Abstract:

Sedimentary organic matterbearing tuff is a special type of tight reservoirs in the Santanghu Basin. The study of sedimentary organic matterbearing tuff in the Santanghu Basin will provide a good opportunity to improve our knowledge of unconventional tight reservoir. Based on thin section examination, field emission environmental scanning electron microscopy (SEM), Xray diffraction (XRD) analysis, measurements of quartz crystallinity index, reservoir physical property, etc., the reservoir characteristics of sedimentary organic matterbearing tuff in Tiaohu Formation are studied in this paper. The results indicate that vitric tuff and crystalvitric tuff are the main rock types of tight reservoirs in Tiaohu Formation. The most important mineral in tuffs is quartz, with average content over 50%. Reservoir space types are mainly micronano scale interparticle pores caused by the devitrification of tuffs, as well as dissolution intraparticle pores, interparticle pores of inorganic minerals and clays, and pores within organic matter. Besides, fractures are also important reservoir space. Tight tuff reservoirs are characterized by high porosity ranging from 10% to 25% and low permeability mainly in the range of (0.010.5)×10-3 μm2. High total porosity is caused by small volume but large quantities of micronano scale pores from the devitrification, and low permeability is attributed to the very small throat radius of pores. The physical property of tuffs is mainly affected by the composition of the volcanic ash and the degree of devitrification which can be characterized by quartz crystallinity index. The physical property of vitric tuff is better than that of crystalvitric tuff. The degree of devitrification is mainly controlled by the depth (temperature) of tuff and the organic acid content.

Key words: tight reservoir, tuff, devitrification, Tiaohu Formation, Santanghu Basin