GEOL. C ROAT. 49/2 197 - 2021 9 Figs. 1 
1 

1 ZAGREI3 1996 

Revie\v paper 

Prospecting for Bypassed Oil and Gas 

Zaki BASSIOUNI ' and Josipa VELIe ' 

Key words: Prospecting, Bypassed oil , Bypassed gas. 

Abstract 
In recent years, il has been increasing ly difficult to economicall y 

explore for hydrocarbons because of geopoliti cal unce rt aint ies and 
low oil pricc~ . In many countri es the remaining po tential prospects 
me in areas which arc difricul\ to access becau se of cli mate, terrain , 
offshore water depth . and/or envi ronmenta l const rai nts. Al so, most of 
the relat ively simple geologic basins have been ex ploited leaving to 
explore areas o f complex geology. In addit ion. Ihe probabil ity of find­
ing large fields in mature basins is understandably low. 

1\ is known that substantia l amounts of hydrocarbo ns has been 
bypassed on a macroscopic and microsco pi c sca le in know n fiel ds. 
Thi s is mainl y due to reservoir heterogeneiti es and complex lithology 
which rcsult in poor rcscrvo ir characterisation. Innovative reservoir 
managcment op ti ons can rcduce the amount of bypassed hydrocar­
bons. Emerging tec hn o logies such as 3-D seismic and hori zon tal 
wells facilitate the locmion o f oil and gas which has been bypassed at 

near-v irgin saturat ion. S uffi cient ly large amou nt s of such hydrocar­
bo ns co uld be economi c <l lI y produced in the ncar term by infi ll 
drilling. Innovati ve improved oi l recovery method s can revive reser­
voirs which were considered depl eted by co nventi onal standard s. 
These emergi ng tec hn olog ies and method s have the potential of 
adding bypasscd o il to thc reserv cs cat egory at a cOSt per barrel much 
lower than the cost of ncw discoveries. 

J. INTRODUCTION 

The 10lal wo rl d o il and gas resource is classi fi ed , 
according to the degree of geologic assurance, as poten­
tially recoverable discovered, and undiscovered. Part of 
the di scovered resources have already been produced. 
The distributi on of the uliimate resource bel ween these 
categories diller from country to country. For the Unit­
ed Stales, a mature oil and gas producer, half of the ul ti­
mate resource have been produced. Future producti on 
wi II come from a va ri ety of sources. Reserves in cur­
rent ly proved fields, plus the growth of these fi elds will 
cOlllribwe half of futurc production. The other half will 
come primaril y from currently und iscovered resources. 

PROCEEDINGS 

Kljucne rijeci: ist raz ivanje, preoslala nafta, preostali 
plin. 

Sazetak 
Isplativo is trazivanj e ugljikovodika odlledavllo je opccnilo posta-

10 otciano bilo zbog gcopoli ti c kih oko lnos li bilo zbog ni ske cije ne 
nafl e. U mll ogim zemljama pronalazc nje preos talih koliCi na nafle i 
pli nl! c ine za mrsc nim nepris tupacnosl lerena, surova klim'l. vel ik e 
dubinc mora i/il i bojazan ad oneciscenja okoli5a. Brojni bazeni. geo­
loski rclalivno jednoslavni i privid no iscrpljeni , sigmno imaj u dijelo­
va sl ozenijc gradc i saSlava koj i bi se mogli dodalno iSlraziv ati. No, 
vjerojatnost pronalazenja veJ ikih polja u nj imaje svakako mal a. 

Znano je da .'i u s tanovite kolicine ug ljikovodika prcostal e u poz­
natirn poljim<l kako u makroskopskim taka i II mikroskopsk im razl11-
jcrima, sto llglavnorn nvisi 0 s loicllo sti grade i titolo skog sas tav a 
lciisl<l, odnos nn 0 losijim rczc rvoarskirn znacajkama. Novi poslupc i 
gospodarenja lez islima smalljuju ko lieinl! preos tal c naft c. Nastupa­
juce tchnologijc kao s to su 3-D sc izmik a i vodoravno buiicnjc 
o mogucavaj u smjestavanje nafle i pHna koja preostaje u go to\,o 
is ko nskOIll zasice njll. Dna bi sc rnogla pro izvoditi II gos podarski 
isplativim kolici nama kao SIO se 10 posli ze i primjericc uli skivanjem u 
buso tine. Suv remcni. savrscniji !lacini o tkri vanja nafl e i plinu mogu 
lIc inil i vr lo zilll irnlji vi mil i ona tci. is ta koja se prema l1obi cajeni111 
mj erilirn a s rnatraju isc rplj cni mu. Na osnov i rez ultala spo mcllutih 
novih lc l1l10 logija i meloda preos tala bi sc nana 1110gla uk ljuciti u pri ­
dobivc za lihe po cijcni ni ioj od cij ene novootkrivcni h kolicina 
ug tj ikovod ika . 

In most mature areas, average inland di scovery is 
becoming smaller and more difficult to find. Deeper 
prospects presenl a challenge of operaling in hi gh tem­
perat ure and high pressure, o f le n corros ive, e nv iron­
me nl. Ex ploring [or undi scovered offshore resources 
requ ire drilling, completion, and produclion operat ions 
in deep water. The cost assoc iated wi th offshore envi­
ronment necess itates the discovery o[ larger to large r 
accumu lations. A fi eld of 4.8 million mJ (30 mill ion 
barrels) may be economi ca ll y viable inlan d but the 
same discovery in 200 melers waters wou ld show mini­
mal return (COX, 1980). 

A large percent o[ the world 's undi scovered 
resou rces lies north of the 60th parallel in Arctic Cana­
da and Russ ia. The cos ts of exp loration , deve lopme nt 

I Louisiana Statc Un iversity, Petrol. Engin. Dep t. , CEBA 3523, Baton Rouge. LA 70803-6417, USA. 

~ Universi ty o f Zagreb, Faculty of Mining, Geology & Petroleum Engi neering, Picroll ijcva 6, P.D.Box 186, IIR - I 0000 Zagreb, Croatia. 



19X 

Cumu lative Production 
9.94 BBbl (34%) 

/' 
Proved Reserves 
1.92 BBbl (6.6%) 

Mobile 
Unrecovered Oil 
553 BBbl (19%) 

Immobile Oil 
I I 78 BBbl (40.4%) 

Total Original Oil-in-Place = 29.17 Billion Bareis 

rig 1 Di stribution of origi nal oil-in-place in Federal Offshore Gulr 01" 
Mex ico, aftcr ,111 lcr report CI.CF. , 1996) . 

and transportation in the Arctic arc vcry high. Hi ghcr 
prices wil l be required to support fut1he r exploration for 
and developme nt o f the Arctic reg ion. Exploring the 
Arctic and other inhospitable and remoiC areas slich as 
the ra in forest will also require the development of eco­
logically sound tcchnologies. 

Th e world rcso urce base is large ly depen dent on 
three rac tors: price of o il and gas, cost of ex ploration 
and development , and availabi lity of lands for explo­
ration. The surface area ye t to be explored is decreasing 
continllous ly , and the price of oil is determi ned by 
forces ex ternal to the petroleum industry. The oil indus­
try forecasts appears to be mov ing towards using a fla t 
o il pr ice. As the resource base continues to mature, the 
cost req uired to explore for and to develop remai ning 
resources is in creas ing. New tec hnolog ies will be the 
key to rcducing the cost. These [actors and considera­
tions are creating a business cl imate that is challenging 
and wi ll necess itate cons iderable change in pri ori ties, 
strategies, and management structure. 

The industry has then to give more consideration to 
bypassed oil. Thi s relatively new term refers to the 
hydrocarbons whi ch have been bypassed on microscop-

ATIle UPDIP 
POTENTIAL 
VOLUME 

SALT 
PROFILE 
AS 
MAPPED 

Fi g. 3 Bypasscd oi l due inaccurate detection of salt bo undary. afte r 
an tCF report ( I.C.F., (996). 

GcologiJ Cro;lIic:l 49f2 

"' 031 
"l! • CD 

0.30 

C 029 

~ 026 
~ 
C 0" 
0 

U 0.26 
0 

" 025 
0 

Q: 0.24 

i5 0.23 

• 
" 022 
0 

U 0" 
1981 '''''' 1987 1990 '993 

Year 

Fi g. 2 Crude oil produclion ( 1981-1 994) of Federal Ofrshorc Gul f of 
Mexico, aner an ICF report ( I. C. F. , 1996). 

ic and macroscop ic scal e in already discovered and 
deve loped rese rvoirs. Oi l bypassed on mic roscopi c 
scal e is the immobile oil trapped in the reservo ir pores 
by viscous and capillary forces and cannot be displaced 
by water. The immobile oil is the target [or number of 
enhanced oil recovery techniq ues particularl y mi sc ible 
gas and chemical fl ood ing. Oil bypassed on a macro­
scop ic scale is the mobil e oi l which remain s in th e 
rese rvoir aft er conventiona l recovery for a varie ty o r 
reasons. The magn itude or bypassed oiJ is signifi cant. 
Fo r ex ample, in federal o ffshore Gulf o f Mex ico it is 
es timated that at the conclusion of conventi onal recov­
e ry techni ques, nearl y two-third s of the known 
resources will remain unrecovered . A 878 mi lli on Ill J 

(5.53 billion barrels) or 19% of OOIP is in the ealego ry 
of mobile bypassed oil (Pi g. 1). Figu re 2 shows that the 
Gu lf of Mexico annual offshore produ ction pea ked in 
1970 and has since fall en dram atically. The noti ceable 
increase in prod uct ion start ing in 1990 is primarily due 
to the application of 3-D seismic, horizonta l wells, and 
othe r tec hnologies wh ich targeted bypassed oi l. The 
different types of bypassed oil and the means for their 
identification and recovery are discussed herea f'ter. 

Fi g. 4 Bypassed oi l due to faulling. aft er an tCF report ( I.C.F. , (996) . 
Thc recently developed 3-D seismi c Icc hnology can prov ide the 
necessary infonnalion to prope rly map Ihe reservoir. 



Ih~sioulli & Velie: Prmpec1 ing for Bypnssed Oil nnd Gas 

all Prooucer 

OIlR,m 

Gas Injector 

, CurrenlOWC 

Water $wepl 
Oi l Zone 

~ 
") o;p Anglo? 

Fig. 5 Schem.Hic of [he double di splaccmcnt proccss (after LEP$K I 
Ct al. . 1995). 

2. BYPASSED OIL DUE TO INA CCU RAT E DELI­
NEATION OF RESERVOIR BOUNDARIES 

One example or inaccurate delineation or geolog ic 
structures is Ihal or sal t inl rus ion s (fig. 3). Optimum 
selcclioll or drilling locations around sal! intrusions is 
hampered by refract ion o f the seismic waves as th ey 

pass through the sa lt body. This refraction tends to dis­
tort the se ismi c images making it d ifficult to pinpoint 
the salt boundary. Much oil has bccn bypassed because 
Ihe sa lt boundaries wcre not clearly visible in seismi c 
imagi ng. Anolher exampl e is tha t of faults which arc 
not de tected by conventional 2 -D seismic technology 
(Fig. 4). As a res ul! o il in an iso lated fault block is 

bypassed. 

3. BYPASSED OIL DUE TO RESERVOIR 
HETEROGENEITY 

Oil reservoirs are produced eithe r by natural or arti­
fic ial d ri ve mcc hanism, in whic h oil is pu shed to thc 
producing wclls by the expanding gas and/or the inva­
s ion or wat e r into the reservoir. The spatial reservo ir 
he te rogenei ty will leave consi derable volumes or oil 
bypassed in isolated pockets. If the bypassed pocket is 
l[lrge enough , it cou ld be produced economically in the 

near te rm simply by drilli ng a new infi ll we ll withou t 
the need ror more compl ex and costl y enhanced o il 
recove ry processes . In a homogeneous system, infill 

drilling accelera tcs product ion. In a heterogeneous sys­
tcm it al so result s in incremental recovery. The recov­
ery is the result or improved reservoi r con tinuit y, 
improved areHI and vertical sweep. Infill drilling is not 
without ri sk and requ ires detai led reservoir description 
to improve chances of success (GO ULD & SAREM , 
1989). 

199 

-- COIled lub,ng 

Bypassed 011 pockets 

Fi g. 6 Combining ad vanced drilling tcchnologies to reach bypassed 
oi l pockets. artc r PEDEN & T WEEDIE ( 1995). 

4. ATTIC OIL AND OIL BYPASSED IN TI-IE 
WATER-SWEPT ZONES 

Attic oi l is th at oi l trapped upclip from the highest 
well in a rese rvoi r. Attic oil is usually limited in sizc to 
justify drilling new wells. Th is bypassed oil is the target 
o r s ide-Iracking or updip gas injection . The updip gas 
injection process for recovering attic oil can be ex pand­
ed into a Double Displacement Process (OOP) to recov­
er the oil bypassed behind the wate r fro n\. The DDP 
invo lves updi p gas injection into a wate r- invaded oil 
eol umn in order to prod uce incrementa l o il. The incre­
mellla l oil results from the mobilisa tion or a fraction of 
Ihe residual o il left behind in the water-s wept zone. 
Gravity-stable gas displacement causes Ihe i'ormalion of 
an oil bank wh ich builds up progress ively as il migrates 
clown the reservoi r towards the producing well s. A sim­
pl ifi ed schelTIalic o f a dipping reservo ir subjected to 
DDP is shown in Fi g. 5. Increment al oil recovery 
depends on the amount of gas injected and thc locat ion 
of a down-dip wells. Favorable condit ions may result in 
the recovery of up to 40% or the initial oil- in -place 
(LEPSK I ct 'II.. 1995). 

Waterllooded reservoirs of Loui siana were screened 
10 dctemli ne thc potcntial of recoveri ng incremcnlal oil 
by CO2 inject ion. Of the 197 waterflooded rese rvoi rs 
screened, 39 look economically a ttrac ti ve. The potential 
incrementa l recovery frolll these reservo irs is about 
11.3 m ill ion m ' (7 1 million barre ls) (DIAZ e t aI. , 
1996). 

5. WATER CONING 

Whenever a well is producing from an oil layer 
overlaying a water layer, the pressu re grad ien ts Illay 
deform the horizontal water-oil con tact into a cone 
shape cont act. The cone height above the ho rizonta l 



200 

, 

! , , 
I 

b"'f' 
.. ~ 

contact depends on the production rate. Above a certain 
critical rate water will enter the well bore. T he water 
coning phenomenon may cause the oil cut to reach its 
economic limit prematurely leaving a signiricant 
amOllnt or mobile oil unrecovered. Producing at a rate 

below critical may be uneconomical. A horizonta l s ide 
track or lateral may present an economic option as a 
much higher flow rate can be realised without deform­
ing the horizontal conlact. Additional well technolo­
gies, namely coil tubing drilling, and under-balanced 
drilling can be combined with horizontal and lateral 
drilling to achieve large improvement in hydrocarbon 
recoveries in such cases (PEDEN & TWEEDIE, 1995) 
(Fig. 6). 

6, BYPASSED OIL DUE TO MISINTERPRE­
TATION OF WELL LOGS 

Potential hydrocarbon-bearing formations are iden­
tified from well log data. Misinterpretation of these 
data, due to the comp lexity or the rock-flu id system, 
may lead to thc bypassing of comple te inte rva ls. Such 
cases arc the target for reinterpretation of original open 
hole logs using newly established pet rophysical models. 
Cased hole logs, such as the Thermal Decay Time 
(TDT) and Ihe Gamma-Ray Spectrometry Tool (GST), 
can also be uscd to investigate the current ilu id content 
in old wells. The reinterpreta tion of old logs was 

oil in 000 t 
gas in 000 000 t 

6000 

5000 

hYdW~f \ 
J" \ 

oil 
- -/, ,,--' , , , 

" , , , 

/" gas/ 
" 

/ ,/,,/ 

4000 

3000 

2000 

1000 

o 
1946 1950 1954 1958 1962 1966 1970 1974 1978 1982 1986 1990 

Fig. 8 Ga s and Crude Oil P rodu ction (in Oil Equivalent) in the 
Republic ofCroalia (after VLAS IC & 13AUK, 1994). 

3B ~ , 

Geologia eroatiea 49/2 

Fig. 7 Shaded Areas Indicating 
Possible Bypassed Oil in a 
Nearly Depleted Reservoir 
in Louisiana, after BOUR­
GOYNE & KIMBRELL 
(1994). 

attempted in a Louisiana field near primary depletion 
(BOURGOYNE & KIMBRELL, 1994). Several possi­
bilities or bypassed oil were identified and ind icated by 

the shaded areas on the cross-section of Fig. 7. The top 
sand is thin and shaly and was not tested for hydrocar­
bon production. New interpretation indicated much 
higher o il saturation than was indicated by initial inter­
pretations . T his thin sand could be a good candidate for 
horizontal well application. The other two sands were 
se lective ly completed. Pockets of mobi le o il were 
bypassed due to uneven advance or watcr and/or 
absence of perforations. Some of these pockets could be 
the target of infill drilling and/or rccomp letion of old 
wells. 

7. BYPASSED OIL POTENTIAL IN 
THE REPUBLIC OF CROA TIA 

Oi l and gas prociuction supplies more than half the 
energy need of the Republic of Croatia. All present oil 
and gas production cOllles from the Pannonian Bas in. 
As of 1994, the Croatian part of the Pannonian Basin 
has produced 78 million tons of oil and 30 billion m3 of 
gas (YLASIC & BAUK, 1994). Prod uc lion his tory is 
shown in r ig. 8. T he sharp drop in oil production in the 

latc eigh ti es is due to occupation of fields in Eastern 
S lavon ia, and reduction of ex ploration act ivities. To 
remedy this critical situation INA is considering more 
aggressive exploration policy wh ich will open domestic 
areas for foreign companies. This option is limited by 
the relatively small area available for exploration. INA 
is also considering acquisition of discovcred reserves 
abroad especia lly in the Mediterranean area, and the 
application of enhanced oil recovery processes 
(YLASIC & BAUK, J994). 

In addition to the above mentioncd options, reserves 
could also be increased by prospecting lor bypassed oil. 
The po tential of th is option can not rea listically be 
de termined before a detailed study is undertaken. The 
Croatian part of the Pannonian Basin contains S4 reser­
voirs. The eight largest of these fields account for 83% 
of total recoverable reserves. Prospecting for bypassed 
oi l should target this manageable number of reservoirs. 

Awaiting such a study, we can point to fcw possibil­
it ies. Analysing primary recovery indicated that an inef­
ficient solution gas drive was predominant in 1110st of 
the fields. Only few fie lds, with 27% of initial recovcr-



Bassiouili & Velie: Prospect ing fo r Bypassed Oil and Gas 

6 7 
-1000 

PI : 

· 1500 

M' , 

M' 0 

'000 M; 

M, ,2 
-2500 

PI 

c=J Oil ~=~=~=~~ Water [I] Impermeable Rock 

able rese rves are producing under cflicient water drive. 
To increase the sweep e fficiency of the encroaching 
water the presen t spaci ng design of production and 
injcction wclls cou ld be modified by infi ll drill ing of 
verti cal or horizontal wclls . Innovative improvcd oil 
recovery me thods such as the double displacement 
process may also contribute to the recovery of bypassed 
oil in the watered-out zones. Even an average increase 
or 20% in the ultimate recovery from these reservoirs 
will result in abo ut 7% increase in the ultimate recovery 
of Croatia. 

Another possibility of the existence of bypassed oil 
is in reservo ir like the Obod reservoir. As shown in Fig. 
9, oil ex ists in zones with secondary porosity embedded 
in an impermeable rock (TURAJLIC er aI., 1979), The 
characterisatio n and delineat ion of reservoirs in such 
environment is rather difficult . The applicat ion of new 
technologies such as 3-D seismics coupled with 
improvements in formation evaluat ion could enhance 
the geologic picture provided by the lower resolution 2-
D seism ic surveys. An improved picture should result 
in additional recovery. 

The most important gas fields in Croatia are Molve, 
Kalin avac, and Stari Gradac (HORVAT, 1995), The 
remaining recoverable reserves of gas in the three fields 
represent more than 80% of the total discovered gas 
rese rves in Croatia. These gas accumulations exist in 
reservoirs of complex lithology, various porosity types 
and generally low permeability. The reservoirs are also 

22 24 

>- \j I 
1-' \..f' / Secondary Porosity 

201 

Fig. 9 Cross scction of 
Obod rcscrvoir, Cro­
atia (aft cr TURAJLlC 
ct aI., 1979). 

characterised by high pressure and high temperatures, 
and the presence of corrosive and toxic gases, such as 
CO2, H2S and HgS. Bccausc of thcse difficulties, the 
existence of significant amounts of bypassed gas is a 
distinct possibility. 

8. CONCLUSIONS AND RECOMMENDATIONS 

Croa tia needs to characterise and evaluate its 
bypassed hydrocarbon potential reserves. This include 
rcmaining unrecovered mobile oil and gas assoc iated 
with complex heterogeneous rescrvoirs , and additional 
recovery potential and related econom ic benefits that 
could result from the application of innovative 
improved technologies. An analytical system needs to 
be developed [or the characterisation and evaluation of 
bypassed hydrocarbons. The systern would consist of 
comprehensive databasc contain ing detailed rock and 
f lu id properties, geologic in formation, and production 
and dcvelopmcnt history for the Pannonian Basin reser­
voirs. Continuation of research and developmcnt efforts 
are needed in the areas of d irect ional drilling and com­
pletion, 3-D seismic imaging, improvcd integration of 
logging and seismic clata, and development of geologic 
models for co mplex environmenl. The research and 
development could effectively be mct through collabo­
rative effort between INA, the university, and govern­
ment. 



202 

9. REFERENCES 

BOURGOYNE. AT Jr. & KIMBRELL. W. ( 1994): 
Development of improved methods for locat in g 
large areas of bypassed oil in Louisiana. - Report 
prepared for DOE under contract #DE-FG07-
891012842. 

COX. 1'.1'. (1980): The Economic Aspects of Marginal 
Field Development in an Onshore Environmcnt.­
SPE paper 9023. 

DIAZ. D .. BASSIOUNI. Z .. KIMBRELL. W. & WOL­
COT!~ , J. (1996): Screening Criteria for Application 
or Carbon Dioxide Miscible Displacement in Watcr­
r100dcd Reservoirs Conta ining Light Oil.- SPE 
paper 35341. 

GOULD. T.L. & SAREM. A.M. (1989): Infill Drilli ng 
for In cremental Recovcry.- Jour. Pel. Tech., March 
1989. 229-237. 

HORVAT. D. (1995): Molve Natural Gas Complex­
Backbone of Croatian Energy Policy.- Nana, 46/1 , 
23-29. Zagreb. 

l.c.r. (1996): An Evaluation of Knowo Remaining Oil 
Resources in Piercement Salt Dome Reservoirs in 
the Gulf of Mexico.- Report prepared by rCF under 
DOE contract #DE-AC22-92BC 14831. Feb. 1996. 

Geologia Croalica 49(2 

LEPSKI. B .• WOLCOTT. J. & BASSIOUNI. Z. 
(1995): Physical Modelling of' the Oil Displacement 
by Gas in Watcrfloodcd Zoncs.- Paper presented at 
SPE Regional Symposium on Improved Oil Recov­
ery in the Gulf Region, AI-Ain, Dubai. 

PEDEN. J.M. & TWEEDIE. A.A.P. (1995): The Appli ­
cation of Horizontal and Other Non-conventional 
Wells for the Improvement of Hydrocarbon Recov­
ery in Both Mature and New Fields.- Paper present­
ed at the Regional Symposiulll on Improved Oil 
Recovery in the Gulf Region, AI-Ain, Dubai. 

TURAJLIC. Y .. MATELJAK. M. & DEVIC. T . 
(1979): Comparative Presentation of Deposits in the 
Oil Fields Benicanci, Obad and Ladislavci -Kucanci 
and Their Studics.- 3rd Annual Meeting of Scientif­
ic Cou nci l for Petroleum of Yugos lav Academy or 
Sciences and Arts, Proceedings, 2, 643-658, Zagreb. 

VLASIC. B. & BAUK, A. (1994): Possibilities of Oil 
and Gas Exploration and Production in the Republic 
01' Croatia. - Nafta. 45/5 . 263-272. Zagreb.