Characteristics of three organic matter pore types in the Wufeng-Longmaxi Shale of the Sichuan Basin, Southwest China

A consensus has been reached through previous studies that organic matter (OM) pores are crucial to porosity in many shale gas reservoirs; however, their origins and types remain controversial. Here, we report the OM pore types hosted in algae, bitumen, graptolite and other fossil fragments in the Wufeng-Longmaxi Formations of the Sichuan Basin, Southwest China. Algae types mainly include multicellular algae, unicellular algae, etc. The OM pores in multicellular algae usually exhibit irregular, bubble-like, spherical and/or elliptical profiles, and their diameters vary between 300 and 800 nm. The shapes of the OM pores in unicellular algae are either irregular or oval, and the pores are hundreds of nanometres in size. The pores associated with solid bitumen are sporadic, isolated and variable in size, ranging from 500 nm to 3 μm. The pores in the graptolite, sponge spicule, radiolarian and other fossil fragments are much smaller and fewer. The pores may only have developed in the surface of the graptolite and bitumen by filling in the biological cavity of the sponge spicule. These new findings provide stronger evidence that multicellular algae are the main hydrocarbon generating organisms of OM pores development.