Probably hit a 'thief zone' or a local Fault that sucked up a...

Probably hit a 'thief zone' or a local Fault that sucked up a lot of their drilling mud. One controls that with adding more coarser material to the mud to block/ barricade any such section.

Caliper log will show over-gauge in that section (ie change of well bore diameter). WL logs should confirm how thick this 'thief' zone was.

Logs will also tell them if they had an water contact at the time, which means Sidetrack & drill in another direction.

U normally get cavities & vugs in dolomitic/limestone reservoirs that suck up a lot of mud. Main thing is they managed to control the mud losses & abandon the current bore.