Pore Characteristics of the Organic-Rich Marine Shales With High Thermal Maturity: A Case study of the Lower Silurian Longmaxi Gas Shale Reservoirs in Southern China

Abstract

The Lower Silurian Longmaxi Formation marine shales, with high organic matter abundance and high thermal maturity, are regarded as the main shale gas reservoirs in China. The pore characteristics of the Longmaxi shales were investigated using samples collected from three investigation wells in southern China. The Longmaxi marine shales are rich in quartz and clay, with average contents of 33.27 wt.% and 32.2 wt.%, respectively. The average TOC content of the samples is 2.32 wt.%, and the average Ro value is 2.4%. Field emission scanning electron microscopy (FESEM), which provides high-resolution images, was used to describe the pore types and structures. Both observation and statistical results indicate that nearly all of the pores are mesopores and macropores, which account for approximately 90% of the total pore volume. Organic matter (OM) pores, interparticle (InterP) pores, and intraparticle (IntraP) pores, are the three main pore types, along with some micro-fracture pores. Typical OM pores within SEM images were extracted by using JMicrovision imaging software and the OM porosity was calculated. OM pores are usually meso- or macropores, and account for a large proportion (approximately 81.4%) of all pores in samples. InterP pores and IntraP pores are often related to rigid particles (quartz, feldspar, or pyrite framboids). Very few InterP or IntraP pores exist between two ductile (clay minerals) particles due to the effects of compaction. Regression analyses suggest that the TOC content is the key factor in pore development for shales with high thermal maturity. The mineral contents (quartz or clay minerals) also contribute to pores development, though their effects are small compared to that of the TOC content. A combination of SEM image analysis, calculations, gas adsorption and regression analysis shows that the TOC content is the controlling factor for gas storage potential, which infers that OM pores play a pivotal role in high thermal maturity shales, largely determining the gas storage potential. Besides, pores related to rigid particles also provide some space for gas storage, though their effects are extremely small compared to that of the TOC content. Pores related to clay minerals show no significant correlations to gas storage potential under the severe influence of the TOC content.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017