Principal characteristics and forming conditions for medium-low abundance large scale oil/gas fields in China

In recent years, some of large scale oil or gas fields with medium-low abundance have been discovered in onshore China. They have such characteristics in common as low porosity and permeability, small oil or gas column, low reserves abundance, and large scale. Studies conducted by the authors show that the formations of large-area reservoirs with medium-low abundance have the following favorable conditions: the large scale sandbodies caused by drag flow in the hinterland of large lacustrine basins, alternating broadly with source rocks like “sandwich-type” architecture of reservoir-source rock association, provide fundamental basis of oil/gas accumulation in large scale; the small oil or gas column and normal to low pressure system reduce the restrict requirement on cap-rock quality, which leads to large scale reservoir formation; the strong reservoir heterogeneity with poor connectivity minimizes gas escape energy and ensures wide oil/gas accumulations even in the area with relatively poor geological conditions; the late uplifting and unloading environment makes it possible for source rock desorb and discharge hydrocarbon to form reservoirs with young generation. Studies indicate that there are nice prospects in discovering large scale lithological reservoirs in the synclinal areas of large terrestrial depression basins of China. This concept has guided the oil/gas discoveries in synclinal areas of Ordos, Songliao, and Sichuan Basins, promoted the great increase of oil and gas reserves and shown a good potential for future exploration. Key words: large scale oil-gas field, large-area reservoir-forming, medium-low abundance, lithological reservoir, new theoretical advance, depression basin