Investigation of fine-grained siliciclastic rocks of different clay content using thermal methods

Abstract

Thermal methods (simultaneous thermal analysis coupled with quadrupole mass spectrometry STA- QMS and retort) were applied to characterization of microporous rocks of different clay content. Two series of samples were investigated: highly diagenetic lower Paleozoic mudstones from Polish Lowlands and less altered Miocene and Paleocene rocks from the Carpathian Foredeep. The aim of the presented work was to differentiate between “free” and “bound” water in the pore space, to estimate the amount of fluids obtained by using retort method in regard to the distinction between free water, capillary water, clay bound water and organic matter fluids. In order to evaluate the retort results it was necessary to determine the temperatures at which a particular kind of water is released. The interpretation was based on the results of STA – QMS investigations together with XRD analysis of the mineral content, cation exchange capacity (CEC) and organic carbon content (TOC) measurements. In STA investigations the loss of mass connected with water release from clay minerals was observed in two temperature ranges: from 40 °C to 150 °C (adsorbed water) and from 440 °C to 620 °C (structural water). When analysing retort measurements, a considerable upward shift of adsorbed water release temperature can be observed. In case of Miocene and Paleocene rocks interpretation results of adsorbed water content indicate the temperature range from 150 to 450 °C, and in case of Lower Paleozoic rocks from 150 °C to 350 °C. Structural water is released at temperatures 500 °C −650 °C. A condensate connected with organic matter was observed within the temperature range 350–500 °C for Lower Paleozoic rock samples of high organic matter content (TOC>2). Taking into account the methodical discrepancies, a satisfactory compliance of results obtained by both methods in regard to the volume of clay bound water and condensate connected with organic matter was found. The results show that thermal methods can be successfully used for evaluation of saturations necessary for proper estimation of reservoir properties.