Abstract
Tight oil has become the focus in exploration and development of unconventional oil in the world, especially in North America and China. In North America, there has been intensive exploration for tight oil in marine. In China, commercial exploration for tight oil in continental sediments is now steadily underway. With the discovery of China’s first tight oil field—Xin’anbian Oilfield in the Ordos Basin, tight oil has been integrated officially into the category for reserves evaluation. Geologically, tight oil is characterized by distribution in depressions and slopes of basins, extensive, mature, and high-quality source rocks, large-scale reservoir space with micro- and nanopore throat systems, source rocks and reservoirs in close contact and with continuous distribution, and local “sweet area.” The evaluation of the distribution of tight oil “sweet area” should focus on relationships between “six features.” These are source properties, lithology, physical properties, brittleness, hydrocarbon potential, and stress anisotropy. In North America, tight oil prospects are distributed in lamellar shale or marl, where natural fractures are frequently present, with TOC > 4 %, porosity > 7 %, brittle mineral content > 50 %, oil saturation of 50 %–80 %, API > 35°, and pressure coefficient > 1.30. In China, tight oil prospects are distributed in lamellar shale, tight sandstone, or tight carbonate rocks, with TOC > 2 %, porosity > 8 %, brittle mineral content > 40 %, oil saturation of 60 %–90 %, low crude oil viscosity, or high formation pressure. Continental tight oil is pervasive in China and its preliminary estimated technically recoverable resources are about (20–25) × 108 t.
Highlights
Tight oil refers to the oil preserved in tight sandstone or tight carbonate rocks with overburden pressure matrix permeability less than or equal to 0.1 9 10-3 lm2
Tight oil is a highlight in global unconventional oil, for which industrial breakthroughs have been achieved in North America
Through case studies based on the tight oil practices in North America and China, major geological features of tight oil are identified and major parameters for the evaluation for ‘‘sweet area’’ are proposed
Summary
Tight oil refers to the oil preserved in tight sandstone or tight carbonate rocks with overburden pressure matrix permeability less than or equal to 0.1 9 10-3 lm (air permeability less than 1 9 10-3 lm). Individual wells generally have no natural productivity or their natural productivity is lower than the lower limit of industrial oil flow, but industrial oil production can be obtained under certain economic conditions and technical measures (Jia et al 2012a, b; Zou et al 2012; Hao et al 2014) Such measures include acid fracturing, multi-stage fracturing, horizontal wells, and multi-lateral wells. Through case studies based on the tight oil practices in North America and China, major geological features of tight oil are identified and major parameters for the evaluation for ‘‘sweet area’’ are proposed. These can provide important reference for continuously promoting the exploration for this important. Sci. (2015) 12:606–617 unconventional oil. ‘‘Sweet area’’ refers to the target area rich in unconventional tight oil which should be developed in priority under current economic and technical conditions
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