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Abstract
Journal of Petroleum Geology, vol.24 (3), July 2001, pp 265-285
Copyright ©
2001 by Scientific Press Ltd. All rights reserved.
REEFS IN THE BASINAL FACIES OF THE ZECHSTEIN LIMESTONE (UPPER PERMIAN) OF WESTERN POLAND: A NEW GAS PLAY
K.
Dyjaczynski*, M. Górski**, S. Mamczur* and T. M. Peryt
+
Zechstein (Upper Permian) reefs have long been known to occur along the margins of the Polish Zechstein Basin where they host important gas reserves. During the last decade, reefal build-ups have also been found in the centre of the basin on pre-Zechstein highs such as the Wolsztyn Ridge in Western Poland. Here, early 2D seismic and subsequent more detailed 3D seismic studies and integrated geophysical and geological interpretations have led to a number of major gas discoveries. The reef complexes in the Koscian– Wielichowo region are thought to have recoverable gas reserves of more than 32B cu m.
The basin-centre reefs have been identified on the basis of palaeogeographic analysis and from the seismo-acoustic properties of the Zechstein Limestone of which they are composed. 3D seismic profiles show that the reefs have irregular shapes in map view. Drilling results indicate that their thickness may exceed 80m, but characteristically their thickness decreases sharply at their margins corresponding to a facies change.
In the Wolsztyn Ridge area, the Zechstein Limestone can be divided into five lithological units which are capped by the Werra Anhydrite. Four of these units reflect deposition in subaqueous environments whereas the uppermost unit originated in a very shallow-water or temporarily subaerial setting. Typical “ecological†bryozoan-stromatolitic reefs occur in the middle and upper (but not the uppermost) parts of the Zechstein Limestone, while the lower parts are dominated by bioclastic packstones and grainstones. Average reservoir porosity is 10–20%. The permeability varies significantly (0-400 mD) but values of a few tens of mD are common.
The occurrence of intraclastic breccia in many boreholes suggests that the reefs may have developed on local palaeoslopes which are generally of tectonic origin. Blocks of contemporaneously lithified Zechstein Limestone as well as steeply-dipping laminar sheets are common in the reefs of the Wolsztyn Ridge and have also previously been recorded in the English Zechstein. Palaeoslopes elsewhere in the Wolsztyn Ridge and possibly also in other parts of the Variscan externides may host similar reefal build-ups which could be of exploration potential.
*PGNiG S.A., O/Zielonogórski Zaklad Górnictwa Nafty i Gazu, ul. Bohaterów Westerplatte 15, 65-034 Zielona Góra, Poland. **Geofizyka Torun, ul. Chrobrego 50, 87-100 Torun, Poland.
+ Panstwowy Instytut Geologiczny, Rakowiecka 4, 00-975 Warszawa, Poland. author for correspondence: [email protected].
| Pool |
Total reserves (million m3) |
Recoverable reserves (million m3) |
Reserves evaluation stage |
|---|
| Koscian South |
12,960 |
10,360 |
Assessed |
| Racot |
190 |
140 |
Estimated |
| Bonikowo |
1,250 |
1,000 |
Estimated |
| Bronsko |
19,300 |
15,400 |
Estimated |
| Wielichowo |
2,100 |
1,600 |
Estimated |
| Ruchocice |
1,300 |
1,000 |
Estimated |
| Paproc West |
3,700 |
3,100 |
Assessed |
| TOTAL |
40,800 |
32,600 |
|
Table 1. Gas reserves in the Zechstein Limestone in the Koscian–Wielichowo region, western Poland.
INTRODUCTION
Reefal build-ups in the basal cycle of the Upper Permian Zechstein Group (the Zechstein Limestone) have been described from various parts of the Southern Permian Basin. They are particularly well known in Germany (Kerkmann, 1969; Paul, 1980, 1996) and England (Smith, 1981a, b, 1994, 1995; Hollingworth and Tucker, 1987). They generally occur along the basin margins; for example in Thuringia (SE Germany), they are located about 2km from the palaeo-coastline (Paul, 1995, Fig. 9). They are also known to occur above highs in the pre-Zechstein topography, as in the Harz Mountains (Paul, 1980). Zechstein reefs show a remarkable variety of geometries (Paul, 1996). Fringing reefs have extensive flats and very steep (sometimes vertical) fore-reef slopes; patch-reefs formed archipelagos with nearly enclosed lagoons; while conical pinnacle reefs were located seawards of the first two types.
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