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

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.

Characteristics and Geological Significance of Organic Matter Veins in Shale Reservoir: A Case Study of the Silurian Longmaxi Formation in Luzhou Area, Sichuan Basin

“…But as a whole, the area of in situ organic matter is large and the boundary between the organic matter and mineral particles is also obvious. The organic matter in the inorganic pores is the most common type of organic matter in the studied section [5], and it is the bitumen generated by in situ organic matter in the process of hydrocarbon generation and expulsion. The area of organic matter in inorganic pores is small and its morphology is controlled by the pores of the mineral matrix before filling, which include intergranular pores, pyrite intercrystalline pores, clay mineral interlayers, and mineral particle cracks (Figure 2c,d).…”

“…Therefore, the pore-forming ability of organic matter is a key parameter in evaluation of unconventional petroleum systems [3,4]. Previous studies have shown that different types of organic matter have great differences in hydrocarbon generation and pore-forming capacity [5][6][7]. At present, the classification methods of over-mature Marine organic matter types are mostly based on morphology from the perspective of organic matter sources, and it is widely believed that bitumen and kerogen are the main components of organic matter [8].…”

New insights into the evaluation criteria for high-quality deep marine shale gas reservoirs in the Longmaxi formation: Evidence from organic matter pore development characteristics

“…Furthermore, lithofacies division determines the high-quality reservoir of the Longmaxi Formation. The dominant lithofacies of the Longmaxi Formation is siliceous shale, which has the highest reservoir quality and the most developed OM pore network (Jia et al, 2021;Wang et al, 2022;Zhang et al, 2022).…”