Some times, there is a significant difference in oil properties, such as API and PVT, in the same reservoir of a field, what could be the reason (s) behind that?
Well,
Multi oil bearing horizons, structural and stratigraphic settings, multi gas drive and type of migrations are the reasons of the difference in oil properties.
Sincerely
I would distinguish migration and quasi-static reasons for variations of oil properties in a reservoir.
Different migration "pulses" (batches of HC of different nature) settling in different part of the reservoir due to reservoir heterogeneity or variation of subsurface properties along migration path might determine 3D PVT variations (cf. Bayan answer).
But quasi static situations also exist. In such case, compositional variations are due to thermo gravitational constraints in an equilibrium state. It is typically the case in hyper critical fluid transition in a thick hc column with very light HC at top and heavier HC at bottom (in which the equilibrium is quasi static only, diffusion and convection occuring at all time). A more common aspect of quasi static situation is the widespread RS vs. Depth variation.
Lookup for Al Shaheen field in Qatar for an example of fairly strong variations of composition in a 3D multi-scale sense.
That can be attributed to the oil alteration during migration and/or in the reservoir Swapan Kumar Bhattacharya can have much more detail about this.
One of the reasons maybe as a result of different inputs/sources contributing to the generation of the reservoir fluids. Also, the variation in the fluids 's properties might be as a result of biodegradation, and diamondoid analysis can indicate the level of degradation of the reservoir fluids.
Abdulkareem is right in listing biodegradation post migration (and not necessary in equilibrium) as a potential source of compositional variations. This biodegradation can be human-induced (anthropogenic) or not.
In addition to other contributions, the different fluid properties in a reservoir is as a result of heterogeneous behavior, mass and momentum transfers of hydrocarbons. The reservoir properties like soil porosity and other retardation factors affect fluid properties of the reservoir. Designers of equipment for production operations consider all these parameters before effective designs could be achieved.
It could be Anisotropy. My interest in Rock Physics increased Geometrically when I started observing that fluids with Same porosity behave differently with respect to some other reservoir properties. In most cases these properties that varied even depended on the porosity to certain extents.
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