- So basic petrophysics. This may be a little bit too basic for some of you but let's just go through it very quickly. The gamma ray is used to identify clean and differentiate clean formation from shale volume, that's its principle application. It measure naturally occurring gamma ray activity which is produced by potassium, thorium, and uranium. The regular, non-spectral, gamma ray measures everything, and is the most commonly run. The problem with it is that if you've got uranium-rich charging of, say, carbonate which happens in, for example, the Niobrara, it'll look like a shale but it isn't. It's uranium charging of carbonates. So the spectral gamma ray separates and isolates potassium, thorium, and uranium which will give you then a much better response for shales, recognizing clay minerals. And to any of you that are involved in recommending logging sweeps, I would strongly, strongly recommend, if the budget will stand it, to run a spectral gamma ray. Here is a simplistic view of how to interpret gamma ray. First of all, this is a schematic of the shale, the shaley and the clean sand. The gamma ray response will be high API units in the shales because of the radioactive clay minerals in there. Whereas in the clean formation, the gamma ray response will be low because there is little, or no, clay minerals. And the interpretation goes that you as the interpreter choose the gamma ray shale and the gamma ray clean, and then convert into volume of shale using one or other of the two equations shown. The linear model just assumes a linear change, the Steiber model assumes a non-linear change, and the Steiber tends to be the one that I think most of us use. It is important to emphasize at this stage that is interpretive. You as the interpreter have to choose gammy ray clean, gamma ray shale. And depending upon your whim, you can make everything clean or everything shale. It's important aspect of petrophysics that it is extraordinarily interpretive. What I usually say is that there may be 35 unknowns that you have to solve, three of which are pretty well defined and you can't do too much so you have to guess the other 27. It's not guesswork, it's experience, but it is very interpretive. Two different interpreters will come up with very different results. The SP is sort of equivalent, it isn't, it's a totally different measurement. The SP log measures currents that are naturally flowing within the wellbore. It was one of the very first logs to be run by the Schlumberger brothers, and was perceived to be a permeability log because qualitatively, it seemed to deflect opposite sands. So it can be used to determine shale volume, as we said, identify permeable intervals, but that's really not its main application. It is the only log that you can get directly at formation water salinity which is real crucial. And here is a equivalent diagram. The SP is caused by disturbance if as you drill the well with an alien fluid in the mud. If the mud filtrate has a different salinity from the formation water, then you will get little cation exchanges going on, and you'll get a little current cell set up at the junction of the wellbore, the permeable cell in the sand, and the shale. And in that case, similar to the gamma ray, you as the identifier will identify a shale point in the shale zone, and then a clean point where you perceive that there is no clay mineral. And again, the standard transform to the shale is a linear transform. Realize, however, that if the mud filtrate has the same water salinity as does the formation, there will be no SP. And if the mud filtrate is more salty than the formation water, you'll get a positive SP, which happens in California. Resistivity is used to identify water saturation. You induce, or force currents into the formation and monitor their responses. And again, sort of referring back to the SP, if you've got a different formation water and you've got invasion, if there's physical invasion of the mud filtrate, you will get an invasion profile, which, again, can be used as qualitative indicator of permeability. It also can be used in unconventionals where you think that the locks are so tight there is no invasion. If you get an invasion profile, it can be a pretty powerful indicator of fractures, because obviously there's invasion along through the fractures. So the resistivity logs measure conductive material in the formation which can be water, both mobile and immobile, water associated with clay minerals, and pyrite. Finally, the porosity logs are logs that really don't measure porosity but that principle petrophysical data coming out of them is porosity. The acoustic log, or sonic log, which measures reciprocal porosity in microseconds per foot or microseconds per meter. The density log which measures the bulk density of the rock, it's a radioactive tool, it's bombarding the formation with gamma rays. And again, equations for both the density and the neutron log are given; you have to know or estimate matrix and fluid properties for both. And then the neutron log bombards the formation with high-energy neutrons which bounce around until they encounter something their same mass, which is hydrogen, and so they are really measuring water. And so this is a powerful application for them. In gas where there's less hydrogen than in oil, you'll get what is called density-neutron crossover, the neutron will read spuriously low. And then finally, the Pe measures photoelectric absorption cross section if used with other logs. Here are some work tools of petrophysicists. The cross logs, first of all, the density-neutron cross logs, and coded on there, porosity increasing from the bottom left to the upper right. And also coded on there, sandstone, limestone, and dolomite, the three principle lithologies in clean formation. Also, an arrow showing you what gas effect is. Gas will move data point to the upper left. And so you might think you are in, let's say, a sandstone but you're not, you're in a gassy limestone. From a petrophysical viewpoint, in terms of porosity, it doesn't matter that much because it moves parallel to isoporosity lines. The right hand graph shows the sonic neutron cross plot; similar kind of thing. A density-neutron is used much more for lithology recognition because it's a better lithology identifier, as we shall see in the next slide. Next slide is showing some matrix plots. You've now eliminated porosity from the equation as best you can. On the right-hand side is a Rho matrix density of the matrix and the sonic transit time of the matrix with anhydrite, dolomite, calcite, and quartz recognized. On the right-hand side, from the viewpoint of a geologist or petrophysicists working in lithologies, it's much more powerful. This is a Rho matrix-U matrix. U matrix is a combination of the density and the Pe log. And you can see a beautiful distinction between quartz, calcite, dolomite in the red, D-O-L is a little hard to read, and anhydrite. And so, if you're working carbonate, it's a really good indicator of lithology. However, be careful, because if you're running in a diorite mud, the Pe log is totally screwed up, and you'll get results that are essentially, unless you are smart enough to correct, essentially meaningless. We are also working on this plot on the right-hand side to look at clay minerals because clay mineral species have different U matrix-Rho matrix positions on that graph. Here is a basic raw data cross plot which is where we start with under panel one, gamma ray and resistivities and calipers and so on. In the second, which is a logarithmic scale, is resisitivity. And if we go down almost to the bottom, we can see that there is a low gamma ray, it's in fact, this is the Niobrara, its the first bench of the Niobrara, you can see an invasion profile, that's separation between the logs, which may indicate either fractures or some kind of permeability going on. And then, the next panels are DT and Pe curves, and then the final one, density, neutron, and density correction. Density correction is important; it's already in the density log, you don't have to put it in there again.