Angel Formation

C. A. Angell, Formation of glasses from liquid and biopolymers, Science, 267, 1924 (1995). 

Fig. 3. Arrhenius plot of the viscosity of several supercooled liquids. The horizontal dotted line, where the viscosity reaches 10 P, is commonly used as a definition of the glass transition. (Reprinted with permission from C. A. Angell. Formation of glasses from liquids and polymers. Science (1995) 267 1924. Copyright (1995), American Association for the advancement of Science.)...

Fig. 3. Generalized stratigraphy for the Dampier sub-basin. Major seismic markers are indicated MUC, Callovian-Oxfordian main rift or breakup unconformity K, near top Angel Formation KA, near top Aptian unconformity. Modified from Kopsen McGann (1985).

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. 4. Geohistory plot for sediments in the Dampier sub-basin (Angel-2). Note that Upper Jurassic Angel Formation sediments underwent rapid initial burial, and that temperatures in this formation exceeded 80 C from the beginning of the Tertiary, and from the Oligocene onwards have been 100 C or greater.

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 Dampier sub-basin no such statistical relationship between hydrocarbon pools and major carbonate-cemented zones is known in Angel Formation reservoirs. 

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 discovery well intersected an 8 2 m net gas column in Upper Angel Formation sandstones. Three appraisal wells established major hydrocarbon reserves at the same stratigraphical horizon, with a maximum gas column of 140 m (Vincent Tilbury, 1988). A thin oil leg was discovered beneath the gas cap in all four wells, but is thickest at Angel-3 (20 m), the only well drilled south of a major ENE-WSW-oriented strike-slip fault zone that traverses the field (Ryan-Grigor Schulz-Rojahn, 1995). The gas composition in Angel Formation reservoirs consists of dominantly methane (80% by molar volume), ethane (6%), propane (3%) and heavier hydrocarbon gases (up to C7+), including minor carbon dioxide (less than 3%) (Woodside, 1971, 1972, 1990). 

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.

Semi-quantitative XRD traces show that Angel Formation sandstones are dominated by quartz and... 

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...

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.

Fig. 17. Wireline log characteristics, smoothed acoustic impedance curve and 3D seismic response over the Upper Angel Formation at Angel-2. Note that the major dolomite-cemented zones (black bars) are identifiable on the basis of neutron, density, resistivity and sonic log profiles. The zones appear as discrete layers at this location, with a cumulative thickness of 164 m, and are not fully cemented but contain some residual porosity. The dolomite-cemented zones occur both above and below the gas-water contact (GWC). The smoothed acoustic impedance curve shows that the zones produce a visible seismic response which is mappable. For an example of a line through the 3D seismic volume see Ryan-Grigor Schulz-Rojahn (1995 their Fig. 10a,b).

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... 

In the Angel Formation, given the marine nature of the sediments, it seems reasonable to assume that... 

Souza et al., 1995). The good general agreement between the isotopic and seismic data provides strong evidence that the Angel Formation dolomite cements precipitated from relatively hot fluids during Miocene times. 

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