--> ABSTRACT: Structural and Diagenetic Controls of a Fractured Carbonate Reservoir - Implications for Oil Deposit Development (A Case Study from the Zechstein of Poland), by Gasiewicz, Andrzej; Mikolajewski, Zbigniew; Slowakiewicz, Miroslaw; #90135 (2011)

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Structural and Diagenetic Controls of a Fractured Carbonate Reservoir - Implications for Oil Deposit Development (A Case Study from the Zechstein of Poland)

Gasiewicz, Andrzej 1; Mikolajewski, Zbigniew 2; Slowakiewicz, Miroslaw 1
(1)Polish Geological Institute, Warsaw, Poland. (2) Polish Oil and Gas Company, Pila, Poland.

In the NW part of Poland, in the Upper Permian (Zechstein) strata occur dolomitized carbonates (Main Dolomite, Ca2) in which 14 oil deposits have been found so far. One of them is Wysoka Kamienska oil field. The complicated Wysoka Kamienska structure is cut by NW-SE trending faults.

Main Dolomite carbonates, which host the oil deposit, are characteristic for carbonate platform slope lithofacies built of laminated limy mudstones, dolostones and dolomitic biolaminites, which can be considered as source rocks.

Maximum thickness of Ca2 profiles here reaches 65m. Almost 90% of the profiles have porosities 0.1-2% but in the last 10% representing upper parts porosities reach 23%. Permeability vary from >1 to 10mD. Total organic content is from ca 0.5% do 1.25% and organic matter is sapropelic. Tiny clusters of fine grained bituminite occur in sapropelic streaks.

Vitrinite reflectance (Ro) values measured in several lense-like carbonate grains syngenetic with the sediment bitumen vary from 0.38-0.62% (mean 0.49%). This suggests low maturity kerogen and low hydrocarbon potential. Ro measured on redeposited grains of vitrinite ranges from 0.79% to 1.85%. Values of Tmax are 414-430°C (close in biolaminites and lime mudstones) what suggests final stage of microbial transformation of organic matter and initial stage of thermocatalytic changes.

Crude oil has a density of 0.877 g/cm³, a viscosity of 40-50cSt (35-44cP) and contain 30-35% of the gasoline fraction, about 5% paraffins and about 1% sulfur. The accompanying gas contains up to 90% hydrocarbons and 5-9% nitrogen.

High porosities are related to fractures filled with oil. Fractures possibly developed by extension tectonics during the Zechstein and were activated during Late Cretaceous/Palaeogene inversion of the Middle Polish Trough. Other porosities come from vugs which were developed at different stages of burial. Most of them are related to late stage of diagenesis and dissolution contemporaneous with fracturing of the carbonate reservoir. Vugs are occluded by late diagenetic anhydrite cement. The most important are therefore fractures in which oil is trapped. Hence Wysoka Kamienska oil field is an example of a fractured carbonate reservoir.

There are two concepts about oil origin: in situ generation and migration from basinal parts. Based on data obtained it is suggested that the oil deposit contains both partly syngenetic HCs and partly epigenetic (the latter related most possibly to fracturing).

 

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.