Dolomite-cemented zones

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.

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... 

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).

In the Gidgealpa Field, the major calcite-cemented zones occur up to 100 m below the present-day oil-water contact (Fig. 7), with the thickest interval in the basal portion of the Namur Sandstone, furthest away from the existing oil pool (Fig. 16). In the Angel Field, major dolomite-cemented zones are absent within hydrocarbonbearing sandstones on the south side of the field (Fig. 20). 

Underclay and overburden sediments are typically clays with about 80% clay and 20% silt and muds with about 50% clay and 50% silt. The most distinct chemical variation in the overburden is the compact siderite- and dolomite-cemented concretion zone, 4.5 m above the base of the Kinneman Creek lignite. 

The cement at the tops of sandbodies may be a direct result of pedogenesis, which occurred at the same time as the development of the pedogenic dolocrete in the fine-grained units. This would occur preferentially at the tops of sandbodies adjacent to zones of active dolocrete pedogenesis. This is probably at least partly responsible for the dolomite cement distribution in the sandbodies. 

Quantitative determination of elemental dolomite composition was carried out on five polished thin sections covered with a thin layer of carbon and using a CAMECA SX 51 electron microprobe at 15 kV, a 20 nA beam current and a 0.2 pm beam diameter. The BSE imaging system linked to the electron microprobe was used to detect zonation in the dolomite cement, and compositional analyses were carried out for each zone. Results were normalized to 100 mol% Fe, Mg and Ca. The precision of the analyses was 100% 2. 

In the Angel Field, Angel Formation waters are characterized by salinities between 35 000 and 37 000 ppm (Woodside, 1972), consistent with this interpretation. Brine-upwelling probably occurred episodically, as indicated by the multiple zoning in the dolomite cement. In this field, whatever the trigger for the major dolomite cementation, the bulk of the cements must have formed in less than 10 million years because the incipient Angel Field structure developed between the Mid-Eocene and the Late Miocene (Fig. 22a-c), and since the Late... 

Dolomite-cemented fractures display a unimodel NNW-SSE orientation and variable distribution, with several zones of frequent, closely spaced examples (Fig. 6). Wave action has eroded the relatively friable surrounding sandstones below the high-tide mark, to leave the cemented fractures standing proud (Fig. 7A,B). Above the tidal limit the dolo-... 

Lateral variations in the eogenetic association may be related to changes in sedimentary conditions or to the starting mineral composition. The development of nitrates was probably controlled in part by sedimentary rhythms. In the border zones of the Triassic Basin of the Sahara where the source of supply (Hoggar Massif) was probably closer by, calcretes were formed and the deposits were cemented by calcite with a heterogeneous structure. In the case of farther removed source areas and a more stable sedimentation the sandstones are characterized by a poikilitic nodular dolomite cement. Variations in the composition of the sandstone cements between calcite and dolomite may be explained by the ionic composition of the solutions in the... 

Both authors calculations also indicated that it is possible for solutions of reasonable compositions for natural waters to produce mixtures of freshwater and seawater that were undersaturated with respect to calcite but supersaturated with respect to dolomite. This observation is a cornerstone for some dolomitization models that are discussed later in this chapter. It is also important to note that the extent of undersaturation which results from mixing is strongly dependent on the initial Pco2 °f the dilute water when it is in equilibrium with calcite. Waters high in CO2 can cause more extensive dissolution. If these waters enter a vadose zone where CO2 can be degassed, they will become supersaturated and calcium carbonate can precipitate. This process provides an excellent mechanism for cementation near the water table. Because the water table can oscillate vertically, a considerable zone of cementation can result. 

Figure 8.16. A hypothetical trend of changes in the stable isotope composition of carbonate cements in different diagenetic environments. A- marine realm B-meteoric realm C- mixing zone D- successively deeper burial for calcite spar E-successively deeper burial for saddle dolomite. B through E are precipitated in progressively hotter waters. (After Choquette and James, 1987.)...

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