1. adj. 
Describing a relatively impermeable reservoir rock from which hydrocarbon production is difficult. Reservoirs can be tight because of smaller grains or matrix between larger grains, or they might be tight because they consist predominantly of silt- or clay-sized grains, as is the case for shale reservoirs. Stimulation of tight formations can result in increased production from formations that previously would have been abandoned or produced uneconomically.
2. adj. 
Secrecy or confidentiality of information. Operators typically try to prevent disclosure of results from exploration wells and will hold any such information "tight". A tight hole is a well whose status and data are not widely disseminated by the operator.
Alternate Form: tight hole