Introduction to Basin modelling with PetroMod™
Mark Wilkinson, University of Edinburgh
Hydrocarbons and Geophysical Exploration, EASC10093
AIMS:
To gain hands-on experience of modern basin modelling software
Test prameters that control maturity in a source rock
Understand and calculate the Time-Temperature Index (TTI) of a source rock
Thanks to Schlumberger for allowing the free use of the PetroMod™ software and
the UKOilandGasData database for teaching
Introduction
Basin models are used in the petroleum industry to answer
questions such as:
 Are source rocks mature?
 How much oil / gas has been generated?
 When did traps form in relation to oil migration?
 How much risk is associated with drilling a
prospect?
We will look at modelling a single well, a so-called 1D
model. The well was drilled in the Magnus Field, in the
Northern North Sea, see map (Wikipedia). More info here:
http://archives.datapages.com/data/specpubs/fieldst3/data/
a016/a016/0001/0050/0095.htm (Shepherd et al., 1990,
AAPG Special Vol 16, p. 95 – 125).
Getting Started
From Learn, download the zipped folder ‘PetroMod_practical’ and copy and paste the
contents somewhere easy to find, we suggest at the top of a drive, e.g your M:drive.
You should have:
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PetroMod_prac (folder)
Comp_log_211_12-1.tiff (source: UKOilandGasData.com)
Locate PetroMod™ (click on the ‘Windows’ flag symbol lower right and type ‘Petro’
into the ‘Search programs…’ box) and launch the software. This opens a window
called the ‘PetroMod Command Menu’.
At the bottom of the window is a button labelled ‘Project Dir’, click this and locate the
unzipped ‘PetroMod_prac’ folder (use the drop down menu labelled with a down
arrow to choose the correct drive on the computer).
Across the top is a series of ‘tabs’, click the ‘1D’ tab followed by the green box /
arrow, or select ‘1D’ from the menu bar and the ‘PetroMod 1D’ option.
This opens the 1D version of PetroMod™ which is used for modelling the burial
history of a single well.
Under ‘File’ select ‘open 1D-model’, and locate the ‘PetroMod_Prac’ folder. Open
this.
The window that opens is the main input area for stratigraphic data. To make things
easier for you, we’ve already input the major stratigraphic units, and some of the
depths.
To get data to complete the table, we will use the ‘Composite’ log for well 211/12-1,
which is a summary of the well:
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locate and launch the programme ‘Irfanview’ [the default Windows Photo
Viewer opens a non-scrollable window, which is a real pain].
open the file ‘Comp_log_211_12-1’ (may be slow)
the top of the log shows information such as where the well is located
In what year was the well drilled? ……………………………………………………
How long did it take to drill? ………………………………………………………………
Look at the section labelled ‘R.T.E.’ (Rotary Table Elevation, i.e. the height of the drill
floor of the rig)– ‘A.M.S.L.’ stands for ‘Above Mean Sea Level’ and is the height of the
drill floor. how high is the rig above the sea? …………………………………………
This is useful as all the depths are measured relative to this.
What do you think that ‘A.S.B.’ stands for? If you can’t guess, look at the log to see
what is at this depth (remember depths are measured from the drill floor, not sea
level). ……………………………………………………………………………………
How deep is the sea here?
………………………………………………………………………………………………
How deep is the well? ………………………………………………………………
Next, look at the Key for lithologies. Sketch the patterns for:
Limestone
Shale
Sandstone
These patterns are used in the column labelled ‘Litholog’ as a summary of the rock
types that were drilled through. This is worked out from the ‘wiggly’ lines plotted
along the log – we will learn about these later in the course.
The depth column is in metres (they are often in feet!). Scroll down to the top of the
Cretaceous (1624m) – what sediments represent the late Cretaceous? Is this what
you might have expected based on your knowledge of UK stratigraphy?
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Scroll down to 2923 – 2985m. This is the reservoir section – the vertical thick black
line shows where core was taken. What lithology is the reservoir and how thick is it ?
(you can get lithology from the patterned ‘Litholog’ column, but also from the notes on
the right hand side – these were written by a mud logger from cuttings).
…………………………………………………………………………………………………
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From the well log, what age is the reservoir?
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What length of core was cut? What % of the well is this (down to the base of the
core?)
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Back to the well model:
The PetroMod™ programme works by splitting the stratigaphy of the well into units
(‘Layers’), each of known thickness and age, each of which gets a line in the input
spreadsheet. To speed things up, we’ve completed the ‘Layer’ column of the
spreadsheet with the names (ages) of the stratigraphy. For each layer, you must
input:
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the age of the base of the layer – from the Table of geological ages below
the depth of the top of the layer – from the well log *** note the depths on the
well log are measured from the drill platform which is above sea level, we
should correct then to depths relative to sea level. We won’t bother, it makes
very little difference to the results.
The boxes that need filling are helpfully shaded pink - the others (grey) will fill
themselves in for you!
If a box becomes yellow then you’ve made a mistake.
Important – you fill in the depth of the TOP of each
layer – but the age of the BASE.
Harland et al. (1989, A Geological Timescale)
To simplify things, we make a few assumptions / simplifications:
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The reservoir interval is Kimmeridgian (all of it – conveniently from start to
finish)
The Coniacian / Santonian strata is again all of the time interval
There is a single unconformity, at 2923m depth - many well logs clearly show these
using the standard ‘wavy line’ symbol – this one doesn’t. Here we will assume that
the unconformity (UC1 in the ‘Layer’ column) is exactly base Tithonian to base
Coniacian
The unconformity layer (‘UC1’) is filled in for you as they are easy to get wrong, and
then the programme will not run. How much erosion has been set for the
unconformity, and how much missing time is there?
…………………………………………………………………………………………………
Lithologies of stratigraphic layers
You must tell the programme the lithology of each stratigraphic unit using the
‘Lithology’ column. PetroMod™ has a wide variety of ready-made lithologies. Only 1
lithology can go in for each unit, so a bit of approximation is needed. The Holocene is
filled in already [‘Shale (typical)’] – use the well log to choose a suitable lithology
for each stratigraphic layer then right-click over each box and select ‘Modify cell(s)’ –
a menu of rock types appears.
You can speed things up if you have cells with identical lithologies by clicking on a
box and dragging the black square in the lower right side (as in Excel).
Running the model
Run the model by clicking on the ‘running’ green man icon on the top line. It will take
a few minutes – don’t panic if nothing seems to happen initially, soon text will scroll
down the window as the programme calculates.
To view the results, use the menu on the leftside. Click on ‘Output’, then check the
box for ‘Default burial plot’ under ‘Burial plots’. This shows how the reservoir got to
its present day depth, from the time of sedimentation to the present day.
Is the burial even? How many times does the basin get rapidly buried?
…………………………………………………………………………………………………
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Now check the box for ‘Porosity’ under ‘Burial plots’. The graph can be moved
around and expanded.
What is the porosity of the reservoir (sand) initially? Hover the cursor over the sand,
and the porosity value appears as text in the lower right corner of the window.
…………………………………………………………………………………………………
What is the predicted porosity of the reservoir at present day? Is this sensible?
…………………………………………………………………………………………………
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What is the predicted porosity of the overlying shale at present day? Is this sensible?
Compare to the figure of porosity versus depth for shales, from the U.S. Geological
Survey Open-File Report 03-329 by Rowan et al. (2003)
…………………………………………………………………………………………………
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Now check the box for ‘Vitrinite Reflectance’ under ‘Depth plots’. What is the
predicted vitrinite reflectance (Ro) of the reservoir?
……………………………………………………………………………………
Analysis of the petroleum in Mangus suggests it came from a Kimmeridge Clay
source with Ro = 0.7 (Shepherd et al., 1990). Why might your value be different –
would you expect a higher or lower number for the reservoir versus the source?
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Varying the model 1 – Water depth
On the left hand menu, select the ‘Input’ menu, and check the box “Boundary
Conditions”. 3 graphs appear (unchecking the box “Main Input” makes more space).
The ‘Paleo Water depth’ plot shows the water depth through time. You can edit this in
the Table (PWD column) – put in your best guess for the depth evolution of the North
Sea, perhaps include:
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The reservoir is deep marine sandstones - potentially 100’s m or even
deeper?
The Cretaceous was a time of high sea levels – hence the Chalk (elsewhere!)
Parts of the North Sea were land during the Paleogene / Neogene
***The depth of the top of the ‘Younger than Pliocene’ layer (in the Input table)
MUST match the revised water depth at present day (i.e. the water must sit on top of
the sediments, with no gap or everlap)! Hence it is input ready for you, do not
edit.
Varying the model 2 – heat flow
An important parameter in basin modelling is the basal heat-flow, i.e. the heat flux
through the sediments, ultimately sourced from radioactive decay in the mantle. This
value depends upon parameters such as the thickness of the crust (thinner crust =
higher heat flow) and so is affected by rifting events such as the one that formed the
North Sea. The model for heat flow can be edited in the same way as the water
depths, but better is to use the model calculator, which is based on a numerical
model by Dan McKenzie of Cambridge:
Click on the button “McKenzie HF…” (the button is below the top menu bar, left of
centre). A window appears with a graph of heat flow through time.
What are the units of heat flow? …………………………………………………………
You can then alter the rifting event that resulted in the deepening of the North Sea –
only alter the TIMING of the rifting for now - to see what you’ve done to the heat flow
click ‘Preview’, and to paste the results into the main model click ‘Start’. Either re-run
the model, or click ‘Reset to defaults’ to get back to where you started.
Does the rifting event affect the early or late part of the burial history?
……………………………………………………………………………………
If you re-run the model now, you should find that the vitrinite reflectance (Ro) has
changed – is it a large or small change?
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The heat flow model affects the maturity of the source rocks in the area – high heat
flow = high maturity = more hydrocarbons (up to a point!). The heat flow can be
calibrated using vitrinite reflectance data (Ro) of samples of fossil wood from core or
cuttings at various depths along the borehole, if we are lucky enough to get some….
Source rock maturity- the TTI
PetroMod™ can calculate the maturity of a source rock – we will use the TimeTemperasture Index (TTI) measure of maturity.
Oil and gas are assumed to be generated from buried organic matter, due to the
effects of temperature acting over geological time. Pressure isn’t important as a
driver (in fact pressure retards gas generation – a common exam error). The rate of
reaction is assumed to be linear with respect to time (diagram below, left), and
exponential with respect to temperature (diagram below, right). This was first
described in the USSR by N.V. Lapotin (1971) and introduced to the West by
Douglas Waples (AAPG, 1980, 64, 916-926). The method is simple but appears to
work!
Maturity is measured by the Time-Temperature Index (TTI) = t . 2n where means
‘the sum of’; t = time interval; n is a function of burial temperature and the equation
is summed over the range of temperatures encountered during burial, in 10 °C
intervals (we used to do this by hand in the practical class, be grateful).
To plot TTI though time, go back to the main Input Screen, and change the lithology
of the reservoir layer to ‘Shale (black)’ (hint – right click on the lithology box and look
under clastic rocks). Change the name of the reservoir layer to ‘Source rock’.
Why black shale?
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Re-run the model (click the running green man again). Wait…
Under ‘Output’ and ‘Time plots’, check ‘Temperature’. This plots temperature through
time for UC1, which we are not very interested in.
Click on the drop-down menu attached to the graph symbol (looks like a small
mountain) below the graph, then click on ‘Select’ and choose ‘Source rock’. This
plots a curve of temperature through time – why is it so complex?
…………………………………………………………………………………………………
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Click on the graph / mountain symbol again and select ‘Add curve’. Click on ‘Overlay
/ Temperature’, select ‘Calibration’ then ‘TTI’ from the menu. Click on ‘Create’ – the
new curve (and a second axis) will appear.
What TTI value do you get for the source rock at present day?
…………………………………………………………………………………………………
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Compare the result to the following empirical values:
TTI = 10 – 160 for oil generation
TTI = 160 – 1000 for condensate
TTI > 1000 for gas.
What type of hydrocarbons is it generating now?
…………………………………………………………………………………………………
Examine the shape of the TTI-time plot. Is the early or later part of the burial curve
more important for the generation of hydrocarbons? Why?
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If we alter the amount of sediment deposited then eroded during the Unconformity, is
this likely to have much effect on the present-day maturity? If you have time you can
test this.
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If you have spare time, go back to the ‘Boundary Conditions’ menu in the ‘Input’, and
set the sea water temperatures to something a bit more realistic – the Mesozoic and
Paleogene were warmer than present, the ice age was colder. Does the predicted Ro
or TTi vary much?
The PetroMod™ software is available for projects and dissertations if anyone is
interested, just let me know.