Evaluation and influencing factors for brittleness of lacustrine shale reservoir: a case study of Qutang Sub-Sag, Subei Basin

Abstract

In order to accurately evaluate the brittleness of lacustrine shale and explore its influencing factors, the mineral compositions, geochemical characteristics and reservoir spaces of the lacustrine shale samples from the second member of Paleogene Funing Formation in the Qutang Sub-Sag, Hai'an Sag, Subei Basin were analyzed by means of whole-rock diffraction analysis, the measurement of organic carbon content (TOC) and vitrinite reflectance, scanning electron microscope (SEM) and triaxial rock mechanics experiment, combining with well logging, strength parameters, mineral compositions and stress-strain curves. The shale is mainly dolomitic and calcite ones with a high content of brittle minerals. The average TOC value is 1.25%, indicating for a mature stage. The reservoir space is composed of ultra-low pores and fractures. The stress-strain curves show a strong characteristic of brittleness. The brittleness evaluation results of different methods have certain differences. The results based on elastic parameters and mineral components are more reliable than that based on strength parameters although each method has its own limitations. The mineral compositions, organic matter abundance, and the degree of storage space development affect the brittleness of shale in the second member of Funing Formation. With the increasing of dolomite content, organic maturity and fracture development, the brittleness of the reservoir increases. The increase in calcite content, organic matter abundance and porosity will weaken the brittleness of the reservoir.

Keywords

• brittleness evaluation
• lacustrine shale
• funing formation
• paleogene
• hai'an sag
• subei basin