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

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.

Differences of Main Enrichment Factors of S1l11-1 Sublayer Shale Gas in Southern Sichuan Basin

In this study, shale cores from 20 wells in the S1l11-1 sublayer of Longmaxi Formation buried in shallow shale (<3500 m) and deep shale (>3500 m) in the southern Sichuan Basin, China were collected to compare their pore structures and gas-bearing properties using multiple experiments. Results showed that the deep layer has relatively lower brittle mineral content, which is disadvantageous in terms of the higher requirements it imposes on hydraulic fracturing. Results also showed that the most important factor controlling the differential enrichment of S1l11-1 shale gas in southern Sichuan Basin is porosity. Moreover, the porosity composition of shallow shale and deep shale has significant differences: the porosity of shallow shale is dominated by organic pores, while for deep shale, both organic and inorganic pores are important. The inorganic pores provide significant storage space for free gas in deep shale; their contribution warrants more attention. We also found that the difference in organic porosity of the shallow and deep shale samples resulted from large differences in pore development ability, while the highest inorganic porosity was concentrated near the optimal mineral composition when the content of quartz plus feldspar plus pyrite was about 70%. This study revealed the primary factor controlling the difference in gas content between shallow and deep shale and detailed the characteristics of microscopic pore structure, providing a basis for the exploration and development of deep shale gas in the Wufeng-Longmaxi Formation in the southern Sichuan Basin.

Characteristics of Organic Matter Particles and Organic Pores of Shale Gas Reservoirs: A Case Study of Longmaxi-Wufeng Shale, Eastern Sichuan Basin

Heterogeneity of organic matter (OM), including size, type, and organic pores within OM, is being recognized along with increasing study using SEM images. Especially, the contribution of organic pores to the entire pore system should be better understood to aid in the evaluation of shale reservoirs. This research observed and quantitatively analyzed over 500 SEM images of 19 core samples from Longmaxi-Wufeng Shale in the eastern Sichuan Basin to summarize the features of OM particles and OM-hosted pores and their evolution during burial. The features of organic pores as well as the embedded minerals within OM particles enables to recognize four different type of OM particles. The organic pore features of each type of OM particles were quantitatively described using parameters such as pore size distribution (PSD), pore geometry, and organic porosity. The PSD of weakly or undeformed porous pyrobitumen indicates that the large organic pores (usually 200 nm to 1 um) is less common than small pores but the major contributor to organic porosity. The organic porosity of OM particles covers a large range of 1–35%, indicating a high heterogeneity among OM particles. Based on analysis of 81 OM particles, the average of organic porosity of the five samples were calculated and ranges from 3% to 12%. In addition, samples from well JY1 have higher organic porosity than JY8. These results helped to reveal how significant the organic pores are for shale gas reservoirs. In addition to presenting many examples of OM particles, this research should significantly improve the understanding of type and evolution of OM particles and contribution of OM-hosted pores to the entire pore system of high to over mature shale.

Data-driven elastic modeling of organic-rich marl during maturation

The elastic modeling of source-rock reservoirs during maturation must incorporate microstructural and geochemical alterations. The common challenge is calibrating the volumetric proportion between each form of organic porosity along with the changes in the bulk and shear moduli of kerogen as a function of maturity. Two forms of organic porosity have generally been observed: (1) spongy and bubble pores inside kerogen and (2) low-aspect-ratio pores or gaps at the interface between shrinking kerogen bodies and the matrix. We have constructed a rock-physics model of organic-rich marl during maturation and calibrated it using rock-physics data sets from controlled pyrolysis experiments of organic-rich marl under stress. We chose these pyrolysis data because the samples provide subsequent changes in porosity and P- and S-wave velocities as a function of maturity, while evidencing minimal grain sliding and mechanical compaction due to their stiff matrix. Our calibration results indicate that spongy and bubble pores should be used as the predominant form in the model regardless of maturity. Our results also indicate a competing effect between increasing kerogen porosity and the increasing moduli of solid kerogen. Kerogen porosity mainly develops throughout the oil windows. Whereas the moduli of solid kerogen increase by a factor of two in the early-peak oil window and remain relatively constant afterward. Consequently, the effective moduli of kerogen experience minimal changes in the early-peak oil window and rapidly decrease to half of the immature values in the late oil window. These calibration results are consistent with several petrophysical and nanoindentation studies on kerogen. Finally, we used the calibrated model to build a rock-physics template of organic-rich marl during maturation. The template was tested with pyrolyzed and naturally matured samples, which showed that our model can be used to characterize reservoir properties across different maturity windows.

The effect of catagenetic maturity on the formation of reservoirs with organic porosity in the Bazhenov formation and peculiarities of their extension

Results of Bazhenov formation deposits organic matter complex investigations to assess its oil generation potential and maturity are presented. Researches were performed by Rock-Eval pyrolysis, rock structure analysis using scanning electron microscopy and organic matter transformation and distribution in the bulk of rocks using luminescent microscopy. As a result of the research, the general pattern for the Bazhenov formation kerogen thermal maturity variability along Western Siberia was confirmed, and the high maturity areas delimitation around the Salym high and the Krasnoleninsky arch was significantly specified. The areas of anomalously high maturity are not related to regional patterns of sedimentation. The possible influence of thermal maturity on the unconventional reservoirs with kerogen porosity formation in the Bazhenov formation and the features of their distribution were established. High thermal maturity that refers to the end of oil window is one of the main condition for the Bazhenov formation oil capacity formation, but knowledge of maturity change through the area pattern is not a sufficient condition for unconventional reservoirs including that with kerogen porosity retrieval.

Criteria of distribution of Bazhenov’s high-carbon formation oil-productive rocks with advanced kerogen porosity

The analysis of Bazhenov high-carbon formation organic matter catagenesis zonal distribution in connection with its own reservoir’s organic matter porosity made in this work. Realized research helped to articulate regional prospecting criteria for oil deposits in Bazhenov’s formation unconventional reservoir with organic porosity.

Geochemical conditions of hydrocarbon accumulation in low-permeability shale sequences

The report is devoted to the study of conditions for the formation of organic porosity–void spaces of organic origin formed during transformation of the organic matter into hydrocarbons, and their role in the formation of hydrocarbon accumulation in low-permeability shale strata. The experience of studying and developing known shale formations of the world testifies to the fact that such strata are hybrid phenomena, that is, they are both oiland gas-bearing strata containing traditional and non-traditional accumulations of hydrocarbons. Based on the results of the programmed pyrolysis by the Rock-Eval-6 method, an estimate has been performed to quantify organic (kerogen) porosity. This estimate allows for determining the forecast retention volume of HCs generated during catagenesis.