Occurrence Space and State of Petroleum in Lacustrine Shale: Insights from Two-Step Pyrolysis and the N₂ Adsorption Experiment

Abstract

The pore structure and occurrence state of hydrocarbons in shale reservoirs are significant factors affecting shale oil production. However, how pore structure affects the occurrence of shale oil remains unclear. This paper aims at analyzing the major distribution space of shale oil with various physical states in shale pores using experimental approaches. Therefore, the original samples and solvent-extracted samples of lacustrine shale from the Permian Fengcheng Formation in Junggar Basin were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), Rock-Eval pyrolysis, and N 2 adsorption/desorption methods. The findings demonstrate that the Fengcheng shale has high oil content, with free oil accounting for 57.17% of the total oil yield. The pore size distribution characteristics of original and solvent-extracted shale samples indicate that shale oil is mainly stored in mesopore pores with diameters between 5 and 40 nm. The threshold limit of pore diameter for the movable oil in shale reservoirs is between 3.5 and 19 nm. Also, by comparing shale oil mobility in various basins, it is found that petroleum mobility occurs when the total oil yield in the shale reaches 6 mg HC/g Rock. Meanwhile, the threshold value of petroleum mobility in different basins is related to the pore shape. Parallel plate pores within shales are more favorable for petroleum mobility. This research presents the mobility of the shale oil threshold, which is critical for shale oil exploration and exploitation.

Distribution of Retained Oil in Shales from the Lucaogou Formation, Jimusar Sag, Northwest China: Insights from Mineralogy and Pore Structure

“…Furthermore, the differences of the pore diameter change after extraction between MIP and NP; results (Figures 5 and 7; Table 2) illustrate that the retained oil is mainly filled in the pores with diameters smaller than 100 nm (especially micropores). The reason is that, in the original samples, the retained oil occupies part of the tiny pore throat space of <30 nm, and part of the pores and throats became empty after solvent extraction process, which enlarges the pore volume of the sample and allows smaller pore to be detected 69 and also decreases the average pore diameter. For the pores with diameters larger than 100 nm, the oil is retained in pores as thin film (Figure 11).…”

Using Fractal Theory to Study the Influence of Movable Oil on the Pore Structure of Different Types of Shale: A Case Study of the Fengcheng Formation Shale in Well X of Mahu Sag, Junggar Basin, China

“…Compared with freshwater lake basins, saline lake basins have the characteristics of mixed lithologies of terrigenous clastic and endogenous carbonates, resulting in complex reservoir physical properties [28]. To date, research on shale oil within the Fengcheng Formation has predominantly centered on shale oil reservoir typology, pore diameter distribution, and organic matter abundance [29][30][31]. However, studies exploring the impact of reservoir mineral composition on shale oil distribution remain limited, with scarce utilization of fractal theory for related investigations.…”

Impacts of Pore Structure on the Occurrence of Free Oil in Lacustrine Shale Pore Networks

“…Previous studies [16,[47][48][49][50][51][52][53] have combined pore characterization techniques with organic matter removal methods to investigate the impact of soluble and insoluble organic matter on the preservation and development of shale pores. Oil extraction with organic reagents significantly increases the pore volume, indicating that oil occupies the pores and affects porosity measurements [16,48,[51][52][53]. Removing kerogen with inorganic reagents yields different changes in pore volume depending on pore type [47,49,50].…”