14
GEOLOGICA BALCANICA, 23. 4, Sofia, August. 1993, p. 65-78. Modelling of subsidence and thermal history of Paleogene sequence in Dolna Kamchija depression} Western Black Sea Basin Elli Chouparova 1 , Noriyuki Suzuki 1 , Peter Bokov 2 1 Department of Geology, Shimane University, Nishikawatssu 1060, Matsue 690, Japan 2 Scientific Research Institute of Mineral Resources, Sitnyakovo Blvd 23, 1505 Sofia, Bulgaria (Received 04. 12. 1992; accepted 19. 02. 1993) E. lfynapo11a, H. Cy3yKu, n. SoKOB- MoOeAupo«auue nozpy:JK:eHUR u mepMaJibHoii ucmopuu nocAeOoBameJI!r nacmu na11eozena B j{oAuo-KaM'lUiiCKoti Oenpeccuu, 3anaouo-lfepm>MopcKuii 6acceiiu. O,!J;HoMepHoo MO,!J;emrposa- HBe norpyJKCHIDl K TCPMll.llbHOll: KCTOpHK C DOMOI.Jlb!O CHCTCMbl MO,li;CITHPOBaHJil( MAT98 npOBO,li;IlJIOCb .lJ.]Ill CKBa- liCHHbl R-64, JloHroJa, c uenLIO HJy'leHKJI HCTOPHH coJpesaHHll nocnep;osareiTbHOCTH naneoreHa. Ka.JIH6pHpo- saHHe MO,li;CITHpOBaHKJI OCHOBbiBaeTCll Ha ,ll;aiiHbiX K3MCpeHHJ:!: JnHMCpH3aUHH CTepaHOB R OrpaJKaTCJibHOJ:!: CDQ- co6HOCTH BHTpHHHTa. J.iCTOPKll norpylKeHKJI llOCITC,li;OBaTCnhHOCTH OCITOlKHlleTCll npHCYTCTBHCM TpeX 3po3HOH- Hb!X COObiTHll: B naneoreKe H HCOreHe. 0cHOBaHHall aa C,li;CJiaHHOM MO,ll;eJIHpOsaHHH, Jp03HOHHall HCTOPHll C ll03- ,li;HCf0 O.IIIIrOUCiia ,11;0 paliHCfO MHOQelia BOCllPHHHMaeTCll HaH60JICC CYllleCTBCHiib!M C06b!THCM .lJ.]Ill HCTOpHH norpylKCHHll H COBpeMCiiHOfO ypOBHll 3peJIOCTK OTJIOJKCHBI!:. MO,li;CJIHpOBaHHe TCpMaJibHOJ:!: HCTOPHH TCCHO CBli- JaHO C MO,!J;eiTHpOBaHHeM naneOTCfiJIOBOTO DOTOKa, KOTOpbiJ:!: C'IHTaeTCll JIHHCJ:!:HO Jaryxa!OIIIHM C BpeMCHeM. MO,li;CJI'bHhiC nepeMCHHhiC - Jp03HOHHall HCTOpKll K renJIOBOH UOTOK - MOfYT KOMITCHCHpOBaTb ce611 B3aHMHO, B HCKOTOpOJ:!: CTeneHH, 'IT06LI o6ecne'IHTb y ,li;OBJICTBOPHTCJibHOO npH6JIHJKeliHe K K3MCpeHHLIM napa.MeTpaM C03peBaHKJI, ,[(onyCKall MHHHManLHOO Jlia'leHKe O,li;HOTO H3 HHX (DOCTOlOiHLill: naneOTCnJIOBOJ:!: llOTOK, pasHLIJ:!: COBpeMeHHOMy HJlH OTCYTCTBHe Jp03HK), MOlKiiO OOJIY'IHTL MaKCHManLHoe B03MOJICHOO JHa'leHHe .lJ.]Ill ,lJ;pyrOrO. PeJym.rarLI MO)J,eiTHposaHHll csH.D;eTCITLCTBYIOT o MaKCHMam.HbiX naneoTeMneparypax Ha rpaHHIJ,e naneoreua- sepXHero MeJia B npe)J.enax 72-79°C. 06'beM paJMLIT'biX OTITOJKeliHJ:I: B Te'leHHe OJIHTOQeH-MHOUeHOBOTO Jpo- 3HOHHOrO C06LITHll, paBHbiil: npH6JIH3KTCJILHO 500 m, C'IHTaeTCll reOJIOfH'ICCKH B03M0lKHbiM. IlO,li;TBep,lJ;H- JIOCb, 'ITO JpeJIOCTb UOCJie,li;OBaTeJlbHOCTH naneoreHa, OCHOBaHHall Ha napaMeTpax OTpaJKaTCJibHOJ:!: CDOC06- HOCTH BUTpHKHTa H CTepaHOBOll: JUHMepHJaUHK, cpaBHHTCJibHO HH3Kall (%R, <0,47%, Epi <0,32). Abstract. One-dimensional modelling of subsidence and thermal history, using modelling system MAT98, is performed for borehole R-64 Longoza to evaluate the maturation history of a Paleogene sequence. The calib- ration of modelling is based on measured data set of sterane epimerization and vitrinite reflectance. Subsidence history of the sequency is complicated by the presence of three erosional events during Paleogene and Neogene times. Based on the performed modelling, the erosional history from Late Oligocene to Early Miocene time is evaluated to be the most important event for the subsidence history and present maturity level of sediments. The modelling of thermal history is closely related to the modelling of paleoheat flow which is assumed to de- crease lineary up to present. The modelled variables- erosional and heat flow histories - can compensate each other to some extent in order to achieve a satisfactory fit with Ihe measured maturity parameters. Assuming minimum value for one of them (constant paleoheat flow eq ual to present day one or no erosion) we can obtain the maximum possible value for the another. The results of modelling show maximum paleotemperatures of the Paleogene - Late Cretaceous boundary in the range of 72°C to 79°C. An amount of eroded sediments du- ring Oligocene-Miocene erosional event about 500 m is proposed to be geologically plausible. The maturity of Paleogene sequence, based of the parameters of vitrinite reflectance and sterane epimerization, is confirmed to be relatively low ( %R 0 < 0.47%, Epi<0.32). Introduction The Dolna Kamchija depression (DKD) is located in the coastal area of the Black Sea (Fig. 1). A great part of this area is si tuated offshore. The region has been object of many investi- gations, mainly concerning the tectonics (At an as o v, 1961 ; K a 1 ink o,l972; B o k ov 5

Modelling of subsidence and thermal history of Paleogene ... · COBpeMeHHOMy HJlH OTCYTCTBHe Jp03HK), MOlKiiO OOJIY'IHTL MaKCHManLHoe B03MOJICHOO JHa'leHHe .lJ.]Ill ,lJ;pyrOrO. PeJym.rarLI

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Page 1: Modelling of subsidence and thermal history of Paleogene ... · COBpeMeHHOMy HJlH OTCYTCTBHe Jp03HK), MOlKiiO OOJIY'IHTL MaKCHManLHoe B03MOJICHOO JHa'leHHe .lJ.]Ill ,lJ;pyrOrO. PeJym.rarLI

GEOLOGICA BALCANICA, 23. 4, Sofia, August. 1993, p. 65-78.

Modelling of subsidence and thermal history of Paleogene sequence in Dolna Kamchija depression} Western Black Sea Basin

Elli Chouparova1, Noriyuki Suzuki1, Peter Bokov2

1Department of Geology, Shimane University, Nishikawatssu 1060, Matsue 690, Japan 2 Scientific Research Institute of Mineral Resources, Sitnyakovo Blvd 23, 1505 Sofia, Bulgaria

(Received 04. 12. 1992; accepted 19. 02. 1993)

E. lfynapo11a, H. Cy3yKu, n. SoKOB- MoOeAupo«auue nozpy:JK:eHUR u mepMaJibHoii ucmopuu nocAeOoBameJI!r nacmu na11eozena B j{oAuo-KaM'lUiiCKoti Oenpeccuu, 3anaouo-lfepm>MopcKuii 6acceiiu. O,!J;HoMepHoo MO,!J;emrposa­HBe norpyJKCHIDl K TCPMll.llbHOll: KCTOpHK C DOMOI.Jlb!O CHCTCMbl MO,li;CITHPOBaHJil( MAT98 npOBO,li;IlJIOCb .lJ.]Ill CKBa­liCHHbl R-64, JloHroJa, c uenLIO HJy'leHKJI HCTOPHH coJpesaHHll nocnep;osareiTbHOCTH naneoreHa. Ka.JIH6pHpo­saHHe MO,li;CITHpOBaHKJI OCHOBbiBaeTCll Ha ,ll;aiiHbiX K3MCpeHHJ:!: JnHMCpH3aUHH CTepaHOB R OrpaJKaTCJibHOJ:!: CDQ­co6HOCTH BHTpHHHTa. J.iCTOPKll norpylKeHKJI llOCITC,li;OBaTCnhHOCTH OCITOlKHlleTCll npHCYTCTBHCM TpeX 3po3HOH­Hb!X COObiTHll: B naneoreKe H HCOreHe. 0cHOBaHHall aa C,li;CJiaHHOM MO,ll;eJIHpOsaHHH, Jp03HOHHall HCTOPHll C ll03-,li;HCf0 O.IIIIrOUCiia ,11;0 paliHCfO MHOQelia BOCllPHHHMaeTCll HaH60JICC CYllleCTBCHiib!M C06b!THCM .lJ.]Ill HCTOpHH norpylKCHHll H COBpeMCiiHOfO ypOBHll 3peJIOCTK OTJIOJKCHBI!:. MO,li;CJIHpOBaHHe TCpMaJibHOJ:!: HCTOPHH TCCHO CBli­JaHO C MO,!J;eiTHpOBaHHeM naneOTCfiJIOBOTO DOTOKa, KOTOpbiJ:!: C'IHTaeTCll JIHHCJ:!:HO Jaryxa!OIIIHM C BpeMCHeM. MO,li;CJI'bHhiC nepeMCHHhiC - Jp03HOHHall HCTOpKll K renJIOBOH UOTOK - MOfYT KOMITCHCHpOBaTb ce611 B3aHMHO, B HCKOTOpOJ:!: CTeneHH, 'IT06LI o6ecne'IHTb y ,li;OBJICTBOPHTCJibHOO npH6JIHJKeliHe K K3MCpeHHLIM napa.MeTpaM C03peBaHKJI, ,[(onyCKall MHHHManLHOO Jlia'leHKe O,li;HOTO H3 HHX (DOCTOlOiHLill: naneOTCnJIOBOJ:!: llOTOK, pasHLIJ:!: COBpeMeHHOMy HJlH OTCYTCTBHe Jp03HK), MOlKiiO OOJIY'IHTL MaKCHManLHoe B03MOJICHOO JHa'leHHe .lJ.]Ill ,lJ;pyrOrO. PeJym.rarLI MO)J,eiTHposaHHll csH.D;eTCITLCTBYIOT o MaKCHMam.HbiX naneoTeMneparypax Ha rpaHHIJ,e naneoreua­sepXHero MeJia B npe)J.enax 72-79°C. 06'beM paJMLIT'biX OTITOJKeliHJ:I: B Te'leHHe OJIHTOQeH-MHOUeHOBOTO Jpo-3HOHHOrO C06LITHll, paBHbiil: npH6JIH3KTCJILHO 500 m, C'IHTaeTCll reOJIOfH'ICCKH B03M0lKHbiM. IlO,li;TBep,lJ;H­JIOCb, 'ITO JpeJIOCTb UOCJie,li;OBaTeJlbHOCTH naneoreHa, OCHOBaHHall Ha napaMeTpax OTpaJKaTCJibHOJ:!: CDOC06-HOCTH BUTpHKHTa H CTepaHOBOll: JUHMepHJaUHK, cpaBHHTCJibHO HH3Kall (%R, <0,47%, Epi <0,32).

Abstract. One-dimensional modelling of subsidence and thermal history, using modelling system MAT98, is performed for borehole R-64 Longoza to evaluate the maturation history of a Paleogene sequence. The calib­ration of modelling is based on measured data set of sterane epimerization and vitrinite reflectance. Subsidence history of the sequency is complicated by the presence of three erosional events during Paleogene and Neogene times. Based on the performed modelling, the erosional history from Late Oligocene to Early Miocene time is evaluated to be the most important event for the subsidence history and present maturity level of sediments. The modelling of thermal history is closely related to the modelling of paleoheat flow which is assumed to de­crease lineary up to present. The modelled variables- erosional and heat flow histories - can compensate each other to some extent in order to achieve a satisfactory fit with Ihe measured maturity parameters. Assuming minimum value for one of them (constant paleoheat flow equal to present day one or no erosion) we can obtain the maximum possible value for the another. The results of modelling show maximum paleotemperatures of the Paleogene - Late Cretaceous boundary in the range of 72°C to 79°C. An amount of eroded sediments du­ring Oligocene-Miocene erosional event about 500 m is proposed to be geologically plausible. The maturity of Paleogene sequence, based of the parameters of vitrinite reflectance and sterane epimerization, is confirmed to be relatively low ( %R0 < 0.47 %, Epi<0.32).

Introduction

The Dolna Kamchija depression (DKD) is located in the coastal area of the Black Sea (Fig. 1). A great part of this area is situated offshore. The region has been object of many investi­gations, mainly concerning the tectonics (At an as o v, 1961 ; K a 1 ink o,l972; B o k ov 5 ~

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0 ~ 10 km ~

~~ ~Cil N ~ • ~

- I 0 50 100 ·-. km

Fig. I. A. General scheme of the main tectonic elements in Bulgaria: R- Rhodope massif; SMM - Ser­bian-Macedonian massif; KE - Kraishtides; SC - Southern Carpathides; B. Location of Dolna Kamchia depression and borehole R-64 Longoza (after D ache v et al., 1988, slightly changed)

et al., 1987; B o k o v, 1989), lithofacies (Shuts k a j a, 1972; Po I s t e r, 1972, unpubli­shed data; B o k o v et al., 1987), distribution of reservoir rocks (D e s h e v, 1974) and gas content (Man de v, 1954; At an as o v & Da sh eva, 1962; V e I e v, 1979). The major interest to this area was attributed to the common gas shows and presence of small deposit. Recently, with the started petroleum exploration in the offshore part of the depression, further investigations in this region are actuated again.

Time and temperature are the most sensitive parameters for maturity proceeding and kerogen conversion. Therefore, the reconstruction of temperature history is very impor­tant for evaluation of petroleum prospects (T i s so t & We It e, 1984). The temperature history of potential source rock results from the subsidence history (burial vs. time) and thermal history (heat flux vs. time) of the basin. One possible way for reconstruction of temperature history is to simulate the evolutions of kerogen and related maturity para­meters (vitrinite reflectance, epimerization of steranes, etc.) and to adjust (calibrate) them with the analytical data set by computer modelling (T i s so t et al., 1987; WapI e s et al., 1991).

Our purpose in this paper is to demonstrate an application of the computer aided ID modelling to evaluate the subsidence, thermal and maturation histories of Paleogene sequence in DKD. The plausible result from this modelling could be expanded to the offshore part of the depression where actual geological data are scarce (Fig. 1).

The modelling system used in our study is a presonal computer aided basin modelling system MAT98 (Suzuki, 1990). The calibration of modelling is based on measured data of vitrinite reflectance and sterane epimerization.

66

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Computer aided integrated basin modelling

Computer aided integrated basin modelling is one of the modern approaches to under­stand and reconstruct the geological and thermal history of a sedimentary basin and eva­luate the petroleum potential. The fundamentals of modelling concepts are introduced in the studies of T i s s o t & E s p i t a 1 i e (1975), T i s s o t & We I t e (1984), G u i­dish et al. (1985), Tis sot et al. (1987); Nakayama (1987), Lerche (1990) and others. Computer aided basin modelling enables in a quantitative manner integration of geological, geochemical and geophysical information. In basin modelling, the sedimen­tary basin is taken as a rock volume with changing boundary conditions and a changing sedimentary fill. The heat transfer into the system is controlled by the layers with different lithologies and thermal conductivity characteristics.

The modelling study proceeds in a following way: determination of the regional geologic framework data (stratigraphy, tectonics, lithology, etc.) to construct quantitatively the conceptual (geological) model; simulation, performed by the mathematical model of a mo­delling system to achieve the predictions of modelling - forward model; calibration and optimization of the conceptual model comparing predictions of modelling (calculated data) with geological observations (analytical data) -inverse model.

Nowadays, there are two main tendencies in the development of modelling process and modelling systems, respectively. One emphasizes on the forward model i. e. construction of the conceptual model. Modelling systems are directed to perform extensive geologic simulations (including migration, accumulation and three dimensional modelling) using huge computers with large memory capacity. Usually, several hours are required to achieve the predictions of modelling.

Another one emphasizes on the inverse model, i. e. on the calibration of predictions achieved by a forward model with present day measured data. Running many times forward­inverse models a best fit could be found. Modelling systems are suitable for personal com­puters which are more operative and the simulations are achieved only for several seconds or minutes.

The approach which we followed in our study is the second one. Modelling system MA T98 (S u z u k i, 1990) is developed based on the common personal computers. The system enables examination of modelling conditions, one-dimensional (1 D) and pseudo­two-dimensional (20) reconstruction of burial and thermal histories as well as numerical analyses of hydrocarbon generation from kerogen. The evaluation of present-day heat flow based on subsurface temperatures or present geothermal gradient and the activation energy distribution of petroleum generation from kerogen also can be easily performed. The calibration of burial and thermal histories can be obtained based on the current kinetic modelling of vitrinite reflectance and sterane epimerization (or other first order kinetic parameters).

Data for calibration of modelling

Calibration of temperature history modelling can be achieved using measured thermal in­dications - paleotemperature indicators (appatite fission track annealing, fluid-inclusion homogenization temperatures) or indicators of total thermal history (vitrinite reflectance, biomarker ratios, spore color). The vitrinite reflectance and the sterane epimerization are used for calibration of temperature history in the present paper. The combination of vitri­nite reflectance and sterane epimerization is favourable for calibration of temperature his­tory because the relative sensitivities of these parameters to time and temperature are different (W a p 1 e s & Machi h a r a, 1991).

To evaluate sterane epimerization and vitrinite reflectance distributions we performed gas chromatography-mass spectrometry (GC-MS) analyses and microscopic measure­ments of polished samples, respectively. The samples for GC-MS analyses were selected

67

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from the clayey parts of the sequence to prevent the influence of possible migration effects. The samples for vitrinite reflectance measurements were chosen based on the macroscopic and microscopic observations of the rocks. Short descriptions of the samples are presen­ted in Table l.

Experimental and analytical results

GC-MS a n a l y s e s. The bitumens were obtained from the powdered samples by ex­traction for 48 hrs in a Soxhlet apparatus with benzene /methanol (9: I). Saturated hydro­carbons were isolated from the extracts using silica gel (MERK 60 PF254) thin layer chro­matography with elution by n-hexane. This saturated hydrocarbon fraction was concen-

Table2

Table 1

Descriptions of the samples and maturity parameters for calibration of modelling

Depth (m) Age I Lithology S /(S + R) %Ro

929.5 Oli mdst 0.05 1262.5 Oli mrl 0.09 0.32 1385 Eoc 2-3 mrl 0.33 1552 Eoc 2-3 mrl 0.35 1677 Eoc 2-3 mrl 0.16 1750 Eoc 1-2 mdst 0.15 1800 Pal2 lmst 0.21 0.44 1936.5 K2 lmst 0.32 0.48 1963 K2 1mst 0.33 0.47

S /(S + R) represents the progress of epimerization at C20 position for C29 steranes with 5a, 14a, 17a configuration. The definition of a parameter given by the corresponding peak numbers from Fig. 2 is S{(S+ R) = 14{(14 + 17).

List of steranes identified on m fz 217 fragmentograms of Fig. 2

Peak No

68

1 2 3 4 5 6

7 ·8 9

10 11 12 13 14 15

16 17 18

Carbon No

27 27 27 27 28 27 29 28 27 27 29 28 28 28 28 29 29 29 29 29 30

Identification

131J(H), 17a(H)- Diacholestane (20S) 131J(H), 17a(H) - Diacholestane (20R) 131J(H), 17IJ(H)- Diacholestane (20S) 131J(H), 171J(H) - Diacholestane 24- Methyl - 131J(H), 17a(H) - Diacholestane (20R) 5a(H), 14a(H), 17a(H) - Cholestane (20S)

± 24-Ethy!-131J(H), 17a(H)- Diacholestane (20 S) + 24-Methyl-13P{H), 17a(H)- Diacholestane (20 S)

5a(H), 1413(H), 171J(H)- Cholestane (20 R) 5a(H), 14a(H), 17a(H) - Cholestane (20 R) 24-Ethyl-13a(H), 17jJ(H) - Diacholestane 24-Methyl(5a(H), 14a(H), 17a(H) - Cholestane (20 S) 24-Methyl-5a(H), 14a(H), 17a(H)- Cholestane (20 R) 24-Methyl-Sa(H), 14IJ(H), 171J(H) - Cholestane (20 R) 24-Methyl-5a(H), 14a(H), 17a(H) - Cholestane (20 R) 24-Ethyl-5a(H), 14a(H), 17a(H) - Cholestane (20 S) 24-Ethyl-5a(H), 141J(H), 171J(H) - Cholestane (20 S)

+ 24-Ethyl-5a(H), 141J(H), 17p(H) - Cholestane (20 R) 2+Ethyl-5a(H), 151J(H), 17P{H) - Cholestane (20 S) 24-Ethyl-5a(H), 14a(H), 17a(H)- Cholestane (20 R) Sterane

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8

929.5 m

1262.5 m

1677.0 m

1936.5 m

Fig. 2. Mass fragmentograms of sterane di stribution . Peak identification is presented in Table 2

trated and injected into a gas chromatograph - mass spectrometer (Shimadzu QP2000A) equipped with a chemical bonded fu sed silica capillary column (OV- 1.25 m x 0.25 mm i. d.). The oven temperature wa s programmed from 40°C to 300°C at 4°C fmm using helium as carrier ga s. Identification of individual steranes was performed by comparison of reten­tion times and mass spectra with tho se publi shed by S e i f e r t & M o I do wan (1978), P h i I p ( 1985), Johns ( 1986), S a k a t a et a!. ( 1988). Relative abundances of steranes

69

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were determined by measuring peak areas on the mfz 217 mass fragmentograms. Exam­ples of sterane distributions on m fz 217 fragmentograms are shown in Fig. 2 and their peak identification in Table 2. The progress of epimerization for C29 -steranes was esti­mated considering the relative increase of 20S epimers and the ratio 20Sf(20R+ 20S). V i t r i n i t e ref 1 e c t a n c e me a s u r e me n t s. To obtain vitrinite particles for reflectance measurements, first wa s applied the procedure for kerogen isolation and afterwards a preparation of specimen for microscopic investigation.

The isolation of kerogen involved removal of the rock matrix (carbonate minerals and silicates) by digesting the samples in hydrochloric and hydrofluoric acids for 18-24 hrs. Using flotation in aqueou s zinc bromide, centryfuging and sieving, the coaly particles were recovered. They were mounted in plastic slides and poli shed on 400, 600, 800 and 1000 grit wheel consequently. Additional fine poli shing was performed with a diamond paste. The measurements were carried out on a Leitz MPV-2 with Orthoplan-POL microphotometer. The mean % R

0 is used in the following discussion.

The a n a I y t i c a I r e s u I t s. They characterize the sequence presented in borehole R-64 to be at relatively low maturity level - with values for sterane epimerization less than 0.33 and vitrinite reflectance- Jes s than 0.47 % (Table 1). These results are consis­tent with the previous works (B o k o v et al., 1987) where the maturity of Paleogene and Upper Cretaceous rocks in NE Bulgaria wa s estimated to be at a protocatagenetic stage (V a s s o e v i c h et al., 1969).

Construction of a model and input data

The computer simulation requires a quantification of all defining parameters during basin development. Construction of a conceptual model comprises the continuous, sequental record of events as a function of time during the evolution of a basin. Each event represents a time span during which one of the three basic geological processes occurs : deposition, erosion, non-deposition (or hiatus). The sediments which have been depo sited during an event and which have not been totally eroded during a later erosional event result in a "layer".

The input data used for the modelling are shown in Table 3. The ages (Ma) of the units were determined considering the paleontological and recent lithostratigraphical investi­gations in Dolna Kamchia region (V a vi 1 ova et al. , 1975; D j u ran o v & Dar a k­c hie v a, 1986; D jura nov, 1987, 1988, 1989, unpublished data). The sedimentary sequence was subdivided into 15 events, in order to simulate the burial history. Several layers were represented as 2 or 3 units in order to allow more control points (Table 3).

During the Tertiary time, three main erosional events occurred in the region- in the end of Late Cretaceous - be ginning of Paleogene, in the end of Paleogene - begin­ning of Neogene and in the end of Neogene times. Their importance and possible overburden will be discussed later. The Alpine tectonic movements connected with rising of the Bal­kanides to the south of Dolna Kamchja depression had not been of such significance in its central zone (well R-64). There are no geological evidences for erosion in that part of the region. Therefore, the event was determined as non-deposition. Submarine erosional processes in the end of Late Eocene- beginning of Oligocene time were similarly expressed in the input data for modelling (B o k o v, 1989).

The thicknesses and lithologies of layers were defined based on the information from well log data and core materials. Interbeddings of different lithologies in a layer were repre­sented with their relative proportion s ( %).

Matrix thermal conductivity data and depth-poro sity relationships for different litho­logies were not available or sufficient. The values shown in Table 4 were approximated based on the published data and considering mineral assemblages of each lithology (A t h y, 1930; Hedberg, 1936; Sclatter&Christie, 1980; Velinov&Boja­djieva, 1981; Sekiguchi, 1984; Deming&Chapman, 1989 ; Brigaud et al., 1990).

70

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Table 3

Initial input data for modelling - borehole R-64 Longoza

Present

No Name of layer Depth and Age (Ma) Assumed Erosion (m)

1 K 1964 72 2 Erosion 1372 66 3 Pal2 1872 60 4 Pal2 1830 59 5 Eoc 1- 2 1788 57 6 Nodepo 1720 54 7 Eoc 2- 3 1720 48 8 Nodepo 1288 40 9 Oli (1) 1288 35

10 Oli(2) 1000 34 11 Oli (3) 850 31 12 Erosion - 123 29 13 Mio (l) 377 13 14 Mio (2) 120 12 15 Erosion - 200 10 16 0 0

Present Depth: At the base of the layer ; Age: Geologic age corresponding to Present Depth ; To: Temperature at sediment {water interface; Lmst:Limestone; Slst: Siltstone; Mrl:Marl; Sdst: Sandstone;

Water To (0 C) Depth Lithology and Proportion

(m) ( %)in unit layer

lO 40 lmst :100 10 0 10 0 lmst :100 10 60 lmst :100 10 60 slst /Mdst :90 /10 10 90 10 200 mrl :100 lO 280 10 300 Mdst /Sdst :65 /35 10 200 Mdst /Sdst :75 /25 10 130 Mdst /Sdst :50 /50 10 0 10 0 Sdst /Mdst :60 /40 10 50 Sdst /Mdst :60 /40 10 0 10 0

K 2- Upper Cretaceous; Pal 2 - Upper Paleogene (Junashka formation) ; Eoc 1-2- Lower-Middle Eoce­ne (Dikilitashka + Beloslavska formations); Eoc 2- 3- Middle-Upper Eocene (Avren formation); Oli - Oligo­cene; Mio - Miocene.

Table 4

Parameters and constants used in modelling

Lithology

Sandstone Siltstone Mudstone Marl Limestone

Kinetic constants

Porosity P ( %) vs. Depth Z (km)

P = 40exp(--0.09400Z) P = 50exp(--0.18326Z) P = 65exp( --0.38065Z) P = 60exp( -0.38065Z) P = 55exp(--0.10034Z)

Matrix Thermal Condicitivity

(meal/em. c. °C)

7.1 5.95 5.6 6.0 6.3

Reversible E (kJfmol)=84 A (1/s)=0.0066 reaction from 0 to 0.54 (Epimeriza-tion , etc.) after A I e x a n d e r et a!. (1986) Vitrinite reflectance modelling EASY %Ro - after S w e e n e y & B u r n h a m (1990)

Water depths of the basin during geologic time were determined based on conside­rations about tectonics, lithofacies (8 o k o vet a!., 1987; 8 o k o v, 1989) and paleontolo­gical observations (D j u r a nov, 1987, unpublished data; K o j u m d j i eva et a!.,

71

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1989). We accepted the concept of deep marine environment of sedimentation during Late· Eocene (Avren Formation) and Early Oligocene time in the central part of Dolna Kamchija depression where R-64 borehole is situated (B o k o v& 0 g n ian o v, 1978, 1979; B o­k 0 v, 1989).

Vitrinite reflectance modelling was performed by EASY %Ra (S w e e n e y & B u r n­h a m, 1990). The kinetic constants of sterane epimerization were adopted from A I e x a n­d e r et al. (1986).

In order to simulate the temperature history, the sediment {water interface tempera­tures during the deposition of each layer are required. Sediment {water interface tempera­tures in our modelling were assumed as constant (10°C) during the geologic time, similar to the present bottom temperatures in the Black Sea region (Er i c k s o n & v o n He r­z e n, 1978). The estimation of present day heat flow requires data about present day tem­perature profile or present day geothermal gradient. We utilized the available data (Com­mitee of Geology, Sofia) for geothermal gradient (2.4°C {100 m) in the region calculated for the depth interval 2300-3400 m.

Results of modelling

Two main problems came forward with the modelling process. First one is related to the erosional history and amount of eroded sediments. Second one is concerned with the pa­leoheat flow history of the basin.

Three main erosional events occurred in the studied area: in the end of Late Creta­ceous -beginning of Paleogene, in the end of Paleogene - beginning of Neogene and in

GEOLOGIC AGE (Ma)

1 000 m Erosion

1000

15 00

2000 20 30

0 m '"0 -t :I: -3 -

Fig. 3 Geohistory diagram of borehole R-64 Longozia in a case of modelled 1000 m eroded sediments during Oligocene-Miocene time. Assumed amounts of overburden during Late Cretaceous-Paleocene and Neogene­Quaternary erosional events are 500 m and 200 m, respectively

72

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Geologic Age ( Ma)

80 70 60 50 40 30

U. Cretaceous Paleocene Eocene Oligocene

MIN H.Fiow - MAX Erosion (1300 m)

0 ........ ---100 ... - - - - - ---------

, , ,

---

20 10

Miocene

Fig. 4. Modelling of paleoheat flow and amount of overburden (refer also to Fig. 6)

0

0.4

0.8

1.2

1.6

2.0

:X: C'D D) --n -0 :e -3 (') D)

~ 3 ~ C'D ~

the end of Neogene time. The ero sional history during the first event does not affect the maturity modelling of a Paleogene sequence. It could have some importance for the matu­ration history of deeper possible source rocks as Lower Cretaceous and Jurassic horizons. The recent erosional event had relatively short duration and the ero sional removal seems not to be so much and thu s le ss significant. The ero sional event in the end of Oligocene­beginning of Miocene time occurred when the horizons were at or near their maximum paleodepths and temperatures, i. e. during th is period the Paleogene sequence has expe­rienced its effective heating time (H o o d et a!. , 1975) (Fig. 3). Therefore, we con sidered this event as the most important one for the present maturity level of sediments and we reduced the modelling of overburden only for the Oligocene-Miocene time.

Heat flow hi story could be reconstructed based on : forward (conceptual) model s ­accepting a priori geological model(s) for heat flow evolution or inverse model s - calib­rating the simulated model s with measured data of thermal indicators. The main publi­shed geological model s compri se: thermal expul sion model (S I e e p, 1971); lithosphere streching model (McKenzie, 1978) ; dike intrusion model (Royden eta!. , 1980) ; deep crustal metamorphi sm model (F a I v e y & Mid dIet on, 1981). These model s require clear geological hi story of the region and still, there a re some di screpancies among them. We followed the second approach, con sidering al so the general geological development of the region.

The tectonic history of Dolna Kamchija region is complex and there are many uncer­tainties and controversial opinions, especially concerning its earlier hi story. The depres­sion is considered as an orogenic foredeep formed during the Middle-Upper Eocene tec­tonic movements connected with the rising of the Balkanides. The Jurassic-Early Cre­taceou s extension, related to the sea floor spreading hi story of the Atlantic ocean is consi-

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dered to be the main tectonic event in the earlier times (Dewey et al., 1973; H suet al., 1977; Lamoine, 1978; Robe r t s & Dixon, 1984).

A relative estimation of the changes in a paleoheat flow could be achieved considering some lithological data, e. g. presence of volcanic rocks. To the south of present day DKD and the Balkanides, intensive volcanic activity is registered to exist during the Late Cre­taceous time in the Srednogorie zone.

These very general geological considerations allow only to presume that during Cre­taceous and Jurassic times, the heat flow most probably had been higher than the present day one. The relationship between heat flow and geologic time is determined with some approximations to be linear (L e r c he et al., 1984). Based on these estimations, we al­lowed the heat flow to decrease lineary with the geologic time up to present.

Even with the above considerations for erosional and heat flow histories, there are still too many possible combinations which will satisfy the calibration with measured data of vitrinite reflectance and epimerization. To solve the problem, our approach was first to determine the extreme limits between these two variables. Assuming no erosion, we ob­tained the highest possible heat flow history. Assuming paleoheat flow to be constant with the present day one (minimum), we obtained the maximum possible amount of eroded sediments during Oligocene - Miocene time (Fig. 4). In this way, we achieved a conside­rable reduction of the potentially acceptable relationships between heat flow history and amount of overburden. Within thus defined extreme limits, there are still several cases which satisfy a calibration with measured data (Fig. 5). The plausible model is among them.

Vitrinite Reflectance(%) Epimerization S/(S+R)

.2 .3.4.5 1 2 " 3 .1 .2 .3 .4

0.5

0 I I I I II· I I

I \~, I ~--~~ ~~----~------~

-E ~ -.s:::::

1.0 -c. Q)

c ~l

1.5 •.

- ~1 2.0 ---t--'---+--t'~-1--1 ----1

I' i

Fig. 5. Calibration of modelling with measured analytical data (dots). The analytical errors for vitrinite ref· leciance measurements and GC-MS analysis are ± 0.07 and ± 0.04, respectively. Dotted area represents the field of calculated range of variation for different modelled cases

74

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Geologic Age (Ma)

;ac j

·....-:; : ~

: ;::.r I!

L_ ________________________ __

I ·-:;;! 1if

Fig. 6. Geohistory diagrams of modelled cases. For the pattern of paleoheat flow, refer to Fig. 4

c CD

"C -::::T -3 -

We demonstrate two cases with assumed overburden of 500 m and 1000 m and heat flow history as shown in Fig. 4 and Fig. 6. The choice of a plausible model should be based on geological considerations or other methods.

Plausibility of modelling

The plausibility of modelling was evaluated considering the regional geological observations and using some additional analytical data.

Maximum temperatures

X-ray powder diffraction analyses of clay minerals for estimation of paleotemperatures was performed in the studied borehole R-64.Results showed that clay minerals are compo­sed mainly of montmorilonite. No presence of montmorilonite-illite mixed-layer mine­rals was confirmed. According to A o y a g i & A sakawa (1984), those results sug­gest that maximum paleotemperatures of the sequence were less than 100°C and they are consistent with the calculated temperatures in the modelling (Fig. 6).

Overburden

Geologically, an estimation of the amount of eroded section is possible if a fully presented sequence is available. A complete sequence of the Oligocene time in NE Bulgaria does not exist. However, in the adjacent regions to the north of Do!na Kamchija depression - Avren step and Varna monocline - the Oligocene sequence is represented by its upper part which is absent in the depression (Chou p a r ova-Pets eva, 1991). That part is also influenced by the erosion but in a less scale and the obserbved maximum thickness is about 400-500 m. Therefore, we could consider that overburden about 500 m is geologically plausible.

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Present heat flow

Basically, calculated and measured value s of pre sent heat flow must be the same. Calcula­ted by our modelling pre sent day heat flow values (0.76 to 0.87 mca1 Jcm2 fsec or 31 to 36 W fm2, Fig. 4) are consi stent with the average values in the Black See region (30 to 40 m W fm2 or 0. 71 to 0.95 mcal fcm2 fsec.). However, they are significantly different from those in the Bulgarian part of the Black Sea (50 to 60m W fm2 or 1.19 to 1.43 mcal fcm2/sec) (E r i c k s o n & von He r zen, 1978 ; D u c h k o v & Kazan t s e v, 1985). We modelled a case with the publi shed present day heat flow of 1.38 mcal fcm2 fsec (58 m W fm2) (D u c h k o v & K a z a n t s e v, 1985) being con stant during the geologic time. Although we a ssumed no erosion at all, the calculated value s for vitrinite reflectance and sterane epimeriza­tion were much higher ( %R0 - 0.59; Epi - 0.54) than the mea sured ones ( %Ra - 0.47; Epi - 0.33). Moreover, we have to give abnormally high thermal conductivities to obtain a present heat flow of 1.38 mcal fcm2 /sec with the condition for geothermal gradient of 2.4°Cf100 m or to increase the gradient. It should be noted that at shallower depth s (above 1000 m), geothermal gradients in Dolna Kamchija depres sion are reported to be higher (2.8-3.1°Cfl00m)(Velinov&Bojadjieva, 1981 ; Petrovet al., 1991). How­ever, many factors influence the temperature s and thermal gradients in this zone where compaction processes a re not completed. U sed in the modelling geothermal gradient of 2.4°Cfl00 m is determined in deeper zone (2300-3400 m) and could be con sidered more reliable.

One of the po ss ible geological rea sons which could interpret higher geothermal gra­dients and heat flows registered in the shallower zone is the effect of heat tran sfer by con­vection. Geological structure of Dolna Kamchija depression is favourable for water move­ment and convection heat transfer. The upward inclination of sediments toward s on­shore, thick Neogene and Paleogene aquifer, a s well a s several regional faults and uncon­formities are observed. Present-day structure of Dolna Kamchija depression should be re­garded in connection with the formation of the Black Sea depression (cauldron-like structu­re) and its rapid subsidence during Pliocene-Quaternary time (Degen s etal., 1978 ; Jasko, 1984). The inten sive sub sidence is proposed to begin a s early as the Oligocene time (M u­r a t o v, 1975). Thus, it seems probable that ovbserved higher present heat flow could be caused by recent geological processes. In this ca se, the maturity modelling of Gretaceous and Paleogene horizon s is not affected by tho se later events.

Summary and conclusions

One dimensional modelling of subsidence and thermal hi stories is performed to evaluate the maturation hi story of Paleogene sequence in Dolna Kamchija depression. An approach of modelling is demon strated when two variables -erosional hi story and heat flow his­tory - are unknown. The modelling of ero sional history wa s reduced to the most impor­tant Oligocene - Miocene erosional event. Paleoheat flow was assumed to decrease li­nearly up to present. First were determined the extreme limits between amount of eroded sediments and paleoheat flow. A ssuming minimum for one of the variables, we obtained the maximum po ssible value for the other. Within determined limits, several candidates for the pla usible model were defined. Based on geological consideration s, a model with an amount of overburden about 500 m wa s propo sed to be a plausible one. As a re sult of the modelling we determined the hi story of maturation of Paleogene sequence. It wa s expected from the previous studie s that the maturity level of Paleogene sediments in DKD (on shore) is re­latively low but the maturation hi story wa s not known. There are many possible models of erosional and heat flow hi story which could lead to the same present level of maturity. We propose a model which is plausible for the modelling conditions used.

Paleogene horizon s continue in the offshore part of the depre ssion where the burial situation is more favourable and a higher maturity could be expected. The plausible model determined onshore could be expanded in the offshore to model the petroleum generation of the sequence there. 76

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Acknowledgment. We thank Scientific Research Institute of Mineral Resources and Committee of Geology, Bul­garia for the permission and providing some initial geological and geophysical data. We are also grateful to A. Hirai and Teikoku Oil Company, Japan for the assistance in performing vitrinite reflectance measurements. This work was supported financially by Japan National Science Foundation Grants 03640642 from Monbusho (Ministry of Education, Science and Culture of Japan). E. Chouparova highly appreciates and acknowledges Monbusho for the scholarship provided.

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