Migration of Reservoir Fluids
Authors: Gussow William C. 
Published: December 2007

Abstract
The application of reservoir and production engineering experience to problems of migration and accumulation of oil and gas, is presented. The principle of differential entrapment is reviewed and updated, and geological evidence for selective trapping of oil and gas is illustrated. This theory, first published in 1953, explains why many good traps are dry while adjacent structures are prolific oil fields. It explains why some traps are gas fields and contain no oil, and why gas is trapped downdip in some areas while synclinal oil occurs in others. The law of gravity explains the gravity distribution of gas, oil and water in a reservoir. However, differential entrapment explains why some oil reservoirs are stratified, and why some oil accumulations are light-gravity condensates while others are medium or heavy crudes.
Under superimposed hydrodynamic conditions, oil and gas accumulations are modified in accordance with the hydrodynamic gradient, causing tilted interfaces, spilling and remigration. When folding and faulting are super-imposed on a basin with oil and gas accumulations, the oil and gas usually remigrate into the new structures in accordance with the principle of differential entrapment, and these new accumulations conform to the existing hydrology. How undersaturated oils are formed, why crudes have different GOR'S, and the origin and significance of saturation pressures and how these are used for determining the time of accumulation are discussed.

Introduction
Migration is fully discussed in the geological literature and is reviewed and updated here for the benefit of the petroleum engineer. Petroleum engineers are well aware that each oil and gas field has special characteristics all its own. This paper discusses the reasons for these differences and how they came about. The oil and gas in an accumulation concentrated from somewhere and have left little trace of their source.
Few petroleum geologists have an engineering background and yet this is becoming more and more essential for a successful exploration effort. The geologist and geophysicist are learning to work together and they realize the need for teamwork in applying their specialties. A similar and much greater effort of cooperation is needed between the reservoir engineer and the exploration geologist to find the remaining undiscovered oil and gas reserves. The engineer should carry his share of the burden and help the exploration geologist find more oil. This mutual understanding and cooperative effort is even more necessary since many engineers are now in management positions charged with making important exploration decisions.
The general application of the structural theory, together with migration, now is accepted by virtually all petroleum geologists. The so-called anomalous occurrences of oil and gas in interconnected traps long baffled the petroleum industry throughout the world. Why are some apparently good structures gas fields rather than oil fields, while others are dry? This is explained by the principle of differential entrapment. This simple principle explains why gas fields may occur in a downdip position and produce little or no oil, while structures farther undip produce oil with little or no gas and others still farther updip are dry. In reality, this carries the structural theory to its logical conclusion.
Following publication of a paper in 1954, two earlier attempts to explain these apparently anomalous occurrences were found, in spite of the fact that 4 man-years had been spent searching the literature without discovering the slightest hint. The work of a Russian geologist came the closest, but it veered off. In 1916 Loyal W. Trumbell, state geologist for Wyoming, set forth ideas almost identical with those developed nearly 35 years later, and yet Trumbell's idea lay dormant except in the minds of a few of his associates and received little attention. Trumbell was certainly on the right track. In 1932 Strong developed an explanation for anomalous occurrences in the Middle East. This was based entirely on the effect of solution gas in the oil. He visualized saturated oil migrating updip without any free gas. With decreasing depth (and pressure), solution gas would be released from the oil. By this line of reasoning, every trap would contain a gas cap at the time of migration. These would be larger and larger basinward since the farther downdip the trap occurred, the more oil would have passed a specific trap. Strong then explained the absence of gas caps in the updip traps by the selective leakage of gas from shallower structures. Pratt wrote that he had been made aware of Strong's idea many years earlier by George M. Lees; and A. I. Levorsen (personal communication) also intimated that he had ". . . bought leases based on the idea many years ago". The effect of solution gas in the oil is believed to be a secondary complication that obscures the underlying principle of differential entrapment.