Three patterns of hydrocarbon accumulation in large tight oil/gas fields are proposed

namely

continuous accumulation (or basin-centered gas type or deep-basin gas type)

quasi-continuous accumulation

and discontinuous (conventional trap type) accumulation.Like the continuous accumulation

the quasi-continuous accumulation also features in large area

without clear boundaries and water lags

reservoir being adjacent to source rocks and pervasive distribution.In addition

non-buoyancy-driven hydrocarbon migration is dominant.The major driving forcers of migration are overpressure

diffusion and capillary pressure

while buoyancy is limited.The migration is largely in the form of non-Darcy surge flow and diffusion flow.However

the quasi-continuous accumulation is different from the continuous accumulation in many aspects.The quasi-continuous accumulation commonly consists of many adjacent medium to small oil/gas reservoirs and the hydrocarbons are in quasi-continuous distribution.Oil/gas and water contact is complex and no regional updip water present.Hydrocarbon charging is mainly vertical and pervasive expulsion.Hydrocarbon accumulation is characte-rized by direct accumulation via primary migration or short-distance secondary migration.The reservoirs get tight prior to or contemporaneous with the hydrocarbon charging and are highly heterogeneous.The traps have certain controlling effect on hydrocarbon accumulation.The continuous unconventional accumulation represented by the basin-centred gas or deep-basin gas and the typical discontinuous conventional accumulation are representative of two end-member types of tight hydrocarbon accumulations in complicated geological settings.Hence some transitional varieties must exist between them and the quasi-continuous accumulation is just such a transitional type that might be a universal and predominated pattern for large oil/gas accumulation to occur in tight formations.In reality

a typical continuous accumulation ought to be that occurs within a source rock interval like shale gas or coalbed methane

while a typical discontinuous accumulation is that occurs within the conventional reservoirs outside or even far away from source rocks

and it is not only strictly confined by conventional traps but also possesses bottom or edge water.The hydrocarbon accumulations in tight reservoirs out of but close to source rocks are predominantly quasi-continuous accumulation

followed by atypical discontinuous accumulations.In contrast

continuous accumulations such as basin-centered gas or deep-basin gas are

if any

uncommon.