Wu Songtao

Progress in China＇s Unconventional Oil ＆ Gas Exploration and Development and Theoretical Technologies

The new century has witnessed a strategic breakthrough in unconventional oil & gas. Hydrocarbon accumulated in micro-/nano-scale pore throat shale systems has become an important domain that could replace current oil & gas resources. Unconventional oil & gas plays an increasingly important role in our energy demand. Tight gas, CBM, heavy oil and asphaltic sand have served as a key domain of exploration & development, with tight oil becoming a ‘bright spot’ domain and shale gas becoming a ‘hotspot’ domain. China has made great breakthroughs in unconventional oil & gas resources, such as tight gas, shale gas, tight oil and CBM, and great progress in oil shale, gas hydrate, heavy oil and oil sand. China has an estimated (223–263)×108t of unconventional oil resources and (890–1260)×1012 m3 of gas resources. China has made a breakthrough for progress in unconventional oil & gas study. New progress achieved in fine-grained sedimentary studies related to continental open lacustrine basin large-scale shallow-water delta sand bodies, lacustrine basin central sandy clastic flow sediments and marine-continental fine-grained sediments provide a theoretical basis for the formation and distribution of basin central reservoir bodies. Great breakthroughs have been made in unconventional reservoir geology in respect of research methodology & technology, multi-scale data merging and physical simulation of formation conditions. Overall characterization of unconventional reservoirs via multi-method and multi-scale becomes increasingly popular and facilitates the rapid development of unconventional oil & gas geological theory, method and technology. The formation of innovative, continuous hydrocarbon accumulation theory, the establishment of the framework of the unconventional oil & gas geological theory system, and the determination of the implications, geological feature, formation mechanism, distribution rule and core technology of unconventional oil & gas geological study lays a theoretical foundation for extensive unconventional oil & gas exploration and development. Theories and technologies of unconventional oil & gas exploration and development developed rapidly, including some key evaluation techniques such as ‘sweet spot zone’ integrated evaluation and a six-property evaluation technique that uses hydrocarbon source, lithology, physical property, brittleness, hydrocarbon potential and stress anisotropy, and some key development & engineering technologies including micro-seismic monitoring, horizontal drilling & completion and “factory-like” operation pattern, “man-made reservoir” development, which have facilitated the innovative development of unconventional oil & gas. These breakthroughs define a new understanding in four aspects: ① theoretical innovation; ② key technologies; ③ complete market mechanism and national policy support; and ④ well-developed ground infrastructure, which are significant for prolonging the life cycle of petroleum industry, accelerating the upgrade and development of theories and technologies and altering the global traditional energy structure.

Shale Gas Formation and Occurrence in China: An Overview of the Current Status and Future Potential

Shale gas is one of the most promising unconventional resources both in China and abroad. It is known as a form of self-contained source-reservoir system with large and continuous dimensions. Through years of considerable exploration efforts, China has identified three large shale gas fields in the Fuling, Changning and Weiyuan areas of the Sichuan Basin, and has announced more than 540 billion m~3 of proven shale gas reserves in marine shale systems. The geological theories for shale gas development have progressed rapidly in China as well. For example, the new depositional patterns have been introduced for deciphering the paleogeography and sedimentary systems of the Wufeng shale and Longmaxi shale in the Sichuan Basin. The shale gas storage mechanism has been widely accepted as differing from conventional natural gas in that it is adsorbed on organic matter or a mineral surface or occurs as free gas trapped in pores and fractures of the shale. Significant advances in the techniques of microstructural characterization have provided new insights on how gas molecules are stored in micro- and nano-scale porous shales. Furthermore, newly-developed concepts and practices in the petroleum industry, such as hydraulic fracturing, microseismic monitoring and multiwell horizontal drilling, have made the production of this unevenly distributed but promising unconventional natural gas a reality. China has 10–36 trillion m~3 of promising shale gas among the world’s whole predicted technically recoverable reserves of 206.6 trillion m~3. China is on the way to achieving its goal of an annual yield of 30–50 billion m~3 by launching more trials within shale gas projects.

Evidence of the Near-Source Accumulation of the Tight Sandstone Gas in Northern Ordos Basin, North-Central China

The tight sandstone gas in Upper Paleozoic Formation of the northern Ordos Basin is a typical giant unconventional tight gas province. Evidences from geochemistry, reservoir geology and pale-otectonic setting all verify that the present-day tight sandstone gas accumulation in the Ordos Basin is the result of near-source accumulation. The evidences are listed as following: tight sandstone gas is mainly distributed in the area with high gas-generating strength; gas composition was not subjected to fractionation; gas saturation significantly decreases with the distance away from the source rocks; gas isotopes suggest their origin is the same and maturity is consistent with in-place source rocks; reservoirs have experienced three types of densification digenesis, including intense compaction, siliceous cementation and calcareous cementation, which took place before the formation of a large amount of tight sandstone gas, forming tight reservoirs with low porosity and permeability, fine pore throat and great capillary resistance; the paleo-structural gradient ratio is small from the main hydrocarbon generation period to present. It is indicated the present distribution of tight sandstone gas in the northern Ordos Basin is the result of near-source and short-distance migration and accumulation.