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Geology/petrophysics...-----SP Log


QUESTION: SIR please clear me the difference between isochore and isopach map?

ANSWER: Dear Muhammad Musa

Isochore maps are prepared with the drilled thickness data which means the thickness is not corrected with respect to the dip of the drilled strata. Isochore maps are usually greater thickness than true stratigraphic thickness.

Isopach maps are those maps prepared with true stratigraphic thickness either the data come from wells or from outcrop. Isopachs are dip corrected maps and it is the measure of the vertical thickness from top to bottom of the bed. Isopach  maps are usually lower thickness than isochore  thickness.

Hope, this may help.

Rating is expected


---------- FOLLOW-UP ----------

QUESTION: sir it means that if dip is added in straitigraphic thicknes then maps become isopach?I am not clear so for.

ANSWER: Dear Muhammad Musa,

Let me give you more insight about it;

An isopach is a line representing equal stratigraphic thickness, and an isopach (or
isopachous) map shows the variations in true stratigraphic thickness of a stratum, formation, or group of formations.

An isochore map shows  contours representing equal drilled thicknesses (or apparent thickness) of formations without regard to true stratigraphic thicknesses.

Understanding the difference of isochore and isopach take that example;

If you drilled a vertical well and encounter a 10 ft horizontal bed with zero degree dip  the isopach and isochore values will be the same (10 ft). If 10 ft  bed dips about forty five degree in either direction you need to drilled 14.1 ft. Now you will see that you have True thickness which was 10 ft and a measured thickness or apparent thickness which is 14.1 ft.

Now take an example of a slanted well with 45 degree well path; the same bed will be drilled with 45 degree well path angle which means this bed will be drilled from its top surface to bottom surface 14.1 ft.

Another example of the outcrop;  walk horizontally across  a bed in the direction vertical to the strike of the bed

You walk 600 ft
Dip of the bed is 20 degree
The true stratigraphic thickness will be 200 ft

Another example

you walk 500 ft.
Dip of the bed is 70 degree
The true stratigraphic thickness will be 470 ft

True Stratigraphic Thickness= sin of Dip X apparent thickness (or horizontal measurement)

Hopefully this may help.



---------- FOLLOW-UP ----------

QUESTION: sir i have started reading petrophysical log and i some confusion. i want to know the behavior of Sp log if:1-we have sandstone bed between two thick shale beds.2-if we are at invaded zone and formation fluid has more salinity then mud filtrate.3-if we have more salinity of mud filtrate then formation fluid?

Dear Mr Muhammad Musa,

Thanks for your question.

Please note that there are other factors as well which effect the SP values such as hydrocarbon content in bed, borehole diameter, Invasion, shale content in permeable bed and bed resistivity. SP needs to be corrected if any of the situation present. Also, keep in mind that SP is very useful log which not only provides quanlitative analysis about permeability, sedimentary facies and also used for  well to well correlation purposes but  also gives quantitative information about Resistivity of water (Rw)and Volume of Shale Vsh)in the beds of interest provided you know Resistivity of Mud filtrate, Bottom hole temperature, Static Spontaneous  Potential (SSP) and shale baseline.Rw and Vsh are important information to derive the Water stauration of the beds in order to determine oil and gas bearing zones.

You questions are responded as follows;

1) The quality of SP data in thin sandstone (less than 3 m)will effect the SP readings which read low and needs to be corrected for bed thickness. If the sand is thicker and homogenous SP will be very minutely affected even if it occurs between twi thick shale beds.

2) If the formation water is more saline means the readings of Resistivity should be lower so you will have Rw < Rmf,  than SP should deflect towards left (negative mV) or termed as normal SP.

3)In case of Rm<Rw or fresher formation water than drilling mud then SP should deflect towards right (positive mV) or termed as reverse SP However, deflection of SP in either direction means the travelling of ions.

Hope, you will not forget to RATE all my answers.




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Syed Tariq Hasany


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