Use of Variations in Strontium Isotope Ratios for Mapping Barriers: An Example from the Troll Field, Norwegian Continental Shelf

GIBBONS, K., Statoil, Stavanger, Norway

Critical to production of the 12- to 28-m-thick oil column from horizontal wells in the western Troll field is an evaluation of the lateral extent of calcite cemented horizons. Calcite cements, representing 10% of the 200-m main reservoir interval, vary from nodules and concretions to stratiform layers 0.5 to 5 m thick.

By mapping the variation in (87)Sr/(86)Sr isotopic compositions of the pore fluids trapped within sandstones, isolated fluid regimes can be identified. Significant differences in (87)Sr/(86)Sr across cemented horizons are used to delineate laterally extensive horizons. In addition, (87)Sr/(86)Sr analyses of calcite cements are used to trace the evolution of pore water compositions since deposition.(87)Sr/(86)Sr isotopic ratios were determined for both calcite cements and pore water leaches from sandstones. A total of 200 samples from ten wells were analyzed. Statistical analysis of the data yielded the following results: (1) Carbonate cements trace the (87)Sr/(86)Sr evolution of pore waters from 0.709, seen in the earliest cements, to 0.711, observed in the latest cements; (2) observe variations in (87)Sr/(86)Sr ratios are independent of primary sandstone composition confirming that differences in (87)Sr/(86)Sr ratios are related to isolation of pore waters; (3) (87)Sr/(86)Sr ratios of both cements and pore water leaches reflect the progressive infilling of hydrocarbons from west to east across the field; (4) significant differences in (87)Sr/(86)Sr ratios are seen at cemented horizons associated with maximum flooding surfaces and sequence boundaries; and (5) laterally extensive horizons are identified in each of the studied wells, some of which can affect production from horizontal wells.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)