The photograph above is of the famous Hibernia Platform located about 200 miles SE of St. Johns, Newfoundland, on the Grand Banks, in 250 feet of water. In terms of gross tonnage this is the biggest offshore platform in the world, which is owned and operated by a Canadian subsidiary of a JV between Exxon, Chevron and several smaller companies. The platform, by the way, is engineered so that it could get clobbered by an iceberg the size of St. Louis and not even flinch.
Two dimensional seismic data was shot across this offshore basin and re-interpretation of the data in the mid 1960's indicated a anticlinal structure existed in lower Cretaceous age limestone at depths of 11,000 feet. Mobil drilled several dry holes on the flanks of the structure in the late 60's and early 70's; the G-55 dry hole found an important fault with throw basin ward, aptly called the G-55 Fault, and in the 1979 Chevron made the Hibernia discovery in its P-15 well, below. The structure below is mapped on the Berriasian sandstone; the P-15 well found an oil water contact in the interval at -2600 feet below sea level. The date of this map is probably 1988 or 89.
Every oil or gas well drilled almost anywhere in the world is wireline logged with an array of different analytical tools and these well logs are the keys to mapping the subsurface accurately and subsequently being about to place future development wells. Those wells add more points of reference in the mapping journey and the picture becomes clearer and clearer.
By comparing well logs to one another the rise, or fall, of the mapping datum can be determined as can additional faulting on the structure. The geologist can determine the nature of the faulting, its throw, or displacement of sands on each side of it, how steep the fault(s) are, whether they will act as seals to trap hydrocarbons, all of that can be done sitting at a desk, correlating well logs, identifying marker beds and creating maps. Sort of like this:
You will please notice my pencil has no eraser on it. When I moved my mapping table the other day in preparation of closing my office there was literally 4 inches of solid eraser crumbs between the table and the wall from 40 years of finding mistakes and having to start all over again. That process, by the way, always starts with the loud word, SHIT ! followed by a holding your head in your hands, staring at nothing. There was other funky stuff back there too, like deteriorated rubber bands, a clear plastic triangle I swore somebody stole 30 years ago, an ugly stickem' note to myself about rate of dip, very stale Fritos and petrified bugs of unknown origins. It took an entire shop vac bucket to clean it up.
Structural geology, by the way, is becoming a lost art, abandoned in the mire of HZ laterals drilled over vast areas, everywhere. Sort of like carpet bombing. But in conventional work, offshore, in the Mid Continent region, the Illinois Basin, etc. geology is still very important, similar to using a Nordic bombsight.
Structural mapping is great source of pride for geologists and because they are often rolled up and placed in map racks, NEVER under any circumstances to be thrown away, maps are made to look nice and painstaking efforts are used to render them immaculate in detail and accuracy. There is nothing worse than having a real geologist look at a map and say, WTF is this, you can't do THAT ?!
You want to crawl under a rock.
Mapping requires great discipline as you are constantly fighting the urge to engage in what we call "creative contouring" to generate a drilling prospect that is not really there. That is a big no, no, particularly when using your own money. If you think you have it right and you are about ready to plop down your family's savings account on a new well...you need to re-map it six ways from Sunday to see which idea makes the most sense.
If you map something that leads to a discovery, or an extension of an existing field, and the well drills out pretty much like you have it mapped, there are several days thereafter where you are truly convinced your shit will never stink again and you will be invited straight to heaven to work in the geology department.
But that generally only lasts until you drill your next dry hole, then you are looking for a flat rock again.
The Hibernia "structure' had over 800 of net, productive Berriasian pay above the oil/water contact and it is expected to recover over 1.4G BO from about 4o wells. Many of those 40 wells are now extended reach wells from the Hibernia platform and steered to targets of undrained reservoir that were subsequently made easier to find thru use of three-dimensional seismic modeling with very high speed computers. Hibernia is now under pressure management.
A structural geologists most used tool.