Tight-Reservoir Micropore Formation and Evolution in Sedimentary Organic-Matter-Bearing Tuff: A Case Study From the Permian Tiaohu Formation in the Santanghu Basin, NW China

Abstract

Recently, a tuffaceous tight oil reservoir with considerable reserves has been found in the Santanghu Basin, northwestern China. The physical properties of the tuff are characterised by high porosity (5–25%) and low permeability (0.01–0.50 mD). This reservoir has the peculiar property of being a sedimentary organic-matter-bearing tuff formed by air-fall volcanic ash in a lake. Its discovery offered an excellent opportunity to further our knowledge of the formation and evolution of tuffaceous tight-reservoir micropores.

This study integrated analyses of organic geochemistry, thin sections, scanning electron microscopy, CT scanning, mineral and element compositions, mercury injections, and measurements of porosity and permeability to determine the characteristics of the formation and evolution of the tuff micropores. (1) The tuff reservoir comprises vitric, crystal-vitric, pelitic, and silicified tuffs, which are characterised by high porosity and big throat, moderate porosity and thin throat, low porosity and thin throat, and very low porosity and moderate throat pore structures, respectively. (2) Of the four types, vitric tuff has the properties best suited to a reservoir, and devitrification of volcanic glasses is the principal mechanism of reservoir micropore formation. The devitrification of vitreous textures within the tuff primarily dictates the reservoir’s characteristics, i.e., the greater the degree of devitrification, the higher the porosity of the reservoir. The tuff of the Permian Tiaohu Formation contains sedimentary organic matter, and organic acids produced during kerogen maturation facilitate the process of devitrification. Vitric tuffs have reservoirs with higher porosity than crystal-vitric tuffs, and the degree of devitrification is generally controlled by the burial depth (temperature) and organic matter (organic acid) content. (3) Evolution of tuff porosity depends mainly on the original composition and burial depth. The porosity of vitric tuff is higher than crystal-vitric tuff at the same depth, but both their porosities initially decrease and then increase with depth. At depths >3000 m, vitric tuff retains the trend towards higher porosity, whereas the porosity of crystal-vitric tuff remains largely unchanged or even decreases.

The results of this investigation could be considered a reference regarding the distribution and prediction of tuffaceous reservoirs in similar situations in other parts of the world.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018