Influencing Factors of Shale Pore Space in Wulalike Formation of Ordos Basin and its Geological Significance of Shale Gas

Key words: 
• Ordos Basin /
• Wulalike Formation /
• Pore space /
• Shale gas /
• Differential enrichment

Abstract: [Objectives] It is important to clarify the coupling relationship between the differences in pore space characteristics of marine shale and lithology, material composition, pore structure, etc., as well as their geological significance for shale gas enrichment, in order to evaluate the gas bearing properties of the Wulalike Formation shale in the western Ordos Basin. This study aims to determine the pore space characteristics and intrinsic coupling factors of the Wulalike Formation shale gas reservoir, and to discuss the geological significance of shale gas in combination with the vertical development characteristics of lithofacies. [Methods] This study conducted analysis and testing experiments related to reservoir physical properties, pore structure, and adsorption capacity. Based on the previous research results and the classification of typical shale lithofacies, the differences in porosity and their key influencing factors of shale with different lithofacies were clarified. With the gas bearing characteristics of shale as constraints, the longitudinal hydrocarbon enrichment intervals of shale were explored. [Results] The overall porosity of the shale in the Wulalike Formation is relatively small, ranging from 0.37% to 5.01%, with an average of only 2.03%. The porosity of the Wulalike Formation shale shows an increasing trend from bottom to top, TOC, quartz content, and the proportion of biogenic silica decrease accordingly. And the correlation between porosity and permeability is not significant. Unlike the Longmaxi Formation shale, the target shale has high biogenic silica and low TOC content as a whole. Due to the low overall TOC content, fewer organic matter pores, and weak hydrocarbon generation and expulsion ability, and biogenic silica mainly occupies small pores and mesopores spaces. The negative correlation coefficients of small pores, mesopores, macropores, and biogenic silicon content are 0.3922, 0.2118, and 0.0403, respectively. As the ratio of biogenic silica to TOC increases, there is no observed trend in the traditional understanding that porosity increases with the increase of TOC. [Conclusion] Poor hydrocarbon generation ability caused by low TOC content and relatively low degree of thermal evolution limits the accumulation of hydrocarbons in the Wulalike Formation shale, and biogenic silica manifested as cementation reducing pore size rather than supporting pore enlargement. Therefore, compared to detrital siliceous shale and detrital mixed shale, biogenic siliceous shale exhibits lower porosity, poorer pore structure characteristics, and poorer fluid mobility characteristics. However, an increase in organic matter content can improve this phenomenon to some extent in the biogenic siliceous shale. The upper part of the target layer is mainly composed of mixed shale lithofacies, which has high debris content and porosity, but low organic matter content and poor hydrocarbon generation and drainage capacity. After the peak hydrocarbon generation period at the bottom, it is easy to cause formation water backflow. Transitioning towards the bottom into biogenic siliceous shale lithofacies, although the TOC content is relatively high, the biogenic siliceous cementation is unfavorable for reservoir performance. Based on the characteristics of pore space, the alternating development of multiple lithofacies shale sites in the upper part of the development site of biogenic siliceous shale is expected to indicate the enrichment site of shale gas under low TOC background.