Conditions and timing of quartz cementation in Brent reservoirs, Hild Field, North Sea: constraints from fluid inclusions and SIMS oxygen isotope microanalysis

The conditions and timing of quartz cementation in the Brent sandstones in the Hild Field, Norwegian North Sea, were constrained on the basis of oxygen isotope microanalysis and fluid inclusion microthermometry combined with optical and cathodoluminescence (CL) microscopy studies. Samples of the Tarbert and the Ness Formations were investigated in three different wells, presently lying at 3.8-4.3 km subsea and at 140-155 degreesC, Quartz cement is present in all wells. Abundance varies from 3% to 26%. and tends to increase with depth of burial. Two distinct, separate episodes of quartz cementation, referred to as Q1 and Q2, are readily distinguished by CL microscopy. The two episodes are also recognized in the bimodal distribution of fluid inclusion temperatures and oxygen isotope microanalyses. The data indicate that Q1 quartz formed in the Early Tertiary (65-35 Ma) at 95-110 degreesC acid 2.3-3.0 km burial depth, and that Q2 mainly precipitated in the Oligocene-Miocene (40-10 Ma) at 125-135 degreesC acid 3.1-3.7 km. Q1 quartz precipitation follows the onset of overpressuring in the reservoir, while precipitation of Q2 seems to slightly precede and accompany the main phase of oil filling (35-10 Ma). Secondary ion mass spectrometry (SIMS) oxygen isotope microanalyses yield delta O-18 delta O-18 values ranging from 17 parts per thousand to 25 parts per thousand, averaging 23 +/- 2 parts per thousand for Q1 and 19.5 +/- 1.5 parts per thousand for Q2. Calculated delta O-18 values of quartz-forming waters are similar for Q1 and Q2, ranging from 1 parts per thousand to 3 parts per thousand, and are similar to the delta O-18 value (approximate to 2 parts per thousand) of present-day formation water in the reservoir. Quartz cementation in the Brent reservoir of the Hild Field occurred during deep burial, at high temperature, from evolved O-18-rich basinal water most likely derived from the Viking Graben. Two episodes of enhanced quartz precipitation are recorded around 100 degreesC and 130 degreesC. No evidence was found for any significant contribution of O-18-depleted Jurassic meteoric water and/or low temperature precipitation of quartz cement. (C) 2001 Elsevier Science B.V. All rights reserved.