Pore throat structure and classification of Paleogene tight reservoirs in Jiyang depression, Bohai Bay Basin, China

Abstract

The pore throat structure characteristics of Paleogene tight sandstone and sandy conglomerate in the Jiyang depression are studied using cast thin section, conventional mercury injection, constant rate mercury injection and micro CT scanning data, and a reservoir classification scheme based on pore throat structure parameters is established. The material composition and structural characteristics of tight reservoirs are analyzed by casting thin section data. The pore throat structure characteristics of tight reservoirs are studied by conventional mercury injection, constant rate mercury injection and micro CT scanning. Ten pore throat structure parameters are analyzed by cluster analysis. Based on the classification results and oil test results, the classification scheme of Paleogene tight reservoirs is established. The Paleogene tight reservoirs in the Jiyang depression have the characteristics of macropores and microthroats, with pores in micron scale, throats in nano-submicron scale, and wide variation of ratio of pore radius to throat radius. The permeability of the tight reservoir is controlled by throat radius, the smaller the difference between pore radius and throat radius, and the more uniform the pore throat size, the higher the permeability will be. The lower limits of average pore throat radius for the tight sandstone and tight sandy conglomerate to produce industrial oil flow without fracturing are 0.6 μm and 0.8 μm, respectively. Reservoirs that can produce industrial oil flow only after fracturing have an average pore-throat radius between 0.2–0.6 μm, and reservoirs with average pore throat radius less than 0.2 μm are ineffective reservoirs under the current fracturing techniques. Different types of tight sandstone and sandy conglomerate reservoirs are classified and evaluated, which are well applied in exploratory evaluation.