August 28, 2014 - Nom Geo
SP log – quick aside: Interpretation
In discussing the SP log I am going to refer you to a publication which has an excellent section on the Spontaneous Potential log. It is "Well Logging For Earth Scientists" By Ellis and Singer and I highly recommend that you buy or borrow a copy.
So on to our sample well.
The SP log being looked is shown below: Note that this is a horizontally exaggerated view of the log.
The deflections to the right indicate where the membrane potential is greatest, that is the Na+ ion is preferentially mobilised through shale and the Cl- ion is preferentially static and does not cross the shale membrane barrier. Hence the negative kicks represent a lowering of the membrane effect and so infers one of two possibilities.
1) A change of the relative salinity between the formation fluid Rw and the borehole mud Rmf.
2) An increase in the permeability of the formation, usually representative of a coarsening facies thus allowing a generalised facies interpretation to be made.
Because it makes sense to travel forward in time, the curve is analysed upwards starting from the lowest depth.
A typical interpretation might look like this;
There is a need to consider first if the SP curve is not being affected by changes in salinity. As we have already discussed apparent formation water resistivity Rwa and equivalent NaCl salinity, the SP log is juxtaposed onto the Rwa results from my earlier post.
The Rwa curve clearly shows that while the SP log curve may at least be partially influenced by changes in Rwa, the movements in the SP curve are best explained by changes in sedimentary facies. Of interest is the fining up sequence between 274 m and 266 m which could be a point bar which has fined-up from a base of clean sand. It could equally be a regressive regime where a delta plain has advanced over beach deposits. The transition immediately above this at 266 to 4264 m suggests a change of facies such as a a lacustrine environment. If this is the case then the later clean sands above this zone which fine up to the coal seam at 254 m could be the result of fluvial plain deposits, gradually fining up to a regional marshes which resulted in the peat deposits which formed the coal seam. This is quite consistent. Unfortunately the SP curve is not a replacement for actual samples of the lithology so there are other plausible scenarios for this zone, for example it could be a marine transgressive event in a back barrier lagoonal setting, eventually returning to the regressive trend and fining up sharply towards the 255 m depth mark which is consistent of the development of a coastal plain where extensive marshes resulted in the deposition of the peat which preceeded its formation into coal. Below shows a neutron porosity curve juxtaposed onto our chart. The neutron is scaled in reverse order and while coal seams are easily resolved, much closer analysis would be required to allow a facies correlation from this log.
Note that there is a negative neutron porosity blip at 270.4 m which means that there is a reduction in H+ ion concentrations, for example a partially gas saturated zone.
What is really required in facies log identfication is to find a way of correlating with another parameter to support this interpretation. The ideal is lithology, however a density porosity curve can serve as an quick analogue.
The really interesting point of the density porosity vs Neutron porosity is that gas zones can be detected. Note that the neutron porosity is by convention shown with a reverse scale so that the density:neutron porosity cross-overs can be more easily identified.However in this interval the only horizons which are known to contain gas are the coal seams where Neutron Porosity actually increases rather than decreasing presumably due to the compressed and minor volume of the adsorbed gas relative to water in the coal (at 252 – 254 m). So we can conclude that many of these empirically derived petrophysical relationships between logs are not applicable to coal which has properties and log responses which are markedly distinct from other sedimentary facies.
The zone just below 270 m is prospective for a occurence of higher Rwa and there could be a structural reason for it occupying this horizon. Interestingly the zone at 282 m has another low density zone which may be fossiliferous and carbonaceous, but too high in mineral content to be considered coal.
What I can conclude is that some of these log interpretation methods work best in marine successions and are useful for water salinity and some understanding of facies relationships.
I am going to leave it there and move on ot a new field as I want to cut my teeth on deeper marine formations. Next time I am going to feature a new set of logs from an offshore basin. Lets see if we can find some oil and gas!