Influence of microscopic fabric on organic matter occurrence and pore development in mudrock: A case study of the Cretaceous Eagle Ford Shale

doi:10.13745/j.esf.sf.2022.5.39

Abstract

Abstract:

The characteristics of organic matter (OM) occurrence and pore development in the Cretaceous Eagle Ford Shale of South Texas, USA were studied at the micrometer scale through analyses of the original oil-window core and artificial gas-window cylinder samples by Ar ion milling/scanning electron micrography (SEM) method, combined with energy-dispersive spectroscopy mapping. Three typical rock fabric types—siliceous-argillaceous seams, foraminifera fossils, and coccolith-rich lenses—were observed under microscope which revealed significantly varying OM occurrences and pore development characteristics. Specifically, in siliceous-argillaceous seams, enriched amorphous kerogen developed few pores within the oil window due to oil sorption until nano spongy pores began to form in large numbers within the gas window as a result of sorbed-oil cracking; whereas structured kerogen, of small quantity, had little or no pore development through the entire petroleum-formation period due to its abnormally low hydrocarbon generation potential. In foraminifera fossil chamber, filling OM represented migrated bitumen that charged during the early stage and developed an abundance of bubbly pores and spongy nanopores within the oil window, and then, with coalesce of nanopores, micron scale large pores began to dominate within the gas window. And in coccolith-rich lenses, filling OM of migrated bitumen charged in the late stage was characterized by abundant spongy nanopores within the oil window, whereas within the gas window abundant mineral intergranular pores re-exposed due to high OM conversion rate. This study demonstrated that the formation and development of OM-hosted pores in marine mudrock were controlled by both OM type and thermal maturity. It provided an important approach to studying the process of hydrocarbon generation, expulsion, and retention as well as revealing the carrier medium of OM-hosted pores and pore preservation mechanisms in mature marine shales in the subsurface by investigating the spatial relationship between microscopic fabric, OM occurrence, and pore heterogeneity in organic-rich mudrock.

Key words: mudrock fabric, OM occurrence, migrated bitumen, OM-hosted pore, thermal maturity, Eagle Ford Shale

CLC Number:

SHAO Deyong, LI Yanfang, ZHANG Liuliu, LUO Huan, MENG Kang, ZHANG Yu, SONG Hui, ZHANG Tongwei. Influence of microscopic fabric on organic matter occurrence and pore development in mudrock: A case study of the Cretaceous Eagle Ford Shale[J]. Earth Science Frontiers, 2023, 30(3): 151-164.

Figures/Tables 8

Fig.1 Eagle Ford Shale oil and gas play distributions across southern Texas, USA and location of shale oil horizontal well. Modified after [37].

Table 1 Geochemical parameters for the Eagle Ford Shale core sample

样品	TOC含量/%	S₁/(mg·g^-1)	S₂/(mg·g^-1)	S₃/(mg·g^-1)	T_max/℃	HI	OI	OSI	PI
抽提前	6.34	8.50	9.02	0.75	446	142	3	143	0.48
抽提后	4.96	0.72	5.51	0.04	459	111	1

Fig.2 SEM (a) and EDX (b) analysis of the Eagle Ford Shale core sample showing fabric characteristics

Fig.3 SEM images showing mineral matrix types and OM occurrence characteristics of the Eagle Ford Shale

Fig.4 Microscopic characteristics of amorphous kerogen and pore development in the Eagle Ford Shale siliceous-argillaceous seams

Fig.5 Microscopic characteristics of structured kerogen and pore development in the Eagle Ford Shale

Fig.6 Microscopic characteristics of migrated bitumen and pore development in the Eagle Ford Shale foraminifer fossil chamber

Fig.7 Microscopic characteristics of migrated bitumen and pore development in the Eagle Ford Shale coccolith-rich lenses

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