Marine Seismic Methods
Seismic exploration offshore uses similar equipment as exploration on land but uses it from a ship (fig. 2.10). A sound source sends sound waves through the water, and formations beneath the seafloor reflect the seismic waves to hydrophones, the marine version of geophones. Most commonly, hundreds of hydrophones trail behind the ship on steel cables (fig. 2.11). Another method of positioning the hydrophones is a vertical-cable survey. In this type of seismic survey, the ship's crew places cables with hydrophones attached in a specific pattern in the ocean. Each cable has an anchor on one end and strong buoy on the other. Then the ship sets off the source of seismic waves in a certain pattern around the cables.
Figure 2.10 The R/V Hollis Hedberg, a geophysical vessel, has logged over 100,000 miles of geophysical data gathering since its christening in 1974. (Courtesy of Gulf Oil Exploration and Production Co.)
A seismic ship can stay at sea several months if necessary. It has a double crew, one for ship operations and one for seismic operations.
If all the surface and subsurface information indicates a strong possibility of hydrocarbons, the oil company may drill an exploratory well or wells. As drilling progresses, the geologist keeps a watchful eye on the underground rock by means of core samples, well logs, and test results. Specialists combine all of the geological information gathered and recorded as the well is drilled with the surface data and the subsurface findings of the geo-physicists in order to try to predict whether any reservoir found has enough oil or gas to justify completing the exploratory well or drilling additional wells in the reservoir.
A log is a record of information about the formations through which a well has been drilled. Exploration companies use several different kinds of logs, and each supplies them with specific information about the particular well and the formations encountered in it.
One of the most common well logs is the driller's log, which provides basic information to the geologist. The driller, the person in charge of drilling operations, keeps a record of the kinds of rocks and fluids encountered at different depths and anything else of interest. Particularly when formations are alternating from soft to hard rock, a driller's log may be a very useful tool to geologists. It keeps track of exactly how long it took to drill through a particular formation, and the drillers can correlate that time record with wells drilled later. An astute driller learns to recognize key formations and reports them on the log.
Geologists also have several methods of getting indirect information about the formations down the hole, using wireline logs. A wireline is a metal line that can be run into the hole with a tool attached to its end. A wireline that can carry electricity to the tool is a conductor line, but in oilfield slang both are often referred to as wireline.
Wireline logging often involves complex calculations and interpretation of the information the tool relays to the surface. The logging specialist uses a computer to compare, or correlate, data from various surveys, print it out in the form of charts or graphs, keep track of the depth of the logging tool, and warn of malfunctions.
Wireline logs gather data in many different ways under many different conditions; however, the procedure for each log is basically the same. An instrument called a sonde is lowered into the wellbore on conductor line. A highly sophisticated electronic instrument, the sonde measures and records electrical, radioactive, or acoustic properties of formations. Then it transmits the signals up the conductor line as it is being raised to the surface at a predetermined speed. At the surface, the wireline unit has equipment—frequently, computers—that translates these signals into graphs that geologists and engineers can interpret. Correlating the information from different types of wireline logs can give the geologist valuable information about porosity, permeability, and the amount of water and petroleum in a formation.
Wireline logging is cheaper, faster, and more accurate than ever due to advances in technology over the past ten years.