Effect of clay and organic matter on nitrogen adsorption specific surface area and cation exchange capacity in shales (mudrocks)

Abstract

The resistivity log and its conventional application is one of the most important analysis used to find oil and gas saturated intervals. In unconventional oil and gas producing rocks, however, this tool and the consequent technique, is affected by many factors and not considered very reliable. Shale reservoir rocks usually have high total specific surface area (TSSA) due to high clay and total organic content (TOC) and nano-scale pores. Resistivity values are rather low and usually not indicative of reservoir zones in high TSSA rocks. Nitrogen adsorption and cation exchange capacity (CEC) are the common techniques to measure TSSA. Clays and organic matter (OM) affect the measured TSSA using either technique. This effect must be taken into account while calculating water saturation using conventional models. In this paper we investigate the mineralogical and geochemical associations of CEC and TSSA and their effects on resistivity in shale reservoirs.We studied samples from oil and gas producing reservoirs such as Bakken, Haynesville, European Silurian, Niobrara, and Monterey formations. CEC was measured using Co(III)-hexamine3+ with the spectrophotometric technique and the equivalent TSSA (CEC-TSSA) was calculated. We also measured the specific surface area using sub-critical Nitrogen gas adsorption technique (N2-SSA). Rock mineralogy, organic matter properties and scanning electron microscope (SEM) images were used to further analyze the data.We find that CEC values are directly correlated with the clay type and content regardless of the OM content or level of thermal maturity. Smectite and illite (when negligible smectite is present) dominate the CEC value in shales. N2-SSA correlates with clay content, especially smectite and illite, but is less sensitive to clay type as CEC. This correlation between N2-SSA and clay content was observed in Bakken (no organic matter), thermally mature (gas window) Haynesville, and low TOC (<2.67 wt%) Niobrara (oil window) samples. We also find that OM significantly affects N2-SSA in two different ways: (1) Blockage of pores and throats by bituminous kerogen, which limits the accessibility of nitrogen to clay surfaces. This effect was observed in thermally immature (oil window) Niobrara (TOC>2.6 wt%) and Monterey shales. (2) Development of nano-scale OM-hosted pores with high surface area mostly for thermally mature (gas window) shales as observed in high TOC (>1.5 wt%) Silurian shales. Correlation with N2-SSA and CEC values revealed that the average charge density for most of the shales in this study varies between 3 and 5 e/nm2 and for some high TOC Niobrara samples can be as high as 32. Relatively higher charge density is due to underestimation of the TSSA by nitrogen adsorption technique. The correlation between SSA/CEC and clay content/type are well studied in the literature. However the results of this study aid in understanding how mineralogy, geological factors, organic matter content and thermal maturity affect this correlation in mud rocks from various reservoirs.