August 31, 2014 - Nom Geo
Shallow Permian Petrophysical logs – putting it all together!
From the previous posts we were manipulating log data to contrast different zones in shallow permian stratigraphy.
We can now attempt to interpret the different zones and how this changes at each depth.
Beginning with the gamma and density signal, a coal seam is present at 252 to 254 m while at ~282 m there is a carbonaceous horizon of coaly shale. At 270 .4 m there is a slight dip in gamma with a small spike in density.
However, there is a marked contrast in resistivity logs where three distinct depth zones can be seen:
– The coal seam at 252 – 254 m
– A zone from 254 to 270 m showing a gap between the borehole wall resistivity and the deeper formation resistivities, possibly a "water zone"
– A zone below 270 m where there is almost no gap between the values of the micro- shallow and deep resistivity log readings.
This infers a change of overall porosity and / or formation water at the 270 m depth.
The initial small pore pressure spike at 270.4 m could be representative of a structuralyinduced pressure cap at the contact between two hydro-geologic zones.
At this 270.4 m pressure spike, taking the negative gamma and neutron spikes, the slight spike in resistivity can often be taken to infer a potential zone of partial gas saturation, as shown below.
This leads one to speculate whether there is a low porosity horizon of low gamma clay trapping a minor amount of gas at this horizon
If one dismisses the change in Rwa being attributable to a change in formation water salinity then the alternative scenario is that the water saturation is less than 100%. Note that gas saturation is ; Sg = 1- Sw
If we assume that there is a water saturation Sw less than 1 but that Rwa is the same throughout the greater zone then we can calculate the Swa value using Archie's law. Our first assumption is that the formation Rwa is the same as the adjacent zone at 0.37 ohm m (result from the earlier post about using Archie's law).
a = 1, n = 2.15, Rwa = 0.37 ohm m (taken from the 3 m horizon below this zone, sonic porosity = 16 % and Rt = 24 ohm m.
and the likely gas saturation is = (100-89%) = 11%.
Realistically we have to also ponder the possibility that the water salinity could be the same all the way down to 279 m. If we then consider the lower Rwa of the lower zone from 274 m to 279 m at 0.13 ohm m. In this scenario and adjusting the equation to replace the Rwa with 0.13 ohm m value then the water saturation drops further to 55%, and our gas saturation is correspondingly higher at 45%.
It is more realistic to input an averaged Rwa value which will change the apparent gas saturation to a value which is intermediate between 11% and 45%.
To review the evidence for the small zone at 270.4 m;
- The drop in Neutron porosity is either due to a partial gas saturation or a drop in porosity!
- The small downwards spike in density porosity which is actually the small spike in density could be due to a denser clay species or compaction. I also like the possibility that it could be a lateritic zone of Fe accretion consistent with a terrigenous coastal facies, and that the densitiy spike is a thin layer containing goethite or siderite.
- The increase in pore pressure using the d' exponent method could be due to: gas, compaction of the clay or it could also be due to a higher density type of clay species such as chlorite or due to a lateritic accretion, which would similarly affect the rate of penetration and could be mis-interpreted as a pore pressure spike!
- The resistivity of the formation has a minor peak which can either be a partial gas saturation, or it could be a porosity decrease.
- The sonic porosity does not show a change in porosity at that point.
- At this stage the issue could be assisted by the inclusion of the gas ratio data, for example the "Pason Gas Ratio Log" which could resolve the duality of the problem.
- In reality there is also no structural data or lithology data and hence the deliberations are constrained to two options of either a gaseous zone, or a zone of low porosity and / or denser lithology.
Finally one must assume that there could be no gas saturation without a low porosity, clay cap so it appears that a low porosity horizon must be present, the only question being, how much, if any gas is present?
- Beginning at 291 m to 280 m where gamma, resistivity and SP logs suggest that shallow marine deposits with higher salinity connate waters are occuring and are gradually transitioning to deltaic or even a barrier front lagoonal deposits.
- At around 270 m this is interrupted by a thin horizon of dense, low potassium clay. I have already speculated that it may be partially saturated gas at around 270.4 m. We can speculate that it is due to a change of clay species which has a higher density, or a clay filled fracture. It may also coincide with a change in lithofacies as suggested by the MRRS resistivity log
- However the SP log suggests that the fining-up coastal facies does in fact continue up until at least the 266 m depth horizon above which there is a distinct change, most likely to a fluvial lacustrine depositional environment with alternating clay-sandy horizons, possibly of a fluvial facies.
- From 264 m we see a coarsening trend up to the 258 m depth which subsequently fines up and culminates in a coal seam at around 254 m to 252 m.
- In the 15 m below the coal seam we can assume that the zone of filtrate invasion has a higher salinity than the formation. It does suggest change in the salinity of connate formation fluid at 270 m.
- A further point to consider is whether the zone 270 m up to 254 m has been influenced by either acidic water leaching from the overlying pre-coal peat deposition, or a facies change resulting in higher porosity?
Note that minor gas occurences can occur in many sedimentary settings and particularly in the vicinity of coal seams and that possibility cannot be discounted.
Also If one takes all of the logs and the resistivity logs it does strongly suggest that different water salinity zones are present.
The take home lesson here is that the most important phase of the interpretation is resolving the correct parameters to try to reduce the number of alternative scenarios. This can be done by analysing more types of log data, actual samples and cores and conducting drill stem tests for gas and permeability in the zone of interest.
I would appreciate any feedback.
I am now going to move to new data sets to see if I can improve on my application of these interpretative methods.