Quartz cementation inhibited by crestal oil charge: Miller deep water sandstone, UK North Sea A. M. E. MARCHAND 1,2, *, R. S. HASZELDINE 2 , C. I. MACAULAY 2 , R. SWENNEN 1 AND A. E. FALLICK 3 1 Fysico-Chemische Geologie, K.U. Leuven, Celestijnenlaan 200C, B-3001 Heverlee, Belgium, 2 Department of Geology and Geophysics, The University of Edinburgh, Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK, and 3 Scottish Universities Research and Reactor Centre, Isotope Geosciences Unit, East-Kilbride, G75 0QF, UK (Received 15 June 1998; revised 11 February 1999) A B S T R A C T : In the Miller Field, diagenetic quartz abundance, isotopic compositions and salinities of quartz-cementing fluids display a distinct pattern which is related to the structural depth of the reservoir sandstones. Quartz cement volumes increase from the crest of the field (average 6.0+1.5%) towards the flanks of the field (average 13.2+2.1%) and directly reduce reservoir porosity. By integrating petrographic observations with results of fluid inclusion measurements and O isotope analyses of diagenetic quartz, the pattern of quartz cementation is seen to be related to the reservoir filling history. Oil filled the crest of the reservoir first and prevented extensive quartz cementation. At greater depth in the reservoir oil zone, quartz overgrowths continued to precipitate until inhibited by the developing oil column. Oxygen isotope compositions of diagenetic quartz imply that quartz cement continued to precipitate in the water zone of the reservoir up to the present day. KEYWORDS: quartz cementation, Miller deep water sandstone, North Sea, diagenetic quartz. The Miller Field is located along the fault-bounded western margin of the South Viking Graben in blocks 16/7b and 16/8b. The reservoir sequence comprises Upper Jurassic sandstones of the Brae Formation, which were deposited as a mid-fan channelized lobe. To the west of the Miller Field, the South Brae Field comprises the proximal slope apron channels of the fan system (Turner et al., 1987). The South Brae and Miller fan deposits form part of an extensive syn-rift play in the North Sea. Near the graben edge in the South Brae Field, the Brae Formation sandstones consist of thick units of mud and sand-supported conglomerates and coarse grained sandstones. Further eastwards in the Miller Field, these sandstones prograde into medium- to fine-grained more distal deposits. In the crest of the Miller Field, the Brae Formation occurs at a depth of 3970 m TVDSS, with the oil-water contact (OWC) at 4090 m TVDSS (Garland, 1993). The Kimmeridge Clay Formation, which laterally interfingers with the Brae Formation sandstones and also directly overlies the main reservoir, provides the source for the hydrocarbons (Rooksby, 1991). As is the case in many North Sea clastic reservoirs, quartz cementation is a major cause of porosity loss in the Brae Formation sandstones in the Miller Field. The aim of this paper is to assess the formation conditions of the quartz cement and to discuss the implications of quartz precipitation for the distribution of porosity. In the crest of the field, porosity can be as high as 23% (Fig. 2a). This is higher than the 8% normally expected at depths Clay Minerals (2000) 35, 205±214 * E-mail: ann.marchand@glg.ed.ac.uk # 2000 The Mineralogical Society