Barbwire Terrace Research Articles

Variation of vertical stress in the onshore Canning Basin, Western Australia

Vertical stress is one of the three principal stresses and is an important parameter in geomechanical studies that are focussed on the prediction of pore pressure, fracture gradients and wellbore stability. Variations of the vertical stress magnitude can be attributed to variations in lithology or diagenetic history, localised uplift and overpressures caused by disequilibrium compaction. This study uses wellbore data from 102 open-file petroleum wells to characterise vertical stress within the onshore Canning Basin of north-western Australia. Vertical stress magnitudes are interpreted from density logs and checkshot data and at 1 km depth below the ground surface range from 20.5 to 25.0 MPa km–1 with a mean value of 22.1 MPa km–1 (s.d. = 1.0 MPa km–1). Significant variation is evident within the calculated stress magnitudes, and when presented spatially, three regions of elevated vertical stress are identified: the Barbwire Terrace, the Devonian reef complexes of the northern Lennard Shelf and the Mowla Terrace. Lithology, abnormal pore pressures and tectonic uplift are investigated as potential mechanisms of the observed variation. Although abnormal pore pressures are identified, no direct correlation between overpressured areas and elevated vertical stress magnitudes is observed. The Canning Basin has an extensive history of uplift; however, there is little evidence for significant recent inversion. While uplift is likely to exert some influence over vertical stress magnitudes in the Canning Basin, the primary cause is interpreted to be lithological: areas of elevated vertical stress magnitude are also areas where thick intervals of carbonate sediments are present.

Sweet-spot mapping through formation evaluation and property modelling using data from the Goldwyer Formation of the Barbwire Terrace, Canning Basin

Abstract The Goldwyer Formation of the Canning Basin has been regarded as a highly prospective shale play. This study assesses the potential prospectivity of this source rock as an unconventional hydrocarbon resource. Considering the sparsity of wells penetrating the Middle Ordovician Goldwyer across the vast under-explored area of the Canning Basin, a basin-wide study of the source rock is not warranted. Goldwyer assessment of the Barbwire Terrace, a subdivision of the Canning Basin, is carried out instead. This assessment includes the estimation of key shale play properties, such as, total organic carbon, total porosity, water saturation, and brittleness index. Each property was estimated from available well data by testing multiple estimation methods. TOC values were derived from multiple regressions of different well data. A simplified Archie's equation was used to estimate water saturation. Density porosity method was primarily used for total porosity estimations. Sonic data along with density were utilized to estimate brittleness index. Each property was then modelled across the Goldwyer Formation within the terrace. This provided geostatistical estimates on the propagation of such properties. In order to generate sweet spot maps for the Barbwire Terrace, averaged maps of different properties were combined in a weighted manner. This approach attempts to simplify the complexity of unconventional resource assessment, which therefore has provided a single product evaluating the prospectivity of the Goldwyer as a hydrocarbon resource. Results have shown that TOC and porosity are mostly the deciding factors for the prospectivity of this source rock, given that their values can be too small where the Goldwyer is deemed non-prospective. Nonetheless, sweet-spot maps show that most prospective zone is the Upper Goldwyer (Goldwyer I), followed by the upper parts of the Lower Goldwyer (Goldwyer III). More specifically, southern flanks of north-western and middle regions of the Barbwire Terrace tend to be more prospective. A stricter approach where cut-off values were applied for each property showed that sweet-spot maps are only prospective in the southern flanks of the middle Barbwire Terrace of Goldwyer I.

Extent and persistence of photic zone euxinia in Middle–Late Devonian seas – Insights from the Canning Basin and implications for petroleum source rock formation

Abstract Late Devonian mass extinctions attributed to extensive anoxia and/or euxinia of the oceans are associated with widespread deposition of organic-rich shales. Also in the epeiric waters of the Canning Basin (Western Australia), photic zone euxinia (PZE) prevailed during the Givetian–Frasnian, with geochemical evidence for PZE on the northern (Lennard Shelf)–, and southern (Barbwire Terrace) margins of the Fitzroy Trough. On the Lennard Shelf, shales record episodic pulses of PZE associated with high algal activity due to enhanced nutrient supply, whereas a restricted marine setting on the Barbwire Terrace is thought to be the main driver for the development of persistent PZE and associated bacterial predominance. Structural evidence indicates that the Fitzroy Trough was a confined basin during the Late Devonian with the possibility of limited ocean circulation. Widespread PZE is expected to have developed in the poorly mixed water column, if the basin received sufficient nutrient supply for enhanced primary production. Notwithstanding the presence of anoxia during deposition of potential source rocks, only two small Devonian-sourced oil fields and numerous oil shows have been found in the Canning Basin. Biomarker assemblages show that the oils produced from the Lennard Shelf fields (i.e. Blina-1, Blina-4 and Janpam North-1) have substantially different molecular compositions to the minor oil recovered from Mirbelia-1 on the Barbwire Terrace. A correlation was established between the Lennard Shelf oils and rock extracts from the Gogo Formation at Blina-1 and McWhae Ridge-1 based on their hopane, sterane and carotenoids abundances. A definitive source correlation was not obtained for the Mirbelia-1 oil, but it did show some genetic affinity to the Givetian–Frasnian extracts from the Barbwire Terrace, suggesting that local source rocks are developed in the region.

Uncovering the Canning Basin: a new comprehensive SEEBASE® model

A new regional scale SEEBASE® model has been produced for the intracratonic Canning Basin, located in the north of Western Australia. The 2017 Canning Basin SEEBASE model is more than an order of magnitude higher resolution than the 2005 OZ SEEBASE version — the average resolution is ~1 : 1 M scale with higher resolution in areas of shallow basement with 2D seismic coverage — such as the Broome Platform and Barbwire Terrace. Post-2005 acquisition of potential field, seismic and well data in the Canning Basin by the Geological Survey of Western Australia (GSWA), Geoscience Australia and industry provided an excellent opportunity to upgrade the SEEBASE depth-to-basement model in 2017. The SEEBASE methodology focuses on a regional understanding of basement, using potential field data to interpret basement terranes, depth-to-basement (SEEBASE), regional structural geology and basement composition. The project involved extensive potential field processing and enhancement and compilation of a wide range of datasets. Integrated interpretation of the potential field data with seismic and well analysis has proven quite powerful and illustrates the strong basement control on the extent and location of basin elements. The project has reassessed the structural evolution of the basin, identified and mapped major structures and produced fault-event maps for key tectonic events. In addition, interpretative maps of basement terranes, depth-to-Moho, basement thickness, basement composition and total sediment thickness have been used to calculate a basin-wide map of basement-derived heat flow. The 2017 Canning Basin SEEBASE is the first public update of the widely used 2005 OZ SEEBASE. All the data and interpretations are available from the GSWA as a report and integrated ArcGIS project, which together provide an excellent summary of the key features within the Canning Basin that will aid hydrocarbon and mineral explorers in the region.

A New Method to Estimate Total Organic Carbon (TOC) Content, an Example from Goldwyer Shale Formation, the Canning Basin

Background:There is an increasing interest in the Goldwyer Formation of the Canning Basin as a potentially prospective shale play. This Ordovician shaly formation is one of the most prominent source rocks in the Canning Basin. One key property to evaluate the prospectivity of any shale oil or gas is its total organic carbon (TOC) richness.Objectives:This study investigates different TOC estimation techniques and validates the reliability of each, aiming to provide a best estimating approach for local and global applications.Method:The limited well distribution in the large area of the Canning Basin makes a basin-wide study not warranted at this stage. A focused look into the Barbwire Terrace was carried out instead. General TOC estimation methods, such as Schmoker and ∆logR were employed for TOC calculation. TOC relationships of single and multivariate regressions were also derived from wireline data and TOC rock sample measurements.Results:Both Schmoker and ∆logR methods tend to overestimate TOC when compared to the available Rock-Eval pyrolysis TOC measurements. The regression approach have shown to provide the best TOC estiamtes for wells in the Barbwire Terrace, where the best multiple regression approach for the terrace and global application was found to be the one derived from gamma-ray (GR), bulk density (RHOB), and sonic log transit time (DT).Conclusion:The generalized nature of the Schmoker method, as it provides a global relationship between density and TOC is probably the main reason why this approach does not provide a good fit in the case of the Goldwyer Formation. Furthermore, the uncertainty associated with the ∆logR method factors, such as the level of maturity (LOM), and resistivity and sonic baselines greatly influence the TOC estimation in this method, and hence, sometimes do not merit a reliable TOC estimation. The multiple regression approach have shown to be most accurate once lithology and compaction information (GR, RHOB, and DT) were incorporated in the regression process. TOC was reliably estimated for wells inside and outside the Barbwire Terrace, and also for wells of a global lacustrine shale. Such derivation have provided a more accurate technical assessment of the shale play and its prospectivity as a potential unconventional hydrocarbon resource.

Sweet-spot mapping of the Goldwyer Formation through formation evaluation and property modelling in the Canning Basin, Barbwire Terrace case study

The Goldwyer Formation of the Canning Basin has been regarded as a highly prospective shale petroleum play. This study assesses the potential prospectivity of this source rock as an unconventional hydrocarbon resource via property modelling. Considering the sparsity of wells penetrating the Middle Ordovician Goldwyer across the vast under-explored area of the Canning Basin, a basin-wide study of the source rock is not justified. Due to a higher well density, assessment of the Goldwyer Formation within the Barbwire Terrace, a sub-division of the Canning Basin, is carried out instead. This assessment includes the estimation of key shale play properties, such as, total organic carbon, total porosity, water saturation, and brittleness. Each property was estimated from available well wireline log data by testing multiple estimation methods. Total organic carbon values were derived from multiple regressions of different well data. A simplified Archie’s equation was used to estimate water saturation. Density porosity method was used for total porosity estimations. Sonic data along with density were utilised to estimate the brittleness index. Each property was then modelled across the Barbwire Terrace, which provided geostatistical estimates on the propagation of each parameter. In order to generate sweet spot maps, averaged maps of the properties were combined in a weighted manner. In the model, the Goldwyer Formation was divided into three layers based on dominant lithology. The uppermost shale dominated layer was predicted by the model to be the most prospective stratigraphic zone. The sweet spot maps highlight the southern flanks of the northern and western part of the Barbwire Terrace as the highest prospective geographic locations. This approach attempts to simplify the complexity of unconventional resource assessment, and has provided a single product evaluating the prospectivity of the Goldwyer as a hydrocarbon resource.

New Geological Insights from the Barbwire Terrace using Falcon5 Data, Canning Basin

The Barbwire Terrace in the Canning Basin has always presented explorers with an enigma. It has long held interest for hydrocarbon explorers and also mineral explorers looking for 'Mississippi Valley Type' sulphide mineralisation, being on the opposite side of the Fitzroy Trough to the Lennard Shelf (host to multiple oil fields and MVT's in the Cadjebut/Kapok area) (Copp, 2008). Seismic interpretation on the Barbwire Terrace has been difficult, not only due to the paucity of modern reflection seismic data, but also due to the difficulty in imaging through the carbonate/dolomites of the Pillara and Nullara sections. The Airborne Gravity Gradiometry (AGG) survey was designed to capture a large comprehensive grid of geophysical information about the southern margin of the Fitzroy Trough. The survey is instrumental in providing a greater understanding of an area of the Canning Basin that is poorly understood, yet has had many hydrocarbon shows and indications. While CGG undertook a more traditional workflow of interpreting the AGG, aeromagnetics and seismic data, a parallel approach, using Geoproxima processing technology (differential geometric analysis for digital data) provided additional insights on features and objects not readily recognisable using traditional colour bar stretches and sun illumination.

An Early Permian brachiopod–gastropod fauna from the Calytrix Formation, Barbwire Terrace, Canning Basin, Western Australia

Taboada, A.C., Mory, A.J., Shi, G.R., Haig, D.W. & Pinilla, M.K., 12.11.2014. An Early Permian brachiopod–gastropod fauna from the Calytrix Formation, Barbwire Terrace, Canning Basin, Western Australia. Alcheringa 39, xxx–xxx. ISSN 0311-5518A small brachiopod–gastropod fauna from a core close to the base of the Calytrix Formation within the Grant Group includes the brachiopods Altiplecus decipiens (Hosking), Myodelthyrium dickinsi (Thomas), Brachythyrinella narsarhensis (Reed), Neochonetes (Sommeriella) obrieni Archbold, Tivertonia barbwirensis sp. nov. and the gastropod Peruvispira canningensis sp. nov. The fauna has affinities with that of the late Sakmarian‒early Artinskian Nura Nura Member directly overlying the Grant Group in other parts of the basin but, as with all lower Cisuralian (and Pennsylvanian) glacial strata in Western Australia, its precise age remains poorly constrained, especially in terms of correlation to international stages. Although the Calytrix fauna lies within the Pseudoreticulatispora confluens Palynozone, the only real constraint on its age (and that of the associated glacially influenced strata) is from Sakmarian (Sterlitamakian) and stratigraphically younger faunas. A brief review of radiometric ages from correlative strata elsewhere in Gondwana shows that those ages need to be updated. The presence of Asselian strata and the position of the Carboniferous‒Permian boundary remain unclear in Western Australia.Arturo César Taboada [ataboada@unpata.edu.ar], CONICET-Laboratorio de Investigaciones en Evolución y Biodiversidad (LIEB), Facultad de Ciencias Naturales, Sede Esquel, Universidad Nacional de la Patagonia ‘San Juan Bosco’, Edificio de Aulas, Ruta Nacional 259, km. 16,5, Esquel U9200, Chubut, Argentina; Arthur Mory [arthur.mory@dmp.wa.gov.au], Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia; Guang R. Shi [grshi@deakin.edu.au], School of Life and Environmental Sciences, Deakin University, Melbourne Burwood Campus, 221 Burwood Highway, Burwood, Victoria 3125, Australia; David W. Haig [david.haig@uwa.edu.au], School of Earth and Environment (M004), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia; María Karina Pinilla [mkpinilla@fcnym.unlp.edu.ar], División Paleozoología Invertebrados, Museo de Ciencias Naturales de La Plata, Paseo del Bosque s/n, 1900 La Plata, Buenos Aires, Argentina.

Tectonic and climatic controls on the deposition of the Permo-Carboniferous Grant Group and Reeves Formation in the Fitzroy Trough, Canning Basin, Western Australia

Interpretation of seismic and well data from the Fitzroy Trough, Barbwire Terrace and Lennard Shelf provides constraint for the tectonic and climatic control on the deposition and architecture of the Permo-Carboniferous Reeves Formation and Grant Group in the Fitzroy Trough.An angular unconformity is recognised at the base of the Reeves Formation, recording the middle Carboniferous Meda Transpressional Movement. This separates two major extensional phases that affected the basin: the Devonian–Early Carboniferous Pillara Extension and the Late Carboniferous Point Moody Extension. The Reeves Formation is interpreted to have been deposited during the Late Carboniferous Point Moody Extension phase, and as such is a syn-rift sequence. Its distribution is restricted to the Fitzroy Trough, with a thickness in-excess of 1875 m deposited in the half-graben, clearly observed to thicken toward the actively moving Fenton Fault. Compressional anticlines underlying the Reeves Formation, and located at left-lateral bends in the Fenton Fault, indicate associated dextral strike-slip movement.Extensional faulting ceased prior to deposition of the overlying Grant Group. Thickening of the Grant Group into the Fitzroy Trough is interpreted to be the result of thermal sag following the main extensional event, and associated compaction of the underlying thick syn-rift sequence and minor displacement along the Fenton Fault.The well-established subdivision of the Grant Group into the Betty, Winifred and Carolyn formations is interpreted to correlate directly with the subdivision of the group proposed by Redfern (1990) on the Barbwire Terrace into Hoya, Calytrix and Clianthus formations. Evidence for climatic control on the depositional style and architecture includes the prominent glacially eroded Base Grant Group unconformity and prominent erosional surfaces associated with facies (diamictites, varved mudrocks) that all suggest a glacial depositional environment. Deglaciation and relative rise in base level gave rise the mudstone dominated middle unit. The absence of glacial signature within the uppermost Clianthus Formation on the Barbwire Terrace (laterally equivalent to the Carolyn Formation on the Lennard Shelf) is attributed to restriction of the remnant ice sheet or glaciers to the margins of the basin, at the final stage of glaciation of the basin.

CANNING BASIN AND GLOBAL PALAEOZOIC PETROLEUM SYSTEMS—A REVIEW

There are more than 131 giant and super-giant oil and gas fields with Palaeozoic source and reservoir that are similar to the Canning Basin. These include Palaeozoic basins of North America, North Africa, and the North Caspian Basin of Kazakhstan and Russia.The productivity of these Palaeozoic petroleum systems depends on timing of generation and preservation of charge. Thick Ordovician, Permian, and Triassic evaporite deposits played a very important role in creating and preserving the North American, north Caspian, and north African giant oil and gas fields, respectively.The Mesozoic–Tertiary charged Palaeozoic systems are typically more productive than the Palaeozoic charged systems as exemplified by the north African basins.The Ordovician sourced and reservoired giant oil fields of the North American Mid-Continent are also highly productive. Within the Canning Basin, Ordovician sourced oil has been recovered on the Barbwire Terrace (in Dodonea–1, Percival–1 and Solanum–1) on the Dampier Terrace (in Edgar Range–1 and Pictor–1) and along the Admiral Bay Fault Zone (in Cudalgarra–1, Great Sandy–1, and Leo–1).The Canning Basin may be the least explored of the known Palaeozoic basins with proven petroleum systems. The Palaeozoic basins of North America are the most explored with 500-wells/10,000 km2 compared to the Canning Basin with only 4-wells/10,000 km2.The presence of five oil fields, numerous oil and gas shows and the well density in the Canning Basin (200 wells in 530,000 km2) suggests that further exploration is warranted. Critical analysis of the distribution of source rock, reservoir, seal, timing of generation versus trap formation and post accumulation modification for each tectonic unit of the Canning Basin is required.

Subaqueous mass flow origin for Lower Permian diamictites and associated facies of the Grant Group, Barbwire Terrace, Canning Basin, Western Australia

The intracratonic Canning Basin is Western Australia’s largest sedimentary basin (>400 000 km2) and has experienced repeated episodes of Phanerozoic extension and subsidence, resulting in deposition of a number of first‐order ‘megasequences’. A major phase of basin extension and sedimentation (Grant Group) occurred in the Late Carboniferous/Early Permian when Australia lay at high palaeolatitudes. Facies analysis of 5000 m of drill core from 25 continuously cored wells in Grant Group strata on the fault‐bounded Barbwire Terrace in the northern Canning Basin identified three facies associations (FAs). These record the predominance of fault‐generated, subaqueous mass flow and sediment reworking. The lowest association (FA I; up to 355 m thick) rests unconformably on tilted older strata and consists of coarse‐grained, subaqueously deposited, sediment gravity flow facies. These include fault‐generated breccias, massive and graded sandstones and conglomerates deposited by turbidity currents and diamictites generated by mixing of different textural populations during downslope remobilization. FA I is overlain abruptly by relatively fine‐grained deposits of FA II (up to 140 m thick), which consist of laminated to thin‐bedded mudstone and sandstone turbidites, recording an abrupt increase in relative water depths. In turn, these facies coarsen upwards and are transitional into shallow‐water, swaley cross‐stratified and rippled sandstones of FA III (up to 125 m thick). The overall stratigraphic succession probably records an initial phase of faulting and accommodation of coarse sediment (FA I), a subsequent phase of rapid subsidence, increasing water depths and ‘sediment underfilling’ (FA II) and, finally, a regressive phase of shoreface progradation. The occurrence of rare striated clasts in FA I suggests reworking of glacial sediment, but no direct glacial influence on sedimentation can be identified.

GEOCHEMICAL CHARACTERISTICS OF PALAEOZOIC PETROLEUM SYSTEMS IN NORTHWESTERN AUSTRALIA

Isotopic and biomarker analyses carried out on Cambrian to Permian oils and source rocks in the Arafura, Bonaparte (Petrel Sub-basin) and Canning Basins have been used to geochemically characterise five distinct petroleum systems within the Larapintine and Gondwanan Petroleum Supersystems. The Larapintine 1 Petroleum System is characterised by isotopically light, free hydrocarbons in the Arafura Basin (613Csat = −32 %o Arafura-1) which have been correlated to kerogens of similar isotopic signature within the Middle Cambrian Jigaimara Formation. The richness and maturity of these source rocks indicate that an effective Larapintine 1 Petroleum System may exist in the northern parts of the Arafura Basin. Larapintine 2 oils, with Gloeocapsomorpha prisca-type signatures, are found on the Barbwire- Dampier Terraces and Admiral Bay Fault Zone in the Canning Basin. These oils can be correlated to source rocks in the Lower Ordovician Goldwyer Formation on the Barbwire Terrace and the Bongabinni Formation in the Admiral Bay Fault Zone by their diagnostic odd- carbon-number preference in the C15—CJ9 n-alkanes. Larapintine 3 oils are derived from Upper Devonian marine carbonates in the Canning Basin and Petrel Sub- basin and have a diagnostic biomarker signature which includes a predominance of steranes relative to diasteranes and abundant gammacerane and 30- norhopanes, similar to those observed in the Upper Devonian Gogo and Pillara Formations. Larapintine 4 oils are derived from Lower Carboniferous marine, clay- rich mudstones in both the Petrel Sub-basin and Canning Basin. They are isotopically light (mean δ13C sat = −28 %o) and have a unique terpane signature which has been identified within the Milligans Formation. Gondwanan 1 Petroleum System hydrocarbons, represented here by the Petrel-4 condensate, have a heavy isotopic signature (δ13C sat = −24 %o) which, coupled with an abundance of the diasterane and diahopane biomarkers, indicates derivation from Permian deltaic source facies. Recognition of the diagnostic geochemical components of each Palaeozoic petroleum system has led to the identification of Permian-like isotopic signatres in some hydrocabon accumulations in the Timor Sea that were previously attributed to Mesozoic sources.

Sedimentary Facies Analysis of an Evolving Glacial Sequence, the Permian-Carboniferous Grant Group, Canning Basin, Western Australia

Abstract Within the Canning basin, the Permian-Carboniferous Grant Group is one of the main hydrocarbon reservoir targets. Previous studies have been limited to the accessible scattered outcrops and sparse subsurface data. However, the recent acquisition of a series of stratigraphic boreholes, located on the Barbwire Terrace, provides fully cored sections through the Grant Group. These have been integrated with wireline log data to allow detailed sedimentary facies analysis and regional correlations. The Grant Group comprises sediments deposited by an extensive icc sheet, which covered the Canning basin during the Late Paleozoic. The cored sections exhibit a complex suite of glaciogenic facies, an evolving glacial sequence, recording the glaciation and gradual deglaciation of the basin, with evidence of periodic icc advantage and retreat cycles. Diamictites, mudstones, and thick sandstones at the base of the section are interpreted to be proximal glacial deposits. The overlying thick mudstone package, followed by a heterolithic/sandstone facies association, are interpreted to be post glacial deposits. The glacial sandstone units have excellent reservoir potential, while interbedded lacustrine mudstones and massive muddy diamictites form effective seals. Analysis of a number of widely spaced wells and the correlation of three recently defined formations, allow these reservoir units to be placed within a stratigraphic framework. This provides an understanding of the reservoir distribution, while displaying the complex nature and spatial variability of individual glaciogenic facies.

Origin of dolomitization on the Barbwire Terrace, Canning Basin, Western Australia

ABSTRACTA thick, areally extensive subsurface sequence of Upper Devonian carbonates occurs on the Barbwire Terrace in the Canning Basin of Western Australia. It is a platform sequence in which most of the shallow water lithologies have been thoroughly dolomitized. Slightly deeper water marls have remained as limestones. The major, regional dolomite type in the sequence is not restricted to peritidal lithologies and forms large thicknesses of dolomite (up to 600 m) with no primary calcite. A small volume of evaporitic, supratidal dolomite is present at one location. This dolomite is derived from highly saline fluids developed in an arid supratidal environment.Replacement dolomite of the regional dolomite type has a xenotopic form, with undulose extinction, and irregular crystal boundaries. In addition, saddle dolomite cements appear to have precipitated contemporaneously with the major phase of replacement dolomite. This suggests the regional dolomite type was precipitated at slightly elevated temperatures. Dolomitized stylolites and cements appear to indicate that dolomitization occurred after cementation and pressure solution.Geochemically, the synsedimentary supratidal and regional dolomite types are quite distinctive. Supratidal dolomites have δ18O values which are significantly higher (δ18O=−2 to +1‰ (PDB)) than the regional dolomite type (δ18O=−9 to −2‰ (PDB)). Assuming the lowest δ18O values for the sabkha dolomite represent replacement in marine waters, the oxygen isotopic composition for Upper Devonian Canning Basin marine dolomite would be around δ18O=−2‰ (PDB).The petrographic and geochemical characteristics of the regional dolomite type support a burial diagenetic origin. However, sources of magnesium in current burial dolomitization models appear insufficient to account for the large volume of dolomite on the Barbwire Terrace. Therefore, it is suggested that dolomitization may have taken place in a near‐surface environment with a major recrystallization event superimposed during burial diagenesis.

The Granulatisporites confluens Oppel-zone and Early Permian marine faunas from the Grant Formation on the Barbwire Terrace, Canning Basin, Western Australia

The Granulatisporites confluens Oppel-zone and Early Permian marine faunas from the Grant Formation on the Barbwire Terrace, Canning Basin, Western Australia

TheGranulatisporites confluensOppel-zone and Early Permian marine faunas from the Grant Formation on the Barbwire Terrace, Canning Basin, Western Australia

A taxonomically diverse plant microfossil assemblage recovered from a fully cored, marine, glacigene sequence of the Grant Formation, Canning Basin, Western Australia, contains 68 palynomorph species derived mostly from ferns, lycopods, gymnosperms and algae. The assemblage has been assigned to the newly defined Granulatisporites confluens Oppel-zone. G. confluens was first described from Late Palaeozoic sequences of the Chaco-Parana Basin, Argentina, and together with other key species indicates close correlation with assemblages from South America, India, Africa and, to a lesser extent, Antarctica. The associated marine fauna is diverse and includes foraminiferids, crinoids, bryozoans, molluscs and brachiopods. Twenty species of Mollusca and Brachiopoda have been identified, including the new species Terrakea capillata Waterhouse. The presence of Strophalosia cf. subcircularis Clarke indicates strong links with younger Asselian faunas of eastern and southern Australia (lower Wasp Head, lower Tiverton an...

HYDROCARBON SOURCE POTENTIAL OF THE GOLDWYER FORMATION, BARBWIRE TERRACE, CANNING BASIN, WESTERN AUSTRALIA

The Goldwyer Formation is widely known from the subsurface Canning Basin, Western Australia. Microfossils of acritarchs, chitinozoans, and conodonts, and macrofossil remains, indicate it was deposited in a normal marine environment in late Early to Middle Ordovician times (Late Arenig to Llanvirn). In Exploration Permit areas 143 and 225, the Formation is subdivided into four lithologic members, informally designated Units 1 to 4 in ascending stratigraphic order. Horizons within Unit 4, immediately underlying the prospective reservoir dolostones of the Nita Formation, are organic rich with between 0.5 and 6 per cent total organic carbon. Generative potential of these horizons, determined by Rock-Eval, averages 13 litres of hydrocarbon per tonne. A conservative estimate of the cumulative thickness of source rock with hydrogen index (HI) values of 300-900, using whole core fluorescence intensity, is 10 m. Thus, under optimum maturation conditions there is potential for generation of an estimated 61 × 109 barrels of liquids from Unit 4 within EP 143 and EP 225. These figures are based on an integrated analysis of 44 core samples from four fully cored 'slim holes' and cores from one conventional oil well. Kero-gen types and measures of organic maturity cannot be determined accurately from the standard Rock-Eval HI/Tmax crossplot. The dominant oil-prone kerogen in Unit 4 is Gloeocapsamorpha prisca Zalessky 1917 and palynological and gas chromatographic/mass spectrometric studies correlate it with Ordovician kerogens from the Baltic Basin, Michigan and Illinois basins, and Amadeus Basin. Gas chromatographic/mass spectrometric source rock-oil correlations show that oils from the Williston Basin (USA), Canning Basin, and Amadeus Basin are derived predominantly from G. prisca. Palaeogeographic reconstructions suggest that these areas lay within 5° north and south of an Ordovician equator and so provide data for further prediction of possible rich hydrocarbon source areas.