CONTROLS OF PRESSURE SOLUTION STRUCTURES ON FLUID MIGRATION - NUCLEAR MAGNETIC RESONANCE STUDIES FROM STRUGA-1 WELL (ZECHSTEIN MAIN DOLOMITE; W POLAND)

References: 16th International Multidisciplinary Scientific GeoConference SGEM 2016, www.sgem.org, SGEM2016 Conference Proceedings, ISBN 978-619-7105-57-5 / ISSN 1314-2704, June 28 - July 6, 2016, Book1 Vol. 3, 785-792 pp

ABSTRACT
Pressure solution structures (PPS) such as stylolites and solution seams have a diversified impact on fluid flow in intersected in Struga-1 well in the Polish part of the Zechstein Main Dolomite (Ca2). Defining the function of PSS in the migration of hydrocarbons is a significant element of reservoir characterization. Petrophysical and mineralogical studies were carried out on sixteen standard thin sections using polarization microscopy. The use of UV light revealed the presence of organic matter and hydrocarbons in both dolomite matrix and the interiors of PSS. The presence of iron oxides has not been recorded inside the PSS. Scanning electron microscopy (SEM), supported by X-Ray microanalysis (EDS), confirmed the presence of clay minerals inside the PSS. The occurrence of framboidal pyrite identified by SEM is related to decay of organic matter. Cathodoluminescence studies enabled us to detect the presence of secondary fluorite reducing the pore space both in dolomite matrix and inside the PSS. Nuclear magnetic resonance (NMR) was used to determine the porosity values in sixteen dry and water-saturated samples with the 2MHz Magritek Rock Core Analyzer. The effective porosity of studied cores varies from 0.8% to 2.1%. The combination of NMR measurements and microscopic observations created an opportunity to conclude that in contrast to stylolites, solution seams exhibit a twofold effect on the movement of fluids. Decreased effective porosity of particular samples is caused by the solution seams, that are greatly filled with insoluble residue. In this case, the clay minerals inside the residue, absorb hydrocarbons observed in the UV. Other solution seams containing fewer insoluble residue form channels for fluids, hence the effective porosity significantly increases.

Keywords: carbonate reservoir rock, pressure solution structures, stylolite, solution seam, nuclear magnetic resonance