scholarly journals Differences in Pore-forming Efficiency among Organic Macerals and Its Restriction against Reservoir Quality: A Case Study Based on the Marine Shale Reservoir in the Longmaxi Formation, Southern Sichuan Basin, China

Lithosphere ◽  
2021 ◽  
Vol 2021 (Special 1) ◽  
Author(s):  
Yunqian Jia ◽  
Denglin Han ◽  
Jizhen Zhang ◽  
Chenchen Wang ◽  
Wei Lin ◽  
...  

Abstract Organic matter pores are of important significance in the shale formation system rich of organic matters. Although a lot of studies have discussed controlling factors of organic matter pores in the past, it still lacks a quantitative analysis on contributions of organic macerals to organic matter pores. In this study, a case study based on the overmature marine facies shale reservoir in the first submember of the Longmaxi Formation of Silurian in the Weiyuan area was carried out. Besides, qualitative and quantitative identifications of organic macerals and their pore development capacity were provided using the scanning electron microscopy (SEM). The results showed that (1) pore-forming efficiency is one controlling factor over pore development of organic matter. Sapropelinite shows the highest pore-forming efficiency (avg. 38.5%) and while the vitrinite, inertinite, and exinite have the lower pore-forming efficiency. (2) The content of sapropelinite is the highest (avg. 82.4%), and the content of sapropelinite is higher in the Long111 and Long113 layers. (3) The content of sapropelinite has a strong positive correlation with the organic surface porosity. (4) Organic surface porosity, organic porosity, and total porosity present basically consistent variations along the vertical direction of single well. Organic surface porosity restricts the organic porosity which is the dominant type in total porosity. Hence, pore-forming efficiency of organic macerals restricts performances of the reservoir.

2018 ◽  
Vol 36 (4) ◽  
pp. 645-664 ◽  
Author(s):  
Qian Pang ◽  
Guang Hu ◽  
Kun Jiao ◽  
Xiucheng Tan ◽  
Hong Liu ◽  
...  

Bio-precursors of organic matter, referring to formerly living precursors, can influence content and distribution of organic pores significantly. However, insufficient attention has been paid in previous studies. To research the impact of bio-precursors of organic matter on shale organic pores, we conducted palynology and thin section analysis, total organic carbon analysis, and N2 gas absorption experiments on the Wufeng and Longmaxi Formations shales and kerogen samples from the Shuanghe outcrop section in southern Sichuan Basin, China. Generally, there are three bio-precursor assemblages being developed from bottom to top in the Wufeng and Longmaxi Formation, namely benthic algae, benthic–planktonic algae, and planktonic algae assemblages. Porosity in kerogen contributes greatly to shale porosity, accounting for 13 − 53% of total porosity. The total porosity and mesopore volume of samples (kerogen and shale) dominated by benthic algae are higher than those by planktonic algae. Pore size distributions of kerogen samples containing mainly benthic algae and planktonic algae are unimodal and multimodal type, respectively, when the pore diameter is larger than 5 nm. The different features between benthic and planktonic algae assemblages could be attributed to their different hydrocarbon generation potential and biological structure. Smaller fractal dimension of pores in kerogen samples mainly containing planktonic algae suggested that the planktonic algae are responsible for smoother pores in shales.


2019 ◽  
Vol 132 (7-8) ◽  
pp. 1704-1721 ◽  
Author(s):  
Yuxuan Wang ◽  
Shang Xu ◽  
Fang Hao ◽  
Baiqiao Zhang ◽  
Zhiguo Shu ◽  
...  

Abstract The organic matter-rich shales in Wufeng-Longmaxi Formation, Jiaoshiba area, Southeast China, are showing a notable petrographic heterogeneity characteristic within the isochronous stratigraphic framework, which lead to vast differences in the mineral composition and organic matter abundance in the adjacent sections of the shale reservoir. The studied shale has been divided into three systems tracts: a transgressive systems tract (TST), an early highstand systems tract (EHST), and a late highstand systems tract (LHST). Multiple-scale petrographic observation and detailed mineralogical and geochemical analyses were combined to investigate the manifestation, origin, and the ways by which the shale heterogeneity is affected. The results indicate that polytropic depositional environments lead to different components in sediment. Subsequently, these differences among shale sections become more apparent through different diagenetic pathways. During the deposition of the section TST, the Hirnantian glaciation and regional volcanism played a crucial role, contributing to the abundant accumulation of fine-grained intrabasinal silica and organic matter. In diagenesis stage, authigenic quartz aggregates derived from siliceous organisms are formed. They filled in primary interparticle pores, forming a rigid particle-bracing structure that provide effective resistivity against the compaction and spaces for organic matter migration and occlusion. Finally, the migrated organic matter left plenty of newly created pore spaces that constituted a great portion of the total porosity of shale reservoir. The depositional process of section EHST is strongly influenced by contour current, which brings about more extrabasinal influx and impoverishes organic matter. In diagenesis stage, the rigid particle-bracing structure could only be preserved in limited areas, since insufficient siliceous supply could not produce enough authigenic quartz. Primary interparticle pores are significantly reduced owing to compaction, leaving less space for later organic matter migration and occlusion. As a result, the total porosity of shale reservoir declines in this section. In a rapid tectonic-uplifting background, the deposition of section LHST is associated with a rapid increase in terrigenous clay minerals, which further dilutes organic matter. Ductile clay experienced strong compaction and then occupies most of the primary interparticle space. Rigid particles are wrapped by a large number of clays, which has destroyed the particle-bracing structure. As a result, the nanoporous system in the shale could not be well preserved.


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