Publications

Compositional analyses of reservoir fluids from oil and gas accumulations in North Sumatra have been compiled to describe and predict the occurrence of C02. Carbon isotope compositions of C02 demonstrate that it is primarily derived from inorganic sources. A set of simple empirical models was developed which utilize reservoir lithology, temperature and pressure to calculate upper and lower limits for C02 concentrations.The initial estimation in all of the models is based on a least squares linear regression of C02 partial pressure and reservoir temperature derived from a study of sandstone reservoirs in the Gulf of Mexico (Smith and Ehrenberg, 1989). This correlation provides an accurate' prediction of C02 concentrations, +I- 15'36, for 90% of the clastic reservoirs examined in North Sumatra. C02 concentrations in carbonate reservoirs show a much wider positive variation from the regression values. The variation between calculated and observed C02 concentrations shows a systematic relationship to reservoir temperature. Reservoirs at temperatures less than 280' F have observed C02 concentrations 15% to 30% higher than the calculated values. At temperatures in excess of 280", F, the range of observed C02 concentrations expands to 0% to 30% higher than the calculated values. These concentration limits contain 85% of the observed C02 values from carbonate reservoirs in North Sumatra.The principal mechanism controlling C02 concentrations in clastic reservoirs is the interaction of silicate mineral transformations and carbonate mineral dissolution. C02 concentrations in gas and oil accumulations reflect local equilibrium between gas, pore water, and mineral phases within the reservoir. In . carbonate reservoirs the absence of silicate minerals allows C02 to be transported within the carbonate aquifer system with little or no alteration. Carbonate reservoirs which are exposed to significant up dip fluid flow will possess C02 concentrations that are representative of the base or entry point of the regional flow system.