Geneza•szczelin odprężeniowych w piaskowcach kwarcytowych kambru środkowego jako kryterium określnia dolnej granicy wieku akumulacji węglowodorów

W. Górecki, W. Strzetelski, J. M. Szwejkowski

Abstract


The origin of relaxation fissures in Middle Cambrian quartzose sandstones as a criterion for determining the earliest age of hydrocarbon accumulation

A description is here given of the morphology and occurrence of relaxation fissures in oil-bearing quartzose sandstones of Middle Cambrian in the Żarnowiec region. The assemblage of textures accompanying the relaxation fissures is reported defining their mutual age relations. The mechanical conditions responsible for the formation of relaxation fissures are also described. The critical depth of erosion necessary to start the propagation of relaxation fissures is computed. This is a basis for determination of the absolute age of the fissures. Hence the lower age limit of hydrocarbon accumulation can be defined indirectly.

Within Polish territory the Peribaltic syneclise covers the NE part of Poland including eastern and central Pomerania along the Baltic coast.

In this region, relaxation fissures of· natural origin occur in Middle Cambrian oil bearing quartzose sandstones. These fissures run horizontally and are rectilinear, arcuate or sinusoidal (Fig. 1). Often they form assemblages showing mutually compensated arrangement (Figs 1-2). The fissure assemblages occur in the sandstone close to clay intercalations (Fig. 1-1) indicating. the actual genetic connection of the relaxation fissures and the horizontal displacements of the plastic clay material in relation to the rigid sandstone rocks (Figs 1-4). The erosion of the overburden and simultaneous change of the regional dip causes the horizontal movement of the clay material. This movement results in the formation of short vertical, fissures associated with the horizontal relaxation fissures (Figs 1-3; Fig. 1-4). Thus they are formed owing to the reaction of the rock to the erosional reductions of the overburden.

The length of relaxation fissures ranges from 1 to 10 cm, their width from 0.2 to 0.5 mm (Fig. 2). As a rule these fissures are not healed. The quartzose sandstones are of brittle material. The experimentally determined values of Young's module of elasticity (E) and those of Poisson's ration (v) are correspondingly: E = 2.1-2.25 105, v = 0.9-0.11 (Figs 8, 10) and E = 1.9-1.95 105 kG/cm2; v = 0.1-17 (Figs 9, 11). Hence it was possible to use Griffith's criterion of fracture (Fig. 6) to compute the minimal thickness of erosional reduction necessary for the formation of relaxation fissures in Cambrian sandstones (Fig. 7). Assuming that the erosion had the character of planar denudation (Fig. 7A) the thickness of the eroded overburden must have been at least 1000-1060 m (see model 7). Within the; Peri-baitic synedise erosion on such a scale may have taken place only between the Silurian and the Lower Permian (Rotliegendes) as the erosion achieved its maximum during that, time. Hence it is reasonable to suppose that the relaxation fissures in the Cambrian sandstones were formed during the Rotliegendes or late Carboniferous. Because of the numerous hydrocarbon traces observable along the unbended fissures it may be concluded that the migration and accumulation of oil within, the Middle Cambrian rock series took place during or after Upper Carboniferous-Roltliegendes time.

On the basis of the computed maximum tensions which may have taken place in a rock subject to extension, it has been possible to compute the maximum depth of the occurrence of relaxation fissures (see formula 9). This depth ranges from 1650 to 1720 m below the denudation surface.

Taking into account the necessary maximum values of tension possible to occur in a rock subject to elasticity, the theoretical maximum depth has been computed at which opened relaxation fissures may be observed (see formula 10). This depth has been estimated at 3800 to 4400 m below the sea level.


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