National Geographic : 1989 Aug
Like detectives, petroleum explorers look for formations that trap oil. The depositional process itself may have buried a sandbar (2). The deformation of a rock layer may have trapped oil in a fold (3) or alongside a salt dome (4) or fault (5). Satellite images (6) and air borne radar (7) help geologists map the earth's surface. Aerial surveys (8) measure magnetic fields, while magnetotelluric surveys (9) measure magnetic and electrical fields. Variations in any field may signal an 6 interesting rock layer. A sniffer W ,, .... (10) detects traces of gaseous hydrocarbons, here bubbling upward from an oil reservoir. Seismic surveys carried out on land (11) and water (12) record differences in how rocks reflect shock waves. They give the clearest geologic picture. Drilling an exploratory well on land (13) or offshore (14)-is the only way to find out exactly what lies underground. As it bores, the bit chips off rock, which drilling fluid carries to the surface for analysis. And, in a process called down-hole logging, a probe lowered into the well detects various proper ties of the rocks it passes through (15). Even with these different types of information, drilling for oil is hit or miss. A well can come tantalizingly close to a field and still strike nothing. "If I had to choose geological knowledge or good luck," says oilman Roy Huffington, whose first overseas well found a major gas field in Borneo, "I'd take good luck every time."