Spatial 87Sr/ 86Sr variations in formation water and calcite from the Ekofisk chalk oil field: implications for reservoir connectivity and fluid composition

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The 87Sr/ 86Sr ratios of formation waters from the chalk of the Ekofisk oilfield were measured using a new technique: extracting salt residues from the interstices of core samples. In the Danian Ekofisk Formation, the chalk matrix itself has 87Sr/ 86Sr and δ 13C values characteristic of Danian seawater, even though δ 18O data indicate the presence of varying quantities of diagenetic carbonate cement. In the Ekofisk Formation, porewater 87Sr/ 86Sr values are very constant (range: 0.70786-0.70810). The formation water probably contains locally derived Sr from carbonate, plus a small contribution from noncarbonate minerals with higher 87Sr/ 86Sr. In the underlying Tor Formation, chalk samples containing a mixture of primary chalk and later cement, have 87Sr/ 86Sr values that rise with depth to a maximum of 0.70865, much higher than contemporaneous seawater. Porewater 87Sr/ 86Sr values increase even more dramatically with depth to a maximum of 0.70988. Both cement and waters have been influenced by Sr with a high 87Sr/ 86Sr probably derived from underlying sedimentary rocks. Comparison with waters brought to the surface together with produced oil indicates that the depthwise increase in 87Sr/ 86Sr reflects an increase in salinity from ∼50,000to∼140,000 mg/l total solute content. This suggests that Zechstein evaporites may be the source of the radiogenic Sr in the Tor Formation. The difference in isotope composition between carbonates and waters in the Ekofisk and Tor Formations indicates that their hydrochemical systems have evolved differently during diagenesis. This is due to the barrier effect of the Ekofisk Tight Zone, a hardground at the base of the Ekofisk Formation which effectively splits the reservoir vertically into two. This study thus provides a new way of determining the positions of permeability barriers in oil reservoirs. Also demonstrated for the first time is the existence of vertical variations in salinity of water in oil. Such variations could have important implications for the calibration of resistivity logs and calculation of the amount of oil in place.