B737 pilot reported fuel imbalance and later identified a fuel leak.

Attributes
ACN	1704308
Time
Date	201911
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Fuel System
Person 1
Function	First Officer Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 166 Flight Crew Total 1799 Flight Crew Type 1799
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Procedural Published Material / Policy Ground Event / Encounter Other / Unknown Inflight Event / Encounter Fuel Issue

Narrative:

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.

Original NASA ASRS Text

Title: B737 pilot reported fuel imbalance and later identified a fuel leak.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.