Distribution of Dissolved Hydrocarbons in Subsurface Waters1
An extensive investigation involving some hundreds of drill-stem tests of water-bearing formations in the Gulf coastal area of the United States has been in progress for over ten years for the purpose of determining the amount and kind of hydrocarbons dissolved in the interstitial water of subsurface formations in petroliferous regions. A primary objective of this study has been to determine the nature of, and degree of, the possible effects of a local accumulation of oil or gas on the amount of total hydrocarbons or of particular constituents dissolved in the nearby water in the same formation or in overlying formations. A secondary objective has been to assemble basic information that might shed light on fundamental questions related to the processes of accumulation of oil and gas.
In order to obtain samples suitably representative of the formation water and to prevent the possible loss of dissolved gaseous hydrocarbons, special techniques have been used to sample the water under essentially formation pressure and to retain the dissolved hydrocarbons with the water in sealed sampling containers. This paper describes the techniques employed and presents data typical of the results obtained.
Appreciable concentrations of dissolved hydrocarbons have been found in the water of most of the sampled formations throughout extensive regions. In some formations the water has been found to be substantially saturated with dissolved gas at the formation pressure; in others, the amount is less than would correspond to saturation at the formation pressure and temperature.
The dissolved hydrocarbons have been found to consist primarily of methane, with ethane and propane present in much smaller concentration, and heavier hydrocarbons in only minute quantities. In general, the concentration of dissolved hydrocarbons in a particular formation increases with depth and increases basinward up to a certain degree, but both regional and local variations from average trend have been found. Local enrichment in dissolved hydrocarbons has been observed in close proximity to some, but not all, oil fields when water from the producing horizon was sampled at locations beyond the edge of the oil accumulation.
Of possible fundamental significance is the large total quantity of hydrocarbon gas, particularly methane, indicated by this work to be dissolved in the waters of the porous formations underlying many thousands of square miles in the region sampled. Concentrations ranging upward to 14 standard cubic feet of dissolved gas per barrel of water have been found.