Development of organic pores in the Longmaxi Formation overmature shales: Combined effects of thermal maturity and organic matter composition

Organic pores are considered to be the dominant contributors to total porosity and hydrocarbon storage in many source rock unconventional reservoirs. In this study, organic petrology, focused ion beam-environmental scanning electron microscopy (FIB-SEM), and low-pressure N-2 adsorption and desorption were used to analyze six shale samples obtained from the Shuanghe section (Changning County) and wells YS118 and JY2 in the Sichuan Basin, China. Results of point-counting under the microscope show that alginite including type I and II is most abundant followed by bitumen and zooclasts of graptolites. The contents of alginite I, alginite II, and bitumen range from 12 vol% to 48 vol%, from 5 vol% to 35 vol%, and from 30 vol% to 48 vol%, respectively. Only few pores were found in graptolites. Thermal maturity, macerals, and palynomorphs have strong influences on the development of organic pores. In the analyzed samples, organic pores (> 10 nm) in shale with a maturity of < 2.5% Ro are larger and rounder than those in shale with a maturity of > 2.5% Ro. In addition, characteristics of organic pore in different maceral types are studied based on SEM and low-pressure N-2 adsorption and desorption. Generally, alginite I and solid bitumen have larger and more abundant organic pores than other macerals. Alginite I and solid bitumen have approximately same porosity (similar to 13.0 vol%) and same mean pore size (similar to 180 nm). Pores in a lginite I are elongated and lattice-like arranged, however, pores in bitumen are spongy and vesicle pores. With respected to alginite I, alginite II has a more complicated pattern of organic-pore development. Some unicellular alginite II grains contain very few nanopores, whereas some aggregated alginite II host plenty of nanoscale organic pores. Detailed studies on the palynomorphs of macerals suggested that alginite I is sourced from benthic algae and alginite II are derived from phytoplankton. By comparing the development of organic pores in different palynomorphs, it was concluded that palynomorphs derived from different algae precursors and with distinct hydrocarbon potentials and hydrocarbon-generating characteristics affect the development of organic pores in alginite macerals in the over-mature stage.