Applied Geochemistry, Vol. 2, pp. 305-319. 1987 0883-2927/87 $3.00 + ,00 Printed in Great Britain Pergamon Journals Ltd.

Organic geochemistry of the sedimentary basins of New Zealand part IV. A biomarker study of the petroleum seepage and some well core

bitumens from the geothermal region of Ngawha Springs

RODERICK J. WESTON and ANTHONY D. WOOLHOUSE Chemistry Division, Department of Scientific and Industrial Research, Private Bag,

Petone, New Zealand

(Received 9 October 1986; accepted in revised form 30 April 1987)

Abstract--The crude oil which seeps to the surface at Ngawha Springs was mature but the oil biomarker parameters indicated that its (unlocated) source rocks had not reached the peak of oil generation. The oil had a mixed marine and terrigenous origin and had not undergone secondary migration except to the surface. The oil was distinguished from diesel oil, which might have polluted thermal springs in the area, by several parameters, specifically those which relate to the source of an oil. The seep oil was concluded to have arisen from accelerated diagenesis of organic material in Tertiary rocks which cap the geothermal system at Ngawha.

A surface asphalt, which had formed from the oil, was lightly degraded. It had absorbed hydrocarbons from immature organic matter near the surface and its hydrocarbon composition was characterised by an abundance of af122R-homohopane. This triterpane was also present in rock bitumens, together with appreciable proportions of flfl-hopanes.

Bitumens extracted from potential source rocks were largely of terrigenous origin. They had characteris- tics of sedimentary organic material which had not reached the oil generation threshold. These rocks did not prove to be the source of the seep oil. One of the bitumens contained a C28-triterpane, the presence of which suggested that the sedimentary organic material had been reworked by bacteria. Such a view was consistent with the geological history of Ngawha Springs.

INTRODUCTION

General geology

TIIE NGAWHA geothermal field is located in an area of young but extinct volcanism and occupies an interest- ing position in the complex of structures forming the Northland Peninsula (Fig. 1). It lies at the centre of four major converging faults which do not actually intersect, but the basin itself is intersected by a large number of smaller faults. Ngawha lies some 20 km west of the exposed Permian-Jurassic (Waipapa Group) greywacke/argillite rocks (Fig. 2), within an extensive area of virtually impermeable Cretaceous- Tertiary allocbthonous sedimentary rocks. These rocks overlie the Waipapa Group unconformably at depths increasing westward from the surface at Kawakawa, through about 550 m at Ngawha, to greater than 3350 m at Waimamaku on the west coast (Sra~NER, 1981). At Ngawha, most of the Cre- taceous-Tertiary rocks are allochthonous but at Waimamaku, the sequence below 2865 m is con- sidered to be autochthonous. The allochthon was emplaced episodically during the Early to Middle Tertiary and was accompanied by the deposition of marine sandstones throughout Northland and which preceded the early Miocene volcanism (Sxlr~El~, 1981). Upper Tertiary to Quaternary basalts erupted through and onto the Cretaceous-Tertiary rocks damming the headwaters of the drainage to the east. This caused a lake to form in which were deposited

some 10 m of carbonaceous lake sediments, derived from erosion of the surrounding Upper Cretaceous rocks. Subsequently, this lake drained to the east. Hot springs and gas vents occupy the sites of vigorous hydrothermal eruptions which are located in the area on which the major faults converge and which is related to the stress field that prevailed during the Late Tertiary-Quaternary volcanism (SKIr~N~I~, 1981).

Stratigraphy

Sedimentation began in the Late Cretaceous, when terrestrial coal measures were deposited. Marine transgression encroached from the west in the Late Cretaceous and Palaeocene, and reached the present day land area about Eocene-Oligocene time. Marine silt-mudstones, sandstones and limestones are the main constituents of the stratigraphic column.

The uppermost 10 m consists of Quaternary lake sediments and some of the strata from 10-50 m contain basalt flows. Between 50-100 m, Eocene coal measures occur, while at 150 m, shales from the same period are found. The stratum from 200-550 m con- sists of siliceous claystone of Late Cretaceous age which has low permeability because of a high clay content. These rocks form an effective cap to the geothermal system. Below 550 m is the Permian- Jurassic (Waipapa Group) greywacke and argillite basement which has almost zero porosity but has

305

306 Roderick J. Weston and Anthony D. Woolhouse

3 0 * S

- 4 0 * S

~,'.E \ l ~,~o.E '

A o

f "~°Oo • I l l

%

I IlllO *

4oo , i km

.Ngawha Spnngs ~ NORTHLAND

EAST

8 0 0 )200 l I I

BASIN

COAST BELT

3 0 " S - -

4 0 " S

WESTERN MARGII GEOSYNCLINE

".'AT

SO" S

. . . . . . . . . . . . .v. . . - . . . . . - . . : . - . . . - . . : . . . . . . . - . . . - . . • ..:.::i:!~:!:!:!:i:i:!:i:!:!:!?!:!:!:::i:i:i:i:!:!:i:!:::

/ :::}!:i!i!!!i : :!::. ,, / SOLANDER, WAIAU ....... )

BASIN ~"

' ~ CAMPBELL

SOUTH

PUKAKI BASIN

BASIN

BASIN

5 0 " S -

1 8 0 ' t I ? 0 * E 180" 1 7 0 " W I 1 1 1 I 1 /

FIG. 1. Oil prospective basins of New Zealand.

fracture permeability (SKINNER, 1981) and forms the geothermal reservoir.

The stratigraphy is highly disordered. Inversions of faunal sequences occur throughout and mixed assemblages are present. Of the springs associated with faulting, only the central Ngawha Springs and a "kerosene" spring on the shores of Lake Omapere are still hot. Copious quantities of gas (mainly CO2) are discharged over a wide area within and about Ngawha.

Occurrence of petroleum

KATZ (1968) pointed out that the only indication of the occurrence of petroleum in the Northland area is the oil seepage at Ngawha Springs. One of the earliest reports of the natural occurrence of oil at Ngawha is

by GRIFFITHS (1898), who reported that the geological features of the area include "a brown decomposed siliceous sinter, containing nodules of calcite, impreg- nated with mineral oil" and among deposits formed by the springs, "several layers of bituminous shale, containing an appreciable quantity of petroleum oil". BELL and CLARKE (1909) observed that "with two exceptions, all the springs of the (Tiger) area are rich in petroleum . . . . On some of the pools on the right side of the small cool stream which traverses the (Tiger) area, floats a thick scum of petroleum which in places, has solidified around the edges". Likewise, FLEMING (1945) reported "much carbonised veg- etation in the Tiger area and oily seeps are frequent".

QtrEm~ELL (1963) opined that the presence of this oil was due solely to the action of hot-spring water on accumulations of organic detritus which were de- posited in Quaternary swamps and was analogous to

Biomarkers, petroleum seepage, Ngawha Springs, New Zealand 307

Scale

Paih~

~o~ t ~ Waimarr~ku ^~ ' \ Ng.-wha

~ \ C k Spr,ngs

Dargaville ~ ~ _ ~

Major ~aults ~ ~ ~'~" Late Tertiary/Quaternary ~ f f ~ ^

~lcan,c ce~s ~ ~ . ~ r [ ] wag~ greywacKes ~,~,~"

FIG. 2. Location of Ngawha Springs.

the oil seepage at Waiotapu. We recently showed, however, that the oil seepage at Waiotapu was the result of accelerated diagenesis of the organic matter in lake sediments there, rather than the result of simple pyrolysis of vegetable matter (Czocr~ANSRA et al., 1986). LEnNER (1965) believed that the Tertiary rocks of Northland were not important in the forma- tion or occurrence of oil except that they could provide reservoir beds and cap rocks. Possible source beds, he maintained, are the carbonaceous shales and dark mudstones which occur in the allochthonous Cretaceous succession. HENDERSOr~ (1937) surmised that the geology of the area was unfavourable for the occurrence of oil in commercial quantities. On the other hand, HAZZARO and MORRIS (1972) believe that the deep coal measures and associated volcanism in the Ngawha region could be the source ofoil and gas, a situation analogous to that in Taranaki, which is part of the same basin (Fig. 1).

Nature o f the petroleum

In March 1983, some 19 tonnes of diesel oil was pumped into geothermal well Ngl3, to dislodge a jammed drill. Much of this oil was assumed to have been dispersed between 250 and 950 m below ground level. During the following winter, residents of the area reported the appearance of larger than usual quantities of oil on surface soil and streams, following

heavy rainfall, and they attributed this phenomenon to the use of diesel oil in the drilling operations, several months earlier.

The purpose of this work was, first, to establish a source and palaeoenvironmental history of the oil at Ngawha Springs through a study of the distribution of chemical fossils (biological markers) in the oil and some rock bitumens; second, to determine whether geothermal energy was responsible for the formation of the oil from either the coal measures or from similarly organic-rich sediments, and third, to estab- lish whether the present-day oil seepages at Ngawha Springs were those of natural petroleum or of diesel oil which had been used in geothermal drilling oper- ations and which might have resurfaced after mi- gration through porous sediinents.

EXPERIMENTAL WORK

General procedures are outlined in a previous paper (CzocxAr~SKA et al., 1986). Experimental results pertinent to this work are listed in Table 1.

Ngawha Springs is located at latitude S. 35o24 ' and lon- gitude E. 173°51 ' (Figs 1, 2). Samples of petroleum were collected from water surfaces adjacent to thermal pools in the Tiger Pool area of Ngawha Springs approximately 1 year after the "oil pollution" was reported. Asphalt was extracted from mud which was located within 10 m of these pools. Core samples were taken from geothermal wells which are located on the (metric) New Zealand Mapping Grid System as follows:

308 Roderick J. Weston and Anthony D. Wooihouse

Table 1. Data for Ngawha bitumens

n-alkane maxi-

PrlnClTPh/nClsPrlPh mum CPI TI "i"2 T3 "1"4 T5 T6 "1"7 Ts S1 $2 $3 $4 $5 $6 $7

Diesel oil 0.64 0.15 5.49 Seepoil 0.66 0.23 1.92 Tiger Pool 50 6.67 1.55

asphalt

Rock extracts Ng2 550 m 0.52 0.59 1.76 Ng4 260 m 0.91 0.67 2.51 Ngl8 230 m 0.79 0.55 2.58

13 1.00 60 0.09 1.44 1.40 2.02 1.43 0.24 23 53 55 1.30 60 20 20 3.0 21 1.00 59 0.21 3.4 0.52 0.79 1.90 0.44 20 45 43 0.70 45 27 39 1.2 29 1.36 55 0.24 3.5 0.32 0.73 7.41 0.32 4 29 40 0.48 30 24 46 0.65

} -- Group 1

-- Group 1I

25 1.49 -- 0.48 2.3 17 2.52 48 0.28 3.9 13 1.49 45 0.31 3.7

0.60 0.99 3.36 -- 13 27 40 0.44 26 23 51 0.51 "[ Group III 0.88 1.01 2.68 0.29 14 37 47 0.65 26 23 51 0.51 f 0.76 0.91 0.91 0.24 38 22 38 0.38 32 22 46 0.70 }--GroupIV

Ti C32 hopane (22S/22S + 22R) % ; T 2 C3017fl(H),21a(H)-hopanelC~o 17a(H), 21fl(H)-hopane; T3 TM/Ts ;'I"4 C27 aflhopane/C30 aflhopane; T5 C29 aflhopane/C3o aflhopane; "1"6 C31 (22R + 22S) aflhopane/C3o aflhopane; T7 C32 (22R + 22S) aflhopane/C3o a~hopane; "Is C31hopane (22S/22S + 22R) %; $1 C29 5a(H),14a(H),17a(H),(20SI20S + 20R) %; $2 C29 (20S + 20R)(aflfl/a~fl + aaa) %; $3 C-~0 (20R)aflfl/aaa; 54 C27/(C27 + C28 -}- C29 ) % ; S 5 C28/(C27 -4- C28 + C29 ) %; S 6 C29/(C27 q- C28 + C29 ) %; S7C27/C29; S4-S 7 apply only to 5a(H),14a (H),17a (H), 20R-steranes.

Well Grid reference Depth (m) Ng2 2,589,627E/6,642,354N 547-550 Ng4 2,587,901 E/6,643,229N 256-259 Ngl8 2,590,286E/6,642,372N 228

These wells are all within 3 km of the Tiger Pool. Core samples were ground to a powder, mixed with sand and then extracted with benzene : methanol (9: 1) for 7 h in a Soxhlet apparatus. The solvent was evaporated and the residue was heated under reflux in pentane for 7 h. After evaporation of the solvent, the residue of pentane soluble material was fractionated as above for the crude oil.

RESULTS AND DISCUSSION

The hydrocarbon samples, described in this paper, were placed into four groups (Table 1). The features which distinguish each group are discussed in each sub-section. The sub-sections describe information relating to the biodegradation, migration, maturity, environment of deposition of organic matter, and the source of the petroleum at Ngawha, as determined by the distribution of chemical fossils (biological mar- kers) in the petroleum and bitumens.

Biodegradation

Of the six samples examined in this paper, only the asphalt surrounding one of the thermal pools had degraded. This asphalt was depleted of all the n- alkanes up to n-Cx8 (Fig. 3) and could therefore be described as lightly degraded (level 2, VOLKMAN, 1984). Degradation was to be expected because the asphalt was exposed not only to bacteria, but also to air, sunlight and geothermal water. As the result of degradation, abnormally high values for the ratios of pristane/n-C17 and phytane/n-Cls were recorded for this asphalt.

Migration

A plot of sterane parameters $1 against $3 (SEXFERr and MOLDOWAN, 1981) for the suite of samples in this

paper was linear, which indicated that no secondary migration of the hydrocarbons had occurred. The gradient of the plot was greater than that discussed by SEXFERr and MOLDOWAN. The steeper gradient was attributed in our earlier work, to the influence of the mineral matrixes of sediments in New Zealand, which was assumed to be different from that of the sedi- ments from which the oils studied by SEXF~Rr and MOLDOWAN were obtained (CzoCHANSgA et al., 1987).

The linearity of this plot is significant from two points of view. First, because there has been no secondary migration, the oil will have seeped to the surface either by passage along a fault or with the assistance of an upward flux of subterranean (geo- thermal) fluid, or both. In neither case would the oil have been in contact with mineral sediments for long periods. Second, if the petroleum was in fact the diesel oil which had earlier been pumped into the ground by way of geothermal well Ng13 and sub- sequently resurfaced, then the diesel might have been expected to pass through the sedimentary strata to some degree in which case some of the hydrocar- bons would have been separated by "geochromato- graphy". These phenomena would have resulted in displacement of the seep oil from the linear $1/$3 plot (S~.tFERT and MOLDOWAN, 1981), but this was not observed.

Neither the oils nor the bitumens at Ngawha have undergone chemical alteration, indicative of second- ary migration, and have most probably risen to the surface with geothermal water.

M a m h ~

The CPI values for the diesel and seep oils demon- strated that these oils were both mature. The CPI value of the asphalt was 1.36 and this value would be much higher if the "C32" alkane is not an n-alkane. The asphalt, therefore, appeared to be considerably less mature than the oil, even though it was probably formed by oxidation and degradation of the seep oil.

Biomarkers, petroleum seepage, Ngawha Springs, New Zealand 309

It is likely, however, that after reaching ground level, the petroleum has absorbed immature organic material from surface mud and detritus. This thesis is developed further below.

The pronounced occurrence of odd-carbon num- bered high molecular weight ( C 2 3 - C 3 1 ) n-alkanes in the rock bitumens afforded CPI values of 1.5-2.5 which indicated that these rocks had not reached the oil generation threshold.

The principle biomarker indices of maturity are the percentage isomerisation (22S/22S + 22R) at C-22 for the C32 afl-hopane (T1), the corresponding isomerisation at C-20 for the C29-steranes ($1) and the percentage isomerisation at C-14 and C-17 in the C29-steranes ($2) (MACKENZIE, 1984).

T~ varies from 0 for naturally occurring hopanoids, to 60% for oils and source rock bitumens which have reached the oil generation threshold. Comparison of the T~ values in Table 1 showed that the diesel oil, the seep oil and the asphalt derived from it, were all mature compared to the rock bitumens which had not reached the oil generation threshold. These results implied that the petroleum could have its origin in strata beneath those from which the rock bitumens were obtained.

S~ varies from 0 for natural steroids to 50--03% at the peak of oil generation. A value of 45% for the seep oil is consistent with the view that the oil source rocks are approaching the peak of their oil generating capability. The diesel oil was somewhat more mature than the seep oil while the asphalt and the 3 rock bitumens were considerably less mature than the seep oil, in agreement with the conclusion drawn from the TI values.

Further refinement of these conclusions is possible from a consideration of the value of $2 which varies

from 0 to 75-80% between the onset and peak of oil generation. An $2 value of 43% for the seep oil showed that the oil source rocks had passed the onset but had not reached the peak of oil genera- tion. $2 values for all the bitumens were fairly consistent and higher than expected, based on the above arguments. MACKENZIE (1984) noted that high values of S 2 have been recorded for immature oils, an observation which has previously been attributed to the presence of bacterially reworked organic material but he indicated that the reason for these high $2 values has not yet been satisfactorily explained.

The values of T1, $1 and $2 for the asphalt were all lower than the corresponding values for the seep oil. This was due to the greater proportion of natural hopanes and steranes having the 22R and 20R con- figurations, respectively, in the asphalt. This obser- vation supports the view derived above from the CPI values that the seep oil has absorbed immature organic material at or near the surface.

A feature of the m/z 191 fragmentograms of the three rock bitumens (Fig. 4) was the abundant occur- rence of a range of 17fl(H), 21fl(H) 22R-hopanes from C29 to C32. The naturally occurring hopanes vanish during diagenesis when they are converted to fla and eventually to afl-hopanes. These transform- ations are usually complete well before the oil gener- ation threshold is reached (MACKENZIE, 1984). The occurrence of flfl-hopanes in the rock bitumens is, therefore, a clear indication of the immaturity of the carbonaceous rocks, a conclusion that concurs with that reached after consideration of the values of T 1 and $1. These observations demonstrated that the carbonaceous rocks extracted in this work were not the source of the seep oil.

Table 2. Assignments of major triterpanes in m/z 191 fragmentograms

Peak Compound Structure

a

b C

d e

f g h i J k 1 m

n

o

p q r

s

t u

v

18a (H)-22,29,30-trisnorneohopane, Ts 17a (H)-22,29,30-trisnorhopane, Tm 17fl (H)-22,29,30-trisnorhopane 17a (H),21fl(H)-30-norhopane 17fl(H),21a (H)-30-norhopane 17ct (H),21fl(H)-hopane 17fl(H),21a(H)-hopane 17a(H),21fl(H)-22S 30-methylhopane 17a(H),21fl(H)-22R 30-methylhopane 17a(H),21fl(H)-22S 30-ethylhopane 17a(H),21fl(H)-22R 30-ethylhopane 17a(H),21fl(H)-22S 30-n-propylhopane 17a(H),21fl(H)-22R 30-n-propylhopane 17a(H),21fl(H)-22S 30-n-butylhopane 17a(H),21fl(H)-22R 30-n-butylhopane 17a(H),21fl(H)-22S 30-n-pentylhopane 17a(H),21fl(H)-22R 30-n-pentylhopane 17fl(H),21fl(H)-30-norhopane 17fl(H),21fl(n)-hopane 17fl(H),21fl(H)-22R 30-methylhopane 17fl(H),21fl(H)-22R 30-ethylhopane a bisnortriterpane

I I I ,R=H III,R-~-H II,R~-----C2H5 III,R~-------C2H 5 II,R~----'CH(CH3)2 III,R~------CH(CHa) 2

II, R~------'CH(CH3 )C2H5

II,R~------CH(CH3)C3H7

II,R~--------CH(CH3)C4H 9

II,R-'--CH(CHa)CsH n

II,R~------CH(CH3)C6H 13

IV,R~-----C2H s IV,R~----CH(CH3)2 IV,R~-----'CH(CH3)C2Hs IV,R~--~CH(CH3)C3H7

310 Roderick J. Weston and Anthony D. Woolhouse

(a)

Pnstane I

Phytane

I I I I

I

I

l i I I I I I I

11 13 15 17 19 21

n- Alkane carbon number

| 1 l

I I I 23 25 27

(b)

P h y ~

I I I I I I I I I 13 15 17 19 21 23 25 27 29

n-Alkane carbon number

I 31

(c)

I I I I I I I I I 17 18 19 21 23 25 27 29 31

n-Alkane carbon number

1 33

FIG. 3. Gas chromatograms of the total saturated alkanes, a. diesel oil; b. seep oil; c. asphalt;

Biomarkers, petroleum seepage, Ngawha Springs, New Zealand 311

(d)

j_i , .

( I ~ I t ,t J , ~ t b 13 14 15 1 19 20 21 23 25 27 29

n- ~l~ne carbon number

I .

I

31 I _

33

( e )

Prist~, l~rYtane I

I l i i i i ! ! ~ 1 , , I ! 1 I I. 12 13 14 15 16 17 18 19 20 21 23 25 27 29 31

n-Aik,~ carbon number

,~f)

I I I_ 1 ! 11 1,2 13 14 15

F'ristane Phytar~ I ! ! i

I . 1 ~ I t J l 17 ~9 21 23 25 27 29 31

n - Alkane c.,arbon numl~-,~

FIO. 3 continued, d. Ng2; e. Ng4; f, Ngl8.

312 Roderick J. Wes ton and An thony D. Woolhouse

d

b J (a)

. . I . . . . I . . . . I . . . . I . . . . I

25 30 35 40 45 Minutes

(b)

i

b d " I

I a a c g k . I e • m n 0

I t I l I I I I I I I I I I I I I 3 4 3 5 3 6 3 7 3B 3 9 4 0 41 4 2 4 3 4 4 4 5 4 6 4 7 4 8 4 9 5 0

Min~es

(C)

d f b c

h S j k t

34 35 36 37 38 39 40 41 42 43 44 45 46

Minutes

FIG. 4. m/z 191 f ragmentograms of the sa turated alkanes. Refer to Table 2 for assignments , a. diesel oil; b. seep oil; c. asphalt;

B iomarkers, petroleum seepage, Ngawha Springs, New Zealand 313

(d)

b

i

d f t

I I I I I I I I I I I I I I 34 35 36 37 38 39 40 41 42 43 4 4 4 5 46 47

Minutes

(e)

r d

I I I I " I I I "1 I I I I I I I I 3 4 3 5 3 6 3 7 3 8 3 9 4 0 41 4 2 4 3 4 4 4 5 4 6 4 7 4 8 4 9

Minutes

(f)

d

c LL A v e g h

r j k

A t. , ,~ u . o

, , i i , , i , , j i i ~ , , j

33 34 35 36 37 315 39 40 41 42 43 44 45 46 47 48

Minutes

F1o. 4 continued, d. Ng2; e. Ng4; f. Ngl8.

314 Roderick J. Weston and Anthony D. Woolhouse

Environment o f sedimentation

The pristane/phytane (pr/ph) ratio is an expression of maturity, especially for coals, but more impor- tantly for oils and bitumens it is an expression of the degree of oxicity of the environment in which sedimentation took place (DIDYK et al., 1978). The diesel and seep oils can be differentiated by several parameters, including the pr/ph ratio. The pr/ph ratio for the diesel oil was 5.5 while the ratio of all the samples from Ngawha ranged from 1.6 to 2.6. These values lie between those for oils which have orig- inated essentially from either an oxic (terrigenous) environment (>3.0) or an anoxic (marine/lacustrine) environment (<1.0) (Hur~T, 1979). The organic material from which the oil at Ngawha has formed was deposited in both oxic and anoxic environments and, therefore, had a mixed origin of both marine and terrigenous sources.

The terrigenous component of the rock bitumens is clearly illustrated by the abundance of high molecular weight odd carbon numbered n-alkanes (C23-C29) in the totally saturated fractions, while the marine com- ponents are expressed by the abundance of the C~6 and Cl7 n-alkanes.

The relative contribution of marine and terrigen- ous organic material to the oils and bitumens can be estimated from the abundance of natural C27, C28 and C:9 (R)-steranes ($4, $5, $6) in the m/z 217 fragmen- tograms (Fig. 5). Because the proportion of C28- steranes ($5) in these extracts is almost invariant (20-27%), the ratio of C27/C29 steranes ($7) can be used to determine whether an oil or bitumen had a

marine ( 5 7 = 2.0), mixed (1.0) or terrigenous (0.5) origin (HuAN6 and MEINSCHEIN, 1979).

From the values of $7 in Table I, it is clear that the diesel oil had a marine origin while the bitumens from the carbonaceous rocks had a largely terrigenous origin. The components of the seep oil are derived from both marine and land sources while the asphalt has been enriched in organic material from surface biota.

Source characters

Several features of the gas chromatograms and mass fragmentograms of the totally saturated hydro- carbon fractions are source-specific and serve to dis- tinguish the four groups of bitumens and, in some cases, to distinguish the individual samples.

The major component of the saturated hydrocar- bon fraction of the bitumen from Ng2 and Ngl8 was a compound which had a retention time between those of nC13 and nC~4. This product is also a minor component of the bitumen from Ng4 but the identity of the product has not been determined.

Two compounds occur in the saturated hydrocar- bon fraction of the bitumen from Ng4 which have retention times in the gas chromatogram between nC2o and nC21, and the more volatile of these prod- ucts is the major component of the total saturated hydrocarbon fraction from Ng4. The structure of these two compounds has also not been determined.

None of these compounds are components of the total saturated hydrocarbon fraction of the seep oil or

Table 3. Assignments of major steranes in m/z 217 fragmentograms

Peak Compound Structure

i

J k I m

n

o

p q r

S

t

13fl(H),17a (H) 20S-diacholestane 13,8(H),17a (H) 20R-diacholestane 13a (H),17fl(H) 20S-diacholestane 13a(H),17fl(H) 20R-diacholestane 24-methyl 13fl(H),17a(H) 20S-diacholestane 24-methyl 13fl(H),17a (H) 20R-diacholestane

"24-methyl 13a (H),17fl(H) 20S-diacholestane 5a(H),14a(H),17a(H) 20S-cholestane "24-ethyl 13fl(H),17a (H) 20S-diacholestane ~5a (H), 14fl(H),17fl(H) 20R-cholestane 15a (H),14fl(H),17fl(H), 20S-cholestane 24-methyl 13a(H),17fl(H) 20R-diacholestane 5a(H),14a(H),17a(H) 20R-cholestane 24-ethyl 13fl(H),17a (H) 20R-diacholestane 24-ethyl 13a (H), 17fl(H) 20S-diacholestane 24-methy15a(H),14a(H),17a(H) 20S-cholestane 24-ethyl 13a (H), 17fl(H) 20R-diacholestane 24-methy15a(H),14fl(H),17fl(H)20R-cholestane 24-methyl 5a (H),14fl(H),17fl(H) 20S-cholestane 24-methy15a(H),14a(H),17a(H) 20R-cholestane 24-ethy15a(H),14a(H),17a(H) 20S-cholestane 2 4-ethy15a (H),14fl (H),17 fl (H) 20 R-cholestane 24-ethy15a(H),14fl(H),17fl(H) 20S-cholestane 24-ethy15a(H),14a(H),17a(H) 20R-cholestane

VI,R~---H VI,R~----H VII,R~--~-H VII,R-~--H VI,R~---Me VI,Rp~-Me VII,R~-.~-Me VIII,R=H VI,R~-----C2H 5 IX,R~-~-H IX,R--H VII,R~Me VIII,R~H VI,R~----C.2H 5 VII,R~----CzH 5 VIII,R~Me VII,R~--~CaH 5 IX,R~---H IX,R~---H VIII,R~Me VIII,R~---C2H5 IX,R~--------C2H 5 IX,R~------C2H 5 VIII,R~----C2H 5

Biomarkers, petroleum seepage, Ngawha Springs, New Zealand 315

a (a)

b de g h

r 20 22 24 26 2 8 3 0

M inu tes

(b)

h

a e ~J

b d i k

C f I n °

I I I I I I I I I ~ 33 ~ 35 36 ~ 38

Minutes

(c) (d)

a

. ~ b de

P qrS

1 i I I I I I I I 31 32 33 34 35 36 37 38 39

Minutes

h

i o o e i k p r s

I I I I I I I I I 31 32 33 34 35 36 37 38 39

Minutes

(e )

m r $ • k no

J p q

1 I 1 I I I I I 31 32 33 34 35 36 37 38

Minutes

f )

J '1 9J k P

<J I l l l l i ° i d i b n= I ' l i l l l l m l . O l , : l / I

I I I I I i I I

31 32 33 34 35 36 37 38

Minutes

FIG. 5. m/z 217 fragmentograms of the saturated alkanes. Refer to Table 3 for assignments, a. diesel oil; b. seep oil; c. asphalt; d. Ng2; e. Ng4; f. Ngl8.

A G 2 : 3 - E

316 Roderick J. Weston and Anthony D. Woolhouse

the asphalt derived from it. These source characters clearly demonstrate that the carbonaceous rocks from which the group III and IV bitumens were extracted were not the source of the seep oil at Ngawha.

Several parameters (T2-T5) derived from the m/z 191 fragmentograms distinguish the diesel from the seep oil. T2 is the ratio of 17fl(H), 21a(H)-hopane to 17a(H), 21fl(H)-hopane at C30. The fla-hopane occurs naturally and the ratio (T2) can vary from 0.5 during diagenesis to 0-0.1 during the early stages of oil generation (MACKENZIE, 1984). AS the diesel has been shown to be a mature oil, the value of 0.09 for T 2 is therefore in agreement with this fact and separates this oil from the seep oil (0.21) and from the rock bitumens (0.28--0.48).

Z 3 (TM fFs) is a parameter which characterises both the source and maturity of an oil (SEIEERT and MOLDO- WAN, 1978). It is used, therefore, to compare the maturity of oils and rock bitumens from within the same basin, or to compare the sources of samples which have the same thermal maturity. Where both the source and maturity of two samples differ, then comparison of the T3 values is not strictly possible.

The ratio of 17a(H) C27-hopane/afl C30-hopane (T4) is also an expression of both source and maturity and the conditions which were discussed for T3, which may be used for comparing two or more bitu- mens, also apply to T4. The ratio of T4 rarely exceeds 1.00 and a significant difference in the value of this ratio for the diesel oil (1.40) clearly separates this oil from the seep oil (0.52).

A feature which characterises the m/z 191 fragmen- tograms of the group III bitumens is the intensity of the peak due to fla-trisnorhopane ((c) in Figs 4d and 4e). The abundance of this C27-hopane was reported by PinEY and GILBERT (1982) as a feature of the bitumens from immature coal (lignite).

The ratio of C29/C30 afl-hopane (Ts) is a parameter which distinguishes oils from different sources (Pvra et al., 1975). The value of this ratio is usually less than 1.0 but a value greater than 1.0 has been recorded for, and has been claimed as characteristic of, bitu- mens extracted from carbonate source rocks (RULLKOTrER et al., 1985) and of oils from the Middle East (PYM et al., 1975). Such a value has not been recorded for any oil or source rock extract so far studied in New Zealand. The values for this ratio clearly separate the oils and bitumens into 3 main groups. This parameter, together with $7, demonstrated that the surface oil at Ngawha Springs was unrelated to the diesel oil.

P,:rhaps the most striking feature of the m/z 191 frabmentograms was the abundance of the C31 aft- (22R)-hopane in the rock extracts, a phenomenon which was most pronounced in the asphalt. Mass spectra of the two afl-homohopane epimers con- firmed their identity and distinguished these Car hopanes from gammacerane (C30) which elutes immediately after aft 22R-homohopane. The value

of the ratio (T8) of S/R epimers at C-22 in the C3a-hopane is normally the same as that for the C32(T1) and C33-hopanes and is an expression of the thermal maturity of an oil (see above). However, PmLV and GILBERt (1982) found an instance where the 22 R-epimer of the C31-hopane was very much more abundant in an oil than its (geological) 22S- epimer. This was especially pronounced where the petroleum had passed through a stratum of immature lignite, which was enriched with the C31 afl-22R- hopane. PHILP and GILBERT (1982) demonstrated that another sample of oil from the same well, but from a deeper reservoir, did not contain the abnormal amounts of this hopane, which had been absorbed by the oil during secondary migration. The great abun- dance of the C31 afl-22R-hopane in immature coal measures, such as peat, has been described by QUIRK et al. (1984). This triterpane is derived from the C32 flfl-22R-carboxylic acid (X) which in turn is formed by the oxidation of bacteriohopanetetrol (XI) between C32 and C33 in the side-chain. The tetrol is present in the bacteria which inhabit surface organic waste and subsequent transformations to the C32 afl-hopane are known to occur rapidly during early diagenesis (QUIRK et al., 1984).

Concomitant with the abundant occurrence of the C31 afl-22R-hopane was the occurrence of the C31 flfl-22R-hopane and its C29 and C30 homologues. This series of triterpanes was clearly apparent in the mlz 191 fragmentograms, especially those of Ng2 and Ng4 (Fig. 4). This series did not extend beyond the C32 flfl-hopane, in line with the above arguments.

From these observations one can conclude that the seep oil originated from mature source rocks, which were not located in the present study, and seeped to the surface where it absorbed hydrocarbons that included traces of flfl-hopanes and particularly the C3~ afl-22R-hopane from peat-like material. The C31 afl-22R-hopane then became concentrated in the asphalt which was formed by degradation of the oil lying on the surface.

A further feature of the distribution of triterpanes in the m/z 191 fragmentogram of the bitumen from Ng18, was the occurrence of a CE8-triterpane. This triterpane occurred only in the Ng18 bitumen and, therefore, distinguished this bitumen from all the others.

The abundance of this triterpane was insufficient to obtain a satisfactory mass spectrum. However, C-GC-MS using SIM at m/z 384 elicited a strong response from this triterpane, while at m/z 177 only a weak response was recorded. These observations indicated that the two methyl groups which had been removed from a parent C30 triterpane were probably both from either rings A/B or rings D/E. The struc- ture and occurrence of such a product viz. 28,30-bis- norhopane is well-documented (SEIFERT et al., 1978; GRANTHAM et al., 1980; RULLKOa-rER et al., 1982; VOLKMAN et al., 1983a, b; KATZ and ELROD, 1983; MOLDOWAN et al., 1984; CURIALE et al., 1985). How-

Biomarkers, petroleum seepage, Ngawha Springs, New Zealand 317

ever, chromatography of a sample of Monterey oil, which contains 17a(H), 18a(H), 21fl(H)-28, 30-bis- norhopane and another triterpane of unknown con- stitution that elutes immediately before the bis- norhopane, demonstrated that the retention time of this unknown tri terpane, relative to 30-norhopane, was identical to that of the bisnortriterpane in the Ngl8 bitumen.

Bisnortriterpanes occur widely in bitumens (RULLKOTTER et al., 1982), sometimes as the major products in the tri terpane fraction and usually only in immature oils or bitumens (GRANTHAM et al., 1980; CURIALE et al., 1985). Recently, NOBLE et al. (1985) demonstrated that bisnorhopane was present in rock bitumens but not in the corresponding rock pyroly- sates and they suggested that the hydrocarbon was never incorporated into the kerogen.

The origin of bisnortriterpanes, and of 28,30-bis- norhopane in particular, has been debated since their discovery, but one explanation which is consistent with all the information to date is that the triterpanes arise from anaerobic bacteria which rework residual organic matter in immature sediments (Gv.Ar~rnAM et al., 1980; KATZ and ELROD, 1983). This thesis is plausible in the present case because allochthonous movement disturbed much of the original sedimen- tary column at Ngawha Springs and, furthermore, Ngl8 was the least mature of the bitumens studied.

Oil source

The possibility that the oil at Ngawha Springs was diesel oil that had resurfaced after having been pumped into a geothermal exploration well was dis- pelled by the present work which showed that the oil occurred naturally. The distribution of biomarkers in the seep oil distinguished it from the diesel oil used in well Ngl3. The parameters T4, Ts, $3, $7, and pr/ph in particular, all distinguished the two oils. These parameters, to a large degree, serve to characterise the source of an oil and the values of these ratios are sufficiently different to conclude that the two oils have different origins.

It is conceivable, however, that during the delay in sampling the petroleum seepage at Ngawha, any diesel oil which might have been dispersed amongst the porous sediments will have been sufficiently diluted to have effectively vanished by the time the samples were taken.

GxcGEr~aAcn (unpublished work) disclosed that the summer preceding the drilling operations was very dry. The mean monthly rainfall between November 1982 and March 1983, inclusive, was 52 mm, whereas the average rainfall over the same period in the past was 109 mm (records of the New Zealand Meteorological Service). As a result, many small springs dried up due to a general decrease in ground water levels. Hydrocarbon production, how- ever, is not likely to be affected by dry weather.

Consequently, a constant rate of hydrocarbon pro- duction coupled with a reduced rate of water dis- charge was expected to result in the accumulation of hydrocarbons underground during dry periods because the hydrocarbons could not be dispersed through hot spring discharges. The increased dis- charges after heavy rainfall during the winter (see Introduction), could then be explained simply in terms of increased mobilisation of the hydrocarbons which had accumulated during the preceding sum- mer.

Results of the work described in this paper demonstrated that the oil seepage at Ngawha Springs is a natural phenomenon and its is therefore unlikely that increased oil production during the winter of 1983 was related to the use of diesel oil in the geother- mal well Ngl3.

Temperature--depth data from the geothermal wells at Ngawha Springs are typical of that recorded in the deepest well, Ngl3, which is located only 300 m from the Tiger Pool (GRAter and McGtrxr~NEss, 1985). The temperature gradient in Ngl3 was constant to a depth of 600 m where the temperature was 230°C. In this region, which "capped" the geothermal system, heat was dissipated by conduction through rocks of poor permeability. From 600 to 1600 m, the tempera- ture was constant. In this depth range, the water reservoir lay within the extensive greywacke base- ment rocks which exhibited good permeability and in which heat was dissipated by convection. From 1600 m, a further constant temperature gradient was observed within rocks of low permeability and a temperature of 301°C was recorded at the total well depth of 2255 m. The average temperature of the primary hot water reservoir at Ngawha Springs was 228°C which was recorded at an average depth of 860 m (Gv, Ar~X and McGuINNEss, 1985).

These data afford an average geothermal gradient (from 11 wells) of 265°C/km. The gradients in indi- vidual wells were higher or lower than this figure depending on the depth at which the temperature was measured, for example, the geothermal gradients in wells Ng2 and Ng4, two of the wells from which rock samples were obtained for this work, were 415°C/km and 308°C/km, respectively (GRAm, 1981). If the surface temperature at Ngawha was assumed to be 20°C and the oil generation was assumed to be in the range 100 + 30°C, then oil source rocks might be expected from these data to be located at depths of 193 +_ 73 m and 260 + 100 m in wells Ng2 and Ng4, respectively. These levels correspond roughly with those at which Tertiary coal measures are found at Ngawha (SKINr~ER, 1981). It is conceivable, there- fore, that diagenesis of the organic material in these strata may be the source of the oil at Ngawha despite the fact that neither rock sample from 256 m (Ng4) nor 228 m (NglS) had reached the oil generation threshold.

Coal measures and dispersed carbonaceous material occur at several positions in the stratigraphic

318 Roderick J. Weston and Anthony D. Woolhouse

column at Ngawha. KATZ (1968) demonstrated for most of the oil basins in New Zealand that source rocks occur at the base of the sedimentary sequence, yet the rock sample from 550 m in well Ng2 was shown to be too immature to generate petroleum. Consequently, depth appears to be no guide to the location of oil source rocks at Ngawha. This confusion is undoubtedly due to stratigraphic mixing which was the result of a combination of allochthonous and tectonic movement.

This work has failed to locate the rocks from which the petroleum at Ngawha is derived but the coal measures in the Tertiary "cap" rocks, which are located immediately above the geothermal water basin, are the most likely source of the oil at Ngawha.

CONCLUSIONS

The crude oil which seeps to the surface at Ngawha Springs was mature but the oil biomarker parameters indicated that its (unlocated) source rocks had not reached the peak of oil generation. The oil had a mixed marine and terrigenous origin and had not undergone secondary migration except to the sur- face. The oil was distinguished from diesel oil by several parameters, specifically those which relate to the source of an oil. The seep oil was concluded to have arisen from accelerated diagenesis of organic material in Tertiary rocks which cap the geothermal system at Ngawha.

A surface asphalt, which had formed from the oil, was lightly degraded. It had absorbed hydrocarbons from immature organic matter near the surface and its hydrocarbon composition was characterised by an abundance of aft 22R-homohopane. This triterpane was also present in rock bitumens together with appreciable proportions of flfl-hopanes. Bitumens extracted from potential source rocks were largely of terrigenous origin. They had characteristics of sedimentary organic material which had not reached the oil generation threshold. These rocks did not prove to be the source of the seep oil. One of the bitumens contained a C28-triterpane, the presence of which suggested that the sedimentary organic material had been reworked by bacteria. Such a view was consistent with the geological history of Ngawha Springs.

Acknowledgements--The authors wish to thank Dr D. S. Sheppard of this Division for providing the oil samples and information on the geochemistry of the Ngawha Springs area; DrW. F. Giggenbach for an unpublished report on the diesel oil problem at Ngawha; Associate-Professor P. R. L. Browne of the Geothermal Institute, University of Auck- land for supplying the rock samples for this work and for information on the geology of the Ngawha Springs area; and Professor R. P. Philp of the Department of Geology and Geophysics, University of Oklahoma (Norman) for a sam- ple of Monterey oil.

Editorial handling: J. Brooks.

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APPENDIX

Structural formulae of organic compounds referred to in Figs 4 and 5

157

R R

? , : . . ~ - . . / ~ C O ~ H

/ ~

OH OH