Lesson Overview: Marine Seismic Exploration

Lesson Overview: Marine Seismic Exploration

Learning Objectives

Introduction to Geology & Geophysicsin Deep Water

Success in Deepwater Seismic Technology • The technology for marine seismic exploration in deep water differs little from that used in shallower depths. • The much higher cost of deepwater exploratory wells could have have precluded exploration. • However, major improvements in seismic technology resulted in a higher success rate. • Deepwater exploration continues despite the very high costs. • The dramatic increase in the success rate has led to fewer dry holes.

Formation of Petroleum Reservoirs Oil and gas reservoirs are formed from microorganisms, plankton, that live and die in the oceans. Their settling to the seafloor together with silt and sand is called sedimentation. GR-98

Formation of Petroleum Reservoirs, cont. Over time and in the presence of heat and pressure, the micro-organisms become hydrocarbons, usually in fine-grained rock called shale.

Migration and Traps Oil and gas migrate away through the permeable rock, unless there is a trap (impermeable rock or salt) to prevent its escape leading to formation of a reservoir. GR-100

Finding the Traps Seismic exploration maps the rock strata in sufficient detail to define structures that may be hydrocarbon traps. WV-21

Finding the Traps, cont. With data from millions of shots, the computer can map the boundaries, i.e., the surfaces between the layers of rock. GR-101

Finding the Traps, cont. GR-102 Geologists must then use other data, such as samples from offset wells, to determine if a structure might contain hydrocarbons.

Exercise How has seismic exploration affected the success of deepwater wells? What natural occurrences can trap hydrocarbons? How do geophysicists find hydrocarbons under the sea? How do micro-organisms become hydrocarbons?

Marine Seismic Vessels and Streamers The newest seismic vessels can tow up to 32 streamers, each several kilometers long with hydrophones every meter or so. GR-93

Marine Seismic Vesselsand Streamers, cont. Multi-streamer vessel GR-176

Marine Seismic Vesselsand Streamers, cont. Precise streamer spacing is critical to the accuracy of the seismic data. GR-175

Marine Seismic Vesselsand Streamers, cont. For controlling their position, the streamers have steerable tail buoys with Differential Global Positioning System (DGPS). GR-94 Controlled Tailbuoy

Marine Seismic Vesselsand Streamers, cont. This device both steers and levels the streamer. GR-174 Sercel Nautilus Seismic Streamer Leveling and Steering

Modern Seismic Vessels • Many geophysicists and geologists use large computers to gather and analyze data onboard the seismic vessel. • This one: • Cruises at 13.5 knots • Can sleep 50 geologists GR-95

Modern Seismic Vessels, cont. 3D concept for a modern seismic vessel GR-97 Ramform Sovereign Seismic Vessel Petroleum Geo-Services (PGS)

4D Seismic: The Fourth Dimension is Time Permanent seismic installations monitor the reservoir during the production period to maximize hydrocarbon extraction Hydrophones for permanent installation in the seabed GR-177 Sercel Optowave™ 4D Seabed Reservoir Monitoring

Exercise What some innovations in seismic vessel design? What is important in streamer design? Describe two tools that help position the vessel. What are hydrophones?

Lesson Overview: Marine Seismic Exploration