organic geochemistry of cuban oils--i. the northern ... geochemi… · organic geochemistry of...

Pergamon

PII: S0146-6380(96)00139-8

Org. Geochem. Vol. 25, No. 8, pp. 475-488, 1996 © 1997 Elsevier Science Ltd

All rights reserved. Printed in Great Britain 0146-6380/96 $15.00 + 0.00

Organic geochemistry of Cuban oils--I. The northern geological province

P. G. C A M P O S ~, J. O. G R I M A L T ~*, L. B E R D I E ~, J. O. L O P E Z - Q U I N T E R O ~ and L. E. N A V A R R E T E - R E Y E S ~

Tetroleum Research Center, Washington 169, Cerro, Havana, Cuba and 2Department of Environmental Chemistry (C.I.D.-C.S.I.C.), Jordi Girona, 18, 08034, Barcelona, Catalonia, Spain

(Received 20 June 1996; returned to author for revision 31 August 1996; accepted 14 October 1996)

Abstract--The isoprenoid, hopanoid and steroid compositions of 15 oils from the most productive oil fields in Cuba were studied to determine source-rock depositional environments and organic matter sources. The oils, which are from the northern geological province of Cuba and can be defined by the position with respect to the overthrust belt, can be grouped into two families: those from the Remedios (I) and those from the Placetas (2) tectonostratigraphic units.

Remedios oils contain 17~(H)-diahopane, high relative amounts of 18~t(H)-22,29,30-trisnorneohopane and diasteranes, which is indicative of generation from clay-rich source rocks. The crude oils of the Pla- cetas Unit exhibit a sterane/hopane composition consistent with a carbonate origin. Nevertheless, these polycyclic hydrocarbons exhibit significant changes in composition, indicating that several organic matter sources, e.g. a carbonate/evaporitic origin of Varadero and Varadero Sur oils, have contributed to the oils from this Unit. The Remedios oils are more mature [evaluated from the C29-~20S/(S + R) and C29-20Rflfl/(flfl + ct~t) sterane indices] than the Placetas oils.

A wide range of biodegradation levels are encountered in these oils (from 0 to 7-8 using the scale derived by Volkman et al. (1983). The high relative abundance of 25-norhopanes is a distinctive feature of Remedios oils. The presence of these compounds in lightly biodegraded or nondegraded oils corresponds to a mixing of paleobiodegraded oil with more recently sourced nondegraded oil in the reservoir. The most biodegraded oil, Cantel, exhibits C27 rr/(rr + sd) and C29-0t~20S/(S + R) sterane ratios, C30 ctfl/(~fl + flct) and Ts/(Ts + Tin) hopane ratios that have been altered by microbial attack. © 1997 Elsevier Science Ltd

Key w o r d s ~ u b a n crude oils, hypersaline crude oils, Jurassic crude oils, acyclic isoprenoid hydrocarbons, hopanes, 25-norhopanes, 17~t(H)-diahopane, steranes, diasteranes, pregnanes

INTRODUCTION

About 25 onshore and offshore oil fields of medium and small size are known in Cuba. These fields are distributed in the western and central parts of the island, situated in the Northern and Southern Geological Provinces, respectively, as defined by their position with respect to the overthrust belt (Fig. 1).

Most of the 21 fields are located in the northern province which encompasses the northern part of the island and adjacent offshore area (Fig. 1). This region is about 1000 km long and 80-100 km wide. Sediments (ca. 10-12km thick) were deposited beginning in Early Jurassic-Cretaceous time follow- ing the structural evolution of a stable continental margin (Echevarria-Rodriguez et al., 1991). The fields are situated in folded and fractured limestones in overthrust sheets. The fragmented distribution of the fields has led to difficulty in modeling the basin and understanding the genetic relationships between

*Author to whom all correspondence should be addressed.

oiffields. The difficulty is augmented by the strong biodegradation of some of the oils.

The oils considered here, Martin Mesa, Caridad, Boca de Jaruco, Varadero, Varadero Sur, Guasimas and Cantel are from the most productive fields. The highest crude oil production corresponds to Boca de Jaruco and Varadero. All these fields were discovered recently, between 1969 (Boca de Jaruco) and 1989 (Martin Mesa). Preliminary data on the hydrocarbon composit ion of some of these oils has been given by Campos et al. (1988) and Grimalt et al. (1991).

This study of the hopanoid and steroid composition of the oils provides a useful insight on source-rock depositional environments. This information contributes to the understanding of oil sources in the Gul f of Mexico (Mello et al., 1995). The oil fields present in the Southern Province will be considered in a future study.

GEOLOGICAL SETTING

Basement rocks in Cuba are composed of marbles and siliciclastic metasediments of Late

475

476 P.G. Campos et al.

L a H a b a n a

7 6 "i

,-.,. Water depth 200m 1 Northern geological province ~!II!~ Southern geological province

N t

1 Martin Mesa 2 Caridad 3 Cantel

4 Boca de Jaruco 5 Guasimas 6 Varadero 7 Varadero sur

Fig. 1. Map showing the two geological Cuban provinces and the location of the oil fields considered in this study.

Proterozoic age (Renne et al., 1989). On top of this, Lower to Middle Jurassic (Callovian)-age continental and lagoonal strata, rich in humic organic matter, were deposited. Carbonate deposition is represented by a number of separate tectonostratigraphic units: Placetas, Camajuani, Rosario and Cayo Loco (Hatten et al., 1989) dominated the Upper Oxfordian to the Turonian. These units are described in detail in Echevarria-Rodriguez et al.

(1991). The Cretaceous was characterized by subsidence

resulting in the accumulation of organic matter. Evaporite sequences formed during continental rift- ing were later deformed into diapiric structures due to sedimentary overburden. Volcanic activity, associated with the formation of oceanic crust along the axis of the proto-Caribbean Basin and the Greater Antilles Arc, occurred in the Lower Cretaceous (Ross and Scotese, 1988). Movement in a north-northeast direction of the Caribbean Plate (Campanian and Palaeocene) resulted in collision of the Cuban Arc with the North American continent, thrusting the volcanic-arc and oceanic-crust over

the continental margin (Placetas, Camajuani and Rosario tectonostratigraphic units). Several strati- graphic levels in the Upper Cretaceous and Paleocene contain shales which are good seals for the accumulations of oil and gas in carbonate reservoirs of Upper Jurassic and Cretaceous ages. Carbonate sedimentation predominated in the post- orogenic stage (Lower Eocene in western Cuba and Middle Eocene in the most eastern part of the Northern Province).

The most important source rocks in the Northern Province are Upper Jurassic to Aptian in age. Less important sources include: (1) Albian-Turonian sequences; (2) Late Cretaceous units; and (3) sin- orogenic Middle Eocene rocks. The oil and gas accumulations discovered so far are associated with the above-mentioned tectonostratigraphic units (Echevarria-Rodriguez et al., 1991).

EXPERIMENTAL

Neutral silica gel (70-230 mesh, Merck) and alumina (70-230 mesh, Merck) were extracted with

Organic geochemistry of Cuban oils--I 477

~0

= <

o

~ g

~ .~.

0

0

~ ~ ~ ~ ' ~ ' ~

~ o

>

. ~ . ~ o o ~ ~ ~ . - ~ . ~ ~ ~ ~ ~ ' ~

dichloromethane-methanol (2:1, v/v) in a Soxhlet apparatus for 24 h. After solvent evaporation, the silica and alumina were heated for 12 h at 120 and 350°C, respectively. Milli-Q-grade water (5%) was added to these adsorbents for deactivation.

The oils were fractionated by column chromatog- raphy in a 34 cm x 0.9 cm i.d. column filled with 8 g of 5% water-deactivated alumina (top) and silica (bottom). The aliphatic hydrocarbons were separ- ated by elution with 20 ml of n-hexane. The fractions were evaporated to dryness and redissolved in iso-octane.

Gas chromatographic analysis (GC) was per- formed with a Carlo Erba Mega Model HRGC 5300 equipped with a flame ionization detector and a splitless injector. A DB-5 capillary column (30 m × 0.25 mm i.d.; film thickness 0.2/~m) was used. Helium was the carrier gas (50 cm s-l). The oven temperature program was from 70-320°C at 6°C min - l . Injector and detector temperatures were 290 and 350°C, respectively. Injection was splitless (iso-octane, hot needle technique) while keeping the split valve closed for 40 s. Nitrogen was used as a make-up gas (30 ml min-l). Detector gas flows were hydrogen (30 ml min - l ) and air (300 ml min-1).

The samples were also analyzed by G C - M S using a Fisons MD-800 instrument. Spectra were obtained in the electron impact mode (70 eV) scan- ning from mass 50 to 550 every second. A HP-5 capillary column (30 m × 0.25 mm i.d.; film thickness 0.25 #m) was used. Helium was the carrier gas (1 mlmin- l ) . The oven temperature program was from 60°C (solvent delay 4min) to 310°C at 4°Cmin - l (holding time 15min). Injector, transfer line and ion source temperatures were 300, 280 and 200°C, respectively. The injection was splitless (iso- octane, hot needle technique) keeping the split valve closed for 48 s.

R E S U L T S A N D D I S C U S S I O N

Bulk composition

The crude oils considered in this study are listed in Table 1. They encompass a wide range of den- sities (from 0.860 to 0.984), API gravity (12-32.5) and sulfur content (0.4-6.7%). In most cases, den- sities, viscosities and sulfur percent values are high. Placetas oils exhibit a higher sulfur content than Rosario oils, 0.40-1.2 and 1.1-6.7%, respectively. As shown in Fig. 2, no correlation is observed between the API gravities of these oils and sulfur content.

Acyclic hydrocarbons

The gas chromatographic profiles of the crude oils listed in Table 1 are shown in Fig. 3. Important differences are observed in terms of biodegradation. Martin Mesa 1 and 7, Guasimas, Varadero and Varadero Sur have well-defined n-alkane envelopes


7.00"

6.00-

5.00-

4.00" n-"

"t-

- - 3 . 0 0 Or)

2.00"

1 . 0 0

which, a priori, correspond to little or no biodegradation. Varadero and Varadero Sur exhibit a smal- ler n-alkane range, C13-C30 , than Martin Mesa 1 and 7 and Guasimas (C12-C36).

Boca de Jaruco 198 and 706 correspond to a degradation step (biodegradation level 3 of Volkman et al., 1983) in which all n-alkanes have been eliminated, but the acyclic isoprenoids remain. The GC profiles of these crude oils are dominated by the regular C]3-C20 acyclic isoprenoids. 2,6,10,14,18-Pentamethyleicosane is present in relatively minor proportions. The structural assignment was based on G C - M S examination; the C25 homol- ogue corresponds to the regular structure and not to 2,6,10,15,19-pentamethyleicosane. These regular isoprenoid hydrocarbons are also the major branched compounds in the nondegraded oils.

Pristane or phytane are the predominant branched compounds in all cases. The relative proportion of pristane vs. phytane (Table 1) is a feature that differentiates the Rosario oils (Pr>Ph) from Placetas oils (Pr < Ph in Boca de Jaruco 198 and 706, Guasimas 18; P r < P h in Varadero and Varadero Sur). The proportion of 2,6,10,14,18-pentamethyleicosane relative to the other regular isoprenoids is also a distinct feature of the nondegraded or lightly degraded Placetas oils, particularly those from the Boca de Jaruco Field.

The greater degraded oils, which are characterized by a large unresolved complex mixture of hydrocarbons, contain little or no acyclic isoprenoids. Caridad 4 and Boca de Jaruco 37 exhibit a biodegradation level of 4-5 (no n-alkanes and very small amounts of acyclic isoprenoids). In the most degraded oils (Martin Mesa 29 and Cantel) regular isoprenoid hydrocarbons have been eliminated. In these cases, triterpenoids and steroids/diasteroids

are visible above the unresolved complex mixture. However, the sterane and hopane distributions of the Cantel oil have been modified by biodegradation, corresponding to a transformation level of 7- 8.

Hopanoid hydrocarbons

Hopane distributions (m/z 191) of the oils listed in Table 1 are given in Fig. 4. Further information is given in Fig. 5, where the m/z 191 and 177 mass fragrnentograms of selected crude oils are compared to illustrate the differences in composition of 25- norhopanes. The hopanes identified in these oils are listed in Table 2. Compounds were identified by comparing mass fragmentograms (m/z 191, 205, 177, 369) with previously reported data (Seifert and Moldowan, 1979; Moldowan et al., 1991), and with crude oils of known hopane composition (Albaiges et al., 1986). The position of the A-ring methyl sub- stituent in the hopanes (numbers 7 and 13 in Table 2) was assigned according to the mass spectral and retention time data reported by Summons and Jahnke (1990).

The series of 17~t(H),21fl(H)-hopanes is dominant and encompasses the C27 (Ts), C29 and C30-C35 homologues which are resolved at the C-22 position into S and R epimers. The ratio between these two diastereoisomers, e.g. C32 22S/(S + R), ranges between 0.53 and 0.62 (Table 3). Except for Martin Mesa 1 (0.53) and Varadero Sur (0.56), the values are higher than 0.58, corresponding to those observed in mature hopane mixtures (Seifert and Moldowan, 1986).

The relative composition of 17ct(H),21fl(H)- and 17fl(H),21ct(H)-hopanes is also maturity related (Mackenzie et al., 1980; Seifert and Moldowan, 1980). In all but one cases the proportion of

Fig. 2. Sulfur composition vs. API gravity of the Cuban oils of the Northern Geological Province.

0.00 lo.oo 151oo 20100 ss:oo 3oloo 3soo

API GRAVITY

Organic geochemistry of Cuban oils---I 479

Martin Mesa 1 15 20

, i , r

~iiil t2, 13 . I,. " : I ,o

15

I .... be' 0 . . . . - -

tlJ,~ j Martin Mesa 29

l a e f~ (hI I,~ , Caridad 4

~ Cantel

Boca de Jaruco 37

gh J,~~,.,.,

h J Boca de Jaruco 198

14 d i

g

c e

12

Boca de Jaruco 706

i

20 Guasimas 18

f I , i [ '

Varadero sur 15

1 4 d / l i t t l i l i l ,

f 120

15

l'tiI[ s0 13 d i

c f . 25

Varadero

Fig. 3. Gas chromatographic profiles of the aliphatic hydrocarbon fractions of selected Cuban oils of the Northern Geological Province: (a) 2,6-dimethylundecane; (b) 2,6,10-trimethylundecane; (c) 2,6,10- trimethyldodecane; (d) 2,6,10-trimethyltridccane; (e) 2,6,10-trimethyltetradecane; (f) norpristane; (g)

pristane; (h) phytane; (i) 2,6,10,14,18-pentamethyleicosane.


5 6

1 2

12 Mart in Mesa 1

17 22 7 18 ~ 26

L I3 [!9t73 .27 30 32

12 Mar t in Mesa 7

I 17 118 22 2':

4 ~ 23 7 30

12 Mar t in Mesa 29 6

l 2 ~J~ 17 4 5 [~19~31182226

14 30 32

12 Boca de Jaruco 37 6

18 22

:3 191, ~ 8x~9 ~ ~ 6 7 30 39

12 Boca de Jaruco 198 6

17 2 I18 22

1 4 ~ 7 9 13 1119,23 26 3,~

12 6

12

12 Caridad 4 6

Boca de Jaruco 706

i 1~8 22 19 3 26

12 Cantel

6 17 18 22

2 I II 123 26

17 2 z •8 22

79 [3 [1912326 30 32-

6 Guasimas 18 12

2 17 ~18 22 26

~ _ ~ r 9 I~4 19 23 7 3q~l 3233

Varadero sur 12

18 17 / 19

2 ~ . ~ ~ 1 ~,.o,,"

._,.,~.~ ~.~,~ ~t&

12 Varadero 6

18 17 19

2 ~ t : J ~ 14 ~ [23 [27 3031 3233

Fig. 4. m/z 191 mass fragmentograms showing the hopane composition of Cuban oils of the Northern Geological Province. Peak assignments are listed in Table 2.


M a r t i n M e s a 1 12

5 6 I m/z 191

, , ¢ . , , m p M t , . . ~ , 4 r w A ~ - - ~ w . . . . . . . . ~ " ~ . . . . i - r - - - - - ' q ~ ' . . , . 1 ~

3 5

l . m/z 177

[~ I 1011 15 1920 24252829

12 M a r t i n M e s a 7

1 /

o 3 u'27 3031 3233 ,

6 m/z 177

6

1 2

3

12 M a r t i n M e s a 29

I m/z 191

7^ [ 1718 22 2" e 9 9 ~23 ~7 3031 • ~ . 32

12 m/z 177

1 2

3

12

| '8 ~3 1.718 22 26 LJ 3

9

C a r i d a d 4

m/z 191

m/z 177

Fig. 5. Representative m/z 191 and 177 mass fragmentograms illustrating hopane and 25-norhopane composition of the Cuban oils of the Northern Geological Province. Peak assignments are listed in

Table 2.

482 P. G. Campos et al.

Table 2. Main hopanes identified in the Cuban oils

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2o 21 22 23 24 25 26 27 28 29 30 31 32 33

18ct(H)-22,29,30-trisnorneohopane (Ts) 17~t(H)-22,29,30-trisnorhopane (Tm) 17ct(H),21/~(H)-25,30-bisnorhopane 17fl(H)-22,29,30-trisnorhopane 17ct(H),21 fl(H)-25-norhopane 17ct(H),21 fl(H)-30-norhopane 2ct-methyl- 17ct(H),21 fl(H)-30-norhopane 17ct(H)-diahopane 17fl(H),21 ct(H)-30-norhopane 17ct(H),21 fl(H)-25-norhomohopane 22S 17ct(H),21fl(H)-25-norhomohopane 22R 17ct(H),21 fl(H)-hopane 2or-methyl- 17a(H),21 fl(H)-hopane 17fl(H),21 ~(H)-hopane 17~t(H),21 fl(H)-25-norbishomohopane 22S 17ct(H),21 fl(H)-25-norbishomohopane 22R 17a(H),21 fl(H)-homohopane 22S 17~t(H),21fl(H)-homohopane 22R Gammacerane 17:t(H),21 fl(H)-25-nortrishomohopane 22S 170t(H),21 fl(H)-25-nortrishomohopane 22R 17ct(H),2 lfl(H)-bishomohopane 22S 17ct(H),21 fl(H)-bishomohopane 22R 17ct(H),21 fl(H)-25-nortetrakishomohopane 22S 17ct(H),21 fl(H)-25-nortetrakishomohopane 22R 17ct(H),21 fl(H)-trishomohopane 22S 17ct(H),21 fl(H)-trishomohopane 22R 17~t(H),21fl(H)-25-norpentakishomohopane 22S 17~t(H),21 fl(H)-25-norpentakishomohopane 22R 17:t(H),21fl(H)-tetrakishomohopane 22S 17ct(H),21 fl(H)-tet rakishomohopane 22R 17ct(H),21fl(H)-pentakishomohopane 22S 17~t(H),21 fl(H)-pentakishomohopane 22R

17ct(H),21fl(H) to 17~(H),21fl(H) +17fl(H),21ct(H)-

hopane is equal or higher than 0.92 (Table 3),

which is characterist ic of mature hopane distri-

but ions (Moldowan e t al., 1994). Cantel oil is an

exception to this general t rend, having a value of

0.86. However, this difference is p robab ly due to

b iodegradat ion , as 17fl(H),21ct(H)-hopanes are

more resistent to microbial a t tack than the

17ot(H),21fl(H) diastereoisomers (Goodwin e t al.,

1983). In this respect, Mar t in Mesa 29, the o ther oil

lacking chromatographica l ly resolved n-alkanes and

acyclic isoprenoids, does not show any appa ren t

decrease in the C3o ~tfll(ctfl + fl~t) ratio, corresponding to a lower b iodegrada t ion level.

Ano the r hopane matur i ty rat io is 18ct(H)- 22,29,30- t r isnorneohopane (Ts) relative to 17~(H)- 22,29,30-tr isnorhopane (Tm; Seifert and Moldowan, 1978). However, this rat io is strongly influenced by the presence of minerals tha t catalyze s tructural re- a r rangement f rom Tm to Ts. Field studies (Rul lk6t ter e t al. , 1984; Philp and Fan Zhaoan , 1987) and pyrolysis experiments ( T a n n e n b a u m et al. , 1986) have shown tha t this pa ramete r is more indicative of source rock mineralogy (e.g. catalytic vs. inert minerals) than thermal maturi ty. Mos t of the Rosar io oils have Ts/(Ts + Tm) ratios ranging from 0.45 and 0.52 (Mar t in Mesa 7 with the low value of 0.23 is an exception), whereas Placetas oils exhibit a considerably lower TJ (Ts + Tm) ratio, 0.19-0.24 (this range excludes the biodegraded oils, Boca de Jaruco 37 and Cantel). The Ts/(Ts + Tm) rat io of these oils are 0.28 and 0.38, respectively, showing a higher Ts conten t than the less biodegraded Placetas oils. To the best of our knowledge, no specific b iodegrada t ion effects have been described for this ratio. However, the t rend towards higher Ts con ten t has also been observed in other studies in which biodegraded and nonbiodegraded oils have been compared (Alexander et al. , 1983; Vo lkman et al. , 1983; Connan , 1984).

The high relative abundance of 25-norhopanes is a distinct feature of mos t Rosar io oils. The relative p ropor t ion and dis t r ibut ion of these demethyla ted hopanes is not un i form (Fig. 5): 25-norhopanes in Mar t in Mesa 1 range f rom C26 to C34 which paral- lels the regular 17cffH),21fl(H)-hopane series. M a r t i n Mesa 29 and Car idad 4 conta in no detectable 25-norhomohopanes , only 17ct(H),21fl(H)- 25,30-bisnorhopane. Finally, no 25-norhopanes are observed in Mar t in Mesa 7. 25-Norhopane distri- but ions which parallel the regular 17~(H),21fl(H)- hopane dis t r ibut ions have also been observed in other crude oils such as the Chongjian-1 oil ( Junggar Basin, China, Z h a n g Daj iang e t al. , 1988) and Cymric Field oils f rom California (Moldowan

Table 3. Hopane indices of the crude oils from North Cuba fields. The relative compositions of all compounds except 17~t(H),21/~(H)-25- norhopane are measured on the m/z 191 fragmentograms

Ts/(Ts + Tin) 25-nor/(25- dia/(dia + hop) 2~tMe/ gam/ ~flC32 22S/ C30 ctfl/(~[J + [~ct) Oil nor + hop) (2~Me + hop) (gam + hop) (S + R)

Martin Mesa 1 0.45 0.44* 0.15t 0.05:~ 0.40§ 0.53¶ 0.96 Martin Mesa 7 0.23 0 0.14 0 0.21 0.58 0.92 Martin Mesa 29 0.50 0.05 0.14 0 0.32 0.60 0.94 Caridad 4 0.52 0.07 O. 15 0 0.35 0.59 0.93 Boca Jaruco 37 0.28 0 0.05 0.09 0.33 0.61 0.93 Boca Jaruco 198 0.21 0 0 0.04 0.37 0.60 0.96 Boca Jaruco 706 0.20 0 0 0.04 0.34 0.62 0.96 Varadero 0.19 0 0 0.06 0.62 0.61 0.92 Varadero Sur 0.24 0 0 0 0.51 0.56 0.94 Guasimas 18 0.20 0 0 0.10 0.40 0.61 0.96 Cantel 0.38 0 0 0 0 0.61 0.86

*(17a(H),21/~(H)-25-norhopane-m/z 177-)/(17~t(H),21fl(H)-25-norhopane-m/z 17% + 17~t(H),21//(H)-hopane -m/z tl7ct(H)-diahopane/(17~t(H)-diahopane + 17~(H),21fl(H)-hopane). :~2~tmethyl-17~t(H),21fl(H)-hopane/(2,~methyl-17~t(H),21fl(H)-hopane + 17~t(H),21fl(H)-hopane). §Gammacerane/(gammacerane + 17a(H),21fl(H)-homohopane 22R). ¶17ct(H),21fl(H)-bishomohopane [22S/(S + R)I.

191-).


Martin Mesa 1 1o

k, I 4 v 911,~ Is 23__

~ 1 6 ~ 192

Martin Mesa 7 10

3 t113 2324 • I 4 67 9~1~l 1~[9211/25 , 8 5

Martin Mesa 29 10

7 19 46 I 1 /~25

1o Caridad 4

i 3 [13

1013 Cantel

llJ 3 7 9 I

4 6 5 20

Boca de Jnruco 371 1o12

" t t 2325 ' %2~

1 3 7 , 20 ~ 2 4 6 13

Boca de Jaruco 198

2 2 346 8 1 7

Boca de Jaruco 706 1012 -

1 Jl 1 , ~ J ,~1 18/211/,25

2 16

Guasimas 18

2 I 1°12 23

"5 8 o o . ~

Varadero sur 1 10 -- 9~ I [12 19" 2 ~ "

2 9 '3 re/

~ 3 6 7 8 1 .27 ~

12 Varadero 1Ol 2~ 5

, , : k J 2 ' 6 ~ 1 20 27

3 4 28

Fig. 6. m/z 217 mass fragmentograms showing the sterane and diasterane composition of the Cuban oils of the Northern Geological Province. Peak assignments are listed in Table 4.


Table 4. Main steranes and diasteranes identified in the Cuban oils

10

11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

27

28

5~t(H), 14fl(H), 17fl(H)-pregnane 5a(H),14fl(H),17fl(H)-homopregnane 13fl(H), 17a(H)-diacholestane 20S 13fl(H), 17~(H)-diacholestane 20R 13ct(H), 17fl(H)-diacholestane 20S 13ct(H), 17/~(H)-diacholestane 20R 24-methyl- 13fl(H), 17ct(H)-diacholestane 20S 24-rnethyl- 13fl(H), 17ct(H)-diacholestane 20R 5~t(H),14~t(H),lTct(H)-cholestane 20S + 24-methyl- 13ct(H), 17fl(H)-diacholestane 20S 5~t(H),14fl(H),17fl(H)-cholestane 20R + 24-ethyl- 13fl(H), 17ct(H)-diacholestane 20S 5ct(H), 14fl(H), 17fl(H)-cholestane 20S 5~(H), 14a(H), 17ct(H)-cholestane 20R 24-ethyl- 13fl(H), 17a(H)-diacholestane 20R 4a-methyl-5a(H), 14fl(H), 17fl(H)-cholestane 20R 24-ethyl-I 3ct(H),l 7fl(H)-diacholestane 20S 24-methyl-5ct(H), 14ct(H), 17ct(H)-cholestane 20S 24 ethyl-13ct(H), 17fl(H)-diacholestane 20R 24-methyl-5a(H), 14fl(H), 17fl(H)-cholestane 20R 24-methyl-5~t(H), 14fl(H), 17fl(H)-cholestane 20S 24-methyl-5ct(H), 14ct(H), 17~t(H)-cholestane 20R 24-ethyl-5~t(H), 14ct(H), 17ct(H)-cholestane 20S 4ct,24-dimethyl-5ct(H), 14fl(H), 17fl(H)-cholestane 20R 24-ethyl-5ct(H),14fl(H), 17fl(H)-cholestane 20R 24-ethyl-5~t(H),14fl(H),l 7fl(H)-cholestane 20S 24-ethyl-5ct(H), 14~t(H),l 7~t(H)-cholestane 20R 4ct-methyl-24-et hyl-5ct(H), 14ct(H), 17~(H)-cholestane 20S* 4ct-methyl-24-ethyl-5~t(H),14fl(H),17fl(H)-cholestane 20S* 4~t-methyl-24-ethyl-5~t(H),l 4ct(H),l 7~(H)-cholestane 20R*

*Tentative identification.

and McCaffrey, 1995). In other cases, the regular 17~(H),21fl(H)-hopanes are absent and only the 25- demethylated hopane series is found (Volkman et al., 1983).

Microbial conversion of hopanes to 25-norhopanes in oil is the process currently invoked for the presence of these compounds in crude oil mixtures (Seifert and Moldowan, 1979; Volkman et al., 1983; Peters et al., 1994). Further evidence of this origin, when the 25-norhopanes parallel the 17~(H),21fl(H)-hopane, has recently been reported (Moldowan and McCaffrey, 1995). The generation of 25-norhopanes is produced by severe biodegradation, when all straight-chain and isoprenoid alkanes, and most of the bicyclic alkanes, have been removed (Philp et aL, 1981, 1982). Martin Mesa 1 contains both 25-norhopanes and n-alkanes, suggesting paleobiodegradation followed by filling of the reservoir by a second pulse of unbiodegraded oil. This process has been proposed to explain the co-occurrence of 25-demethylated hopanes and n- alkanes in Australian crude oils (Volkman et al., 1983).

Another distinct feature of Ro ario oils is the constant ratio (0.14-0.15, Table 3) of 17ct(H)-diahopane/(17ct(H)-diahopane + 17~(H),21fl(H)-hopane), independent of their level of biodegradation. The formation of this rearranged hopane is currently attributed to clay catalysis (Moidowan et al., 1991).

Two attributes, a high gammacerane content and high C35/C34 homohopane ratios, differentiate

Varadero and Varadero Sur oils from others in the Northwestern Cuban Province, suggesting that they are derived from a hypersaline source. Gammacerane is present in all crude oils mentioned in Table 1 except the Cantel oil. This compound occurs in variable amounts in many oils of different origin (Moldowan et al., 1985), and so its presence is not, in principle, indicative of specific origins. However, the oils originating from hypersaline source rocks tend to have large amounts of this tri- terpenoid relative to the content in 17~(H),21fl(H)- hopanes (Moidowan et al., 1985; Mello et al., 1988). Another feature characteristic of marine-evaporitic sourced crude oils is the higher proportion of C35 hopanes relative to the C34 homologues (Ten Haven et al., 1985, 1987; Connan et al., 1986; Fu Jiamo et al., 1986; Grimalt et al., 1991), although this latter feature may also be encountered in oils originating from normal marine salinity environments (Mello et al., 1988; de Leeuw and Sinninghe Damste, 1990).

Other features related to hypersaline sourced crude oils include the predominance of C22-C32 even-to-odd carbon numbered n-alkanes, high phytane/pristane ratios (Ten Haven et al., 1985, 1987, 1988; Connan et al., 1986; Fu Jiamo et al., 1986; Mello et al., 1988;) and high abundances of 2,6,10,14,18-pentamethyleicosane (Mello et al., 1988). In these two oils no odd-even carbon number preference is observed, pristane and phytane are in similar amounts (0.71 and 0.96 in Varadero and Varadero Sur, respectively, see Table 1) and, although they contain 2,6,10,14,18-pentamethyleicosane, they do not have higher relative abundances of this isoprenoid than other Table 1 oils.

Another feature that differentiates Rosario and Placetas oils is the higher relative abundance of 2ct- methylhopanes in Placetas oils. The C30 and C3~ homologues of these methylhopanes (Table 2) have been identified based on their mass spectra. They show an intense m/z 205 fragment which is a diagnostic ion with respect to possible coeluting compounds such as 18~t(H)-30-norneohopane (C29Ts; Moldowan et al., 1991). These 2~-methylhopanes are commonly found in carbonate sediments, although they are not diagnostic markers of this lithology (Price et al., 1987).

Steroid hydrocarbons

The sterane composition of Table 1 oils is shown in Fig. 6 (peak identifications are given in Table 4). Identification was made by comparison of the m/z 217, 218 and 259 mass fragmentograms with pre- vious reports (Seifert and Moldowan, 1979) and with those of crude oils of known sterane composition (Albaiges et al., 1986). The position and stereochemistry of the A-ring methyl substituted steranes were determined by comparison of selected mass fragmentograms (m/z 217, 231, 372, 386, 400


Table 5. Sterane indices of crude oils from North Cuba fields. The relative compositions of all compounds are measured on the m/z 217 fragmentogram

Percentage of preg/ C29 ~gaC29 4ctMe-/ C27,29 rr/ Oil (preg + ster + dias) flfl/(ltfl + act) 20S/(S + R) (4ctMe- + ster) (rr + sd)

C27 C28 C29

Martin Mesa 1 39* 14 47 0,42:1: 0.54~ 0.475 0ll 0,69** 34t 25 35 0 ,61t t 30 26 50 0.37 0.56 0.44 0 0.63 31 30 39 0.53

Martin Mesa 29 41 21 38 0.48 0.60 0.49 0 0.65 35 36 29 0,68

Caridad 4 42 19 39 0.53 0.58 0.43 0 0.65 36 33 31 0.68

Boca Jaruco 37 44 21 35 0,49 0.54 0.39 0.18 0.31 40 25 35 0.19

Boca Jaruco 198 50 19 31 0,54 0.57 0.41 0.17 0.21 39 29 32 0.065

Boca Jaruco 706 47 19 34 0.53 0.53 0.40 0.15 0.20 41 30 29 0.20

Varadero 45 14 40 0.52 0.48 0.32 0.30 0.16 37 34 29 0.08

Varadero Sur 32 24 44 0.62 0.48 0.37 0.20 0.39 34 26 40 0.33

Guasimas 18 49 18 33 0.76 0,57 0,41 0 0.39 39 29 32 0.26

Cantel 29 22 49 0.37 0.46 0.46 0 0.62 26 34 40 0.51

Martin Mesa 7

*C27 C2s C29 5a(H),14ct(H),l 7ct(H)-cholestanes 20R. tC27 C28 C29 5a(H),14fl(H),l 7fl(H)-cbolestanes 20S. :~(5a(H),14fl(H),17fl(H)-pregnane + homopregnane)/(5ct(H),14fl(H),17fl(H)-cholestane 20R + pregnane + homopregnaue + 13fl(H),

17ct(H)-diacholestane 20S). ~(24-ethyl-5~t(H), 14fl(H), 17fl(H)-cholestanes-20S + 20R-)/(24-ethyl-5a(H), 14fl(H), 17fl(H)-cholestanes-20S + 20R- + 24-ethyl-5a(H),

14a(H),17ct(H)-cholestanes-20S + 20R-). ¶24-ethyl-5ct(H),14ct(H),17a(H)-cholestane [20S/(S + R)]. ll(4a-methyl-5ct(H),14fl(H),17fl(H)-cholestane 20R)/(4ct-methyl-5~(H),14fl(H),17fl(H)-cholestane 20R + 5a(H),14fl(H),17fl(H)-cholestane

20S). **(13fl(H),17~(H)-diacholestanes -20S + 20R-)/(13//(H),17a(H)-diacholestanes -20S + 20R- +5~(H),14ct(H),17~(H)-cholestane

20S + 20R-). tl'(24-ethyl-13fl(H),17a(H)-diacholestane 20R)/(24-ethyl-13fl(H),17~(H)-diacholestane 20R + 24-ethyl-5~(H),14~(H),17~(H)-cholestane 20R).

and 414) with those reported by Summons et al. (1987, 1988) and Summons and Capon (1988). The absence of 24-n-propylcholestanes was determined by mass spectral examination of the m/z 414 molecular weight steranes and from the lack of re- sponse in the m/z 304 mass fragmentogram trace (Moldowan et al., 1990). However, the identifications of the methylsteranes are tentative in the absence of G C - M S - M S (m/z 414--,217) data. Examination of the rn/z 358 mass fragmentogram did not show significant concentrations of C26 ster- aries. The pregnanes were identified by comparison with the retention time and mass spectral data reported by Wingert and Pomerantz (1986) and Jiang Zhusheng et al. (1990).

The m/z 217 profiles shown in Fig. 6 exhibit the important differences in the sterane distributions in the Rosario and Placetas oils. The Rosario oils contain a high proportion of diasteranes to steranes [0.63-0.69; (13fl(H),17~t(H)-diacholestanes - 20S + 20R-)/(13fl(H),17ot(H)-diacholestanes 20S + 20R- + 5ct(H),14~(H),17ct(H)-cholestane 20S + 20R-), Table 5] than the Placetas oils (0.16- 0.39, excluding Cantel oil). These rearranged ster- aries are represented by the 20S and 20R 13fl(H),17ct(H)-diacholestanes and the less abundant

OG 2 5 / 8 ~

13~(H),17fl(H)-diacholestanes. The higher proportion of diasteranes in the Cantel oil is consistent with extensive biodegradation, resulting in elimin- ation of steranes with respect to diasteranes (Seifert and Moldowan, 1979).

The high abundance of rearranged steranes in crude oils is indicative of generation from clay-rich source rocks (Rubinstein et al., 1975; Sieskind et al., 1979; Grantham and Wakefield, 1988). The higher content of diasteranes in Rosario than Placetas oils is consistent with the differences in hopanoid composition related to catalytic effects, namely higher Ts/Tm ratios and 17ct(H)-diahopane content.

The 20S vs. 20R 24-ethyl-5~t(H),14ct(H),17ct(H)- cholestane (C29-~205/(5 + R)) and the 5~t(H),14fl(H),17fl(H)- vs. 5ct(H),14~(H),17~t(H)-24- ethyl-cholestane 20R (C29-20Rflfl/(flfl + ct~)) ratios were used to determine the thermal maturity of these oils (Table 5). Both indices, 0.32-0.47 and 0.48-0.60, respectively, show that none of the crude oils from Table 1 has reached full maturity with equilibrium ratios of 0.55 and 0.75, respectively (Seifert and Moldowan, 1981; Mackenzie, 1984). The Remedios oils exhibit higher maturation, 0.44- 0.49 and 0.54-0.60 for C29-~t~20S/(S + R) and C29 o


20Rflfl/(flfl + as) than those from the Placetas Unit, 0.37-0.41 and 0.48-0.57. Varadero and Varadero Sur, the two hypersaline-sourced oils from this tectonostratigraphic unit, show even more immature ratios of 0.32-0.37 and 0.48, respectively.

The Cantel oil has not been included in this comparison. According to the C29-~20S/(8 + R) ratio this oil is the most mature of the Placetas oils. However, as mentioned above, the strong biodegradation of this oil has altered the sterane composition. The 24-ethyl-5~(H),14~(H),17c~(H)-cholestane 20R diastereoisomer has been depleted in relation to the other 24-ethyl-cholestanes (Alexander et al., 1983), shifting the C29-~208/(8 + R) index towards higher maturity values.

Variable relative amounts of pregnanes are found in the Table 1 oils. Higher proportions of these compounds relative to the sterane content have been related to higher maturity (Wingert and Pomerantz, 1986; Huang Difan et al., 1994), higher biodegradation (Jiang Zhusheng et al., 1990) or inputs from marine/hypersaline organic matter (Fan Pu et al., 1991). The differences observed in this study do not show any clear relationship with any of these features. The lowest pregnane ratio is found in Martin Mesa 7 and Cantel oil, preg/ (preg + ster + d i a s ) = 0.37 (Table 5), the latter being the oil with the highest degree of biodegradation. On the other hand, the more mature oils from the Rosario unit (0.37 0.53) do not show signifi- cantly higher ratios than Placetas oils (0.37-0.76). The hypersaline oils, Varadero and Varadero Sur, have ratios of 0.52 and 0.62, respectively, which cannot be differentiated from the others based on the pregnane content. There is no obvious expla- nation for the high pregnane content, 0.76, in Guasimas 18 oil.

The relative abundance of 4~-methyl steranes is another means of differentiating the oils. These methylated compounds are believed to be derived from dinoflagellates (Robinson et al., 1984). The relative abundance of 4~-methyl-steranes to steranes, e.g. the [4ct-methyl-5ct(H),14fl(H),17fl(H)-cholestane 20R)/(4~t-methyl-5~(H), 14fl(H), 17fl(H)- cholestane20R + 5ct(H), 14fl(H), 17fl(H)-cholestane 20S] ratio, (Table 5) can be used as a source indicator (Shi et al., 1982). 4~-Methylsteranes are below the detection limit in most of the oils in this study; however, this is with the exception of several Placetas oils, the Boca de Jaruco and the hypersaline oils. The former have a distinct and rather uniform composition (4~Me-/(4~tMe- +ster) index: 0.15-0.18), while the latter show the higher indices, 0.30 and 0.20, for Varadero and Varadero Sur, respectively. The occurrence of high relative abundances of 4~-methylsteranes is typical of marine evaporitic crude oils (Philp and Fan Zhaoan, 1987; Mello et aL, 1988).

CONCLUSIONS

The oils examined in the Northern Cuban Geological Province show distinct origins. Those from the Rosario Unit (Martin Mesa I, 7 and 29 and Caridad 4) have a relatively low sulfur content (0.4-1.2%), pristane>phytane, relatively high Ts/ (Ts + Tin) ratios (0.23-0.52), a higher proportion of 17~(H)-diahopane/hopane (dia/(dia + hop) = 0.14- 0.15), a higher diasteranes/steranes (C27 rr/ (rr + sd) = 0.63-0.69), and a lower relative abundance of 2a-methylhopanes/hopanes and 4~t-methylsteranes under the detection limit. Many of these features, namely high Ts, 17~t(H)-diahopane and diasterane content, indicate generation from clay- rich source rocks (e.g. shales). In contrast, the crude oils from the Placetas Unit exhibit lower TJ (T~ + Tin) ratios (0.19-0.24), lower diasteranes (0.16-0.39) and no detectable 17a(H)-diahopane, suggesting a carbonate source.

However, the oils from the Placetas Unit do not exhibit a uniform composition. Important differences are observed, which indicates diverse oil sources. The high gammacerane and 4a-methylsterane content and high C35/C34 hopane ratio of the Varadero and Varadero Sur oils indicate generation from carbonate-evaporitic organic matter. The specific 4ct-methylsterane content of Boca de Jaruco oils, 0.15-0.18, is also indicative of a distinct organic matter source.

The Rosario and Placetas oils can also be differentiated by the degree of thermal maturation. This is reflected in C29-~20S/(S + R) and C29-20Rflfl/ (tiff + ~ ) sterane ratios which exhibit higher values for Rosario oils, 0.44-0.49 and 0.54-0.60, than Placetas oils, 0.37-0.41 and 0.48-0.57, respectively. However, based on these parameters none of these oils have reached full maturity.

Biodegradation has altered several of the oils in this study. The well-defined n-aikane distribution of Martin Mesa 1 and 7, Guasimas 18, Varadero and Varadero Sur suggests no biodegradation. Boca de Jaruco 198 and 706 have biodegradation levels of 3 (using the scale of Volkman et al., 1983). The low amounts of acyclic isoprenoids in Caridad 4 and Boca de Jaruco 37 correspond to a biodegradation level of 4-5. The Martin Mesa 29 and Cantel oils exhibit some alteration of the sterane and hopane distributions. Changes are particularly pronounced in the Cantel oil, where the C27 rr/(rr + sd), Cz9- ~t~20S/(S + R) sterane and the C30 ~fl/(ctfl + fl~) and Ts/(T~ + Tm) hopane ratios have been affected by biodegradation.

A distinct feature of the Rosario oils is the high relative abundance of 25-norhopanes, which is gen- erally indicative of strong biodegradation. The presence of these compounds in mildly biodegraded or nondegraded oils corresponds to the mixing of severely biodegraded oil with undegraded oil during accumulation in the reservoir.


Associate E d i t o r - - M . E. L. Kohnen

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