Characterization of geochemical properties and factors controlling the pore structure development of shale gas reservoirs

Abstract

The pore structure is an important parameter to understand the transport and storage capacities in shale. There is very limited research reported on the controlling factors of pore development in lacustrine shales to date. This study investigated pore developments and hydrocarbon generation potential of lacustrine Roseneath and Murteree shale formations. Although these shale formations are proven shale gas reservoir in the Cooper Basin, Australia, pore development studies are still rare. The geochemical properties measured on core samples include total organic contents (TOC), thermal maturity, hydrocarbon generation potential, and mineral composition by X-ray diffraction (XRD). Field-emission scanning electron microscopy (FE-SEM), Focused Ion Beam milling scanning electron microscopy (FIB-SEM), and low-pressure nitrogen adsorption-desorption isotherm experiments were used to quantify and qualify pore structure. Furthermore, the factors were analyzed that control pore development through statistical analysis. The results indicate that organic contents for the Roseneath shales (0.9%–1.74% Ro%) and Murteree shales (1%–1.7% Ro%) range between 0.81% and 5.36% and 0.51%–6.69%, respectively. The dominant mineral phases are quartz, clay (Illite, Kaolinite), and siderite, with variable quantities of the studied formations. Rock-Eval pyrolysis suggests Kerogen Type III and implies mostly late oil generation to dry gas generation potential in both shale formations. In Roseneath shale, total surface area (SSA) and total pore volume (TPV) varies from 2.25 cm2/g to 5 cm2/g and 0.005 cm3/g – 0.02 cm3/g, respectively. In Murteree shale, SSA and TPV vary from 3.5 cm2/g to 6.35 cm2/g and 0.005 cm3/g to 0.032 cm3/g, respectively. The pores exhibit unimodal to bimodal distribution with peaks at 2–4 nm, 90–120 nm, and 170–180 nm. The mesopores are dominant in studied formations, followed by macropores and micropores. Mesopores have the largest contribution to specific surface area and pore volume. The desorption hysteresis loop on the N2 adsorption-desorption curve indicates the presence of permeable and impermeable pores, as most of the samples belong to H2 and H3 type hysteresis loop. The pore size of intraparticle and interparticle various greatly; however, the pores with a diameter of 2.5 nm, 18 nm and 49 nm are common in both formations. The organopores contribute to micropores' development, and most organic pores have a pore diameter of 1.5 nm and 1.8 nm. The total pore volume (TPV) and specific surface area (SSA) are positively correlated with organic matter contents, quartz, and thermal maturity in both shale formations. However, clay minerals are positively correlated with SSA and TPV, which indicate that clay minerals have a significant contribution to Roseneath shale's pore development. The siderite mineral contributes to intraparticle pores development and positively correlated with SSA and TPV in Murteree shale. The thermal maturity significantly enhances pore's development by creating organopores and inorganic pores in both shales, as evidenced through statistical analysis and image analysis. The study has significant importance in understanding the pore structure and gas storage mechanism in shale.