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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.

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The big success in marine shale gas exploration and production made China the third country worldwide to commercialize shale gas development. However, the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in and around the Sichuan Basin are currently the only targets that have realized shale gas industrial development. Great challenges are emerging since tremendous shale gas resources of marine facies, continental facies, and transitional facies that are trapped in new areas and multiple other formations are yet to be successfully developed. Thus, we find it a great necessity to provide suggestions on shale gas exploration and development in China, which hopefully can be helpful for global shale gas exploitation. To meet this goal, this work provides a critical review on the history and current status of China’s shale gas exploration and development and summarizes key practical experiences. In the light of characteristic analysis of typical industrial gas fields and wells, research status, problems and challenges, along with suggestions on pivotal scientific issues are addressed including the development of organic-rich shales, reservoir types and characteristics, shale gas content, and the main controlling factors on shale gas enrichment. Further, future directions of shale gas exploration and development are nailed down, incorporating three levels: areas to improve development technology, areas to seek exploration breakthrough, and areas to conduct preliminary studies. The normal-pressure and deep shale gas retained in the Wufeng and Longmaxi Formations in and around the Sichuan Basin are the first level, which are the most realistic resources that can be commercially developed. For the normal-pressure shale gas, detailed research on the sweet spot selection, drilling–encounter ratio enhancement, and cost minimization by advanced technologies are most imperative; for the deep shale gas, state-of-the-art technology to maximize the stimulated reservoir volume of lateral wells is the key. Gas resources in other shale formations in the Sichuan Basin and its periphery such as the Cambrian marine shales, Permian transitional shales, and Jurassic continental shales are the second level, which have the greatest prospective to claim exploration breakthroughs, while shale gas resources in other basins or regions still demand grand scientific and technological tasks for exploration and development preparation. All in all, as a country with diverse shale gas types and such intricate geological and surface conditions, the summary of China’s shale gas exploration and development practices is of vital significance that will not only shed light on China’s shale gas development directions but also provide references for the shale gas industry in other countries and regions.

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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 th...

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