Eromanga basin

Non-metamorphic equivalents of this Th-free mineralization may be sought in some rare limestone-hosted U-occurences, such as the Jurassic Todilto lacustrine formation in the Grant Uranium Belt (Rawson Richard 1980), the Cretaceous Toolebuc marine formation in Eromanga Basin (Ramsden 1982), the Mesoproterozoic Vempale marine formation in Cuddapah Basin (Sinha et al. 1989), and the Cretaceous Probeer marine formation in the Huab deposit (Hartleb 1988). 

Ramsden, A.R. ET AL 1982. Origin and significance of the Toolebuc gamma-ray anomaly in parts of the Eromanga Basin. Journal Geological Society of Australia, 29, 285-296. 

Alexander R., Larcher A. V., Kagi R. 1., and Price P. L. (1988) The use of plant-dervied biomarkers for correlation of oils with source rocks in the Cooper/Eromanga Basin system, Australia. APEA J. 28, 310-324. 

Eromanga Basin clues to petroleum migration and entrapment. APEA J., 1993, 63-76. 

Attention is focused on two Australian petroleum fields, located in different geological settings more than 2500 km apart the Gidgealpa Field in the Jurassic-Cretaceous Eromanga basin of Central Australia (Fig. lA) and the Angel Field, located in the Dampier sub-basin (Carnarvon basin) of Australia s North West Shelf (Fig. 1B). The case studies exemplify the occurrence of major poikilotopic carbonate-cemented zones in sandstones buried to depths between 1500 and 3000 m, namely the... 

Fig. 1. Location maps of (A) the Gidgealpa Field in the Eromanga basin, and (B) the Angel Field in the Dampier sub-basin, Carnarvon basin. Both fields occur along broadly NE-SW-trending anticlinal trends. The small black squares in (A) represent the location of the wells listed in Table 1.

Cooper basin sediments consist of glaciofluvial, fluvio-lacustrine and deltaic elastics (Fig. 2) (Bat-tersby, 1976 Thornton, 1979). The Jurassic section of the Eromanga basin sequence was deposited in an intercratonic basin sag, and is made up of non-marine elastics deposited under fluvial, deltaic and lacustrine conditions. The Early Cretaceous Murta Member to Cadna-Owie Formation (Fig. 2) show deposition changing from lacustrine to marine, with marine conditions prevailing from the Aptian until the Upper Albian, when a return to paralic and fluvio-lacustrine conditions is indicated (Senior et al., 1978 Armstrong Barr, 1986). 

Fig. 2. Stratigraphical column for the Cooper/Eromanga basin province. The seal integrity of the regional Nappamerri Group (Triassic) is a major controlling factor in the distribution of Permian-sourced hydrocarbons in Jurassic sandstones. Much of the Jurassic Eromanga basin sequence forms part of the regional J-aquifer system of the Great Artesian Basin (Fig. lA), including the Namur Sandstone (Habermehl,...

Fig. 5. Geohistory plot for Eromanga basin sediments (Gidgealpa-7). Note that the Upper Jurassic Namur Sandstone and younger Mesozoic sediments underwent rapid initial burial, the same as in the Angel Field area (Fig. 4).

Fig. 6. Petroleum systems of (a) the Dampier sub-basin (Carnarvon basin) and (b) the Cooper-Eromanga basin province. Note that both geological provinces are characterized by late-stage (Tertiary) compression that triggered trap development in Mesozoic sequences, broadly synchronous with peak hydrocarbon generation and migration from older sequences.

Based on the petrophysical review of 475 wells on 169 structures, 50% of all Eromanga basin structures with major carbonate-cemented zones in Jurassic reservoirs have oil pools in reservoirs of the same age. In contrast, only 13% of structures without identifiable carbonate cement in Jurassic reser-... 

Table 1. Examples of major carbonate-cemented zones as identified from sonic and bulk density logs and lithological descriptions of cuttings for a variety of structures in dilferent parts of the Eromanga basin (Fig. lA). Note that the carbonate-cemented zones in Jurassic sandstones vary in cumulative thickness, from a few metres (e.g. Strzelecki-10) to about 110 m (Spencer West-1), including over short distances (e.g. Strzelecki). All wells occur near the Cooper basin margin, or along major fault-bounded structural trends where the regional Nappamerri Group seal (Triassic) underlying the Jurassic sandstones is incompetent or missing...

Fig. 8. Histogram of 5 C frequency distribution for (a) calcite cement in the Lower Namur Sandstone of the Gidgealpa Field (b) calcite cement in the Namur Sandstone and the Adori Sandstone, its lateral equivalent, for different petroleum fields, including the Big Lake, Kema, Marana, Moomba, Spencer, Strzelecki, Tantanna and Warana Fields (see Fig. 1 A) (c) siderite cement in Jurassic elastics (d) Cooper basin CO2 gases (e) Eromanga basin CO2 gases. Compiled from various sources (as shown). Note that the 5 C character of Eromanga basin calcite cements is similar to that of Cooper basin carbon dioxide gases. See text for explanation.

In the Eromanga basin this conclusion is supported by the strong statistical correlation between the occurrence of major calcite-cemented zones and oil pools in Jurassic sandstones. This concept is particularly valuable if a correlation can be proved between the size of the hydrocarbon pools and the size of the carbonate cement volume (intensity and areal extent of high-amplitude events). 

In cases where Eromanga basin structures contain major carbonate-cemented zones but are without oil discoveries in Jurassic sandstones, it is possible that either the hydrocarbons have leaked into younger sequences via partially breached seals, or that drilling has been off the crest of these structures and that updip oil pools remain to be discovered (B. Jensen-Schmidt, MESA, personal communication). However, as the relationship between the oil pools and the major calcite-cemented zones is only indirect, not all calcite-cemented zones will be associated with oil pools. 

Poikilotopic carbonate cements can reduce reservoir porosity in relatively clean, massive sandstones over large areas, of the order of at least 300 km (Angel Field). Based on log characteristics, major carbonate-cemented zones can attain a cumulative thickness of at least 165 m in marine sandstones (Dampier sub-basin, Carnarvon basin) and 110 m in fluvial sandstones (Eromanga basin). The total volume of carbonate cement in petroleum fields can approach 1 km, as exemplified by. the Angel Field case study. 

Anderson, A. (1985) A geoseismic investigation of carbonate cementation of the Namur Sandstone in the Gidgealpa Field, Eromanga Basin. BSc Honours thesis. University of Adelaide, Department of Geology and Geophysics. 

Armstrong, J.D. Barr, T.M. (1986) The Eromanga Basin—an overview of exploration and potential. In Contributions to the Geology and Hydrocarbon Potential of the Eromanga Basin (Eds Gravestock, D.I., Moore, P.S. Pitt, G.M.). Geol. Soc. Aust. Spec. Publ., 12, 25-38. 

Bone, Y. (1989) Paleotemperature analysis, Eromanga Basin, South Australia—fluid inclusion microthermometry auto-fluorescence microscopy and cathodolumines-... 

Boult, P., Theologou, P. Foden, J. (1997) Capillary seals within the Eromanga Basin, Australia— implications foe exploration and production. In Seals, Traps and the Petroleum System (Ed. Surdam, R.C.). Mem. Am. Ass. Petrol. GeoL, Tulsa, 67, 143-167. 

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