Tight oil reservoirs are low-permeability oil accumulations in non-source rock reservoirs that can produce economic flow only through massive fracturing or other special measures. Their absolute permeability is less than 2×10

-3

μm

2

and porosity less than 12%. Tight oil reservoirs are widely accepted as the continuous-type accumulations of unconventional hydrocarbons. Based on our studies on the Ordos Basin in China

we found that oil reservoirs in the lower and middle plays of the Triassic Yanchang Formation on the Yi-Shan slope are mostly tight or near-tight sand oil accumulations. Their oil accumulation patterns are neither the conventional lithologic accumulations nor the continuous unconventional accumulations

instead a transitional type that we refer to as the quasi-continuous accumulation. The quasi-continuous hydrocarbon accumulation can be defined as extensive accumulations in tight reservoirs with neither defined boundaries nor water legs and controlled by unconventional trapping mechanisms. Studies indicate that the quasi-continuous tight sand oil accumulations in the Ordos Basin have the following characteristics:(1)oil distribution is extensive geographically in quasi-continuous form without a defined boundary;(2)the hydrocarbon generation covers vast area and the charging is highly intensive;(3)the reservoirs have unfavorable physical properties and are highly heterogeneous;(4)traps are unconventional that exist in the transition form between conventional shaped traps such as the structural and stratigraphic traps and the shapeless trap or no trap at all;(5)the oil-water contact is unclear and no obvious edge/bottom water present;(6)the formation pressure system is complex and underpressure is predominant;(7)the migration and accumulation of hydrocarbons are short-distanced and driven principally by non-buoyant forces;(8)the accumulation and distribution of oil pools are rarely controlled by local structures

but primarily by source rock and reservoir conditions;(9)the preservation conditions are excellent and reservoir variations are unobvious; and(10)the resource potential is significant. The quasi-continuous accumulation model indicated that the exploration potential of this kind of reservoirs is large

but the traditional thinking of geological studies and exploration must be adjusted accordingly to fully tap their potentials.