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2005-12-01

Boring coring.


George wants to see some core coming out of the ground. Well, no names, no pack-drill, but here's a few pix from a recent coring job. (When I've finished unpacking them and cutting off the identifying marks.)
Coring is a pretty uncommon activity these days - the only time it's really necessary is to get undamaged samples of a reservoir for porosity/ permeability measurement, and even then the results are somewhat suspect because the core is typically flushed through with liquid filtrate from the drilling mud, and that can damage the porosity of the rock sample.
An interesting job I was on a few years ago was where the client wanted to compare the vertical and horizontal permeability of the reservoir. Here, even if there were porosity damage, it shouldn't differentially affect the vertical versus horizontal properties. Has that file finished extracting yet? Nope.

Half an hour later ... well actually the photos I took on that coring job were much more boring than I'd remembered. Only one worth bothering with really, and none of the actual coring operation (unless you want photos documenting that the core barrels had sub-standard orientation markings - didn't think so).
The photo is of the cut end of a section of core, which I photographed in the driving rain of a wet and mosquito-ridden evening. The blade used to cut the core has cut more-or less straight and planar, so the wiggling of the fine beds of dark material indicates that there is irregular bedding here. In the NE of the image you can see that one of the beds is thinning quite substantially over a small distance. (You can also see most of a centimeter scale bar). The saw nick in the NW of the image is where we broke off a chip for microscopic examination.
Then we wire-clipped a cap onto the end of the fibre-glass barrel, nailed it onto a box with it's neighbours, and packed it onto the back of a truck to go to the core analysis lab, never to be seen again (at least, not by me).

2005-11-28

Day in the (death) of a worm

Click for full-size image?
Just been digging around in the rock pile, and I found this rock which I'd recently described in a USENET post:

The cohesiveness of consolidated mud would 
provide a comparatively resistant surface over 
which coarser-grained sediments could be 
transported as a package. 
Actually, thinking about it, I've a nice specimen 
in my rock pile showing a similar event from 
100-odd million years ago. Mid-jurassic of the 
Isle of Wight, there was a muddy sea-bottom 
with a rich fauna of burrowing worms chomping 
away buried in their nice muddy burrows. Then 
along came a flood of silty sand which buried 
and killed off the worms leaving a series of 
sandy casts of the top ends of the burrows on 
the *under* surface of a circa-2cm thick sand 
bed.
One hundred million years later along comes a 
superannuated hippie of a geology student who 
extracts said thin sand bed from a 10-odd metre 
high cliff of similarly interbedded muds and 
sands, and carefully appreciates the day in 
the death of these worms before spending 
hours in the pub (and back in the honours 
year student labs) carefully picking out the 
bits of clay from between the casts of the 
burrows.

"AA" size cell and a ruler to give scale objects. The autofocus of the camera seems to be slipping, and there's no manual alternative. I'm thinking that I can almost justify getting a new camera.

2005-11-29
Carsten described these as "escape structures". I don't think so - I think they're burrows which the worms (lugworm or equivalent) died in after getting a belly full of sand. The underlying mudstone bed contains sub-vertically oriented sand tubes dispersed through the mud, representing the worms which swallowed a sand meal, stopped eating, swallowed (separating the swallowed sand from the continuous sheet), then died.
Please note that this photograph is of the UNDERSIDE of the slab as originally found, in a normally oriented sediment sequence.

2005-03-22

Silverpit structure

There has been a bit of discussion lately on the interpretation of the "Silverpit" structure in the Southern North Sea.
A couple of years ago a paper was published in Nature about a circular set of faults surrounding a sub-surface depression in the top-Cretaceous reflector. The interpretation was that this was an impact structure. A summary of the paper can be found at the Geological Society website (if you're really interested, I can email you the paper). Well, there have been alternative interpretations published over the last few months, re-interpreting it as a sagging structure caused by withdrawl of Zechstein salt at depth. John Underhill (of Edinburgh University) has presented data orthogonal to the original section (link above), which does put a very different interpretation on the structure.
It still doesn't look right though - the "central peak" structure in the original paper is still a very anomalous structure, and Underhill's interpretation of thickness variations in the overburden to the salt is inconsistent - the Cretaceous thins in one direction and the Jurassic in another, combining to give the appearence of synsedimentary halokinesis. It still looks odd to me.
I am very surprised at a seismologist of Phil Allen's experience not looking out-of-plane though.
(In other comments since I first wrote this, Mr Allen claims he did, but disagrees with Underhill's interpretation.)