determination of hydrocarbon prospective areas in the tuzgolu (saltlake) basin, central anatolia, by...

Journal of Petroleum Science and Engineering 62 (2008) 36–44

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Journal of Petroleum Science and Engineering

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Determination of hydrocarbon prospective areas in the Tuzgolu (Saltlake) Basin,central Anatolia, by using geophysical data

Attila Aydemir a,⁎, Abdullah Ates b

a Turkiye Petrolleri A.O. Mustafa Kemal Mah. 2.Cad. No: 86, 06100 Sogutozu, Ankara, Turkeyb Ankara University, Faculty of Engineering, Department of Geophysical Engineering, 06100, Besevler, Ankara, Turkey

⁎ Corresponding author. Tel.: +90 312 207 2342; fax:E-mail address: [email protected] (A. Aydemir).

0920-4105/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.petrol.2008.07.005

a b s t r a c t
a r t i c l e i n f o
Article history:
Tuzgolu Basin is the large Received 20 November 2007Accepted 13 July 2008
Keywords:Tuzgolu BasinSouthern Aksaray DepressionSultanhani and Eregli DepressionsSouthern Tuzgolu HighSuluklu–Cihanbeyli–Goloren AnomalyVertical derivativeAnalytical Signal

st interior basin in Central Anatolia, Turkey, with significant hydrocarbonindications in outcrops and exploration wells. However, there is no commercial discovery since 1959 which isthe beginning year of exploration activities. Because of the poor seismic quality, all available geological-geophysical data and methods should be used in integration with each other to carry out explorationactivities. In previous studies, the basin was modeled three dimensionally (3D) using gravity data by theauthors of this paper and results of the modeling study were published recently. The model results can beevaluated to determine probable hydrocarbon generation zones. Gravity anomalies in this region exhibitmany inflections that could be prospective locations for hydrocarbons. In this study, the gravity data weresubjected to the vertical derivative in order to isolate inflections and to determine concealed structurally highareas in the subsurface that could have hydrocarbon potential. These potential subsurface structures werealso compared with the Analytical Signal map produced from the aeromagnetic anomalies to reveal if theyare originated from a magmatic intrusion. Finally, it was determined that all exploration wells were drilledoff-structure and none of these potential subsurface structures was tested. According to correlation of theprevious well locations and determined subsurface structures in or around the hydrocarbon generationzones, it is possible to claim that the Tuzgolu Basin with no previous discovery remains prospective forhydrocarbon exploration activities in the future.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

The Tuzgolu (Salt Lake) Basin is located in Central Anatolia, Turkey.It is the largest interior basinwhich is related with the Haymana Basinto the north and they are connected to each other through a channelshaped small basin named Tersakan Basin (Aydemir, 2005). All basinscan be considered collectively as the Central Anatolian Basins locatedto the south of Ankara (Fig. 1).

The Tuzgolu (Salt Lake) Basin has been subjected to the hydrocarbonexploration activities since 1959 (Rigo de Righi and Cortesini, 1959).Thirty (including re-entries)wells were drilled and about 5500 km two-dimensional (2-D) seismic data have been acquired since then. TheTurkish Petroleum Corporation (TPAO) has performed the majority ofexploration activities in the basin. There are plenty of published studieson the geology and hydrocarbon potential of the Tuzgolu Basin by TPAOgeoscientists and academicians. Some of the prominent studies are asfollows: Bailey and McCallien (1953), Unalan et al. (1976), Sengor andYilmaz (1981), Gorur et al. (1984,1998), Gursoy et al. (1998), Ates (1999),Ates and Kearey (2001), Coskun (2004), Aydemir (2005), Aydemir andAtes (2005), Ayyildiz (2000, 2006). The Tuzgolu Basin has beenmodeled

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three dimensionally (3-D) by Aydemir (2005) for the first time to revealthedeep structure of the basin, and themodeling resultswere publishedconsidering the Haymana, Tersakan and Tuzgolu Basins separately byAydemir and Ates (2006a). Recently, three-dimensional (3-D) gravitymodel results of the Tuzgolu Basin have been compared withgeochemical property maps of the major source rock, the Karapinar-yaylasi Formation (Aydemir, 2008). Gravity anomalies in the TuzgoluBasin exhibit many inflections that could be evaluated as the potentialsubsurface structures for hydrocarbon exploration. The present studyshows the correlationbetween the derivative of the gravityanomalies incomparisonwith thehydrocarbongeneration zones in thegravitymodelmap. Afterwards, these potential areas were compared with theAnalytical Signal map of magnetic anomalies to reveal if these subsur-face structures have a magmatic origin. Finally, it was determined thatnone of the previous exploration wells penetrated these potentialsubsurface structures and results show that the Tuzgolu Basin remainsprospective for the hydrocarbon exploration in the future.

2. Geological setting

The Tuzgolu (Salt Lake) Basin is located on the Kirsehir Block inCentral Anatolia. Surface of the study area is mostly covered bysediments of the Neogene and Quaternary units (Fig. 1). They are

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Fig.1. Generalized surface geologymap. Study area is surrounded by a rectangle. Modified fromGursoy et al., 1998. KEF: Kirikkale–Erbaa Fault, CATB: Central Anatolian Thrust Belt, LT:Lake Tuzgolu, SAF: Sereflikochisar–Aksaray Fault, EF: Ecemis Fault. Numbered symbols of the geological units; 1: Quaternary Volcanics, 2: Pliocene Volcanics and volcanoclastics,3: Mio–Pliocene Volcanics and volcanoclastics, 4: Ophiolitic mélange, 5: Tokat Massif, 6: Kirsehir Massif, 7: Cover units (Neogene and Quaternary).

37A. Aydemir, A. Ates / Journal of Petroleum Science and Engineering 62 (2008) 36–44

surrounded by the Kirsehir Metamorphics to the east and theKutahya–Bolkardagi Metamorphics of the Menderes–Taurus Platformto the west. Ophiolitic, mafic–ultramafic rocks originated from theprevious oceanic crusts take place along with the suture zones to thenorth. Southern part of the basin is covered by the CappadocianVolcanic Complex. Other sedimentary units older than the Neogeneare observed around the eastern boundary of the Tuzgolu Basin in verylimited zones so that they are not illustrated in the surface geologymap. Only major tectonic unit on the surface is the right lateral strike–slip Sereflikochisar–Aksaray Fault (SAF) extending along the easternboundary of the Lake Tuzgolu (Fig. 1).

General stratigraphic columnar section of the Tuzgolu Basin isillustrated in Fig. 2. Metamorphic basement of the Kirsehir Block wasoverlain by terrestrial redbeds and conglomerates of the KartalFormation. The Haymana Formation deposited in deeper parts of thebasin at the same time (Late Maastrichtian) with a dominantlithology of turbidites and other deep sedimentary materials. Lime-stones of the Asmabogazi Member of this formation were depositedon margins as shallow equivalent. In Paleocene and Eocene, theTuzgolu Basin had relatively shallow marine environment andsediments of this period are represented by shallow-marineturbidites of the Karapinaryaylasi Formation which is mainlycomposed of shales, siltstones and sandstones. Rock-evaluation andpyrolysis results indicate that this formation is the major source rock

in the basin having very high TOC (0.82% in average) and potentialyield values in the mature to extremely mature stages. Organicmatter types are generally Type II and III. The hydrocarbongeneration potential (S1+S2) indicates mainly natural gas generationtogether with the oil generation potential in some places coincidingto the deepest part of the basin (Aydemir, 2008). Reefoidal limestonesof the Caldag Formation developed on the paleo-highs as shallowequivalent of the Karapinaryaylasi Formation. They are bothconsidered as main reservoir units in the basin in case of reefoidaldevelopments preserved their reservoir properties from the destruc-tive diagenetic influences, and they were targeted in previousexploration wells. It is very difficult to distinguish the Early Eoceneunits from the Paleocene turbidites by means of lithological changesin the Tuzgolu Basin so that they are collectively named as theKarapinaryaylasi Formation (Dellaloglu and Aksu, 1984). However,the Early Eocene units named the Eskipolatli Formation aredistinguished from the Paleocene Kirkkavak Formation in theneighbouring Haymana Basin to the north. End of the mid-Eoceneis a collision and accretion time of all blocks and plates formingAnatolian assemblage (Gorur et al., 1998). As a result of this majortectonic gathering, the Tuzgolu Basin shallowed and became alacustrine environment giving way to terrestrial, lacustrine andevaporite deposition at the end of Lutetian. The Yassipur Formation isthe sedimentary unit of this period and it is composed of coarse

Fig. 2. Generalized stratigraphic columnar section of the Tuzgolu Basin. Abbreviations;PL–Q: Pliocene–Quaternary, AM: Akbogaz Member, CM: Cavuskalesi Member,KM: Karamollausagi Member, ABM: Asmayaylasi Member, KCC: Kirsehir CrystallineComplex, BP: Baranadag Pluton.

38 A. Aydemir, A. Ates / Journal of Petroleum Science and Engineering 62 (2008) 36–44

grained sandstones and siltstones intercalated withmarls and a thickevaporite unit named Akbogaz Member (Fig. 2). The KochisarFormation (Oligo–Miocene) overlies unconformably all Eoceneunits in the Tuzgolu Basin. The Haymana Basin was exposed duringOligo–Miocene, as a result, the Yassipur and Kochisar Formationscan not be observed around this basin. Finally, the Mio–PlioceneCihanbeyli Formation unconformably overlies all of the older rocksand covers almost all over the northern andwestern part of the studyarea (Fig. 1). The lithology of this unit is composed of conglomerate,sandstone, siltstone, claystone and lacustrine limestone. It alsocontains volcanic units alternated with the sedimentary units insome places.

3. Geophysical data

3.1. Gravity data

Gravity data were collected by the General Directorate of MineralResearch and Exploration of Turkey (MTA) and measurements weretied to the Potsdam (981260.00 mgal) base value. After surveying;latitude, free-air, Bouguer, topographical and tidal corrections wereapplied to the measured gravity data. Then, the data were gridded at2.5 km interval. The gravity anomaly map of the Tuzgolu Basin isgiven in Fig. 3 with the 5 mGal contour interval. Topography of thestudy area is almost flat excluding the volcanic mountains ofHasandagi, Melendizdagi and the vicinity to the east of the studyarea. However, there are no topographical changes on the surface toinfluence the gravity anomalies for most of the Tuzgolu Basin,although there are lots of anomalous changes on the gravity datato indicate depressive areas and some relatively high subsurfacestructures. Additionally, some inflection locations on the anomalycontours can also be observed and these inflections could beevaluated as potential traps or prospective areas for hydrocarbons.However, they have no indications on the surface and no clearevidences on seismic sections.

3.2. Aeromagnetic data

Aeromagnetic data were acquired by the MTA measuring totalcomponent of the geomagnetic field along profiles in the N–Sdirections with 2 km line spacing. Flight altitude is about 600 m andthe sampling interval is approximately 70 m. Surveyed values werereduced to October 1982 reference value and they were subjected tothe daily variation and direction error corrections. “InternationalGeomagnetic Reference Field-IGRF” values were calculated using aprogram developed by Baldwin and Langel (1993). The areaincluding the Tuzgolu Basin was bounded by the Zone-3 in thestudy of Aydemir and Ates (2006a). Residual magnetic anomaly mapof this zone after IGRF values removed is given in Fig. 4 as the studyarea.

An obvious NW–SE trending broad magnetic anomaly isobserved in this figure crossing the western part of the studyarea. This anomaly has been named as the Suluklu–Cihanbeyli–Goloren Anomaly by Aydemir (2005) and two-dimensional (2D)modeling of this anomaly was performed by Aydemir and Ates(2006b).

3.3. Well data

Most of the 30 hydrocarbon exploration wells including 7 re-entries were drilled by the Turkish Petroleum Corporation (TPAO) inCentral Anatolia. Some of them having shallow total depth (TD) valueswere drilled for stratigraphic purposes. Unfortunately, no economicalhydrocarbon discoveries have been realized from any of the explora-tion wells until now. Well-composite logs used in this study wereprovided by the General Directorate of Petroleum Affairs (GDPA). Well

Fig. 3. Gravity anomaly map of the Tuzgolu Basin and surrounding area. Contour Interval: 5 mGal. GD: Goloren Mountain. Borehole abbreviations; TG: Tuzgolu Wells (1–10), 2:Karapinar-2, 3: Karapinar-3.


locations and the Aksaray city center are illustrated in Figs. 3 and 4 andthe following figures.

4. Three-dimensional (3D) gravity modelling results

The sedimentary fill of the Tuzgolu Basin is mainly composed ofshales, sandstones and limestones deposited on isolated paleo-highs,together with the evaporitic units deposited on the ancient shallowplaces and coastal plains. Basement units are composed of schists,marbles and graywackes. Density difference between the sedimentarybasin fill and the basement was obtained as 0.25 gr cm− 3 for 3Dgravity modeling (Aydemir and Ates, 2006a). Density contrast is thekey parameter for model construction which was performed by usinga computer program developed by Cordell and Henderson (1968).Gravity model map of the Tuzgolu Basin with the Aksaray city center,important volcanic eruption centers (mountains) and well locations isgiven in Fig. 5.

Three main depression zones in the Tuzgolu Basin were observedas hydrocarbon generation zones: a) Southern Aksaray, b) Eregli and,c) Sultanhani Depressions. The Southern Tuzgolu High which is abasement-related tectonic high is located between the SouthernAksaray and the Sultanhani Depressions. The GolorenMountain that islocated on the southeastern edge of the Suluklu–Cihanbeyli–GolorenMagnetic Anomaly (Figs. 3 and 4) separates the Sultanhani and Eregli

Depressions. Previous exploration activities were concentrated only inthe Sultanhani Depression and partly in the Eregli Depression.However, the largest depression in the basin is the Southern AksarayDepressionwhich is an unexplored area with no borehole and seismicdata. Possible reason for the absence of any exploration activity onthis depression could be the younger volcanic cover on this area(Fig. 1). Deep hydrocarbon exploration wells were drilled along thewestern margin of the Sultanhani Depression, but only two shallowwells (TG-9, TG-10) were drilled in the Eregli Depression to obtainsubsurface geological information and for the stratigraphy control.Most of the seismic data were also acquired on the SultanhaniDepression which is composed of two sub-depressions; the exploredone to the north named as the Sultanhani Depression-North (SD-N)and unexplored one to the south named as the Sultanhani Depression-South (SD-S). Depth values obtained from the 3D gravity model werecorrelated with the seismic and well data recently by Aydemir andAtes (2006a) and a good consistency was observed.

5. Vertical derivative of gravity anomalies and interpretation

Vertical derivative in the potential field methods is an applicationto distinguish the effects of local masses in an ambient, regional fielddata. The reason for usage of derivative can be explained as follows;effects of local and relatively small masses are often concealed and

Fig. 4. Residual magnetic anomaly map of study area. Contour Interval: 50 nT. Borehole abbreviations; TG: Tuzgolu Wells (1–10), 2: Karapinar-2, 3: Karapinar-3. S–C–G Anomaly:Suluklu–Cihanbeyli–Goloren Anomaly.


included in the response of more regional, large masses. It is especiallyvalid for the basinal effect of the gravity data. First and secondderivative applications are used to differentiate deeper and moreregional concealed masses, and to resolve and accentuate shallowsources, respectively. The mathematical expression of the derivativecan be given as follows (Robinson and Silvia, 1981):

Let the potential field is represented by

U x; y; zð Þ ð5:1Þ

The Laplace equation is given as

j2U x; y; zð Þ ¼ 0 for zN0 ð5:2Þ

or

A2UAx2

þ A2UAy2

þ A2UAz2

¼ 0 ð5:3Þ

This would introduce the boundary condition (observed data onthe surface) U (x, y, 0) which is known, but also the other boundarycondition ∂U (0, 0, z)/ ∂z which is not known. In order to obtain thesolution, the Fourier Transform can be used instead of the Laplace

Transform and the Transfer Function can be obtained for the solutionof problem. Transfer Function in three-dimension can be given asfollows;

ed k2xþk2yð Þ1=2 ð5:4Þ

Here;

d distance (in z direction)kx wave-number in the x directionky wave-number in the y direction

From this representation, the vertical derivative of the potentialfield can be performed by the operator (Gunn, 1975) given below:

1n

k2x þ k2y� �1=2

� �nð5:5Þ

n: rank of the derivative and it can be given as n=1 for the first verticalderivative.

In this study, the gravity anomaly map was subjected to the firstvertical derivative in order to determine concealed local tectonichighs (structures) in the subsurface of the Tuzgolu Basin. However, theresidual magnetic anomaly map of the study area was also

Fig. 5. Three-dimensional gravity model of the Tuzgolu Basin. Contour interval: 1 km. Lows from 6 to 13 km are hachured. SAF: Sereflikochisar–Aksaray Fault, GD: GolorenMountain,SD-N: Sultanhani Depression-North, SD-S: Sultanhani Depression-South, Borehole abbreviations; TG: Tuzgolu Wells (1–10), 2: Karapinar-2, 3: Karapinar-3. Contours in peripheralband have been suppressed because of edge effects arising from 3D modeling procedure.


transformed to the Analytical Signal to understand if these buried,regional structures are resulted by magmatic intrusions. The Analy-tical Signal transform exhibits the exact locations of causative bodiescreating magnetic anomalies, and it is independent of their directionof magnetization (Blakely, 1996). Analytical Signal can be expressed asthe vertical and horizontal gradient of the magnetic anomaly and it isformulated in 3-D case by

A x; yð Þ ¼ AMAx

iþ AMAy

jþ AMAz

k ð5:6Þ

Where,M is the magnitude of magnetic anomaly, i, j and k are unitvectors in the x, y and z directions, respectively (Bilim and Ates, 2003).

The amplitude function of the Analytical Signal can be given asfollows;

jA x; yð Þj ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAMAx

� �2

þ AMAy

� �2

þ AMAz

� �2s

ð5:7Þ

First vertical derivative map of the gravity anomalies is given inFig. 6 which shows some important negative and positive contourclosures. Negatives are illustrated with blue and positives with redcolors. These contour closures have not been tested by exploration

wells, whether they are prospective or not. Moreover, they have notbeen investigated with a derivative approach until this study. Incomparison with the 3D modeling results, the Southern Tuzgolu Highis the most important and longest subsurface structure in the TuzgoluBasin and there is an apparent contour closure on the SouthernTuzgolu High to the southwest of TG-7 well in Fig. 6. Similarly, thesecond important high is extending in the NW–SE direction betweenthe Eregli Depression and Northern Eregli Depression to the south(Fig. 5). A contour closure consistent with this high can be observed tothe north of TG-10 well in the vertical derivative map (Fig. 6). There isalso another contour closure to the south of Aksaray city center whichcould be a potential structure in terms of the hydrocarbon exploration(Fig. 6). It is located to the east of Aksaray-1 well and by theSereflikochisar–Aksaray Fault. The other contour closure to the east ofprevious one could be interesting, but it is probably located on the up-thrown block of the fault which is originally a normal fault dipping tothe west. Common characteristics of these closures mentioned aboveare that they do not coincide with any magnetic contour closures inthe Analytical Signal map of the aeromagnetic data (Fig. 7), and thatthey are all away from any of aeromagnetic anomalies (Fig. 4).

There are also some other contour closures as shown in Fig. 6, butthey are thought to be magmatic in origin because of their locationsextending around the magnetic anomalies, and they are not suggested

Fig. 6. First vertical derivative map of the gravity data in the Tuzgolu Basin. Contour Interval: 0.5 mGal/km. Contours in peripheral band have been suppressed because of edge effectsarising from derivative procedure.


to evaluate for the hydrocarbon potential. Additionally, these contourclosures aremostly located around the volcanic outcrops. For instance,two contour closures to the south of Goloren town are located on theGoloren Mountain (GD in Fig. 5) which is a volcanic cone on thesurface. There is another apparent closure to the west of these twinclosures. However, that closure is located to the west of the southernpart of Sultanhani Depression-South (SD-S) and to the south ofKarapinar-3 well. The metamorphic basement was encountered at thedepth of 1315m in this well after a section of Mio–Pliocene sediments.As a result, this closure can not be suggested to investigate for thehydrocarbon potential. It is probably a shallow and local basementrelated high. Other contour closures around themountains, Hasandagiand Melendizdagi can also be interpreted to be originated from themagmatic intrusions.

6. Conclusions and discussions

The Tuzgolu Basin is the largest andmost important interior basin inCentral Anatolia for the hydrocarbon exploration. It has three importantdepressions covering vast areas (Aydemir and Ates, 2006a) and thesedepressions can be evaluated as kitchen areas for the source rockdeposition and hydrocarbonmaturation. Sediment deposition since theLate Cretaceous to Quaternary is generally composed of turbiditic unitsincluding some formations bearing important source rock potential

and some others with good reservoir properties. Seal problem does notexist in the basin. However, there is not any economical discoveryrecorded up to date. The seismic quality is generally poor and explora-tion strategies require using all available geological–geophysical dataand methods in integration with each other.

There are two important and basement-related subsurface struc-tures; the larger one in the middle of Tuzgolu Basin named theSouthern Tuzgolu High located between the Sultanhani and SouthernAksaray Depressions, and the second in the middle of two sub-depressions of the Eregli Depression. Their extension similaritybetween the sub-depressions removes the hydrocarbon migrationproblem, because they are located on the migration pathwaysbetween the possible kitchen areas from both sides. Verticalderivative map of the gravity data (Fig. 6) includes some contourclosures on these structures indicating prospective areas for thehydrocarbon exploration. There are also some fault related contourclosures around the Sereflikochisar–Aksaray Fault that could also beprospective. However, some other contour closures can also beobserved around the magnetic anomalies, but they were evaluated asmagmatic intrusion related structures, because the Analytical Signal(AS) map of the aeromagnetic data also contains highs on the samelocations.

The Southern Aksaray Depression is the most important anddeepest part of the Tuzgolu Basin. According to the 3D modeling

Fig. 7. Analytical Signal (AS) map of the aeromagnetic anomalies in the Tuzgolu Basin.


results, an average of 8–9 km thick overburden is expected above themetamorphic basement. This thickness is sufficient for the over-burden pressure required for the hydrocarbon generation andgeothermal gradient results of previous studies (Ates et al., 2005)support the expectations of hydrocarbon generation in this zone.However, this zone has been neglected for hydrocarbon activitiespossibly because of the volcanic cover on the surface. It takes placebetween two extinct volcanos, Hasandagi and Melendizdagi. Mean-while, this depression deserves more detailed and further studies forthe hydrocarbon exploration. Although, existence of magmaticactivity around this important depression is risky for the hydrocarbonbearing, it can also be advantageous for the maturation process ofhydrocarbons if there is reasonable distance from the magmaticintrusions.

There are about 30 exploration wells (more than 15 of them aredeep wells) drilled in the Tuzgolu Basin and other related, neighbour-ing basins. Apparent hydrocarbon shows were recorded in most wellsand uneconomical amount of gas was also tested in Karapinar wells.However, none of deep exploration wells are located on theprospective contour closures observed on the subsurface structures,in comparison with the gravity derivative map. As a conclusion, it ispossible to say that the Tuzgolu Basin has not been tested properly andcan be evaluated as an unexplored basin in spite of previousexploration activities. Additionally, existent geochemical studies are

also consistent with the geophysical modeling results. In order toreveal hydrocarbon potential of the Tuzgolu Basin, an intensiveseismic data acquisition activity should be programmed focusing onthe contour closures observed in this study and a drilling campaignshould also follow the interpretation of new data to be acquired in thefuture.

Acknowledgement

We would like to express our sincere thanks to the GeneralDirectorate Mineral Research and Exploration (MTA) for the use ofgravity and magnetic data that were provided for a TUBITAK Project(Project No: YDABCAG-118). Seismic sections and well compositelogs were also provided for the same project by the General Directoryof Petroleum Affairs (GDPA). Authors would also like to thank Mr.Serdar Demiralin for reading and critising the manuscript beforesubmission.

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determination of hydrocarbon prospective areas in the tuzgolu (saltlake) basin, central anatolia, by...

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