Abstract The exceptionally low density of deep penetration shows that the Appalachian basin in Pennsylvania remains largely unexplored and geologically unknown. It still can be considered one of the major domestic areas with significant potential for new petroleum reserves. The thickness of the Paleozoic section ranges from less than 5,000 ft (1,520 m) in northwestern Pennsylvania to possibly 30,000 ft (9,140 m) northeast of Harrisburg. The sedimentary section averages 12,000 ft (3,660 m) in thickness in the Appalachian Plateau province and 25,000 ft (7,620 m) in the Valley and Ridge province. The upper part of the section is mostly clastic rocks, but the lower part is carbonate with some clastic rocks. Only half of the stratigraphic section has been evaluated in 40 percent of the 26,000-sq mi (67,340 sq km) Appalachian Plateau area in Pennsylvania. In the other 60 percent of the area, more than 10,000 ft (3,050 m) of favorable section associated with a wide variety of structures remains to be adequately appraised by drilling. During the past 100 years, interest has been focused essentially on the sandstone reservoirs—i.e., the multiple, shallow Upper Devonian producing units and the Oris- kany and Medina. Where these rocks are present in areas of exploration for deeper reservoirs, they are an added incentive. Where productive, they may be the deciding factor in making an exploratory venture profitable. However, most unevaluated potential Paleozoic strata in the Appalachians consist of a great variety of carbonate rocks that have an equal variety of potential reservoir zones, nearly all of which have contained shows of gas locally. Barrier- and patch-reef zones and porous oolitic, algal, fossiliferous, dolomitized, and calcarenitic zones can be identified; some are apparently of large areal distribution and others are discontinuous. These zones are the prime targets for future exploration.