Geological characteristics and sweet area evaluation for tight oil

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 > 35A degrees, 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) x 10(8) t.