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Although the water depths and hole conditions
are extreme, Schlumberger has engineered methods to operate
effectively in these environments. For example, last November,
Schlumberger Logging While Drilling tools and Wireline logging
tools were used to effectively evaluate formations in over
16,000 feet (4800 meters) of water near Costa Rica. To put
this in perspective, what is now considered "deep water"
in offshore oil exploration is limited to depths which range
from 5000 to 7000 feet (1500-2100 meters) of water.
In August of 1996, drilling off the coast of Northern California,
holes were drilled into superheated formations where borehole
temperatures approached 375 degrees Fahrenheit (175 degrees
C). Standard logging tools are built to stand up to a maximum
350 degrees F (160 degrees C). Wireline logging tools specifically
designed to endure such harsh environments were used to
make sophisticated measurements. The data collected gave
scientists insights about some physical properties, such
as rock strength, geological and sedimentary processes,
and even tectonic anomalies.
The JOIDES Resolution is not just in the business of making
holes, but also in collecting and analyzing data. Data is
accumulated in two ways:
- Physical core samples are continuously being taken,
usually on every hole that is drilled (this is very unusual
by oil industry standards), and
- Wireline and Logging While Drilling (LWD)* services
are used to put measurement devices into the hole itself
to make measurements and send data to the surface in the
form of a "log."
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| A log showing layers of sand and
shale |
A log can be thought of as a long graph showing depth
on the vertical axis and the response of the logging sensors
to the geological subsurface on the horizontal axis. Since
there is no riser, all wireline logging equipment must be
run into the hole through the drillpipe itself, which is
very unusual.
Because the drillship is dynamically positioned and is
subject to effects from the motion of the seas, accurate
depth determination when using Logging While Drilling services
is yet another challenge. On most drilling rigs, depth is
tracked with some fixed reference on the seafloor to be
used as a permanent datum. What was necessary on the JOIDES
Resolution was to be able to record accurate depth to within
fractions of a foot with over 3 miles of ocean between the
ship and the sea floor. A special software program was developed
which employs both tidal wave data and data from several
ship sensors to help achieve the depth accuracy needed to
interpret the measurements from the resistivity and nuclear
tools. This accuracy must be be at the "inch"
level to be of any real value.
The combination of physical core samples and wireline,
or LWD, logging data give the scientists a more complete
picture of what is happening in the earth's subsurface.
Sometimes core recovery is poor (maybe only 50% recovery),
and it is not always certain exactly to which depth the
core samples belong in the hole. It is under these circumstances
that the logging data can be used to correlate with the
physical core samples, putting the uncertain pieces on depth,
and giving insights into the nature of the missing core
samples. One of the more popular wireline logging tools
in use on the ship is a scanning tool which actually makes
an image of the inside of the borehole. It creates the picture
using microresistivity measurements with a vertical resolution
of .2 inches. Depth accuracy is very important for this
kind of measurement, as you can imagine that a measurement
with such precision would not be of much use if the scientists
did not know exactly where it came from. When the geological
strata have undergone significant changes (i.e. layers of
rock being folded, uplifted, or disturbed by tectonic processes)
it is important to know the orientation of the core sample
with respect to the hole, and this information can be provided
by correlation of the core sample to the scanned logging
image.
An example of the kind of interpretation possible came
from Leg 171b. The objective of the cruise was to test the
theory that a large asteroid had struck the Yucatan Peninsula,
throwing so much dust into the atmosphere that it changed
the climate and caused the death of the dinosaurs. Core
samples were taken from several holes off the coast of Florida.
Scientists analyzed the layers of sediment below the impact
and discovered that the sediments were rich in the skeletal
remains of tiny organisms. The layer of sediment known as
the KT Boundary (where the evidence of the impact was suspected
to be) contained elements that are typically found in asteroids;
thus when the asteroid struck the earth, it threw tons of
rock dust into the air, which inevitably ended up on the
ocean floor as sediment. The layers of sediment immediately
above the KT boundary indicated very little fossilized remains
of microorganisms, thus attesting to the millions of years'
lifelessness which followed the asteroid's collision with
the earth.
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