GEOMODELLING FOR DUMMIES

1 INVENTORYING DATA

Before starting to the project, knowing the available data is a must. The data then to be classified based

on its format for examples seismic data and well data. After that the data is classified again in a more

detailed classification, for example:

(-) Seismic Data is classified into 2d seismic data and 3d seismic data that should be put in the different

folders

(-)Well data can be classified based on different purposes, for example:

- ) The availability of all data in the well (wireline, core, biostratigraphy, checkshot, etc.)

WELL CAL DT GR RES(RXO) NPHI RHOB SP CHKS DPTM CORE BIOSTRATIGRAPHY

WELL A V V V V - V V - V V -

WELL B V V V V V V V - V V -

WELL C V V V V V V V - V - -

WELL D V V V V - V V - V - -

WELL E V - V - - V V - V - -

WELL F V V V V V V V - V - -

- ) Determining the well heads which consists of wells name, coordinates of the well (X,Y), EKB

(Elevation of Kelly Bushing), and MD (Measured Depth)

2 IMPORTING DATA

After all data are inventoried properly, the next thing that must be done is to start the software to process

all the needed data to perform geomodelling where in this tutorial, the software that is used is Petrel

2008. In Petrel, the first thing to be done is to open the project settings in the project tab. The purposes

is to set the project according to our target and the available data. Care must be taken in determining the

unit and coordinates since it will affect all of the will-be-imported data. Units encompass every unit that

will be used in the project where the determination of it is according to the data ergo it must be fit with

the data. Coordinates are the location in this world (preferably UTM) where the project is undergone.

Once the data has been imported and subsequently there is a change in the project settings, it will not

affect the data whatsoever.

Well X Y EKB End Depth

Well A xxxx.xxx yyyyy.yyyy x y

Well B xxxx.xx yyyyy.yyyy x y

Well C xxxx.xxx yyyyy.yyyy x y

Well D xxxxx.xxxx yyyy.yyyy x y

After the unit and coordinates have been set properly, the next step to be done is to import all the needed

data into petrel. The imported data depends on the purpose of the project where in this tutorial the

purpose is to calculate the hydrocarbon reserve by using two primary data which are 3D seismic data and

several well data.

-) Seismic data either it is 2D or 3D is mostly in the form of SGY or SEG-Y, a basic seismic format which can

be directly imported. The section of seismic of data can be viewed in the interpretation window under

window tab. In here, both section of the seismic data (xline and inline can be viewed). In the picture below

can be seen that the section is the part of xline in line 400 where the red circle points out that the seismic

unit is in ms (millisecond).

-) Well data also can be imported to Petrel but before inserting all of the wireline data, well heads must

first be inserted with the well heads as the type of file. The well heads are the data that has been stated

in table 2. After the well heads has been imported the next step is to import the wireline data by simply

importing the LAS (Log ASCII Standard) file as the common file that has wire line data in it in each well

that its well heads has already been defined before.

After the well heads have been properly imported the next step is to insert the wireline data of each well

that is needed in the project either in the form of LAS file or DLIS file. In this point also, the wireline data

that is needed can be filtered after the data has been imported. After the needed wireline data has been

imported, it can be seen by opening the well section in the window tab. Care must be taken in order to

determine the type of depth either it is TVD, TVSDD, or MD.

The imported data then should be viewed in the form of basemap. The basemap itself will provide us with

the image of the exact location of seismic data or the well data or even both of them in the specific grid

and scale which should be viewed to explain data limitation in both quantity and coverage.

If core or cutting description is available, it can be imported to the Petrel manually by using the comment

log. Comment log will create a separate logs column beside the electric log to give additional information

either about the description of core or cutting and any other additional data that are recognized important

to be showed.

Picture above explains the use of comment log to show cutting description beside electric log where the

top and base of the description should be defined first.

3 WELL TO WELL CORRELATION

After the data has been all properly imported the next step will vary which depends on the purpose of the

modelling. However, in this tutorial the next step is to perform well to well correlation. To start performing

well to well correlation, choose the well correlation option under stratigraphy modelling under process

tab on the down left corner. There are several steps that can be done even inconsecutively:

1. Determining the direction of the cross section. This depends on the results that the correlators

want to see from the correlation. The direction can cut and be perpendicular with the regional

trend of the structures, for example, to see the faults offset or faults occurrence from the

correlation, this may be referred as the structural cross section. It also can be performed in the

regional sedimentation direction if the available well data that are to be correlated are abundant

so the rough picture of the lateral distribution of facies and the reservoir continuity could be

interpreted albeit the accuracy is still low, supposedly, this is may be referred to stratigraphic

cross section. Defining the direction of a cross section can also be having a zigzag/ random

direction whereas it may affect the dips value. The value will no longer be a true dip but it will

change into apparent dip (having a lesser value than the true dip).

2. Determining the wireline data that will be used for correlation. The most used wireline data for

correlation is gamma ray (GR) since it shows a good distinction between lithology comparing with

other wireline data and if sequence stratigraphy is to be used as the main concept in performing

correlation, GR may give a good explanation in defining the fluctuation of relative sea level

(system tracts, parasequence, sequence) and yielding a good distinctive pattern that relative easy

to be geologically interpreted albeit it must still be accompanied by other data.

3. Determining the horizon that will be correlated. The horizon that will be correlated varies which

depends on the purpose of the project and also the availability of the data. Several example of

the horizons are:

a. Top sand; a horizon that is commonly found either in marked log or mud log where it

defines the top of sand reservoir interval. Correlating the top of sand from geologic

aspects may be similar with correlating the transgressive surface (TS) albeit some other

boundaries may also be applied (flooding surface for example).

b. SRM (shale resistivity marker); a horizon that is rarely used, presumably, but it is

explained by Bischke, 1991 that this horizon is good to be correlated. The reasons are

stated below:

Sand sometimes has a poor continuity where it is bad in correlating wells

Deposition of clays or muds are commonly in quiet water/ environment where

the coverage is extremely vast and consistent

Rapid changes in stratigraphic thickness in shale are not common

From geological aspects, SRM is more or less similar with maximum flooding surface

(MFS).

c. Top Formation; Formation is tantamount with lithostratigraphy where the top horizon

represents the top of a certain lithostratigraphy. Its basically similar with top sand (based

on lithology also) but it has a bigger coverage where top sand is limited to only sand

reservoir.

d. Sequence stratigraphy boundaries; basically this horizons are easier to be explained since

it applies directly the geological principles, sequence stratigraphy. The horizon is a

boundaries with each boundary gives an explanation of the start or the end of such

geologic events. Some of those boundaries are:

Sequence Boundary (SB). Usually representing the start of the deposition of

lowstand system tract where there the rate of sedimentation is higher than the

rate of the rise of relative sea level which is causing regression/ forced regression

(shoreline moving seaward) to happen. It is basically an erosional surface that is

caused by relative sea level fall which is also defined as unconformity. In GR log,

it is commonly marked as a horizon where there is a shifting from high GR pattern

to low GR pattern.

Maximum Flooding Surface (MFS). Usually representing the end of the

deposition of transgressive system tract and the start of the deposition of

highstand system tract. It is basically a surface boundary where the highest

relative sea level is achieved in a sequence. The end of retrogradation (shoreline

moving landward) where the rate of sedimentation is lower than the rate of the

rise of relative sea level which is causing transgression. it is commonly marked as

a horizon where there is a maximum value of GR.

Transgressive Surface (TS). Usually representing the start of the deposition of

transgressive system tract and the end of the deposition of lowstand system tract

where it marks the initiation of relative sea level rise. In GR log, it is commonly

marked as a horizon where there is a shifting from low GR pattern to high GR

pattern.

4. Correlating the horizon. Applying geological concepts in correlating the horizon is a must which

is to depict real subsurface conditions. Correlating horizon means that correlating horizons based

on the same chronostratigraphic framework which can be well explained by applying sequence

stratigraphy boundaries in correlating the wells. Correlation also is done by interpreting the

similar electric log pattern between one well with the others either below or above the horizon.

The electric log pattern is well-explained in Kendall, 2003. However, if the horizon has been

determined before such as top sand, correlation can also be done but it lacks of geological

explanation since correlating sandstone only may lead to mistakes if its not accompanied by the

same chronostratigraphic framework. Several questions might arise when correlating wells, for

example:

Inconsistency of bed thickness in the same interval. This may occurs because of several

reasons which are stated below:

Wrong horizon to horizon correlation where for example the MFS is correlated

with SB hence the interval is not in the same chronostratigraphic framework, this

is why the division of horizon, using geological principles, is very important to

avoid such mistakes.

Facies change also may explain the occurrence of bed thicknesss inconsistency

but commonly it occurs when the change of thickness is not rapid for example

the change from coastal facies (sandstone dominated) into central basin facies

(mudstone facies) in estuary depositional environment.

The occurrence of geological structures. Geological structures, for example,

anticline has two different strata; the flat one (crest) and the dipping one (flank)

that notwithstanding the true stratigraphic thickness (TST) has the same value in

every strata but when the wells were drilled vertically means that the thickness

that is derived is true vertical thickness (TVT). The apparent value of TVT in the

flat one (TST=TVT) the dipping one (TST

5. Restoring the section. Restoring the section may be optional but it helps correlator in knowing

the condition of the subsurface prior to such tectonic activities. The horizon that is to be restored

should be the youngest horizon hence it may restores to the condition in the subsurface before

the latest tectonic activities therefore the lateral distribution of the facies could be interpreted.

Practical corner