Page 38: of Maritime Reporter Magazine (April 2005)

38 Maritime Reporter & Engineering News

Offshore Technology Annual

The search for oil and gas is a challenge that carries a high financial risk. Wells may cost more than $50-70 million each and be drilled to nearly six miles below the ocean's surface. In the early days of energy explo- ration, discoveries were often a matter of luck. Even today, the U.S. Energy Information Administration estimates 13% of all holes drilled in the U.S. do not find commercial quantities of oil and gas (down from 37% in 1973). Advanced geophysical and geological tools such as three-dimensional seismic imaging, sophisticated well-logging tools and techniques, com- puter-based petrophysical analysis, and paleontology reduce the risk associated with locating oil and gas reserves.

In the past, geoscientists had to rely on two-dimen- sional seismic data interpretation to define potential hydrocarbon prospects. The images scientists assem- bled were useful, but less than complete. Today, how- ever, advanced three-dimensional seismic data are pro- viding a more detailed understanding of a prospect.

Seismic data are obtained by using hydrophone or geo- phone receivers. These receivers record the time it takes for a signal or sound wave (caused by an air pulse) to travel from the source to the receiver. As the signal or wave travels, it reflects off the layered sedi- ments it encounters and returns to the receiver. The source and receiver are then moved in a grid pattern across an area as the process is repeated.

The data generated through this method are then processed to yield a three-dimensional image or "pic- ture" that represents the subsurface strata and geologic features (faults, salt domes, etc.) in the area of investi- gation. By estimating the velocity of sound through the various subsurface strata and the travel time of the sig- nal from the source to the receiver, the depth to a par- ticular reflector can be estimated. Geoscientists then translate the seismic data into maps, cross-sections, and three-dimensional models of potential hydrocar- bon accumulations or prospects.

In the evaluation of a new lease block, exploratory wells may be drilled to test subsurface strata and struc- tures for potential hydrocarbon accumulations. Logs obtained from these exploration wells (see figure on page 3) provide information that can enhance the three-dimensional seismic picture of the area. Well logs are continuous recordings of specific physical properties of the subsurface strata encountered during drilling of an exploratory well. Such logs may be obtained by wireline logging, which consists of lower- ing a logging tool into a well and recording data as the tool is raised from the bottom of the well. Logs can

Searching for Oil and Gas

Prospecting With New Tools

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