Organic Matter Types of the Wufeng and Longmaxi Formations in the Sichuan Basin, South China: Implications for the Formation of Organic Matter Pores

Organic matter (OM) pores are crucial to porosity in many shale gas reservoirs, but the origin and types of OM pores remain controversial. In this paper, the OM types are systematically identified and analyzed in the Wufeng–Longmaxi Formations from wells JY 1 and JY 2 in the Jiaoshiba gas field, Sichuan Basin. The results indicate that the OM comprises several hydrocarbon-generating organisms, such as various algae (multi- and unicellular algae, colonial algae, acritarch, etc.), graptolite, sponge spicule, and other fossil fragments, as well as several amorphous OM types, such as solid bitumen and strongly compacted algae. The OM pores have inherited the morphology and structure of multicellular algae and are commonly hundreds of nanometers, some of which even reach micrometer size, exhibiting irregular, bubble-like, spherical, and/or elliptical shapes. In the unicellular algae, only a small amount of OM pores are observed, which are isolated and distributed randomly. The OM pores, either irregular or oval, are tens to more than 100 nm, which are developed by the arrangement gap between the unicellular algae, and some are generated inside the unicellular algae by hydrocarbon generation and expulsion. Two types of OM pores developed in solid bitumen, including bitumen–spherulite pores and vesicular pores. The bitumen–spherulite pores are formed by the arrangement of nanoscale bitumen–spherulite with a pore diameter of nanometer scale. The vesicular pores are formed by gas generation and expulsion after oil cracking, and the shape is mostly sporadic, isolated with various sizes ranging from 500 nm to 3 μm. The OM pores in graptolite, sponge spicule, radiolarian, and other fossil fragments are much fewer in quantity and smaller in size. The OM pores may have only developed on the surface of graptolite. The pores are commonly developed in the walls of the fossil fragments and in the solid bitumen by filling in the biological cavity of the sponge spicule. Therefore, it is concluded that the OM types are the pivotal causes of different OM pore types and properties. Multicellular algae are most beneficial to OM pore development, serving as the major producer of OM pores. In the profile of the Wufeng–Longmaxi shales, the vertical variation of OM types and OM pores are diverse in different graptolitic zones. The dominant hydrocarbon-generating organisms in the WF2–LM4 graptolitic zone are multicellular algae and graptolite, followed by a small number of unicellular algae, sponge spicule, radiolarian, and other fossil fragments. The OM pores are mainly developed in multicellular algae and graptolite, with only a few developed in solid bitumen and unicellular algae. In the LM5–LM8 graptolitic zone, however, the hydrocarbon-generating organisms are primarily unicellular algae, with little multicellular algae, graptolite, sponge spicule, and other fossil fragments. The OM pores are mainly developed in unicellular algae and solid bitumen. The new findings provide evidence to support the proposal that multicellular algae are the main hydrocarbon-generating organisms controlling the OM pore development. Moreover, the WF2–LM4 graptolitic zone is the target interval for shale gas exploration and development in the Upper Ordovician and Lower Silurian Formations in the Sichuan Basin and its surrounding areas.