Without much context, it may be an adjective derived from 'reef'...

http://www.glossary.oilfield.slb.com/Display.cfm?Term=reefal
reefal:
Alternative to 'reef'

http://www.glossary.oilfield.slb.com/Display.cfm?Term=reef

1. n. [Geology] ID: 417

A mound, ridge, or buildup of sediment or sedimentary rock, most commonly produced by organisms that secrete shells such as corals. Reefs are typically taller than the sediment that surrounds them, resistant to weathering and wave action, and preserved within sediment of a different composition. Carbonate reefs form in a limited range of temperatures, water depths, salinities and wave activities, so their occurrence can be used to interpret past environmental conditions. Because the rocks that surround reefs can differ in composition and permeability, porous reefs can form stratigraphic traps for hydrocarbons. Porosity of reefal limestones depends on post-depositional diagenetic changes.