UPPER CAMBRIAN AND TREMADOCIAN SEDIMENTS IN THE NAROL AREA (SOUTHERN LUBLIN REGION) – SOURCE AND RESERVOIR OF SHALE GAS?

Abstract

Petrologic investigations of Upper Cambrian and Tremadocian deposits were carried out in the Narol region (southern Lublin region) in prospecting for shale gas accumulations. The observations and analyses were made using a polarizing microscope, luminoscope (CL) and scanning microscope (BSE, SE, EDS, SEM-CL). The following analyses were performed: CL-spectral analysis of quartz, X-ray structural analysis of clay fraction, and pyrolytic analysis of organic matter. The rocks under study are represented mainly by clay-silt shales with sandy interbeds. They belong to the epicontinental siliciclastic association deposited on an extensive shelf subjected to tidal and storm action. The shales consist largely of illite, and the silt fraction is represented by quartz with a small admixture of feldspars. Quartz cement is common (growths and aggregates of authigenic quartz), while carbonate cement (calcite, Fe-dolomite/ ankerite and siderite), as well as pyrite, kaolinite and phosphate cements are rare. The shales reveal microporosity in the form of “microchannels” paralleling illite plates, and within mica packets. The micropores (1–2 µm in size) are observed in both the carbonate cement and organic matter. As a result of deep burial and intense diagenetic processes, the organic matter has undergone strong alteration (max. Ro = 2.5%). The vitrinite reflectance index and pyrolitic analysis of organic matter, as well as the highly ordered illite structure, indicate the maximum palaeotemperatures in the range of 120–150°C. The rocks show numerous fractures healed with carbonates and/or quartz. Some of the fractures that run parallel to the lamination (or more rarely those running perpendicular or at a high angle) have remained open and are potential pathways of hydrocarbon migration. Pyrolytic analysis shows that the shales do not represent source rocks. It is supposed that they do not represent reservoirs for unconventional hydrocarbon accumulations.