Quantitative Characterization of Organic Pore Structure from Gas Adsorption in Lower Cretaceous Lacustrine Shales in the Songliao Basin, NE China

Abstract

Organic matter (OM) pores are widely considered to be important for gas storage and transportation. In this work, we quantitatively analyze the pore structure of OM and its controlling factors through geochemical and petrologic analyses, optical microscope, OM isolation, and adsorption isotherms. These analyses were carried out on lacustrine shale samples from the Lower Cretaceous Shahezi Formation, which is located in the Changling Fault Depression in Songliao Basin. The results show that the content of soluble OM (SOM) is low, accounting for 0.26%-3.75% of total OM. The contribution of pore development from SOM itself is limited. After extraction of SOM by chloroform, pore volume (PV), specific surface area (SSA), and average pore diameter (APD) exposed to gas molecules greatly increase. The existence of SOM has an obvious effect on pores of >10 nm, especially the clay mineral-related pores that contribute the most to the total PV. The content of kerogen is higher than SOM and accounts for 9.9%-65.5% (averaging 24.0%) of total PV in bulk shale, only second to minerals. More importantly, kerogen is the dominant contributor to organic PV, accounting for 95.8%-99.7% (averaging 98.3%) of the total organic PV. The pores developed in the kerogen peak at 0.4-0.7 nm and 10-30 nm. The solid bitumen (SB) followed by vitrinite and inertinite in kerogen contributes the most to the total kerogen PV.

Insights into Controls of Mineralogy and Pore Structure on the Density of Methane Adsorption Phase in Shales under Supercritical Conditions

“…Shale is a complicated geomaterial, composed of a wide range of organic macerals (e.g., type I, II, and III kerogen, bitumen, and pyrobitumen), minerals (e.g., typically, quartz, feldspar, calcite, dolomite, pyrite, and clay minerals), and pore sizes (<1 nm to 1 μm, over 3 orders of magnitude). ,− ,− All the factors result in the heterogeneous nature of shale that is complicated further by subsequent diagenesis after sedimentation and render difficulties in an accurate estimation of subsurface shale gas. ,− ,− Within a shale matrix, OM and clay minerals are regarded as functional minerals to adsorbed gas due to well-developed micropores (<2 nm) and mesopores (2–50 nm), which provide significant pore volume (PV) and specific surface area (SSA) for subsurface adsorption. ,, The type, enrichment, and thermal maturity of OM were abundantly reported to impact adsorption capacity in the past decade. ,, Total organic carbon (TOC) content was observed positively correlated to methane adsorption capacity and total gas content . Microporosity associated with OM and clay minerals was also suggested contributing to a high capacity of methane adsorption .…”

“…10,25,26 The type, enrichment, and thermal maturity of OM were abundantly reported to impact adsorption capacity in the past decade. 19,27,28 Total organic carbon (TOC) content was observed positively correlated to methane adsorption capacity and total gas content. 27 Microporosity associated with OM and clay minerals was also suggested contributing to a high capacity of methane adsorption.…”