Abstract A sand-packed cell was used to determine the pressure-volume relationship of confined oil and gas by applying both increasing and decreasing pressures. Then the same hydrocarbon fluid was given a routine PV analysis over the same pressure and temperature ranges. The relationship between pressure and hydrocarbon volumes proved to be practically the same for all runs. Equilibrium conditions between phases were very rapidly established within the sand-packed cell. A small, closed reservoir was produced until free gas developed in the oil leg; then the reservoir was gas-injected to restore a third of the pressure decline. By volumetric balance considerations it was indicated that as the pressure was raised the oil dissolved gas; and within experimental error, the gas maintained the oil saturated. It was concluded that for both the laboratory and reservoir tests, the dispersed gas in the oil maintained the oil saturated during pressure increase. Introduction In Eastern Venezuela, the question of resaturation of reservoir oil in the oil leg through pressure build-up has often arisen. Should oil leg resaturation take place, increased effective permeability will develop with increasing pressure. Decreased viscosity and increased oil formation volume factor from increased solution of gas in oil, would lead to reduced residual oil. These benefits would be reflected in increased ultimate recovery and improved well productivity. However, they could be limited, e.g., active water or solution gas drives. In the case of water drive, loss by backflow of oil into the water leg might result from pressure build-up. With an active solution gas drive that has led to a relatively high free gas saturation, to attain equal reservoir volumes of residual oil through gas injection, higher abandonment gas:oil ratios are necessary at higher pressures. The cost of handling greater volumes of gas could outweigh the advantages of an increased formation volume factor associated with a higher pressure. There are a number of conflicting articles (pro and con) in the literature concerning resaturation.
The presence of oil in certain geologic traps and its absence in many others which at first appear favorable emphasizes the need of exhaustive studies of the controlling factors involved in the accumulation of oil. Comparison of productive and non-productive traps in areas where source beds appear alike leads toward certain deductions concerning migration of oil. These deductions must be checked against various observations and experimental evidence. Although contrary to some theories that have been widely held, it seems necessary to conclude that oil must experience a comparatively early origin and migration. Also that increasing overburden and compaction seem to be mainly responsible for migration of the oil into the reservoir. The lateral variation in thickness of over urden seems to be the main source of forces which cause and give direction to the movement of fluids within the reservoir. Provided conditions remain favorable there seems to be no need to proscribe limits as to distance of migration. End_of_Article - Last_Page 1514------------
The writers advocate the classification of the American Permian as a system composed of four series, named, from oldest to youngest, the Wolfcamp series, the Leonard series, the Guadalupe series, and the Ochoa series.
This paper is intended as a commentary on past and present classifications and correlations of the Pennsylvanian rocks of north-central Texas, with suggestions regarding solution of some of the regional problems involved. It is apparent that the problems are too complex and numerous to expect early settlement. The following major premises and conclusions are presented: 1. The Pennsylvanian rocks of north-central Texas may now be correlated with considerable certainty with those of other regions. 2. A time-stratigraphic classification in agreement with a standard time-stratigraphic classification for the system would have important advantages. 3. The principal standard section of reference should be established in the northern Appalachian region, with time-stratigraphic divisions based, in part, on data available from other regions, particularly the Mid-Continent. 4. The standard and regional time-stratigraphic divisions should be based on broad studies of structural and lithologic data, as well as on presently available paleontological evidence, in the hope of establishing widely recognizable, distinctive, and equivalent time-stratigraphic divisions which would require a minimum revision of boundaries heretofore used in the various major Pennsylvanian provinces. 5. Division of the Pennsylvanian system into six series appears appropriate, the major Pottsville and Pittsburgh divisions of the northern Appalachian region each being divided into three series, known, respectively, in north-central Texas as the Springer, Morrow, and Lampasas and the Strawn, Canyon, and Cisco. 6. The highest position used for the boundary between these lower and upper major divisions in Pennsylvania appears to agree closely with the usage of the last three decades in north-central Texas, where the boundary between the "subsurface Bend" and the Strawn has been placed above the "Caddo" limestone of oil-field terminology. 7. The top of the Caddo limestone occurs at a variable position in this region. The upper boundary of the youngest Caddo limestone has been used herein as a boundary between the Lampasas and the Strawn series. It is thought to be approximately equivalent to the Dornick Hills-Deese boundary of the Ardmore basin, the Boggy-Thurman boundary of eastern Oklahoma, the Tradewater-Carbondale boundary of the Illinois Geological Survey Bull. 67 (1942), the long-established Kanawha-Allegheny boundary of West Virginia, and the boundary recommended in Pennsylvania on the basis of recent studies. A somewhat lower boundary may ultimately prove more useful.
GENERAL STATEMENT The numerous problems involved in the scientific classification and nomenclature of rock units have long engaged the attention of geologists. The appended partial list of papers dealing with this subject sufficiently indicates its scope and importance. Gradually, certain guiding principles have come to be recognized generally, and practices deemed good in the classification and nomenclature of rock units have become fairly well established. There is much room, however, for development of the principles and especially for the more widespread application of what seem to be the best practices in using the principles. The larger geological surveys have, necessarily, given consideration to the formulation of rules to provide for their own publications a comparative uniformity in the method of classifying and naming the rock units treated by them. But such rules have not been propounded and widely disseminated with intent to promote their use by geologists in general. . . .
