~ Pergamon Org. Geochem. Vol. 21, No. 3/4, pp. 359-371, 1994

Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved

0146-6380/94 $7.00 + 0.00

Organic geochemical characterization of bitumens, seeps, rock extracts and condensates from Tanzania

F. MPANJU and R. P. PHILP School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, U.S.A.

Abstract--Several bitumens, oil seeps, and rock extracts from several areas of Tanzania have been characterized by a variety of geochemical techniques. Samples analysed included liquid seeps from Pemba, Ruvuma, bitumen extracts from sandstone outcrops from the Wingayongo area, cores and cuttings from Kimbiji East-l, Mnazi Bay-l, Pemba-5 wells and a condensate from Songo Songo-1 well. Samples from two areas, Wingayongo and Msimbati (Ruvuma) were particularly noteworthy and are discussed in detail in this paper. The extracts of the Wingayongo sandstone outcrop are virtually devoid of n-alkanes and steranes; hopanes are dominated by isomers with the flfl-stereochemistry suggesting an unusual source material or maturity history for this sample. The Msimbati seep samples are also characterized by high concentrations of immature hopanoids present as both hydrocarbons and fatty acids. The carbon isotopic compositions for the saturate and aromatic fractions are exceedingly light with 6 ~3C values of -47 and - 59%, respectively. From the results of this study it is proposed that these seeps do not represent the biodegraded residues of mature crude oil but rather are a type of paraffin dirt where bacteria have utilized thermal methane as a substrate and biosynthesized the various compounds found in these samples. Evaluation of the biomarkers in a condensate from Songo Songo suggests the source rocks contained Type II/III kerogen. The only successful correlation that could be made was between a seep on Pemba Island and the Campanian/Maastrichtian formation of the Kimbiji East-1 well. This same seep did not correlate with extracts from the nearby Pemba-5 well and similarly the Ruvuma seeps did not correlate with extracts from the nearby Manzi Bay-I well.

Key words--Tanzania, seeps, bitumens, biodegradation, GCIRMS, hopanes, steranes

INTRODUCTION

Tanzania lies between 1 ° and 11 ° south on the East Coast of Africa and covers an area of 945,000 km: in which nine sedimentary basins have been recognized (Fig. 1). Liquid hydrocarbons have been discovered in some of these basins although not in substantial amounts, possibly due to the low levels of exploration activity. Most of the wells that have been drilled are scattered over a wide area (Fig. 2) and potential source rocks have been found in the Permian, Juras- sic, Cretaceous (Maastritchian/Campanian) and Ter- tiary (Eocene). Two gas fields have been discovered on Songo Songo Island in the Coastal Basin and Mnazi Bay in the Ruvuma Basin. Exploration ac- tivity for oil/gas occurrences in Tanzania has, in the past, been limited with most of the activity, over the past 30 or 40 years concentrated in the coastal areas particularly the offshore Ruvu, Mandawa, and Ru- vuma Basins, and the Rufiji Trough (Fig. l). As part of a concerted effort by the Tanzanian Government to attract new exploration ventures in their country, samples of oils, seeps, and cores have been character- ized in some detail using various organic geochemical techniques. Oil seeps and bitumen have been reported in three areas of Tanzania. The most northerly occurrence is on Pemba Island; the central discovery is the bituminous sandstone at Wingayongo in the Rufiji Basin and the southernmost discovery is in the Ruvuma Basin (Fig. 2). Another seep, not described

in this paper has been reported in Lake Tanganyika. The seeps on Pemba Island (Fig. 2) and some of the seeps in the Ruvuma Basin have been characterized as seeps of degraded crude oils. The Wingayongo sandstone outcrop extract and two seep samples from the Ruvuma Basin show unusual hydrocarbon distri- butions. This paper will discuss the geochemical characteristics of these samples and the attempts made to correlate them with their suspected source rocks. The nature of the unusual seeps will also be discussed.

The first area of interest is the so-called Wingay- ongo seep located 160 km SSW of Dar-es-Salaam on the flanks of the Rufiji Basin (Fig. l). The extracts of this exposed sandstone and seep are unusual in that the chromatograms are dominated by hopanes, some with the immature flfl-stereochemistry, and virtually devoid of steranes and n-alkanes. The sample col- lected from the Ruvuma Basin (Fig. 1), namely the Msimbati seep was characterized by an extremely light carbon isotopic composition and a rather un- usual terpane distribution. It had been suggested previously (unpublished reports of Tanzanian Pet- roleum Development Corp.) that samples from the Msimbati area were genuine oil seeps. However, in view of their unusual isotopic composition and the fact that large quantities of gas are produced in this region it is proposed in this paper that some of these so-called "seeps" are probably biogenic in origin. Bacteria have utilized the gas as a substrate and

359

360 F. MPANJU and R, P. PHILP

produced a paraffin dirt composed of hydrocarbons and fatty acids dominated by hopanoid structures. This is a relatively unusual occurrence and to the best of our knowledge there is only one other account of so-called paraffin dirt in the literature (Simoneit and Didyk, 1978).

In addition to these rather unusual seeps, results are also presented from analyses of more conventional seeps and source rock extracts recov- ered from the Pemba area as well as the Songo Songo condensate and rock extracts from the Kimbiji East well.

E X P E R I M E N T A L

heights were determined from the chromatograms and used for relative quantitation purposes.

Gas chromatography mas.~ ,v)ectrometry (G('-MS)

The distributions of steranes and terpanes were obtained with a Finnigan TSQ-70 interfaced to a Varian 3340 GC, equipped with a 30 m x 0.25 mm i.d. DB-5 column using He as carrier gas, The GC conditions were as follows: injection temperature 300C" column temperature isothermal at 40 'C and programmed at 10 C/rain to 140C and then to 3 0 0 C at 3 C / m i n and held isothermally for 20 min. The ion source temperature was 200C; electron energy and current were 70 eV and 200/tA, respectively.

Gas chromatography (GC)

The saturated hydrocarbon fractions were analysed by GC, using a Varian 3300 GC equipped with a DB5 capillary column (30 m x 0.25 mm i.d.) and helium as carrier gas. The oven temperature was isothermal at 80°C for 5 min, and then programmed at 4°C/min to 290°C and held for 20min. Peak

i 7 ; UGANDA / Lak , ,z---'

" '1 - - _ 7 - "

JKWA

BASIN

('arbon isotopic composition

The seeps, extracts and bitumens and their saturate and aromatic fractions were statically combusted according to the methods of Sorer (1984). Following combustion, the tubes were opened on a vacuum line (2 -¢ 10 ~ torr) and water was removed by trapping at - 70 C (Neslab Cryocool CC-60 cooling unit). The

KENYA ( r/" ' - - " ' - J ~ ~ N a i r o b i • \ A F R I C A

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Fig. 1. Location of Tanzania and its major sedimentary basins.

li:7

Bitumen, seeps, rock extracts and condensates characterization

I . . . . - . . . . - . . . . - . . . . - . - , . . . . . . . - 3 8 . . . . . , - , , , , - , . ~ 0 " i I i ~1%\ I / %r xx I / ~1 xx I / \ ~ 1 x\ I / \ ~ 1 % \ I ! \1 ~.x I I / \1 x \ \ ~ l i l t s ~ 1 ~ \ \ \ \ \ \ \ x \ \ x \ \ \ ~ '

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T A N Z A N I A : W E L L L O C A T I O N S

~0"

Fig. 2. Location of exploration wells, drilled in the coastal areas of Tanzania, and oil seeps.

362 F. MPANJU and R. P. PHILP

CO2 was collected in a gas sampling ampoule at -196~C (liquid N2) and then transferred directly to the inlet system of a Finnigan MAT Delta E isotope ratio mass spectrometer equipped with a 90°C sector magnetic deflector. The stable carbon isotopic com- position is expressed using the standard ,5 notation relative to PDB.

RESULTS AND DISCUSSION

Rufiji Basin

Wingayongo is located about 160kin SSW of Dar-es-Salaam on the flanks of Rufiji Basin (Fig. 1) and is the site of an oil seep which has migrated through faults in the region. Nearby in the Wingay- ongo-1 borehole, the top 40 m of the Lower Creta- ceous Kipatimu Beds are stained with bitumen which may be all that remains of a breached palaeoreser- voir. The Rufiji Trough is a major east-west trending basin in the centre of the coastal area of Tanzania which has considerable potential for the discovery of hydrocarbons and yet remains significantly underex- plored. It covers an area of approx. 16,000 km 2 and extends some 200 km westward from the coastline to the Ulanga Basement Spur. The sedimentary section generally thickens from west to east with over l0 km of Karroo and post-Karroo deposits in the deepest parts of the basin. The outcrop pattern reflects the easterly plunge with Jurassic rocks cropping out on the western flanks, Lower Cretaceous over much of the Lukuliro area, Upper Cretaceous at Kichi and Ruaruke and Neogene in the Rufiji Delta. Although the evolution of the Rufiji Trough has much in common with the other basins of coastal Tanzania, in effect it is a composite basin comprising part of the "Failed Rift System" in the west and a "'Jurassic Rift" Basin in the east. The Rufiji Trough is mostly within the Rufiji River floodplain and delta and is consequently low lying apart from hills in the south- east and southwest.

Despite the presence of the Songo Songo gas field offshore and the significant oil saturated sandstone at Wingayongo, the Rufiji Trough is the least explored of the Tanzanian basins. No wells have been drilled in the trough proper although Lukuliro-I was drilled by Shell on the southern flank. Wingayongo-l, a stratigraphic borehole drilled by BP on the northern flank of the basins, discovered bitumen in the upper- most 40 m and a gas flare was tested.

The source rock potential is poorly known but the oil saturated sandstone at Wingayongo provides direct evidence of an oil-prone source at depth which is thought to be derived from the Bajocian Makarawe Shale. Further source rock potential exists in the Lower Jurassic (Nondwa Formation) and the Cam- panian black shales of the Ruaruke Formation. The Middle Jurassic source rocks were probably at peak maturity in the late Cretaceous thereby charging structures developed earlier. The Campanian shales

may be presently mature in the Rufiji Delta and along the coastal zone of the Rufiji Embayment.

The seep is located at an isolated sandstone out- crop on Wingayongo Hill which was formerly pre- sumed to be of Upper Jurassic age but is now known to be Neocomian (Kapatimu Beds). Sour gas at Wingayongo Hill associated with bitumen has been known for many years. The Kapatimu Beds (com- monly known as Wingayongo series) are a rhythmi.. cally bedded, deltaic sequence about 762 mm thick This triangular block elevated by faulting comprises whitish, medium grained to coarse grained sand- stones with patches of' brown, bituminous staining at the surface.

In this study the Wingayongo bitumen was ex- tracted and fractionated using thin layer chromatog- raphy into saturate, aromatic and NSO fractions. GC and GCMS analysis of the saturate fractions revealed several unusual features not commonly observed in bitumens. First, the saturate chromatograms are to- tally devoid of n-alkanes and dominated by twt~ major peaks and several others in the triterpane region [Fig. 3(a) and (b)]. It is of interest to note that in an earlier unpublished report (TPDC/Statoil, 1990), samples from this same outcrop were tbund to, contain significant n-alkanes although the terpane distribution was identical to that shown for the samples examined here. It is possible that the samples examined previously were collected from greater depths than the samples used in this study. The current samples may have been exposed to the atmos- phere for longer periods of time to relatively high temperatures leading to the loss of the more volatile n-alkanes. The possibility of extensive biodegrada- tion cannot be totally eliminated However. in one of the present samples, traces of n-alkanes still show ~ predominance of the lighter homologues and the Pr/Ct7 and Ph/C~ ratios are similar to those in the unpublished report. This lends to eliminate the possi- bility of biodegradation which would lead to prefer- ential removal of n-alkanes over isoprenoids. The second unusual feature of the Wingayongo bitume13 samples is their lack of detectable steranes. An m 217 chromatogram reveals few components but the signal-to-noise ratio is poor and no structures can be confidently assigned to any of the peaks.

The major componenls of the saturated hydro- carbon fraction are hopanoid triterpanes, as seen from the GC trace and the m/= 191 chromatogram [Fig. 3(a) and (b)]. The abundance and distribution of the hopanes is unusual (Fig. 3) with the fraction being dominated by the 17~(H),21fl(H)-28,30-bisnorho- pane (Grantham et al.. 1983). There are only a few' known cases, notably the Monterey Shale (Mold- owan et al., 1984) and oils from the North Sea. where this compound is the major component of either an extract or oil. 17ct (H),21fl (H)-Hopane is the second most abundant component and the only extended hopanes present are the C32-22S + R pair, with virtu- ally no trace of the C3~ or C , -C , s extended hopanes.

Bitumen, seeps, rock extracts and condcnsatcs characterization 363

WINGAYONGO BITUMEN

C28

TOTAL SATURATES

(RD)

C3o

INCREASING TIME - - - >

~. 188"

80"

CO

WINGAYON@O

M / z 191

C28

0 . . . . , . . . . I . . . . ' . . . . I . . . .

580 1008

C3o

C32

i . . . . I . . . . L I . . . . I . . . . i • •

1588 2080

INCREASING TIME - - - > Fig. 3. (a) G-C-FID chromatogram for the saturate fraction from the Wingayongo bitumen. (b) m/z 191 chromatogram for the same sample. (Major peaks are hopanoid-typc triterpanes-carbon numbers are

given on chromatograms).

This type of distribution has not been reported in the literature to the best of our knowledge, and defies a conventional explanation. None of the regular biodegradation pathways have ever produced this type of hopane fingerprint and hence this distribution

poses a dilemma. There is no good evidence to suggest biodegradation despite the paucity of n-alka- nes. The presence of low concentrations of A ~3'~s- neohopenes and flfl-hopanes, as well as those hopanes described above, suggests a relatively low

364 F. MPANJU and R. P. PHILP

maturity for this sample. A number of possibilities can be put forward for the origin of these bitumens, none of which however is totally convincing:

(1) Highly weathered seep--unlikely due to pres- ence of low concentrations of low molecular weight n-alkanes. The distribution of the hopanes is unlike any previously observed for biodegraded samples. However one possibility that cannot be entirely ruled out is that the products observed here are the finger- print of the microbial community that has utilized the exposed bitumens as a carbon source. One could rationalize the unusual hopane distribution as being a mixture of components from the microbial commu- nity plus unaltered products present in the original bitumen.

(2) Unusual source materials---unlikely since the source would have to be dominated by bacteria which produced the hopanes, with few, if any, additional sources of organic matter. A predominance of the C32-bishomohopanes is also unusual and whilst it may be partially explained by the nature of the depositional environment, it is more likely a reflection of source input.

(3) Bitumen formed at very' low levels o f maturity - possible but again the problem lies in the unusual distribution of the hopanes. There are unpublished reports of other samples from this region being dominated by the flfl-hopanes, although none of these isomers were observed in the samples examined in the present study.

The aromatic fractions from these samples were also fairly complex mixtures, but did not show any significant concentrations of hopanoic acids, as methyl esters, as did the Ruvuma samples described below. Hence, the Wingayongo bitumens are unique and if any information is to become available on their source material, it is essential that the origin of the terpane fingerprint be better under- stood.

Although it is not proposed to discuss the source rock data in great detail in this paper, it is important to note that extracts of samples from around 2700 m in the Pemba-5 well show a similarity to the Wingayongo samples. In particular, the Pemba-5 samples are characterized by significant quantities of C28 bisnorhopane and the presence of the C32 extended hopanes in higher concen- trations than the C3~ extended hopanes. Concen- trations of the C33-'C35 extended compounds are extremely low. Whilst the Wingayongo bitumen and Pemba-5 extracts are not identical, their high concentrations of C28 bisnorhopane plus similarities in extended hopane distributions suggest partial similarity of their source materials. This in turn would lend further support to the idea that the extended hopane distribution in the Wingayongo bitumen probably does not result from biodegradation.

Ruvuma Basin

The Ruvuma Basin is situated in the southeastern part of Tanzania adjacent to the Mozambique border and covers an area of about 16,000 km 2. The onshore portion of the basin is covered by an undulating alluvial plain dipping gently to the coast. Offshore there is a narrow shelf between 10 and 15 km wide which includes many coral islands, reefs and bars. To the west the basin is bounded by the Masasi Base- ment Spur and is separated from the Mandawa Basin by the east-west trending Ruvuma Saddle.

The basin lies towards the southern end of the large East African Karroo Rift System which developed southwards from Somalia as Madagascar rifted from the mainland during the break-up of Gondwanaland. Originating in late Karroo times, the rift became an area of regional subsidence in which Jurassic, Lower Cretaceous and then a complete post-Aptian drift sequence accumulated.

The presence of oil-prone source rocks at depth within the Ruvuma Basin has been inferred from the occurrence of the oil seep at Msimbati Island and the minor shows found in several wells and boreholes in the area (Tanzanian Petroleum Development Corp., unpublished data). Seismic sections reveal that equiv- alents of the Lower Jurassic Nondwa black shales, the Makarawe shale and the Campanian black shales all exist at depth and provide potential source inter- vals

A gas seep was reported on Msimbati Island and a well drilled by AGIP in 1982 discovered gas which flowed up to 14 million cubic feet per day from Oligocene sands. This gas is thought to be almost entirely biogenic. Three relatively shallow wells (90 m) were also drilled in this area but all were suspended due to blow-outs. High concentrations of hydrogen sulphide were detected during the drilling of these three wells. The carbon isotopic compo- sitions of Nyuruko and Lipwapwata gas seeps are

~C = - 3 4 to -28.6%o PDB for methane and - 16.3 to -- 12.0%o PDB for ethane and the hydrogen isotope 6D ratio ranged for methane from - 146 to - 122%o SMOW indicative of thermogenic gas.

Four seep samples from Nyuruko, Makukwa, Lip- wapwatawere and Mikindani, on Msimbati Island (Fig. 2), were collected and examined from this area for the current study. Three were found to be very unusual and one appeared to be a more typical oil seep. GC and GC-MS analyses of the Nyuruko and Mikindani seep oils suggest that they are genuine, mature crude oils which have undergone varying degrees of biodegradation. The m/z 191 terpane chromatogram of Nyuruko seep shows the presence of the C29 and C30 hopanes in almost equal concen- trations plus an additional component labelled peak X which is probably the 29,30-bisnorhopane (Fig. 4). The presence of the 30-norhopane series would also account for the enhanced concentration of the C29 hopane. Similar characteristics were noted by Palacas

Bitumen, seeps, rock extracts and condensates characterization 365

et al. (1984) in carbonate-sourced oils. The C35 ex- tended hopanes were present in slightly higher con- centrations than the C34 homologues, as observed by McKirdy et al. (1983) in oils derived from carbonate source rocks. However, more recently it has been shown by Moldowan et al. (1992) that an enhanced concentration of the C35 extended hopane also reflects highly reducing conditions in the original deposi- tional environment. The m/z 218 chromatograms show that the concentration of the steranes decreases in the order C27 > C29 > C2s consistent with a marine source (Fig. 4). The Mikindani seep (Hole 31) also

has the appearance of a biodegraded bitumen. The m/z 191 chromatogram shows the presence of 18a (H)-oleanane, a characteristic feature of Nigerian crude oils (Whitehead, 1974; Ekweozor et al., 1979a, b) and common in other Tertiary oils (Palmer, 1984). The relative sterane concentration decreases in the order C29 > C2s > C27 suggesting that the bitumen was derived from source rocks containing terrigenous organic matter. The most significant property which distinguished the Makukwa and Msimbati samples from the others is the exceedingly light isotopic composition of their saturate and aromatic fractions

100%

29

m / z 191

X

23"

| I w I " / "

30

t i 29

m / z 218 28

INCREASING RETENTION TIME - - - >

Fig. 4. The m/z 191 and 218 chromatograms for the Nyuriko seep. Note the presence of the 30-norhopanes with compound X being the 29,30-bisnorhopane.

366 F. MPANJU and R. P. PHILP

01 ( J . m

El

E O k_ .<

L_) 1,1

~O

- 2 0 . 0 0

- 5 0 . 0 0

- 4 0 . 0 0

- 5 0 . 0 0

O ¢ • 0

* Songo Songo * Pemba seep • P e m b a 5 - Mandawa * K i m b i j i ,, W ingayongo , Nyu ruko - Makukwa

M s i m b a t i ® M a k a r a w e

- 6 0 . 0 0 . . . . . . . . . i . . . . . . . . . i . . . . . . . . . i . . . . . . . . . I . . . . . . . . . i . . . . . . . . . i

-50.00 -45.00 -40.00 -35.00 -30.00 -25.00 -20.00

613C Saturates Fig. 5. BuLk isotopic compositions for the various seeps and extracts are shown on this Safer plot. Note

the exceedingly light isotopic composition for the two seep samples from Msimbati and Makukwa.

(Fig. 5). The Msimbati sample with values of ]3C = - 4 7 and -59%0 for the saturates and aro-

matics, respectively, is particularly noteworthy. GC and GC-MS analyses of the saturated hydro-

carbons from the Msimbati seep reveal that these are a relatively simple mixture dominated by hopanes (Fig. 6). As with the Wingayongo sample, the concen- tration of the steranes is negligible and will not be discussed further in this paper. Analysis of the Msim- bati saturated hydrocarbon fraction by GC-MS in both the full scan and MID modes revealed that the hopanes are dominated by compounds with the im- mature tiff and fl~t configurations at C-17 and C-21. No evidence was found for the presence of any ctfl isomers nor is there evidence for extended hopanes above C3~. Hence, again we have a rather unusual situation of a seep totally devoid of n-alkanes and steranes, and dominated by "immature" hopanes. Furthermore, the aromatic fractions of these "seeps" also contain copious quantities of flfl-hopanic acid, as the methyl ester, indicative of low thermal matu- rity (Fig. 7). It is proposed that both the hopanes and the hopanoic acids were synthesized by bacteria utilizing thermogenic gas, known to be present in the area, as a substrate. Supporting evidence for this hypothesis is provided by the isotopic composition of the individual hopanes. The 6 ~3C values of the indi- vidual components are extremely light (Fig. 8) rela-

tive to values expected in mature thermogenic oils and appear more characteristic of biogenic samples. As already mentioned there is a preponderance of gas in the region of these "seeps" and some very shallow wells were abandoned as a result of gas blow-outs.

Hence, it is suggested that in the case of the Msimbati seep, bacteria utilize the methane as a substrate and biosynthesize various hopanoids. A similar proposal involving the use of fossil methane as a substrate for various heterotrophic organisms in abyssal brine springs was discussed by Paull et al. 0989). In their study of the isotopic compositions of individual components from the Messel Shale, Hayes et al. (1989) indicated that a normal kinetic isotope effect of up to 25%0 is associated with the assimilation of CH4 by methanotrophic bacteria, which infers that they will be more strongly depleted in 13C than their source. The extent of depletion depends on the efficiency with which the bacteria are able to capture the CH4. If methanotrophic bacteria are operative in the Msimbati region, and the extent of depletion is in the range mentioned above, it must be assumed that these bacteria are using gas of thermal origin as their primary substrate and not biogenic gas which would be typically much lighter.

A second "seep" sample collected from Makukwa had similar characteristics in its saturated hydro- carbon fraction which was dominated by the hopanes

Bitumen, seeps, rock extracts and condensates characterization 367

and particularly the /~fl- and flct-hopanes with an almost total absence of n-alkanes and steranes [Fig. 6(b)].

The Msimbati and Makukwa samples are also distinguished by their aromatic fractions which con- tain a number of hopanoic acids, as methyl esters

(Fig. 7). These compounds are again typical of those expected in aromatic fractions of immature sediments and support the idea that the seeps are formed by bacterial utilization of natural gas seeps.

As far as we are aware there is only one other report in the literature of a so-called paraffin-dirt.

MAKUKWA

SATURATE H / C 30"

30" I i

I

I I~l [[a-27 ~ II

30"

MSIMBATI SATURATE H I C

6 -27

WI-31

N4CRF_4WNG ~ TIME - - - >

Fig. 6. GC chromatograms for the saturate hydrocarbon fractions from (a) the Msimbati seep and (b) the Makukwa seep. Both arc dominated by the flfl-hopanes as indicated on the chromatograms.

368 F. MPANJU and R. P. PHILP

MAKUKWA

AROMATIC FRACTION

(FID)

MSIMBATI

AROMATIC FRACTION

118-C. HOPANOIC ACID

INCREASING RETENTION TIME - - - >

Fig. 7. GC chromatograms for the aromatic hydrocarbon fractions from (a) the Msimbati seep and (b) the Makukwa seep. The fractions are characterized by the presence of hopanoic acids.

The paper by Simoneit and Didyk (1978) reported such an occurrence in the Chilean Andes. The distri- bution of its hydrocarbons is significantly different from those observed herein. However, this is not altogether surprising in view of the different climatic conditions at the two seep locations and the fact that the Tanzanian sample was collected from a coastal swamp whereas that from Chile occurred in a moun- tainous region at high altitude and a cold and dry climate.

Songo Songo condensate

A condensate was collected from the Songo Songo well in the coastal basin (Fig. 2) and analysis of this condensate by gas chromatography showed a typical condensate distribution with traces of n-alkanes up to

nC25 (Fig. 9). Its distributions of tricyclic, tetracyclic and pentacyclic terpanes suggest a mixed source or Type II and III kerogen. Correlation of this conden- sate to potential source rocks on the basis of bio- marker distributions was difficult due to its high maturity.

Pemba Island seep and Kimbiji East-1 well

The Pemba Island seep (Tundaua) occurs within the offshore coastal basin of Tanzania which covers an area of 45,000 km 2 (Fig. 2). The seep was reported in 1984 after BP Shell had drilled 5 dry wells in the area in 1958. Dead oil and a potential source rock with TOC of 7.39% was found in the Pemba-5 well.

Gas chromatograms of the saturated hydrocarbon fractions of four Pemba seep oils have n-alkane

Bitumen, seeps, rock extracts and condensates characterization 369

MSlMBATI SEEP

l iD CHROMATOGRAM

WITH GC- IRMS DATA

0-27

- 63 .68

l " I J L l k • _ • n _ - l J _ _ . J . . ! a , ~ .

s.

30"

o

~ 60.31

INCREASING RETENllON 11ME--->

Fig. 8. FID chromatogram indicating the isotopic composition of individual hopanes obtained from the analysis of this fraction by gas chromatography-isotope ratio mass spectrometry.

distributions typical of a mature oil which has not undergone severe biodegradation although the chro- matograms show signs of an enhanced hump of unresolved complex mixtures of hydrocarbons. Pr/nC~7 ratios in the range of 0.52 to 3.82 and

Ph/n C~s of 0.57 to 3.03 indicate that these samples are in the early stages of biodegradation. The m/z 191 chromatograms show an abundance of tricyclic ter- panes, suggesting a possible contribution from Tas- manites to the source rocks (Aquino Neto et al., 1983,

SONGO SONGO CONDENSATE

17

INCREASING RETENTK)N TIME - - - > Fig. 9. GC traces from the Songo Songo condensate.

O(3 21-3/4--K

370

100%

100%

F. MPANJU and R. P. PmLP

2 9 m / z 191

30

KIMBIJI EAST 1 27 m / z 2 1 8

. . . . , . . . . , . . . . , . . . . , - . . , . . . . , . . . . , . . . . , . . . . ,

KIMBIJI EAST 1

~ L

3o m / z 191 20 PEMBA SEEP

, , . . , . , . . . . . , . . . ,

m / z 2 1 8 27 PEMBA SEEP

INCREASING RETENT ION T I M E - - - >

Fig. 10. Correlations between the Pemba seep and the sample at 3364-76 m from the Kimbiyi East-I well based on the m/z 191 and 217 chromatograms.

1992), and supported by a relatively high abundance of C27 steranes (Fig. 10).

The biomarker distributions of the seeps did not correlate with any of the extracts from the upper 3002 m of the sequence in the nearby Pemba- 5 well. Below this depth no biomarkers were detected due to high levels of thermal maturity. However, the hopanes and steranes of the Pemba seep did correlate well with an extract from the Maastritchian/Campanian interval of the Kimbiji East-1 well (Fig. 10) which is therefore a possible source rock.

CONCLUSIONS

Reexamination of petroleum and source rock samples from various areas of Tanzania has led to the discovery of seep samples with unusual geochemical characteristics, particularly their hopane distri- butions. It is proposed that in one of these areas, Msimbati, some of the seep samples are not of thermal origin but probably correspond to the so- called paraffin-dirt previously reported in Chile. It is further suggested that these seeps are formed by bacteria utilizing thermal methane as the substrate and biosynthesizing various hopanoid compounds, including hopanoic acids and hopenes. Support for this hypothesis comes from the anomalous 3~3C isotopic values, absence of steranes and n-aikanes, and predominance of hopanes with an immature stereochemistry which would not be expected in a seep of mature crude oil.

Bitumen extracts from the Wingayongo sand- stone outcrop are quite different and most probably formed as a result of an active oil seep or unusual weathering of Mesozoic-sourced hydrocarbons. Samples analysed from the same area in the early 1980s contained alkanes but had the same unusual distribution of hopanes, and absence of steranes, as reported herein. The samples examined in the present study were surface samples and lack of n-alkanes may result from biodegradation or the fact that the samples used in the earlier study were collected from a greater depth. At this time we do not have a good explanation for the unusual hopane distributions. These are dominated by C28-bisnorho- pane, and only the 22S and R C32 hopanes are present in the extended hopane series. No other oils in the region have such hopane distributions. To add to the mystery the Ruhoi River seep discovered 5 km from Wingayongo had perfectly "normal" biomarker dis- tributions.

On the basis of the limited data set presented herein that the Pemba seep (Tundaua) appears to be a genuine seep possibly sourced from the Maastrichtian/Campanian or an older formation. In the Ruvuma Basin, the Nyuruko seep is possibly sourced from Cenzoic source rocks and the gas from Mesozoic/Paleozoic source rocks. The unusual Makukwa seep resembles paraffin dirt formed by bacteria utilizing methane as a substrate for the synthesis of hopanoic acids and hopenes. The Nyu- ruko seep has a marine origin and is different from other seeps in the area.

Bitumen, seeps, rock extracts and condensates characterization 371

REFERENCES

Aquino Neto F. R., Trendel J. M., Restle A., Albrecht P. and Connan J. (1983) Occurrence and formation of tricyclic and tetracyclic terpanes in sediments and petroleum. In Advances in Organic Geochemistry 1981 (Edited by Bjoroy M. et al.), pp. 659-667. Wiley, Chichester.

Aquino Neto F. R., Trigiiis J., Azevedo D. A., Rodrigues R. and Simoneit B. R. T. (1992) Organic geochemistry of geographically unrelated tasmanites. Org. Geochem. 18, 791403.

Ekweozor C. M., Okogun J. I., Ekong D. E. V. and Maxwell J. R. (1979a) Preliminary organic geochemical studies of samples from Nigeria Delta, Nigeria, Part l: analyses of crude oils, of triterpanes. Chem. Geol. 27, 11-28.

Ekweozor C. M., Okogun J. I., Ekong D. E. V. and Maxwell J. R. (1979b) Preliminary organic geochemical studies of samples from Nigeria, Part 2: analysis of shales for triterpane derivatives. Chem. Geol. 27, 29-37.

Grantham P. J., Posthuma J. and Baak A. (1983) Triter- panes in a number of Far Eastern crude oils. In Advances in Organic Geochemistry 1981 (Edited by Bjoroy M. et al.), pp. 675-683. Wiley, Chichester.

Hayes J. M., Takigiku R., Ocampo R., Callot H. J. and Albrecht P. (1989) Isotopic compositions and probable origins of organic molecules in the Eocene Messel shale. Nature 329, 48-51.

McKirdy D. M., Aldridge A. K., Ypma P. J. M. (1983) A geochemical comparison of crude oil from pre-Ordovician carbonate rocks. In Advances in Organic Geochemistry 1981 (Edited by Bjoroy M. et al.), pp. 99-107. Wiley, Chichester.

Moldowan J. M., Seifert W. K., Arnold E. and Clardy J. (1984) Structure, proof and significance of stereoisomeric

28,30-bisnorhopanes in petroleum and petroleum source rocks. Geochim. Cosmochim. Acta 48, 1651-1661.

Moldowan J. M., Sundararaman P., Salvatori T., Alajbeg A., Gjukic B., Lee C. Y. and Dcmaison G. J. (1992) Source correlation and maturity assessment of select oils and rocks from the Central Adriatic Basin (Italy and Yugoslavia). In Biological Markers in Sediments and Petroleum (Edited by Moldowan J. M., Albrecht P. and Philp R. P.), pp. 370-401. Prentice-Hall, Englewood Cliffs, N.J.

Palacas J. G., Anders D. E. and King J. D. (1984) South Florida basin--a prime example of carbonate source of Petroleum Geochemistry and source rock potential of carbonate rocks. AAPG Studies in Geology No. 18 (Edited by Palacas J. G.), pp. 71-96. AAPG, Tulsa.

Palmer S. (1984) Effect of water washing on Cl~+ hydro- carbon fraction of crude oils from NW Palawan, Phillip- ines. Am. Assoc. Pet. Geol. Bull. 68, 137-149.

Paull C. K., Martens C. S., Chanton J. P., Neumann A. C., Coston J., Jull A. J. T. and Toolin L. J. (1989) Old carbon in living organisms and young CaCO 3 cements from abyssal brine seeps. Nature 342, 166-168.

Simoneit B. R. T. and Didyk B. M. (1978) Organic geochemistry of Chilean paraffin dirt. Chem. Geol. 23, 21~,0.

Sofer Z. (1984) Stable Carbon isotope compositions of crude oils: application to source depositional environ- ments and petroleum alterations. Am. Assoc. Pet. Geol. Bull. 68, 3149.

TPDC/Statoil (1990) Analysis of sediments and gases from the Ruruima Basin, SE Tanzania. Restricted report pre- pared by IKU/Sintef Group.

Whitehead E. V. (1974) The structure of petroleum pentacy- danes. In Advances in Organic Geochemistry 1973 (Edited by Tissot B. and Bienner F.), pp. 225-243. Technic, Paris.