Types, characteristics，genesis and prospects of conventional and unconventional hydrocarbon accumulations:taking tight oil and tight gas in China as an instance
ZOU Caineng 1,2 ZHU Rukai 1,2 WU Songtao 1,2 YANG Zhi 1 TAO Shizhen 1 YUAN Xuanjun 1 HOU Lianhua 1 YANG Hua 3 XU Chunchun 4 LI Denghua 1 BAI Bin 1,2 WANG Lan 1
It is an inevitable trend for oil and gas industry to transform exploration & development domain from conventional hydrocarbon accumulations to unconventional hydrocarbon ones, which are obviously different in types, geological features and genesis. Conventional petroleum focuses on accumulation mechanism, and the key answer is whether the trap contains petroleum, otherwise, unconventional petroleum focuses on the reservoir space, and the key answer is how much the reservoirs capture petroleum. Unconventional hydrocarbon resources are mainly characterized by continuous distribution and no natural oil and gas production from per well. Currently, unconventional problems occur in the exploration and development of conventional hydrocarbon resources, thus it is necessary to transform unconventional hydrocarbon resources into new “conventional” hydrocarbon resources. With technology development, unconventional hydrocarbons can be transformed into conventional ones. Generally, conventional hydrocarbon deposits consist of structural and litho-stratigraphic hydrocarbon reservoirs, where oil and gas are distributed in an isolated structure or a larger structure group with clear trap boundaries and pore-throat systems in millimeter-micrometer scale. Oil and gas in this case accumulate by buoyancy to form hydrocarbon pools. However, unconventional hydrocarbon accumulations, including tight sandstone oil and gas, tight carbonate oil and gas, shale oil and gas, coal-bed methane, oil shale, oil sand, hydrate, etc., are distributed continuously or quasi-continuously in basin's slopes or centers. Commonly, they are characterized by source-reservoir paragenesis and have no distinct trap boundaries. Pore-throat systems in nanometer scale are well-developed in unconventional reservoir rocks, and related hydrocarbons are mainly detained in situ or migrate for a short distance into reservoirs that are close to source rocks because buoyancy is limited. The present paper systematically analyzed geological characteristics and exploration potential of tight hydrocarbons in some typical basins of China, where pores in nanometer scale with partial micrometer-millimeter pores dominate the reservoir space of unconventional hydrocarbons, the diameter of reservoir pores is 5~200 nm in gas shale, 40~500 nm in tight oil limestone, 50~900 nm in tight oil sandstone and 40~700 nm in tight gas sand. In terms of the rapid development of globe petroleum industry and nano-technology, a concept of nano-hydrocarbons is proposed in this paper that indicates that “nano-hydrocarbon” is the development direction of oil and gas industry in the future, urgently requiring developing vicarious technologies, such as nano-hydrocarbon perspective viewing mirror, nano-hydrocarbon displacement agent and nano-hydrocarbon exploitation robots. Petroleum intellectualization times will come in following.
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