Narrative:

We received a call from the cabin that they and some passengers had witnessed sparks coming from the left engine and then stopping. We noticed engine indications were in the normal range but oil quantity was lower than previous and oil pressure; while in normal range; was less than the right engine. The engine seemed to be operating well at that time.got in touch with dispatch and asked if he could find out if maintenance control was able to see any down-linked engine events. Maybe that would show something irregular around the time of the witnessed event. They couldn't. We were then patched to dispatch and maintenance control on commercial radio and verbally talked to them. We were advised by maintenance control that there was no low limit to oil quantity and it could go to zero but we didn't have to shut the engine down until oil pressure was at the 'magic number' of 13 psi. We continued monitoring oil quantity and saw that it was slowly getting lower. The consensus was to continue on our route and monitor the engine.we had decided on a few alternate airports on our flight path to use as diversion airports should the time come that an engine shutdown was necessary. During this time we had been reviewing all pertinent procedures we would need if/when the engine needed to be shutdown while also discussing the situation in general.approaching ZZZ it was becoming apparent that the rate of oil loss would not allow us to continue to ZZZ1. ZZZ2 was suggested but that was too far to be considered nearest suitable airport. It was decided that ZZZ would be our diversion airport as there was a plane available and closest to us with personnel there to assist. Flight attendants were briefed and passengers were made aware of the precautionary nature; at that time; of our diversion.first officer (first officer) was PF (pilot flying) at this time and made arrangements to go to ZZZ3 initially in a brief error in communication. I noticed this and we confirmed with the dispatcher's message that ZZZ was our airport and we corrected our destination with center. At some point during the decent the oil quantity went to zero. Oil pressure was fluctuating between 22-28 psi. The APU was also started to save time if we were to need it. We had set up for runway 02C and had previously run non normal landing calculations in the event we did approach on a single engine.I had resumed PF duties and shallowed our decent for two reasons. First; we were still 40 miles from ZZZ and second I wanted to add thrust to see where the oil pressure would be before we leveled off closer to the airport. First officer brought up the idea to review the procedure for a failure while on short final to the runway. That procedure was reviewed. About 10 miles from the runway the oil pressure started to flash red at 13 psi and we decided it was time to secure the left engine. [Advised ATC] at this time. First officer did a very good job performing the checklist while I continued toward the runway. All was complete and configured by the time we turned final at the glide slope intercept. Approach resulted in an uneventful landing. Arff was there to look over the plane and with no concerns we then proceed to the gate.after plane was secured at the gate I had made calls to the dispatcher who ended up being a different one who we had been coordinating with. The chief pilot called and discussion about flying the other plane to ZZZ1 was initiated. The first officer and I answered the human factors questions he asked. I was unaware that it was fairly standard practice to remove the crew from all subsequent flying after an even like this. First officer had that understanding but I wasn't getting the message he was trying to get across to me.after getting off this call I received a call from the chief pilot who essentially did the right thing and removed us from the ZZZ1 flight we thought we were ok to do. His judgment was correct that in hindsight I was not in the state I believed to be in and rest not more flying was the best call. This was apparent the further away I got from this situation.then another call from the crew coordinator and he had us flying the crew's (who had flown the other plane into ZZZ) remaining schedule which would have extended us 2 days beyond our trip. This was contested and amended to have us layover in ZZZ.after all this I received a call from maintenance control who wanted some more details about the engine parameters before and after shutdown to help assess the course of maintenance. He mentioned he did not see an elb (electronic logbook) report yet and that he would put it in for me since I had totally forgot due to the multiple distractions I created by making a phone call before report the mechanical issue.overall CRM (crew resource management) made this event 'uneventful'. Flight attendants; company communications system and pilots sharing information and ideas and making decisions based on all resources available to us. Synergy on the flight deck was apparent as between the two of us a comprehensive plan was made and executed with little to no (apparent) stress.

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Original NASA ASRS Text

Title: B737 Captain reported an engine oil system anomaly which resulted in an engine shutdown; and a successful diversion to a suitable alternate airport.

Narrative: We received a call from the cabin that they and some passengers had witnessed sparks coming from the left engine and then stopping. We noticed engine indications were in the normal range but oil quantity was lower than previous and oil pressure; while in normal range; was less than the right engine. The engine seemed to be operating well at that time.Got in touch with dispatch and asked if he could find out if Maintenance Control was able to see any down-linked engine events. Maybe that would show something irregular around the time of the witnessed event. They couldn't. We were then patched to Dispatch and Maintenance Control on Commercial Radio and verbally talked to them. We were advised by Maintenance Control that there was no low limit to oil quantity and it could go to ZERO but we didn't have to shut the engine down until oil pressure was at the 'magic number' of 13 PSI. We continued monitoring oil quantity and saw that it was slowly getting lower. The consensus was to continue on our route and monitor the engine.We had decided on a few alternate airports on our flight path to use as diversion airports should the time come that an engine shutdown was necessary. During this time we had been reviewing all pertinent procedures we would need if/when the engine needed to be shutdown while also discussing the situation in general.Approaching ZZZ it was becoming apparent that the rate of oil loss would not allow us to continue to ZZZ1. ZZZ2 was suggested but that was too far to be considered nearest suitable airport. It was decided that ZZZ would be our diversion airport as there was a plane available and closest to us with personnel there to assist. Flight attendants were briefed and passengers were made aware of the precautionary nature; at that time; of our diversion.FO (First Officer) was PF (Pilot Flying) at this time and made arrangements to go to ZZZ3 initially in a brief error in communication. I noticed this and we confirmed with the Dispatcher's message that ZZZ was our airport and we corrected our destination with Center. At some point during the decent the oil quantity went to ZERO. Oil pressure was fluctuating between 22-28 PSI. The APU was also started to save time if we were to need it. We had set up for Runway 02C and had previously run non normal landing calculations in the event we did approach on a single engine.I had resumed PF duties and shallowed our decent for two reasons. First; we were still 40 miles from ZZZ and second I wanted to add thrust to see where the oil pressure would be before we leveled off closer to the airport. FO brought up the idea to review the procedure for a failure while on short final to the runway. That procedure was reviewed. About 10 miles from the runway the oil pressure started to flash red at 13 PSI and we decided it was time to secure the left engine. [Advised ATC] at this time. FO did a very good job performing the checklist while I continued toward the runway. All was complete and configured by the time we turned final at the glide slope intercept. Approach resulted in an uneventful landing. ARFF was there to look over the plane and with no concerns we then proceed to the gate.After plane was secured at the gate I had made calls to the dispatcher who ended up being a different one who we had been coordinating with. The Chief Pilot called and discussion about flying the other plane to ZZZ1 was initiated. The FO and I answered the Human Factors questions he asked. I was unaware that it was fairly standard practice to remove the crew from all subsequent flying after an even like this. FO had that understanding but I wasn't getting the message he was trying to get across to me.After getting off this call I received a call from the Chief Pilot who essentially did the right thing and removed us from the ZZZ1 flight we thought we were OK to do. His judgment was correct that in hindsight I was not in the state I believed to be in and rest not more flying was the best call. This was apparent the further away I got from this situation.Then another call from the Crew Coordinator and he had us flying the crew's (who had flown the other plane into ZZZ) remaining schedule which would have extended us 2 days beyond our trip. This was contested and amended to have us layover in ZZZ.After all this I received a call from Maintenance Control who wanted some more details about the engine parameters before and after shutdown to help assess the course of maintenance. He mentioned he did not see an ELB (Electronic Logbook) report yet and that he would put it in for me since I had TOTALLY FORGOT due to the multiple distractions I created by making a phone call before report the mechanical issue.Overall CRM (Crew Resource Management) made this event 'uneventful'. Flight attendants; Company Communications System and pilots sharing information and ideas and making decisions based on all resources available to us. Synergy on the flight deck was apparent as between the two of us a comprehensive plan was made and executed with little to no (apparent) stress.

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.