May I suggest that you look at other analogous datasets with n > 25, e.g. the North Sea basin or Gulf of Mexico, before making some firm conclusions about whether Pareto or Lognormal works best. A lot of this information is in the public domain, one can browse the Websites of the UK DTI, Norwegian Petroleum Directorate, or the Danish Energy Agency for public domain field information. Certainly, at first blush, the log normal seems to make more sense than the other, you have your few giant fields (Brent, Statfjord, Ekofisk, etc), lots of middle sized fields, and many more small pimples in the North Sea that have yet to be developed.
Cheers Syed On Wednesday, August 31, 2005, at 11:33PM, Beatrice Mare-Jones <[EMAIL PROTECTED]> wrote: >Hello list > >I am a PhD student looking at developing a statistical model to predict >the size-distribution of an area's oil and gas fields. > >It is clear that previous investigators prefer either a Pareto power law >or a lognormal distribution to approximate field-size distributions. > >The data I am using does not look like it comes from a Pareto distribution >- which I explain as being a result of undersampling - which previous >investigators have reported - that undersampling occurs because the small >fields are not sampled or recoded. However by using basin-modelling >software to simulate oil and gas fields (for the same basin that my >discovered empirical data comes from) I notice that this sample is also >undersampled - that is fields under a certain size are not being simulated >- which is probably due to the resolution of my input data but what is >interesting is that the undersampling actually occurs throughout all the >size ranges - including the medium to larger sizes - which I would not >have expected. Like the discovery dataset (n = 25) the simulated dataset >(n = 140) looks like it is more from a lognormal distribution than a >Pareto distribution. > >My conclusion is that without being able to say that a Pareto is better >than a lognormal and vise-versa it appears only logical to use both >distributions. > >Geologically there does not seems to be a reason why a modal size (greater >than what is detectable by exploration methods) of fields should exist - >which would be the case if the data was from a lognormal distribution - >except if the distribution is highly right skewed (at the small field >size) and the mode is actually just below the detection of size. > >Geologically there does seem reason for fields to become so small that >they become entities (that trap oil and gas) - and this relationship may >be better approximated by a Pareto. > > >The Pareto and lognormal form is similar but maybe one is better to >approximate field sizes than the other. >My question is do you think a Pareto distribution better approximates an >oil and gas size distribution than a lognormal (or vise-versa) and if so >why. > > >I am currently working on goodness of fit test to throw some more light on >this - but if anyone has any thing to say I'd appreciate some comments. > >Thank you, > >Kind regards > >Beatrice > >Geological and Nuclear Sciences >New Zealand >www.gns.cri.nz > > >* By using the ai-geostats mailing list you agree to follow its rules >( see http://www.ai-geostats.org/help_ai-geostats.htm ) > >* To unsubscribe to ai-geostats, send the following in the subject or in the >body (plain text format) of an email message to [EMAIL PROTECTED] > >Signoff ai-geostats >
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