Dolomite-cemented zones Angel Field

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... 

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). 

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... 

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