THE SOUTH MYLDZHINSKOYE STRUCTURAL UPLIFT AND ITS ASSOCIATED OIL DEPOSIT

N. V. KOPTYAEV, Yu. S. MINDIGALEEV, L. A. PUKHLYAKOV

(Presented by Professor A. V. Aksarin)

The oil-bearing structures of Western Siberia, as a rule, are very gentle. The dip angles of their flanks usually do not exceed 1–2°. The South Myldzhinskoye structural uplift is, in this respect, somewhat exceptional. Thus, on reflecting horizon II-a, which corresponds to the lower part of the Maryanovskaya Formation (Upper Jurassic), this uplift represents a gentle anticline; however, in its steepest parts—for example, in the area of well 24—the dip angles of its flanks reach 4°. In the study of other surfaces, still larger dip angles are revealed. In particular, the base of the Maryanovskaya Formation in well 22 has an absolute elevation of −2311 m, while in well 24 it has an absolute elevation of −2439 m. Thus, the difference in absolute elevations here is 128 m. Taking into account that the distance between the wells here is approximately 1 km (Fig. 1), and assuming that the subsurface relief between them is close to a plane, we find that the dip angle of this relief exceeds 7°. If, however, it is assumed that the base of the Maryanovskaya Formation between the wells under consideration has certain irregularities, then its dip angles in individual segments must be still greater. What, then, caused such a sharp increase in the dip angles of the flanks of the South Myldzhinskoye structure?

To answer this question, core from well 24, taken below the absolute elevation of −2300 m, was studied. The dip angles of the bedding planes in it proved to be highly varied, namely: at a depth of −2583 m, 20°; at a depth of −2601 m, 16°30′; at a depth of −2606 m, 2°; and so on. Meanwhile, the zenith angles of the well at these points are 2°00′, 1°30′, and 1°30′, respectively. Thus, the true dip angles of the beds at the points under consideration lie within 18–22° in the first case, 15–18° in the second case, and 0°30′–3°30′ in the third case. It follows from this that the bedding planes in question do not correspond to the general direction of dip of the beds in the area under consideration (which, as noted above, lies within 4–7°). Apparently, well 24 intersected a zone of cross-bedded rocks. On this basis, it may be assumed that this well was drilled in the channel of some ancient river that flowed here in a meridional or nearly meridional direction.

Well 22, drilled on the dome of the South Myldzhinskoye structure, showed signs of oil-bearing capacity in the interval with absolute elevations of −2008 to −2047 m (Tarskaya Formation). The total thickness of this formation

reaches 90 m; the reservoirs in it are fine-grained calcareous sandstones, whose average porosity ranges from 21.5 to 26.0%, and permeability from 20 to 700 millidarcies.

After perforation, replacement of the clay mud by water, and lowering of the fluid level in the well to 220 meters below the wellhead, an oil flow was obtained. However, after several hours the well watered out and began to produce a mixture of oil and water. With a total rate of 80 m³/day (through an 8-mm choke), the amount of oil was about 50 m³/day and the amount of water about 30 m³/day. The oil is low-sulfur (sulfur content 0.12%), paraffinic (paraffin content 19.54%), specific gravity 0.8173 g/cm³, initial boiling point 59°C. Fractional composition of the oil: up to 150°—17%, from 150° to 200°—10%, from 200° to 250°—9%, from 250° to 300°—10%, residue—54%. The water is mineralized; total mineralization reaches 17604 mg/liter; specific gravity 1.0114 g/cm³; pH—7.6.

For explaining the fact that the well at first produced clean oil and then began to produce oil with water, several hypotheses were proposed. The most acceptable of them is that according to which well 22 turned out to have been drilled near the oil-water contact. In accordance with this is the fact that well 24, which encountered the Tarskaya Formation at a greater depth, produced no oil at all (it produced only mineralized water). Apparently, the oil deposit associated with the South Myldzhinskoye structural uplift is of comparatively small size.

Fig. 1. Structural map of the South Myldzhinskoye uplift on reflecting horizon II-a (Maryanovskaya Formation), scale 1:200000.

1—wells that produced oil, 2—wells that did not produce oil, 3—South Myldzhinskoye uplift, 4—Verkhnesalatskoye uplift, 5—contours of the bed on reflecting horizon II-a (Maryanovskaya Formation), 6—fault, according to seismic-exploration data.