By taking a quick glance at any well log a lot can be determined about the rock. Formations/Hydrocarbons provide different gamma ray, density, neutron and/or resistivity tool responses making it easy to get a detailed look into what is downhole. Typical Sandstone, Limestone and Dolomite formations
Sandstone
Pe = 1.81
Gamma Ray <20 API
NPor = DPor (SS Scale)
Limestone
Pe =5.08
Gamma Ray <20 API
NPor = DPor (LS Scale)
NPor > DPor by 6-9% (SS)
Dolomite
Pe = 3.14
Gamma Ray <20 API
NPor = DPor (DOL Scale)
NPor > DPor by 12-14% (LS)
HydroCarbon (Oil)
An oily formation can be spotted typically by an increase in resistivity over a watery formation with a pinching effect on the porosity curves. This pinching effect becomes more noticeable when the logs are plotted on the correct matrix for that formation.
Ex. A possible oil hydrocarbon zone in a sandstone formation, plotted using a sandstone matrix as shown by the PE of ~1.8.
HydroCarbon (Gas)
A gas formation can be spotted also with an increase in resistivity over a watery formation, but mainly due to the crossover shown by porosities. The crossover is due to Gas Effect, which supresses the neutron curve due to low hydrogen content.
Ex. Shows a possible gas hydrocarbon zone in a sandstone formation, plotted using a sandstone matrix as shown by the PE of ~1.8.
Coal
A coal, having a great amount of bound water, creates a distinct high porosity reading on both the neutron and density porisity curves. High resistivities are also seen due to this bound water. Expected PE for a coal is very low at only 0.2.
Anhydrite
Anhydrites are typically spotted using the porosity curves. As shown below in a sandstone matrix, the neutron reading hovers close to 0% pu while the density reads -15%. Being that anhydrite is that much denser than a Sandstone formation. PE for an anhydrite is typically close to 5.
Heavy Minerals (Barite)
Heavy minerals such as Barite area spotted mainly using the PE curve (Density tool). Being such a heavy mineral, it is very dense and has an extremely high PE of 267.
Shale
Shales are very noticeable when using both porosity curves as well as the resistivity curves. With no invasion present the resistivity curves overlay while the porosity curves spread apart.
The spreading of the porosity curves is due to Shale Effect, which exaggerates the neutron curve due to high hydrogen content of the clay itself.
Metal Downhole
Tools such as your induction can provide erratic responses when metal is found in the hole. Casing shavings could be a cause for this type of response.
To do more in depth calculations on a given formation, including porosity matrix adjustments as well as water saturation, calculators are available.
