PergamonOrg. Geochem. Vol. 29, No. 1-3, pp. 713-734, 1998 ~) 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain P I h S0146-6380(98)00132-6 0146-6380/98/$- see front matter Oi l -source correlations as a tool in identifying the petroleum syst ems of the southern Taroom Trough, Austral i a KHAL E D R. AL- AROURU' - ' t , DAVI D M. MC KI R DY' * and C HR I S T OP HE R J. B OR E HAM 3 'Organic Geochemistry in Basin Analysis Group, Department of Geology and Geophysics, University of Adelaide, Adelaide SA 5005, Australia, ~National Centre for Petroleum Geology and Geophysics, University of Adelaide, Adelaide SA 5005, Australia and 3Australian Geological Survey Organisation, GPO Box 378, Canberra ACT 2601, Australia Abstract--A geochemical study of crude oils and samples of various Permian, Triassic and Jurassic organic-rich rock units from the southern Taroom Trough was undertaken to test the prevailing Permian-source hypothesis for the petroleum reserves of the Bowen/Surat Basin. Seventy three core and cuttings samples were screened using organic petrography, total organic carbon analysis, Rock-Eval pyrolysis, and solvent extraction. Saturated and aromatic hydrocarbon fractions of selected source rocks, kerogen hydrous pyrolysates, and thirteen oils were then analysed by gas chromatography, gas chromatography mass spectrometry and isotope ratio mass spectrometry, for the purpose of oil-to- source correlation. On the basis of their bulk carbon isotopic compositions and terpane, sterane and aromatic biomarker signatures, two oil families were identified. Those oils sourced by the Snake Creek Mudstone and reservoired in the Showgrounds Sandstone (both of Middle Triassic age) at Roswin North and Rednook are assigned to the "Snake Creek-Showgrounds" petroleum system. The remaining oils belong to the "Blackwater-Precipice" system. These originated in the Late Permian coal measures of the Blackwater Group but are produced from the Precipice Sandstone and other reservoir rocks of Permian to Jurassic age along the southeastern and southwestern margins of the trough. © 1998 Elsevier Science Ltd. All rights reserved Ke y wo r d s - - o i l - s o u r c e correlation, terpanes, steranes, carbon isotopes, petroleum system, Taroom Trough INTRODUCTION The first commer ci al hydr oc a r bon r esour ces in Aus t r al i a were di scover ed in 1900 at Ro ma in t he Sur at Basi n ( El l i ot t and Br own, 1989). The Sur at and Bowen Basi ns (Fi g. l a) t oget her compr i se one o f Aus t r al i a' s mor e pr ospect i ve ons hor e pet r ol eum pr ovi nces and bot h ar e i mpor t a nt pr oducer s of oi l and gas. The Bowen Basi n al so host s l arge reserves o f st eami ng coal . Over one hundr e d oil and gas dis- cover i es have been ma de in t hese basins, mos t of t hem l ocat ed al ong t he east er n and west er n mar gi ns of t he sout her n Ta r o o m Tr ough (Fi g. l b). The ori - gin o f t hese pet r ol eum occur r ences has been of i nt er est to l ocal expl or er s f or mor e t han 30 years, and is still bei ng debat ed. The ma j or effect i ve sour ce r ock uni t s in t he Bowen/ Sur at pr ovi nce are consi der ed by ma ny researchers t o be conf i ned t o t he Pe r mi a n sect i on (Fi g. l c, Thoma s e t a l . , 1982; Ha wki ns e t al . , 1992; Bor eham, 1995; Car mi chael and Bor eham, 1997). I n *To whom correspondence should be addressed. Tel.: +61-8-8303-5378; Fax: +61-8-8303-4347; E-mail:
[email protected]. tPresent address: School of Earth Sciences, Macquarie University, NSW 2109, Australia. t he l at t er paper , t he Mi ddl e Tr i assi c Mo o l a y e mb e r Fo r ma t i o n is i dent i fi ed as a pot ent i al source r ock f or pet r ol eum, and based on its l i ght c a r bon i sot o- pi c compos i t i on, t he Re d n o o k oil has been r el at ed t o a Tri assi c sour ce ( Bor eham, 1995). However , t he l acust r i ne Snake Cr eek Muds t one , a me mbe r of t he Mool a ye mbe r For ma t i on, is bet t er known as a l at er al l y- cont i nuous seal whi ch effect i vel y f or ms a bar r i er t o ver t i cal mi gr at i on f r om est abl i shed Pe r mi a n sour ce r ocks ( Gol i n and Smyt h, 1986). Hy d r o c a r b o n accumul at i ons in t he over l yi ng Tr i assi c and Jur assi c f or mat i ons suggest t he exist- ence of a sour ce ( or sources) ot her t han t he Per mi an, as l ong l at er al and ver t i cal mi gr at i on pat hs have t r adi t i onal l y been consi der ed unl i kel y ( Thoma s e t al . , 1982). The Snake Cr eek Muds t one is t he mos t ma t ur e of t he possi bl e al t er nat i ve sources, al t hough its t r ue hydr oc a r bon- ge ne r a t i ng pot ent i al has not yet been adequat el y eval uat ed. Thi s paper r epor t s t he resul t s o f a geochemi cal st udy desi gned t o assess t he r el at i ve cont r i but i ons of Tr i assi c and Pe r mi a n sour ce r ocks in t he s out her n Ta r o o m Tr ough t o t he oil reserves of t he Bowe n/ Sur a t Basin. I n par t i cul ar , a det ai l ed i nvest i - gat i on of t he Tr i assi c Snake Cr eek Muds t one and t he Per mi an Back Cr eek and Bl ackwat er Gr oups 713 714 Khal ed R. Al -Arouri e t al . u! sel 3 l e J n S - - - ~ 7 I - - u ! s e 8 ueMot 3 0 0 A . , u / L L / L L / L / L i l L ~ L ~ L / L - - 0 ~ o o • • • • 0 ~ ~ ~ z ~ z ~ O ~ - ~ ~ 1 ~ ~ ~ o ~ ~ O ~ z ~ ° 1 7 o = ~ I~ t i l e 0 ~ ( ~ W 0 ~ o ~ o ~ ~ i - ° > L L O eq o4 ~1 ¢q c~l (%1 cq (D sno3ov_L: ~l : l O I Ol SSv~I nr OISSVII:::I.L NVllall:l:=ld .~o%~ O I O Z O S : I I A I O I O Z O : I V - I V d O-IS I ON, MI ONOO9 ~ P 7 7 ~ ~ ~ =+ ~ -~o~ ~ - o ~ + ~ + ~ o o ~ + (3 + "1- u~ . _- - - - - - - - ] i ~ ~ ~ ~ + - + e ~ ~ . ~ - ~ - - + + ~ .~...~ + ~ =..~ + ' r , , . . . . + • ~ o , ~ , ~ o o . , o , ~ * ' = ~ + ~ _~ ~ ~ + ' : , . ' - ' . ~ " .... • / • ~ - - ~ , ~ ' Y / ' - ~ - : ~ " i =~ ; . . . . . . . . V " : v/e .... ' / ~ . . . . . . . ~ o ~ 5 . P ~ " . ~ ' ? ~" ; e ~ ~. ~..------..-:"~,,~ ~ , , - ' ~ " ~ ~" % i - - - - - w , - " • " ~ ~ =~ . . . . . F Y, - - + .: .. :' o ~ , . x ~ . ~ - - . 9 = z o " . . . . . . ~ ~ ' ~ I~ " ° ° ' r,, ~ ~ ~ " ~ i . . . " ~ ~ ~ : I -- • I ~ / ) / + - ; I r ~ ~ r a ~ n--l - i E I E : D~N~SON Z Z Om r r LU u~Z ( # ) l i d \ : > ~,£11 , e! ,.= # = 0J O O .=..~ = 0 = ~J ,-4 ~b L~ Petroleum systems of the southern Taroom Trough, Australia 715 was under t aken, based on organi c pet r ogr aphy, t ot al organi c car bon analysis, Rock- Eval pyrolysis, sol vent ext ract i on and bi omar ker anal yses of satu- r at ed and ar omat i c hydr ocar bons in represent at i ve sampl es from 43 expl or at i on wells (Fig. l b). Sealed- t ube hydr ous pyrol ysi s was also carri ed out on kerogens i sol at ed from end-member Snake Creek or gani c facies and t he resulting pyr ol ysat es were compar ed with the ext ract s and oils in terms of t hei r bi omar ker and car bon i sot opi c composi t i ons. Thi rt een pet rol eums were sampl ed and anal ysed chemi cal l y and i sot opi cal l y for the pur pose of oil- source correl at i on. The overall ai m of this st udy was t o expl ai n the observed di st r i but i on of oil and gas t hr oughout the Tar oom Tr ough in the cont ext of pet r ol eum systems, with a view to del i neat i ng t hose areas most prospect i ve for furt her discoveries. GEOLOGICAL SETTING The Tar oom Tr ough was i ni t i at ed as a forel and basi n duri ng the l at est Car boni f er ous or Earl y Per mi an in response t o a compressive force accom- panyi ng subduct i on al ong Aust r al i a' s east ern conti- nent al mar gi n ( Mi ddl et on and Hunt , 1989). The t r ough is the east ern depocent re of the Bowen Basin, and is bounded by the Burunga- Goondi wi ndi Faul t System in the east and the Wal get t - Roma Shel f in the west (Fig. l a and b). The thickness of its Permo-Tri assi c sedi ment ary succession exceeds 9 km al ong its N- S axis in the cent ral par t of the sout hern Tar oom Tr ough (Tot t erdel l e t a L, 1992). The sequence (Fig. l c) compri ses shallow mar i ne and t errest ri al sediments, i ncl udi ng t hi ck deposi t s of bl ack coal. The sout hern por t i on of the Tar oom Tr ough (between l at i t udes 25°S and 29°S) covers an ar ea of ca. 50,000 km 2 and is unconf or mabl y overl ai n by t he Jur assi c- Cret aceous Surat Basin succession. The Back Creek Gr oup section (~3 km thick) compri ses shallow mar i ne t o paral i c car bonat es, siliciclastic and vol ca- niclastic sediments ( Fi el di ng e t al . , 1990). The over- l yi ng Gyr anda For mat i on was deposi t ed duri ng a mar i ne transgression. The ensuing regression l ed to deposi t i on of extensive coal and car bonaceous facies of the Bl ackwat er Gr oup in al l uvi al plain, del t ai c and paral i c settings (Tot t erdel l e t al . , 1992). Deposi t i on of Earl y Triassic, fine-grained, red beds ( Rewan Gr oup) was fol l owed by about 5 km of Mi ddl e Triassic sedi ment s compri si ng the Showgr ounds Sandst one, Snake Creek Mudst one and Mool ayember For mat i on which accumul at ed in fluvial, lacustrine and fluviodeltaic envi ronment s, respectively. A pr edomi nant l y freshwat er lacustrine deposi t i onal setting is i nferred for the Snake Creek Mudst one on the basi s of its lithofacies, sedimen- t ar y structures, mi crofl ora and organi c facies (Hawki ns e t al . , 1992; A1-Arouri, 1996). Af t er maj or Lat e Triassic uplift and erosi on, renewed subsi dence led to the accumul at i on of up t o 2.7 km of fluvio-lacustrine and shallow mar i ne sedi ment s in the overl yi ng Surat Basin duri ng the Jurassi c and Ear l y Cret aceous (Fig. lc; Thomas e t aL, 1982; El l i ot t , 1993). SAMPLES AND METHODS Seventy t hree core and cuttings sampl e s of pot en- t i al source r ock (mudst one, siltstone, shale and coal), rangi ng in age from Earl y Per mi an to Mi ddl e Jurassic, were chosen from 43 expl or at i on wells l ocat ed t hr oughout the sout hern Tar oom Tr ough (Fig. l b and c). Par t i cul ar at t ent i on was pai d to the Snake Creek Mudst one which was sampl ed in 40 wells. Twelve oils and one condensat e f r om reser- voi rs of Per mi an t o Jurassi c age in the Sur at and Bowen Basins were selected for the pur pose of oil- source cor r el at i on (Fig. l b and c). Al l r ock sampl es were subj ect ed to screening analyses, i ncl udi ng de- t er mi nat i on of t ot al organi c car bon (TOC) cont ent usi ng a Leco car bon analyser, and Rock- Eval pyrol - ysis using a Gi r del I FP- Fi na Mar k 2 i nst rument ( Model 59). Pol i shed sections mount ed in aral di t e resin were exami ned under reflected white light and UV- exci t at i on using a Lei t z Or t hol ux I I mi croscope fitted with oil i mmersi on objectives. Macer al analysis was per f or med in accordance wi t h the Aust r al i an St andar d AS 2856 ( St andar ds Associ at i on of Aust ral i a, 1986). To furt her i l l ust rat e the nat ur e of the organi c mat t er in some samples, a Philips 1000 scanni ng el ect ron mi croscope was utilised. Bitumen, or ext ract abl e organi c mat t er (EOM), was ext ract ed f r om powder ed r ock sampl es (up to 50 g) wi t h an azeot r opi c mi xt ure of di chl oro- met hane ( DCM) and met hanol (93:7) in Soxhlet ap- par at us for 72 h. Ext ract s and oils were deasphal t ed and t hen f r act i onat ed i nt o sat ur at ed hydr ocar bons, ar omat i c hydr ocar bons and NSO compounds by open- col umn liquid chr omat ogr aphy on silica gel/ al umi na, eluting successively with pet r ol eum ether, pet r ol eum et her / DCM (40:60) and DCM/ met hanol (35:65). Sat ur at ed hydr ocar bons were furt her separ- at ed i nt o n-al kanes (silicalite-adduct; SA) and branched-cycl i c hydr ocar bons (silicalite non- adduct ; SNA) using silicalite powder (West e t al . , 1990). Gas chr omat ogr aphy (GC) was carri ed out on the SA fract i ons of bot h the ext ract s and the kerogen pyrol ysat es. A Var i an 3400 gas chr omat ogr aph fitted wi t h an OV101 fused silica col umn ( 2 5 mx 0 . 2 5 mm i.d.) was used. The oven was pr ogr ammed from 60 to 300°C at 4°C/ mi n and held at the final t emper at ur e unt i l all compounds had eluted. Hydr ogen was used as the car r i er gas. The SNA fract i ons were anal ysed in the mul t i pl e reac- t i on moni t or i ng ( MRM) mode usi ng a VG Anal yt i cal Aut oSpec- Ul t i maQ mass spect romet er fitted wi t h a Car l o Er ba 8060 gas chr omat ogr aph 716 Kh a l e d R. Al - Ar o u r i e t al . and an A200S Aut osampl er , and cont rol l ed by a VG OPUS dat a system. The gas chr omat ogr aph was equi pped wi t h a 50 m x 0.2 mm i.d. HP Ultra-1 crossl i nked methylsilicone capi l l ary column. Samples (in hexane) were injected (splitless mode for 1 min) at 28°C. The oven was hel d at 50°C for 2 mi n before its t emper at ur e was raised to 150°C at 15°C/min, then to 310°C at 3°C/min, and held iso- t hermal for 28 min. The carri er gas was hydrogen. I ons were generat ed by el ect ron i mpact with an accel erat i ng vol t age of 8 kV. The t ot al analysis time was 90 mi n with a cycle time of 1.82 s. Ker ogen concent rat es were obt ai ned from six represent at i ve sampl es of the Snake Creek Mudst one. Sol vent -ext ract ed r ock powder was exhaust i vel y macer at ed with hydrochl ori c and hydrofl uori c aci d t o remove the mi neral mat t er. This was followed by cent ri fugat i on and/ or fl ot at i on in DCM to concent rat e the kerogen. Seal ed-t ube hydr ous pyrol ysi s of the kerogen concent rat e ( ~50mg) was carri ed out accordi ng to the pr ocedur e of Bor eham and Powell (1991). The pyrol ysat es were fract i onat ed i nt o sat ur at ed hydro- carbons, ar omat i c hydr ocar bons and NSO com- pounds in the same manner as the extracts. Sat urat es were furt her separat ed i nt o SA and SNA fractions. The oil and ext ract fract i ons (i.e. sat urat ed hydr ocar bons, ar omat i c hydr ocar bons, and NSO compounds) , kerogen concent rat es and kerogen pyr ol ysat e fract i ons (sat urat es and aromat i cs) were anal ysed for t hei r stable car bon i sot opi c compo- sition. Samples ( 2mg) were sealed with copper oxi de in evacuat ed quart z tubes and heated at 950°C. The resulting CO2 was anal ysed in a Eur opa Scientific i sot ope- r at i o mass spect romet er. Results are expressed as per mil relative to the PeeDee Belemnite (PDB) st andar d. RESULTS AND DI SCUSSI ON Source richness, kerogen type and maturity Back Creek Group. Earl y Per mi an mudst ones of the Back Creek Gr oup, exami ned in four wells ( Cockat oo Cr eek- l , Burunga-1, Meeleebee-1 in the nor t her n par t of the st udy area, and Flinton-1 in the south), have fai r to very good organi c richness (TOC = 0. 7- 4%) but a poor hydr ocar bon yield ( < 500 ppm) and, at best, are onl y a fai r oil source (Fig. 2a). The Back Creek Gr oup cont ai ns essen- tially gas-prone Type I I I / I V kerogen (Fig. 2e) which is mat ur e in the sout h ( Ro=0. 87%) and generally over mat ur e in the nor t h (Tmax=460-505°C, Ro = 1. 5-2. 3%). These high mat ur at i on levels have obscured the ori gi nal maceral composi t i on, and resul t ed in n-al kane di st ri but i ons maxi mi si ng in the l ower mol ecul ar weight range. Therefore, all its lip- tinites will have been convert ed t o rank-mi cri ni t e (Fig. 3b). Low hydrogen index values ( HI = 3-44, Fig. 2e), poor genetic pot ent i al (S1 + $2 < 2 k g hydr ocar bons/ t onne) , mi ni mal ext ract abl e C15 + hy- dr ocar bon yields (2-11 mg/ g TOC) and low pr i st ane/ n- hept adecane (<0.6) and phyt ane/ n-oct a- decane (_<0.2) values are all consi st ent with overma- t ure organi c mat t er of low reactivity. These dat a i mpl y t hat the bul k of the Back Creek section is within the gas window; and t hat liquid hydrocar- bons were capabl e of being generat ed from its kero- gen and have al r eady been expelled from these rocks. At Scot i a- l , j ust nor t h of Burunga-1 (Fig. l b), gas is bleeding from the Gyr anda For - mat i on, whereas oil traces were observed in fi'ac- tures within tuffaceous sandst one of the Back Creek Gr oup, and in fract ured basement . A mari ne influ- ence in the l ower par t of t he Back Creek section is i ndi cat ed by a resi dual even-carbon-number pre- domi nance at Ci6, Cj s and C20 in its n-alkanes and by pr i st ane/ phyt ane values < 1 (e.g. sample from Cockat oo Creek-1 in Fig. 5). Blackwater Group. Lat e Permi an coals and car- bonaceous shales of the Bl ackwat er Gr oup have TOC and Ci s~ hydr ocar bon cont ent s characteristic of fair to good oi l -source rocks (Fig. 2b). Their gen- etic pot ent i al (SI + $2 = 12-193 kg hydr ocar bons/ tonne) and hydrogen index values (HI = 120 290, Fig. 2e) reveal the presence of bet t er quality Type I1/1II kerogen with some abi l i t y to generate oil and gas. Mat ur i t y ranges from initially mat ure in the sout h of the st udy area (Tm,,x=431-443°C; P I - 0.08-0.12) and al ong the flanks of the Tar oom Tr ough to within the oil window at Bur- unga-1 (T, ..... = 4 4 4 461"C; PI = 0.20 0.25). Fur t her nort h at Gl enhaught on-1 this uni t has entered the wet gas wi ndow (Ro = 1.6%, Thomas et al., 1982; Hawki ns et al., 1992). These source rocks cont ai n abundant liptinite (mai nl y cutinite, resinite, l i pt odet ri ni t e and rare sporinite) in which fluor- escence is subdued, pr obabl y due t o expulsion of much of their hydr ocar bons (Fig. 3c). Snake Creek Muds'tone. Wi t h the exception of siltstone intervals in three wells (TOC_< 0.7%), all 41 samples of this Mi ddl e Triassic lacustrine uni t have good to very good organi c richness ( TOC = 1- 4%) and hydr ocar bon contents indica- tive of a fair to very good source for oil (Fig. 2c). It cont ai ns i ni t i al l y mat ur e (T, ..... =429 440°C) Type I l l or Type l l / I I l kerogen (Fig. 2f) with a fair to good pet rol eum pot ent i al (Sl + $2 = 4 6 kg hydr ocar bons/ t onne) . Vitrinite reflectance measure- ment s indicate t hat this unit has passed the gener- at i on t hreshol d for Type I I I kerogen ( Ro=0. 7%) over the axi al par t of the t rough, with the highest reflectance recorded at Inglestone-1 (Fig. 4a). This maxi mum mat ur at i on level cor r esponds to the earl y mat ur e stage of hydr ocar bon generat i on for Type I I / I I I kerogen. A hydr ogen i ndex- cont our map for the Snake Creek Mudst one in the sout hern Tar oom Tr ough (Fig. 4b) shows t h a t its highest hydr o- Petroleum systems of the southern Taroom Trough, Australia 717 10000, E ~ooo: IO00- T" ~oo so~ 1 0 o. 1 / / / GAS LEAI~ J Y SOURCE ~=A~ROiL > , / < /4 L ~ N - BARREN O ¢ B a c k C r e e k G r o u p o.s ~. oTO C %s.o ~o.o ~o ~oo (b) 10 000! ~ ~o l O 0."1 B l a c k w a t e r G r o u p • = . . . . = • - - i . . . . , - - - i . . . . O.B 1. 0TO C %5.0 ,0.0 so 100 D O - , O 0 ' 00- ~2 0 0 5 0 1 1 0 o. 1 (c) • S n a k e C r e e k M u d s t o n e o.5 1. oTO C o/oS.o l o. o so 1oo C O - 0 0 - ~)0. ;00. 00 50 10 0.1 ; a " ° ; n C T ' o r % U r ° " O Pr eci pi ce Sa n d s t o n e o.s ,.oTO C %s.o I o. o so ,oo 1 o o o 1 o o o , o o o - ' \ " ' \ 8 o 0 - ,- L a t e P e r m i a n c o a l a o o - ,~" • f a c i e s ( i n c l . B C M " ,/\ a n d G y r a n d a Frn) x 7oo- I I - - ,' ~ x 7 0 0 - I I , " - ~ ~ ; ~ O E a r l y P e r m i a n ~ S ~ = s o o " \ " / ~ a c ~ C r e e k ~ r o u ~ - - = e o o - / ~ s o o - 5 0 0 - ~ ' ~ SOOi t ~ ~ ~ l T z . ~ . ,~ " , ~ ~ ' ~ J u r a ~1:} " . . . . = ~ o o WCM ~ / , : 3 0 0 - " I _a e P e r m i a n ? " , " T r i a s s i c ' I : . : w ~ I . . ' ~ ' " " S C i ~ , " 2oo- 2 0 o - ', .'.~: i ||| ~ , . I ~ . . . . P . . . . . . 100--~ ||| . . . . _ " , , = , ~ . . ~ % ~ , ~ - - - ~ , . - - ~ - - ~ = - - : . ~ . , ~ . . . . - , , , = , - ' , T " , , 380 430 480 S30 580 3 8 0 4 3 0 Tm~× (°C) (f) J u r a s s i c W a l l o o n [3 C o a l M e a s u r e s ( W C M ) T r i a s s i c S n a k e • C r e e k U u d s t o n e ( S C M ) , Fig. 2. Source-rock richness plots for (a) Back Creek Group, (b) Blackwater Group (including Gyranda Formation), (c) Snake Creek Mudstone, and (d) various Jurassic formations. Panels (e) and (f) show the HI versus Tma× plots for the Permian and Triassic-Jurassic rock samples, respectively. ' 4Ao ' s~o ' T m a x ( ° C ) 718 Khaled R. AI-Arouri et al. car bon- gener at i ng pot ent i al ( HI > 200) is devel oped in the sout hwest ern par t of the t rough in t he vicin- ity of Bor ah Creek-3 and -4, Tinker-2, Inglestone-1 and Fl i nt on-1 (see also Smyt h and Mast al erz, 1991). Out si de this "sweet spot " the Snake Creek Mudst one has hydrogen indices in t he range HI = 39-182. Pet r ogr aphi c exami nat i on revealed t hat t hose samples with t he highest hydr ogen indi- ces cont ai n l i pt i ni t e-ri ch di spersed organi c mat t er which bears a close resembl ance to Type I I kerogen. The abundant liptinite compri ses mainly l amal gi ni t e (Fig. 3d), cutinite and l i pt odet ri ni t e, with rare to common spori ni t e and resinite (Fig, 3e). Di nofl agel - lares, acri t archs and Bo t r y o c o c c u s - l i k e al gae are oc- casi onal l y recogni sabl e within a highly fluorescing gr oundmass (A1-Arouri, 1996). J ur as s i c uni t s. Coal s and car bonaceous mud- stones from vari ous Jurassi c units (Evergreen For mat i on, Precipice Sandst one, and Wal l oon Coal Measures) were shown t o have fair oil pot en- tials (Fig. 2d). However, they are i mmat ur e (Tm~,x -< 430°C) t hr oughout the Surat Basin (Fig. 2f; see also Thomas e t al . , 1982) and t herefore cannot be effective source rocks for any of its oils. Pe t r ogr aphi c evi dence (~i c oi l gener at i on in t he S n a k e Cr e e k Mu d s t o n e Gi ven its kerogen t ype and observed mat ur at i on levels (Fig. 4), the Snake Creek Mudst one may be expected to be actively generat i ng pet rol eum within a relatively small area of the sout hern Tar oom Trough. Certainly, some of its liptinites and per- hydr ous vitrinites have passed their nomi nal oil generat i on t hreshol ds (Ro = 0.45% for cert ai n kinds of resinite, 0. 5% for desmocol l i ni t e and bi t umi ni t e, and 0. 6% for spori ni t e and cutinite: Cook, 1982; Teichmfiller and Dur and, 1983). Even at this low mat ur i t y level, kerogen rich in such t hermal l y labile maceral s can generate significant amount s of light oil and condensat e (Snowdon and Powell, 1982). Mi cr oscopi c observat i ons (e.g. Fig. 3f) confirm t hat hydr ocar bon generat i on from the Snake Creek or- gani c mat t er has indeed st art ed, with the best gener- at i ng pot ent i al devel oped on the sout hwest ern edge of the t rough. This is i ndi cat ed by intense yellow- fluorescing liptinites and brown-fl uoresci ng desmo- collinite and telocollinite, with many phyt ocl ast s appar ent l y expelling t hei r hydr ocar bons i nt o the sur r oundi ng medi um. Vi t ri ni t e fluoresces when it is hydrogen-ri ch, i ndi cat i ng good oi l -generat i ng abi l - ity. Al t ernat i vel y, vitrinite may fluoresce when its mi cr opor es are i mpr egnat ed by liquid hydr ocar bons generat ed from associ at ed liptinites ( Mukhopadhyay and Hat cher, 1994). Extensive mi cri ni t i sat i on in a few sampl e s ma y indicate pre- vious hydr ocar bon generat i on (Stach e t al . , 1982). Exsudat i ni t e and/ or oil is present in telocollinite fractures and fusinite cell lumens (Fig. 3f). In many samples, fluorinite is seen dissolving in the mount - ing medi um to form oil haze. Oil haze surrounds cutinite, resinite, fluorinite and alginite, whereas oil dr opl et s (live oil!) have mobi l i sed and mi grat ed i nt o t he sur r oundi ng rock mat ri x. Pe t r o l e u m g e o c h e mi s t r y Dat a on the bul k chemi cal and i sot opi c compo- sition of the oils are present ed in Tabl e 1. Source and mat ur i t y- dependent bi omar ker paramet ers of the oils and source rocks are summari sed in Tabl e 2. Al kane chr omat ogr ams represent at i ve of two differ- ent oil si gnat ures are shown in Fig. 5. Al kane pr o- files of represent at i ve Permi an, Triassic and Jurassic source rocks are also di spl ayed for compari son. Selected mat ur i t y- dependent and source-specific bio- mar ker ratios, based on bot h sat urat ed and aro- mat i c hydr ocar bons, are pl ot t ed in Fig. 6. Ex pul s i on ma t u r i t y . As est i mat ed from t hei r bio- mar ker isomer rat i os (C31 hopane 22S = 57-64%; C3o f l ~/ ~f l hopane < 0.14; T~/Tm =0. 40-1. 32; C29 sterane 20S = 55- 62%: Tabl e 2; Fig. 6a) and calcu- l at ed vitrinite reflectance derived from methyl- phenant hrene index ( MPI , Radke and Welte, 1983) measurement s (Table 1), most of the oils were expelled from t hei r source rocks earl y in the con- vent i onal oi l -condensat e window, and at reasonabl y si mi l ar mat ur i t y levels (Re = 0. 62-0. 75%). The Red- nook- 1 crude appear s to be a somewhat l at er expul- sion pr oduct ( Rc =l . 04%) , consistent with its descri pt i on as a condensat e. Wi t h this one excep- tion, the cal cul at ed vitrinite reflectance values obt ai ned here are within the range report ed by Boreham (1995) for a l arger set of Bowen/ Surat oils. Oi l Jami l i es . Two different families of crude oils can be recognised. Those assigned to Fa mi l y 1 are the very light (65c' APi) pet rol eums from Roswin Nor t h- I and Rednook- l , bot h reservoired in the Triassic Showgrounds Sandst one. Each has a non- waxy, uni modal n-al kane profile (maxi mum ar ound n-tridecane, Fig. 5), al t hough their respective pris- Fig. 3. Photomicrographs of dispersed organic matter. Plates a and b (incident white light): texto-ulmi- nite and sporinite, intensely micrinitised and extensively pyritised, in the Back Creek Group, Cockatoo Creek-1 (x625). Plate c (fluorescence mode): phytoplankton and lamalginite in Back Creek Group, Burunga-1 (×1563). Plate d (fluorescence mode): dull-fluorescing resinite in carbonaceous shale facies of the Blackwater Group, Burunga-I (×625). Plate e (fluorescence mode): lamalginite (la) associated with pyrite framboid (py) in Snake Creek Mudstone, Muggleton-1 (x625). Plate f (fluorescence mode): resi- nite (r), sporinite (sp) and cutinite in Snake Creek Mudstone, McGregor-1 (×625). Plate g (white light): exsudatinite (dark grey) filling cell cavities of fusinite in the Snake Creek Mudstone, Tiggrigie Creek-1 (×1563). Plate h (fluorescence mode): same field of view as (g). Fig. 3. Scc caption on p. 718. P e t r o l e u m s ys t e ms o f t he s o u t h e r n T a r o o m Tr o u g h , Au s t r a l i a " - - ' - - ' - N - - - C ) ~dO- I S I { JNI MI O UU~) ~ / ~ -- + ×g ~O ~O _~- + + A V + ~ .I. 0 (5 • . . . . . . . . . . . . . o . 0 . . . . . . . . . . . . . . . . . . . . . . . . . ~+ O. i f) ~ g ° l ! ! i x:i "7 o4 ~ + 2 , ~ 14. ,=J "~ IE uJ ~ " , - nO / ~ / I ol ~0 o o - - I LLI ¢,t) f / Z A V I'~ co o + o + O t r I t l T / / \ / 1 / I . i . . J " r k~ 721 O O em g O E- : ~. ~ . ~ ¢~ , . ~ 09 ~. ~ e~ e~ g , 4 ~b 722 Khaled R. A1-Arouri et al. I I t t ~ e n o t ~ e ~ I I I I I I I I I I I " 7 ooooo 6 ' r . o t ane/ phyt ane rat i os are quite different ( pr / ph = 5.6 and 1.7). Thi s l at t er di screpancy may be largely due to differences in mat ur i t y at the time of pr i mar y mi grat i on. Mos t i mpor t ant l y, the carbon i sot opi c composi t i ons of t hei r sat ur at ed and ar omat i c hydr ocar bon fractions are relatively light (613C~ - 27%0, Tabl e 1). The remai ni ng oils in Tabl e 1 all bel ong to Fa mi l y 2. Mos t are of lower API gravity (40-47 °) t han the mi nor Fami l y l crudes, and have n-al kane di st ri but i ons similar to t hat of the Cogoon River West-1 oil (Fig. 5). Thei r pr i st ane/ phyt ane rat i os are uni forml y high ( pr / ph = 4 6). Al t hough their reservoir format i ons differ, rangi ng in age from Permi an to Jurassic, all unal t ered Fami l y 2 oils are relatively waxy (Hawki ns e t al . , 1992; Boreham, 1995). The Riverslea-1 oil ( not shown in Fig. 5) is clearly bi odegraded, as i ndi cat ed by its l ack of n-alkanes and i soprenoi ds. Nevertheless, its pol y- cyclic bi omar ker s and car bon i sot opi c composi t i on (613C~-24%0, Tabl e 1) prove its close rel at i onshi p to the Fami l y 2 oils, which appear to possess a charact eri st i cal l y heavy i sot opi c signature (see next section). Oils from bot h families are charact eri sed by high sat ur at ed/ ar omat i c hydr ocar bon rat i os (sat / ar om = 3 23, Tabl e 1) t oget her with high pri st ane/ phyt ane r at i os (pr/ ph = 4-6), typical of oils derived from vascul ar pl ant remai ns deposi t ed in a pre- domi nant l y oxic envi ronment (Powell and McKi r dy, 1973; Cl ayt on, 1994). The t errest ri al nat ure of the oils is reflected also in their sterane di st ri but i ons ( C 2 9 > C 2 8 ~ _ _ C 2 7 steranes: Tabl e 2; Fig. 6b and c), and moder at e to high hopane/ st er- ane rat i os (hop/ st er = 1.2 7). Lower values for the l at t er r at i o ( h o p / s t e r - 0.6 0.7) in the Wi l ga-2 (Fig. 7a) and Washpool -1 oils are at t ri but ed to preferent i al bi odegr adat i on of hopanes. Al t hough steranes are known to under go bi odegr adat i on before hopanes, the reverse does also occur; and where this has happened (as in the case of these two oils) the affected hopanes are convert ed to 25- nor hopanes (Peters and Mol dowan, 1993). Thus, in s i t u bact eri al al t erat i on of these oils has enhanced the relative abundances of the mor e resistant hopa- noid compounds (viz. 25-norhopanes, 18c~(H)-30- nor neohopane and met hyl hopanes), whereas the steranes appear to be unaffected. The absence of the mari ne al gal bi omarker, 24-n-propyl chol est ane, from all the oils examined furt her emphasises their t errest ri al origin. The oils exhibit moder at e to high abundances of di ast eranes relative to steranes (C29 di a/ st er = 1-2, Fig. 7). This composi t i onal feat ure is associ at ed with high relative concent rat i ons of di ahopane (C3o di ah/ hop = 0.1 0.6) and 18e(H)-30-norneohopane (C29Ts/30-NH = 0.1 0.4: Tabl e 2, Fig. 7) i ndi cat i ng deposi t i on of t hei r pr ecur sor t errest ri al organic mat - ter in a clay-rich, oxic suboxi c envi ronment (Peters o o [ - o o = o~ e-i ~a Pe t r ol e um s ys t e ms o f t he s out he r n T a r o o m Tr o ug h, Aus t r a l i a o o o o c o o o o ~ o o ~ o o o o ~ ~ ~ ~ , ~ ~ ~ ~ ~ ~- ~- - ~ ~ ~ , ~ , ~ ~ ' ~ ~ - ~ , 7 ~ , ~ ' ¢ ° ~ " * ~ ' ~ ' ~ , ¢ , ~ ~ ~ - ~ ~ , ~ , ~ - ~ , - ~ - ~ ~ , - ~ , ~ - ~ , . ~ - ~ _.~ _ ~ ~ ~ ~ ~ ~ - = . . . . . . . . . . . - ~ ~ ~ ; 2 2 2 ~ e e o e ~ e e e e e e e u . . . . ~ ~ ~ - - ~ ' . . . . ~ Z , ~ , 7 ~ - ~ ~ " * , , ~ ~ ~ f f - , 9 ~ , 9 ~ : ~ z . _ ~ ' ~ ~ ~ ~ 723 724 X W f f l ._~ .~_ D,- , ~ - 0 i i r , ~ ~ e- X ( M ~ - - r - - m . d J m @ X I l l O o o • ~ ' r " ~ - ~ ,,~ ~ o x ~t Kha l e d R. AI - Ar our i e t al . _~ ×co ~ . O ~ - T ! ! 0 "I b ~ m q ! . . _ W o o D E 0 X ° ' l 1 , i O e - O z e - o g.~_~ " 9 _ 2 ' e - o ~ - O $ ; o O O O , I Z u O < Ld P e t r o l e u m s ys t e ms o f t h e s o u t h e r n T a r o o m T r o u g h , Au s t r a l i a 725 ( a ) 2.0 1 . 5 - 1 . 0 - 0 oo 0. 5- 0.0 0.0 ( c ) C 2 7 s t e r a n e 0 .-" (300 .,°"" o o ,-~ © % , , , , 1.0 2.0 3, 0 4. 0 =_P_P_~O~ ,. ( 1 ~ 2 0 R "-'29 5.0 1 2 1 0 ~ 8 e- ( t " 5 ¢ ° i , 2 ( b ) ._o x O I . I I [ ] c1 o * ~I o - - A l g a l - - ~ ± - - L a n d P l a n t - I < I ! 0.1 1 1 0 1 0 0 C 2 9 / C 2 7 S t e r a n e C 2 8 st er ane BCG calcareous mudstone facies 40 ' BWG carbonaceous ~l., ~ shale facies f " ,~ _. C29 methyl sterane Legend ~ ( d ) ~ ] Snake Creek Mudstone / Btackwater Group / X BT::¢koCr eTe;u~ r°up / X C 2 9 st er ane C2s methyl sterane C30 methyl sterane 0.2 0 ._1 -0.2 -0. 4 - ( e ) I I Lower limit of 1-MP in Jurassic Surat Basin sediments (Boreham, 1995) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . r . . . . . . . . . . . . . . . . . . . . . i [] Lower limit of 1-MP in Jurassic Eromanga sediments (Alexander et al., 1988) I O 0 o ~ ) 4 0 0 11 5 c 1. 0 ,1.0 - ,2.0 - I f ) Lower limit of retene in Jurassic E~ L"O-- 6 O1J D 5 nanga sediments (Alexander el al., 1988) O [] D % 41, ¢, I ' i i ' -0.8 -0.4 0 0 4 0.8 -I .0 -0 5 0.5 L^^ 1,2,5-TMN 1,7-DM P Log uy 1,3,6-TMN X Fi g. 6. P l o t s o f ( a) s t e r a n e m a t u r a t i o n i n d i c a t o r s , ( b) p r i s t a n e / p h y t a n e ve r s us C29/ C27 s t e r a n e s , (c) r e l a - t i ve a b u n d a n c e o f C27 , C28 a n d C29 N0(~ s t e r a n e s , ( d) C28, C29 a n d C3o 4c( - met hyl - 20R s t e r a n e s , a n d (e, f) a r o ma t i c h y d r o c a r b o n s o u r c e p a r a me t e r s ( a f t e r Al e x a n d e r e t a l . , 1988) f o r s o u r c e r o c k s a n d oi l s f r o m t h e s o u t h e r n T a r o o m T r o u g h . 1 . 0 726 Khal ed R. Al-Arouri et al. ( a ) q IZ't To R ' I , , / . 1 1 2 5 i 2 Cm2"7z 3 7 0 . 4 191 8 ,3 0 19.7 0 ~ : " • ' " " " , ' ' : ' z ' ~ - ' ' - T ' T ' t - : ~ " " " ; ' ' ~ . r ~ , . . . . . c ~ 3 8 4 . 4 t 9 1 R'I 2930 2830~ 1 9 .4 , ! . . . . . . . . . . . . ~ , L ~ , . - . . . . , . . . . . . . . . . . . . lo o ] 2 ~ t c2o R ] J ]aB° T m /z 3 9 8 . 4 191 0 2 9 , I ~ , 2 9 s 5 2 .9 . . . . . , . . . . . , . . . . . , . . . . . , . . . . . L . . . . . b . . . . . , . . ~P I C3o 'o o " 0 3 o . / 2 ' 1 2 . 4 1 9 1 ~ ' _ , _ L _ L 0- , . . . . , . . . . . P . . . . . . . . . . . . . . . , . . . . . , " , ~oo" No [ XT I ~ _ C3o °~'~° R I m /z 4 1 6 . 4 191 6 . 4 _ _ .E 'o o 7 2 5 -N H I ~ l I I 0 2 0 R ~ m /z 3 9 8 . 4 177 n " o l ~ ~ . ~ . . . . 2 0 . 6 R I ~ 5 7 ~ z 4 1 2 177 0 - - . = 4 : , . ' , . - , • , , . ' ~ - , . . . . . . . 5 0 : 0 0 5 2 : 0 0 5 4 . ' ~ 5 6 : 0 0 5 8 ~ 0 1 ~ 0 ~ 0 1 : 0 2 : 0 0 1 : 0 4 : 0 0 ' 0 0 7 I p ~ ' s R I [ [ ~ 0 2 7 / II 1 m /z 3 7 2 .4 2 1 7 p~s I ] 1 o o . s R I C2n~,z8 3 8 6 . 4 2 1 7 15.9 0 , ~ , \ : : ~ _ " / . T ' . ' , , : , ' : " ? ' T - , , " ~ f f - , - , , , , . . : ~ , , , . . . . , II II ~ R oos I I 1 0 0 o . 4 8 . ' 0 0 5 0 : 0 0 5 2 ~ 0 5 4 ~ 5 6 : 0 0 5 8 : 0 0 1 : 0 0 : 0 0 1 : 0 2 : 0 0 R e t e n t i o n T i m e ( m i n u t e s ) ( b ) , . A t T ' T R I C2rff7z 3 7 0 . 4 191 X T 1 °°" l _ 1 C ~ 7 z9 .1 R I 3 8 4 . 4 1 9 1 2 9 ,3 0 2 8 ,3 0 I I o . . . . . . . . . ~ . ~ L _ J ~ L r L A , , , lo o ~P R I m /z 3 9 8 - 4 191 R I m /z 4 1 2 . 4 191 100 o . . . . . . . . . . . . . . . . . . . . . V 3,'°: ~ - , . , . . . . . , . . . . . , No R I C3m'°z 4 1 6 . 4 191 C 3 0 C h e ila n th a n e s 19.5 0 , . . . . . , . . . . ~ * . . . . . " i ~ . . . . ~ . . . . . w ' ' q [ c,, o ! . . . . . ~ - . . ~ . , ~ " , - . ~ ! . - . . . . . . . . . . . . . . . 100- I R I XT xTCr~zl 412 . 4 1 7 7 c - .................. ~ ~ , 50:00 52~0 54:00 56:00 5 8 :0 0 1 :0 0 :0 0 I g 2 : 0 0 1 ~ 1 :0 0 ' 0 7 ~aS ~ ' t ° ~ s o . , ff3 ,6 .4 2 1 7 p a $ o . . . . 4 8 : 0 0 5 0 : 0 0 5 2 : 0 0 5 4 : 0 0 5 6 : 0 0 5 8 : 0 0 1 : 0 0 : 0 0 1 : 0 2 : 0 0 R e t e n t i o n T i m e ( m i n u t e s ) Fig. 7. MRM chromat ograms showing the distributions of terpanes and steranes in oils from (a) Wilga- 2, and (b) Roswin North-1. See text, Tabl e 2 and Appendix for explanation of peak labels. XT = cross-talk. Also shown for each trace are the carbon number, metastable t ransi t i on monitored and the scale fact or for relative intensity (larger number = greater relative peak height). Petroleum systems of the southern Taroom Trough, Australia 727 and Mol dowan, 1993). The inferred argi l l aceous charact er of the source rocks is consi st ent with the low 30- nor hopane/ hopane r at i os ( < 1) of the oils; and with t hei r lack, or very low concent rat i on, of 29, 30-bi snorhopane (cf. Cl ar k and Philp, 1989). Ot her specific bact eri al bi omar ker s (including 2e- and 3/~-methyl-17e(H)-hopanes; 28,30- and 29,30- bi snorhopanes; and 2e- and 3/~-methylsteranes, Peters and Mol dowan, 1993) occur ubi qui t ousl y in all the Per mo- Tr i assi c oils and sediments anal ysed and, therefore, are of no help in distinguishing the two oil families. However, differences in t hei r re- spective di st r i but i ons of bicyclic sesquiterpanes and 4e-met hyl st eranes do al l ow for mol ecul ar di scri mi - nat i on between the oils of Fami l i es 1 and 2 (see next section). Oi l - s our ce r o c k cor r el at i on Tri assi c oi l s ( F a mi l y 1 ) . A positive cor r el at i on exists between the C14 16+ bicyclic al kane distri- but i ons of t he Fami l y 1 oils (represent ed by the Roswin Nort h-1 crude) and shales of the Snake Creek Mudst one. Bot h oils and source rocks have abundant Ct4 15 rearranged and regul ar bicyclic al kanes while C16+ homol ogues are nearl y absent (Fig. 8). These oils are also charact eri sed by the near absence of tricyclic t erpanes (including C30), agai n resembl i ng the Snake Creek extracts (A1- Ar our i e t al . , 1995). This may i mpl y generat i on from a less mat ur e source r ock (cf. Peters and Mol - dowan, 1993) and/ or an ent i rel y different organi c facies (cf. Seifert and Mol dowan, 1978). The l at er expl anat i on is favoured here since the oils in this family appear t o have been generat ed at different mat ur i t y levels (Table 1). Al t hough the Roswi n Nort h-1 oil has a distinc- tive sterane di st ri but i on (C27>>C28, Fig. 7), the Fami l y 1 oils cannot be di st i ngui shed from t hose of Fami l y 2 on t he sterane t er nar y pl ot (Fig. 6c). Interestingly, however, the two oil families exhi bi t quite different relative concent rat i ons of C30 met hyl steranes, with the highest abundances being f ound in the Fami l y 1 oils (Fig. 6c). This may be anot her, al bei t subtle, i ndi cat i on of a common source for Fami l y 1 oils. Al so, the Triassic oils have mar kedl y higher abundances of 4a-met hyl st eranes, i ncl udi ng t ent at i vel y identified di nost erane (4c~S/ 3f l S ~ 1; dino/3/~S ~ 0. 1-0. 2: Fig. 9). El evat ed abundances of 4c~-methylsteranes are observed in many Snake Creek Mudst one extracts (Fig. 9), provi di ng furt her t est i mony t o the origin of t he Fami l y 1 oils from this Triassic unit. The presence of di nost erane is consi st ent with ot her evidence of mari ne i ncursi ons to the Snake Creek pal aeol ake al ong the nor t hwest and sout heast margi ns of the sout hern Tar oom Tr ough (e.g. acri t archs at Gl enhaught on- 1, Tiggrigie Cr eek- l , Amool ee-1 and Cabawi n- l : A1- Ar our i , 1996). Pe r mi a n oi l s ( F a mi l y 2 ) . The oils of this family (e.g. Bellbird-1, Fig. 8) are likewise enriched in C14 15 bicyclic al kanes, i ncl udi ng rearranged dri - manes, at the expense of t hei r C~6+ homol ogues, with the l at er being either absent or present as mi nor component s, consi st ent with t hei r origin from coal and/ or shale source rocks (cf. Pal acas e t al . , 1984; Nobl e e t al . , 1986). Vari at i ons in the dri - mane di st ri but i ons of different Per mo- Tr i assi c lithofacies suggest t hat abundant homodr i manes and met hyl homodr i manes are a charact eri st i c fea- ture of cal careous mudst ones deposi t ed under less oxic, and pr obabl y mor e saline, condi t i ons (A1- Ar our i e t al . , 1995). The absence of this class of ses- qui t erpanes in the present suite of Tar oom Tr ough oils and condensat es precl udes the cal careous facies from bei ng t hei r source (Fig. 8). The tricyclic t erpane di st r i but i ons of these oils can be used as evidence for t hei r generat i on from the car bonaceous facies of the Bl ackwat er Gr oup. Bot h cont ai n abundant C~9 and C20 chei l ant hanes which pr edomi nat e over t hei r C21 and C23 homol - ogues and C30 hopane, at t est i ng to t hei r deri vat i on largely from higher pl ant precursors which were deposi t ed in a pr edomi nant l y oxic envi ronment (Peters and Mol dowan, 1993; AI - Ar our i e t al . , 1995). In the cal careous mudst ones of the Back Creek Gr oup the reverse rel at i onshi p occurs (C21 and C23>>CI9 and C20 cheilanthanes). On the basis of the relative abundances of C27, C28 and C29 steranes (Fig. 6c), the Bl ackwat er shales appear to be poor l y cor r el at ed to any of the Fami l y 2 oils. However, enhanced relative abun- dances of t he l ower-mol ecul ar-wei ght steranes in the oils may be due to f r act i onat i on duri ng mi- gr at i on of the oil. Al t ernat i vel y, the e f f e c t i v e source rocks may be mor e mat ur e equi val ent s of the Bl ackwat er car bonaceous shales anal ysed in this study. The l at t er expl anat i on is favoured since a shift to l ower homol ogues is known to be mat ur i t y- rel at ed (e.g. Curiale, 1992). Hi gh concent rat i ons of 2~- and 3fl-methylsteranes rel at i ve to t hei r 4a- met hyl isomers is anot her feat ure t hat charact eri ses most of the oils and Per mi an sediments in t he Bowen/ Surat Basin. Al l the Per mi an oils have low relative abundances of 4~-met hyl st eranes ( 4 ~ S / 3 f l S ~ 0.3), and are devoi d of di nost er ane (Fig. 9). Si mi l ar met hyl st er ane di st r i but i ons are f ound in t he car bonaceous shales of the Lat e Permi an section. Two of t he Fami l y 2 oils show mi nor, but signifi- cant, depar t ur es f r om this Per mi an met hyl st erane pat t ern. The Tayl or - 12A oil exhibits a somewhat hi gher concent r at i on of 4c~-methylsteranes t han the ot her Per mi an oils ( par amet er 27, Tabl e 2); and the Merroombi l e-1 condensat e cont ai ns a t race amount of di nost er ane ( par amet er 30, Tabl e 2). Thus, these two pet r ol eum accumul at i ons appear to have received a smal l pr opor t i on of t hei r hydr ocar bon charge from the Triassic source kitchen. 728 ~o ° o~ IDO Khaled R. A1-Arouri et aL -2 [ O £ N q ~ ~,.,,..Jq I " ~ ~ . ~ ~ [ N ~ _~-~ ~- °~ ! o ~ (%)/q!sue~,ul e^!i~lehl °i ~° ~ ° 0 L ~ 0 . R 0 L Z 0 Petroleum systems of the southern Tar oom Trough, Aust ral i a 8 s e - ~E 0 O O " 0 e- o e" i m E L _ a . 8 i ¢, - o z t - ~ O t r o O 0 D 0 ¢D.. cO CXl e- g) f r o ~._~ O0 ZZ co ee~ Ol ~° ° e~Sg ~ E E E E ¢ = - ~ ' ~ - ~ ~gg~g . ~ II II II II e" c~. em v ~ .0_ co. CO ":" f.O I " a:~ co ~ 0 O 0 _~o mO e - o q~ "lD , - & O O ~oo OO I I ~ t - - ~ r - . o ~Onn O t - OO Z Z 09 '~" e., o [..., 0 "T a . 0 0 ~ ".~, • - o c,.)~ .=..~ sl P~ t g o o 729 730 Khaled R. A1-Arouri e t al . A J ~ q0 R C~ co <> v <> <> <> [ ] 0 R , 0 0 .o • ~ u u ~ O O l " I . . . . . . . . . . . . . . . . . , , . . ..,." : . " ' , , \ i i " ' ) / - , , , , , : ; C < ::, ........... 6- _ _ o o ..... ~ .......... lel.. - - - ~ • ' ~ ~ 0 i 6" .... . _ o ~ .~ I ~ - ~ . . . . 0 E o o ~ e u e ~ q d / e u e ~ s ! J d ~ . ~ ® • , • • • • • 0 0 0 ~ ~ ~ ~ o~ o 0 c~O o ' ~ c O ~ o J o 3 c~ Petroleum systems of the southern Taroom Trough, Australia 731 Ar o ma t i c s our ce p a r a me t e r s . The met hyl phenan- threne and t r i met hyl napht hal ene di st ri but i ons of selected oils from bot h families are consi st ent wi t h the inferred pre-Jurassi c age of their respective source rocks. I n the two ar omat i c source affinity di agrams devised by Al exander e t al. (1988) to dis- tinguish oils of Jurassic and Permi an origin in the Cooper / Er omanga Basin, the Tar oom Tr ough crudes pl ot in (or i mmedi at el y adj acent to) the lower left quadr ant (Fig. 6e and f). Not e t hat , in t erms of its ar omat i c hydr ocar bon signature, the Triassic Fami l y 1 oil from Roswi n Nort h-1 oil (sample 4) is i ndi st i ngui shabl e from the Permi an Fami l y 2 oils. S t a b l e carbon i s ot opi c compos i t i on. Assumi ng the ar omat i c fract i on to be represent at i ve of the whol e oil (Schoell, 1984), eleven out of thirteen oils have very similar 613C values, averagi ng -24.3%o (range - 25. 0 to -23.6%0, Tabl e 1). The Roswin Nort h-1 oil (~13C=-26.8%0) and Rednook-1 condensat e (613C = -27. 4%o), by way of cont rast , bear dis- tinctly light i sot opi c signatures which clearly differ- entiate t hem from the ot her (Permi an) oils (Fig. 10). It is wor t h not i ng t hat the aforement i oned Triassic cont r i but i on to the Tayl or - 12A and Merroombi l e-1 pet rol eums is insufficient to affect t hei r i sot opi c sig- nat ures which t herefore remai n heavy. Genet i c rel at i onshi ps between oils and t hei r sources can be demonst r at ed using a St ahl (1978) pl ot , a pr i st ane/ phyt ane rat i o versus 6~3C pl ot , and a Sofer (1984) pl ot (Fig. 10a-c). These di agr ams in- dicate t hat the Per mi an oils are genetically rel at ed and best mat ch the Permi an source r ock kerogens (613C = - 2 4 . 6 to -22.9%0) and extracts. The Triassic oils pl ot separat el y and can be tied to a Triassic source (Snake Creek Mudst one: kerogen ~ 1 3 C = - 2 8 to -26%0). The combi ned effects of mat ur i t y (cf. Si monei t e t al . , 1981) and organi c facies (cf. Lewan, 1986) differences best expl ai n the observed vari at i ons in 613C values for the Triassic and Permi an facies. I t is i nt erest i ng t hat 13C-de- pl et i on of this magni t ude appear s to be an Aust ral i a-wi de feat ure of Triassic mar i ne settings (Summons e t al . , 1995). Nor mal l y, oils show 6~3C vari at i ons of up t o 1.5%0 relative to t hei r source r ock bi t umens (Peters and Mol dowan, 1993) or kerogens of similar mat ur - ity. In fact, this is what is observed here for bot h the Permi an and Triassic oils when compar ed wi t h t hei r respective Permi an and Triassic extracts, ker o- gens and kerogen pyrol ysat es (Fig. 10). The ar o- mat i c fract i ons of the oils clearly obey this 1.5%o- maxi mum-di fference rule when compar ed to t he aromat i cs of t hei r source r ock bi t umen. The kerogen and ext ract sat urat es fract i ons of a coal from the Jurassi c Wal l oon Coal Measures have car bon i sot opi c composi t i ons falling within the range for Per mi an oils and source rocks (Fig. 10c). However, the possi bi l i t y of the Wal l oon bei ng a source for these oils is precl uded on t he basis of its i mmat ur i t y (Fig. 21); and by the l ack of enhanced concent rat i ons of the charact eri st i c Ar aucar i acean conifer resin bi omar ker s (viz. 1,2,5-trimethyl- napht hal ene, 1-met hyl phenant hrene, 1,7-dimethyl- phenant hrene and retene, Fig. 6e and f) in any of the oils. Re c ogni t i on o f p e t r o l e u m s y s t e ms A pet r ol eum system st art s when a source rock generates hydr ocar bons ( Magoon, 1992). Thus, the presence of hydr ocar bons is pr oof of a pet r ol eum system, the essential elements of which are the source, reservoir, seal and over bur den rocks ( Magoon and Dow, 1994). Pet r ol eum systems can be classified accordi ng to the kerogen t ype or the age of t hei r source rock ( Magoon, 1992). Ident i fi cat i on of two oil families within the set of oils anal ysed necessitates the existence of (at least) two di st i nct pet r ol eum systems in the Tar oom Trough. Tr i as s i c p e t r o l e u m s y s t e m. Thi s system involves Triassic source and reservoi r rocks in a very l oca- lised ar ea of the sout hwest ern par t of the t rough. I t encompasses the Roswi n Nor t h and Rednook pet - rol eum pool s (as well as par t of the Mer r oombi l e and Tayl or accumul at i ons), all reservoi red in the Showgrounds Sandst one, and t he Snake Creek Mudst one source r ock in the vicinity of Borah Creek-3 and 4, Ti nker-2, Ingl est one-I and the adj a- cent deeper par t s of the t rough. This system can, therefore, also be called the "Snake Creek-Show- gr ounds pet r ol eum syst em" (A1-Arouri e t al . , 1998). Its l at eral extent is, most likely, confined to the ar ea of the best hydr ocar bon generat i ng pot ent i al as out - lined in Fig. 4b. Here, the Snake Creek source rocks have at t ai ned an appr opr i at e t hermal mat ur - ity for (and di spl ay pet r ogr aphi c evidence of) active hydr ocar bon generat i on, as discussed previ ousl y. Where j uxt aposed with carri er and reservoir rocks, this source r ock uni t has cont r i but ed to the oil found in the underl yi ng Showgr ounds Sandst one. The measured mat ur i t y of the Rednook condensat e (Rc = 1.04%) suggests t hat the Snake Creek Mud- st one has, in fact, at t ai ned somewhat higher mat u- r at i on levels t han i ndi cat ed in Fig. 4a. These mor e mat ur e areas of the Snake Creek ki t chen most likely occur al ong the axis of the Tar oom Tr ough where no dri l l hol es have yet penet r at ed the unit. The lim- ited ar ea (Fig. 4) and average thickness (19 m) of mat ur e source r ock is consi st ent with the rel at i vel y mi nor amount s of hydr ocar bons di scovered in this pet rol eum system (6.1 x 104 m 3 oil, 4.6 × 104 m 3 gas: AI - Ar our i e t al . , 1998). Pe r mi a n p e t r o l e u m s y s t e m. This system, unl i ke the Triassic system, is of wide areal and strati- graphi c extent. It includes the source rocks of the Bl ackwat er Gr oup, which are mat ur e t o over mat ur e t hr oughout much of the Tar oom Trough, and all 732 Khaled R. A1-Arouri et al. Family 2 oils and related hydrocarbons produced from reservoirs of Permian to Jurassic age along the southeastern and southwestern margins of the trough (AI-Arouri et al., 1998). This geochemical study has proven that oils in this system are derived mainly from carbonaceous shales of the Blackwater Group. Most of the oil in this system is reservoired in the Precipice Sandstone' (Fig. lc). Hence, this sys- tem can conveniently be called the "Blackwater-Pre- cipice petroleum system". Its discovered reserves are 7. 2x 106m 3 oil and 1.4x 101°m 3 gas (A1- Arouri et al., 1998). Thermal maturation and petroleum generation in both systems, as simulated kinetically (A1-Arouri et al., 1998), suggests that generation of hydrocarbons in the Permian-sourced petroleum system started at about 175 Ma and ended at 90Ma, whereas the Snake Creek Mudstone commenced charging its Triassic reservoir about 50 Ma later (125-75 Ma). In the north, the bulk of the generated hydrocar- bons were expelled well before the main defor- mation event (Late Triassic-Jurassic), whereas, in the south, generation post-dated the development of trap structures, making the southern regions more prospective for hydrocarbons. CONCLUSIONS The major potential source rocks in the Taroom Trough are marine mudstones in the lower part of the Permian section (Back Creek Group), and coals and carbonaceous shales in its upper part (Blackwater Group). Both the marine and nonmar- ine lithofacies are organic-rich and have attained sufficient maturity for oil and gas generation. However, using terpane and sterane biomarkers and isotopic data, the major source of the trough' s oil was shown to be the Late Permian Blackwater Group. These isotopically heavy oils (613Carom~-24%o) occur widely throughout the study area, mostly in the Precipice Sandstone but also in other reservoirs of Permian to Jurassic age. They are therefore assigned to the "Blackwater- Precipice" petroleum system. In addition, a previously unrecognised subsidiary oil source was identified, the Middle Triassic lacus- trine Snake Creek Mudstone. This moderately or- ganic-rich unit was deposited in a largely suboxic freshwater setting. It exhibits its best generating po- tential (good quality Type II/III kerogen of appro- priate thermal maturity) in a very localised area between Tinker-l, Inglestone-1 and Flinton-1, and in the adjacent deeper parts of the southwestern trough. This unit has sourced the isotopically lighter oils ( 6 1 3 C a r o m ~ - 27%o) in the Roswin North and Rednook fields (and also contributed to the Taylor and Merroombile accumulations), all of which are produced from the Showgrounds Sandstone. These new Triassic oil-source corre- lations demonstrate the existence of a second sys- tem, the "Snake Creek-Showgrounds" petroleum system in the southern Taroom Trough. Acknowledgements--This research forms part of a Ph.D. study by the senior author who gratefully acknowledges financial support from an Overseas Postgraduate Research Award and a University of Adelaide Postgraduate Research Scholarship. 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Paramet er APPENDI X Key To Biomarker Parameters Listed In Table 2 Deri vat i on Maturity-dependent parameters 1 %22S 2 Mor / Hop 3 T~/Tm 4 %20S %22S/(22S + 22R)-C3t homohopane = 100 x 17c~(H),21/J(H) 22S homohopane/17:~(H),21/?(H) 22(S + R) homohopane C30 moretane/C30 hopane - 17[3(H),21~(H)-hopane/17c~(H),21/~(H)-hopane C27 18ct(H)-22,29,30-trisnorneohopane/C~7 17~(H)-22,29,30-trisnorhopane % 20S/(20S + 20R)-C29 sterane - 100 x 5cffH), 14c~(H), 17:~(H)-24-ethylcholestane 20S/ 5~(H),14(H)~,17~(H)-24-ethylcholestane 20(S + R) %[~[3/(tq[~ + 7c0-C29 sterane = 5:~(H),14/~(H),17/~(H)-24-ethylcholestane 20(S + R)/ [5~(H),14/~'(H),17/qH)-24-ethylcholestane 20(S + R) + 5~(H),14c~(H),17~(H)-24-ethylcholestane 20(S + R)] Source-specific parameters: hopanes 6 25NH/ Hop 7 30NH/ Hop 8 29, 30BNH/ Hop 9 28, 30BNH/ Hop 10 2 + 3MeH/ Hop 11 2MeH/ 3MeH 12 C~9T~/30NH 13 C30 Di a/ Hop 14 Hop/ St er C29 17~(H),21/J(H)-25-norhopane/C30 17c~(H),21/~'(H)-hopane C29 177(H),21fl(H)-30-norhopane/C3o 17a(H),21//(H)-hopane 29,30-bisnorhopane/C30 17a(H),21/i(H)-hopane 28,30-bisnorhopane/C3o 17~(H),21/J(H)-hopane C3t (2a + 3/$)-methylhopane/C3o 17:~(H),21/$(H)-hopane 2a-met hyl hopane/ 3/ / -met hyl hopane 18~(H),21fl(H)- 30-norneohopane/C 29 17a(H),21/~(H)-30-norhopane C30 17~(H)-diahopane/C3o 17~(H),21/~:(H)-hopane hopanes/steranes, where hopanes = I~C_~7 34 hopanes and moretanes - C27 t ri snorhopanes ( 7' , + 7',11) + C2s bi snorhopanes (28,30-BNH + 29,30-BNH) + C29 (30-NH + 30- normoret ane) + C3o (hopane + moretane) + C3t 34 (homohopanes + homomoretanes); and steranes = 1~C27 2~ [5~(H),I4~(H),I7~(H)-20(S + R) + 5~(H)A4[I(H),I7[I(H)-20(S + R)] steranes Source-spec(fic parameters: steranes and methyLs'terane.~ 15 C27: C28: C29 5~(H),14~(H),17~(H)-cholestane 20R: 5~(H),14~(H),17~(H)-24-methylcholestane 20R: 5c~(H),I4~(H), 17~(H t-24-ethylcholestane 20R 16 C 3 0 / C 2 9 5~(H),t4~(H),17c~(H)-24-n-propylcholestane 20R/5~(H),14c~(H),17~,t(H)-24-ethylcholestane 20R 17 Di a/ St er 5c~(H),13/~'(H),17~(H)-dia-24-ethylcholestane 20(S + R)/[5~(H).14:~(H),17~(H)-24-ethylcholestane 20(S + R) + 5:~(H),14/~(H),17//(H)-24-ethylcholestane 20(S + R)] 18 C28 4-Met hyl st erane 100 × 4~-methyl-5c~(H),14c~(H),17~(H)-cholestane 20R/EC28 3. 4:~-methylsteranes 19 C29 4-Met hyl st erane 100 x 4c~-methyl-5:~(H),14c~(H),17~(H)-24-methylcholestane 20R/ZC2s 30 4:~-methylsteranes 20 C30 4-Met hyl st erane 100 x [4~-methyl-5~(H),14~(H),17~(H)-24-ethylcholestane 20R + 4ct,23,24-trimethyl- 5:~(H), 14~(H),I 7c~(H)-cholestane 20R]/ZC28 30 4~-methylsteranes 21 4Mest/Ster 4~-methyl-5~(H)A4~(H),17~(H)-24-ethylcholestane 20R + 4~,23,24-trimethyl-5c~(H),14~(H),17c~(H)- cholestane 20R/5~(H),14c~(H),I 7c~(H)-24-ethylcholestane 20R 22 2 + 3Mest/Ster (2c~ + 3/t)-methyl-5~(H),14c~(H),17~(H)-24-ethylcholestane 20R/5~(H),14ct(H),17~(H)-24- et hyl chol est ane 20R 23 4~S/313S 4~-methyl-5c~(H), 14~(H),I 7~(H)-24-ethylcholestane 20S/3[:t-methyl-5~(H),14c~(H),17c~(H)-24- ethylcholestane 20S 24 Dino/3flS 4c~,23 R.24S-trimethyl-5c~(H), 14c~(H), 17:~(H)-cholestane 20R/3[4-methyl-5~(H),14~(H),17a(H)-24- et hyl chol est ane 20S