Characteristics of Organic Matter Pores and the Relationship with Current Pressure System of Lower Silurian Longmaxi Shales in Dingshan Field, Southern Sichuan, China

Qian Cao,Ke Jiang,Zhongshan Yin,Zhentao Wen,Xingzhi Wang,Minghui Qi,Andrea Brogi

doi:10.1155/2021/9967479

Abstract

Organic matter pores (OMP) provide significant storage space for hydrocarbons in lower Silurian Longmaxi shales in the Dingshan field of southern Sichuan, China. The distributions of organic matter and the different OMP structure parameters were characterized through Ar-ion polishing, scanning electron microscopy (SEM), and image analysis software for shale samples of different wells. The research results indicated that organic matter has been divided into two categories based on its occurrence, location, and its relationship with authigenic minerals: organic matter in situ and migrated organic matter. OMP for organic matter in situ are mainly micropores mostly arranged isolatedly, while in migrated organic matter pores show larger sizes and higher roundness. The development of OMP in samples is predominantly controlled by the formation pressure. The existence of overpressure alleviated the stress on the rock skeleton, causing the compaction of some migrated organic matters to lag or decrease. This played a positive role in protecting the development of pores in the interior and edge of the rock skeleton, and it can also induce the development of microfractures in shale. The protective effect of formation pressure on organic pores was provided for understanding the exploration and exploitation of Longmaxi shales in the study area.

Highlights

In recent years, investigations on the pore type, pore size, and pore structure characteristic parameters of mud shale have been conducted providing significant achievements under an increasing interest in unconventional hydrocarbons [1,2,3]
Combined with the transmission electron microscope (TEM) observation, the animal debris in the samples mainly included graptolites and foraminifera, which were partially degraded in the deposition stage and partially transformed into amorphous dispersed organic matter and retained (Figures 2(c)–2(f))
The following conclusions were drawn from this study: (1) Based on the petrological characteristics, genesis, and morphological distribution of different organic matters, the organic matter could be divided into two types: organic matter in situ and migrated organic matter

Summary

Introduction

Investigations on the pore type, pore size, and pore structure characteristic parameters of mud shale have been conducted providing significant achievements under an increasing interest in unconventional hydrocarbons [1,2,3]. Previous studies on organic-rich shales, such as Barnett, Woodford, Marcellus, and Wufeng-Longmaxi shales, have found that porosity in organic matter represents a significant fraction of the reservoir space [4,5,6]. The formation and evolution of organic matter pores play an important role in the storage and migration of oil and gas in shale reservoirs, increasing the specific surface area and pore volume of shale providing storage space for shale gas [6,7,8]. The occurrence of solid bitumen is generally regarded as the secondary product from oil-prone kerogen by thermal degradation and hydrocarbon generation, that is, bitumen and hydrocarbon can be generated in a source rock and migrated to another [9]. He claimed that migrabitumen (“solid petroleum bitumen”), as a high viscosity fluid, is often distributed in

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Discussion

Conclusion