Tight Reservoir Characteristics of Sedimentary Organic-Matter-Bearing Tuff and Influencing Factors of Petrophysical Propertyin the Santanghu Basin, Northwest China

Abstract

Abstract

The discovery of the Permian Tiaohu Formation reservoir in the Santanghu Basin of Northwest China, offered an excellent opportunity to further enhance our knowledge of the tuffs' reservoir characteristics. The reservoir has the peculiar property of being a sedimentary organic-matter-bearing tuff. Commercial oils have been obtained from hydraulic fracturing of horizontal wells.

Based on cores, thin section, scanning electron microscope (SEM), cathodoluminescence (CL), X-ray diffraction (XRD), quartz crystallinity index, and porosity and permeability, the tuff reservoir characteristics are studied. The main affecting factors of petrophysical property are analyzed.

There is mainly vitric tuff in the Tiaohu Formation, the original ash components of which are primarily vitric fragments. This study found that most quartz grains in the vitric tuffs are non-luminescent under cathodoluminescence, indicating they were mostly formed at low temperatures(<300 °C), namely products of devitrification in the diagenesis stage. Pores are difficult to observe in thin sections of the tight sedimentary organic-matter-bearing tuff. However, well-developed micropores are found under SEM. The sizes of the individual pores are small, but their numbers are large. Pore size is mainly on the micron–nanometre scale.

Tuff reservoirs are characterized by high porosity and low permeability. The porosity of the tuff ranges mainly from 5–25% and air permeability is <1.0 mD under atmospheric pressure conditions. The tuff reservoir petrophysical properties are related to the devitrification of volcanic glass in the tuff. Individual interparticle pores within the tuff, produced by devitrification, have small sizes but are large in number, resulting in high total porosity. The fine granularity and small pore throat radii (mainly 0.02∼0.12μm) of the tuff lead to very low permeability. The physical properties are influenced mainly by the original composition of the tuff and the degree of devitrification which can be characterized by quartz crystallinity index. And the content of organic acid and burial depth (temperature) are the two most important factors affecting the devitrification degree of the tuff.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016