X-ray micro computed tomography characterization of porosity in Rotliegend sandstones on the northern slope of the Wolsztyn Ridge, western Poland

Abstract

Natural gas in the Polish Rotliegend Basin occurs in porous and permeable aeolian sandstones, and traps are mostly structural. Lithological traps are rare and occur on the northern slope of the Wolsztyn Ridge where fluvial and aeolian sandstones overlap with alluvial facies. Both fluvial and aeolian sandstones are reservoir rocks in this area. The X-ray micro computed tomography (MCT) results and microscopic observations indicate that the sandstones in lithologic traps on the northern slope of the Wolsztyn Ridge form highly compartmentalized vertical reservoir rocks composed of four types of sandstones. The profiles are dominated by very low-porosity fluvial sandstones (F2), and low-porosity aeolian and fluvial sandstones (A2, F1). The A1 type of sandstones with high porosity (10%) occurs only in some sections of the profiles. The most important diagenetic processes that reduced porosity were compaction and cementation by carbonate cements. All studied sandstones were subjected to the same diagenetic processes. However, each of the processes ran with varying intensity in different types of sandstones. Detailed analyses of pore distribution by MCT methods with respect to primary depositional or lithofacies effects, and secondary diagenetic effects, help to understand the 3D geometry of pores and pore shape-size distributions. The results can be used in the studies of other sandstones with a different origin and age