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1 2006

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Hydrocarbon generating potential and their migration versus the diagenetic changes in Upper Jurassic and Lower Cretaceous sediments in the Polish Lowlands

Wołkowicz K., Grotek I.

Volumina Jurassica

2006

Vol. 4, no. 1

261-262

The material studied came from selected boreholes, mainly in the Mid-Polish Trough: Korabiewice PIG 1, Magnuszew IG 1, Narol PIG 1, Narol PIG 2, Oświno IG 1, Poddębice PIG 2, Strzelno IG 1 and Tomaszów Lubelski IG 1. In the Upper Jurassic section, marine carbonates predominate (e.g. Radlicz 1997). At the Jurassic/Cretaceous boundary, the basin shallowed substantially, and carbonate-sulphate deposits formed. During the subsequent stage of Lower Cretaceous sedimentation, carbonate and siliciclastic rocks were deposited (e.g. Marek 1997). The sediments underwent diagenetic processes influencing their reservoir properties. In the Upper Jurassic carbonate rocks, dissolution of many constituents, especially bioclasts, led to formation of numerous pores. Also heavy dolomitization enhanced their secondary porosity. Chemical compaction produced stylolite sutures, acting as possible migration ways for fluids. The reservoir properties were negatively affected by mechanical compaction and cementation, due to denser packing of grains and filling the pores with calcite and anhydrite. Diagenetic processes led to formation of rocks with various levels of porosity or very poorly permeable (<0.1mD). In the Lower Cretaceous strata, important results of decreased porosity are due to compaction and cementing. In siliciclastic rocks, grains were protected against destruction by compaction by their early diagenetic overgrowths of quartz and chlorite. In carbonates, the grains were similarly protected by calcite spar envelopes or micritic covers. While early cementation helped preserving primary porosity, late cementation led to blocking of the pores between grains and crystals. In arenites increased porosity was due to dissolution of unstable constituents, mainly feldspars. Through diagenesis, the Cretaceous sandstones obtained strongly porosity and permeable. The studied Upper Jurassic and Lower Cretaceous strata contain organic matter, mainly humus type, with poor hydrocarbon generating potential. The degree of their transformation indicates immature (Lower Cretaceous) or early (Upper Jurassic) hydrocarbon generating stageCretaceus deposits. (Figs 1, 2). Only the most changed Upper Jurassic rocks from the Poddębice PIG 2 borehole in their bottom part achieve the main stage of petroleum generation. Locally, they are enriched in organic carbon (>1.5% Corg), thus can be regarded as very poor or poor bedrock for oil generation. Upper Jurassic rock studied showed low maturity of organic matter to hydrocarbon generation, with limited migration potential. The Cretaceous strata analyzed contain organic matter at the immature stage. However, noteworthy is the presence of sandstones with high filtration properties.