RESERVOIR AND SOURCE‐ROCK CHARACTERISATION OF THE EARLY PALAEOZOIC INTERVAL IN THE PERIBALTIC SYNECLISE, NORTHERN POLAND

Abstract

Detailed investigations have been conducted on samples from 2,750m of conventional cores from 18 wells penetrating the Cambrian siliciclastics in northern Poland. The results of sedimentological, petrological and geochemical analyses were combined with a computeraided regional evaluation of the Polish onshore sector of the Peribaltic Syneclise. Our results may help in the successful future identification of Cambrian prospects in northern Poland.The Middle Cambrian reservoir sandstones consist of quartzites in which low porosities (5% ‐ 8%) and permeabilities (5 to 24 mD) were preserved only in oil‐bearing structures in the more deeply‐buried, western part of the Peribaltic Syneclise. Isotopic studies suggest crystallisation temperatures for the pore‐plugging quartz cement of between 53d̀C and 78 d̀C. These temperatures were not reached in the shallower, eastern part of the basin, where porosities of up to 20% are present in the quartz arenites which constitute potential reservoir units.A 150‐m thick, organic‐lean (0.1‐l.0 wt. % TOC) Middle Cambrian shale below the quartzites, and an up to 20‐m thick, organic‐rich (4.5–10 wt. % TOC) Late Cambrian shale unit above the quartzites, are regarded as source rocks for the oil accumulations in the Cambrian reservoirs. The organic matter in these two shale units consists of abundant grey, lenticular particles and finely‐dispersed bituminite‐like constituents. The reflectance intensity of these bitumen particles is a useful parameter for evaluating the source rocks' maturity. Geochemical investigations have proved the (potential) source rocks to be immature in the shallower, eastern part of the Peribaltic Syneclise, and to have reached the “oil window ” in the more deeply‐buried western part of the basin.The extractable organic matter is dominated by saturated hydrocarbons. Late Cambrian extracts can be differentiated from Middle Cambrian samples by their higher content of aromatic compounds. Reservoired Cambrian oils and extracts from Cambrian source rocks could not be distinguished by triterpane “fingerprints ”. Both are characterised by a well‐developed sequence of tri‐ and pentacyclic compounds, with no variation in their relative abundances.Burial‐history modelling indicates that rapid subsidence of the Cambrian deposits to depths of more than 2,500m occurred during the Silurian in the western part of the basin. In the eastern part, however, a much more moderate burial history is inferred. Here, the Cambrian interval lies at depths of only 1,500 to 2,200m. An analysis of the basin's geothermal history indicates that porosity‐plugging quartz cementation in the Cambrian quartzites commenced in the western Peribaltic Syneclise during this rapid Silurian burial. In this western region, the source rocks reached the “oil window” during the Devonian. Hence, migration and accumulation of the generated hydrocarbons could preserve only the remaining pore space, which had already been drastically reduced. This genetic model implies that petroleum traps were formed before or during the Carboniferous.Because the thick Silurian marlstones acted as an efficient regional seal, the generated hydrocarbons must have been trapped somewhere in the underlying Early Palaeozoic sequence. Oil stains in Cambrian quartz arenites in the organic‐geochemically immature eastern part of the basin point to long‐distance, up‐dip migration towards the basin's eastern margin. A scarcity of structural traps and reduced shalyness of the eastwards‐coarsening, Early Palaeozoic sequence are adverse features, relative to a high degree of preservation of the hydrocarbons generated. The search for subtle diagenetic traps, however, may prove to be challenging.