Stratigraphy and mineralogy of Unit H, Nagoorin South oil shale deposit

Stratigraphy and mineralogy of Unit Nagoorin South oil shale deposit John

H. Patterson

and Arthur

H,

W. Lindner”

CSIRO, Division of Coal and Energy Technology, Lucas Private Mailbag 7, Menai, NS W 2234, Australia *Arthur Lindner and Associates P/L (Received 18 February 1992; revised 26 July 1992)

Heights

Research

Laboratories,

Mineralogical and chemical analyses are presented for a continuous sequence of samples over Unit H, Nagoorin South oil shale deposit. X-ray diffractometry and electron microprobe analysis were used to determine the mineralogy and the composition of carbonate minerals. The mineralogy and geochemistry are similar to those of other Tertiary oil shales, except that the composition of siderite-type minerals is unusually variable with stratigraphic depth. The stratigraphy is defined in Unit H and the subunits are characterized. Three types of siderite are recognized, depending upon the subunit and conditions during deposition and diagenesis. Magnesian siderites range in composition from 13 to 53 % magnesium carbonate. (Keywords: geochemistry;

oil shale; siderites)

The Nagoorin and Nagoorin South oil shale deposits are contiguous within the Nagoorin graben, _ 70 km south of the port of Gladstone in Queensland. Indicated in situ resources total 3.1 billion barrels of oil based on the modified Fischer assay’. Predominantly carbonaceous oil shales occur in a thick sequence of Tertiary sedimentary rocks (named the Nagoorin beds2) largely covered with a veneer of Quaternary alluvium. The Tertiary sequence includes cannel coal, carbonaceous shale and brown oil shale (lamosite) interbedded with shale, siltstone, sandstone, conglomerate and minor limestone. It is intruded by minor basalt dykes and sills. Analysis of samples from core and auger holes at 60 locations, aggregating > 14 000 m drilled, indicates that the NNW trending, 16 km long Nagoorin graben is fault-bounded on both flanks and contains a west-dipping sedimentary pile, which has been sourced from a southerly direction. Differential movement along the bounding faults has resulted in a half-graben, plunging south at the north end, so that the older units subcrop along the eastern boundary of the graben and across its northern end. Strike- and cross-faulting also occurs within the graben, further modifying the subcrop pattern (Figure I). Eight stratigraphic units have been recognized within the sequence (Figure I). They are informally named, commencing with an incompletely penetrated, undifferentiated basal sandstone (Unit B), succeeded by Units C to J in decreasing age, aggregating more than 870m in thickness. Gravity data suggest that the total Tertiary sequence may be considerably thicker than this. Five of the units (C, D, E, F and H) contain significant amounts of organic-rich shales (including lamosites, cannel coals and carbonaceous shales) and claystones.

Presented at the 6th Australian Oil Shale Workshop, 1991. The University

of Queensland.

0016 2361.93’06 0863 06 I 1993Butterworth -Heinemann

Queensland,

Ltd.

5-6 Australia

December

Cross-faulting (close to the mineral permit boundary that delineates the Nagoorin and Nagoorin South deposits) has been inferred to account for the distribution of units in the subsurface, which results in Units H and J being restricted to the Nagoorin South deposit. Units B to G occur in the Nagoorin deposit, with B, F and G common to both deposits. The youngest units are preserved in Nagoorin South, which is also nearest the source of elastic fill with a higher content of total and coarser-grained elastics. A notably smaller proportion of the shale oil resource (- 15%) is therefore contained in the Nagoorin South deposit. The older, deeper, thicker, more distal, organic-rich units present in the Nagoorin deposit are believed to wedge out against, or onlap, the coarser-grained (and organically barren) fill encountered at depth in all the Nagoorin South drill holes”. The regional geology, stratigraphy, organic petrography and major rock types that occur in the Nagoorin deposit have been described elsewhere4. Unit H, present only in Nagoorin South, remains as an organic-rich unit which has not been previously investigated. It is the most widespread high-grade unit in the Nagoorin South deposit and therefore has some commercial interest. It ranges from 33 to 41 m in thickness and is predominantly a lamosite, but contains two persistent and distinctive bands of carbonaceous oil shale. The unit was subdivided into Subunits Ha to Hd (in decreasing age) in the course of this study (Figures 1 and 2). Organic petrological and mineralogical studies of Nagoorin oil shales have been reported4.“. Five dominant rock types were recognized: cannel coal, carbonaceous shale, sideritic- carbonaceous shale, claystone, and lamosite. These rock types recur in a number of cycles within the units and refect depositional environments ranging from pedogenic to swampy and lacustrine. The present work provides a more detailed study of the inorganic geochemistry and mineralogy of the various rock types within Unit H ofthe Nagoorin South deposit.

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863

Figure 1

Interpreted

subcrop

geology

of the Nagoorin

NAGOORIN SOI OIL SHALE DEPOSIT

and stratigraphy

NAGOORIN

Dolerite/Baaalt

DUEENSLAND

I

L

‘=

j

g

South

I II,, and the Nagoorin

I

/

_‘,

. .

GR;;;c

oil shale deposits,

/‘f$S’i

I

and cloycy rondrtenc,

oil SholC wth

in

graben

port.

Dominanlly rondstonc,minor slItstone, madsfono, kdroqenous mu6slone, oil shale and carbonaceous oil shale. Conqlomcrolc

oil shale and CorbonOceou$

car.?

DESCRIPTION

conqtomcrotc. Mo¶tly coIcorcou¶. M,na,e,ru9inarr mo,,,inp,

Mudflonc

LITHOLOGICAL

Interbedded

Nagoorin

., ;.

/

Stratigraphy

and mineralogy

of Unit

H, Nagoorin Kerogen

40

0

I5

0

Kerogen

I ,IIIL, f’@e

230

,I,,,l,,

4

8024

Calcite

1

,,,I

Dolomite

South

oil shale

deposit:

J. H. Patterson

and A. W. Lindner

and Minerals, wt% 0 .

3 ,,I,,,

601

,I,,,,

2

Sub Unit 30

IO

200

15

300

20

40

Ca-siderite~Mg-siderite

I

Hd

l---

Hc

Hb

Ha

Figure 2 Silicate and selected mineral concentration

profiles

over Unit N Nagoorin

EXPERIMENTAL A continuous sequence of 2 m samples over Unit H, borehole NSD 17, 50-86 m (samples 25052-25069) was studied. Additional samples were taken of siderite and dolomite layers observed within the section. General chemical and mineralogical procedures were as previously described6. Major inorganic element analyses were obtained by X-ray fluorescence spectrometry. X-ray diffractometry (XRD) and scanning electron microprobe analyses were used to determine the mineralogy. Mineralogical estimates were made for coal, kerogen, siderite, pyrite and total carbonates by calculation from the chemical analyses for organic C, mineral C, and total S respectively. Other semiquantitative mineral estimates were made from the chemical analyses and XRD patterns in comparison with the Unit H composite sample 27740. RESULTS

AND DISCUSSION

Stratigraphy

The stratigraphy of Unit H has been defined, based on drill log data and the mineralogical results described below. Four subunits, Ha to Hd, have been recognized in order of decreasing age as follows: Subunit Ha: calcareous lamosite with Mg-siderite and ostracods and gastropods abundant in parts, and thin dolomite layers observed at the base; thickness 6-18 m; oil yields 25-180 LTOM. Subunit Hb: carbonaceous oil shale and silty mudstone with negligible carbonates; thickness 8-17 m; oil yields 15-50 LTOM. Subunit Hc: lamosite with Ca-siderite and rare ostracods and gastropods; thickness 7710 m; oil yields 5g-100 LTOM. Subunit Hd: carbonaceous oil shale; thickness l-3 m; oil yields ~20 LTOM.

Element

South borehole

abundances

NSD

17

and associations

The results of chemical and mineralogical analyses for the composite oil shale sample of Unit H (27740) and concentration ranges for the various subunits of Unit H, borehole NSD 17, are given in Tub/e 1. The chemical element abundances are not unusual for an oil shale and generally appear comparable with those of Rundle or Stuart oil shales’j. The total S concentrations (0.3-2 wt%) are above normal abundances found in sedimentary shales but are not unusual for other Tertiary oil shales. The chemical composition varies little within each subunit, but there are some differences between the various subunits. Most major elements are probably grade-controlled, but carbonate-related species (CaO, MgO, FeO, CO, and Mn) differ in the three subunits. Subunit Ha (68.45587.15 m) is characterized by high CaO, MgO and CO, concentrations, associated with the minerals calcite and dolomite. The carbonaceous Subunit Hb is typically low in carbonates, whereas in Subunit Hc the carbonates are sideritic. Both Subunits Hc and Ha are somewhat enriched in FeO, CO, and Mn relative to Subunit Hb (Table I). Mineralogy

The general mineralogy over Unit H is comparable with that previously reported for other Tertiary oil shale deposits6,‘, but varies with stratigraphic depth. The major minerals observed in most samples are smectite, kaolinite, quartz, opaline silica, feldspar and pyrite. Minor carbonate minerals include calcite, Ca-siderite, Mg-siderite and dolomite, which vary in concentration with stratigraphic depth. Mineral concentration-depth profiles are shown in Figure 2. Carbonaceous samples in Subunits Hd and Hb contain negligible calcite and little Ca-siderite, except for the sample at 5c-52 m. This is a mixed carbonaceous-lamosite sample and it could well be that the siderite occurs only in the lamosite

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Stratigraphy Table 1

and mineralogy

Chemical

and mineralogical

of Unit H, Nagoorin analyses

and concentration

South oil shale deposit: J. H. Patterson and A. W. Lindner ranges” for subunits

of Unit H, Nagoorin

South deposit

Subunit Hc

Hb

Composite 27740

Ha

sample

Average

shale

Major components (wt%) 12.8-13.8 4749.5

Al,& SiO

0.44.9

Fe

41.3 15.1

5.9-22.6 3246.5

44450 I I-20.8

0.34.9

0.551.7

1.0 4.25

58.5 15.0

Fe0

4413.9

1.8&2.8

3.24.8

MgQ

l-l.6

0.68-0.73

0.9-4.3

1.48

2.5

cao

0.84.3

0.5-1.15

0.9556.6

2.14

3.1

Na,O

0.330.7

0.18-0.4

0.3-1.1

0.48

1.3

K,Q

0.8%1.1

0.5-I

0.771.8

1.0

3.2

0.551

0.5-0.8

0.7

0.77

O.lkl.5

l-7

2.57

0.574.7

TiOz

24

CO,

0.5-1.1

Total S Organic

0.3-I

7.5512.5

C

2-2.65

H

53-100

Oil yield (LTOM)

4419

2-3

1.8-3.5

1749

24-179

Organic matter and minerals (wt%) I l-18 Kerogen Cannel

l&27

coal

0.6-l .9

7-18

6627.5 _

1.1

1.86 79 17

12-20

7-14

X-17

12

Kaolinite

9-14

IO-30

5517

20

Smectite

30

Quartz

30-35

3540

28-45

Illite

l-2

O-2

I-2

2

Feldspar

l-10

l-3

l-4

5

Pyrite

l-2

0.5-1.5

l-3

1.8

Calcite

O-I

2-8

3

Dolomite

O-6 _

G5

0.5

Ca-siderite

225

O-1

o-2.5

I

O-2

0.552.5 G9

I 5

Mg-siderite Opal-CT “Fe calculated

3-10 on basis that total S occurs

as pyrite and that the remainder

of Subunit Hc. This lamosite is characterized by relatively high concentrations of Ca-siderite (- 5%) and the absence of Mg-siderites and calcite (Figure 2). This probably reflects relatively shallow and oxidizing lacustrine conditions close to a swampy lake edge. Calcareous lamosites (Subunit Ha in Table I) contain 226% calcite and l-3% Mg-siderite rather than Casiderite. This reflects lacustrine conditions suitable for the proliferation of ostracods and gastropods. Ferroan dolomite was also observed in some samples, and a massive 0.2 m band was recognized at 83.75 m. The trends in carbonate mineralogy with stratigraphic depth have been discussed above and are shown in Figure 2. In summary these are: the occurrence of Ca-siderite within Subunit Hc; negligible carbonates in the carbonaceous shales of Subunit Hb; the occurrence of calcite and Mg-siderite within Subunit Ha; and the occurrence of ferroan dolomite and high-Mg-siderite at the base of Subunit Ha. Overall, the mineralogy of Unit H, Nagoorin South is essentially comparable with that of Rundle and Stuart oil shale!?. Scanning electron microprobe analyses of carbonate minerals Electron microprobe analyses were performed on all samples that contained carbonate minerals. Wavelengthdispersive X-ray spectra were used to determine the

866

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of total Fe should

0.24

11.7

be expressed

_ as Fe0

chemical compositions of the carbonate minerals. Unit H is unusual in the range of carbonate minerals observed-calcite, dolomite, Ca-siderite and Mg-siderite ~ and there are some clear trends with stratigraphic depth. The chemical compositions of the carbonate minerals affect the temperature and extent of decomposition in the retort and combustor and were of particular interest in the present work. The lower sections of Unit H are calcareous and mineralogically similar to RundleeStuart oil shales. In Nagoorin South, Unit H, two types of calcite have been observed, primary (fragments of ostracods and gastropods) and altered ostracods. The ostracod fragments are essentially pure calcite. Altered ostracods are notably observed in the interval 5658 m and show variable concentrations of up to 20 wt% Fe0 at the edges. This is attributed to replacement of calcium, mainly by iron during exposure of primary ostracods to solutions rich in iron during diagenesis. Massive dolomite is observed over a thin 0.2 m section at the base of Subunit Ha in borehole NSD 17. Microprobe analyses show it to be ferroan dolomite with two phases. Predominant and slightly darker grains (Ca0.49Mg0.4,Fe0.0,Mn0.01COJ appear to have been cemented together in a matrix of brighter ferroan dolomite (Ca,,,,Mg,,,,Fe,,,,Mn~,~~CO,). Both phases are ferroan dolomites, although the iron-rich phase is on

Stratigraphy

and mineralogy

of Unit H, Nagoorin

South oil shale deposit: J. H. Patterson and A. W. Lindner

CaC03

Ca-siderites

2

M&O3

4

6

8

10

12

14

16

18

20

22

24

I-lm

Figure 3 Compositions in selected samples

of individual

siderite

and Mg-siderite

grains

the boundary between ferroan dolomite and ankerite. Similar massive layers observed in the Stuart deposit contain less iron and are essentially pure dolomite, and are considered to be formed by penecontemporaneous precipitation during periods of pedogenesis and desiccation of pre-existing lake sediment@. The composition of such secondary carbonates reflects the composition of pore liquors as precipitation proceeds. Thus it would appear that availability of magnesium decreased relative to both iron and calcium in the final stages of desiccation and precipitation. As described above, variability in the composition of the iron carbonates, Ca-siderite and Mg-siderite, has proved to be a particular feature of Unit H. Average chemical formulae were calculated from the microprobe analyses. The range in chemical composition of the Nagoorin South siderites is illustrated in Figure 3. It is evident that samples 25053 and 25065 contain Ca-siderite and Mg-siderite respectively, whereas sample 25068 contains a range of Mg- to high-Mg-siderites. Calcian siderite grains are generally small (5-10 ,um) and are mainly observed in the top section of borehole NSD 17, 50-60 m (Figure 2). The typical composition is CO,, with some variability in Fe o.82Mg,.,,Cao.osMn,.,7 manganese content. The lower section of Unit H, borehole NSD 17, appears to be mineralogically comparable with Rundle and Stuart oil shales, and this is confirmed by the rare occurrence of Ca-siderite and the predominance of Mg- and high-Mg-siderites. Magnesian and high-Mgsiderites are somewhat larger (5-20 pm) in size and are widely dispersed in the lamosite matrix. The calcium content of the siderite grains is sensibly constant throughout (Figure .3), so that magnesium replacement of iron is the dominant variable within Unit H. The compositional range is 13-53 wt% MgCO, and there is a trend to higher Mg contents with increasing stratigraphic depth. The siderite compositions range from Fe o.71Mgo.l,Ca,.,,Mn,.,,CO, to FeO.,,MgO.&aO.,s Mn 0,04COLI in Subunit Ha. Thus the siderites in Nagoorin South, Unit H, are essentially comparable with those observed in various sections of the Condor, Rundle, Stuart, and Yaamba deposits’. However, the siderite composition differs in the stratigraphically deeper horizons of the Nagoorin deposit5 and averages Fe o.94Mg,.,,Ca,.ozMn,.,2 CO,. Siderite composition affects decomposition during retorting’, and this aspect

Figure 4

Magnesium

and iron profiles across

a Mg-sidcrite

grain

is considered elsewhere8 for the Nagoorin South and Nagoorin deposits. Samples from the basal 6 m of Unit H contain both Mg- and high-Mg-siderites. The two Mg-siderites generally occur as separate grains, and both types are observed in oil shale particles 1 mm in size and have therefore formed at the same stratigraphic level. However, several mixed grains occur in which central areas of high-Mg-siderite are surrounded by Mg-siderite. This may represent secondary growth of Mg-siderite on an earlier generation of high-Mg-siderite crystals or a change in magnesium availability in the pore liquors as crystallization proceeded. The latter explanation is favoured by observations of gradually decreasing magnesium (and increasing iron) concentrations towards the edge of rhombohedral and mixed grains of high-Mgsiderite. This is clearly indicated by the X-ray intensity line scans shown in Figure 4. This suggests continuous growth with decreased magnesium availability relative to iron in the pore liquors as crystallization proceeded in early diagenesis. X-ray diffraction patterns did not resolve separate peaks for Mg- and high-Mg-siderite, further supporting a continuous range of Mg-siderite compositions rather than overgrowths of Mg-siderite on high-Mg-siderite. CONCLUSIONS 1. The general mineralogy and geochemistry observed for Unit H of the Nagoorin South deposit are comparable with those of Rundle and Stuart oil shales. Mineralogical residences for major elements are the same, and differences with stratigraphic depth relate mostly to changes in the relative amounts of the carbonate minerals. Minerals identified as significant in processing of Nagoorin South oil shales include smectite, kaolinite, quartz, calcite, Ca-siderite, Mgsiderite, opaline silica and pyrite. 2. Four subunits with different mineralogy have been characterized with increasing depth in borehole NSD 17: Subunit Subunit Subunit Subunit

Hd: Hc: Hb: Ha:

carbonaceous shale lamosite containing Ca-siderite carbonaceous shale, negligible carbonates lamosite containing calcite, Mg-siderite and sometimes dolomite.

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Stratigraphy

and mineralogy

of Unit H, Nagoorin

South

Ranges of chemical and mineralogical compositions determined for three subunits of Unit H show that variations in kerogen and cannel coal contents and in the carbonate minerals, calcite, ferroan dolomite, Ca-siderite and Mg-siderite are particularly significant in relation to decomposition during retorting and combustion. In the Nagoorin graben, four types of siderite have been recognized, depending upon the unit and conditions during deposition and diagenesis: siderite, Ca-siderite, Mg-siderite and high-Mg-siderite. Iron carbonate content varies considerably and is especially concentrated in thin layers of sideritic-carbonaceous shale in Unit C and in the lamosites of Unit H. Similar siderites have been observed in other Tertiary oil shales of eastern Queensland. Typical chemical formulae were determined for each type. Siderite mineralogy is particularly complex for Subunit Ha, Nagoorin South deposit. The magnesium content of Mg-siderites increases with stratigraphic depth, and Mg-siderite and high-Mg-siderite coexist within the basal section. Mg-siderite and high-Mg-siderites generally occur as separate grains, but occasionally mixed grains comprising Mg-siderite surrounding the high-Mg-siderite are observed. High magnesium concentrations in central regions are gradually reduced towards the extremities and there is an inverse relation between magnesium and iron concentrations.

868

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oil shale deposit: J. H. Patterson

and A. W. Lindner

Microprobe and XRD analyses suggest that this may reflect changes in magnesium availability as crystallization proceeded in early diagenesis. ACKNOWLEDGEMENTS This work was partly funded by a collaborative research agreement with Southern Pacific Petroleum NL and Central Pacific Minerals NL, supported by NERDD Project 1282. The authors thank the following CSIRO colleagues for analytical work: P. Udaja, A. K. Hutchings, S. B. Weir, A. Martinez and K. M. Kinealy. Thanks are also due to Y. Farrar, G. D. McOrist and J. J. Fardy of ANSTO for the INAA analyses, and to J. G. Allcock of CPM for the figures. REFERENCES Gannon, A. J. and Wright, B. C. In Proc. Fifth Australian Workshop on Oil Shale, Lucas Heights, 1989, p. 3 Dear, J. F., McKellar, R. G. and Tucker, R. M. Geological Survey of Queensland, Report No. 46, 1971 Henstridge, D. A., Ivanac, J. F., Lindner, A. W. and O’Dea, T. R. In Abstract, Seventh Australian Geological Convention, 1984, p, 237 Henstridge, D. A. and Hutton, A. C. Fuel 1987, 66, 301 Patterson, J. H., Hutton, A. C. and Henstridge, D. A. Fuel 1988, 67, 1357 Patterson, J. H. and Henstridge, D. A. Chem. Geol. 1990, 82, 319 Patterson, J. H., Hurst, H. J. and Levy, J. H. Fuel 1991, 70, 1252 Hurst, H. J., Levy, J. H. and Patterson, J. H. Fuel, 1993, 72, 885