Angel Field

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.

The Angel Field is located at the northern end of the NE-SW-oriented Madeleine Trend in the Dampier sub-basin, which forms part of the Silurian to Holocene Carnarvon basin, offshore Western Australia (Fig IB). The Madeleine Trend is a major fault-controlled anticlinal feature that follows the main depositional axis of the Dampier subbasin and along which several other oilfields are located, including the nearby Cossack and Wanaea Fields (Fig. IB). The stratigraphy of the Dampier sub-basin consists of marine and deltaic elastics and Tertiary carbonates (Fig. 3). 

In the Angel Field, Angel Formation temperatures exceeded 80 °C from the Pliocene onwards, and first attained 100°C during the Oligocene (Fig. 4). During the Miocene, temperatures were close to the present-day maximum burial temperature (100°C-104 C). 

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).

Both the Angel Field and Gidgealpa Field represent structural four-way dip closures. The Angel Field is mapped at reservoir (Angel Formation) level as a broad, low-relief drape closure with three separate... 

In the Gidgealpa Field the Namur Sandstone is sealed by the Early Cretaceous shales of the Murta Member of the Mooga Formation (Figs 2 and 7). In the Angel Field, Angel Formation reservoirs are sealed by the conformable Neocomian shales of the Barrow Group (Fig. 3). 

The Angel Field contains 30 billion m of gas and 50 million barrels of condensate (Playford, 1975). 

Fig. 9. Stratigraphical cross-section, Angel Formation, Angel Field The Upper Angel Formation is defined by the relatively clean, massive gamma-ray response, representing stacked mass-flow sandstones of marine origin that extend down to the Mid D. jurassicum boundary. All four wells encountered gas and condensate reserves within this stratigraphical interval (see Fig. 20 for a location map). Only at Angel-2 were major dolomite-cemented zones intersected (shaded), which cannot be explained by facies variations between the well locations, based on GR log motives and core descriptions of Upper Angel Formation sandstones.

Table 2. Derivation, lithological and textural characteristics of core samples and cuttings derived from the Upper Angel Formation in the Angel Field and the Namur Sandstone in the Gidgealpa Field...

Table 3. Semi-quantitative bulk-rock XRD results for Angel Formation reservoirs in the Angel Field, and Namur...

Sandstone samples from the Gidgealpa Field. The dominant carbonate cement is dolomite in the calcite in the Gidgealpa Field Angel Field, and ... 

Fig. 11. Paragenetic sequence for the Upper Angel Formation, Angel Field. Dolomite cementation was a relatively late, pore-filling diagenetic event that occurred, at least in part, synchronously with microfracturing and hydrocarbon migration (see Fig. IOC). Later-stage anhydrite cement precipitated in some Angel Formation sandstones at Angel-2 and, to a lesser extent, at Angel-4.

Table 4. Bulk-rock carbon and oxygen isotope results for dolomite cement in the Angel Formation, Angel Field. Precipitation temperatures for dolomite cement were calculated using the fractionation factor of Northrop Clayton (1966) and assuming a marine composition for the original 8 0 pore water (6 0 = 0%o). When integrating the calculated dolomite precipitation temperatures (93-97 °C) with the geohistory plot for Angel-2 (Fig. 4) an Eocene to Late Miocene age for the dolomite cement is suggested, closely matching the seismic evidence (Fig. 22c)...

Fig. 12. Ternary diagram showing the elemental composition of CaO, MgO and FeO (mol%) for the dolomite cement in Angel Formation reservoirs, Angel Field.

In the Angel Field (Angel-2), bulk density can be as high as 2.6 g/cm in dolomite-cemented sandstone intervals, compared with 2.35 g/cm in sandstones without significant dolomite cement (Fig. 17). Examination of resistivity character shows that the MSFL, LLS and LLD curves each... 

In both fields, wireline log correlation shows that the major carbonate-cemented zones share the following characteristics (i) they are found in clean stacked sandstone sequences (ii) they occur in discrete layers, separated by porous sandstone intervals (iii) they are intimately associated with hydrocarbon pools (iv) the thickness and relative spatial configuration of the carbonate-cemented zones vary over relatively short distances and are unrelated to facies controls (Figs 9 and 16). The major difference between the carbonate-cemented zones in each field is their stratigraphical position in relation to the hydrocarbon pools. In the Gidgealpa Field, the calcite-cemented zones occur below the oil-water contact in the lower and middle portions of the Namur Sandstone, in a stacked sandstone sequence (Fig. 16). In the Angel Field, the dolomite-cemented zones occur in the upper part of a massive sandstone sequence (Fig. 9), both below and above the... 

In both the Angel Field and Gidgealpa Field, the acoustic impedance produced by the major... 

The 3D seismic data show that four different sets of faults occur at reservoir level in the Angel Field area (Fig. 20). The most prominent are ENE-WSW-oriented strike-slip faults that concentrate in a zone 1 km wide south of Angel-1, Angel-2 and Angel-4. 

The fault zone traverses the width of the Angel Field. Other secondary faults include NW-SE-trending normal faults, NNW-SSE-trending strike-slip faults, and NNE-SSW-trending reverse faults. 

The western and eastern boundaries of the high-amplitude zone are irregular and less well defined, and do not appear to be associated with any faults. The high-amplitude zone is restricted to the northern and northeastern sectors of the Angel Field, with the major proportion of the high-amplitude zone extending well beyond the present-day field outline, towards the northeast. 

Amplitudes are strongest near the crest of the Angel Field structure/Madeleine Trend, and decrease downslope until they phase out completely in more basinal areas of the Angel Field, well below... 

Isochron maps produced for the interval between the top Angel Formation horizon and several shallower reflectors, including reflectors at Mid-Eocene, Mid-Miocene and Late Miocene level, reveal the structural evolution of the Angel Field area (Fig. 22). The reflectors used here are present over the entire northwestern margin of Australia, and lack evidence of major erosion (Apthorpe, 1988). 

Isochron maps between these reflectors and the top Angel Formation horizon show that no time closure was available at reservoir level in the Angel Field area until the Late Miocene (Fig. 22c). A... 

Calibration of seismic results with wireline log data shows that the carbonate cement volume is several times larger in the Angel Field than in the Gidgealpa Field area. 

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