Experimental investigation of the pore structure of triassic terrestrial shale in the Yanchang Formation, Ordos Basin, China

Abstract

The Yanchang Formation was deposited in a terrestrial environment in the late Triassic period. It consists of organic-rich shales that possess hydrocarbon resources for both conventional and tight gas sand plays in the Ordos Basin. The identified source rocks have become horizontal drilling targets as an active shale gas play. Although limited, the current study suggests that the physics of its porous media is far more complicated than that of marine shales in either North America or China, which calls for detailed characterization. In this study, a series of laboratory experiments were conducted on 20 samples from Chang 7 and Chang 9 members of the Yanchang Formation to characterize the pore structure of the terrestrial shale, including the mercury injection capillary porosimetry (MICP) test, low-pressure N2 adsorption test, X-ray diffraction mineralogy (XRD) test, and geochemical analysis. Moreover, petrographical analysis was performed to obtain field-emission scanning electron microscopy (FE-SEM) images in order to identify different types of pores, as well as their correlations with mineral constituents and organic matter.The mineralogical analysis (XRD) reveals that the major constituting minerals of the samples are clay minerals, followed by quartz and feldspar. In addition, the percentages of carbonate minerals are found to be relatively low, which is distinctively different from the mineral makeup of common marine shales. Pyrite exists in all samples, varying from 1% to 12%, while the total organic carbon (TOC) is in the range of 3.5–7.14% and the vitrinite reflectance (Ro) from 0.88 to 1.39%. The low-pressure N2 adsorption analysis shows that the total pore volume (TPV) ranges from 0.180–5.236 × 10−3 cm3/g, and the specific surface area (SSA) changes from 0.209 to 13.601 m2/g. The pore size distribution (PSD) attained from the low-pressure N2 adsorption test reveals multimodal peaks between the 1.8 nm and 3–8 nm intervals, suggesting that most of the pores are mesopores.From the FE-SEM observations, it is discovered that the organic matter or pyrite nodules occupy the macropore space (3–10 μm) formed at current burial depths. Organic pores are found to have a pore size ranging from 30 to 50 nm. Moreover, inter-crystalline pores exist within pyrite nodules with a pore size from 0 to 50 nm. These observations are in good agreement with the bimodal pore size distribution measured from the low-pressure N2 adsorption analysis. Furthermore, the formation of macropores is likely controlled by the interplay between multiple factors, such as clay content, silicate minerals, carbonate minerals, and organic matter. In conclusion, the findings of this study provide a better understanding of pore structures in terrestrial shales, and help to evaluate the storage capacity and transport capability of terrestrial shales, in general.