Oil & Gas Geology ›› 2021, Vol. 42 ›› Issue (6): 1321-1333.doi: 10.11743/ogg20210607

• Petroleum Geology • Previous Articles     Next Articles

Characteristics of and factors influencing organic pores in the Lower Silurian Longmaxi Formation, Fushun-Yongchuan area, Sichuan Basin

Xiya Yang1,2(), Chenglin Liu1,2,*(), Wenping Liu3, Haolin Ren1,2   

  1. 1. State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum(Beijing), Beijing 102249, China
    2. College of Geosciences, China University of Petroleum(Beijing), Beijing 102249, China
    3. Exploration and Development Research Institute of Southwest Oil & Gasfield Company, PetroChina, Chengdu, Sichuan 610046, China
  • Received:2021-07-09 Online:2021-12-28 Published:2021-12-16
  • Contact: Chenglin Liu E-mail:18801321355@163.com;lclzgx@126.com

Abstract:

The Lower Paleozoic marine shale sequences are widely developed in the southern part of Sichuan Basin. Among others, the Longmaxi Formation shale possesses high-quality source rocks favorable for shale gas enrichment. We discuss the characteristics of and factors influencing the organic pores in the Lower Silurian Longmaxi Formation, Fushun-Yongchuan area, based on coring data obtained from the drilling of organic-rich shale, as well as related analysis and tests. The research results indicate that the organic matter abundance of the Longmaxi Formation shale is high with a TOC content ranging from 2.45% to 5.59%, and its thermal evolution reaches a high-to-over-mature stage with a vitrinite reflectance (Ro) of 1.97% to 2.23%, providing a favorable basis for the growth of organic pores. As important reservoir space for shale gas, most organic pores are round-, oval- or concave-shaped under scanning electron microscopy (SEM) observation, and a few occur in strips and/or irregular polygons; all pores are smooth-edged. Besides, nitrogen adsorption and high pressure mercury intrusion porosimetry (MIP), show that these organic pores are relatively small in size, most of which are supposed to be ascribed to mesopores, accounting for about 57%. Then, we quantitatively analyze the organic pores across the study area by establishing a relationship model between porosity of organic pores and spectral gamma-ray (GR) logs (U), and the results show that the porosity of organic pores varies between 1.10% and 3.64%, with an average of 2.15%. There are multiple factors related to the development and preservation of organic pores. The correlation between organic pore porosity with contents of clay minerals, brittle minerals and TOC, is set up and shows that the porosity of organic pores is positively correlated with contents of brittle minerals and TOC but negatively with clay mineral content. Compaction has a destructive effect on the preservation of organic pores, and a higher formation pressure coefficient is conducive to the preservation of organic pores.

Key words: organic pore, brittle mineral, shale, Longmaxi Formation, Fushun-Yongchuan area, Sichuan Basin

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