Factors influencing the specific surface areas of argillaceous source rocks

Abstract

This paper integrates methods of nitrogen adsorption (N2-BET) and ethylene glycol monoethyl ether (EGME) to present a comprehensive understanding of the specific surface areas (SSAs) of bulk rocks and clay fractions (<2μm) in argillaceous source rocks, and employs the X-ray diffraction method to discuss the characteristics and influence factors of SSAs of argillaceous source rocks in order to service for unconventional petroleum exploration and exploitation. The methods of N2-BET and EGME can be used to obtain external surface area (N2-BET SSA) and total surface area (EGME SSA) within argillaceous source rock, respectively, and the difference between the total and the external surface area yields the internal surface area (internal SSA). The internal SSA accounts for over 80% in EGME SSA. Silty mudstone and argillaceous siltstone have larger internal and external surface areas than mud-bearing siltstone. Further, clay minerals (smectite in particular) make the greatest contribution to the internal, external and total surface areas (especially the internal and total surface areas) of source rock, whereas detrital minerals and carbonate minerals have a negative effect on each specific surface area. What is noteworthy is the fact that an abrupt increase in carbonate mineral content results in remarkable decreases in internal and EGME SSAs. These features therefore indicate that the differences in rock constituents (clay, detrital minerals and carbonate minerals) and mineral compositions affect the variations in SSAs characteristics of argillaceous source rocks. By plotting the SSAs vs. depth in bulk rocks and clay fractions, the variation extent of SSAs in clay fractions is better than that in bulk rocks. For either bulk rocks or clay factions, the value of internal SSA/N2-BET SSA changes abruptly from 20 in the shallow-part (above 1500m) to 5–10 in the deep-part (below 2000m). The evolution of minerals during diagenesis creates variability in the mineral content with burial depth, which causes a subsectional evolution of SSAs in the vertical. As the surfaces within argillaceous source rock are closely correlated with the occurrence of organic matter or hydrocarbon, analyses of the difference between internal surface area and external surface area give the result that the SSAs of argillaceous source rocks are influenced by the factors of rock types, mineral compositions, diagenesis, etc., so attention to specific mineral assemblage and burial depth are of great significance to enhance the success rate of petroleum exploration and development in argillaceous source rocks, particularly in the research of unconventional petroleum systems.