Peter Fitch
Making wiggles - life with the physical properties of deep sea sediments!
Peter Fitch
Physical Properties & Downhole Tools Specialist
I am currently a 4th year PhD student at the University of Leicester. My PhD research is investigating the variability (or heterogeneities) in the rock properties (petrophysics) of limestone oil and gas reservoirs, and in turn relating this to fluid flow and storage properties.
Sailing as a physical properties specialist on IODP Expedition 320 has provided me with a fantastic opportunity to investigate rock properties from a completely different geological setting, and also to be involved in the actual measurement and initial processing of this data - not to mention living on a ship in the middle of the Pacific for eight weeks.
Life on ship has become strangely normal: rarely walking in straight lines, greeting people with "good morning" throughout the day as different shifts begin, eating breakfast at midnight, the sound of "core on deck!"...
My job on the JOIDES Resolution is as a physical properties specialist - we drill a borehole into the ocean floor and then various measurements are made on the sediments (core) which we bring back up onto the ship. When the core arrives on deck we run it through a series of scanners that measure the following properties:
- Bulk density of the sediment: a function of the rock type/mineralogy, pore space and fluid content.
- Magnetic susceptibility: the presence and amount of magnetic grains.
- Sound wave velocity through the sediment: generally the more compact, rigid, and dense the sediment then the faster sound waves can pass through it.
- Resistivity: how well the rock, and any fluid present, can conduct electrical currents.
- Gamma radiation - the decay of naturally occurring radioactive particles within the sediment depends upon the lithology and mineralogy (typically the presence of mudstone with uranium, thorium and potassium particles).
Once the cores have been run through the various scanners we then measure the thermal conductivity, which can be used to investigate heat flow in the sediments, by comparing them with temperatures measured down in the borehole itself.
Peter Fitch making p-wave velocity measurements.
Next the core sections are cut in half: one half goes to the sedimentologists and palaeomagnetists for further measurements, detailed descriptions and archiving, and the other half is used for discrete sampling. Along with the geochemists and palaeontologists we take one sample per 1.5 metre section. Our sample is then weighed, dried for 24 hours at 105oC, re-weighed and then put into a pycnometer to measure the volume.
From these measurements we can calculate various rock properties such as the grain density (indicative of mineralogy) and porosity. Weighing a sample at sea is not an easy task, because with every rise and fall of the boat, the weight will change slightly (sometimes up to 30%!). To combat this we have similar equipment to that used in space research where these forces can be measured and compensated for.
Before the working half of the core is sealed away we measure discrete sound wave velocity, or compressional P-wave, in 3 directions (the x-, y-, and z- axes) by inserting sets of transducers and contact probes into the core. This allows us to investigate how properties change in different directions through the core (known as anisotropy).
Scanner equipment known as the 'Whole Round Multi-Sensor Logger' which measures non-contact resistivity, P-wave velocity, magnetic susceptibility, bulk density and natural gamma radiation.
Once all these measurements have been completed for each hole, we then check the data quality and filter where necessary. When a site is completed (normally three holes per site) we can then download the data and make graphs and plots (or wiggles!) of the various data types.
This is when we can compare our discrete measurements with the scanner measurements, allowing us to confirm and calibrate the different data types. The last task for each site is to write a site report detailing methods, data trends and any initial findings in relation to the other shipboard research groups.
So we are all kept fairly busy through our 12 hour shifts! It is really amazing how much data we have collected over the past seven weeks, and completing five reports in the same time... imagine if all research could occur at similar rates!
Core with moisture and density samples about to be taken.
On ship, our core measurements of physical properties are used to help calibrate the well logs measurements (devices run down the borehole itself), and are used by the stratigraphic correlators to check we are drilling and coring the correct depths, so that adjustments can be made in the second and third holes drilled at the site to ensure complete coverage (the overarching aim of the expedition is to recover a complete section through these Cenozoic sediments to allow for detailed investigations of climate change).
Our physical property measurements also provide information which can be integrated with the sedimentologists core descriptions to aid our understanding of changes in the sediment composition, porosity and also geochemistry. Most of the sediments that we are seeing on this expedition are carbonate-rich ooze, radiolarian-dominated ooze and clays.
These three lithologies have specific properties which we can be identified and used to track them down the borehole, across sharp and gradational boundaries. For example, the density measurements can be combined with geochemical analysis of calcium carbonate content to interpret the rates at which this sediment was deposited through time, which is dependant on a range of factors including the productivity of the oceanic micro-beasties and the amount of dissolution at the sea floor!
My PhD has used lots of well log and core data provided by my industrial sponsor, so to have been part of this team taking measurements and seeing the actual sediments these values come from has been a real eye opener, which I hope will feed directly into my current research.
As well as providing a great opportunity to investigate different sediment types with different physical properties, sailing on the JOIDES Resolution has provided a fascinating opportunity to discuss science findings with a very diverse group of geologists, palaeoceanographers and technicians, allowing me to explore where my current knowledge and understanding can fit into this massive and important research topic of climate change, which is quite different to my normal oil and gas exploration focus!
Posted on 5 May 2009
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