Geochemical characteristics of Tertiary oils derived from siliceous sources in Japan, Russia and U.S.A

PII:

Org. Geochem. Vol. 27, No. 7/8, pp. 523±536, 1997 # 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0146-6380/97 $17.00 + 0.00 S0146-6380(97)00073-9

Geochemical characteristics of Tertiary oils derived from siliceous sources in Japan, Russia and U.S.A ALEXANDER CHAKHMAKHCHEV*1{, MASARU SUZUKI2, AMANE WASEDA2 KUNIAKI TAKAYAMA1 1

Japan National Oil Corporation (TRC), 2-2, Hamada 1-chome, Mihama-ku, Chiba 261, Japan and 2 JAPEX Research Center, 1-2-1 Hamada, Mihama-ku, Chiba 261, Japan

(Received 3 September 1996; returned to author for revision 14 February 1997; accepted 4 August 1997) AbstractÐSiliceous sourced Tertiary oils from the Circum-Paci®c area of Japan, Russia and the U.S.A. have a heavy carbon isotope composition, monomodal n-alkane distributions, and nearly identical regular sterane compositions with a predominance of C27 homologues. These are consistent with open marine depositional environments dominated by diatomaceous organic matter. However, a number of alkane and biomarker parameters such as Pr/Ph, CPI, relative concentration of 28,30-bisnorhopane, and the C35/C34 homohopane ratio indicate more oxic depositional environments for the source rocks of Japan and Russia. In contrast to the California Monterey Formation sourced oils, petroleums with low maturity levels from the North Sakhalin basin, Russia and the Akita basin, Japan have lower concentrations of asphaltenes and sulphur and are characterized by higher API gravities. A correlation of extractable organic matter from source rocks vs the least matured petroleums demonstrates that oil expulsion in siliceous shales of the Akita basin occurs at a maturity level corresponding to Ror0.65%, which is in the range of the conventional oil window (Ro=0.6±1.1%). # 1997 Elsevier Science Ltd. All rights reserved Key wordsÐMonterey oils, California, Akita oils, Japan, Sakhalin oils, Russia, biodegradation of oil, low maturity generation, biomarker characteristics, siliceous shale source rocks

INTRODUCTION

Oil basins of the Circum-Paci®c region, including the Akita of Japan, North Sakhalin of Russia, and Santa Maria and Santa Barbara-Ventura of the United States, are located at young continental margins characterized by active geological processes. In these basins, rapid rates of sedimentation and active tectonic movements resulted in the formation of petroleums with unique geochemical characteristics. The well known Miocene Monterey Formation, California and its analogs the Onnagawa Formation, Japan and the Oligocene±Miocene sequence of Sakhalin Island, Russia are considered e€ective source rocks. These formations have common features, such as young age (Miocene± Oligocene), enrichment in biogenic silica and signi®cant diatomaceous input in their organic matter. It is of great interest that siliceous formations from the Circum-Paci®c area have coincident source rock±reservoir properties (Isaacs, 1984). *Present address: LUKOIL, Department of geology and exploration, Sretensky blv., 11 101000 Moscow, Russia {Author to whom correspondence should be addressed. 523

Previous geochemical studies by King and Claypool (1983), Magoon and Isaacs (1983), Curiale et al. (1985), Sakata et al. (1988), Bazhenova and Aref'ev (1990), Peters and Moldowan (1993), Waseda and Nishita (1994), Chakhmakhchev and Suzuki (1995), Chakhmakhchev et al. (1996) and Popovich and Kravchenko (1995) demonstrated a number of characteristics of siliceous sourced oils from the di€erent basins of the Circum-Paci®c region. Extensive data were published on bulk properties, isotopic and biomarker compositions of the Monterey oils of California. Japanese petroleums are documented mainly in domestic research journals. Among the siliceous sourced oils from the Circum-Paci®c area, the least studied are Russian petroleums from Sakhalin Island. Di€erent mechanisms for the generation of immature oils from siliceous sources have been proposed; however, none has been adopted unanimously. For example, Orr (1986) suggested that sulphur-rich Type II-S kerogen from the Monterey Formation of the Santa Maria basin generates heavy asphaltenene rich oils at lower thermal exposure than typical Type II kerogen. The cleavage of weak sulphur±carbon bonds in kerogen during early stages of maturation were considered as the

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In the present study, we investigated biomarker distributions of representative petroleums from the Circum-Paci®c region of Japan, Russia and the western United States (California), in order to reveal similarities and distinctions among oils from the di€erent basins characterized by biogenic siliceous source rocks. Based on their geochemical characteristics, we attempted to reconstruct depositional environments and the type of organic matter source. We also discuss the timing of oil expulsion along with geochemical data of the petroleums and the organic matter of the source rocks.

GEOLOGICAL SETTINGS

Fig. 1. Location map of oil and source rock samples from Akita basin, Japan used in the present study. The possible source rock samples were collected from the YuriokiChubu well.

principal mechanism of kerogen decomposition. Orr also suggested that oils inherit their high sulphur content and heteroatom distributions from the Type II-S kerogen. Baskin and Peters (1992) also emphasized the critical role of organic sulphur in the early generation of oil from a sulphur-rich Monterey kerogen and suggested a two-step process for petroleum generation that included initial bitumen generation from kerogen and subsequent oil formation by decomposition of the bitumen. Taguchi et al. (1988) proposed a mechanism for the expulsion of immature hydrocarbons from siliceous sources in the Akita basin at temperatures of 30±508C and burial depths of 500±800 m. It was suggested that the expulsion of hydrocarbons was closely related to the gelatinization and crystallization of amorphous silica, and the ejection of alumosilicate components and organic matter. Bazhenova and Aref'ev (1990) demonstrated the possible formation of immature oils during diagenesis in siliceous rocks of the Okhotsk basin in Eastern Sakhalin. However, they found Sakhalin oils to be poorer in sulphur and richer in oxygen compared with Monterey oils. In contrast to the above-mentioned studies, Kruge (1986) suggested that oils in some shallow Monterey reservoirs are not indigenous, but were generated in the source rocks at depths of more than 3400 m and at vitrinite re¯ectance levels >0.5%. It is important to note that a number of recent laboratory experiments demonstrated that the kinetics of hydrocarbon generation of the Monterey Formation are not governed by organic sulphur content alone, and the temperatures for the onset and peak generation of the Monterey are similar to those calculated for typical Type II source rocks (Jarvie and Lundell, 1993; Reynolds et al., 1995).

Akita basin The Akita basin is located in the inner belt of the Japanese island arc (Fig. 1). The igneous and metamorphic basement rocks are Cretaceous to early Cenozoic. Tertiary deposits accumulated as a result of rapid subsidence and more than 6000 m of Tertiary sediments are present in this basin. They are composed mainly of tu€aceous sandstones and dark gray siliceous and argillaceous mudstones. In the Akita basin, the Onnagawa and Nishikurosawa Formations of Middle and Late Miocene ages are considered to be e€ective source rocks. Figure 2 shows a geologic column for the Akita basin with age, sediment type and occurrence of oil and gas. These deposits are characterized by a high content (up to 60±70%) of biogenic silica. The organic matter in each formation is predominantly Type II kerogen. Paleogeothermal gradient of the Akita basin at maximum burial depth is estimated at 3.0± 4.58C/100 m. Most of the oil and gas±oil ®elds are related to steeply-dipping reverse faults and folds trending northeast±southwest. Hydrocarbon accumulations are found in both clastic and volcanic reservoirs. Detailed geological descriptions of the Akita basin and the geochemical characteristics of possible source rocks and oils were published by Hirai (1980), Sakata et al. (1988) and Kikuchi et al. (1991) (and references within). North Sakhalin basin North Sakhalin basin occupies the northeast coast of Sakhalin and the adjacent area of the Okhotsk Sea (Fig. 3). The great majority of oil ®elds relate to anticlines with longitudinal and transverse faults. Almost all oil and gas deposits have been discovered in Miocene and Pliocene deltaic sediments of the paleo-Amur river. The level and number of faults control the phase of the hydrocarbon accumulations. Anticlinal structures characterized by intensive faulting usually contain only oil pools with low gas/oil ratios since conditions for gas trapping are poor.

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525

Fig. 2. Generalized stratigraphic framework of the North Sakhalin basin, Russia and the Akita basin, Japan.

Lithology, organic carbon content and maturity of possible source rocks in North Sakhalin

Fig. 3. Location map of oil samples from the North Sakhalin basin, Russia.

It is believed that two possible source sequences of Miocene and Oligocene ages exist in the North Sakhalin basin (internal, unpublished report of IGIRGI, 1992). The ®rst is related to Middle and Lower Miocene sequences, including the Uinisk, Okobyk and Daginsk Formations (Fig. 2). These sequences are mainly shallow marine and coastal± marine deposits composed of clays, siltstones and sandstones. Organic carbon content in the clays range from 1.0 to 1.5% and maturity levels vary from low to moderate, corresponding to %Ro=0.50±0.75. In some portions of the basin, Middle and Lower Miocene sequences contain siliceous shales deposited in deep marine environments. For example, at the Okruzhnoe oil ®eld, Lower Miocene ±Oligocene siliceous shales are the reservoirs, analogous to some source±reservoir sequences of the Monterey Formation, California, U.S.A. (Ammosov, 1975; Ivanov et al., 1988; Agapitov et al., 1991; Kodina et al., 1989). Argillaceous and siliceous Olicogene shales are considered to be the main source of the petroleum in the Sakhalin. These sediments of the Daehurinsk and Machigairsk Formations, deposited in a marine environment, have moderate organic carbon contents of 0.9±1.5% and variable maturity levels corre-

526

Alexander Chakhmakhchev et al. Table 1. Bulk geochemical properties of oils from the Circum-Paci®c area of Japan, Russia and U.S.A.

API8 Asphaltenes d13CS (%)

Monterey oils, California

Akita oils, Japan

Sakhalin oils, Russia

33.8±6.7 High ÿ21.9 to ÿ23.3 0.50±7.58

35.8±48.0 Low ÿ21.7 to ÿ22.8 0.14±0.36

33.0±43.2 Low ÿ23.9 to ÿ25.6 0.10±0.45

See, for example, Curiale et al. (1985); Orr (1986); Kodina et al. (1989); Agapitov et al. (1991); Schoell et al. (1992); Waseda and Nishita (1994) and references therein.

sponding to %Ro=0.70±1.6%. Among four selected petroleums from Sakhalin, only oil from the Okruzhnoe ®eld has been correlated to the siliceous source rocks of Lower Miocene±Oligocene age based on biomarker and carbon isotope characteristics (Kodina et al., 1989; Bazhenova and Aref'ev, 1990, 1994). For the other oil samples, we suggest a siliceous Tertiary source based on similarities in their geochemical characteristics to other oils with known siliceous sources. It is important to note that these four samples have very low terrestrial input and are likely to derive from clastic-poor source rocks. However, the samples selected do not represent all varieties of Sakhalin oils, others of which have higher plant input and advanced maturity levels (Chakhmakhchev and Suzuki, 1995; unpublished data). Formation of oils with terrestrial input is controlled by the proximity of the paleoAmur river which contributed a great deal of clastic sediment (clay, sand) and terrestrial organic matter to Sakhalin area. Santa Maria and Santa Barbara-Ventura basins For a detailed description of Santa Maria and Santa Barbara-Ventura basins (California), their geology, origin, type and geochemistry of source rocks and oils, readers are referred to King and Claypool (1983), Magoon and Isaacs (1983), Isaacs (1984, 1992), Curiale et al. (1985) and Curiale and Odermatt (1989) and references therein. The California coastal area oils have bulk and geochemical similarities, such as low gravities, high sulfur and asphaltene concentrations, and low maturity levels. However, it should be noted that not all Monterey oils are characteristic of those from the Santa Maria and Santa Barbara-Ventura basins. Monterey oils from the San Joaquin basin can be di€erent in geologic setting, mode of formation and physical and chemical properties (Baskin, pers. commun.).

SAMPLES AND METHODS

Twelve oils and four possible source rocks were selected for this comparison study based on geochemical screening of a larger number of analyzed samples from the Circum-Paci®c area (Figs 1±3). Oil samples were studied from the Akita basin, Japan, the Okhotsk basin (Sakhalin), Russia and

the Santa Maria and Ventura basins, U.S.A. All have low to moderate maturity levels as evaluated by hydrocarbon indicators. Powdered core samples (<200 mesh) of rocks were extracted with C6H6/MeOH (9:1) in a Soxhlet apparatus. Elemental sulphur was removed by activated copper during extraction. The extracts and crude oils were subjected to column chromatography on silica gel (Wakogel C-200). The saturate fraction was eluted by n-hexane (35 ml). The saturate fraction was analyzed using a HP5890 GC and Finnigan Mat TSQ 700 GC/MS equipped with a 30 m  0.25 mm i.d. fused silica capillary coated with DB-5 (J and W). The oven temperature was held at 408C for 2 min and programmed from 40 to 808C at 108C/min, then from 80 to 3008C at 48C/ min, then held for 30 min at isothermal. The mass spectrometer was operated at an electron energy of 70 eV, an ion source temperature of 2008C and transfer line of 3008C. Ions monitored were m/ z = 217 (steranes), and m/z = 177, m/z = 191 (triterpanes). Identi®cation of biomarkers was performed by comparison of GC retention time data and mass spectra with published data (Philp, 1985; Peters and Moldowan, 1993 and references therein). Relative concentrations in gas chromatoarams and mass fragmentograms were determined based on peak heights. Vitrinite re¯ectance (%Ro) measurements were performed.

RESULTS

Bulk properties The bulk properties of oils from the CircumPaci®c area are diverse, showing a wide range of API gravities and asphaltene and sulfur contents (Table 1). In comparison with the Monterey petroleums, oils from Japan and Russia are much lighter (API = 33±488) and relatively poor in asphaltenes and sulphur (S < 1.0 wt%). In general, Tertiary oils from the study areas are characterized by heavy (positive) isotope composition (Magoon and Isaacs, 1983; Waseda and Nishita, 1994), possibly re¯ecting a dominant diatomaceous organic matter source. However, Sakhalin petroleums show slightly lighter isotope compositions in the range of d C13= ÿ 23.9 to ÿ25.6-. An explanation for this variation is given below.

Geochemical characteristics of Tertiary oils derived from siliceous sources

527

Fig. 4. Gas chromatograms of the saturate fractions for three oils from the Akita basin, Japan, the North Sakhalin basin, Russia and the Santa Maria basin, U.S.A. Lower Pr/Ph ratio and CPI < 1 suggest anoxicity in the source rock depositional environments of the Santa Maria basin.

Alkane distributions in oils Non-biodegraded and slightly degraded oils from the Circum-Paci®c area of Japan, Russia and the western U.S.A. have some similar n-alkanes distributions. Saturate fractions of the oils demonstrate a monomodal distribution at a maximum among the lighter hydrocarbons (C11±C19), and biomarker peaks visible in the gas chromatograms (Fig. 4). However, Monterey reservoired oils are distinguished by a predominance of even-numbered nalkanes and a carbon preference index (CPI) lower than 1.0, whereas oils from Russia and Japan show

CPI values above 1.0. Signi®cant di€erences can be observed in the Pr/Ph ratios of these petroleums, with values of 1.36±1.76 in oils from Japan and Russia and 0.71±1.15 in oils from California. In some low maturity oils from Russia and the U.S.A. relative concentrations of C19 and C20 isoprenoids are high (Table 2). Sterane distributions in oils Tertiary oils from the study areas demonstrate very similar distributions of regular steranes. The carbon number predominance in steranes is

C27 (%) C28 (%) C29 (%) 20S/20S + 20R bb/bb + aa dia/reg C21+C21/reg

C29/C30 (ab) C28/C30(ab) Oleanane/ C30(ab) 22S/ 22S + 22R, C31 Ts/Ts+Tm, C27 ba/ab + ba, C30 C35/C34

5 6 7 8 9 10 11

12 13 14 15

0.35 0.12

0.31 0.20

0.57

0.54

0.51

1.22

0.60 0.21 0.12

41.4 30.0 28.6 0.39 0.43 0.31 0.27

nd 1.41 nd nd

$

0.51 0.30 0.65

43.2} 27.6} 29.2} 0.85 0.17 0.48} 0.66}

nd nd nd nd

*

0.95

0.26 0.12

0.53

0.58 0.12 0.07

44.3 32.2 23.4 0.40 0.43 0.36 0.24

>1 1.26 1.98 2.04

%

0.68

0.27 0.14

0.56

0.62 0.11 0.07

45.5 32.6 21.9 0.32 0.31 0.12 0.15

>1 1.60 2.26 1.58

%

Okruzhnoe 1900±1929

Yabase conf.

0.73

0.33 0.12

0.53

0.75

0.21 0.14

0.55

% Alkanes >1 >1 1.41 1.48 1.19 1.08 1.07 0.85 Steranes m/z = 217 42.0 36.1 33.9 35.2 24.1 28.7 0.47 0.35 0.53 0.35 0.54 0.20 1.03 0.17 Hopanes m/z = 191 0.53 0.52 0.12 0.07 0.33 0.20

%?

Amarume 1000

0.83

0.31 0.13

0.54

0.53 0.08 0.24

38.5 34.9 26.6 0.42 0.42 0.33 0.42

<1 1.53 0.85 0.57

%

Yurihara conf.

0.78

0.16 0.13

0.56

0.79 0.08 0.15

39.1 34.8 26.1 0.42 0.38 0.21 0.18

<1 1.69 1.14 0.71

%

Hashimoto 900

1.02

0.24 0.12

0.60

0.55 0.34 0.05

43.2 33.1 23.7 0.43 0.51 0.29 0.32

<1 0.94 0.91 0.91

%

Four Deer 1463±1890

1.01

0.25 0.13

0.58

0.52 0.18 0.07

40.5 33.4 26.1 0.33 0.38 0.17 0.13

<1 0.86 1.70 2.08

%

Barham Ranch 1219

St Maria

1.37

0.25 0.09

0.57

0.61 0.55 0.13

42.5 30.9 26.6 0.40 0.52 0.18 0.27

<1 0.71 1.57 2.12

%

Point Pedernales 2011

1.45

0.28 0.09

0.57

0.62 0.69 0.13

41.6 34.5 23.9 0.45 0.54 0.34 0.44

<1 1.15 0.66 0.61

%

South Elwood 1067

St.BarbaraVentura

*Biodegraded oils. $Heavily biodegraded oils. %Undegraded oils. 1: 0.5(C23ÿC33)odd/(C22ÿC32)even + (C23ÿC33)odd/(C24ÿC34)even). 2: Pristane/phytane. 3: Pristane/normal ÿ C17. 4: Phytane/normal ÿ C18. m/z = 217. 5: aaC27(20R)/(aaC27(20R) + aaC28(20R)a(C29(20R)). 6: aaC28(20R)/(aaC27(20R) + aaC28(20R)a(C29(20R)). 7: aaC29(20R)/(aaC27(20R) + aaC28(20R)a (C29(20R)).8: aaC29(20S/20S + 20R). 9: C29(bb(20S + 20R)/bb(20S + 20R) + aa(20S + 20R)). 10: (diasteranes labelled as dia)/(C27+C28+C29)aa20R. 11: (C21+C22)/(C27+C28+C29)aa20R. }: Calculated based on 20S isomers. m/z = 191. 12: abC29norhopane/abC30hopane. 13: 28,30-bisnorhopane/abC30hopane. 14: oleanane/abC30hopane. 15: abC31(22S)/(abC31(22S) + abC31(22R)). 16: 18a(H) ÿ 22,29,30-trisnorneohopane/ (18a(H)22,29,30-trisnorneohopane + 17a(H)-22,29,30-trisnorhopane). 17: baC30hopane/(abC30hopane + baC30moretane). 18: C35(22S + 22R)/C34(22S + 22R)homohopanes.

18

16 17

CPI Pr/Ph Pr/n-C17 Ph/n-C18

Biodegradation

East Ekhabi 1360±1372

Akita

Table 2. Biomarker geochemical characteristics of oils from the Circum-Paci®c area of Japan, Russia and U.S.A.

North Sakhalin

Oil ®eld depth Katangly 135± Ekhabi 945± (m) 143 958

1 2 3 4

N

Indicators/ basin

528 Alexander Chakhmakhchev et al.

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529

Fig. 5. Sterane (m/z = 217) distributions in oils from the Akita basin, Japan, the North Sakhalin basin, Russia and the Santa Maria basin, U.S.A. demonstrating similar source organic matter.

C27>C28>C29 (Figs 5 and 6; Table 2). Abundant C28 steranes are common for marine sourced young Cenozoic oils (Grantham and Wake®eld, 1988). Diasteranes are present in low concentrations (dia/ reg = 0.12±0.54) and their relative content seems to be controlled by maturation, because it correlates well with the 20S/(20S + 20R) ratio maturity parameter (Fig. 7) (Waples and Machihara, 1992, and references therein). Maturity parameters such as 20S/(20S + 20R) and bb/(bb + aa) C29 steranes

range from 0.32 to 0.47 and 0.31 to 0.54, respectively, indicating low to moderate levels of thermal transformation (Seifert and Moldowan, 1981). The (C21+C22)/(C27+C28+C29) sterane ratio increases in petroleums with maturation and correlates with 20S/(20S + 20R) parameter as shown by Sakata et al. (1988). Two peaks in m/z = 217 mass fragmentograms were identi®ed as C26 steranes (norcholestanes) based on their mass spectra and comparison with published traces (Fig. 5) (Moldowan et al.,

530

Alexander Chakhmakhchev et al.

Fig. 6. Triangular diagram showing nearly identical distributions of C27, C28, C29 regular steranes in selected oils from the Circum-Paci®c area.

1991). Earlier, abnormally high abundances of these compounds have been found in extracts from the highly siliceous Onnagawa formation, Japan by Suzuki et al. (1993). Diatoms were suggested to be a precursor of C26 steranes in the organic-rich Onnagawa shales. Hopane distributions in oils In several respects, triterpane distributions in oils from the Circum-Paci®c area are similar to each other. The oils show almost identical distributions of tricyclic compounds in m/z = 191 mass fragmentograms (Fig. 8). The C29 (norhopane)/C30 (hopane) values of 0.51±0.79 detected in petroleums are common for siliclastic sourced oils (Peters and Moldowan, 1993). Moretane and age-diagnostic oleanane are present in relatively low concentrations. The maturity parameter Ts/(Ts+Tm) of

C27 hopanes is low and varies insigni®cantly, ranging from 0.16 to 0.35. Monterey oils are distinguished by high concentrations of the 28,30 -bisnorhopane (C28) and C35 homohopanes, which are considered to be indicators of highly reducing anoxic environments (Katz and Elrod, 1983; Peters and Moldowan, 1993). In the m/z = 191 mass fragmentograms of the Akita basin petroleums, a peak eluting between the C24 and C25 tricyclics was identi®ed as tetracyclic 10b(H)-des-A-oleanane (Woolhouse et al., 1992). This compound was shown to correlate positively with the Pr/Ph ratio and oleanane contents in petroleums from Japan (Chakhmakhchev et al., 1996). Its mass spectrum has a fragment ion of m/z = 330, which is characteristic of tetracyclic terpanes. These are possibly derived from degradation process acting on various triterpenoid precursors present in the higher plant during early diagenesis (Aquino Neto et al., 1983; Philp and Gilbert, 1986; Kvenvolden et al., 1991). Low concentrations of 10bP(H)-des-Aoleanane and oleanane in Japanese oils indicate insigni®cant terrestrial input. Biodegradation It is of great importance to distinguish petroleums altered by biodegradation when products of early generation are discussed. Relatively viscous and heavy oil produced from a depth of 135±143 m in the Katangly ®eld, Sakhalin is depleted in nalkanes and acyclic isoprenoids. The other distinguishing feature of this biodegraded oil is an abnormal distribution of steranes (20S>>20R) because 20S isomers are more resistant to biodegradation than their 20R homologues (Fig. 9). Hopane biomarkers also demonstrate an unusual distri-

Fig. 7. DIA/REG vs 20S/(20S + 20R) and (C21+C22)(C27+C28+C29) vs 20S/(20S + 20R) plots showing that relative concentrations of diasteranes and C21+C22 steranes in oils depend on maturity.

Geochemical characteristics of Tertiary oils derived from siliceous sources

531

Fig. 8. Hopane (m/z = 191) distributions in oils from the Akita basin, Japan, the North Sakhalin basin, Russia and the Santa Maria basin, U.S.A. Compared with Californian coastal oils, Russian and Japanese oils are relatively low in 28,30-bisnorhopane and C35 homohopanes.

bution. In this oil, norhopane and hopane (ab) are severely reduced and the more biodegradation resistant oleanane and C35 homohopanes are relatively abundant (Moldowan and McCa€rey, 1995). In addition, 25- norhopanes (heavy biodegradation indicators) were detected in this oil by m/z = 177 and m/z = 191 mass fragmentograms (Peters and Moldowan, 1993). The Katangly sample represents a group of shallow biodegraded petroleums which

should not be considered as immature petroleums resulting from early generation. Organic matter and depositional environment of the source rocks for petroleums from the Circum-Paci®c area Alkane distributions with a maximum concentration at C15±C17. predominance of C27 steranes, and heavy carbon isotope composition of ÿ21 to

532

Alexander Chakhmakhchev et al.

Fig. 9. Sterane (m/z = 217) and hopane (m/z = 177 and =191) distributions in the heavily biodegraded Katangly oil (depth 135±143 m).

ÿ25- are suggestive of open marine depositional conditions for the source material involved (Curiale et al., 1985). Diatom organisms are often considered as organic matter precursors for petroleums from the Circum-Paci®c area (Giger and Scha€ner, 1981; Suzuki et al., 1993). Slightly lighter isotope composition of carbon of the Sakhalin oils may re¯ect a di€erence in water temperature during sedimentation (Sackett, 1989). The North Sakhalin basin is located at relatively higher latitudes than the Akita, Santa Maria and Santa Barbara-Ventura

basins. In this area cold water diatom assemblages are thought to be responsible for the isotope fractionation resulting in a shift toward lighter isotope composition of ÿ23 to ÿ25-. Low diasterane contents in petroleums are commonly derived from source rocks that are poor in elastic material, such as siliceous shales (Curiale et al., 1985). More than likely, the presence of oleanane in oils re¯ects their young age, but the low concentration of this biomarker in this case suggests negligible terrestrial input. Absence of signi®cant

Geochemical characteristics of Tertiary oils derived from siliceous sources

533

terrestrial input is supported by the low concentrations of moretane in oils with low maturity and the sterane distribution. The most extensive variations are observed among parameters of oxic conditions (redox potential) during deposition of the source sediments. Thus, coastal Monterey oils are characterized by the lower Pr/Ph ratio, predominance of even-numbered alkanes (CPI < 1), high contents of 28,30-bisnorhopane and high values C35/C34 homohopanes (>1.0). These parameters indicate highly reducing to anoxic marine environments of Monterey source rocks and di€er signi®cantly in oils from Japan and Russia, suggesting more oxic depositional environments in the source rocks (Fig. 10). Maturity indicators of oils Fig. 10. Pr/Ph vs C35/C34 homohopanes as indicators of redox potential in source depositional environments of oils from the Circum-Paci®c area. .

Japanese and Russian oils expelled from siliceous source rocks at the top of the main phase of oil generation are characterized by low to moderate maturity levels evaluated by di€erent biomarker

Fig. 11. Alkane distributions and maturity parameters in extracts of possible source rocks, YuriokiChubu well in the Akita basin. The least matured oils from the Akita and Sakhalin can be correlated to the organic matter of source rocks buried at depths of 3500±4000 km based on isoprenoids/alkanes, 20S/(20S + 20R) of steranes and 22S/(22S + 22R) of hopanes ratios.

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parameters. For example, none has its 20S/ (20S + 20R) ratio of C29 steranes close to equilibrium. Oils show a low Ts/(Ts+Tm) ratio of C27 terpanes and predominance of isoprenoids over nalkanes in GC patterns. Abundant biomarkers observed in the saturate fraction gas chromatograms also suggest indirectly low maturity. However, low maturity indicators do not characterize all Tertiary oils from the Circum-Paci®c area. For example, several petroleums from the nearby Niigata basin, Japan have relatively higher maturity levels, as indicated by 20S/(20S + 20R) ratio of 0.56±0.57 (Suzuki et al., 1987; Chakhmakhchev et al., 1996). This variation in maturity of Niigata and Akita basins oils was explained by di€erent timing of expulsion between siliceous and argillaceous source rocks (Suzuki et al., 1987). The Niigata basin is geographically close to the Akita, however, it is distinguished by a di€erent source rock lithology (Miocene argillaceous shales of the Teradomari and Nanatani Formations). It is important to note that unlike coastal Monterey sourced petroleums, oils from Russian Sakhalin and Japanese Akita basin oils are relatively light and poor in asphaltenes and sulphur. Although some siliceous sourced Akita and Sakhalin oils were shown be relatively rich in aromatic sulphur compounds, total sulphur content in petroleums from Japan and Russia never exceeds 0.5% (Chakhmakhchev and Suzuki, unpublished data). The timing of primary migration in the Akita basin, Japan To understand the timing of primary migration of oils derived from the siliceous sources, we correlated hydrocarbon distributions of oils and extractable organic matter of possible source rocks from the Akita basin, Japan. Figure 11 shows alkane distributions of hydrocarbon extracts from the source rocks with maturity levels in the range of Ro=0.55 to 0.74% from the Yurioki-Chubu well in the Akita basin. As can be seen the least matured Akita and Sakhalin oils can be correlated to the organic matter of source rocks buried at the depth of 3500± 4000 m based on isoprenoid/alkane, the 20S/ (20S + 20R) sterane ratio and the 22S/(22S + 22R) ratio hopane. It is less likely that the shallower rocks are an e€ective source of oil, since they are low in n-alkanes and are characterized by extremely low values of biomarker maturity. This oil±source rock correlation provides evidence for petroleum expulsion in siliceous shales of the Akita basin at maturity levels corresponding to Ror0.60±0.65%. Earlier, approximately the same maturity levels and depth intervals were suggested for the onset of oil generation in the Monterey Formation of the west San Joaquin basin (Kruge, 1986). Data obtained show that some oils with low maturity indicators

from the Circum-Paci®c area can be expelled from siliceous source rocks at the top of the main phase of oil generation. However, the timing of expulsion is in the range of conventional intervals for catagenesis, corresponding to Ro=0.6±1.1%. CONCLUSIONS

Hydrocarbon compositions of siliceous sourced oils from the Circum-Paci®c area are similar, and may suggest open marine depositional environments of the source rocks and diatomaceous organic matter sources. However, the depositional environment of the possible source rocks in Sakhalin, Russia and Akita basin, Japan, are more oxic compared with Monterey oils. The viscous Katangly sample from the Sakhalin produced from 135±143 m depth is an example of heavily biodegraded oil, which should not be considered a product of early generation. Oils with indications of low maturity from Russia and Japan have relatively high API gravity and are poor in asphaltene and sulfur. Geochemical correlation of extractable organic matter of source rocks and the least mature oils demonstrated that the petroleum expulsion in siliceous shales of the Akita basin occurs at the top of the main phase of oil generation corresponding to Ror0.60%. Associate EditionÐM. E. L. Kohnen AcknowledgementsÐWe thank IGIRGI, Moscow (Institute of Geology and Exploitation of Combustible Fuels) for providing the oil samples from North Sakhalin and their geological description, Caroline Isaacs of USGS for helpful discussions and providing the Monterey oils, Satoshi Sasaki and Hisashi Ishida of JNOC (Japan National Oil Corporation) for managerial support and JNOC for permission to publish this study. An earlier version of the manuscript bene®ted from critical review by M. Abrams of Exxon Ventures (CIS) Inc. The authors wish to express their sincere gratitude to the referees David K. Baskin of Chevron Petroleum Tech. Co, Albert G. Holba of ARCO Exploration and Production Tech. and Math E. L. Kohnen of Shell for their excellent recommendations which improved the paper.

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