PROBABILITY CLASSES AND GEOLOGICAL EVENTS IN GEOLOGICAL
PROBABILITY CALCULATION
VJEROJATNOSNI RAZREDI I GEOLOŠKI DOGAĐAJI U IZRAČUNU GEOLOŠKE
VJEROJATNOSTI
Author:
Tomislav Malvić1, Miro Đureković1 i Igor Rusan2
1INA-Oil
Industry Plc., E&P of Oil and Gas, Reservoir Engineering & Field Development
Department, Šubićeva 29, 10000 Zagreb
2INA-Oil
Industry Plc., E&P of Oil and Gas, Exploration Department, Šubićeva 29, 10000
Zagreb
Key words: Geological probability, categories, events, probability classes, Pannonian
Basin
Ključne riječi: geološka vjerojatnost, kategorije,događaji, vjerojatnosni razredi, Panonski
bazen
Sažetak:
U radu su opisani temeljni principi izračunavanja geološke vjerojatnosti otkrića ležišta
ugljikovodika. Prikazana metodologija primijenjena je za procjenu potencijalnog otkrića u
hrvatskom dijelu Panonskog bazena. Upotrijebljen je deterministički pristup koji uključuje
pet geoloških kategorija (zamku, rezervoar, matične stijene, migraciju I očuvanje
ugljikovodika), procjenjujući na taj način geološke događaje kojima je određena geološka
građa naftnog sustava. Načinjena je znatna redukcija vjerojatnosnih razreda te je izračun
moguće provjeriti na različitim lokalitetima ili stratigrafskim jedinicama za koje je načinjen
proračun. Odabrani vjerojatnosni razredi uključuju pet numeričkih I tekstualnih vrijednosti, I
to redom: 1.00 za dokazani, 0.75 za vrlo pouzdano određen događaj, 0.50 za prilično
pouzdan, 0.25 za nepouzdani te 0.05 za neopaženi ili nepostojeći geološki događaj.
Metodologija je transparenta, odnosno može jednostavno biti primijenjena u gotovo bilo
kojem dijelu Panonskog bazenskog sustava.
Abstract:
This paper considers the basic principles of geological probability calculation for
hydrocarbon reservoir. Presented methodology is applied for evaluation of potential
hydrocarbon discovery in Croatian part of Pannonian Basin. Applied deterministic approach
included five geological categories (traps, reservoirs, source rocks, migration and
hydrocarbon preservation), estimating geological events that could describe geological
settings of petroleum systems. Significant reduction of probability classes make possible
easily check different plays or prospects that could be estimated. Applied probability
classes included five numerical and textual values as follows: 1.00 for proven, 0.75 for
highly reliable, 0.50 for fairly reliable, 0.25 for unreliable and 0.05 for impossible prediction
or missing geological event.
Calculation procedure is made in the form of freeware
computer program. Such transparent methodology can easily be applied in almost any part
of Pannonian Basin System.
INTRODUCTION
The goal of geological probability calculation is finding an appropriate numerical value,
which can describe the probability of new hydrocarbon discoveries on observed area. This
procedure could be applied both in exploration and/or development of hydrocarbon
reservoirs.
Geological probability is defined as multiplication of the main geological factors or
categories that define petroleum system. Reliability of data has the extraordinary
importance. There is always balance between invested funds and collected data depends
on reliability of geological database, predicted size of potential discovery and company
budget. First step in that process of several activities is estimation of geological probability,
calculated from acquired data.
Testing dataset was collected from regional studies published for southern part of the
Drava depression and the Bjelovar subdepression (Malvić, 1998, 2003). This mature area
included all relevant geological categories stratigraphically described in Neogene and preNeogene sediments (Figure 1). Thickness of Neogene-Quaternary clastic sediments is
mostly up to 3000 meters, with Mesozoic and Palaeozoic rocks in the basement. The
highest hydrocarbon potential is attached to Lower and Middle Miocene coarse-grained
clastites, accompanied with fractured and weathered basement rocks in unique
hydrodynamic unit. Lower and Middle Miocene reservoirs and traps are relative shallow,
located between 800 and 1500 meters. The second reservoir unit consists of Lower
Pontian sandstone members. Unfortunately, these sandstones are characterized with very
instable reservoir properties and depth shallower than 1000 meters, what makes possible
to hydrocarbon escaping along the faults as well as water degradation.
Figure 1. Categories of hydrocarbon system in analyzed area (Malvić et al., 2005)
Source rocks (Ottnangian to Badenian ages) are assumed inside two major synclines of
subdepression, on depth between 1600 and 2500 meters. Catagenesis phase is reached
only in the deepest parts, and it is why significant part of hydrocarbons migrated from
northwestern part of the Drava depression. There are proven source rocks represented by
mudstones, marls and siltites of Lower Miocene to Badenian age at depths greater than
3000 meters (Baric et al., 1998).
The potential for new hydrocarbon systems was calculated via five independent geological
categories: (i) Structures; (ii) Reservoirs; (iii) Migration; (iv) Source rocks; and (v)
Preservation of hydrocarbons. Calculation can be done for any play or prospect. Brown
and Rose (2001) defined play as operational and prospect as economic unit. Rose (2001)
and White (1992) described play as several, geologically similar, fields, discoveries or
prospects. Here, the term play was used as substitution for stratigraphic interval(s) where
are already discovered economic hydrocarbon reserves. There are two such plays: (a)
basement rocks and Miocene breccia and (c) Pannonian and Pontian sandstones. Mostly
structural traps inside analyzed area were selected from structural maps and palinspastic
sections (Malvic, 1998, 2003).
GEOLOGICAL PROBABILITIES VIA CATEGORIES
Geological probability calculation procedures are well-known and simple tool for petroleum
system description for decades. Generally, such calculation could be done allowing
expert(s) to estimate probabilities of particular geological categories, using numerical
values in the range 0.0-1.0 and using geological probability tables published for different
petroleum provinces around the world. The approach applied in this paper was based on
evaluation of five geological categories (according White, 1993):

Category 1: probability of TRAP;

Category 2: probability of RESERVOIR;

Category 3: probability of SOURCE ROCKS;

Category 4: probability of MIGRATION;

Category 5: probability of HC PRESERVATION.
Multiplication of these categories final result in Probability of Success of discovery:
POS  p(trap)  p(reservoir )  p( source _ rocks)  p(migration)  p( HC _ preservation)
(1)
The appropriate geological database, including characteristic geological categories and
events, was derived from available dataset published in earlier regional study of the
Bjelovar subdepression (Malvic, 1998, 2003). This database was organized in Access, and
linked to executive computer code programmed in DelphiTM language (Brkić et al., 2006),
called GeoProb Modeling 1.0.2 (Figure 3). The program is freeware.
Figure 3. POS calculation in GeoProb Modeling 1.0.2
Each geological category could be evaluated from the different type of data, with very
various qualities. Seismic mostly plays major role as base for structural and sometimes
also stratigraphic interpretation. Addition important information is eventually existing
production in analyzed play or close to estimated prospect. If such production exists,
potential discovery can be classified as potential (not only hypothetical).
PROBABILITY CLASSES
Geological categories are always described through several probability classes). Each
class has unique discrete numerical values in the range 0 and 1 that describes probability
to occur selected event(s). We selected (only) five classes to correlate probability event by
classification of reserves. As we know reserves as proven, possible, speculative and so on,
there is deeply geological sense that such ranking cab be also applied in determination of
petroleum system that include such reserves. It means that geological event or category
can also be proven, highly or fairly probable, hypothetical or missing. According to such
qualification geological events are described as:

1.00 for event in play/prospect described as “Proven”;

0.75 for event in play/prospect described as “Highly reliable prediction”;

0.50 for event in play/prospect described as “Fairly reliable prediction”;

0.25 for event in play/prospect described as “Unreliable prediction”;

0.05 for event in play/prospect described as “Missing/Undefined parameter”.
There is selected 62 geological events (and more than 100 options) though 5 categories
and special database (Figure 3), as part of computer program GeoProb 1.0.2.
Figure 3. Database with 62 geological events classified in five probability classes
DISCUSSION
Present computer program can be used for geological probability calculation in any
Neogene petroleum zone (play or prospect) in the southern part of Drava depression or
the Bjelovar subdepression. Also, it can be applied in both in the entire Sava and Drava
depressions, without any or with minor modifications represented in carefully reselection of geological events.
The proposed geological database represents is based on the five probability classes:
1.00 (proven), 0.75 (highly reliable predicted), 0.50 (fairly reliable predicted), 0.25
(unreliable predicted) and 0.05 (missing geological event).
It would be useful presented methodology upgrades by additional tool for estimation of
seismic and production data. Both of these two groups included important information
on reliability of data used for geological events selection.
REFERENCES
Brkić, N., T. Malvić and I. Rusan, 2006, GeoProb Modeling 1.0 (computer program):
Zagreb, www.mapconsult.net (freeware).
Brown, P. J., and P. R. Rose, 2001, Plays and Concessions – A Straightforward Method for
Assessing Volumes, Value, and Chance (poster): AAPG Annual Meeting, Search
and Discovery article #40024.
Malvić, T., 1998, Strukturni i tektonski odnosi, te značajke ugljikovodika šire okolice
naftnoga polja Galovac-Pavljani: Zagreb, Master thesis, Faculty of Mining, Geology
and Petroleum Engineering, 111 p.
Malvić, T., 2003, Oil-Geological Relations and Probability of Discovering New Hydrocarbon
Reserves in the Bjelovar Sag: Zagreb, Ph.D. Dissertation, Faculty of Mining,
Geology and Petroleum Engineering, 123 p.
Malvić, T., J. Velić, and Z. Peh, 2005, Qualitative-Quantitative Analyses of the Influence of
Depth and Lithological Composition on Lower Pontian Sandstone Porosity in the
Central Part of Bjelovar Sag (Croatia): Geologia Croatica, v. 58, p. 73-85.
Rose, P. R., 2001, Risk Analysis and Management of Petroleum Exploration Ventures:
Tulsa, AAPG Methods in Exploration Series, 12, 164 p.
White, D. A., 1992, Selecting and Assessing Plays, in R. Steinmets, ed., AAPG Business of
Petroleum Exploration: Treatise of Petroleum Geology, Chapter 8: Tulsa, p. 87-94.
White, D. A., 1993, Geologic Risking Guide for Prospects and Plays: AAPG Bulletin, v. 77,
p. 2048-2061.