subsurface geology and resource exploration
TRANSCRIPT
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Hydrocarbon traps are the s tructures, s tratigraphic differences or permeability and poros ity contrasts
(e.g. unconformities, faults and folds) that force the accumulation of hydrocarbons in high
enough concentrations to be viable for extraction. The hydrocarbon accumulation is
known as apool.
Salt domes are large mass es of salt (low density) that dome upward and deform the more dense
rocks around them, creating structural traps on their margins. These are very common in the Gulf
of Mexico and can be located on the seafloor by the hum mocky or "bumpy" hill-like features theycause on the seafloor surface. They are also found on land (e.g. Avery Island, Louis iana).
How do geologists decide where to drill for oil?
Using a ll the available data, geologists will try to find traps and assess the poss ibilities of oil being there.
Even if they decide oil is there, the possible amount of oil and the total cost of drilling for it and refining it
mus t be compared to the amount of money that will be made by selling the final product.
Well log data is the information that comes from previously drilled wells. Geologists have identified
the rocks and fossils that come up in the "cuttings" as a well is drilled.
Well A Well B
500
1000
1500
2000
2500
3000
Depth
(feet)Sea Floor
Notice that the geology between the wells is inferred from the well log data.
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Seismic data can be used with existing well log data to interpret the geology between the wells .
A source (like airguns,vibration plates or even explosives) sends s ound wave energy down into the ear
Receivers(called "geophones") measure the time it takes the wave energy to be reflected back off of
different layers.
Source Receivers
The resultingseismic section is li ke a cross section, but the vertical axis is the two-way travel time,
the time it takes the waves to go from s ource to receiver. Avelocity model mus t be applied to
convert the time data into depth below the surface. Then a geologis t can correlate seismic "reflectorsto the geology seen in well logs.
The waves will travel at different speeds according to the different densities of the different layers.
Processing of
seismic data
Subsurface
geology
Seismic section -
ready to interpret!
Seismic data
acquisition
The seismic reflectors could be
depositional contacts, unconformities,
faults, contrasts in densities, or other
features. A geologist will combine all
the available data - well logs, surface
data from maps, knowledge of the
physical properties of different rocks,
etc. to then make reasonableinterpretations of what the seismic
reflectors represent. With the cross
section created on the seismic
section, evaluations can be made of
the most likely places to find
petroleum.
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You are a geologist with Got Gas? Oil Company (GGOC). You have anopportunity to explore for oil in the Gulf of Mexico, and you have new seismicdata to interpret along with well log data.
You must tell your boss, Big Earl Spindletop, where the oil is and if it iseconomical to recover it. In other words, if can you make money on it! Use thewell logs and seismic lines to do the job!
Remember with seismic data, a source has produced sound waves thatpropogate beneath the seafloor of the Gulf. The waves travel at different speedsthrough different media, and it is the density contrasts between adjacent layersthat show up as discontinuities in the seismic data.
Medium Seismic Wave Velocity [ft/sec]Water 4950
Sandstone 9750Limestone 16,170
Avg. Sedimentary Rocks 11,000Salt 15,000
Granite 16,040Basalt 18,990
The formula for converting depth to time and vice versa in seismic data is:
Z = V x T Where: Zis depthVis wave velocityTis time
To convert the seismic data, which are in two-way travel time into depth belowthe seafloor, you must use an average velocity or velocity model that representsthe geology seen in the well logs. Then you can use the well logs, which are in
depth below seafloor, to determine which seismic discontinuities are caused byformation contacts, faults etc.
Remember, two-way travel time is the amount of time it takes for the waves totravel fromleaving the source, bounce off a subsurface discontinuity and travelback to the receivers. You must take this into account when calculating thedepths.
Well Logs
1. Using the well log cuttings, identify and list the rock types from eachformation on the well log data sheet.
2. Determine which seismic wave velocity should be used for two-waytravel time conversions.
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