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Genetic potential of the Histria petroliferous basin

SARAMET MIHAI

Department of GeologyAl. I. Cuza University

Bulevardul Carol I, Nr.20A, Iasi 700505ROMANIA

[email protected]

RADUCANU RAZVAN

Department of MathematicsAl. I. Cuza University


[email protected]

CATUNEANU OCTAVIANFaculty of Science

University of Alberta, Tory 3-117Earth & Atmospheric Sciences

[email protected]

CHIRILA GABRIEL

Department of GeologyAl. I. Cuza University


[email protected]

Abstract: - The paper identifies geochemically the petroliferous system Histria on the Romanian shelf of the Black Seafrom the extension the orogene from north Dobrogea. This petroliferous system consists from the following twosubsystems: the first is of generation-expulsion type and is formed by the oligocene source rock and the second one isof migration-accumulation type and is formed by the accumulations of gases and oil which belong to the cretacic andeocene natural reservoirs. We have noticed through Rock-Eval analyses that the organic matter from the source rocksis of continental nature and generates gases and at the same time is of algae nature (lacustrine and/or marine) andgenerates oil and gases.

Key-Words:- Rock-Eval analysis, petroliferous basin, subsidence, hydrocarbons.

1 Introduction and geological background

The Dobrogea county of Romania consists of thefollowing tectonic units: North Dobrogea Orogen,Central Dobrogea Massif and South DobrogeaPlatform. These units extend eastwards beneath theBlack Sea into the Romanian shelf, and are separated by major faults Sf. Gheorghe, Peceneaga-Camena,Capidava-Ovidiu and Intramoesic (Clrai-Fierbini,Fig.1). The shelf portion of the North DobrogeaOrogen consists of three thrust sheets, with thethrusting direction towards north-west: the Tulceathrust, the Niculitel thrust and the Macin thrust. Thesestructural elements form the basement for the

sedimentary succession that accumulated during theevolution of the Black Sea in the offshore extension ofthe North Dobrogea Orogen. Within this tectonicsetting, the northern portion of the Romanian offshoreincludes a continental shelf and a deeper-water basin;the latter defines the Histria Basin (HistriaDepression), which has actively received sedimentsstarting with the Aptian. The continental slope thatoutlines the Histria Basin is partly controlled by basement faults (e.g., the Peceneaga-Camena fault tothe south). This slope formed in relation to theextensional regime that accompanied the opening ofthe intra-cratonic rift basin of the Black Sea during thelate Aptian-Albian.

Fig. 1 Location of the Romanian offshore

All hydrocarbon-bearing reservoirs which are pre-sently known within the Histria Basin (Sinoe, Delta,West Lebda, East Lebda and Pescru fields) weredeposited during the Albian-Eocen interval.The Histria Basin entered a compressional regimestarting with the beginning of the Oligocene.

The transition from extension, which dominated theevolution of the basin until the end of the Eocene, to

WATER AND GEOSCIENCE

ISSN: 1790-5095 68 ISBN: 978-960-474-160-1

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WELL DEPTH TOCo S1o S2

o fcio = 1 0.085 (S1

o + S2o) / TOCo

- m % mgHC/g rock -812.5 1.16 0.18 0.44 0.95531

Sinoe 922.5 1.85 0.15 1.23 0.937952.5 1.38 0.20 1.07 0.922813

Lebda V. 1082.5 1.60 0.18 1.80 0.890

1087.5 1.03 0.15 0.77 0.92431Sinoe 1202.5 1.07 0.08 0.53 0.951Mean - - - - 0.929

Table 1. The computation of the initial fraction of residual organic carbon from the oligocene source rock

compression starting with the Oligocene, generatedsignificant tectonic inversion, and was accompanied byhigh rates of subsidence in the basin. This tectonicinversion resulted in the uplift of the basement alongthe edge of the Histria Basin during the Oligocene, aswell as in the progradation of the depocenter. Highsediment supply following the Oligocene basin-margin

uplift resulted eventually in the complete fill of theAlbian-Eocene depression [4].Oligocene sediments in the Histria Basin are generallyof pelitic character. The Oligocene Sea was initiallyopened to the east and north-east of the Histriadepression. In the center and the north-west part of thedepression, the environmental conditions were typicalof a restrictive, closed basin of anoxic type.Subsidence within the basin led to rapid burial of thesediments and the preservation of organic matter,conducive to the formation of source rocks.Following burial, the source rocks entered a

geothermic field adequate to the maturation andexpulsion of hydrocarbons. Starting with the lateLower Miocene, the Oligocene source rocks havegenerated and expelled hydrocarbons. These hydro-carbons accumulated in traps with reservoirs ofEocene, Upper Cretaceous and Albian age, followingthe process of primary and secondary migration. TheOligocene shales played both the role of source rocksin the Histria Basin, and seal for the underlying Albianto Eocene reservoirs. This relationship between theAlbian Eocene reservoirs and the Oligocene sourcerocks and seals defines the petroleum system of the

Histria Basin. This petroleum system has been studied by means of chemical analyses of the oil producedfrom the Sinoe, Delta, West Lebda, East Lebda andPescru fields, as well as of the natural bitumencollected from the Oligocene source rocks.

2 The generation and expulsion of hydrocar-

bons: the methodWe consider a source rock analyzed through combustion. By using this method, one can measure theamount of total organic carbon content TOC as the

ratio between the mass of organic carbon C obtainedduring burning and the mass mrs of the source rock:

,rsm

CTOC= [%] or [g OC/100 g rock]. (1)

From relation (1) we obtain the quantity Cof organiccarbon:C = 10 TOC mrs. [mg OC]. (2)A source rock stores a certain quantity of organicmatter which may vary through time as the organic

matter matures and hydrocarbons are released. If weanalyzed the rock through combustion at an initialreference moment in time, when the initial quantity oftotal organic carbon is TOCo, then the correspondinginitial quantity of organic carbon is:Co = 10 TOCo mrs, [mg OC]. (3)As the source rock is buried gradually during theevolution of the sedimentary basin, it may reach anadequate geothermal field that may lead to thematuration of the organic matter and to the expulsionof hydrocarbons. It is assumed that the rock hasalready released a certain quantity of hydrocarbonswithin the sedimentary basin at the moment ofextraction from the producing fields.The quantity of organic carbon corresponding to theexpelled hydrocarbons OCis given by the differencebetween the initial quantity of organic carbon Co andthe residual (present-day) quantity of organic carbon Cas determined through combustion on the analyzedsource rock:OC = (Co C),[g OC] (4)From (2), (3) and (4) we have:OC = 10 (TOCo TOC) mrs, [mg OC] (5)

or:OC = 10TOCo (1TOC / TOCo)mrs, [mg OC]. (6)We define the following factor of hydrocarbonsexpulsion:fex = 1 TOC / TOC

o (7)

From (6) and (7) we have:OC = 10 fex TOC

o mrs, [mg OC], (8)

or:OC/mrs = 10

fex TOCo, [mg OC/ g rock], (9)

Relation (9) enables us to calculate the quantity ofhydrocarbons expelled from the source rock. Theconversion of the quantity of organic carbon OC/mrs

into expelled hydrocarbons HC/mrs is possible usinga w coefficient.


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WELL DEPTH TOC S1 S2

fci TOCo fex fex

ph fexph

- m % mg HC/g rock - % - - -1487.5 0.95 0.11 1.19 0.884 1.00 0.0500 0.0111 0.005510

Tomis 1832.5 1.49 0.09 1.17 0.928 1.49 0 0 0817

Lebda V.2067.5 1.31 0.09 1.31 0.909 1.34 0.0224 0.0050 0.0025

816Lebda V. 2215.0 3.93 0.71 22.12 0.506 7.21 0.4549 0.1871 0.09363497.0 2.27 0.50 3.93 0.834 2.53 0.1028 0.0228 0.011475

Coblcescu 3995.5 1.87 0.27 4.29 0.793 2.19 0.1461 0.0341 0.0170Table 2. The initial content of total organic carbon and the expulsion factors from the oligocene source rock

According to Pepper and Corvit [3] this coefficientdepends upon the number of carbon atoms in themolecule, and varies between 0.75 and 0.87 mgOC/mg HC (0.75 for methanol and 0.87 for n-alcanswith infinite length).Cooles et al. [1] and Lewan et al. [2] considered anaverage value ofw = 0.85 mg OC/mg HC. Accordingto this approach, the quantity of hydrocarbons expelledfrom a source rock is:HC/mrs=10 fex TOC

o/0.85 [mg HC/ g rock]. (10)

In order to estimate the quantity of hydrocarbonsexpelled by a source rock using relation (10) it isnecessary to know its initial total organic carboncontent TOCo. The amount of initial total organiccarbon of the source rock may be estimated by usingan algorithm as described below.We consider a source rock analyzed through com-bustion and pyrolysis (Rock-Eval analysis). The ana-

lysis through combustion allows the measurement ofthe total organic carbon content TOC; pyrolysis allowsone to determine the S1, S2 and S3 peaks, which havethe following meaning: The S1 peak represents the free (mobile) hydro-carbons generated by the organic matter in the sourcerock; The S2 peak represents the fixed hydrocarbons gene-rated by the organic matter in the source rock as aresult of pyrolysis; The S3 peak relates to residual organic matter whichdoes not contribute to the generation of hydrocarbons.

According to the principle of mass balance, thefollowing relation is true:TOC = S1 + S2 + S3. (11)The TOCis expressed in terms of % or gOC/100grock;the S1 and S2 peaks are expressed in terms ofmgHC/grock; and the S3 peak is expressed in terms ofmg CO2/g rock. In order to homogenize relation (11)we consider the following homogenization factor:w = 0.85 mg OC/mg HC. Consequently:10 TOC=0.85 (S1 + S2) + S3, [mg OC/g rock]. (12)We denote asfci the fraction of residual organic carbonfrom the total organic carbon content:

fci TOC =0.1 S3, [mg OC/g rock] (13)

From relations (12) and (13) we have:fci TOC = TOC 0.085 (S1 + S2) (14)andfci = 1 0.085 (S1 + S2) / TOC. (15)The following relation between the ratio of totalorganic carbon content and the ratio of the corres-ponding fractions of the residual organic carbon can beestablished:TOCo/TOC = fci

o / fci, (17)or:TOC

o= fci

o/ fci TOC, [%]. (18)

The following workflow can be used to determine theinitial total organic carbon content TOCo in the sourcerock: We perform analyses of combustion and pyrolysis onrocks from the same sedimentary basin, which aresimilar from a petrographic standpoint with the studiedsource rock, but with immature organic matter. We

thus determine TOCo

and the S1o

and S2o

peaks. Wethen use these values in relation (16) in order todetermine the fraction fci

o of residual organic carbonfrom the initial content of total organic carbon forevery immature rock. We then calculate their meanvalue:fci

med= 1/n fci

o (19)where (n) is the number of the analyzed samples; We use the TOC, S1 and S2 values known from theanalysis of the actual source rock in relation (15) inorder to calculate the fci fraction of residual organiccarbon from the total organic carbon content;

We then use the relation (18) written under thefollowing form:TOCo = fci

med/ fci TOC, [%] (20)to determine the initial content of residual organiccarbon of the analyzed rock.

3 The generation and expulsion of hydrocarbonsThe Oligocene sedimentary formations from theHistria Basin consist generally of compact shales,immature organic matter and thus are representativefor the initial conditions of formation of the sourcerock. Table 1 presents the initial content of organic

carbon TOCo and the values of the S1o and S2o peaksfor each sample.


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Fig. 2 Expulsion factor obtained through dry and hydro pyrolysis (Rock-Eval)

We then use the relation (16) to calculate the fractionsfci

o of residual organic carbon from the initial contentsof total organic carbon. The calculated mean value forthe fractionsfci

o isfcimed

= 0.929.Table 2 includes samples that contain mature organicmatter and are likely to have expelled hydrocarbons inthe past. Samples in the table are listed according todepth, and the corresponding values ofTOCand of theS1 and S2 peaks are indicated as well. We use therelations (15), (20) and (7) to compute the fraction fciof residual organic carbon from the total organiccarbon, the initial content in total organic carbonTOCo, and the expulsion factorfex.Lewan et al., [2] estimated the quantities of expelledhydrocarbons from the New Albany shale in theIllinois Basin. They noticed that the expulsion factor ofthe hydrocarbons determined according to the Rock-Eval pyrolysis method is overestimated. They proposed that a more realistic expulsion factor may bedetermined by using a hydro pyrolysis technique.

According to Lewans et al. [2] experiments, a quan-tity of 300500 g of rock (with cuttings ranging in sizefrom 0.5 cm to 2 cm) is introduced in a reactor whichcontains water at a constant temperature of 250365oC. During hydro pyrolysis, water heating takes placein a closed circuit installation during a 3 to 6-day timeinterval. The pressure within the reactor may vary between 10 and 20 MPa, depending on temperature,the content of organic matter of the source rock andthe duration of heating. The pressure is the result of thewater vapours and of the presence of gases at the upper part of the reactor. If the rock sample is a potential

source rock subjected to adequate conditions andduration for the experiment, the rock expels a quantity

of oil that accumulates at the surface of water in thereactor. The molecular and isotopic composition of theexpelled oil is similar to that of natural oil. Thissuggests that the analysis through hydro combustiondoes simulate closely the natural process ofhydrocarbon expulsion from source rocks. Theanalysis of the New Albany shales has demon-stratedthat the expulsion factor through hydro pyrolysis issmaller than the one obtained from Rock-Eval pyrolysis (Fig.3). We have used this conclusion tocalibrate the results obtained in the case of theOligocene source rocks in the Histria Basin. Figure 3illustrates the correlation between the expulsion factordetermined through the Rock-Eval method and theexpulsion factor determined through hydro pyrolysis.The mathematical expression of this correlation isgiven by:

fexph

= 304. 80210-6

+ 0.252828fex 0.781901fex2

++ 6.27273fex

3 13.65fex4 + 11.7165fex

5.

Table 3 presents the values of the expulsion factorsfexph by taking into account the hydro pyrolysis. Even

though hydro pyrolysis simulates the natural expulsionof hydrocarbons from the source rock, the expulsionfactors obtained through hydro pyrolysis seem tooverestimate the natural ones. Considering the varia- bles which may lead to an overestimation of theexpulsion factors determined through hydro pyrolysis(e.g., the increased solubility of the water from thenetwork of natural bitumen; the decrease in pressureduring the experiment; and the thermal expansion ofthe hydrocarbons) it was proposed that the expulsion

factor under natural conditions is half of the expulsionfactor measured with the method of hydro pyrolysis.


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WELL DEPTH TOCo fexph HC/mrs TYPE OF ORGANIC MATTER

- m % - mg HC/g rock -10

Tomis1487.5 1.00 0.0055 0.065

817Lebda V

2067.5 1.34 0.0025 0.039Continental, produce gases

816Lebda V 2215.0 7.21 0.0936 7.939 Marine, produce parafinic and condensed oil

3497.0 2.53 0.0114 0.33975Coblcescu 3995.5 2.19 0.0170 0.438

Continental, produce gases

Table 3. The unit quantities of expulsion hydrocarbons from the oligocene source rock

Fig. 3 Expulsion factor hydro depending with ex-pulsion factor from Rock-Eval pyrolysis

Based on this conclusion, as well as consideringrelation (10), we have calculated in Table 3 the

quantities of the hydrocarbons expelled in the case ofthe Histria Basin. It can be observed that the sourcerocks from the wells 10 Tomis, 817 West Lebda and75 Coblcescu have expelled small amounts of gases,while the sample from the 816 Lebda Vest well hasexpelled a much higher quantity of oil and gases.

4 ConclusionsThe petroleum system of the Histria Basin developsalong the offshore extension of the North Dobrogeaorogen, within the deep-water setting of the Back Sea(Fig. 1). This system has a hydrocarbon generation-

expulsion component, represented by Oligocenesource rocks, and a migration-accumulation compo-nent that involves reservoirs of Albian to Eocene age.This study focuses on the geochemical analysis ofthe Oligocene source rocks. Rock-Eval analysesindicate a complex origin for the organic matter inthe source rocks, from a continental nature, gene-rating gases, to a lacustrine and/or marine algalnature, generating oil and gases.The expulsion potential of these rocks has beenestimated to 0.0390.438 Kg gases/t rock and to7.939 Kg oil/t rock.

References[1] COLLES G.P.,MACKENZIE A.S.,QUIGLEY T.M.(1986), Calculation of petroleum masses generatedand expelled fron source rocks, Org. Geochem., 10,p. 2516-2521.[2] LEWAN M.D., COMER J.B., HAMILTON-SMITH

T., HASENMUELLERN.R., GUTHRIE J.M., HATCHJ.R., GAUTIER D.L., FRANKIE W.T. (1995), Feasi- bility Study of Material-Balance Assessment ofPetroleum from the New Albany Shale in theIllinois Basin, U. S. Geological Survey Bulletin2137, Washington.[3] PEPPERA.S.&DODD T.A.(1995), Simple kine-tic models of petroleum formation, Part II: oil/gascracking, Marine and Petroleum Geology, vol. 12,no. 3, p. 321-340.[4] ARAMET M., GAVRILESCU G., CRNGANU C.(2007), Quantitative estimation of expelled fluids

from Oligocene rocks, Histria Basin, WesternBlack Sea, Marine and Petroleum Geology, doi:10.1016/j.marpetgeo.2007.05.


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