Re: paleonet Sieving of ditch cuttings samples

[Aidan Karley <aidan_karley@xxxxxxx>]

In article <E1CSAhi-00057H-00@newt.nhm.ac.uk>, Paleonet Digest wrote:
> I am about to process some ditch cuttings samples for palynology.  It 
> is usual to dry-sieve samples, prior to their chemical maceration, to 
> remove the larger pieces as these are likely to be caved.  Can anyone 
> recommend which size is optimal for this first sieving?
>
       That you're having to dry the samples suggests that it's from a 
*very* recent well. Have you actually seen the cuttings in question? The 
last decade or so worth of cuttings samples that I've either caught 
personally at the wellsite, or when I've been supervising mudloggers in 
their bulk sample catching and producing the well log. The large 
majority of UKCS wells these days are drilled with PDC bits, and the 
typical "cutting" is actually a low-grade cataclastite. (Traces of 
glazed mylonite, verging on the pseudotachylite are by no means 
unknown!)
       Routinely I work with on-site palynologists (and their 
palaeo-prep technicians), and I've never noticed them picking out any 
cavings, because they're very difficult to distinguish. I remember 
seeing an OU video a few years ago with a "talking head" from Robrtson's 
Research showing you how to pick the bits of black caved shale from the 
nice white Chalk cuttings. I burst into laughter at how inappropriate it 
was. Thinking back on it, if the sample in question wasn't "prepared" 
for the show, then I would diagnose that they'd got a quite serious 
problem with shale caving (and therefore, with general wellbore 
stability). If I'd seen that coming up on the well my concern (and my 
responsibility to report these concerns) would be with the stability of 
the wellbore, the possibility of losing the string and the well, and 
potentially going into a pressure control situation.
       I'll reply to a (consultant) palynologist of my acquaintance 
off-list, but his email might be dead.
       
       Short answer - in my experience, people filter the cavings at the 
identification stage, not at the sample preparation stage. Presumably it 
is more time-efficient. (In these circumstances, an answer is typically 
required ASAP, and spending an hour manually picking "caved" from 
"fresh" cuttings, if it could be done, would be unacceptable.

-- 
 Aidan Karley,
 Aberdeen, Scotland,
 Location: 57°10'11" N,  02°08'43"  W (sub-tropical Aberdeen), 0.021233