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

Abstract

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 {sup 87}Sr/{sup 86}Sr isotopic compositions of the pore fluids trapped within sandstones, isolated fluid regimes can be identified. Significant differences in {sup 87}Sr/{sup 86}Sr across cemented horizons are used to delineate laterally extensive horizons. In addition, {sup 87}Sr/{sup 86}Sr analyses of calcite cements are used to trace the evolution of pore water compositions since deposition. {sup 87}Sr/{sup 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 {sup 87}Sr/{sup 86}Sr evolution of pore waters from 0.709, seen in the earliest cements, to 0.711, observed in the latest cements; (2) observed variations in {sup 87}Sr/{sup 86}Sr ratios are independent of primary sandstone composition confirming that differences in {sup 87}Sr/{sup 86}Sr ratios are related to isolationmore » of pore waters; (3) {sup 87}Sr/{sup 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 {sup 87}Sr/{sup 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.« less