A series of reservoirs are examined to assess the state of their contained fluids in particular with respect to the extent of thermodynamic equilibrium in the reservoir. In addition, this paper resolves the long-standing puzzle as to why tar/bitumen deposition is at the crest of the field in some reservoirs and at the oil-water contact in other reservoirs. Substantial systematic fluid variations are found utilizing Downhole Fluid Analysis (DFA) as the enabling technology. This study employs the cubic equation of state (EoS) for gas-liquid analysis and the Flory-Huggins-Zuo EoS and the Yen-Mullins model of asphaltenes for analysis of dissolved solid - solution equilibria of (live) reservoir crude oils. ‘Young’ fluid systems exhibit huge, non-monotonic variations of fluids (and solids), while moderately aged fluid systems exhibit monotonic yet grossly disequilibrium properties and the ‘aged’ reservoirs are fully equilibrated even when in massive scale. Nevertheless, these old reservoirs retain significant fluid and organic solid variations as a result of sequential fluid-related processes in geologic time.

The dynamical behaviors of fluids within reservoirs to account for these variations are obtained by linking a simple understanding of petroleum, especially its asphaltene nanocolloidal and thermodynamic properties, with simple concepts from fluid mechanics. In particular, the location of tar deposition in reservoirs is clarified when formed due to asphaltene instability upon a secondary reservoir fluid charge. Tar deposition can be formed upstructure for rapid gas charge as is regularly seen in young reservoirs, or can be formed at the oil-water contact for a slower gas charge as seen in many older reservoirs. Most importantly, the state of the reservoir fluids within the context of geologic time is shown to be tightly coupled to key reservoir concerns for production. Thus, the understanding of the geologic context of the reservoir can be utilized to optimize reservoir evaluation such as for wireline formation testers. The expanding capabilities of DFA coupled with formidable advances in petroleum science have revealed dramatic systematic variations of reservoir fluids which can be understood in simple ways. These new methods are becoming indispensable for optimization of production.