SPE 105333

Missed, Bypassed, and Underestimated Hydrocarbon Traps: Analysis of BHT Records

Reveals “Undiscovery Wells” in North Arabia
Muhammad W. Ibrahim1, Target Exploration

Copyright 2007, Society of Petroleum Engineers

Global hydrocarbon exploration history recorded many cases of
th
early "dry holes" or "dry" anomalies becoming hydrocarbon
This paper was prepared for presentation at the 15 SPE Middle East Oil & Gas Show and
Conference held in Bahrain International Exhibition Centre, Kingdom of Bahrain, 11–14 March producers at later dates. Examples are numerous; such as well
2007.
K1-13 of Libya, which was classified as "dry hole" at completion
This paper was selected for presentation by an SPE Program Committee following review of date, then proved to be an oil producer at later dates is probably
information contained in an abstract submitted by the author(s). Contents of the paper, as
presented, have not been reviewed by the Society of Petroleum Engineers and are subject to one of many potential "undiscovery wells" in Middle Eastern and
correction by the author(s). The material, as presented, does not necessarily reflect any
position of the Society of Petroleum Engineers, its officers, or members. Papers presented at
North African (MENA) countries.
SPE meetings are subject to publication review by Editorial Committees of the Society of
Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper
for commercial purposes without the written consent of the Society of Petroleum Engineers is Geological and Geothermal Frameworks
prohibited. Permission to reproduce in print is restricted to an abstract of not more than
300 words; illustrations may not be copied. The abstract must contain conspicuous
Bottom-hole temperature (BHT) recordings using maximum
acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. recording mercury-in-glass thermometers is one of the oldest
Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.
geophysical well logging tools, and early exploration BHT
measurements are correlatable with present-day BHTs
Abstract
irrespective of the BHT logging device (e.g. optical fiber sensors,
The search for hydrocarbons in Middle Eastern and North
Carnahan et al, 1999). At present there are thousands of BHT
African (MENA) countries began essentially as an exploration
readings in records of old suspended, P&A, wet, tight or dry
for giant oil fields, and several exploration wells may have been
boreholes in every producing basin. Temperature data can be
suspended, plugged and abandoned and declared "dry" under
used as a quick look method to investigate early dry holes as
giant-prone early exploration strategies, obsolete logistics,
some of these wells may be commercially producible now.
drilling technology, and production methods. Some of these
"dry holes" are now proven oil producers or "undiscovery wells".
The studied area is located northeast of the Proterozoic Arabian
Geothermal gradient anomalies associated with hydrocarbon
Shield, on the northeastern Arabian Shelf and south-southwest of
traps have been recognised since the early days of modern
the Late Tertiary Taurus-Zagros suture zone (Long. 39.00 ºE to
hydrocarbon exploration. The goals of this study are: (1)
48.00 ºE and Lat. 29.00 ºN to 37.00 ºN). More than 150 oil and
mapping the geothermal gradients of North Arabia, (2)
gas fields have been discovered within the study area.
delineating geothermal gradient fairways and anomalies of oil
and gas traps, then (3) identifying and reviewing similar
An exclusive computer programme was used to create BHT
anomalies in the same area as they may be indications of un-
database, correct, test the statistical significance of BHT
drilled, under-rated or bypassed hydrocarbon traps.
measurements, calculate and plot Compensated Geothermal
In this study an exclusive computer programme was used to
Gradients (CGG) and Extrapolated Surface Temperature
create BHT database, correct, test the statistical significance of
Intercepts (ESTI) of 50 exploration and development wells in the
BHT measurements, calculate and plot Compensated
study area.
Geothermal Gradients (CGG) and Extrapolated Surface
Temperature Intercepts (ESTI) of 50 exploration and
Published geothermal studies on Northern Arabia include a
development wells in the study area covering onshore Kuwait,
generalized geothermal gradient map (Klemme, 1975), a
NW Saudi Arabia, Iraq, and parts of W. Iran, E. Jordan and E.
geothermal gradient contour map of South Iraq (Ibrahim, 1984),
Syria (Long. 39.00 ºE to 48.00 ºE and Lat. 29.00 ºN to 37.00 ºN).
and a geothermal gradient and heat flow maps of southern
The study recognised 30 Proven geothermal gradient anomalies
Turkey (Unlan and Ungur, 1979). Sporadic geothermal gradients
of hydrocarbon entrapment (i.e. associated with proven oil and
and BHT data were published by Al Saad (1986) and Ibrahim
gas fields), and used their model to identify and delineate 3
(1994b) on southern Iraq, Al-Habba et al (1994) on Western Iraq
Potential, 6 Probable and 5 Possible geothermal gradient
and Al Shadidi et al (1995) on Northern Iraq. Beydoun et al
anomalies of hydrocarbon entrapment in the area.
(1994) and Abu Ajemieh (1989) published some geothermal
There are thousands of BHT data dormant in borehole records of
gradients and BHT data on eastern Jordan, Abu Rabee (1986) on
every producing basin that can be used in similar analysis to
Kuwait, Cole et al (1994) on northern Saudi Arabia and Sirriya
investigate old dry holes for commercially producible traps.
(1986) on Syria.
1
Currently with the R&T Group of KOC, Ahmadi, Kuwait.
2 SPE 105333

The bottom-hole temperature recording device (maximum Subsurface Fluid Migration and Geothermal Anomalies
recording mercury-in-glass thermometers) is one of the oldest Whether the hydrocarbon moves from the source rocks to
(un-evolved) geophysical well logging tools. Early BHT accumulate in reservoir rocks as molecular solution, colloidal
measurements therefore are correlatable with present-day dispersion, solution of organic acid or as separate phase expelled
bottom-hole temperature measurements. At present there are out of networks of micro-fractures; it must have moved along
thousands of BHT measurements in records of early suspended, with or transported by an enormous quantity of hydrodynamic
P&A, wet, tight or dry boreholes in every producing basin. Some underground water.
of these wells may be commercially producible under present Compacting sediments in young basins expel most of its
logistics, technological, economical or geo-political compaction water upwards. Older, pre-compacted basins with
environments. As geothermal gradient anomalies associated with recharge intakes and discharge outlets have some of the
hydrocarbon traps have been recognised since the early days of recharged water moving upward through the ancient compaction
modern oil exploration, this study is aiming to recognize them routes (Roberts, 1981).
(as well as un-drilled geothermal gradient anomalies of In nature there are no absolute aquiclude, but aquitards and
hydrocarbon entrapments) from genuine dry holes in the area. aquifers. Aquitards are usually heat insulators (poor heat
conductors) and aquifers are heat transmitters (heat convectors).
In this study all available BHT measurements and geothermal The nearest to an aquiclude are thick salt and anhydrite sequence
gradients of exploration and development wells in the study area (good heat conductors) as in the Jurassic and Miocene salt-
will be used to: (a) contour-map the geothermal gradients of the evaporite of North Arabia.
study area, (b) identify geothermal gradient anomalies associated Static aquifers have no heat exchange, but very slow movement
with proven hydrocarbon traps, then (c) use the defined of underground water (as slow as 1 mm/day) can produce a
geothermal model of producing wells to identify and delineate significant thermal change (i.e. geothermal anomaly) in the
potential, probable and possible (drilled, partially drilled and shallower parts of the aquifer (Adivarahan, 1962, Bredehoef and
undrilled) anomalies indicative of focused subsurface fluid Papadopolous, 1968; Dominico and Palaciauskas, 1973; Straus
migration sites (i.e. the usual site of oil or gas traps). and Schubert, 1977).
Hydrocarbon traps function as focal points for migrating connate
Fifty wells within the study area were selected for this study, or recharge waters from the surrounding lows into, through or
because they have had a number of logging runs sufficient to past the trap; leaving behind the hydrocarbon phase.
generate a number of BHT readings that justify further analysis. The trap's incipient low pressure gradient leads to higher salt
The above database was supplemented with additional 53 concentration, higher hydrocarbon saturation, higher geothermal
corrected forced geothermal gradients (i.e. with one regional gradient (Roberts, 1980, 1981) and (if hydrocarbon is entrapped)
surface temperature). They were treated in the mapping higher porosity than the surrounding aquifer.
procedure as control points of lesser statistical significance.
Geothermal gradient anomalies associated with hydrocarbon
The Compensated Geothermal Gradient (CGG-ESTI) traps are primarily a function of the intensity of water movement
Method through the trap. Other factors, such as preserved high (over
Geothermal gradient contour maps are commonly used in the oil pressured) reservoir porosity, structural amplitude and
exploration industry to illustrate the thermal status of subsurface entrapment of poor conductive oil and gas in the reservoir pore
strata and source rocks maturation. The maps reflect variations in space exert smaller thermal changes on the overlying rocks.
the rates of increase of temperature with depth; due to one, or Source rocks in many basins (such as the Mesopotamian
combinations of the followings: Structural Terrace) are still within the hydrocarbon generation
1. Areal variations in heat flow due to crustal thickness, age, type phase, where migrating hydrocarbon must share the same
and tectonics (Vyskocil, 1979; Gupta, 1980). migration routs with compaction and post-compaction
2. Tectonics, depth, age and composition of basement rocks subsurface waters.
(Carslow and Jaeger, 1959; Hamza and Verma, 1969). Subsurface water migration into or past hydrocarbon trapping
3. Variations in rock's bulk saturated thermal conductivity, such closures may start before, during or, less frequently after
as: changes in matrix mineralogy, clay contents, permeability, hydrocarbon generation. Such migration routs and focusing
porosity, pore contents, subsurface pressure, fracture orientation points that feed hydrocarbon traps may continue after the end of
and density (Lewis and Rose, 1970; Ball, 1982; Chapman, et al, hydrocarbon migration and entrapment, and so does the asso-
1992). ciated geothermal anomalies. Therefore, high CGG-low ESTI
4. Surface and subsurface structures such as: anticlines, horsts, anomalies signals, and give away focus sites (traps) of
grabens, faults, fracture fields, igneous, mud and salt intrusions subsurface water (and associated hydrocarbon) migration into
(Selig and Wallick, 1966; Lachenbruch, 1978). shallower thermally convective reservoirs as in SE, NE Iraq and
5. Thermal variation due to cross-formational or lateral SW Iran.
hydrodynamic movements of subsurface fluids in response to
compaction, structural deformations and topography have been In old pre-compacted basins with deep traps; vertical water
identified as an important cause of geothermal anomalies (Beck, movement along young faults may seep and dissipate entrapped
1929; Jaeger, 1965; Lachenbruch, 1978; Toth, 1980; Ibrahim, hydrocarbons. There high CGG-low ESTI anomalies may signal
1983; Hitchon, 1984; Meyer and McGee, 1985; Meister et al breaches of cap rocks, low impedance seals, and may be accom-
1992). panied by re-migration, seepage and/or loss and destruction of
the entrapped hydrocarbon as in the Hit area in WSW Iraq.
SPE 105333 3

High CGG-low ESTI as well as Low CGG-high ESTI are 1. Re-examining anomalous borehole's shows record, lithological
reflections of the regional geothermal backgrounds in sedimen- descriptions, stratigraphic correlation, structural interpretation,
tary basins, or indeed, as in the Middle East; within one basin. well site report, wire-line logs, DST and drilling records; as such
The CGG/ESTI cross-plot can reveal the nature of the geother- wells may have been declared "dry" under past logistics,
mal background; expressed as the CGG-ESTI zone where the exploration, production or economic environments, improperly
majority of the dry holes clusters. It also display a separate CGG- logged, log-analyzed or badly tested.
ESTI cluster of suspended and/or hydrocarbon producing wells
2. Reviewing seismic records for fresh or alternative
(if present), reflecting the geothermal environments conducive to
stratigraphic or structural interpretation, re-processing or
hydrocarbon migration and entrapment.
additional seismic coverage, as the anomalous "dry hole" may
have been positioned off the flank of a nearby trap or stopped
The CGG and ESTI limits of the thermally anomalous wells may
short of a deeper trap.
vary from one area to another within a single basin depending on
the nature and intensity of the proven CGG-ESTI anomalies 3. Deepening old boreholes to test deeper (newly realized)
identified by the CGG/ESTI cross-plots. In the study area the exploration target(s) in areas where shallow reservoirs were
geothermal cross-plot limits of West Iraq, Eastern Jordan and hitherto the only targets.
NW Saudi Arabia may be different from those of Kuwait and SE 4. Focusing human resources on studying a limited number of
Iraq. The presented CGG-ESTI limits are the optimum limits for open or released blocks that display promising CGG-ESTI
several thermo-tectonic areas. anomalies instead of dissipating efforts on several areas.

Potential Applications of the CGG-ESTI Method Several case studies have shown that the interim CGG-ESTI plot
In the absence of sharp lateral variations of deep crustal heat of a borehole is identical to the final CGG/ESTI cross-plot
flow, lateral and vertical changes in rock facies, porosity and realised after hydrocarbon discovery at TD.
permeability would express gradual and gentle variations in the
regional (background) geothermal gradient. If the geothermal gradients of the explored area are known (from
Structural deformation or stratigraphic irregularities that focus CGG-ESTI maps and cross-plots); then one can model the
cross-formational fluid flow exert the most prominent localized expected thermogram, CGG and ESTI of a proposed well as a
effects and induce local anomalies on, otherwise a static thermal hydrocarbon producer or as a dry-hole. Accordingly; an
state of the subsurface (Meinhold, 1971; Toth, 1980; Roberts, anomalous interim CGG/ESTI cross-plot can be used during
1980, 81; Hitchon, 1984; Meyer and McGee, 1985). drilling to provide an additional input to the decision tree to
In sedimentary basins, lateral and vertical heat convection and justify: continue drilling to test deeper secondary target(s), side
mass transportation of heat via fluid flow is known to cause, tracking or abandoning the well; when the shallow first objective
associate with and enhanced by the entrapment of poorly proved to be dry, wet, etc.
conductive hydrocarbons. Such events super-impose mappable
geothermal anomalies on the regional (background) geothermal A CGG-ESTI anomaly is not an evidence of un-reached trap -
gradient contours (Meinhold, 1971; Roberts, 1980, 81; Toth, particularly if there are no source rocks to start with. However;
1980; Ball, 1982; Ibrahim, 1983, 94a, b; Hitchon, 1984; Metre properly interpreted, the CGG-ESTI anomalies can delineate
and McGee, 1985; Jones, 1980, 84; Meister et al, 1992; B- migration and entrapment sites, trends, fairways, and re-
lackwell, 1986; Carstens and Finstad, 1981; Andrews-Speed et migration or dissipation paths of subsurface fluid.
al, 1984).
Compensated Geothermal Gradient Map of Northern
Convective and mass transportation of heat has been recognised Arabia
as the major mechanism of subsurface heat exchange and will Producing wells produced distinctive high CGG-low ESTI
eventually replace conductive heat transfer models in basin clusters on the cross plot analysis. However, the similarity of
analysis, (Carlier, et al. , 1994). the geothermal signature of dry holes with the CGG/ESTI
Geothermal gradient anomalies associated with hydrocarbon cross-plot cluster of suspended heavy oil wells made their
entrapment have been recognised since the early days of explo- separation impossible via the regionally optimum limits (<2.5
ration (e.g. Beck, 1929). Therefore if the model of geothermal ºC/100 m and >35 ºC ESTI).
gradient anomalies accompanying hydrocarbon entrapments is This study found that the background gradient of most of the
defined, it is only logical to reconnaissance geothermal gradient study area to be conducive to oil migration and entrapment. This
map for similar anomalies in the same area or basin ,(Ibrahim, is rather unusual, as most of the studied basins have relatively
1986, 1988, 1992, 1994a, 1994b; Meyer and McGee, 1985), limited prospective areas and larger non-prospective areas.
Hulen, et al, 1994). Such anomalies may be associated with The small number of BHT data in north Iraq, Eastern Syria and
undiscovered traps, satellite traps, field extensions or bypassed Western Iran forced the author to relay on conventional
traps in plugged and abandoned "dry holes". geothermal data (i.e. single surface temperature gradients) in
generating the geothermal gradient contours there.
Potential application of the CGG-ESTI method is in screening The absence of ESTI data and the presence of single surface
large number of wells in oil basins for dry holes with anomalous temperatures also affected the resulted surface temperature
CGG/ESTI cross plots and areas which may provide justification contours causing a lack of ESTI anomalies.
for:
4 SPE 105333

Proven CGG-ESTI Anomalies Using the CGG-ESTI data of 21 dry holes, 62 producing and 20
An example of proven CGG-ESTI anomalies is Proven Anomaly suspended wells this study identified and delineated the regional
No. 25. The “W” Anomaly in SW Kuwait, Covering the geothermal fairways of gas and oil, and isolated areas of heavy
location of KOC's suspended oil well W-1. This is a closed 27.7 oil and dry closures in North Arabia.
ºC ESTI contour in a 3.1-3.2 ºC/100 m CGG area, induced by the The study recognised 30 Proven geothermal gradient anomalies
CGG-ESTI values of well W-1. of hydrocarbon entrapment (i.e. associated with proven oil and
gas fields), and used their model to identify and delineate 3
Potential CGG-ESTI Anomalies Potential, 6 Probable and 5 Possible geothermal gradient
An example of potential CGG-ESTI anomalies is Potential anomalies of hydrocarbon entrapment within the study area.
Anomaly No. 3. The “X” Anomaly in SW. Iraq, which is a
closed 2.1 ºC/100 m contour in a 28-32 ºC ESTI area. The Summary and Conclusions
anomaly was induced by the statistically significant CGG-ESTI Hydrocarbon exploration in the Middle East and North Africa
values of the same well (1.9/30). began essentially as “an exploration for giant oil fields.
Well X-1 of BPC was declared D&A in 1960. However, its Therefore, several exploration wells may have been suspended,
geothermal signature is that of a "probable flank well". A review plugged and abandoned and declared "dry" under giant-prone
of the original final well log revealed that it tested high pressure early exploration strategies, obsolete logistics, drilling
gas and little water from the prolific Cretaceous Nahr Umr technology, production methods, economical reserves limits, risk
(Burgan) sandstone reservoir. BPC concluded that there is a trap evaluation or political climate.” (Ibrahim, 2000).
in the vicinity. This paper illustrates a case study of using a quick-look analysis
The anomaly is probably that of a heavy oil trap-edge well. This and mapping of the geothermal gradients of North Arabia for the
was confirmed by reviewing the original final well log and test purpose of identifying the geothermal signature (CCGG-ESTI
records. The well tested oil from the Cretaceous Ratawi cross-plot cluster) of proven oil and gas fields, then use it to
Reservoir (400 BOPD) and the Jurassic Najmah carbonates (330 classify similar anomalies associated with the dryholes as
BOPD of 25.2 APIº Oil). potential, probable and possible missed, by-passed and un-
reached hydrocarbon traps (undiscovery wells) within the area
Probable CGG-ESTI Anomalies according to their degree of similarity with the recognized
An example of probable CGG-ESTI Anomalies is Probable geothermal signature of the proven oil and gas fields.
Anomaly No 6. The “Y” Anomaly in NW Saudi Arabia, The results and regional exploration consequences of this study
which is a closed 2.9 ºC/100 m contour in a 24-28 ºC ESTI area. can be summaries by the followings:
The anomaly was induced by the statistically significant CGG- 1. The study found relatively high geothermal gradient
anomalies to be associated with most of the known oil and gas
ESTI values of ARAMCO's D&A stratigraphic test well Y
fields in the study area, as well as some anomalous "dry holes".
(2.95/27).
2. The CGG-ESTI method is a practical quick-look
Well Y was drilled to a TD. of 700 m in Devonian rocks. More
method to recognize such wells among hundreds or thousands of
recent discoveries in the Basal Silurian/Upper Ordovician rocks genuine dry holes.
in the vicinity of this well should justify deepening or re 3. An old "dry hole" with anomalous CGG-ESTI may
evaluating the drilled anomaly of this well as the new structural indicate an "undiscovery well" and subsequently provide
contour map of the area proved it to be a closed structural high at justification for:
the Ordovician/Silurian contact (Mahmoud et al, 1990 and Cole A. reviewing drilled seismic anomaly for alternative
et al, 1994). interpretation; as the hole may have been wrongly
positioned,
Possible CGG-ESTI Anomalies B. re-examining the well as it may have been suspended or
An example of possible CGG-ESTI Anomalies is Possible declared "dry" under past logistics or exploration
Anomaly No. 2. The “Z” Anomaly of N. Iraq. This is part of a conditions,
larger proven Ain Zalah Anomaly. Neither BHT nor geothermal C. deepening of the borehole to test deeper potential
gradient data are available for this well, which was scouted as reservoir in areas where shallow reservoirs were the
dry hole. A review of the original final well log was not possible primary targets,
due to lack of information. D. to continue drilling for deeper targets when shallow
targets proved wet, dry or tight, this is in conjunction
Discussion with other indicators when and if the hole's interim
High geothermal gradient anomalies are associated with oil and CGG-ESTI is anomalous,
gas fields in basins all over the Globe. They can be used to E. selecting new exploration acreage in areas with
identify hydrocarbon migration fairways and potential favourable regional geothermal background and
"undiscovery" wells, such as K1-13 of Mobil Oil (now in anomalies.
Concession NC149 of Sirte Oil) in Libya and in other MENA
countries (Ibrahim, 2000). Added to other exploration methods, the CGG-ESTI maps and
Anomalous geothermal gradients are known in rocks overlying cross-plots should improve risk assessment of basins, blocks,
hydrocarbon traps. The anomalies are mainly caused by focused prospects, seismic anomalies and proposed or revised wells
migration of heat-convective subsurface fluids into shallow
reservoir closures.
SPE 105333 5

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