Energy & Fuek 1992,6,666677
666
Generation and Expulsion of Hydrocarbons from a Paris Basin Toarcian Source Rock An Experimental Study by Confined-System Pyrolysis Ph. Blanc and J. Connan* Elf Aquitaine, Centre Scientifique et Technique Jean Feger, Avenue Larribau, 64018 Pau Cedex, France Received February 20, 1992. Revised Manuscript Received June 11, 1992
An experimental study of the generation and the expulsion of hydrocarbons from a Paris Basin Toarcian source rock has been carried out by means of confined pyrolysis in gold sealed tubes at 330 "C. Emphasis has been laid on the influence of pressure conditions (from 170 to loo0 bar). The results of experiments undertaken under vacuum have also been considered. Rock as well as isolated organic matter samples have been heated this way, for 6 h to as long as 14 days. Every class of effluents has been recovered gas, gasoline, water, and expelled and residual oils, as well as the solid residue, which corresponds grossly to the remaining kerogen. Several obvious examples are shown of the positive effect of pressure on the expulsion efficienciesof the c13+oily products and its negative effect on the hydrocarbon generation from organic matter. High-pressure conditions also favor the expulsionof polar/macromolecularcompounds. Participation of the mineral matrix in the production of carbon dioxide and water has been evaluated, and the influence of pressure on the COz generation from organic and/or mineral origins analyzed. A secondarycracking phenomenon has been revealed, taking place between 24 h and 5 days of heating, giving birth to light products and an insoluble pyrobitumen. This secondary process affects the expelled oil more than the residual one. Lastly, a reverse trend of the effect of pressure on the evolution of organic matter is suspected between 550 and loo0 bar, which could in fact result from mixed competitive reactions, either favored or unfavored by high-pressure conditions. In any case, pressure has a direct influence on the nature and the quantity of the products generated and expelled; therefore, this parameter should be taken into account for correct modeling of generation and expulsion of hydrocarbons from sedimentary rocks.
Introduction The complexity of phenomena involved during the natural evolution of organic matter with burial brought organicgeochemiststo undertake simulation of maturation under laboratory conditions. Lots of studies have been performed, based on the principlethat artificialmaturation can be suitably described by pyrolysis experiments. Confined-system pyrolysis has been shown to reproduce natural evolution more accurately than either open or closed but unconfined pyrolysis, at least for type I1 and type I11 related organic matter.'* The role of water still remains controversial. Hydrous pyrolysis is generally considered as reproducing natural maturation properly because water is likely to avoid the formation of olefins among the obtained pr~ducta.~-'lHowever, other re(1) Monthioux, M.; Landais, p.; Monin, J. C. Org. Geochem. 1985,8, 275-292. (2) Monthioux, M.; Landais, P.; Durand, B. Adv. Org. Geochem. 1985 1986 10,299-311. (3) Monthioux, M. Fuel 1988,67,843-847. (4) Landais, P.; Monthioux, M. Fuel Process. Technol. 1988,20,123132. (5) Landais, P.; Monin, J. C.; Monthioux, M.; Poty, B.; Zaugg, P. C.R. Acad. Sci. Paris l989,308,II,1161-1166. (6) Bbhar, F.; Leblond, C.; Saint-Paul, C. Rev. Zmt. Fr. Pbt. 1989,44, 387-411. (7) Lewan M. D.; Winters, J. C.; McDonald, J. H. Science 1979,203, 897-899. (8)Hoering, T. C. Org. Geochem. 1984,5, 267-278. (9) Jones, D. M.; Douglas, A. G.; Connan, J. Energy Fuels 1987, 1, 468-476. (10) Weres, 0.; Newton, A. S.; Tsao, L. O g . Geochem. 1988,12,433444. (11) Eglinton, T. I.; Douglas, A. G.; Rowland, S. J. Adv. Org. Geochem. 1987 1988,13, 655-663.
0887-0624/92/2506-0666$03.00/0
searchers estimate that water is a mere hydrogenating agent and that hydrogen can be provided by the effluents themselvesin the case of confined pyr0ly~is.l~~ As a matter of fact, high-pressure conditions could play a role in this phenomenon.12 Oil formation from kerogen maturation is not the only center of interest for the petroleum geochemist. Once it is generated, other phenomena have to occur in order for oil to be expelled from the source rock. Expulsion simulation is therefore another important s u b j e ~ t , lall ~-~~ the more since the mechanisms of primary migration are still subject to c o n t r o ~ e r s y . ~ The ~ J ~ only way to compensate short (in comparison with natural conditions) heatingtimes is to perform high-temperatureexperiments. It is therefore necessaryto develop kinetic models to make the link between natural and experimental processes.l* However, though it is well established that temperature is the main parameter governing such models,l9pressure seems to be involved not only in the expulsion phase14J6 but also during the maturation one.121s27 It has, however, (12) Dominb, F. Energy Fuels 1989,3,89-96. (13) Burnham, A. K.; Braun, R. L. Adv. Org. Geochem. 1989, 1990, 27-39. (14) Lafargue, E.;Espitalib, J.; Jacobsen, T.; Eggen, S. Adu. Org. Geochem. 1989 1990, 16, 121-131. (15) Takeda, N.; Sato, S.; Machihara, T. Adv. Org. Geochem. 1989 1990,16, 143-153. (16) Durand, B. Adv. Org. Geochem. 1987, 1988,13,445-459. (17) Tiseot, B. P. Rev. Imt. Fr. Pkt. 1988,43,143-153. (18) Ungerer, P. Adv. Org. Geochem. 1989, 1990,16,1-25. (19) Tieaot, B. P.; Pelet, R.;,Ungerer,P. Am. Assoc. Pet. Geol. Bull. 1987, 71, 145-1466, (20) Cecil, B. C.; Stanton, R. W.; Robbins, E. I. Am. Assoc. Pet. Geol. Bull. 1977,61, 775.
0 1992 American Chemical Society
Energy & Fuels, VoZ. 6, No.5,1992 667
Generation and Expulsion of Hydrocarbons
very rarely been introduced todate in kinetic modeling,28129 even when secondary cracking is involved.30 We report here the results of the artificial maturation of a Paris Basin Toarciansource rock in gold-sealedtubes submitted to pressure ranging from 170 to lo00 bar. Experimentsperformed in glass tubes under vacuum will also be referred to. The influences of both the duration of heating and the pressure on the catagenesis as well as on the expulsionof the hydrocarbons have been reviewed. Experiments carried out on thejsolated organic matter have also been undertaken. This paper presents the preliminary results of such an experimental simulation.
Experimental Section Samples Presentation. Core samples from the immature Grimonviller source rock (Ro= 0.35%) in the Paris Basin Toarcian were selected (about 5 m deep). Rock-Eval analyses show that they contain type II organic matter (HI = 600 mg HC/g TOC and 01= 15mg COdg TOC, as average values), with a-'2 value around 424 OC. The average initial organic carbon content is 7 % ,with 2 96 sulfur (LECOanalyzer). X-ray diffractionshows that these rocks are made mainly of quartz (12 % ) and carbonates (calcite 27 % ,dolomite 1% ,siderite 1%). They are also rich in pyrite and clays. Acid attacks with HF and HC1 enable the recovery of the organic matter, free of clays and carbonates. Artificial Maturation Procedure. Each core is separated intotwo parts: one is used for artificial maturation experiments while the other serves as a witness for unheated sample analysis. About 1.5 g of rock is placed in a gold tube which is sealed under argon. Gold has been chosen since it is chemically inert, can be submitted to a wide range of temperatures and pressures, and transmits extemal pressure perfectly to the compounds inside due to its good ductibility. For the organic matter samples, a quantity of 100 mg is used. The gold tubes are then placed in autoclaves; description of the temperature and pressure measurements and controls is given in Landais et al.31 Accuracy for temperature measurement is 1 2OC, whereas pressure is controlled with a precision of 1 5 bar. A temperature of 330 "C has been chosen; it is high enough to enable kerogen cracking but limit secondary cracking processes on generated products for convenient laboratory time scales. Pressure conditions were 170,550, and lo00 bar and heating times were 6 h, 12 h, 24 h, 5 days, and 14 days. Products are analyzed at the end of each experiment. The choice of time-temperature conditions has been carefully studied, in order to reproduce natural maturation processes properly within the oil window.= Experiments under vacuum have been undertaken following a closed-system pyrolysis procedure. A rock sample (about 15 g) is placed under vacuum in a glass tube the volume of which is 45 cm3. The same temperature and durations of heating that were previously used are applied. At the end of the pyrolysis, the glass tube is broken, the pressure registered, and effluents recovered and analyzed. In fact, actual reaction pressure is not really restricted to 0 bar since reactants' pressure has to be taken (21)Mc Tavish, R A. Nature 1978,271,648-650. (22)price, L. C. Chem. Geol. 1982,37,215-228. (23)Vandenbroucke,M.;Durand,B.;Oudin,J. L. Adu. Org. Geochem. la1 1983,147-155. (24) Goffh, B.;Villey, M. Bull. Minkral. 1984,107,81-91. (25)Sajg6, C.;McEvoy, J.; Wolff,G. A.; Horvhth, Z. A. Adu. Org. Geochem. 1985 1986,10,331-337. (26)Domine, F. Org. Geochem. 1991,17,619-634. (27)Price, L. C.;Wenger, L. M. Am. Assoc. Pet. Geol. Bull. 1990,74, 743. (28) Braun, R. L.; Burnham, A. K.Energy Fuels 1990,4,132-146. (29)Domine, F.;Enguehard, F. Org. Geochem. 1992,18,41-49. (30)Ungerer, P.; Bbhar, F.; V i b a , M.; Hewn, 0. R.; Audibert, A. Adv. Org. Geochem. 1987 1988,13,857-868. (31)Landais, P.; Michels, R.; Poty, B.; Monthioux,M. J. Anal. Appl. Pyrol. 1989,16,103-115. (32)Landais, P.Org. Geochem. 1991,17,705-710.
metallic borer
/
\,
*pump
/
GC injection loop 30cc
/
I
\
I trap (-100°C)
capillary tube for recovery of gasolines +water
I
isothermal chamber (50°C)
liquid N2 trap for gases (-196°C)
\
liquid Nz (-196°C)
Figure 1. Schematicrepresentationof the light fractionrecovery device. into account. This pressure ranges from 0.36 bar for the 6-h experiment up to 1.58 bar for the 14-day pyrolysis. Recovery and Analysis of the Light Fractions. We developed in our laboratory a very simple method to recover gas, gasoline, and water fractions evolved from our maturation simulations in gold-sealed tubes (a similar method is used for glass tube experiments). This method can be used for both rock and pure organic matter samples. Figure 1describesthis recovery system. The gold tube is placed in a cell equipped with a moving metallic head which is used to bore a tiny hole inside the tube. The cell is heated at 50 "C and connected to a glass line submitted to vacuum with pumping during one night (1O-g mbar). Two cold traps are on the line; the first one is cooled with a mixture of liquid nitrogen and ethanol a t a temperature of -100 "C and the second one with pure liquid nitrogen at -196 "C. The glass tube entering the latter is a chromatograph injection loop of 30 cm3 and contains silica gel that has been dried at 170 OC under vacuum. A glass capillary tube, which had been previously weighed, is placed between the boring cell and the liquid Nz/ alcohol trap. The whole device is placed in an isothermalchamber at 50 "C. When the gold tube is drilled, the effluents begin to enter the line, without any pumping. The hydrocarbonsin the CgCl3range and water are condensed in the f i i t trap at -100 "C. Argon and the more volatile compounds including methane and nitrogen move to the second trap to be condensed. A special silica gel, free of cobalt chloride (blueindicator for tracing water trapping), is used to avoid reaction by sulfur species (H2S). The injection loop is then disconnected from the vacuum line and placed at the head of a chromatograph for analysis. We used a preparative column, Porapak Q, 1m X l/2 in. i.d., and a thermal conductivity detector (TCD). In that system,each compound can be recovered in order to undergo isotopic analyses. Quantitative data are obtained with extemal standards. The analysis begins at -25 "C and reaches 200 "C. Gasoline water fraction is recovered by cryogenic trapping; the liquid Ndalcohol cooling is removed and the glass tube is warmed. Since the preweighed capillary glass tube is cooled a t -196 "C, the compounds migrate from the warm to the cold source and are thus trapped. The capillary tube is then sealed and weighed. We have thus a direct mass measurement of the fraction containing the C d 1 3 hydrocarbons and water (water can be quantified by measurement of the hydrogen volume evolved from reacting with an hydride such as CaH2, and C d l 3 hydrocarbons analyzed by GC; this will be dealt with in a further publication). Carbon and oxygen isotopic analyses have been obtained on the gas fraction since preparative chromatography is used. 613C data are given for CHI, C2H6, and C3Ho as well as for C02. The values are expressed as 960 with regard to the Pee-Dee belemnite (96OlPDB). 6lSo data are obtained for C02, expressed as 460 with regard to the standard mean ocean water (96OlSMOW). Isotopic analyses from the acid treatment of carbonates will also be used. Residual gasolines, i.e., still trapped inside the rock, are also analyzed using a thermovaporization method. The sample is finely ground (about 100 mg for a sample containing 1096 organic matter) and thermodesorbed at 220 "C during 10 min under a helium stream. Effluents are trapped in liquid nitrogen and then brought back to room temperature before injection in a
+
668 Energy & Fuels, Vol. 6, No. 5, 1992
Blanc a n d Connan
Table I. Checking of the Validity of the Light Fractions Recovery Method: Comparison between Weights of Recovered Effluents and the Weight Loss of the Cells before and after Boring. sample ref
El 1A El 5A E2 1A E5 1A E5 2A1 E5 2A2 E5 3A E5 4A1 E5 4A2 E5 5A1 E5 5A2
sample type
R. R. R. R. R. R. R. R. R. R. R.
gas, ml, mg 16.4 31.1 10.4 7.1 10.9 10.1 12.1 21.0 15.6 26.7 23.1
gasoline + water, m2, mg 85.7 69.0 37.4 28.8 48.9 44.9 54.2 92.8 46.7 116.2 132.1
ml + m2 = m3,
mg 102.1 100.1 47.8 35.9 59.8 55.0 66.3 113.8 62.3 142.9 155.2
weight lose after boring, ms mg 105.9 130.6 47.1 34.5 52.8 52.9 65.6 115.0 62.8 141.1 156.0
7.7 7.3 O.M. 4.3 3.0 8.5 9.0 O.M. 3.8 4.7 11.0 12.5 O.M. 5.5 5.5 22.3 22.3 O.M. 8.2 14.1 24.3 26.8 O.M. 9.1 15.2 7.5 7.4 O.M. 3.9 3.6 E4 1A 13.4 13.5 O.M. 6.3 7.2 E4 2A 18.8 15.6 O.M. 7.7 7.9 E4 3A 23.6 26.9 O.M. 8.2 15.4 E4 4A 13.2 14.1 5.5 7.7 E6 1Al O.M. 19.8 18.3 O.M. 8.9 9.4 E6 2A1 32.1 29.0 O.M. 11.3 17.7 E6 3A1 41.2 43.7 O.M. 18.0 23.2 E6 4A1 See Table I1 for sample code meaning; R. = rock sample, O.M.= organic matter sample. E3 1A E3 2A E3 3A E3 4A E3 5A
30-m Apiezon L column equiped with a flame ionization detector (FID). Oven temperature increases from 40 to 220 OC at a rate of 2 OC/min. Quantitative analysis is obtained with reference to an external standard. This method is not applied to organic matter samples since the latter are previously ground before pyrolysis; furthermore, results on rock samples show that these residual gasoline8 are relatively minor compounds. Bulk Geochemical Analysis on the Solid Phase. Three kinds of bulk geochemical analyses have been performed on the solid residue. LECO analysis, elemental analysis and Rock-Eval pyrolysis. LECO analyzer gives the totalorganic carbon and the sulfur contents. TOC values are also obtained with the carbon module analyzer of the Rock-Eval 11. Sl,S2,and Ss peaks and T,, are registered and hydrogen and oxygen indexes are calculated. Rock-Eval experiments have been carried out on the whole and on the chloroform extracted rocks as well as on the total isolated organic matter samples. Elemental analyses gave C, H, 0 contents. ChloroformWashing a n d Extraction. In order to approach the oil fractions expelled,the gold tubes containing rock samples are opened with great care (after recovery of the light effluents) to avoid powdering; a rock cube is recovered and plunged for 10 s in 20 cm3of CHCl3. This treatment allows the cube to be washed a t only its external surface and avoid real extraction of molecules trapped inside the rock. The internal surface of the gold tube is also washed with CHCb and the two fractions are added. Once the expelled oil fraction is thus obtained, grinding of the rock is undertaken and organic extract is obtained using Soxtec extraction with CHCls. Pure organic matter samples are directly extracted. Liquid a n d Gas Chromatography. The constitution of both expelledand residual oils is obtained usingan Iatroecanapparatus (thin layer chromatography-FID coupling). This method, which requires a low quantity of the product, gives ratios of saturated hydrocarbons, aromatics and polarS.99 Improvements have been performed which allow one to calculate the quantification of the asphaltene fraction in another set of experiments; saturates, aromatics, asphaltenes, and resins are thus obtained in a twostep procedure. Preparative medium-pressure liquid chromatography is also used to quantify these different fractions to perform a gas chro(33) Berrut, J. B.;Jonathan, D.Characterization ojHeauy Crude Oik and Petroleum Residues; Technip: Paris, 1984;pp 400-405.
m3
- m4,
(ma - m,)/m,,
mg
%
-3.8 -30.5 +0.7 +1.4 +7.0 +2.1 +0.7 -1.2 -0.5 +1.8 -0.8
-3.6 -23.4 +1.5 +4.1 +13.3 +4.0 +1.1 -1.0 -0.8 +1.3 -0.5
-0.4 -0.5 -1.5 0 -2.5 +0.1
-5.2 -5.6 -12.0 0 -9.3 +1.4 +0.7 -17.0 -14.0 -6.4 -7.6 -9.7 -5.7
+os
-3.2 -3.3 -0.9 -1.5 -3.1 -2.5
matographyof saturated and aromatic hydrocarbons. Conditions for the liquid chromatography are as follows: MPLC 100,column NICOPREP filled with silica, grain size 40 pm, pressure 6-7 bar maximum, n-hexane as solvent. Conditions for gas chromatographic analyses are as follows: 50 m X 0.21 mm i.d. columns, coated with SE 52 (methylsilicone);film thickness 0.11 pm; temperature program from 80 to 300 OC at 1.6 OC/min, with hydrogen as a carrier gas.
Results and Discussion Validity of the Light Fractions Recovery Method. In order to check the validityof our light fractionsrecovery system, a mass balance calculation was carried out (Table I). The summation of the gas and gasoline water masses is compared in each case to the weight loss of the sealed gold tube once it has been bored (weight measurements are madejust before and after boring). With the exception of sample El 5A (which was bored without the 50 O C isothermal room), the weight of the gas and gasoline + water fractions which have been recovered corresponds to the weight loss of the gold tube. If the two largest discrepancies are taken away, the mean absolute difference between the calculated mass and the weight loss is 1.6 mg for the rocks and 1.5 mg for the organic matter samples (see Table I). These values correspond to relative differences of 2.2% for the rocks and 7.1% for the organic matters (whichis due for the latter to the very s m a l l amount of sample analyzed, 100 mg). Furthermore, since no systematic trend can be observed in the figures of Table I, it can be concluded that the method we developed to recover the light pyrolysis effluents is adequate. Analysisof the Light Effluents. The different yields of products have been expressed as a percentage of the initial total organic matter contained in the samples (TOW). This enables direct comparison between the different fractions and between the rock and the organic matter samples. Such yields are given in Table 11. Gas yields range between 8 and 18% T O W for the rock samples, against 4-1296 TOM0 for the organic matter samples. It appears clearly on Table I1 that there is a
+
Generation and Expulsion of Hydrocarbon8
Energy &Fuels, Vol. 6, No. 5, 1992 669
Table 11. Yieldr in Products Obtained after Pyrolyris Exwrimentsa ~~
-
COz,
Pas,
sample ref EllA E31A E12A E32A E13A E33A E14A E34A E15A E35A E21A E41A E22A E42A E23A EX3A E24A E44A E51A E61Al E52A1 E52A2 E53A E62A1 E54A1 E54A2 E63A1 E55A1 E55A2 E64A1
sample type
R.
O.M. R.
O.M. R.
O.M. R.
O.M. R.
O.M. R.
O.M. R.
O.M. R.
O.M. R.
O.M. R.
O.M. R. R. R.
O.M. R. R.
O.M. R. R.
O.M.
press.,
%
ClC6HC,
9.7 4.5 8.8 4.0 10.3 5.9 16.7 8.7 18.2 9.6
0.8 0.5 0.8 1.4
3.4 5.7 4.1
3.8 11.7 3.8
0.6 0.5
6.2 2.8
TO&
170 170 170 170 170 170 170 170 170 170
time 6h 6h 12 h 12 h 24h 24 h 5d 5d 14 d 14 d
550 550 550 550 550 550 550 550
6h 6h 24h 24 h 5d 5d 14 d 14 d
8.0 4.1 9.8 6.6 13.1 8.2 14.8 8.6
lo00 lo00 lo00 lo00 lo00 lo00 lo00 lo00 lo00 lo00 lo00 lo00
6h 6h 12 h 12 h 24 h 24h 5d 5d 5d 14 d 14 d 14 d
8.6 4.1 10.0 9.9 9.9 6.5 12.5 13.0 8.1 18.2 16.4 11.6
bar
%TO&
1.2
%
CAM+
k26~
0
residual Cl3+ %TO& 38.5 39.3 41.9 57.8 59.2 53.1 40.0 35.2 35.7 24.2
2.3 0 10.1 0 6.7 0 0.9 0
30.1 32.3 57.0 52.9 37.9 34.0 33.5 26.0
32.4 32.3 67.1 52.9 44.6 34.0 34.4 26.0
5.3 0 8.4 6.0 7.3 0 8.3 13.9 0 3.0 3.2 0
32.5 31.1 38.0 35.9 36.4 47.9 36.4 29.5 41.0 28.6 28.1 24.1
37.8 31.1 46.4 41.9 43.7 47.9 44.7 43.4 41.0 31.6
%TO&
%TO&
%TO&
7.9 2.7 7.6 2.7
60.6 3.2
0.6
2.1
30.8
0.4
3.4
4.0 27.0 5.9 38.3 14.9 40.3 16.1
3.7
3.8
0.5 0.7 0.8 0.8 1.0 1.5 2.3 2.6 2.5 4.4 3.6 4.8
6.6 2.9 8.4 7.9 8.0 4.0 9.5 10.0 4.5 12.5 11.9 5.3
34.6 5.6 44.8 44.1 42.3 6.7 55.3 40.7 12.8 79.4 93.8 15.0
3.2
exwlled &S+,
TO&
28.7 3.8 26.6 7.6 29.7 8.3 41.0 16.2
1.6
residual Cdl3i
3.7 3.5
0
4.0 0
0.7
3.4 0
3.2
5.8 0
11.1
1.3 1.9 2.9
5.5 2.2 1.4 0.5 3.2 3.6 5.6 9.1 6.4
2.3
total CIS+,
%TO& 40.6 39.3 45.9 57.8 62.6 53.1 45.8 35.2 38.0 24.2
generated petroleum, s-
%TO& 47.0 65.8 64.9 58.8 49.9 40.2 67.1 50.5 50.8 40.8
61.1 61.9
31.3
24.1
50.7
TO& = initial total organic matter content in the sample considered.
noticeable contribution of the mineral matter to the production of gas in the pyrolyzed rocks. This is more clearly highlighted by the yields in gaseous hydrocarbons (C1to C5) and carbon dioxide; while the former are almost identical for rock and organic matter samples, the latter are indeed higher for the rocks. Obviously, the C02 which evolved from the pyrolysis of the Paris Basin Toarcian source rocks partly stems from the decomposition of carbonates. Direct comparison between rock and organic matter samples made it possible to obtain an evaluation of the proportion of “mineral C02” in the total C02 as being between 50 and 68%. This ratio tends to be more or less independent from the duration of heating, but unfavored by high-pressure conditions (about 66% as an average value at 170 bar, against 54% at lo00 bar). Furthermore, pressure seems to favor the production of C02 from an organic origin and to hamper the production of C02 from a mineral. Additional information is nevertheless required for absolute statement. Mineral contribution to the production of C02 is ah0 shown on Table I11representing the volumic compositions of gases. In fact, C02, which is the main gaseous product, represents between 70 and 75% for short heating times (6-12 h) and 50 and 60% for long heating times (14 days) with rock samples, but only between 45 and 60 % and 30% , respectively, for the organic matter samples. The C1CdCO2 ratio also reflects the mineral contribution to CO2 since it is always higher for organic matter samples than for rock samples. Moreover, this ratio tends to increase with an increased duration of heating, confirmingthereby that C02 is mainly a product of early generation, whereas hydrocarbon generation takes place later. This is exemplified by the chromatogramsof gasesevolved from organic
matter samples heated under 170 bar during 6 h, 24 h, 5 days, and 14 days, (Figure 2). Isotopic data on c143hydrocarbons and C02 are given in Table IV. It can be noticed that the 613Cof C02 evolved from organic matter samples tend to be heavier when the duration of heating increases (from about -26%/PDB after 6 h for about -24%/PDB after 14 days). This can be explained by the fact that 12C-12C bonds are more easily broken than 13C-12C thus, the first C02 obtained is richer in 12Cthan the one obtained after a long heating period. However, the situation is not so clear in the case of methane, which tends to be lighter when the duration of heating increases (from -42%/PDB to -45%/PDB); other phenomena could be involved such as the contribution of secondary methane issued from cracking of the oil,%isotopic exchanges with C02 or oxidation by water. This difference between CH4 and C02 could also reflect the fact that carboxylicgroups are richer in 13Cthan alkyl chains (cf. below), which could make the comparison between the breaking of 12C-12Cand 12C-13Cmore sensitive in the case of the production of C02, for statistical reasons. The main result of the carbon isotopic study lies in the differences registered for carbon dioxide of rock and organic matter samples: between -5 and -10%/PDB for the rocks, against-25%/PDB for the organic matters. Here again, the contribution of C02 occurring from the decomposition of carbonates seems to be a reasonable explanation since carbonates are known to be isotopically heavier than organicmatter (an isotopic exchangebetween (34) Sackett, W. M.; Nakaparksin, S.; Dalrymple, D. Adu. Org. Geochem. 1 W 1970, 37-53. (35) Kharaka,Y.K.;Carothers, W. W.; Roeenbauer, R. J. Geochim. Cosmochim. Acta 1983,47, 397-402. (36) Clayton,C. J. Org. Geochem. 1991,17,887-899.
670 Energy & Fuels, Val. 6, No. 5, 1992
Blanc and Connun
Table 111. Analyses of Gams (%/volume) sample ref 1A EllA E21A E51A
sample type R. R. R. R.
time 6h 6h 6h 6h
press., bar
C1,%
Cz,%
cs, %
170 550 lo00
10.5 6.1 6.6 6.1
4.7 2.2 2.5
4.0 1.4
2.1
1.4
0.6 0.7
0
c4,
1.8
%
1.5
Ca, %
HC,%
coz, %
Has, %
0.9 0.1
21.6 11.8 11.5 10.5
71.1 77.5 72.8 67.7
6.8 tr. tr.
2.1
Na+Ar, %
% HC/ % coz
10.6 15.6 21.8
0.30 0.16 0.16 0.16
0.4
2A E12A E52A1 E52A2
R. R. R. R.
12 h 12 h 12 h 12h
0 170 550 lo00
12.0 7.2 7.7 7.0
6.9 3.0 3.1 2.9
4.1 2.0 2.5 2.0
3.0 1.5 1.0 1.1
1.4 0.6 0.3 0.2
27.4 14.3 14.6 13.2
60.3 80.9 75.4 71.1
11.9 0.8
0.4 4.7 10.0 15.6
0.45 0.18 0.19 0.19
3A E13A E22A E53A
R. R. R. R.
24h 24h 24h 24 h
0
13.3
6.9
4.5
3.2
1.5
29.4
57.1
13.2
0.3
0.51
170 550 lo00
10.3
5.0
3.4
1.3
0.2
20.2
75.2
4.6
0.27
4A E14A E23A E54A1 E54A2
R. R. R. R. R.
5d 5d 5d 5d 5d
0 170 550 lo00 lo00
16.8
9.2
8.0
3.9
1.9
39.8
45.9
0.1
0.87
15.0 14.3
8.9 8.3
5.4 5.8
1.4 2.4
0.1 0.5
30.8 31.3
62.2 60.7
6.6 8.1
0.50 0.52
5A E15A E24A E55A1 E55A2
R. R. R. R. R.
14 d 14 d 14 d 14 d 14 d
0
21.5 16.3
11.9 8.7
8.1
1.9 0.2
35.7
38.4 59.9
13.6
6.2
4.6 4.3
48.0
170 550 lo00 lo00
0.1 4.4
1.25 0.60
18.8 19.0
10.5 10.7
6.6 5.5
1.4 1.0
0.7 0.2
38.8 36.4
55.9 56.8
0.0 0.1
6.0 6.7
0.68 0.64
E31A E41A E61A1 E32A E33A E42A E62A1 E34A E43A E63A1 E35A E44A E64A1
O.M. O.M. O.M. O.M. O.M. O.M. O.M. O.M. O.M. O.M. O.M. O.M. O.M.
6h 6h 6h 12h 24 h 24 h 24 h 5d 5d 5d 14 d 14 d 14 d
170 550 lo00 170 170 550 lo00 170 550 lo00 170 550 lo00
13.4 14.3 15.3 18.7 17.8 16.5 16.1 23.6 23.8 23.6 29.2 31.7 28.6
4.7 5.1 6.3 8.6 10.6 8.3 15.8 17.5 13.7 13.7 15.7 14.8 15.8
2.6 2.3 4.4 5.7 4.0 8.3 4.1 7.2 8.6 7.2 8.3 7.2 9.7
0.3 0.4 1.8 1.4 0.6 1.2 0.7 1.1 2.8 1.3 1.5 1.7 2.2
45.2 53.7 58.1 51.3 43.0 40.8 47.4 29.6 33.4 38.9 29.1 32.8 29.9
8.7 7.1
0.1 0.0 0.2 0.1 1.6 0.7
21.0 22.1 27.8 34.5 33.0 34.5 36.8 51.0 49.6 45.8 55.0 55.6 56.7
25.2 17.3 14.1 14.1 14.5 12.6 9.3 8.6 7.2 8.4 8.1 5.9 4.4
0.46 0.41 0.48 0.67 0.77
COZ and carbonates could also be involved). Oxygen isotopic composition of COZ also exhibits systematic differences when evolved from rocks (around +45960/ SMOW)or from organic matters (around +35960/SMOW). However, isotopic exchanges with water are likely to influence values. Lastly, the differences in 613Cbetween COZand gaseous hydrocarbons (C%, CZ&, and C a s ) for the organicmatter samples (e.g., mean values for COZand CH4 are about -25 and -44960, respectively) are in agreement with previous studies showing that carboxyl groups are isotopically heavier than pure hydrocarbonated compound^.^^^^^ In particular, the mean differenceof 19960registered between COZand CH4 is fully corroboratedby isotope fractionation calculations between COZand CH4 produced during the decarboxylation of acetic acid at 300 "C ( ~ O % O ) .More~~ over, the values of 613C for COZand CH4 obtained in the latter study are identical with ours. Hence, methane and carbon dioxide recovered from our pyrolysis experiments on organic matter samples both seem to stem mainly from carboxylic acid thermal cracking-typeprimary reactions. This result could contribute to explain the fact that, with increasing maturation, 13C enrichment in COZmatches with a depletion of 13Cin CH4. It has been observed that carbonates from the Grimonviller rocks tend to be isotopicallylighter after the artificial (37) L&tolle,R.; Balabane,M.; Gaveau,B.;Monthioux,M. C. R.Acad. Sci. Paris 1985, 300,11, 13-16.
0.3 0.2 0.4
14.2
9.5 12.0 6.6 10.8 9.9 6.9 7.8 5.7 9.1
0.85
0.78 1.72 1.49 1.18 1.89 1.70 1.90
maturation process. Taking +0.7%/PDB as a mean value of 613C of carbonates before pyrolysis, this value reaches about -1.5960/PDB for the carbonates contained in the solid residue after pyrolysis (Chennaux, G., private communication). On the basis of the isotopic data of COZ from rock and organic matter samples as well as the ratios of COZfrom organic or mineral origin, we have calculated the b13C of the COZevolved from the thermal decomposition of carbonates, following the relation:
where R = rock and OM = organic matter. Results show this COZto be isotopically very heavy (around 7960/PDB), which is in agreement with the fact that the residual carbonates are isotopically lighter after pyrolysis. A ratio of thermal decompositionof carbonates can be obtained using these data. If we call this ratio X, the following relation can be written 613c0,2HCI before pymlylis
= XPCO,~pyrolyrl
+ (1- x)613c0,2-
-
(2)
HCI dter pymlyah
with 613C032-(pyrolysis)= 6 W 0 2 previouslycalculated in relation 1. Results (whichhave to be considered with caution since we took average isotopic values and estimation of COZ from mineral or organic origins) indicate that between about 4 and 12% of the carbonates could have been
Energy I&Fuels, Vol. 6, No. 5,1992 671
Generation and Expulsion of Hydrocarbons Table IV. Isotopic Data on Gaseous Hydrocarbons (Ct. . Cl.-_Cd-. and Carbon Dioxide 6180 960 r 6'SC 96OIPDB SMOW sample sample press., ref tvue time bar CI CI CS COI C02 6h 0 -41.0 -34.4 -34.0 -9.7 +51.0 1A R. -4.8 +47.0 170 -42.2 -36.4 6h E l l A R. 550 -42.1 -36.1 -34.9 -6.1 +43.0 6h E21A R. -32.7 lo00 -40.8 E51A R. 6h 0 -41.7 -35.0 -34.0 -6.2 +51.7 2A R. 12h 12h 170 -42.1 -36.0 -35.0 -4.9 +48.2 E12A R. +43.5 E52A1 R. 12h 550 -40.8 -34.2 -29.5 -5.3 12h lo00 -41.3 -33.7 -33.0 -5.2 +45.7 E52A2 R. +52.0 24h 0 -43.0 -34.8 -34.1 -5.1 3A R. 170 24h E13A R. 550 24h E22A R. 24h lo00 -41.9 -34.7 -32.5 -6.0 +45.7 E53A R. +52.0 0 -43.5 -35.4 -33.4 -5.6 4A R. 5d 5d 170 E14A R. 550 5d E23A R. -7.0 +44.0 lo00 -43.7 -35.9 E54A1 R. 5d 5d lo00 -43.6 -35.7 -33.6 -6.6 +45.2 E54A2 R. 14d 0 -43.9 -34.9 -33.0 -4.0 +50.5 5A R. E15A R. 14d 170 -43.9 -35.0 -33.3 -11.0 +23.0 14d 550 E24A R. E55A1 R. 14d lo00 -44.3 -34.7 -32.7 -8.0 +43.0 -7.3 +42.0 E55A2 R. 14d lo00 -44.3 -34.9
(a)
__
E31A E41A E61Al E32A E33A E42A E62A1 E34A E43A E63A1 E35A E44A E64A1
O.M. O.M. O.M. O.M. O.M. O.M. O.M. O.M. O.M. O.M. O.M. O.M. O.M.
6h 6h 6h 12h 24h 24h 24h 5d 5d 5d 14d 14d 14d
170 -42.2 -35.7 550 -42.1 -36.1 lo00 -43.0 -36.2 170 -42.9 -36.1 170 -43.3 -36.7 550 -43.7 -36.5 lo00 -43.6 -36.5 170 -44.5 -35.2 550 -45.8 -35.3 lo00 -44.2 -35.7 170 -44.9 -34.6 550 -45.1 -37.4 lo00 -44.4 -34.3
-33.6 -34.6 -34.3 -34.2 -34.1 -33.9 -32.8 -33.4 -32.2 -31.3 -32.3 -32.0
-27.7 -24.7 -25.5 -25.7 -25.4 -25.0 -25.3 -24.4 -22.8 -24.0 -24.2 -24.2 -24.1
+36.0 +35.5 +36.0 +38.5 +35.3 +35.0 +34.5 +36.0 +35.0 +34.0 +35.0 +35.0 +35.0
destroyed during pyrolysis. When the duration of heating increases, so does this ratio. Moreover, these results confirm that pressure seems to be unfavorable to the decomposition of carbonates, e.g., 8.4% are decomposed at 170bar and 6.6% at 550bar after 6h, against only 4.7% at lo00 bar after 24 h. At 10oO bar, 7.5% decomposition is reached only after 5 days of heating. This phenomenon could be explained by reactions following the Le Chatelier's principle, briefly, in that case, increase of pressure is compensated by the decrease of gaseous products in the chemical equilibrium. However, direct decomposition of calcite within our temperature-pressure conditionsseems impossible. A temperature of lo00 OC should be reached to yield C02 with a pressure of 1 bar. A mechanism involving silicate has been envisaged on the basis of thermodynamicdata to explain the formation of COz from calcite: CaC03 Si02 C02 + c&io3. The validity of this mechanism is nevertheless doubtful since wollastonite has never been observed. The quantitative data obtained by this isotopic approach are of the same order of magnitude than those resulting from a direct mass balance calculation between "mineral" C02 and CaC03 which should have been destroyed [wt of CaC03destroyed = (wt of COzmin X 100)/44)1: about 4% of the carbonates could have yielded carbon dioxide through thermal cleavage (average value). Yields in gasoline + water are also indicated on Table 11, along with residual gasoline (C&13 hydrocarbons obtained after thermodeaorption). Like C02, a comparison between rocks and organic matters provides evidence of
+
I
CO,
GrW.*S%TQ(O
Q m w .Ll%TQlO
I
I
Figure 2. Gas chromatograms of the gaseous producta expelled from the confined pyrolysis of organic matter samples under 170 bar (at 330 "C)during (a) 6 h, (b)24 h, (c) 5 days, and (d) 14 days.
anoticeable participation of H2O of mineral origin (mainly coming from the clays),to the gasoline + water fractions generated from rocks; this water represents between 60 and 90% of the total gasoline + water fraction. Since quantitative separation of water and hydrocarbons has not yet been conducted, we will not enter into detaila concerning the expelled and residual gasoline fractions analysis. This will be the subject of a further publication. Analysis of the(&+ Oil Fractions. Yields in residual and expelled C13+ oil fractions are indicated in Table 11. Generally, yields in expelled products are rather low compared with residual ones. It can be observed in both series that, for a given pressure condition, yields reach a maximum value, generally between 24 h and 5 days of heating. The decrease observed afterwards can be accounted for by the existenceof a secondarycrackingprocess affecting the oil, leading in particular to the formation of a solid residue, insoluble in chloroform (pyrobitumen). Total oil yieldsare comparablefor rock and isolated organic matter samples submitted to the same analytical procedure, though generally lower for the latter. This justifies the direct comparison done on both series, though we are aware that some mineral and/or organic adsorption should be considered. Lastly, total "petroleum" yields are also given in Table 11;they represent in fact the sum of all the products generated from pure organic matter samples. These yields range from about 40 to 67% of the initial organic matter. Maximum "petroleum" generation is reached for a longer heating time when pressure is increased (12h at 170 bar, 24 h at 550 bar, and 5 days at 10oO bar),but this feature has to be considered with caution due to the precision of the experimental data. Residual and expelled CIS+oil yields, expressed in % of the initial rock weight before pyrolysis in Table V, have been used to calculate oil expulsion efficiencies. Results, expressed in % (Table V), clearly showthat pressure seems favorable to the expulsion of c13+products (from about 1 % for experiments under vacuum, up to 15-25 % under lo00 bar) and that for a given pressure condition a maximum of expulsion is reached after 5 days of heating. The latter observation tends to indicate that the secondary crackingphenomenon affscta more extensivelythe expelled fraction than the residual one. Results of Table V are better illustrated in Figure 3, which represents the expulsionefficiency lines (330 OC, &e duration of heating) as a function of pressure. It may be considered that a strong external pressure should require a stronger fluid pressure inside the rock to enable their expulsion. However, the favorable effect of
Blanc and Connan
672 Energy &Fuels, Vol. 6, No. 5, 1992 Table V. Yields in Extracted and Expelled Cis+ Oils Expressed in Initial Rock Weight bedore Pyrolysis. Oil Expulsion Efficiencies = Expelled/Total Generated Oil Ratios extracted expelled generated expulsion press., oil, oil, oil, efficiency, time bar %rock" %rock" %rock" % 1.5 0.04 2.60 2.56 6h 0 5.2 3.46 3.28 0.18 170 6h 7.2 2.90 2.69 0.21 6h 550 14.0 0.45 3.22 2.77 6h lo00 12 h 12 h 12 h
0 170 lo00
3.79 3.54 3.12
0.04 0.34 0.61
3.83 3.88 3.73
1.0 8.8 16.4
24 h 24 h 24 h 24 h
0 170 550 lo00
4.11 5.43 5.73 3.34
0.09 0.32 1.02 0.67
4.20 5.75 6.75 4.01
2.1 5.6 15.1 16.7
5d 5d 5d 5d
0 170 550 lo00
4.00 4.03 3.67 3.32
0.11 0.58 0.65 1.11
4.11 4.61 4.32 4.43
2.7 12.6 15.0 25.1
14 d 14 d 14 d 14 d
0 170 550 lo00
2.53 3.06 3.55 2.86
0.02 0.19 0.10 0.31
2.55 3.25 3.65 3.17
0.8 5.8 2.7 9.8
0
Initial rock before pyrolysis. 25
r
5 n
-20 F?
>. V
z
.
. '
DAYS
24 HOURS
12 HOURS
L
l
4 DAYS
" ' ~ " " " " " ' ~
400 600 800 1000 PRESSURE (bar) Figure 3. Regression lines between oil expulsion efficiencies and pressure conditione. All experimenta have been carried out at 330 O C .
pressure toward expulsion could be explained by a modification of the phase conditions. In fact, whereas water, carbon dioxide, and hydrocarbons form a gaseous supercritical phase at 170 bar, this supercritical phase behaves as a liquid phase at higher pressures, and the polar-rich bitumen becomes more mobile.3* Therefore, if we consider that the different phases are expelled in proportion to their mobility, high-pressureconditions are favorable to the expulsion of oil (hydrocarbonsand polar compounds) but also of water and carbon dioxide; such are the results that haye been observed. Organic extracts from rocks, isolated organic matters, and expelled oils from rocks have been analyzed using an (38) Connan, J.; Montel, F.; Blanc, Ph.; Sahuquet, B.; Jouhannel, R. Abstract, 15thInternational Meeting on OrganicGeochemistry, Mancheeter, 1991, pp 14-15.
SATURATES POLARS Figure 4. Gross compositions (saturates - aromatics - polars) of extracted oils in rocks and organic matters pyrolyzed under 550 bar: evolution with increasing pyrolysis time.
Iatroscan apparatus to establish their content in saturates, aromatics, and polars, the latter being divided into asphaltenes and resins (alsoquantified). Severalparametera have been calculated,such as yields in hydrocarbons (ppm of rock or % of TO&), saturates to aromatics ratios (Sl A), and asphaltenes expulsion efficiencies. All these data (not given here) have led to relevant observations which are summarized below. Typical evolution of the gross composition of organic extracts from rocks and organic matter with increasing heating time is given in a saturates-aromatics-polars triangular diagram in Figure 4, (e.g., a set of experimenta conducted under 550 bar). T w o phenomena are clearly documented 1. Organic extracts of rocks are always richer in polar compounds than organic extracts of isolated organic matters. This feature is probably due to the fact that the less polar compounds are preferentiallyexpelledfrom rocks and that adsorptionphenomena of polar specieson organic matter trapped them in the case of organic-richsamples.39 2. A two-step evolution is observed for both rock and organic matter samples: the first stage of enrichment in polar compounds is due to the release of polar structures from the kerogen at an early catagenetic level (C-0 and C-S bonds are easier to break than C-C ones); then a progressive enrichment in hydrocarbons is observed in a second step. These laboratorypathways follow the natural trends observed in fields studies. It may be added that the progressive depletion in polar compounds between 6 hand 14 days of heating is ascribed to a depletion in asphaltenes, as clearly seen in Figure 6 (hydrocarbons-resins-asphaltenes triangular diagram);it is assumed that asphaltenes undergo disproportionation processes leading to hydrocarbons on one side (mainly aromatics) and resins on the other side. In particular, the resin content stays at a constant level (30%) due to the fact that these structures are also able to release hydrocarbons when being heated (along with polar gases such as COz and H2S). A typical example of the pressure effecta is given on Figure 6 (e.g., experiments conducted during 24 h). In particular, the saturates-aromatics-polars triangular diagram (Figure 6a) shows that the expelled oils are always relatively richer in hydrocarbons than the residual ones, a result that is in agreement with natural observations. (39)Ma",A.; Juzwa, M.; Betlej, K.; Sobkowiak, M.Fuel Process. Technol. 1979,2,36-44.
Energy & Fuels, Vol. 6, No. 5, 1992 673
Generation and Expulsion of Hydrocarbons
1
* UNHEATED ROCK I BHOURS
SATURATEStAROMATICS A
!
24 HOURS 5 DAYS
AL 5
-
0
5
-
0
DURING OF HEATING (DAYS)
0
vvvvvvvvv\ 54
100
DURING OF HEATING (DAYS)
(00
0
RESINS
ASPHALTENES
Figure 5. Gross compositions (hydrocarbons - asphaltenes resina) of extracted oils from organic matter samples pyrolyzed under 550 bar: evolution with increasing pyrolysis time.
0
54
SANRATES
100
POURS
7i-J JQWD
ZM)
kW
W
200
400 800 800 PRESSURE (BARS)
E41
0
1000
'
'
200
'
'
100
'
'
so0
'
'
800
'
1oc
PRESSURE (BARS)
Figure 7. Saturates to aromatics ratios: (a) Evolution as a function of pyrolysis time for extracted hydrocarbone from rock samples; (b) Same as above for expelled hydrocarbons; (c) Evolution as a function of pressure for expelled hydrocarbons; (d) Evolution of (expelled S/A)/(residual S/A) as a function of pressure. W
LPW
PRESSURE (bor)
Figure 6. (a) Gross compositions of expelled and residual oils from rock samples pyrolyzed during 24 h evolution with increasing pressure. (b) Evolution of the amount of extracted hydrocarbons from rock samples pyrolyzed during 24 h, as a function of pressure.
Therefore, this result provides a confirmation of the validity of our experimentalprocedure. Even more, results on both expelled and residual oils indicate that a pressure increase tends to enhance the relative content in polar compounds with, possibly, a reverse trend between 550 and lo00bar. This has also been observed in oils from the organic matter samples. This result has been extended a t a quantitative level by measuring the absolute amount of the hydrocarbons aa a function of pressure. An example is given in Figure 6b for the same experimental conditions as in Figure 6a. From these results, it is concluded that a pressure increase is unfavorable to the formation Of C13+ hydrocarbonsthrough thermal evolutionof organicmatter. Though this result is not systematicallyobservable under each experimental condition, this conclusion is believed to be valuable since it is generally observed for rock and organic matter samples, for both residual and total (residual + expelled) oils. It has moreover also been observed from hydrous pyrolysis e~periments.~7 The last type of observation deals with the particular influence of pressure on the nature of expelled oils. Figure 7 shows its effecta on the S/A ratio. This ratio tends to increase with heating time, for both the residual and the expelled oils from rocks (Figure 7, a and b), as also seen in nature. It can be noticed that the first evolutionary stage consistsof a decrease of this S/Aratio,which is likely due to the neogenesis of aromatic moieties. The S/A ratio in expelled oils shows a decreasing trend with pressure,
Table VI. Expulsion Efficiencien of CIS+Oils and Anphaltenes (Expelled/Generated Products Expressed in 9%) 170bar 550bar 6 h 12h 14d 6h
Cis+oil asphalt"
6.2 3.8
8.8 6.4
5.8 4.7
7.2 7.5
6h
lo00 bar 12h 24h 5 d
14d
14.0 16.4 16.7 26.1 9.8 11.2 15.5 20.0 33.3 13.3
whatever the duration of heating (Figure7c). This feature suggests that pressure is unfavorable to the thermal evolution of organic matter by a slowing down of some cracking reactions (please note, as a reminder, that the natural trend is for the S/A ratio to increase with maturation). Moreover,pressure could favor the expulsion of polar compounds. In that respect, the evolution of the expelled (S/A)/residual@/A) ratio as a function of pressure (Figure 7d) is clearly indicative. Results under lowpressure conditions (0 and 170bar) are in agreement with most observationsin geological systems; Le., saturates are preferentially expelled in comparison to aromatics since the former are less polar. On the contrary, high-pressure condition results (550 and, most of all, lo00 bar) show a reverse phenomenon: polar compounds are preferentially expelled (S/A ratios are lower for expelled oils than for residual ones). These data have been completed by the calculation of the asphaltene expulsion efficiencies. It clearly appears that, under low-pressure conditions, asphaltene expulsion efficiencies are lower than CIS+oil fraction expulsion efficiencies, i.e., a result in agreement with what could be expected under natural condition. On the contrary, under high-pressure conditions, expuleion efficienciesof wphaltenes tend to be higher thanthose of the CIS+ oil fraction (cf. Table VI). Similar results have
Blanc and Connan
674 Energy & Fuels, Vol. 6, No. 5,1992
(d) 1170
(a) (UNHEATED]
- 24
BARS
EOURSJ
BARS
-
6 HOURS[
(e)
1170 BARS
-
5 DAYS1
( C ) 1170 BARS
-
12 HOURS]
(f)
(170 BARS
-
14 DAYS/
( b ) 1170
*
Ph = phytane ' I n-alkanes Pr = pristane Figure 8. Gas chromatograms of saturated hydrocarbone extracted from (a) unheated rock sample, as well as from rock samples pyrolyzed under 170 bar during (b) 6 hours, (c) 12 hours, (d) 24 hours, (e) 5 days, and (014 days. also been observed in experiments carried out on full size
core samples from Grimonviller.3'3 Detailed analyses, at a molecular level (GC, GC-MS), will be the subject of a further publication on gas, gasolines, residual and expelled C13+ saturates and aromatics. However,we would like to report some preliminaryresults which seem important. Figure 8 shows the gas chromatograms of the c13+ saturated hydrocarbon fractions extracted from the rock samples heated under 170 bar. The c13+alkane profiles confirm that our artificial maturation device reproduces pretty well the natural evolution of organic matter; in particular, (1)the progressive dilution of isoprenoids and polycyclichydrocarbons by the n-alkanes generated from cracking of the kerogen; (2) the step-by-step vanishing of the odd-predominance in the C d 3 7 range to obtain a rather smooth distribution after 14 days of heating; (3) the progressive shift of the n-alkane distribution toward the lower molecular weight compounde;and (4) the reversal of the pristane to phytane ratio when heating, alsoobeerved
in coal samplesof increasing rank,40reflecting a difference in the timing of generation of these isoprenoid hydrocarbon~.~~ Figure 9 focuses on the (2294237 range of the n-alkane distributions from residual extracts of rocks heated during 24 h under 0,170,660, and lo00 bar. It shows that, with increasingpressure, both the odd-even predominance and the concentration of polycyclic compounds become more obvious. Two calculated parameters have come to support this assumption: an odd-even index (OEI) calculated in the C29437 range, and the Cm-hopaneln-Cm ratio as a measurement of polycyclic hydrocarbon relative concentration. Both exhibit a progressive increasewith increasing pressure (Figure 9). Thia result can be interpreted as a negative influence of pressure on the evolution of organic matter, since both high odd-even predominance and high polycycliccompound concentrationare interpreted rather (40!Radkee, M.;Schaefer, R. G.; Leythaeuaer, D.; TeichmMer, M . Geochrm. Coamochrm. Acta 1980,44,1787-1800. (41) Brooks,J. D.; Gould, K.;Smith, J. W .Nature 1969,222,267-269.
Energy & Fuels, Vol. 6, No. 5,1992 676 --#-UNHEATED ROCK 8HOURS
+-+-24 HOURS 5 DAYS 14 DAYS
-A-
w Z
300
u
(e)
2 4 EouR8
- 550 BARS]
0
> I
1
?:.-.-;-
200
s!.,
--#.
m+
A
100
0 400
420
440
I
,
-....
,("
m
, ....,
Ob
--..._. 460
480
Tmax ('C)
Figure 10. HI vs-'2 diagram for unextracted solid residues of rock samples pyrolyzed during 6 h, 24 h, 5 days, and 14 days: influence of pressure on Rock-Eval data.
Figure 9. Partial gas chromatograms focusing on the high molecular range of the saturated hydrocarbons extracted from rock samples pyrolyzed during 24 h under (a) vacuum, (b) 170 bar, (c) 550 bar, and (d) lo00 bar.
asa token of immaturity. These data confirm the previous results suggesting that pressure was unfavorable to the generation of hydrocarbons. Comparison of the chromatogramsis likely to enable a quantification of the effect of pressure on the profile of alkanes (in particular in the high molecular weight range), though we are aware that it is rather speculative. As an example, a resembling distribution is obtained after 6 h under vacuum, 12 h at 170 bar, and 24 h at lo00 bar (other results for a 12-h heating have not been reported in the paper since experiments after 550 bar have not been conducted for this duration). In the same way, the distribution obtained after 5 days under vacuum resembles the one after 14days under 550 bar. However, other durations of heating between 24 h and 5 days should be considered for a more suitable and accurate comparison. Analysis of the Solid Residues. Results of the analyses of the solid residues remaining after heating of the samples and recovery of expelled fractions (i.e., gas, gasoline + water and c13+ expelled oil) have to be considered with caution. In fact, the low quantities of isolated organicmatter (between100and 150mg) subjected to artificial maturation experimentshave made it difficult (oreven impossiblein somecases)to recover pure kerogen, free of mobile organic matter, so as to undertake valuable further analysis. Therefore, results obtained deal with the unextracted organic matter. The followingis a review of our main preliminary results. Elemental analyses of the unextracted solid residues have confiimedthat our artificialmaturation experiments reproduce properly what is observed in nature. In particular, both H/C and O/C ratios tend to decrease when
increasing the duration of heating confirming the progressive loss of H2O and C02 reported on Table 11. Plots of these ratios in a Van Krevelen diagram show that, whereas the unheated sample contains a type I1 organic matter at a late diagenesis stage (viz., at a vitrinite reflectance about 0.4%, heated samples follow the maturation pathway of type I1 kerogens up to a stage of advanced catagenesis where vitrinite reflectance reaches 1.3%.42Previous has shown that some discrepancies could exist between natural and artificial series concerning this atomic H/C vs O/C trend, but such an observation results from an open-system pyrolysis device. The most important results in the Rock-Eval pyrolysis data are illustrated on Figure 10 (HI vs T ,, diagram for rocks). One may notice two main observations: 1. For a given pressure, when the duration of heating increases, hydrogen index tends to decrease (from about 600 to 150 mg HC/g TOC) and Tmartends to increase (between 425 and 445 "C). These results resemble the natural evolution of organic matter with maturation. 2. For a given duration of heating, when pressure increases from 0 to 550 bar, so does HI, but the latter decreases between 550 and lo00 bar. This result has also been observed for the isolated organic matter samples. Since a decrease of HI is interpreted as a sign of increasing maturity, it is assumed at least between 0 and 550bar that a pressure increase is unfavorable to the evolution of organic matter. The decrease of HI registered between 550 and lo00 bar is poorly understood. As a reminder, this reverse trend between 550 and lo00 bar was also observed as far as the relative content in polar compounds in oils was concerned. Though we have not found any univoqual explanation until now, even from a thermodynamic point of view, a reasonable proposal could be given by Mallinson et alS4 In their calculational study of the (42) Tissot, B.P.; Welte, D. H. Petroleum Formation and Occurrence, 2nd ed.;Springer-Verlag: Berlin, 1984. (43) Lo,H.B. Org. Geochem. 1991,17,415-420. (44) Mallinson, R.L.; Burnham, A. K.; B r a y , R. L.;Westbrook, C. K. Organic Geochemistry. Aduances and Apphcationa m Energy and the Natural Enuironment; Manchester University Press: Manchester and New-York, 1991; pp 309-312.
676 Energy & Fuels, Vol. 6, No. 5, 1992 Table VII. Calculation of the High Molecular Weight Compounds Contribution to the 82 Peak in Rock-Eva1 Analyses. SzHR SzEHR (mgHC/ (mgHC/ Su-Sm, sample press., gHR) gHR) mgHC/ (Su-Sm)/ ref bar time = S u =Sm gHR Su,% 2.72 5.1 53.78 51.06 0 2.84 4.6 62.01 59.17 0 17.47 44.1 39.62 22.15 E l 1A 170 6 h 18.30 58.4 13.04 E l 2A 170 12h 31.34 6.24 19.38 75.6 E l 3A 170 24 h 25.62 4.11 16.79 80.3 20.90 E l 4A 170 5 d 9.74 87.0 1.46 E l 5A 170 14 d 11.20 40.42 26.08 14.34 35.5 E2 1A 550 6 h 26.15 81.2 6.05 E2 2A 550 24 h 32.20 18.97 2.04 16.93 89.2 E2 3A 550 5 d 1.71 11.72 87.3 E2 4A 550 14 d 13.43 39.56 26.16 13.40 33.9 E5 1A lo00 6 h 14.63 17.20 54.0 E5 2A1 lo00 12 h 31.83 13.77 45.2 16.69 E5 2A2 lo00 12 h 30.46 19.15 65.0 10.93 E5 3A lo00 24 h 29.48 15.36 79.8 19.24 3.88 E5 4A1 lo00 5 d 15.13 81.3 18.62 3.49 E5 4A2 lo00 5 d 2.04 10.40 83.2 E5 5A1 lo00 14 d 12.14 1.44 E5 5A2 lo00 14 d 10.85 9.41 86.7 a
HR = heated rock; EHR = extracted heated rock.
effect of pressure on hydrocarbon cracking they show a reverse kinetic trend between 300 and 10o0 atm when heating at 200 "C. It has been observed that a large contribution to the SZ peak of Rock-Eval was brought by extractible high molecular weight compounds.4648 In our study, this contribution has been estimated by a comparison of Sa values from unextracted and extracted rock samples. Results. (Table VII) show that if the contribution of extractible high molecular weight compounds represents only 5 % of the SZ peak of the unheated rock, this contributionranges from about 35-45 % for samplesheated for 6 h and up to almost 90% in the case of rocks heated for 14 days. However, whereas this contribution is enhanced when the duration of heating increases, the absolute amount of extractible high molecular weight ) compounds registered in the SZ peak (S2A - S ~ Bgoes through a maximum value (after a 24-h heating) for a given pressure. This feature is explained by a two-step process: first, a generation of high molecular weight entities (e.g., asphaltenes, resins) from cracking of the kerogen and then a generation of hydrocarbons by cracking of these entities themselves. This phenomenon is well illustrated on Figure 11which shows a perfect correlation between high molecular weight compounds calculated from Rock-Eval pyrolysis and polar compounds calculated by Iatroscan (Le., asphaltenes plus resins) whatever the pressure conditions. Maximum values are reached after 24 h of heating. Since these soluble high molecular weight compounds are likely to generate hydrocarbons during maturati0n,4~they should be taken into account when evaluating the petroleum potential of sedimentary rocks. (45) Espitalil, J.; Deroo, G.;Marquis, F. Rev. Inst. Fr. Pet. 1986,41, 73-89.
(48)Tarafa, M. E.; Whelan, J. K.; Farrington, J. W. Org. Ceochem.
1988.12.137-149. . ~ ~ . (47) Delvau, D.; Martin, H.; Leplat, P.;Paulet, J. Ado. Org.Ceochem. 1989 1990,16, 1221-1229. (48) Wilhelms,A.; Larter, S. R.; Leythaeuser,D.Org. Geochem. 1991, 17. 361-354. ---
Blanc and Connun
0 UNHEATED ROCKS H HEATED 8 HOURS 0 HEATED 12 HOURS HEATED 24 HOURS A HEATED 5 DAYS 0 HEATED 14 DAYS 4
u
* w $ 0o
10
20
30
40
50
POURS IATROSCAN (mg/g rock)
Figure 11. Analyeis of the organic extract: correlation between the content of high molecular weight compounds (Rock-Eval analyea of unextracted and extracted rock samples) and the content of polar compounds (Iatroscan analyses of residual oils).
Therefore, Rock-Eval data obtained on unextracted rocks and illustrated on Figure 10 appear to correctly reflect this generative potential as a whole. It should be added that the hypothesisof an unfavorable effect of pressure on the genesis and evolution of hydrocarbons was previously proposed after examination of natural case histories.22p24This phenomenon could explain the occurrence of CU+ hydrocarbons at depths and temperatures a t which only gases and pyrobitumenshould be expected.22*50Experimental studies of artificial maturation of alginite and ligniteZ6as well as kinetic studies of molecular models pyrolysis also tend to favor such an hypothesis. Domin6I2and Domin6 et alqS1showed that high pressure decreases the overall pyrolysis rate of n-hexane, whereas Enguehard et aLS2concluded that increased pressure favors addition reactions in the case of dibutyl ether pyrolysis. Recent studies28*4have laid emphasis upon the difficulty to observe pressure effects under laboratory conditions; depending on pressuretemperature conditions, pressure can either accelerate or inhibit some cracking reactions. These authors, however, work with pure model compounds and their results are therefore unlikely to reflect exactly the complexity of the reactions which can be involved when organic matter is pyrolyzed. As a matter of fact, several kinds of reactions can take place during heating, more or less interrelated, leading to a given quantity of oil, with a given composition as well as a given molecular distribution. Even if pressure actually seemsto be unfavorable to the evolution of organic matter as a whole, it is likely that some kin& of reactions can be accelerated whereas some others might be inhibited by high-pressure conditions, thus leading to apparently contradictory results. As an example, the decrease of the
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~
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(49)Tmot, B. Churacterizotion ofHeouy Crude Oils and Petroleum Residues, Technip: Paris, 1984; pp 3-18.
(50)Price, L. C.; Clayton, J. L.; Rumen, L. L. Org. Ceochem. 1981,3, 59-77. (51) DominB, F.;Marquaire, P. M.; Muller, C.; C h e , G. M.Energy Fueb 1990,4, 2-10.
(52) Enguehard, F.; Kreaemnnn, S.; Domin6, F. Adu. Org. Geochem.
1989 1990,16,155-160.
Generation and Expulsion of Hydrocarbons
hydrogen index of samples pyrolysed under lo00 bar in comparison with those treated under 550 bar does not necessarily mean that the former pressure condition is more favorable to the evolution of organic matter than the latter. In fact, the favored production of carbon dioxide from organic origin observed under 10oO bar may explain the decrease of the polar compound content in the oil, in particular the macromolecular ones such as resins which have been shown to largely contribute to the SZpeak.
Energy & Fuels, Vol. 6, No. 5, 1992 677
(e.g., aromatics compared with saturates among the hydrocarbons, and asphaltenes compared with hydrocarbons among the C13+ oil). Lastly, a reversal trend of the effect of pressure between 550 and lo00 bar at 330 OC has been noted, though not well understood. This could be the result of interrelated competitive reactions. From a general point of view, these results show that the pressure parameter should be taken into account for appropriate modeling of generation and expulsion of hydrocarbons from sedimentary rocks.
Conclusions Confined-system pyrolysis of rocka and of the corresponding isolated organic matter in gold sealed tubes has shown that the generation and expulsion of hydrocarbons could be correctlyreproduced under laboratoryconditions. As preliminary results, these experiments have made it possible to estimate the participation of both mineral and organic matters in the production of carbon dioxide when heating. Emphasis has been laid on the favorableinfluence of pressure on the expulsion efficiency of the C13+ oil fraction and on its unfavorable influence on the hydrocarbon generation from organic matter. The latter can be interpreted by a retardation effect of pressure on cracking reactions or a direct influence on the nature of the generated compounds. Moreover, high-pressure conditions have been shown to easethe generation and expulsion of the most polar compounds in each kind of fractions
Acknowledgment. P. Landais and R. Michels are acknowledged for the pyrolysis experiments conducted at the Centre de Recherches sur la GBologie de 1'Uranium (CREGU, Nancy). We are also grateful to J.-Y.Cariou, who largely contributed to the gas and gasoline recovery device,to A. Doumerguewho prepared some of the samples, and, in a more general way, to all our colleagues in Elf Aquitaine for the multiple analyses. P.B. acknowledges the Centre National de la Recherche Scientifique (C.N.R.S.) and Elf Aquitainefor a postdoctoralfellowship. The management of Exploration Directorate of Elf Aquitaine kindly gave permission to publish this paper. Registry NO. W ,14762-74-4;"0,14797-71-8;CHI, 74-82-8; C&, 74-84-0;Cas, 74-98-6;CdH10, 106-97-8;C a i 2 , 109-66-0; coz,124-38-9;H ~ S7783-06-4; , N,7727-37-9;~ r7440-37-1. ,
