ORIGINAL PAPER

Diagenetic controls on evolution of porosity in the PermianIllawarra Coal Measures, Southern Sydney Basin, Australia

Fahad Al Gahtani

Received: 13 June 2013 /Accepted: 24 October 2013# Saudi Society for Geosciences 2013

Abstract Petrography of the Illawarra Coal Measures wasdescribed by thin section, scanning electron microscope andX-ray diffraction techniques. Sandstone composition of theIllawarra Coal Measures includes mostly lithic grains withminor quartz, feldspar, mica and heavy minerals. TheIllawarra Coal Measures consists of litharenite and rarelysublitharenite. Quartz includes monocrystalline and polycrys-talline grains while the feldspar includes both K-feldspar andplagioclase. Volcanic, sedimentary and chert rock fragmentsare present in the Illawarra Coal Measures. Thin-section po-rosity occurs in all units, particularly the coarse-grained de-posits. Secondary porosity is more common than primaryporosity in the Illawarra Coal Measures. Thin sections andscanning electron microscopy were used to describe diagenet-ic alterations and their influence on porosity in Illawarra CoalMeasures. These diagenetic alterations include compaction,quartz overgrowths, authigenic clay minerals, carbonate ce-ment and authigenic feldspar. In the Illawarra Coal Measures,shale, siltstone and fine-grained sandstone are common in theWilton Formation, Bargo Claystone, Darkes Forest Sand-stone, Allans Creek Formation, Unnamed Member Threeand Unnamed Member Two. These units have low porosityand form lithological seals and confining layers. Medium- andcoarse-grained sandstone is common in the Kembla Sand-stone, Lawrence Sandstone Member and Loddon Sandstoneand contains low porosity. Thus, gas or water may be presentin these formations. The Tongarra Coal, Allans Creek CoalMember, Wongawilli Coal, Hargrave Coal Member, CapeHorn Coal Member, Balgownie Coal Member and Bulli Coalare the main sources for gas in the Illawarra Coal Measures.

Keywords Petrography . Diagenesis . Porosity . Primaryporosity . Secondary porosity . Authigenic clayminerals .

Carbonate cement

Introduction

The petrology of the Illawarra Coal Measures has previouslybeen studied by a number of authors (e.g. Cusack 1991;Bamberry 1992). Bamberry (1992) described the petrographyin the Sydney Subgroup showing quartz and lithic suites. Henoted that the sandstones of Marrangaroo and Blackmans FlatConglomerates and the Wilton Formation represent the quartzsuite. The quartz suite is also characterised in the uppermostErins Vale Formation from the Thirroul area and sandstone inthe basal Wilton Formation. On the other hand, all sandstoneswhich overlie the Tongarra Coal represent the lithic suite(Bamberry 1992). In the Wongawilli Seam, petrography andcomposition of kaolinitic claystones were described byLoughnan (1971). Diagenesis of the Eckersley Formationwas previously studied by Cusack (1991) who provided adiagenetic sequence. Previous studies of porosity in theIllawarra Coal Measures are rare to absent.

Bowman (1974) interpreted the coal-bearing sediments ofthe Pheasants Nest Formation as fluvial deposits in a deltaicsetting. Moderately sorted, flat bedded and burrowed, fine- tomedium-grained sandstone, deposited in bays and lagoons, isa common feature in the Erins Vale Formation according toBowman (1980).

The Sydney Subgroup was deposited mainly in a fluvialenvironment on a distributive delta floodplain (Bowman 1980).Bunny (1972) and Bowman (1974) interpreted the Wilton For-mation as floodplain deposits associated with a fluvial environ-ment. The deposits of meandering fluvial and floodplain envi-ronments were interpreted for the sequence from the BargoClaystone to the Allans Creek Formation (Bunny 1972;

F. Al Gahtani (*)Ministry of Petroleum and Mineral Resources, Riyadh, Saudi Arabiae-mail: fahadfm_99@hotmail.com

Arab J GeosciDOI 10.1007/s12517-013-1166-0

Bowman 1974). Two carbonaceous intervals with sequences ofclaystone-siltstone are described in the Allans Creek Formationaccording to Bamberry et al. (1995). The sequence from the baseof the Kembla Sandstone to the Wongawilli Coal representsupward fining from a fluvial channel to a coal swamp environ-ment (Jones 1983). Deposition of the Loddon Sandstone oc-curred as a fining upward succession in a meandering fluvialsystem (Roche and Hutton 1998).

The purpose of this study is to investigate the diagenetichistory of the reservoir sandstones and the influence of diage-netic alterations on primary and secondary porosity in theIllawarra Coal Measures of Southern Sydney Basin, Australia.

Geologic setting

The Sydney Basin forms the southern division of the Sydney–Gunnedah–Bowen Basin (Fig. 1). The study area consists ofpart of the southern Sydney Basin in the Illawarra district ofNew South Wales, Australia (Fig. 2).

Many studies have discussed the nomenclature of theIllawarra Coal Measures (e.g., Bowman 1970; Bunny 1972;Bowman 1974; Carr 1983; Bamberry 1992). The Illawarra CoalMeasures are classified into two subgroups, which include theCumberland Subgroup and the overlying Sydney Subgroup.

The Cumberland Subgroup contains the Pheasants NestFormation and the Erins Vale Formation (Jones 1983). ThePheasants Nest Formation is present at the base of the Cum-berland Subgroup and overlies the upper Shoalhaven Group(Table 1). The thickness of the Pheasants Nest Formation is75 m (Bowman 1980) and it includes lithic sandstone, thincoal seams and plentiful conglomerate in the Robertson area(Hutton and Bamberry 1999). The Erins Vale Formation rep-resents the uppermost formation of the Cumberland Subgroupand is 37 m thick (Bowman 1980). It consists of conglomerateinterbedded with coarse-grained sandstone while bioturbatedsiltstone (Kulnura Marine Tongue) is recorded in the upperpart of the Erins Vale Formation (Hutton and Bamberry 1999).

The Sydney Subgroup is the upper subgroup of the IllawarraCoal Measures and contains lithic sandstone, siltstone,mudrock, tuff and coals. The Wilton Formation is present atthe base of the Sydney Subgroup and consists of coarse sand-stone, siltstone and coal. It has a thickness that varies between15 and 30 m (Bowman 1980). Two members recognised in theWilton Formation are the Woonona Coal Member and theWanganderry Sandstone Member. Carbonaceous claystone,coal and tuffaceous claystone are present in the Woonona CoalMember (Roche 1997). It has a maximum thickness of 3 m(Bowman 1974). Also, Roche (1997) noted that theWanganderry Sandstone Member included medium- to verycoarse-grained quartzose sandstone, claystone, carbonaceoussiltstone and pebbly conglomerate. It has a maximum thicknessof about 22 m at Yerranderie (Hutton and Bamberry 1999).

The Tongarra Coal overlies the Wilton Formation and con-sists of carbonaceous claystone and interbedded coal, sandstoneand tuff. It has a thickness that ranges from 2 to 3 m (Bowman1974). It underlies the Bargo Claystone, which is overlain by theDark Forest Sandstone (Bowman 1974). The Bargo Claystoneand the Dark Forest Sandstone were defined as members of theAppin Formation but Bunny (1972), Bowman (1974; 1980) andBamberry (1992) showed that they have extensive development,thus they are recognised as formations. Bunny (1972) showedthat the Bargo Claystone contains dark grey to black claystone,while light grey, fine-grained, lithic to quartz-lithic sandstone arerecorded in the Dark Forest Sandstone. They have an averagethickness of about 15 and 10 m, respectively (Bowman 1974).The Austinmer Sandstone Member was identified at the base ofthe Bargo Claystone by Bowman (1970). The HuntleyClaystone Member occurs just above the base of the BargoClaystone (Hutton and Bamberry 1999).

The Allans Creek Formation overlies the Darkes Forest Sand-stone and contains four coal intervals separated by sandstone,claystone and siltstone (Hutton and Bamberry 1999). It is 8 mthick (Bowman 1974). The Kembla Sandstone overlies theAllans Creek Formation and has a maximum thickness of about23.8 m in the Southern Coalfield. It includes medium- to verycoarse-grained lithic sandstone and interbeds of siltstone andclaystone (Roche 1997). The Wongawilli Coal overlies theKembla Sandstone and includes the Farmborough ClaystoneMember which is widespread in the Southern Coalfield(Hutton and Bamberry 1999). The Wongawilli Coal containsbands and splits and has a thickness of between 4 and 11 m(Arditto 2003).

Coals, shales, and interbedded sandstones of the EckersleyFormation overlie the Wongawilli Coal (Bowman 1970). TheEckersley Formation is composed of the Novice SandstoneMember, Woronora Coal Member, Hargrave Coal Member,Cape Horn Coal Member and Balgownie Coal Member(Bowman 1974). Bamberry (1992) indicated new changes inthe stratigraphy when he introduced the BurragorangClaystone, above the Cape Horn Coal, and the Loddon Sand-stone Members at the top of the Eckersley Formation. Roche(1997) noted that the Lawrence Sandstone Member existsbetween the Cape Horn Coal and Balgownie Coal Member,and contains lithic sandstone, claystone, siltstone and con-glomerate. It has a maximum thickness of about 16 m in theSouthern Coalfield. It is overlain by the Balgownie CoalMember which consists of coal and carbonaceous claystone(Roche 1997). The Loddon Sandstone has subsequently beenelevated to formation status (Herbert 1995). It occurs betweenthe Balgownie Coal Member and the Bulli Coal and containslithic sandstone, claystone, siltstone and conglomerate(Hutton and Bamberry 1999). The Bulli Coal is present attop of the Illawarra Coal Measures and has a thickness thatvaries from 0 to 4 m (Bowman 1974). It includes coal andcarbonaceous claystone (Roche 1997).

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Fig. 1 Structural-element map of the Bowen-Gunnedah-Sydney Basin system and the Surat Basin. Structural elements after Exon (1976), Beckett et al. (1983),Balfe et al. (1988) and Yoo (1988). Superimposed on the structural elements is the position of the Meandarra Gravity Ridge (from Krassay et al. 2009)

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Methods

In this study, field work included collection of 131 coresamples selected from 7 wells drilled by BHPBillitonIllawarra Coal in the southern Sydney Basin. These samplesinclude 71 samples of sandstone, 21 samples of siltstone and39 samples of shale,

Petrographic microscope studies were completed for 76samples. Before preparation of thin sections, 76 samples werevacuum impregnated with blue dyed resin for the purposes ofdescription and study of the porosity under the microscope.

X-ray diffraction analysis was used to study 122 samples of finetomedium-grained sandstone, fine-grained sandstone, siltstone,shale, tuff, coal and carbonate cement. These samples wereprepared for X-ray diffraction analysis using a Philips(PW3710) diffractometer (Cu Kα radiation, 35 kV, 28.5 mA)to determine the percentage of each mineral in fine samples,and clay minerals in the sandstone (oriented samples of <2 μmclay fractions. A JEOL JSM-T330 scanning electron micro-scope (SEM) was used to examine about 28 samples to deter-mine morphology, textural relationships, mineral composition,porosity, and diagenetic aspects of the sandstone samples.

Fig. 2 Location of the SydneyBasin and the Coalfields within it(from Grevenitz et al. 2003)

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Results

Petrography

The framework composition of the Illawarra Coal Measureswas classified using the QFL diagram according to Folk(1968). The Illawarra Coal Measures consists mostly oflitharenite with rare sublitharenite (Q29.5%, F2.9%, R 67.6%;Fig. 3a). The origin of the Illawarra Coal Measures wasrecognised based on the diagram from Dickinson (1985).The Illawarra Coal Measures were derived from lithic

recycled to transitional recycled and quartzose recycledprovenance areas (Qm25.1%, F3.2%, Lt71.8%; Fig. 3b).

Quartz is main mineral in the sandstone and plays a role in thedetermination of source area (Fig. 4a). Detrital quartz is com-posed of both monocrystalline and polycrystalline quartz grains.Quartz occurs in the Illawarra Coal Measures ranging in abun-dance from 1.2 to 49.4 % (Appendix 1). Feldspar is the secondmost important mineral in sandstones, following quartz. Feldsparimportance lies in determining the provenance of the sandstone.Feldspar grains include fine-grained K-feldspar and plagioclase.Lithic grains are composed of rock fragments and chert in the

Table 1 Stratigraphy of Southern Coalfield (modified after Bunny 1972; Bowman 1974, 1980; Carr 1983; Bamberry 1992)

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Illawarra Coal Measures (Fig. 4b–e). Chert grains are includedwithin rock fragments because distinguishing between felsicvolcanic rock fragments and chert grains is not easy. So, chertgrains are classified with volcanic rock fragments in this unit.Lithic grains are present in the Illawarra CoalMeasures and rangefrom1.9 to 90.5% (Appendix 1). Chert grains are common in theIllawarra Coal Measures and have a microgranular texture(Fig. 4c and e). Chert grains are recorded in amounts up to90.5 % (Appendix 1). Mica includes biotite and muscovite, withthe latter being more common than the former (Fig. 5a). Theheavy minerals are present in trace amounts and consist ofhornblende, rutile, zircon and tourmaline.

Diagenesis

Thin section and SEM analysis were used to describe the dia-genesis of all units in the Illawarra Coal Measures. Authigenicminerals comprise quartz overgrowths, authigenic clay minerals,carbonate cement, authigenic feldspar and dissolution.

Diagenetic minerals

Quartz cementation

Quartz cement is seen using both thin section and SEMtechniques. It is uncommon in the Illawarra Coal Measureswhich includes more detrital lithic grains than detrital quartz.Thus, quartz cement is volumetrically negligible. Quartzcement percentages range from 0 to 2 % (Appendix 1).

Quartz cement exists as syntaxial overgrowths around detritalquartz grains or is arranged as crystals oriented perpendicular tothe quartz grains (Figs. 4a, 5b and 6a). It also occurs as a pore-filling cement in most units of the Illawarra Coal Measures.Quartz overgrowths decrease in abundance in sandstones thatinclude high percentages of carbonate cement (Umar et al. 2011).

The presence of authigenic clays also affects the overgrowthhabit of authigenic quartz (Rossi et al. 2002; Umar et al.2011). Thus, precipitation of quartz overgrowths is difficult insamples with greater contents of clay minerals. This meansthat grain-coating clays are important in preventing the de-velopment of quartz overgrowths, and in some samples theyenhance reservoir quality. Double quartz overgrowths weredetected in some samples (Fig. 5c), whereas fluid inclusionsoccur rarely in the succession. Fluid inclusions, clay coatingsand iron oxides may be observed separating quartz cementfrom detrital quartz grains whereas in some samples, thedistinction between quartz overgrowths and detrital grains isnot clear.

Authigenic clay minerals

Authigenic clay minerals are described from SEM data andinclude kaolinite, illite, mixed-layer illite/smectite and chlorite.

Kaolinite (0.2–68.7 %) is distributed in all units in theIllawarra Coal Measures. It fills pores between grains, includ-ing primary and secondary pore spaces, and exists as bookletsand vermicular aggregates (Fig. 6b). Kaolinite is coated bychlorite in some samples and is intergrown with some detritalclay. Illite is also associated with kaolinite in some samples.Kaolinite is mostly authigenic, indicated by its high level ofcrystallinity (Cusack 1991). The conversion of kaolinite intodickite is observed by scanning electron microscope. Thistechnique showed that dickite is characterised as blocky crys-tals, with thick smooth surfaces, thus it is different fromkaolinite which is in booklets and vermicular aggregates ofthin crystals with etched surfaces.

Mixed-layer illite/smectite (0.2–48.5 %) is usually the mostcommon clay mineral in the Illawarra Coal Measures(Fig. 6c–f). It was not identified in thin sections of samplesbut SEM showed that mixed-layer illite/smectite coats most

a bQ

F R

Quartz arenite

Subarkose

ArkoseLitharenite

Feldspathic litharenite

Sublitharenite

Lithic arkose

Qm

F Lt

Transitional continental

Craton interior

Basement uplift

Quartzose recycled

Transitional recycled

Lithic continental

Mixed

Dissected Arc

Undissected Arc

Transitional Arc

Fig. 3 a Classification of the Illawarra Coal Measures (after Folk 1968).Q quartz, F feldspar, R rock fragment, b The provenance of the Illawarra CoalMeasures (after Dickinson 1985). Qm monocrystalline quartz, F feldspar, Lt rock fragment+chert

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detrital and authigenic grains, such as detrital quartz grains,carbonate cement, quartz overgrowths and altered feldspar.Mixed-layer illite/smectite is also recorded as a pore-fillingcement.

Illite (0–17.9 %) occurs as fibrous crystals that are mainlyoriented perpendicular to detrital grain surfaces (Fig. 6d). Itexists as a pore-filling cement in some samples where it ispartly incorporated into quartz overgrowths (Umar et al.2011). In some samples, it is observed as high birefringentpatches. Some detrital grains and pores are bordered or coatedwith illite. Grain-coating illite is present in two forms as ultra-thin layers and thin mat-like crystals.

Chlorite (0–2.9 %) is rarely present in the Illawarra Coal Mea-sures (Fig. 6e). It is identified as scattered platelets inpseudomatrix oriented perpendicular to grain surfaces and asrims around overgrowth grains. Also, it exists as a replacivemineral in detrital grains and encloses some quartz overgrowths.

Carbonate cement

Carbonate cement occurs in the Illawarra Coal Measures andvaries between 0 and 60.4 % (Appendix 1). It is generally thefirst cement in the Illawarra Coal Measures and includesankerite, siderite, calcite, and dolomite.

Fig. 4 a Quartz cement (Qo) ispresent as overgrowth aroundquartz grain (Q) and lithic grains(white arrows). b Poikilotopicsiderite (S) is coating a volcanicrock fragment (VRF). c Detritalgrains are partly coated by largecrystals of ankerite (A) which alsofills pores. d Small dolomitecrystals (D) fill pore space andcoat the margins of volcanic andsedimentary rock fragments(VRF, SRF). e Poikilotopiccalcite cement (Ca) occludes alarge pore and exists as a coatingon the margins of detrital grainssuch as volcanic rock fragment(VRF), sedimentary rockfragment (SRF) and chert (Ch). fConcavo-convex grain contacts(red arrow) are observed

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Siderite (0–51.1 %) is present as large euhedral, subhedraland anhedral crystals (≥20 μm) in most samples (Fig. 4b).Large crystals are blocky and poikilotopic and exist aspore-filling cement and as partial grain coatings on detritalgrains such as quartz and rock fragments. Small crystals ofpore-filling siderite are observed in some samples. Also,siderite cement is present as a replacive mineral in quartzand feldspar in some samples. Siderite is usually nowstained by iron oxide which may be derived from alterationof ilmenite or from iron hydroxides (Karim et al. 2010).More siderite is found in fine-grained samples than in thecoarser grained samples, thus it decreases observably with

increasing grain size. Fluid inclusions are present in thesiderite cement but are uncommon.

Ankerite (0–46.7 %) is observed as a cement and replace-ment mineral (Fig. 4c). It appears as small crystals (10–20 μm) and large crystals (≥20 μm). In both cases, itoccurs as a pore-filling cement and coats some detritaland authigenic grains, such as quartz, feldspar, lithic grainsand quartz overgrowths. In some samples, ankerite cementis noted as a replacement mineral in detrital grains. SEMdisplays euhedral ankerite with a rhomohedral shape occur-ring as a pore-filling cement.

Fig. 5 a Deformation ofmuscovite (Mu) between quartzgrains, indicating mechanicalcompaction. b Quartz cement(Qo) is present as overgrowtharound quartz grain (Q) and ispartly coated by ankerite cement(A). Margins of very fine volcanicrock fragment (VRF) andsedimentary rock fragment (SRF)are coated by ankerite cement (A).c Quartz overgrowth (Qo) ispartly coated by late diageneticcalcite (Ca) cement which alsooccurs as coatings on detritalgrains such quartz (Q) and chert(Ch). d Secondary porosity(white arrows) result fromdissolution of siderite (S) cement.e Feldspar dissolution resultssecondary porosity (white arrow)

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Calcite (0–44.7%) exists in most samples as large euhedral toanhedral crystals (≥20 μm; Fig. 4e). Also, it fills large andsmall pores in some samples and the margins of some grainsare coated by calcite cement. Detrital grains such as quartzmay be replaced partially or totally by calcite cement in somesamples. The presence of fluid inclusions is rare in the calcitecement.

Dolomite (0–11.1 %) is less widespread as a carbonate ce-ment, although it occurs in all units of the Illawarra CoalMeasures (Fig. 4d). It is usually present as small crystalswhich are anhedral to euhedral. However, it was observed asreplacive rhombs in some framework grains and as a pore-filling cement. Some detrital grains are coated by dolomitecement as shown by thin section.

Authigenic feldspar

In the Illawarra Coal Measures, authigenic feldspar is uncom-mon but it was observed in thin section and SEM analysis infew samples.

Diagenetic Sequence

Compaction

Mechanical and chemical compactions are shown in bothsandstone and siltstone samples in the Illawarra Coal Mea-sures (Figs. 4f and 5a). Chemical compaction is determinedthrough the increased grain contacts in the sequence (Fig. 4f).

Fig. 6 a Quartz overgrowth (Qo)fills pore and is partly coated bylate diagenetic illite clays (whitearrow), thus porosities (redarrows) are preserved. bKaolinite (Ka) occurs as bookletsand vermicular aggregates and isassociated with dickite. c Welldeveloped ankerite cement (A) ispartly coated by late mixed-layerillite/smectite (Mix). Also,primary pore (red arrow) occursin the Kembla Sandstone. dAuthigenic kaolinite (Ka) isintergrown with authigenic illite(ill). Also, primary pores (redarrows) are recorded in theKembla Sandstone. e Scatteredplatelets of late diagenetic chloriteclays (Chl) post-date quartzovergrowth (Qo) and preservesprimary pore (red arrow) in theLoddon Sandstone. f Latediagenetic mixed-layer illite/smectite (Mix) is precipitated onsiderite cement (S) and quartzovergrowth (Qo). Porosities (redarrows) are recorded

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Three types of contacts, including concavo-convex, suturedcontacts and long contacts, provide evidence for chemicalcompaction in the Illawarra Coal Measures whereas pressuresolution is rare in the succession, indicated by point contactsand long contacts (Wolela 2009). Where mica is present it isbent because of physical compaction (Fig. 5a). The presenceof pseudoplastic deformation of mud intraclasts, ductile grainsand grain arrangement provides evidence of mechanical com-paction. Compaction is uncommon in some samples due to thepresence of widespread carbonate cement. Compaction isincluded in both the early and late stages of diagenesis.

Cementation

Authigenic clay minerals

Authigenic clay minerals are characteristically oriented per-pendicular to the surface of detrital grains when they occur aspore-linings and pore-fillings. In the Illawarra Coal Measures,the disaggregation and chemical breakdown of lithic constit-uents is the source of clays (Bamberry 1992).

Fluvial deposits include predominantly grain-coating clays(Worden and Morad 2003). Grain-coating clays may be re-corded as mechanical infiltration (El-Ghali et al. 2006), oc-curring as coatings on detrital and overgrowth grains. Kaolin-ite, mixed-layer illite/smectite, illite and chlorite are present asearly diagenetic minerals in the Illawarra Coal Measures.

Authigenic clay minerals, such as kaolinite, illite, chloriteand mixed-layer illite/smectite commonly fill intergranularpore spaces. The occurrence of kaolinite as grain coatings ondetrital grains or as a pore-filling cement indicates that kao-linite was precipitated as an early diagenetic mineral (Fig. 6band d). The vermicular texture of kaolinite also indicates theorigin of kaolinite at an early stage (Abouessa and Morad2009). The textural characteristics of illite such as fibrouscrystals, ultra-thin layers and thin mats also suggest an earlydiagenetic origin for the illite. The presence of illite in the formof pore-fillings and grain coatings are also interpreted as earlyauthigenic illite (Fig. 6d).Mixed-layer illite/smectite is recordedas grain coatings on most detrital grains and this demonstratesits formation during early diagenesis (Fig. 6c). Also, authigenickaolinite is engulfed, and thus pre-dates, authigenic chlorite.

Pore-water is the source of kaolinite which occurs in the formof booklets (Wolela 2009). Low pH and low ionic strengthwaters are recorded as supportive factors for the precipitationof kaolinite (Worden and Burley 2003; Hammer et al. 2010).Authigenic kaolinite can be produced by dissolution of carbon-ates, feldspar and unstable minerals in lithic fragments (Zhanget al. 2009). Alteration of volcanic rock fragments, breakdownand alteration of feldspar, illitization of matrix constituents andearlier kaolinite can be the sources of illite and chlorite. Replace-ment of feldspar may also contribute to the formation of illite(Umar et al. 2011). In some samples, carbonate grains are coated

by clays, indicating that precipitation of the latter was a result ofdissolution and alteration processes (Bai 1991). The formation ofsmectite clays occurs during weathering, particularly in arid tosemi-arid climatic conditions (El-Ghali et al. 2009a).

Many evidences showed chlorite as a pore-filling and grain-coating cement, indicating its precipitation during early diagene-sis. The occurrence of chlorite as grain coatings may be due todetrital mineral transformation or the formation of eogenetic clayminerals as suggested by (Gier et al. 2008). Authigenic chloritegrain coatings are characterised by the presence of iron (Blochet al. 2002). Chlorite growth is enhanced when the temperatureexceeds 60–70 °C (Worden and Morad 2003). Many studieshave shown that the relative timing of authigenic chlorite precip-itation in sandstone occurs during early diagenesis, soon after theperiod of deposition and prior to complete compaction (Blochet al. 2002; Billault et al. 2003). Chlorite may be precipitated asresult of ions which are derived from alteration of frameworkgrains such volcanic rock fragments (Bai 1991). Dissolution ofvolcanic rock fragments and feldspar also play a role in theformation chlorite in secondary pores by contributing Mg, Feand Si ions (cf. Umar et al. 2011). The precipitation ofMg and Feionsmay be recorded as early diagenetic (Sur et al. 2002) and thisleads to the interpretation that pore-filling chlorite and chloritegrain coatings are early diagenetic.

Carbonate cementation

Carbonate cementation mainly post-dates the authigenic claysand occurs as grain coatings around detrital grains. This supportsthe concept that carbonate cement is early diagenetic (Fig. 4b–e).The early formation of carbonate cement is also indicated by thelarge crystal size of the pore-filling carbonate cement. Theseresults indicate that carbonate cementation takes place duringearly diagenesis. Unformed ductile grains are observed in sand-stone rich in carbonate cement, supporting the occurrence ofcarbonate cement as early diagenetic according to Salem et al.(2000). Also, the occurrence of early diagenetic carbonate ce-ment is confirmed by the loose grain packing of the sandstone(cf. Salem et al. 2000). Quartz overgrowths are rare in theIllawarra CoalMeasures, which is rich in carbonate cement. Thisindicates that carbonate cement was precipitated during earlydiagenesis, and pre-dates the development of quartz overgrowthsaccording the interpretation of Odigi and Amajor (2010).

Pore-filling siderite and grain-coating siderite indicate thatsiderite also formed during early diagenesis. Lee and Lim(2008) showed that the formation of siderite during earlydiagenesis is evidenced by the existence of pore spaces whichwere large enough for the development of euhedral sideritecrystals. However, the precipitation temperature for sideritewas raised, supported by high Mg and low Ca which arecharacteristic in the siderite cement (Abouessa and Morad2009). Also, the high Fe/Ca supports an interpretation of theprecipitation of siderite cement within the early stages of

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diagenesis (Lee and Lim 2008). The large crystals of calcitecement and poikilotopic calcite cement as well as the loosegrain packing seen in thin sections also indicates that calcitecementation occurred during early diagenesis (cf. Lee andLim 2008). The solubility product can be exceeded as a resultof increasing the carbonate ion and this supports the precipi-tation calcite as an early stage diagenetic mineral.

Dissolution/alterations of unstable detrital grains

Several types of dissolution are recorded in the Illawarra CoalMeasures as the products of mid- to late-stage diagenesis(Fig. 5d-e). These are quartz dissolution, feldspar dissolution,rock fragment dissolution and carbonate dissolution. The dis-solution of feldspar, carbonate cement and lithic grains pro-vides secondary porosity.

Authigenic quartz

Quartz overgrowths represent a mid to late authigenic mineralin this succession and the occurrence of quartz cement asovergrowth around detrital quartz grains supports this inter-pretation (Figs. 4a, 5b and 6a). The temperature suggested forthe precipitation of quartz overgrowths is between 90–130 °C(Morad et al. 2000) whereas in other studies it was suggestedto be between 40–60 °C (McBride et al. 1988; Salem et al.2000). Quartz overgrowths post-date the carbonate cementand authigenic clayminerals. Also, quartz overgrowth crystalsdo not include corrosion and this interpretation support thatquartz overgrowth occurred as a late authigenic mineral.

Several studies have shown that quartz cementation isderived from silica released by carbonate mineral replacementof quartz grains, feldspar dissolution and the transformation ofclays (Rezaee and Tingate 1997; Kim et al. 2007). Also,illitization of smectite, kaolinitization and K-feldsparalbitization are recorded as sources of silica in other studies(Luo et al. 2009). In this study, pressure dissolution is record-ed as the source of quartz cement (cf. Kordi et al. 2011). Thedistribution of grain coatings, infiltrated clays andpseudomatrix control the quartz overgrowth distribution (cf.El-Ghali et al. 2009a).

Late carbonate cementation

Thin section studies have indicated that ankerite cement wasprecipitated again after the quartz overgrowths since it partial-ly coats quartz overgrowths in some samples; thus, it is also alate authigenic mineral (Fig. 5b). Also, late-stage sideritecement has re-precipitated as new cement following dissolu-tion of early siderite cement (Fig. 5d).

Late poikilotopic calcite may be formed because of car-bonate grain or cement dissolution (cf. Al-Ramadan et al.2004). The occurrence of euhedral quartz overgrowths that

are embedded in calcite cement indicate that the quartz over-growths developed in pores prior to calcite cementation (Umaret al. 2011; Fig. 5c). Also, late-stage calcite includes minorMg and Fe that may support the precipitation of calcite duringthis stage (Sur et al. 2002). Also, the solubility product can beexceeded as a result of increasing carbonate ions by evaporationof vadose or near-surface phreatic ground water, suggesting thatthe precipitation of calcite cement during late diagenesis post-dated the quartz overgrowths.

Also, Fig. 5d showed new late authigenic siderite cementhas re-precipitated after dissolution of early siderite cement.Other studies have shown that late-stage diagenetic sideritecement has high Mg and Ca contents (Rossi et al. 2001).

Late mixed-layer illite/smectite and chlorite

Some authigenic clay minerals were precipitated again afterthe quartz overgrowths (Fig. 6a, c, e–f). Mixed-layer illite/smectite is developed on quartz overgrowths and carbonatecement (Fig. 6c and f). This indicates that mixed-layer illite/smectite is precipitated during late diagenesis. Detrital inher-itance from K-, Ca- and Mg-rich silicates under the influenceof alkalinity conditions suggests the precipitation of mixed-layer illite/smectite and chlorite as late-stage diagenetic min-erals after the development of quartz overgrowths. Also,quartz overgrowths may be coated by chlorite, indicating thatthe latter also formed as a late diagenetic mineral.

In conclusion, the main diagenetic stages represented by theIllawarra CoalMeasures, arranged in a chronological order, are:

1. Mild compaction2. Early authigenic clay minerals3. Precipitation of carbonate cement4. Dissolution and alteration5. Secondary silica overgrowths6. Late carbonate cement7. Late authigenic clay minerals

Porosity data

Thin section and SEM showed that porosity is comprised ofboth primary and secondary porosity in the southern SydneyBasin. The determination of microporosity by microscope isnot easy in most of the studied samples (El-ghali et al. 2009a).

Thin-section porosity

In the Illawarra Coal Measures, total thin-section porosityvaries between a trace and 15 % (Appendix 1).

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Pore types

Primary porosity

In this unit, primary porosity varies between 42 and 330 μm insize and consists of intergranular porosity (Fig. 6d). It rangesfrom 0 to 4 % (Appendix 1).

Secondary porosity

The size of secondary porosity is not very different fromdissolved grain size and varies between 20 and 220 μm. Thismeans that pore size and grain size are often similar.

As indicated by Smosna and Sager (2008), the originalgrain may be inferred by two characters of secondary porosityin sandstone, including size and shape. The dissolution ofunstable grains, particularly feldspar and volcanic rock frag-ments, as well as carbonate cement increases the percentage ofsecondary pores in sandstones. The secondary porosity variesbetween 0 and 11 % (Appendix 1).

Four types of secondary porosity were formed in theIllawarra Coal Measures including intergranular porosity,intragranular porosity, mouldic pores and oversized pores.Secondary intergranular porosity is seen in sandstone, occur-ring between grains due to dissolution of pore-filling carbon-ate cement and it is difficult to distinguish from primaryintergranular porosity (Fig. 5d). The dissolved detrital grains,such as feldspar and volcanic rock fragments, contribute to theformation of intragranular porosity (Fig. 7a–b). Also, partiallydissolved feldspar is clearly present associated with some sec-ondary pores and results in mouldic pores in sandstone (cf.Lima and De Ros 2002; Luo et al. 2009; Fig. 7b). Oversizedpores occur in thin section and are also affected by totaldissolution of several detrital grains in sandstone (Zhang et al.2007). Corroded traces of feldspar occur in both mouldic poresand oversized pores. Also, packing inhomogeneity andhoneycombed grains are observed associated with secondarypores.

The influence of diagenetic alteration on porosity

Diagenetic alteration that controls the reservoir quality in thesouthern Sydney Basin basically includes compaction, car-bonate cement, authigenic clay minerals, dissolution andquartz overgrowths (Salem et al. 2000; Salem et al. 2005;Zhang et al. 2007; Luo et al. 2009).

Compaction

Compaction is the main reason for porosity loss in theIllawarra Coal Measures. Chemical and mechanical compac-tions are bothmoderate to high and play a dominant role in the

porosity reduction in the Illawarra CoalMeasures. Mechanicalcompaction is recorded between framework grains and isrepresented by deformation of ductile grains (mica and rockfragments) and mud intraclasts as the main factors that con-tribute to the porosity reduction.

In sandstone rich in detrital lithic fragments, the wide-spread presence of ductile grains in most samples indicatesthat most porosity was reduced by mechanical compaction.Thus, pore space is not commonly available for quartz pre-cipitation. So, the influence of mechanical compaction onporosity reduction is strengthened by the presence of ductilegrains in the Illawarra Coal Measures (Ehrenberg et al. 2008).Also, the common occurrence ofmica andmudrock fragmentsin the Illawarra Coal Measures supports the influence ofcompaction to reduce porosity (cf. Luo et al. 2009). Porespaces were partly or completely filled by pseudomatrix,which is formed by mechanical compaction and has abilityto reduce porosity in the Illawarra Coal Measures (El-Ghaliet al. 2009a). Its importance is stronger in sandstones that havefew remaining lithic grains (cf. Al-Ramadan et al. 2004). Thisillustrates the influence of sandstone composition on thin-section porosity measurements in the Illawarra Coal Mea-sures. Mansurbeg et al. (2009) showed that mechanical com-paction is an important cause of porosity loss, supported bythe presence of bent mica flakes, carbonate grains and plasticdeformation of ductile sedimentary grains. Modern alterationand extension of detrital mica, especially in outcrop samples,is another significant factor in the partial porosity loss in theIllawarra Coal Measures, as shown by Hlal (2008). Also,detrital mica contributes to the partial porosity loss throughthe development of intergranular pressure dissolution ofquartz grains (cf. Hlal 2008). Mechanical compaction reducesporosity during early diagenesis and had more influence insamples with low contents of early carbonate cement in var-ious units of the Illawarra Coal Measures (cf. Umar et al.2011). This indicates that the reservoir quality was reduceddue to mechanical compaction during early burial in thestudied succession. For example, the Loddon Sandstonecontains rare carbonate cement, thus mechanical compactionhad more influence on porosity whereas the influence ofcompaction is less apparent with the more abundant carbon-ate cement in the Darkes Forest Sandstone. Mechanicalcompaction strongly affects porosity, particularly in theKembla Sandstone and Loddon Sandstone, which are dom-inated by clays with very low quartz contents (cf. Ehrenberget al. 2008). Grain rearrangement is also important to reduceporosity where lithic grains are absent, particularly in quartz-ose samples (Worden et al. 2000). Mechanical compactionalso reduces early secondary porosity particularly beforequartz cementation.

The occurrence of long, concavo-convex and sutured graincontacts shows the influence of chemical compaction onporosity reduction in the Illawarra Coal Measures. The

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interpenetration of grains provides important evidence of po-rosity reduction by chemical dissolution in the Illawarra CoalMeasures. Also, chemical compaction is more common in theabsence of carbonate cement in the reduction of porosity (cf.Kim and Lee 2004).

In general, the porosity that is reduced by mechanicaland chemical compaction cannot be recovered, as shownby Wolela (2009). In some samples, secondary porosity iscompletely absent which may also be due to compaction.This indicates that the secondary porosity in these sampleswas created before compaction had ended and then itbecame compacted, thus it disappeared (cf. Kim and Lee2004). The influence of compaction on porosity is stronger

in coarse-grained and moderate sorted sandstone. Also, along burial period and the abundance of lithic grains inthe Kembla Sandstone and Loddon Sandstone are impor-tant factors that enhanced the influence of compaction toreduce porosity.

In the Wilton Formation, Bargo Claystone and DarkesForest Sandstone the influence of compaction on reservoirquality is rare because of the abundance of early diageneticcarbonate cement in these units. Thus, this confirmed thatporosity is mainly reduced by compaction of poorly cementedunits. The proportion of porosity reduction under the influencecompaction during burial can be determined by the ratio oflithic to quartzose grains.

Fig. 7 a Secondary porosity ledto the dissolution of a rockfragment (white arrow). bFeldspar is totally dissolved,forming oversized porosity (whitearrow). c Microporosity (whitearrow) results fromkaolinitization of feldspar

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Carbonate cementation

After the influence of compaction on porosity, carbonatecement is the second main factor causing porosity loss in theIllawarra Coal Measures (Ketzer et al. 2003). In general,carbonate cement is the main type of porosity loss that maybe recoverable as result of dissolution of carbonate cement asshown by Wolela (2009).

In the coarse-grained deposits, carbonate cement hascaused a significant decrease in the porosity and it is alsosignificant in the secondary porosity increase, via dissolution,in the Kembla Sandstone, Lawrence Sandstone and LoddonSandstone. Thin-section porosity may also be destroyed byeogenetic carbonate cement, which is observed as a pore-filling cement in the coarse-grained deposits (e.g. El-Ghaliet al. 2009b; Umar et al. 2011). Ankerite cement is distributedin both large and small pores and is the most importantcarbonate cement in the porosity loss in these coarse deposits.Also, porosity is lost in the coarse-grained deposits because ofthe occurrence of dolomite and calcite cements (El-Ghali et al.2009a, b) and eogenetic siderite (cf. Bertier et al. 2008) inpores spaces. This indicates that the abundance of carbonatecement prevents the development of porosity in the coarse-grained deposits as shown by Gier et al. (2008). Also earlycarbonate cement is a barrier to the development of quartzovergrowths (cf. Umar et al. 2011) and intense mechanicalcompaction (cf. Rossi et al. 2001) in the coarse-grained de-posits. When widespread, the early carbonate cementcompletely fills both large and small pores and reduces po-rosity, whereas if there is only minor early carbonate cement itcan still support the framework grains against compaction.

In some samples from the Kembla Sandstone, Law-rence Sandstone and Loddon Sandstone, low porosity ispreserved by early carbonate cement which is significantin inhibiting the development of early mechanical com-paction. In later diagenesis, dissolution of carbonate ce-ment contributes to the development of reservoir qualitywhen and where it creates the low quantities of secondaryporosity in these units (e.g. Zhang et al. 2008; Chi et al.2003; Zhang et al. 2007). However, where early cemen-tation was abundant and almost completely occluded pri-mary porosity, later dissolution was compensated by re-precipitation of calcite cement and no new porosity wascreated. New secondary porosity was generated if thedissolution of this early calcite cement was not compen-sated by new cement formation. Similar results wereobserved in many studies, for example, the study by Chiet al. (2003) in the Late Carboniferous strata in the Mar-itimes Basin. In other cases, carbonate grains are effectivein the development or destruction of porosity. They maybe dissolved and result in secondary porosity or, in othercases, they are dissolved and then re-precipitation ascarbonate cement (cf. Hlal 2008).

In some samples pore-filling carbonate cement completelyreduces porosity in the Illawarra Coal Measure. In othersamples, open pore space can be observed as a result of partialcarbonate cementation or later dissolution of carbonate ce-ment (cf. Zhang et al. 2008). In pores with incomplete filling,low porosity can also be observed in some samples as shownby Al-Ramadan et al. (2004). Late diagenetic pore-fillingcarbonate cement is minor in some samples and in this caseit has little influence on reservoir quality.

In fine-grained deposits, early carbonate cement is presentas a pore-filling cement that reduces porosity in these deposits(cf. Frohlich et al. 2010; Al-Ramadan et al. 2004). In thesedeposits, porosity loss is not recoverable by dissolution ofcarbonate cement.

Authigenic clay minerals

Authigenic clayminerals affect thin-section porosity but are ofless influence than compaction and carbonate cement in theIllawarra Coal Measures. In coarse-grained deposits, repre-sented by the Kembla Sandstone, Lawrence Sandstone andLoddon Sandstone, authigenic clay minerals, includingmixed-layer illite/smectite, kaolinite, illite and chlorite, haveinfluenced the thin-section porosity. They were observed byscanning electron microscope as pore-filling, pore-bridgingand pore coating clays, thus they decrease the porosity inthese units. In general, pore-filling and pore-lining clays pre-vent the preservation or development of porosity in these units(cf. Wolela and Gierlowski-Kordesch 2007). Fibrous mixed-layer illite/smectite is recorded as a pore-filling cement, re-ducing primary porosity in these units. Vermicular booklets ofkaolinite crystals are also present in primary and secondarypores and also reduce both primary and secondary porosity(cf. Borgohain et al. 2010). Primary porosity also decreases inthese units as a result of pseudomatrix, which is generated bydeformation of ductile lithic grains, or the development of latediagenetic kaolinite and compaction in some samples. Chlo-rite is observed to fill primary pores and causes a porosity lossin these units (e.g. Zhang et al. 2007; Al-Ramadan et al. 2004).It also fills intergranular secondary pores leading to a porosityreduction. On the other hand, illite exists as a pore-fillingcement (Al-Ramadan et al. 2004; Mckinley et al. 2011) andas pore-bridges between grains; consequently it can reduceporosity in the sandstone. In general, pore-filling mixed-layerillite/smectite and pore-filling chlorite cause a greater reductionof porosity in coarse-grained deposits than pore-filling kaoliniteand pore-filling illite. This porosity reduction by pore-fillingclays is unrecoverable in the Illawarra Coal Measures.

In coarse-grained deposits and according to scanning elec-tron microscope studies, chlorite, illite and mixed-layer illite/smectite are present as grain coatings and prevent the devel-opment of quartz overgrowths, thus minor primary porosity ispreserved in these deposits. In many samples, scanning

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electron microscope analysis showed grain-coating mixed-layer illite/smectite as a barrier to repel the growth of quartzovergrowths, thus retaining low porosity. Also, the develop-ment of quartz overgrowths around quartz grains is preventedby thick grain-coating chlorite, which preserves low porosityas result of this process in coarse-grained deposits (e.g. Al-Ramadan et al. 2004; Salem et al. 2005; Zhang et al. 2008;Armitage et al. 2010). Continuous layers are created by grain-coating chlorite on the surface of detrital grains adjacent tointergranular pore spaces, thus this process prevents the growthof quartz overgrowths and preserves porosity. Primary porositycan also be preserved by grain-coating illite, which also preventsthe development of quartz overgrowths in coarse-grained de-posits (Moraes and De Ros 1990; Bloch et al. 2002; Abouessaand Morad 2009). Grain-coating mixed-layer illite/smectite andgrain-coating chlorite have more influence on quartz over-growths than secondary kaolinite and illite in the coarse-grained deposits. Thus, they are more important in the preser-vation of porosity. Also, quartz cement is influenced more byauthigenic clay coatings than by detrital clay rims. Authigenicclays coating are more continuous than detrital clay rims that arepresent as a result of mechanical infiltration (Taylor et al. 2010).In general, quartz overgrowths increase with small quantities ofgrain-coating clays and is reduced by more abundant grain-coating clays. Grain-coating clays are more important in thepreservation of porosity, particularly in the presence of carbonatecement and quartz overgrowths, which prevent the developmentof compaction (cf. Salem et al. 2000). Microporosity is presentin coarse-grained deposits due to kaolinitization of feldspar (e.g.Bertier et al. 2008; Abouessa and Morad 2009; Fig. 7c).

In fine-grained deposits, authigenic clay minerals are dom-inant and prevent the preservation or growth of secondaryporosity. Kaolinite, illite, chlorite and mixed-layer illite/smectite are distributed through the pores and cause a deteri-oration of porosity in these deposits. The presence of grain-coating clays generally does not preserve porosity in fine-grained deposits (Zhang et al. 2010). This is because theyare thin and discontinuous or because they are very rare, thusthey were not able to inhibit diagenetic alteration, such quartzovergrowths. Thus, the influence of grain-coating clays onquartz overgrowths is weak in these samples.

Dissolution

Dissolution of grains is recorded as a mid- to late-stage dia-genetic process that generates and preserves minor secondaryporosity during burial of the Illawarra Coal Measures. Thisprocess has less influence on porosity than compaction; car-bonate cement and pore-filling clay minerals, but it has moreinfluence on the measured porosity than grain-coating clays.In the Kembla Sandstone, Lawrence Sandstone and LoddonSandstone, secondary porosity is observed where the abun-dance of lithic grains and the presence of feldspars allow the

development of secondary porosity. Thin sections showingpartial to complete dissolution of detrital grains, such asfeldspar and rock fragments, supports the generation andpreservation of minor secondary porosity in these sandstones(cf. Ketzer et al. 2003; Al-Ramadan et al. 2005). Feldspargrains were partly dissolved in some samples resulting inminor secondary porosity that is preserved in these samples(cf. Luo et al. 2009). This provides evidence that the dissolu-tion of detrital feldspar is significant in the enhancement ofsecondary porosity in the Illawarra Coal Measures (e.g. Limaand De Ros 2002; Rossi et al. 2002; Ketzer et al. 2003; Duttonand Loucks 2009). Secondary porosity, which formed bypartial dissolution of coarse plagioclase grains, is higher thanthat produced by fine plagioclase grains in the Illawarra CoalMeasures. Also, secondary porosity, created by dissolution ofvolcanic rock fragments is more than by dissolution of sedi-mentary rock fragments because the former dominate thelatter. Also the glassy volcanic detritus is much more unstablethan the quartz and clays forming the sedimentary clasts.Here, the role of composition has a major influence on poros-ity generation. The presence of feldspar and lithic grains canenhance porosity through their dissolution. Also, dissolution ofcarbonate cement is a mechanism to increase porosity in thestudied succession (cf. Chi et al. 2003) and can be formed afterquartz overgrowth precipitation in the Illawarra Coal Measures(Rossi et al. 2001). In some samples, large sections of open,well-connected secondary intergranular pores can be producedby the dissolution of calcite cement (cf. Mansurbeg et al. 2008).

In the Illawarra CoalMeasures, dissolution of detrital grains,such as feldspar and lithic grains, is more important in theformation of secondary porosity than dissolution of cement.This indicates that the formation of secondary porosity bydissolution of detrital grains is volumetrically subordinate inthe Illawarra Coal Measures. However, the determination ofsecondary porosity generated by framework grain dissolution iseasier than recognising secondary porosity resulting from pore-filling cement dissolution (Taylor et al. 2010). In some samples,dissolution of feldspar did not result in secondary porosity dueto the precipitation of kaolinite in the resultant pores. Thismeans that kaolinite may be formed to fill secondary porescreated by dissolution of feldspar (cf. Bertier et al. 2008). Also,dissolution of feldspar may result in secondary pores that canalso be filled with carbonate cement as shown by Umar et al.(2011). The latter may have prevented the development ofporosity in the Illawarra Coal Measures. Another reason is thatmechanical compaction also effectively causes the collapse ofthe secondary porosity after dissolution (Wilkinson et al. 2001;Dutton and Loucks 2009). The abundance of mechanical com-paction may indicate that secondary porosity was more com-mon than primary porosity. However, if only the inside of thefeldspar grain is subjected to solution porosity (cf. Zhang et al.2007) this may indicate why feldspar dissolution is not effectivein the development of porosity in some cases. In some samples,

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secondary pores that result from dissolution of polycrystallinequartz are filled by carbonate cement. Thus, they are also noteffective in the development of porosity.

Organic acids, CO2 and dehydration of clay minerals alsocontribute to the dissolution of grains and cements and thegeneration of secondary porosity (Zhang et al. 2007). The matu-ration of organic matter is the source of organic acids and CO2

(Zhang et al. 2007). Also, Chi et al. (2003) showed that reactionsbetween inorganic minerals and meteoric water contribute to thedissolution of grains and cements and generation of secondaryporosity. Unstable grains are dissolved by percolation of gravity-driven acidic meteoric water and groundwater, forming second-ary porosity (Wolela and Gierlowski-Kordesch 2007). Also,meteoric water has a main role in the dissolution of carbonatecement to generate secondary porosity (Chi et al. 2003).

Quartz overgrowths

In the Illawarra Coal Measures, quartz overgrowths only have aminor influence on primary porosity because of the abundanceof ductile lithic grains and carbonate cement that control poros-ity. Also, quartz grains are not abundant in the Illawarra CoalMeasures, thus there is little opportunity for quartz overgrowthdevelopment. This suggests that the influence of quartz over-growth on porosity is low. Minor quartz cementation occurs insome pores and is not a significant mechanism for the porosityloss. In the Kembla Sandstone, Lawrence Sandstone andLoddon Sandstone, scanning electron microscope studiesshowed that grain coatings prevented quartz overgrowth devel-opment (Fig. 6a, e–f). The inhibition of quartz overgrowthsmeans that they have almost no influence on the preservation ofprimary porosity (cf. Salem et al. 2005; Zhang et al. 2007).

Reservoir Potential of the Illawarra Coal Measures

Porosity was measured for the Illawarra Coal Measures. Po-rosity is important to determine groundwater and petroleumreservoirs. Porosity with structure, control the potential for gasin these units. Extractable groundwater and gas may be pres-ent in the units which contain high porosity whereas it wouldbe unavailable in units which contain low porosity. Also,potential oil and gas sources will be shown in this part.

In the Illawarra Coal Measures, thin-section porosity iscompletely absent in the Wilton Formation, Bargo Claystone,Darkes Forest Sandstone, Allans Creek Formation, UnnamedMember Three and Unnamed Member Two which arecharacterised by shale, siltstone and fine-grained sandstone.Thus, these formations are less porous, they are poorly sortedand fine-grained. Carbonate cement and authigenic clays arepresent in the shale, siltstone and fine-grained sandstone aspotential barriers to prevent the preservation of porosity. An-kerite cement and mixed-layer illite/smectite have considerable

influence on reservoir quality, thus pore spaces availablefor hydrocarbons occurrences are absent. These factorsrepresented by lithology, diagenetic alteration, sortingand grain size indicate that these formations are litho-logical seals and confining layers in the Illawarra CoalMeasures.

Also, changes in facies and grain size play a role ininfluencing gas migration and groundwater movement in theIllawarra Coal Measures. Freeze and Cherry (1979) showedthat the hydraulic gradient is changed in aquifers as result ofvariations in facies. The Kembla Sandstone, Lawrence Sand-stone Member and Loddon Sandstone were deposited in flu-vial environments that were not dissimilar to the environmentsof many other typical fluvial groundwater and hydrocarbonreservoirs. Medium- to coarse-grained sandstone, siltstoneand shale units are present in the Kembla Sandstone, Law-rence Sandstone and Loddon Sandstone. Medium- to coarse-grained sandstone units are more common in the channelfacies than siltstone or shale. They are moderately to wellsorted, with rounded to subrounded grains. These featurescan preserve moderate porosity in medium- to coarse-grained sandstone and indicate that these units probably con-tain gas or water (Wolela 2009). Well logs confirmed thepresence of porosity in the medium- to coarse-grained sand-stone. Diagenetic alteration has influenced the reservoir qual-ity, and hence has affected the possibility of hydrocarbonoccurrences. Carbonate cement is also commonly present inthe medium- to coarse-grained sandstone where it fills porespaces, reducing thin-section porosity. Clay minerals are pres-ent in two forms—as detrital matrix and authigenic clays. Theinfluence of the latter on porosity is stronger than the former.Mechanical compaction is also common in the medium- tocoarse-grained sandstone and further reduces thin-section po-rosity. All these factors affect reservoir quality, and thus thepore space available for gas or water is probably low in themedium- and coarse-grained sandstone. Dickey (1981)showed that the migration of hydrocarbons is reduced by thepresence of clay minerals. This indicates that mechanicalcompaction, carbonate cement and authigenic clays all leadto the production of effective seals. Dissolution of grainsresults some secondary porosity in the medium- and coarse-grained sandstone, thus increasing the pore spaces availablefor gas or water. According to Ryan (2005), an abundance ofvolcanic rock fragments in a sandstone commonly leads tolow porosity, destruction of hydrocarbon reservoirs and theproduction of permeability traps. In this study, volcanic rockfragments are common in sandstone beds in the KemblaSandstone, Lawrence Sandstone and Loddon Sandstone, thusthey are contain hydrocarbon traps adjacent to low porosityzones.

Thin-section porosity is absent in the floodplain shale andsiltstone units that form lithological seals and confining layersin the Kembla Sandstone, Lawrence Sandstone and Loddon

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Sandstone. These shale and siltstone units are poorly sortedand fine-grained. Schoen (1996) showed that the preservationof primary porosity can be affected by grain size and sorting.This indicates that poor sorting and fine grains prevent thepreservation of visible porosity in shale and siltstone units.Also, secondary mixed-layer illite/smectite, kaolinite and illiteare distributed through the shale and siltstone units and furtherreduce the porosity. Thus, pore spaces available for gas orwater probably are absent in these units. This indicates thatshale and siltstone beds form local vertical permeability sealsin and between the Kembla Sandstone, Lawrence Sandstoneand Loddon Sandstone.

The Tongarra Coal, Allans Creek CoalMember,WongawilliCoal, Hargrave Coal Member, Cape Horn Coal Member,Balgownie Coal Member and Bulli Coal all contain coal seamgases and are recognised as the main source of gas in theIllawarra Coal Measures. They result gas to the Kembla Sand-stone, Lawrence Sandstone and Loddon Sandstone which areunderlain by Allans Creek Coal Member, Cape Horn CoalMember and Balgownie Coal Member, respectively. Thus,gas may migrate from the Allans Creek Coal Member to theoverlying Kembla Sandstone and from the Cape Horn CoalMember to the overlying Lawrence Sandstone. Also, gas maymigrate from the Balgownie Coal Member to the overlyingLoddon Sandstone.

Gentle synclines and faults are present in the Illawarra CoalMeasures and have influenced the migration of gas or move-ment of water. Synclines may be recorded as the potentialbarriers to gas flow in the Illawarra Coal Measures. Also,faults are important for draining or preserving hydrocarbonsin a reservoir. In some individual units, reservoirs may berestricted by faults, which are present as seals and breakanticlinal reservoirs according to Dickey (1981). Also, faultshave probably influenced groundwater movement in theIllawarra Coal Measures. Faults can prevent free groundwatermovement and form semi-isolated groundwater potentialwithin the Illawarra Coal Measures. This indicates that thepath of the groundwater movement can be changed as result ofthe occurrence of faults.

Conclusions

The Illawarra Coal Measures is mostly litharenite with raresublitharenite. The Illawarra Coal Measures plot in the lithicrecycled to transitional recycled and quartzose recycled prov-enance fields. In the Illawarra CoalMeasures, quartz and lithicgrains are derived from the New England Fold Belt andan eastern volcanic arc with minor contributions from theLachlan Fold Belt.

Petrographic data demonstrated that lithic grains are com-mon in the Illawarra Coal Measures whereas feldspar grainsare sparse in this unit. Quartz includes monocrystalline and

polycrystalline quartz grains whereas feldspar grains consistof K-feldspar and plagioclase. Rock fragments are volcanic orsedimentary. Mica includes more common muscovite thanbiotite. The heavy minerals comprise hematite, hornblende,rutile, zircon and tourmaline in trace percentages.

Thin section and scanning electron microscope analyseswere used to describe the alteration and diagenesis in theIllawarra Coal Measures. Results showed that carbonate ce-ment is most common in the Illawarra Coal Measures.

In the Illawarra Coal Measures, authigenic minerals includ-ed quartz overgrowths, authigenic clay minerals, carbonatecement and authigenic feldspar. Quartz overgrowths are un-common whereas authigenic clay minerals are characterisedby kaolinite, illite, mixed-layer illite/smectite and chlorite.Siderite is the most common carbonate cement in the IllawarraCoal Measures.

In the Illawarra Coal Measures, thin-section porosity isobserved in some samples whereas density porosity is rep-resented in most samples. The study noted the presence oftwo types of porosity in the Illawarra Coal Measures. Theyare primary and secondary porosity. The latter occurs athigher percentages than former in the Illawarra CoalMeasures.

The influence of mechanical compaction on thin-sectionporosity is stronger in the Illawarra Coal Measures. Car-bonate cement is prevalent in the Illawarra Coal Measures,thus it has an important influence on thin-section porosity.In the Illawarra Coal Measures, pore-filling clays are dom-inant and reduce thin-section porosity in this unit. Also,grain-coating clays preserve thin-section porosity in thisunit. In the Illawarra Coal Measures, lithic and feldspargrains are common, thus secondary porosity caused byunstable grain dissolution is more abundant in the IllawarraCoal Measures.

In the Illawarra Coal Measures, individual units represent-ed by the Wilton Formation, Bargo Claystone, Darkes ForestSandstone, Allans Creek Formation, UnnamedMember Threeand Unnamed Member Two have very low thin-section po-rosity values. These formations are characterised by poorlysorted shale, siltstone and fine-grained sandstone which havelow visible porosity. Thus, these formations are recorded aseffective water and hydrocarbon seals and confining layers inthe Illawarra Coal Measures. On the other hand, the KemblaSandstone, Lawrence Sandstone and Loddon Sandstone arerepresented by medium- and coarse-grained sandstone unitswhich have low thin-section porosity and are moderately towell sorted. Thus, they would contain gas or water in thesubsurface. In the Illawarra Coal Measures, coal seam gas iscommonly presented in the Tongarra Coal, Allans Creek CoalMember, Wongawilli Coal, Hargrave Coal Member, CapeHorn Coal Member, Balgownie Coal Member and Bulli Coal.These formations probably are the source of gas in the KemblaSandstone, Lawrence Sandstone and Loddon Sandstone

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Appendix 1 Point count results of samples from the Illawarra Coal Measures

Samples Q F RF Ch Mu HML C Ma Po

Qo Cac

S Ca 0 D Tcarb P1 P2 TPo

LDSS—EAW 156 (14) 14.1 2.8 60.5 47.9 0 0.6 1.1 1.3 1.3 1.5 0 3.9 20.8 0 0 0

LDSS—EAW 156 (15) 12.7 1.8 52.3 40.4 0 0.5 1.1 0.7 0.8 0 1.7 6 17 1.5 7 8.5

LDSS—EAW 30 (30) 22.9 2.2 44.3 23.2 1 0.9 2 2.6 2 0.5 0 6.3 19.5 0 0 0

LDSS—EAW 30 (31) 20.2 2.4 48.4 33.3 0 1.4 0.7 3.5 3.2 0 1.1 16 6 0.5 4.2 4.7

LDSS—EAW 30 (32) 22.4 1.5 51.3 35.8 0.4 0.7 0.4 2.2 1.9 0 0 4.7 18.2 0 0 0

LDSS—EDEN 124 (13) 9.5 1.9 62.4 54.2 4 0.9 0.5 1.2 0.6 0 0 1.8 18.7 0 0 0

LDSS—EDEN 124 (14) 11.2 2 54.1 45.6 0.4 0.9 0.9 0.3 0.4 0.7 4 10.4 18 0 2 2

LDSS—EDEN 124 (15) 9.2 1.4 56.1 47.1 1.7 1.2 0.4 0.8 0.4 0.3 0.6 3.4 22.8 0.7 3 3.7

LDSS—EDEN 124 (16) 8 1.2 58.3 47.7 0.5 1.1 0.4 4 0.3 0 0.2 5.6 19.2 0.3 5 5.3

LDSS—EDEN 125 (15) 13.5 0.8 37.5 35.4 4 1.6 0.3 3.1 2.6 1 0.9 7.9 33.3 0 0 0

LDSS—EDEN 125 (16) 12.9 1 39.1 32.5 0.8 1.3 0 1.1 0.5 0 10.3 25.3 14 1 4.5 5.5

LDSS—EDEN 125 (17) 11.1 1.1 36.6 29.8 0 1.4 0.2 2.1 0.5 0 1 6 41.3 0 2 2

LDSS—EDEN 126 (16) 12.4 1.1 57.7 43.4 4.2 0.9 1 0 0.4 0.5 0 0.4 22 0 0 0

LDSS—EDEN 126 (17) 13.4 2.4 51.4 38 0.5 1.2 0.3 2 1.1 0 0.5 7.8 21 0.5 1.5 2

LDSS—EDEN 126 (18) 15.3 2.6 53.2 41.4 1.4 1 0.3 1.5 1.5 0 0 3 23 0 0 0

LDSS—EDEN 127 (15) 20.8 2.1 56.2 48.5 0.3 1.1 0.7 3.2 0.3 0 0 3.5 15.3 0 0 0

LDSS—EDEN 127 (16) 10.1 2 65.2 57.9 0.4 0.8 0.1 0.2 0.1 0.5 1.1 2.5 16.3 0 1.2 1.2

LDSS—EDEN 127 (17) 10.3 2.1 56.6 50.7 0.5 0.8 0.2 0.3 0 1.5 0.8 2.1 22.8 0.5 4 4.5

LDSS—EDEN 127 (18) 10.1 1.9 59.2 54.1 0.5 1.2 0.2 0.6 0.1 0 0.6 2.2 15 1.5 8 9.5

LRSS—Surface 1 18.6 2 55.5 50.7 0 1 0.7 0.8 4.6 0 0 10 12 0 0 0

LRSS—Surface 3 26.4 1.6 52.7 49.2 1.1 1.4 0.6 0 0 0 0.9 0.9 15.3 0 0 0

LRSS—Surface 4 16.7 0.2 71.7 67.4 0 0.5 0.7 1.2 0 0 0 1 .2 8.8 0 0 0

LRSS—EAW 156 (18) 18.2 1.5 27.2 19.2 0.4 1 0.7 5 0.5 0.5 0.5 6 44.3 0 0 0

LRSS—EAW 156 (19) 9.4 2.4 42.7 35.6 0.3 0.5 0.2 1 0.2 0.5 11.1 25.3 15.6 0.5 3 3.5

LRSS—EAW 156 (20) 10.4 1.7 41.5 35.7 0 1.2 0.2 3.7 0.7 0 3.3 17.4 24.3 0.5 2.5 3

LRSS—EAW 30 (33) 32.8 2.2 14 12 1.6 0.5 0.8 2.4 1.8 0 0 4.5 43.2 0 0.2 0.2

LRSS—EAW 30 (34) 31.3 2.5 29.7 26.3 0 0.4 0.6 3.3 1.9 0 0.8 13.1 21.3 0 1 1

LRSS—EAW 30 (35) 49.4 3.2 15.1 13.2 1.8 0.8 1.1 8.9 0.6 0 0 9.5 18.7 0 0 0

LRSS—EDEN 124 (18) 7.8 1.2 45 36.7 0.3 0.6 0.1 0.1 1.3 29.4 10.2 41 4 0 0 0

LRSS—EDEN 124 (19) 17.7 2.1 42.2 32.4 0.3 0.7 0.8 2.2 0.4 2.9 0.9 6.4 29.6 0 0 0

LRSS—EDEN 125 (37) 15.3 1.4 37.5 32.4 1.5 0.5 0.7 1.5 1.5 7.5 1.6 12.1 15.9 4 11 15

LRSS—EDEN 125 (19) 18.4 2 53.3 42.4 1.1 1.3 0.8 3.5 2.7 6.4 1.3 13.9 6.3 0.3 2.4 2.7

LRSS—EDEN 126 (19) 17.9 2.5 49.4 46.2 2.2 1 0.4 2 0.4 3 2.4 7.8 18.8 0 0 0

LRSS—EDEN 126 (20) 12.9 1 7.1 7.1 1.8 1.2 0.3 14.2 3.2 0 0 17.4 58.2 0 0 0

LRSS—EDEN 126 (21) 10.8 0.9 5.2 5.2 1.3 0.7 0.3 0.8 8.2 17.3 6.6 32.9 47.8 0 0 0

LRSS—EDEN 127 (19) 11.5 1 1.9 1.2 0.9 1.2 0 3.3 0.3 0.4 0.5 4.5 78.8 0 0 0

LRSS—EDEN 127 (20) 12.7 1.6 17.2 12.7 0.3 0.6 0.2 0.3 0.2 15 8.7 24.2 43.2 0 0 0

UNM2—EAW 30 (51) 19 1 16 3 1.1 0.3 0.4 5.8 0.3 12.4 6.2 24.7 37.5 0 0 0

UNM3—EAW 156 (22) 12.1 1.3 47.4 39.3 0.1 0.9 0.2 14.2 0.9 0 3.3 18.4 19.5 0 0 0

UNM3—EDEN 126 (22) 12.8 2.3 6.9 6.9 1.8 0.5 0.3 4.2 2.6 8.5 3.1 18.4 57 0 0 0

UNM3—EDEN 127 (21) 13.4 1 16.1 13.1 1 0.6 0.3 1.6 0.4 14.4 6.8 23.2 44.4 0 0 0

WWCO—EDEN 124 (22) 16.3 1.1 41.7 37 0 1.3 0.8 2.5 0 2 0.3 4.8 34 0 0 0

KBSS—Surface 1 5.8 0.7 69.8 65 0 1.5 0.1 0 3.8 0 1.9 5.7 16.4 0 0 0

KBSS—Surface 3 13.4 0.9 40.4 37.1 0 1.4 0.5 0 23.3 2.6 1.2 27.1 16.3 0 0 0

KBSS—Surface 5 9.2 0.8 67.2 52.9 0 0.7 0.2 0 17.6 0.3 0 17.9 4 0 0 0

Arab J Geosci

which are more porous and permeable. For example, gas maymigrate from the Allans Creek Coal Member to the overlyingKembla Sandstone and from the Cape Horn Coal Member tothe overlying Lawrence Sandstone. Also, gas may migratefrom the Balgownie Coal Member to the overlying LoddonSandstone. Porosity is higher in the Loddon Sandstone than inthe Kembla Sandstone and Lawrence Sandstone. Thus, thevolume available for gas or water in the Loddon Sandstone isprobably greater than in the Kembla Sandstone and LawrenceSandstone.

Acknowledgments This work was supported by the University ofWollongong. Samples were collected by BHPBilliton Illawarra Coalwho provided samples from study area. Also, they supported mewith well logs, maps and additional information. Thin sections,scanning electron microscope photos and X-ray diffraction analyseswere completed by the laboratories in the University of Wollongong.I would like to thank the library staff in the University of Wollon-gong who helped me in searching for information and borrowingscientific journals from other universities. Also, I would like to thankreviewers by Arabian Journal of Geosciences who examined thisarticle and they are Dr. Ihsan and Dr. Mohamed Khalifa (Universityof Malaya, Malaysia).

Appendix 1 (continued)

Samples Q F RF Ch Mu HML C Ma Po

Qo Cac

S Ca 0 D Tcarb P1 P2 TPo

KBSS—Surface 6 1.2 0.8 90.5 90.5 0 0 0 0 1 1.6 3.7 6.3 0 0.2 1 1.2

KBSS—Surface 7 8.2 1.4 74.4 70 0 1.1 0.2 0 5 0.3 0 5.3 8.3 0 1 1

KBSS—EAW 156 (25) 19.9 3 50.9 44.2 0.8 0.8 0.8 2.3 0.4 0.2 0 2.9 20.7 0 0 0

KBSS—EAW 156 (26) 17 0.5 52.4 46.2 0 0.4 0.8 1.3 0 0.8 0.5 2.6 23.5 0.5 2.2 2.7

KBSS—EAW 156 (27) 11.6 1.5 60 45.7 0 0.9 0.5 1.4 0.4 3.3 1.4 6.5 18.2 0 0.7 0.7

KBSS—EDEN 124 (23) 7.7 1 47.7 34.8 0.1 0.9 0.5 2 11.1 16.8 4.7 34.6 7.5 0 0 0

KBSS—EDEN 124 (24) 4.6 1.1 63.9 53.3 0 0.9 0.1 0.3 3.5 8.9 1.7 14.4 11.3 0.3 3.4 3.7

KBSS—EDEN 124 (25) 6.2 1.1 59.8 50.7 0.1 1.1 0.2 0.5 1.4 6.9 1.2 10 16.8 0.5 4.2 4.7

KBSS—EDEN 125 (21) 11.2 0.5 5.4 5.2 0.3 2.7 0.2 1.2 0 0 0 1.2 78.2 0 0 0

KBSS—EDEN 125 (22) 5.3 1.4 49.3 26.3 0 0 0 1.8 8.8 23.5 5.2 39.3 0.5 1 3.2 4.2

KBSS—EDEN 126 (23) 16.4 2.1 36.1 29 1.1 1.2 0.5 2.2 1.1 17.6 5.6 26.5 15.8 0 0 0

KBSS—EDEN 126 (24) 9.8 1.8 64.5 42.5 0 1.6 0.2 0.5 1.7 5.1 1.5 8.8 12.3 0 1 1

KBSS—EDEN 126 (25) 18.1 2 53.1 43.5 0.3 1.2 0.7 2.2 0.4 0 0 2.6 20.8 0 1.2 1.2

KBSS—EDEN 127 (22) 22.7 1.8 42.7 37 0.9 0.6 0.5 1.2 0.4 12.7 2.7 17 13.8 0 0 0

KBSS—EDEN 127 (23) 9.4 1.5 66.7 58.6 0 0.7 0.2 0.6 0 10.3 1.3 12.2 8.8 0 0.5 0.5

KBSS—EDEN 127 (24) 19.3 1.9 38.6 35.9 1.1 0.8 0.4 4.2 1.3 0.6 0 6.1 31.8 0 0 0

ACFM—Surface 1 12.9 0.2 2 2 0.9 1.3 0.2 0 0 0 0 0 67.2 0 0 0

ACFM—Surface 3 16.4 1.5 67 66.3 1.5 0.7 0.5 0.3 0.4 0.1 0.1 0.9 5.8 0 0 0

ACFM—EDEN 124 (28) 8.2 1.8 29.9 24.9 0.8 0.6 0.3 0.3 9.9 21.6 3.3 35.1 23.3 0 0 0

ACFM—EDEN 126 (28) 14.3 2.4 56 49.9 0 1.6 0.7 0.2 0 1 0.3 1.5 23.3 0 0 0

DFSS—EDEN 124 (30) 7 1.8 32.5 25 1.5 1 0.1 1.8 23.4 12.3 0.8 38.3 17.8 0 0 0

DFSS—EDEN 125 (27) 14 1 24.8 22.3 2.1 1.6 0 51.1 0 0 0 51.1 5.4 0 0 0

DFSS—EDEN 125 (28) 9.6 2.1 26 25 0.2 1 0 2.3 44.7 6.2 1.8 55 6.1 0 0 0

DFSS—EDEN 126 (36) 15.6 2.5 37 30.6 0 0.9 0.5 3.7 0 19.9 4.9 28.5 15 0 0 0

DFSS—EDEN 126 (29) 17.1 2.3 40.8 32.6 0 1.5 0.5 0.4 3.9 17.2 7 28.5 9.3 0 0 0

DFSS—EDEN 127 (29) 9.8 0.5 7.5 6.5 0.2 1.4 0.2 3.2 2.2 46.7 8.3 60.4 19.9 0 0 0

DFSS—EDEN 127 (30) 20.8 2.2 42.9 39.1 1.2 0.8 0.5 1 0.2 10.1 3 14.3 17.3 0 0 0

BGCS—EDEN 127 (33) 17.3 0.2 4.2 4.2 0.2 0.2 0.4 16.2 0 17.4 4.9 38.5 39 0 0 0

WTFM—EDEN 124 (35) 11.8 1.3 44.1 35.1 0 0.7 0.2 0.9 4.3 8.3 5.4 18.9 23 0 0 0

WTFM—EDEN 126 (34) 15.9 1.2 8.4 3 1.7 1.3 0.7 8.1 7.8 2.5 1.8 20.2 50.2 0 0 0

WTFM—EDEN 127 (35) 16.6 0.7 21.5 12 0 0 0.4 0 7.1 8.8 5.1 21 39.8 0 0 0

Q quartz, F feldspar, RF rock fragment, Ch chert,Mu muscovite,HML heavy minerals, Qo quartz overgrowth, Carb carbonate cement, S siderite, Cacalcite, A ankerite, D dolomite, Tcarb total carbonate cement, P1 primary porosity, P2 secondary porosity, TPo total thin-section porosity

Arab J Geosci

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