Stratigraphic Analysis of Source-Bed Occurrences and Reservoir Oil Gravities

A knowledge of hydrocarbon source-bed occurrences and lithologic types is fundamental to an understanding of the distribution of oil accumulations. A study of the relations between source-bed lithology and the crude oil in producing reservoirs in the southeastern United States and West Texas was undertaken to provide geologic data on factors affecting oil gravities and the time and direction of migration. This study shows that distinct oil-gravity characteristics can be correlated with source-bed lithologic types, and that the depth of burial which is required to convert oil to gas condensate also influences the oil-gravity characteristics. In addition, the study shows that an approximate overburden of 2,000 feet is needed for flush migration from the source bed. Statistical analyses, theory, and major worldwide oil-field examples verify the dominant role of shale, marl, and clay as source beds for the underlying reservoirs. In contrast, shale-free carbonate sequences contain oil accumulations whose source is an underlying shaly, fine-textured, or phosphatic limestone. Crude oil in the Ellenburger of West Texas originated in shale source beds in the Simpson, Woodford, and Lower Pennsylvanian, each of which exhibits a distinctive curve showing the variations between oil gravity and depth. Other primary source-bed sequences in the Permian basin are Pennsylvanian dark gray shale, Wolfcampian and Leonardian basinal terrigenous clastic sediments (dark shale and siltstone), and Guadalupian basinal and lagoonal terrigenous elastics.