Rozwój procesów stylolityzacji i deformacji epigenetycznych w aspekcie roponośności piaskowców kwarcytowych kambru środkowego w rejonie Żarnowca

Wojciech Strzetelski


The evolution of stylolitization and epigenetic deformations in the Middle Cambrian oil-bearing quartzose sandstones in the area of Żarnowiec (Northern Poland)

The Middle Cambrian oil-bearing series in the area of Żarnowiec (Polish Eastern Pomerania) consists of quartzose sandstones interbedded by black claystones, dn which abundant occurrences of stylolites have been noted. It has been found that the evolution of stylolitization proceeded in two phases: a) Older phase associated with microstylolites 1 of the first generation. The microstylolites are generally primitive wave-like in shape (Fig. 1—1, 1—2), with the seam breadth varying from 0.1 to 0.2 mm, the amplitude of flutes 0.5 mm and the amplitude of wave less than 0.7 mm. They originated on the basis of thin clay inserts and laminae. b) Younger phase associated with macrostylolites which grew up on the basis of deformed clay interbeddings, 1—3 mm in thickness. The macrostylolites in question have a primitive wave-like or suturated shape (Fig. 5—1), the breadth of seams is 0.2—0.5 .mm, the amplitude of wave 0.5’—5 mm. Phase (b) also witnessed the rise of microstylolites of the second generation (Fig. 4—2). The formation of macrostylolites was attended by deformation of thicker clay laminae and inserts, which process gave rise to semi-stylolitic structures (Fig. 6—lc, 6—2c), deformed clay interbeddings (Fig. 6—la, b, c; Fig. 6—2a, b), as well as to cloud-dike patterns (Fig. 7). Diapirs and wedged-in clay and sandy structures originated at the same time (Fig. 8). They are defined as epigenetic features. Between the phases (a) and (b) of stylolitization inclined stylolites were formed, growing up on the basis of Clay inserts associated with the original inclined lamination of sandstones. They are: inclined microstylolites (Fig. 3—lb, 3—3), inclined knotty stylolites and slide-down structures (Fig. 3—4). Inclined microstylolites are generally younger than the first-generation horizontal microstylolites of phase (a) (Fig. 3—5). Fissures of 'tectonic origin have not been discussed in the present paper. Mention has only been made of open relaxation fissures which usually have a horizontal or slightly inclined position and are sinusoidal-wavy or arch-like (convex up- or downwards) in shape. They owe their origin to reduction of the overburden pressure due to erosion. It is feasible that relaxation fissures in the Middle Cambrian sandstones are a result of intense erosion which took place in this area in the period between the Silurian and Late Permian. The structures found in the Middle Cambrian oil-bearing series in the area of Żarnowiec arose in the following sequence:
1. microstylolites of the first generation and some semi-stylolitic structures (phase „a”),
2. inclined microstylolites,
3. inclined relaxation fissures and slide-down structures,
4. horizon tad relaxation fissures,
5. macrostylolites and some microstylolites of the second generation (phase ,,b”), and the bulk of semi-stylolitic Structures and deformed clay interbeddings.
Bituminous traces in the second phase of stylolitization are very scarce compared with abundant traces of hydrocarbons found along the first-generation microstylolites. The horizontal relaxation fissures also contain ample traces of hydrocarbons. Intense lateral migration of hydrocarbons took place presumably in the period between (4) and (5). The first-generation stylolites acted as main channels for the lateral migration of oil and gas, forming simultaneously an impermeable barrier for vertical migration. The microstylolites in the Middle Cambrian terrigenic series of the Baltic Syneclise are thought to have attained their typical form at a depth more than 2700 m.

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