The term API Gravity is a measure of the relative lightness or heaviness of petroleum liquids when compared with water. Wikipedia offers a good discussion of its usage: http://en.wikipedia.org/wiki/API_gravity (accessed July 7, 2011).
API Gravity has reported for many of the oil fields in Indiana, and where available it is included as a data field in the Oil Field Summary of the Field and Production Summary.
Although the IGS reports an API Gravity value for many oil fields in the PDMS, we are not quite sure how the reported values were determined. We believe the values were obtained from crude oil purchasers. In 1955, the first annual report "Oil development and production in Indiana during 1954" was published. API Gravity was reported for nearly all of the 224 oil fields, sometimes a single API Gravity value was reported for the field, other times a range of values was reported for the field.
Spot-checking a few annual reports up to the final year for which they were reported, 2002 (J. Cazee, 2004), gravity values were still being reported through the 1960s, but stopped being reported in the 1970s.
Cazee, J. T., 2004, Petroleum exploration, development, and production in Indiana during 2002: Indiana Geological Survey Mineral Economic Series 49, 50 p.
Dawson, T. A., and G. L. Carpenter, 1955, Oil development and production in Indiana during 1954: Indiana Geological Survey Mineral Economics Series No. 1, 11 p.