.  The tug towed us uneventfully to gate.  We sent an Emergency Locator Beacon about the fuel imbalance and discussed the problem with Maintenance at the gate.
B737 pilot reported fuel imbalance and later identified a fuel leak.