2d basin modeling and petroleum system (okui, 1997)

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PA97 - PO - 15

INDONESIAN PETROLEUM ASS0CIATION

Proceedings of the Petroleum Systems of SE Asia and Australasia Conference, May 1997

K EY TO SUCCESSFUL PETROLEUM SY STEM ANAL Y SIS :UNDERSTANDINGOF INPUT PARAMETERS IN 2D BASIN MODELING

AkihikoObi*

HNTRODUCTION

Basin modeling techniques are giving new insights to

oil and gas exploration, since they can integrate many

processes with quantitative evaluation on the historyof sedimentary basins (Figure I). Many geological and

geochemical processes are too slow and compies for

human beings to integrate quantitatively. But the

evolution of computer techniques enables us to

simulate and visualize these processes in human time

and space scales.

Explorationists generally develop several hypotheses

or scenarios during evaluations. The artificial

experiment nE basin modeling can compare these

hypotheses. Since each module forming the whole

basin modeling package is developed through physicaland chemical knowledge, basin modeling can provide

constraints and reality checks on these hypothesis and

hence reduce exploration risk.

Quality, accuracy and reality of the simulations

depend not only on the model itself, but also on input

parameters and the numerical scheme. In this paper, I

woulcl 1 c': !o discuss pitfalls in 2D basin modeling.

Successful modeling can be only accomplished by the

tuning of input parameters with geological

understanding, not by using default values in

commercial software packages.

In this paper, the properties of shale (such as

permeability, relative permeability, capillary pressure),

and the kinetic parameters for source rocks are

discussed.

METHODS

The modeling in this paper was conducted by JNOC's

* Japan National Oil Corporation

two-dimensional three-phase fluid flow basin

modeling, "SIGMA-2D" (Okui et al., 1994, 1996).

SIGMA-2D (2-Dimensional Simulator for Integration

bf Generation, Migration and Accumulation) is a finite

difference code developed by the TechnologyResearch Center of Japan National Oil Corporation.

SIGMA-2D can simulate the generation, migration

and accumulation of oil and gas (three-phase fluid

flow) in a two-dimensional cross section (Figure 2).

The modeling is divided into three categories;

Geological, Generation and Migration (Figure 3).

The geological modeling is responsible for the

reconstruction of burial and compaction of sediments,

tectonic and hydraulic fracturing, fluid flow and heat

flow. Compaction is calculated based on effective

stress law and fluid flow condition, which is governedby Darcy's law. Pressure increase is achieved either

by sediments loading, fluid expansionorhydrocarbon

generation. Tectonic fracturing is calculated by

simplified strain'analysis and hydraulic fracturing is

predicted based on pore pressure distribution.

Conductive and convective heat flow creates the

temperature distribution in the section.

The generation modeling is responsible for the

calculation of maturation of organic matter (vitrinite

reflectance and sterane epimerization) and generation

of oil and gas. A first-order kinetic reaction model isapplied for these calculations and multiple parallel

reactions are used for the generation.

The migration modeling is responsible for the

calculation of expulsion, secondary migration, PVT

condition (dissolution of fluid) and sealing

(accumulation). Expulsion and secondary migration is

calculated based on Darcy's law using relative

permeability concepts. Maximum dissolved capacity

of gas into oil is calculated based on pressure and

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© IPA, 2006 - Proceedings of an International Conference on Petroleum Systems ofSE Asia and Australasia, 1997

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temperature for certain oil and gas types. Free gas

which corresponds to the excess of this capacity can

migrate as a separate phase. Migrating oil and gas can

be trapped based on capillary pressure concepts.

Four governing equations with the terms related toabove phenomena are solved simultaneously in each

time step at each grid point in the section (Figure4).These equations are mass conservation equations for

water; oil and gas, respectively, and an energy (heat)

conservation equation. Rock and fluid properties have

to be given as input parameters. The rock properties

related to fluid flow such as permeability, relative

permeability and capillary pressure are primarily given

as a function of porosity for each lithology. The

properties related to heat flow such as thermal

conductivity and heat capacity are given as a function

of porosity, temperature and pressure.

SIGMA-2D has been applied to over 25 basins

throughout the world, mainly to basins in Japan and

Asia, but also to these in the North Sea and Middle

East.

RESULT S AND DISCUSSION

VitriniteReflectance

Simulation of hydrocarbon generation requires

assumptions regarding the thermal history in a basin.

Thk thermal history is generally calculated by

basement heat flow, surface temperature and thermal

conductivity of rocks in basin modeling (Waples et

al., 1992). Basement heat flow is so sensitive to

thermal history that it is usually calibrated by

measurable parameters. For present heat flow, actual

temperatures recorded at wells during electrical

logging or testing are used. Maturity indicators such

as vitrinite reflectance are used for the calibration of

paleo-heat flow, since it increases as a result of the

sum of heat energy from past to present.

Relatively low vitirinite reflectances compared to high

geothermal gradients are widely observed in Southeast

Asia. This combination results in low paleo-heat flow

with a rapid increase since Pleistocene to the present.

This spiky heat flow suggests an activation of rifting

since Pleistocene, which can not be geologically

justified.

Fluorescence alteration of multiple macerals (FAMM)

analysis (Wilkins et al., 1992, 1995) is a method to

measure fluorescence alteration of various macerals

without considering the type. The results can be

converted to true vitrinite reflectance by the cross-

plots of the degree of the alteration against the

intensity, since the relationship for standard vitrinitewas established. The advantage of this method is to

eliminate the uncertainties of identifying macerals in

conventional vitrinite-reflectance measurements, such

as skill of the technician, cavings and reworking. The

effect of chemical composition on the reflectance can

also be eliminated.

FAMM analysis on the samples from Southeast Asia

indicated that many vitrinite-reflectance measurements

appear to be suppressed (Waples et al., 1997). The

combination of a vitrinite-reflectance profile suggested

by FAMM analysis with temperature measurementsjustify an exponential decay or high constant

basement heat flow, which is more consistent with the

geological setting in Southeast Asia. The importance

of this is that modified thermal history calculates

earlier generation of oil and gas. In the Khmer Trough

of Cambodia, the difference is about eight million

years, which allows vertical oil migration for 800m

even if the flow rate is as low as m/year. (Okui

et al., 1997).

K inetic Parruneten

Oil generation occurs as a degradation of kerogen and

gas generation mainly occurs as a degradation of oil.

Most basin modeling software packages adopt first-

order parallel reactions to express these processes

(Tissot et al., 1987; Ungerer, 1990). Each reaction is

described by reaction kinetics in this model and the

kinetic parameters include activation energy and

frequency factor. The activation energy is the energy

required to activate the reaction and may vary

according to the strength of a bond in chemical

compounds. Since various chemical compounds

construct a kerogen, the activation energies are given

as a distribution for each type of kerogen (Tissot et

al., 1987; Ungerer, 1990). It is expected that the

activation energy distribution in source rock varies

according to the type of organic matter and

depositional environment.

About three hundreds measurements of kinetic

parameters in the Akita Basin of J apan indicate that

the activation energy distribution varies even within a



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marine Type I1 kerogen. Kerogen with more sulfur,

nitrogen and oxygen content exhibits a lower and

wider distribution, which is interpreted by the

assumption that these elements are expected to

weaken the chemical bonds in the kerogen. Source

rock with lower activation energy distribution isrecognized at the basin margin where high organic

'productivity and an anoxic environment due to

upwelling are suggested.

A simulation in the Akita Basin applying a lower

activation energy distribution for the marine Type I 1kerogen indicates earlier initiation of oil generation

even though the peak is not much different from the

conventional Type11 Earlier initiation of generation

may allow oils to migrate further in a basin.

KineticParameters for Coal

Kinetic parameters are generally derived from the

result of pyrolysis experiments such as hydrous

pyrolysis and rock-eval pyrolysis. In this type of

experiment, high temperature is generally applied to

compensate for the time required to generate

hydrocarbon. Moreover, the rock-eval is generally

conducted under an open system. These conditions

force the hydrocarbon generated in the experiment to

be expelled out of the source rock sample, since the

viscosity of generated hydrocarbon becomes very low

due to high temperature.

Pepper (1 991) and Pepper and Corvi (1 995) suggested

that the initial oil generated, especially in coals,

should be adsorbed in a kerogen, maybe due to

polarity of the oil. The adsorbed oils will be cracked

to gas by additional heating with burial and. expelled

as gas phase. I t is difficult to reproduce the adsorption

phenomena by any pyrolysis experiment. Therefore,

the activation energy distribution derived from these

experiments should be used with caution, especially

for coals.

Pepper (1991) proposed maximum adsorption capacity

as 200 mgHC/gTOC. Therefore, one of the methods

to apply the activation energy distribution from the

experiments is to take 200 mgHC/gTOC from the

lower part of the distribution and add them above the

activation energy corresponding to the oil cracking

range, which enables gas generation directly from the

kerogen. The application of such a modified activation

energy distribution to a basin in Southeast Asia

indicated that much more gas is generated and

migrated, which is consistent with the distribution of

gas fields in that basin.

Absolute Permeability for Shale

Absolute permeability is a principal rock property

which controls fluid flow in a basin. This property is

generally described by the Kozeny-Caman equation,

which indicates that the permeability decreases as

porosity decreases. Since the Koreny-Carman equation

was derived from the theoretical consideration of

repacking of spheres, this should be keep in mind

when applying to rocks that have suffered chemical

diagenesis.

SIGMA-2D's application in the Akita Basin of Japan

revealed that the Kozeny-Carman equation for shalecan not reproduce existing overpressuring in this

basin. More rapid decreaseofpermeability is required

to simulate the overpressuring in this basin, which is

interpreted as being due to diagenetic cementation of

zeolite and quartz in the throats of pore system. Itwas

found that the overpressured rocks contain more

zeolite and quartz than clay; these minerals were

originally deposited as volcanic glass and diatoms in

deep marine environments, respectively.

Relative Permeability for Shale

Relative permeability is a convenient rock property to

model multi-phase fluid flow in a basin. Laboratory

measurements can be done on reservoir rocks, but not

on fine-grained rocks. The analogue of reservoir rocks

is generally applied for fine-grained source rock and

seal rock in 2D basin modeling.

SIGMA-2D's application in the North Sea revealed

that the analogous curve can not reproduce enough

expulsion of oil and gas, and hence enough

accumulation confirmed by drilling. For medium to

lean source rock, it becomes more serious, as

demonstrated by the application to the Niigata Basin

of Japan.

It was found that only the curve with high irreducible

water saturation can simulate a consistent result (Okui

and Waples, 1993). This new curve was predicted by

careful examination of various curves from reservoir

rocks. It was suggested that fine-grained rocks contain

much more irreducible water due to micro-porosity



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and bonded water at the surface of grains and thereby

it is required to fill most of pore space with oil to

create dominant oil flow.

In the North Sea simulation, this new curve

successfully simhated downward expulsion of oilfrom Upper Jurassic source rocks (Draupne and

Heather Formations) to Middle Jurassic carrier beds

(Brent Formation). The contribution &om both source

rocks to charged volumes in traps was consistent with

biomarker composition in this simulation.

Capillary PRSSUR or Shale

Capillary pressure plays an important role €or sealing

oil and gas in a basin. This rock property is primaly

given as a function of pore-throat size. Since the

throat size decreases as compaction proceeds, sealingcapacity (capillary pressure) should be given as a

function of depth (or porosity).

If a trap receives oil at shallower depth, the leaking of

oil can start earlier, resulting in a shorter oil column.

For fine-grained seal rock, as well as source rock, it

is also not necessary to fill most of pore space with

oil. This combination simulated the leaking of oil

from Middle Jurassic reservoirs to the Tertiary for the

North Sea application.

A nhydnte

Anhydrite is generally considered as a complete seal,

for which very low permeability is given. SIGMA-

2D's application to the Middle East indicated that very

low permeability for anhydrite seals caused the

dipping of the oil/water contact of an accumulation

due to strong waterflow in horizontal direction, which

is not consistent with the actual distribution of oil.

Careful observation of an anhydrite core revealed that

a network of dolomitic mudstone exists in the

anhydrite, showing chicken-wire structure. Mercury

injection tests on this anhydrite indicated that the

pore-throats of this network were large enough to leak

water. A new application, given higher permeability to

the anhydrite bed, demonstrated a relatively flat

oillwater contact, which is consistent with reality.

Furthermore, the simulation indicated that oil can

migrate vertically through anhydrite beds before

severe compaction.

CONCLUSIONS

Two-dimensional basin modeling is one of the best

tools to evaluate the petroleum system. Since basin

modeling is a computer simulation technique, not only

the models but also the input parameters determinethe accuracy of the evaluation. However, existing

commercial software packages only supply

generalized default values. Therefore, the users have

to be careful to apply these values directly, and the

tuning of the input parameters with understanding of

those background is a key to successful two-

dimensional basin modeling.

REFERENCES

Okui, A . and Waples, D.W., 1993, Relativepermeability and hydrocarbon expulsion from source

rocks, In : A.G. Dore et al. (eds), Basin Modelling :

Advances and Applications, Elsevier, 293 -301.

Okui, A ., Hara, M., Fu, H. and Takayama, k.,1996,

SIGMA-2D : A simulator for the integration of

generation, migration, and accumulation of oil and

gas. Proceedings of VIIIth International Symposium

on the Observation o€the Continental Crust Through

Drilling, 365-368.

Okui, A,, Hara, M. and Matsubayashi, H., 1994, Theanalysis of secondary migration by two-dimensional

basin model "SIGMA-2D" (abstract), 1994 AAPG

Annual Convention Official Program, 227-228.

Okui, A,, Imayoshi, A. and Tsuji, K., 1997, Petroleum

system in the Khmer Trough, Cambodia, this volume.

Pepper, A.S., 1991, Estimating the petroleum

expulsion behaviour of source rocks: a novel

quantitative approach. In : England, W.A. and Fleet,

A . . (eds) Petroleum migration, The Geological

Society, Special Publication, 59, 9-31.

Pepper, A S. and Corvi, P.J ., 1995, Simple kinetic

models of petroleum formation. Part 111: Modelling an

open system, Marine Petrol. Geol. 12, 417-452.

Tissot, B., Pelet, R. and Ungerer, B., 1987, Thermal

history of sedimentary basins, maturation indices and

kinetics of oil and gas generation, Bull. Am.

Assoc. Petrol. Geol. 71, 1445 -1466.



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Ungerer, P., 1990, State of the art of research in

kinetic modelling of oil formation and expulsion,Org.Geochem. 16, 1-25.

Waples, D.W., Suizu, M., and Kamata, H., 1992, The

art of maturity modeling, Part2: Alternative modelsand sensitivity analysis, Bull. Am. Assoc. Petrol.

Geol. 76, 47-66.

Waples, D.W., Ramly, M . and Leslie, W., 1997,

Implication of vitrinite-reflectance suppression for the

tectonic and thermal history of the Malay Basin,

Proceedings Volume Kuala Lumpur 1994 AAPG

International Conference -- Southeast Asian Basins:

Oil and Gas for 21st Century, Kuala Lumpur,

Geological Society of Malaysia, (in press)

Wilkins, R.W.T., Wilmshurst, J.R., Russell, N.J.,Hladky, G., Ellacott, M.V. and Buckingham, C.P.,

1992, Fluorescence alteration and the suppression ofvitrinite reflectance, Org. Geochem. 18, 629-640.

Wilkins, R.W.T., Wilmshurst, J.R., Hladky, G.,Ellacott, M.V. and Buckingham, C.P., 1995, Should

fluorescence alteration replace vitrinite reflectance as

amajor tool for thermal maturity determination in oil

exploration?, Org. Geochem. 21, 191-209.



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FIGURE 2 - Three Phase Fluid Flow System. SIGMA-2D can simulate the generation, migration andaccumulationof oil and gas (three-phase fluid flow) in a two-dimensional cross section.



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- GAS GENERATIONby KINETIC MODEL- GAS EXPANSION & COMPRESSION- MAXIMUM DISSOLUTION OF GAS IN OIL

- RELATIVE PERMEABILITY for GAS- CAPILLARY PRESSUREOf GAS- GAS FLOWby DARCY'S LAW

STRUCTURE OF S IGMA-ZDMASS CONSERVATION OF WATER MASS CONSERVATION OFOIL

- COMPACTIONof SEDIMENTS- WATER EXPANSION&COMPRESSION

- FRACTURING- POROSITY REDUCTION- PERMEABILITY CHANGE- RELATIVE PERMEABILITY forWATER- WATER FLOWby DARCY'S LAW

ItMASS CONSERVATION OF GAS

~- OIL GENERATIONby KINETIC MODEL- OIL EXPANSION & COMPRESSION

- DENSITY & VISCOSITY OF OIL

- RELATIVE PERMEABILITY for OIL- CAPILLARY PRESSUREof OIL

- HC FLOWby DARCY'S LAW

- CONDUCTIVE HEAT FLOW- CONVECTIVE HEAT FLOW

- VlTRlNlTE REFLECTANCE

- STERANE EPIMERZATION

+-F IGURE4 - Structure of SIGMA-2D. Four governing equations with the terms related to above

phenomena are solved simultaneously in each time step at each grid point in the section.

2d basin modeling and petroleum system (okui, 1997)

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