Multiple generations of high salinity formation water in the Triassic Sherwood Sandstone: Wytch Farm oilfield, onshore UK

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The origin and heterogeneity of oilfield formation water in the Lower Triassic Sherwood Sandstone at Wytch Farm in the Wessex Basin, UK, have been investigated using production data, detailed water geochemistry and O, S and H stable isotope data. The formation waters are highly saline, NaCl-type brines with TDS values of up to 230,000 mg/L. There is a general decrease in salinity from the flanks of the field to the crest with Cl − decreasing from about 136,000 to 109,000 mg/L. The Cl/Br ratio of the water shows that salinity was largely derived from the dissolution of Upper Triassic continental evaporites found off-structure to the west and north of the field. The water in the field had a meteoric source although variation in δ 2H values suggests that there may be meteoric waters of different ages in the oilfield, reflecting recharge under different palaeoclimatic conditions. At the crest of the field, aqueous SO 4 2 - resulted from dissolution of anhydrite in the reservoir. In contrast, in other parts of the field there is an indication that some of the dissolved SO 4 2 - was derived from oxidation of pyrite at some point on the recharge path of meteoric water to the field. There were two meteoric influx events bringing different Cl − concentrations and different δ 2H values. The first was probably before the Eocene oil influx and could have occurred in the Lower Cretaceous or early Tertiary. The second meteoric influx event probably occurred after or during oil migration into the Wytch Farm structure since the second meteoric water is found at the flanks of the field adjacent to the regions where salt is found in the stratigraphy. The preservation of heterogeneities in oilfield formation water compositions suggests that there has been little aqueous fluid movement or diffusive flux for over 40 million years. Mass flux has been restricted by density stratification within the aquifer and the very low effective permeability for the aqueous phase in the oil-filled reservoir.