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Notes

11

Lithology and Porosity

Lithology/Porosity

Schlumberger 1999

Notes

The next major step in the procedure is lithology identification. Lithology data gives information on porosity and other parameters.This is an essential step as from the lithology comes porosity and other parameters.Lithology is generally split up into simple, dirty and complex. A simple lithology is one containing a single mineral. A dirty lithology includes shale and complex is a mixture or combination of mixtures.In nature the complex lithology is the one which is most common. However in a lot of evaluation the matrix can be considered as simple or at worst dirty.

22


Lithology General

Lithology of a formation can be:

Simple

Dirty

Complex

Odd minerals can also be present, such as micas in sandstones or anhydrites in carbonates which

complicate the problem further.

Notes

The lithology is a vital piece of information, both for interpretation purposes and for reservoir performance hence it is important to evaluate. Evaluation starts during the drilling process with cuttings analysis. This is then enhanced with logs and possible core data.

33


Lithology Determination

The lithology can be obtained in several ways:

From the cuttings (depth problems).From local knowledge (good during development).From the known depositional environment (good in general basis).From a log Quicklook (good starting point).From individual log readings (difficult if there are no areas of zero porosity).From crossplots (the best method).

Notes

The two key tools are the neutron and density. These form the basis of most analysis techniques including a number of computer based algorithms.Valuable additional data comes from the natural gamma ray spectroscopy especially in complex situations.

44


Lithology Tools

Most tools react to lithology - usually in conjunction with the porosity.

Major lithology tools are:

Neutron - reacts to fluid and matrix.Density - reacts to matrix and fluid.Sonic - reacts to a mixture of matrix and fluid, complicated by seeing only primary porosity.Spectroscopy - identifies shale types and special minerals.Geochemical logging, identifies 10 elements;

K, U, Th, Al, Si, Ca, S, Fe, Gd, TiFrom these the exact mineralogy can be computed.

Magnetic resonance is a porosity only tool; it does not react to the matrix.

Notes

Crossplots are an essential tool in log evaluation. They come in two varieties, a simple frequency plot of one log against another or a z-axis plot with a third in an effective z-axis.Virtually any log can be plotted against another.Additional control of the data plotted is made by using a cut-off, for example the calliper, eliminating bad hole sections which may mask the true formation readings.Zoning plots also helps fine tune the analysis.

55


Crossplots

Combines properties from both measurements, thus eliminating ambiguities.The most common crossplot is the Density Neutron.

Notes

In order to analyse a formation for lithology a model is created.

Taking a formation composed of a single mineral and a single fluid, water, in the pore space,equations can be written for the bulk density and neutron porosity. These have as an unknown the porosity, the other parameters are known. Hence this can be solved for the porosity.

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VolumeFormation model:Water-bearing, mono-mineral.

This formation can be described by the density tool and the neutron tool.

2 equations for 1 unknown:system is over-determined.

for limestone: Nma = 0for sand: Nma = 0.04

( ) += 1mamfb( ) += 1mamfn

Notes

The solution in graphical form is a straight line going from the matrix point ( porosity = 0 ) to the mud filtrate point ( porosity = 100% ). This line is sealed in porosity. Hence for any point falling on this line the porosity is known.

For example the point n = 20, b = 2.3, the porosity is 20%. The inputs to this equation ma and f and ma, and f have to be known for a correct solution.Any point falling on the line has to satisfy the inputs. If the line is the sandstone line, then a clean point landing on this line must be sandstone.

77


Crossplot Solution

The plot is a straight line from the matrix point to the 100% porosity, water point. It is scaled in porosity.

Notes

The lithology lines are guidelines ; it should not be assumed that all three minerals are present at all times. For example in a known shaly sand the only relevant line is the sandstone line, which in a carbonate the limestone and dolomite lines are used.

In shaly sands the only two minerals normally present (basic interpretation) are sandstone and shale. A clean water bearing point will fall on the sandstone line, the shale point will be down towards the bottom right of the plot. Any other point will fall between these two extremes and can be called shaly sand.

88


Neutron-density Crossplot

This crossplot has b plotted against the corrected neutron porosity.Fluid density in this plot is 1.0g/cm3.

Notes

The density neutron plot shown is one of a pair found in the chart books. The difference between the two is the fluid density ( salinity ) used; in one fresh water ( 0ppm ) with a f=1.0g/u, in the other salt saturated ( 250 kppm ) with a f=1.19g/cc.The plots represent the latest work in characterising the neutron tool measurements. The major change from older versions is the shape of the sandstone and dolomite lithology lines. Old charts had these incorrectly curved.

The difference between the two charts is seen most clearly on the dolomite line. This is due to the formation salinity effect on the neutron porosity measurement.

99


Neutron-Density Crossplot

This plot is the same as the previous one except that the fluid density here is 1.19 g/cm3.

Notes

1010


Dual Mineral model

= + V + V B mf m1 m1 m2 m2 = + V + V

N N mf m1 N m1 m2 N m2

1 = + V + Vm1 m2

3 unknown : , V , V , 3 equations

system is just determinedm1 m2

(Material Balance Equation)

The model now has a mixture of the two minerals and water. Each mineral has a volume Vm and the water occupies the porosity . The equations for the neutron porosity and bulk density describing this model are written in terms of the known quantities ( mf, m1, m2 ) and the unknowns Vm1, Vm2 and . A third, material balance equation has to be added in order to solve this system.

Notes

1111


Dual Mineral plot

The plot now has two lines, one from each matrix point

The equi-porosity lines join the lines, any point falling between can be assigned its porosity

the zero porosity line is scaled in ratio (or percent) of the two minerals. This can be extended to the water point. Points falling inside the lines can be subdivided in mineral percent

The graphical solution to the plot is a pair of straight lines going from the matrix points to the mud filtrate point. Each line is scaled in porosity. Equi-porosity lines join the two lines together. The line joining the two matrix points goes from 0% Mineral1 ( 100% Mineral 2 ) to 100% mineral1 ( 0% Mineral 2 ) This is scaled in mineral percentage. Each of the parallel Equi-porosity lines can be scaled in the same way. Lines could be drawn joining all these mineral percentage points and terminating at the mud filtrate point.

Notes

1212


Dual mineral plot expanded

This is the useful area of the plot. Any point on this plot lying between the two lines can be given a porosity and a lithology percentage. For example a point n = 25, b= 2.4 lands on the 20% Equi-porosity line about one third of the way along. Hence this point has a porosity of 20% and is composed of one third mineral2 and two thirds mineral 1.

This means that any mixture of minerals can be analysed for porosity and lithology as long as the two components are known. The usual case for this in a carbonate with mixtures of limestone and dolomite This mixture only very rarely has shale associated with it and hence this analysis can be used.

Notes

1313


Crossplot example

This is a typical frequency crossplot

The lines are the limestone, sandstone and dolomite lithology lines

The basic crossplot has two logs plotted on the x and y axes. There are frequently lithology lines that can be used to simplify the interpretation. Other zones, such as the shale zone, can be marked for convenience.

The example shows a frequency crossplot over a large interval in a well. The majority of the points line around the limestone line with some towards dolomite and some towards sandstone. The formation also contains light hydrocarbon. In order to properly analyse a plot of this complexity the interval has to be zoned into relevant sections starting with the water zone and progressing to the hydrocarbons. In the water there will be no hydrocarbon effect and the lithology can be well defined

Notes

This example z-axis plot is a neutron porosity plotted against the bulk density with a gamma ray in the z-axis. Colour is used to distinguish the changing gamma ray values, as shown by the scale on the side of the plot.If, as is normally the case, increasing gamma ray indicates increasing shaliness, this plot clearly shows the trend from the clean lithology line towards the shale zone.

1414


Z-axis Plot

Notes

1515


Other Crossplots

There are numerous other crossplots to identify minerals using combinations of tools.

ma - Umab - PeMID plot (n, b, t)MN plot (n, b, t)

The z -axis is used for clarification.

The number of possible crossplots is endless, any log can be plotted against any other or a mixture of other. These plots are the major lithology crossplots using the logs which react primarily to the mineralogy ( and porosity ) of the formation.

Notes

This is the simplest lithology plot. It combines the Pe with the b; since the former has only a small porosity dependence, the regions of different lithology. It is often used in conjunction with the neutron - density plot to clarify difficult interpretations.

1616


Pe - b Crossplot This plot is ideal to identify the lithology in conjunction with the neutron density plot.Note that the Pef does have a small porosity dependence.

Notes

1717


b - Pe Crossplot Example

A similar plot than the previous one , this simply has the measured bulk density plotted against the Pe. The latter has little porosity effect hence the lithology lines are nearly vertical. The different lithologies will plot on or near their respective lines. Unfortunately the Pe value for shale and other minerals is usually in the same range as the clean lithologies hence they are difficult to distinguish. ( Shale values range from 1.8 -6, Feldspar 2.8 -3.1 ). Some of the heavy minerals have a Pe value that is extremely high, hence a small amount does show ( e.g.. Zircon, Pe = 69! )

Notes

1818


ma - Uma crossplot

A number of minerals are presented in this view of the plot.

Crossplot uses a combination of the density, neutron and Pe curves. ma comes from the density-neutron crossplot and Uma from b.Pe. This version of the crossplot shows expected zones for a number of the heavier minerals. Note that some zones are very precise while others cover a large spread of values. This reflects the possible combinations of these substances.

Notes

1919


ma - Uma example 1

This is the more conventional view of this plot.

The example is plotted on the standard ( chartbook ) scales and includes the three major sedimentary rocks as well as the lighter minerals. The grid allows a computation of the relative value of each of the limestone, sandstone and dolomite. Some computer programs make use of this to produce a lithology description.

This plot is useful in mineral identification as the majority of clean points plot in the triangle, shale plots below, usually well defined leaving a lot of minerals clearly marked.

Notes

2020


ma - Uma example 2

The Pe is added in the z-axis to give an extra dimension.

Adding a Z axis to the plot can enhance the usefulness.

Notes

2121


Matrix Identification Plot

The Matrix Identification Plot uses neutron, density and sonic data as inputs. An apparent crossplot porosity is found on a density-neutron and a sonic neutron crossplot.

The values are entered into the relevant section of the following chart and the values of tmaa and maa read;

Notes

2222


MID plot

The MID, Matrix Identification Diagram, is an older type of lithology identification, using not the Pe but the sonic slowness in conjunction with the density and neutron. This type of plot is useful when the Pe is ruled out by barite in the mud,

The axes, maa and maa are obtained from auxiliary plots using the neutron porosity. This resultant plot has definite regions for each of the major minerals and some minor ones. Other minerals can be placed on the plot by computing their respective values of maa and tmaa.For example: Feldspar, ma = 2,52, n= -3, t =69.

Muscovite = 2.82. n= 22, t =55.

Notes

The MN plot is the oldest of the combination diagrams using, once again the bulk density , neutron porosity and sonic. M & N are calculated from the formulae

With the relevant values for t fluid and f fluid.

The major minerals are represented as well as the directions for shale, secondary porosity etc. Secondary mineral points can be computed as for the MID plot.

2323


MN plot

The MN plot uses data from the neutron, density and sonic logs to solve complex lithology.Used when Pef is not available or as extra information.

M = tf tb fN = nf nb f

Notes

2424


MN plotThe example shows a typical MN plot in a carbonate with the lithology points falling on and between the calcite and dolomite points. In addition some points also trend up in the direction expected of secondary porosity.

Notes

Geochemical logging with the large number of outputs can produce a large number of plots. Some of these are used in sandstone classification, other as shown help in identifying minerals in complex sandstones.

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Other Plots

The Geochemical logging tools produce a number of outputs that are best analysed using crossplots. The result is a detailed mineralogy.

Two plots are shown here.

Others are Al - Si, Al - Ca,Al - Fe, Ca - Si, K - Si.

Any of the measured elements can be put in the z-axis for trend spotting.

Notes

2626


Sonic Density Crossplot

Sonic-Density

Sonic slowness is plotted against the bulk density for both the Wyllie and Rayner- Gardner-Hunt equations. It is obvious that this chart is useless for the identification of the lithology and hence the computation of porosity as the lines are too close together. The addition of the two transforms adds considerably to the problem. However the crossplot can be used for the mineral identification and shale computations as all the clean lithologies plot together.

Notes

2727


Neutron Sonic Crossplot

Sonic slowness is plotted against neutron porosity for both transforms. There is a good separation between the lines of the major lithologies and hence this chart could be used to compute lithology and porosity. This is only possible, however if the transform has been correctly selected. For example the point n=20, t=65 falls on the dolomite line at =12.5% using the Wyllie equation but at 50% limestone / dolomite at = 16% if Raymer -Gardner - Hunt is chosen.

Notes

The geochemical method is very similar to that employed in core analysis except it is measured in situ. The result is a highly accurate porosity.

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other lithology porosity methods

The Geochemical logging tool measures GR yields which are transformed into element

weight percentages using an oxide closure model.

Si - main element in sandstones, also present in clays.

Ca - main element in limestones, also present in dolomite and in some feldspars.

Fe - present in clays and some heavy minerals.Al - present in all clays.

Using a knowledge of the mineral composition a mineralogy is obtained.This works best in sandstones.From the mineralogy (expressed in Volumes, e.g. Vlim) the precise grain density can be calculated.Then, using the density log, the porosity can be

computed.

Notes

The hydrocarbon effect causes the crossover effect seen on the neutron - density logs. The logs are no longer reading the correct porosity. When plotted it results in the effect shown. If the lithology is known the points can be corrected to give the true porosity.

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Hydrocarbon Effect

The presence of light hydrocarbons especially gas, in the invaded zone seriously affects the main porosity tools, the density and neutron.Both tools are calibrated to read correctly in water-filled rock.Light hydrocarbon has a lower hydrogen index, hence the neutron reads low and the low density of the fluid makes the density low.Points exhibiting this problem plot above and to the right of the lithology line on the crossplot.

Notes

The correction can be made using this chart. The density porosity involves knowing the lithology and hence ma. The value of Shr (=1-Sxo) has to be computed assuming the the 1 value is correct. In most cases this is close enough to give a good answer. This method assumes that the tools are reading in the invaded zone.

3030


Hydrocarbon CorrectionA line is drawn from the neutron porosity to the density porosity (computed using the relevant matrix value). This porosity, 1, is connected to the origin of the Shr, plot. The read here is subtracted from 1 to give the corrected porosity.

6 lthporo1

Documents

lithology data

complex lithology

simple lithology

notesthe lithology

dirty lithology

lithology identification

major lithology tools

neutron porosity