Microstructure Evolution of Organic Matter and Clay Minerals in Shales with Increasing Thermal Maturity

Abstract

As the two important components of shale, organic matter (OM) and clay minerals are usually thought to strongly influence the hydrocarbon generation, enrichment and exploitation. The evolution process of OM and clay minerals as well as their interrelationship over a wide range of thermal maturities are not completely clear. Taking Yanchang (T3y), Longmaxi (S1l) and Niutitang (Є1n) shales as examples, we have studied the microstructure characteristics of OM and clay minerals in shales with different thermal maturities. The effects of clay minerals and OM on pores were reinforced through sedimentation experiments. Using a combination of field emission scanning electron microscopy (FE‐SEM) and low‐pressure N2 adsorption, we investigated the microstructure differences among the three shales. The results showed that both OM and clay minerals have strong effects on pores, and small mesopore (2–20 nm) is the dominant pore component for all three samples. However, the differences between the three samples are embodied in the distribution of pore size and the location. For the T3y shale, clay minerals are loosely arranged and develop large amounts of pores, and fine OM grains often fill in intergranular minerals or fractures. Widespread OM pores distribute irregularly in S1l shale, and most of the pores are elliptical and nondirectional. The Є1n shale is characterized by the preferred orientational OM‐clay aggregates, and lots of pores in the composites are in the mesopore range, suggesting that over maturity lead to the collapse and compaction of pores under huge pressure of strata. The results of the current research imply that with increasing thermal maturity, OM pores are absent at low maturity (T3y), are maximized at high maturity (S1l) and are destroyed or compacted at over‐mature stage (Є1n). Meanwhile, clay minerals have gone through mineral transformation and orientational evolution. The interaction of the two processes makes a significant difference to the microstructure evolution of OM and clay minerals in shale, and the findings provide scientific foundation in better understanding diagenetic evolution and hydrocarbon generation of shale.

Influences of Primary Moisture on Methane Adsorption within Lower Silurian Longmaxi Shales in the Sichuan Basin, China

“…The shale particles and dry spectroscopically pure potassium bromide (KBr) were completely mixed with a mass ratio of 1:200. It is worth noting that the occurrence between organic matters and inorganic minerals of shales is extremely complex. − Accordingly, to mitigate the potential negative effect of minerals of shales on oxygenic functional group measurement, the mixed shale particles and KBr were fully ground in an agate mortar to ultrafine powder prior to FTIR analysis. In our study, the number and resolution of scan are set as 32 and 4 cm –1 , respectively.…”

High-Temperature-Induced Pore System Evolution of Immature Shale with Different Total Organic Carbon Contents

“…At 25–300 °C, the effect of temperature removes the free water in the pores of the immature shale and at the same time causes the inorganic minerals to be finely adjusted under the action of thermal stress. Due to the strong sensitivity of clay minerals to water, water molecules may occupy the adsorption position in mesopores and block the pore throat through dispersed or dissolved clay minerals; therefore, the volume of the mesopore increases from 25 to 300 °C. , This process has relatively little effect on the pore structure, and the volume and specific surface area of the small pores increase slightly. At 300–400 °C, the kerogen begins to transform into polymer insoluble matter at a certain temperature, and the linear polymer-insoluble matter is decomposed into asphaltene and dry distillation gas at high temperature .…”

Quantitative Characterization and Differences of the Pore Structure in Lacustrine Siliceous Shale and Argillaceous Shale: a Case Study of the Upper Triassic Yanchang Formation Shales in the Southern Ordos Basin, China

“…26,27 The overall shape of the basin is rectangular, extending from north to south, and there are six tectonic units: Yimeng uplift at the north of the basin, Weibei uplift in the southern basin, Tianhuan depression and thrust belt in the west part of the basin, and Jinxi curved fold belt and central Yishan slope at the east and center of the basin, respectively. 26,28 The study area covers a part of the southern area of the Yishan Slope, named Luohe study area, with an area of approximately 3834 square kilometers (Figure 1A). The differential subsidence of the basin occurred in the late period of the Indosinian movement and began to tilt from the northwest to southeast during the Late Triassic period.…”

“…The Ordos Basin covers parts of central and western China, with a total area of 320,000 square kilometers, ranking second among the basins in China. , The overall shape of the basin is rectangular, extending from north to south, and there are six tectonic units: Yimeng uplift at the north of the basin, Weibei uplift in the southern basin, Tianhuan depression and thrust belt in the west part of the basin, and Jinxi curved fold belt and central Yishan slope at the east and center of the basin, respectively. , The study area covers a part of the southern area of the Yishan Slope, named Luohe study area, with an area of approximately 3834 square kilometers (Figure A).…”