The effect of hydrocarbons on quartz cementation: diagenesis in the Upper Jurassic sandstones of the Miller Field, North Sea, revisited Nils Einar Aase and Olav Walderhaug Statoil ASA, ST-FH, N-4035 Stavanger, Norway (e-mail: nea@statoil.com; owald@statoil.com) ABSTRACT: Variable quartz cementation and porosity distribution in the Upper Jurassic Brae Formation deep-water sandstone reservoir of the Miller Field (UK, North Sea) is mainly controlled by the amount of coatings on the quartz grains. Oil was possibly present in the pore space for the last 40 Ma, but had no significant effect on preserving porosity in the oil leg relative to the water leg. Samples with anomalously high porosities, which commonly occur in the shallowest sandstone intervals, are microquartz coated, and it is a misinterpretation to explain these high porosities as due to hydrocarbons inhibiting quartz cementation. Porosity preservation due to microquartz coating is quite common in Upper Jurassic sandstones in the North Sea. Kinetic modelling of quartz cementation with the Exemplar program correctly predicts observed quartz cement volumes in the Brae Formation outside the microquartz-coated zones, when high quality petrographic data, including abundance of clay coatings, are provided as input. Significant overprediction of quartz cement volumes occurs only for samples where scanning electron microscope analysis shows microquartz coatings to be present. Failure to recognize grain coating as one of the major parameters controlling quartz cementa- tion may lead to incorrect geological models of reservoir quality and selection of inappropriate exploration or production strategies. KEYWORDS: Miller Field, diagenesis, porosity, hydrocarbon effect, microquartz coating INTRODUCTION Several workers claim to have observed sandstone reservoirs where quartz cement is less abundant in the gas or oil zone than the water zone due to hydrocarbons having inhibited quartz cementation (Füchtbauer 1961; Prozorovich 1970; Hawkins 1978; Maher 1981; Dixon et al. 1989). In the last decade, Saigal et al. (1992), Gluyas et al. (1993), Burley (1993), Worden et al. (1998), Marchand et al. (2000, 2001, 2002), Haszeldine et al. (2003), Wilkinson et al. (2004) and Carr & Petersen (2004), amongst others, support the idea that diagenetic reactions in sandstones can be slowed or stopped by emplacement of hydrocarbons and expulsion of pore water. On the other hand, several workers document little or no difference in the reservoir quality between oil and water zones (Ehrenberg 1990; Giles et al. 1992; Ramm & Bjørlykke 1994; Bjørkum & Nadeau 1998). Fluid inclusion studies (Walderhaug 1990, 1994; Midtbø et al. 2000) indicate that quartz cementation has continued up to the present day in many hydrocarbon-filled sandstones. The fact that quartz grains are normally water-wet also suggests that dissolution, transport (diffusion) and precipitation of silica will take place, even if the pore space is mainly filled by hydro- carbons (Lowry 1956; Rittenhouse 1971; Bjørkum et al. 1998). Also experimentally, high oil saturation has been shown not to inhibit the growth of quartz cement under high pressure/temperature conditions (Teinturier & Pironon 2004). Very little quantitative evidence has been presented in favour of hydrocarbons preserving reservoir quality. Furthermore, facies changes may occur over the same interval as the hydrocarbon–water contact, thus obscuring the effects of hydrocarbons. Obtaining statistically meaningful data from similar lithologies in both the hydrocarbon and water legs is a common problem. However, Marchand et al. (2000, 2001, 2002), claim to have ‘clearly demonstrated qualitatively and quantitatively that quartz cementation is greatly retarded by the presence of oil in the pore space’ in upper Jurassic sandstones from the Miller Field in the North Sea, despite oil- and water-zone samples being ‘texturally and mineralogically similar’. If the conclusions of Marchand et al. (2000, 2001, 2002) are correct, then this is of major importance to the petroleum industry. In the current authors’ opinion, there are no previous studies that convinc- ingly demonstrate a large reduction of quartz cementation due to the presence of hydrocarbons and, therefore, a petro- graphic re-examination of the Miller Field sandstones studied by Marchand et al. (2000, 2001, 2002) has been undertaken, with emphasis on the abundance of quartz cement and corresponding porosity loss. SAMPLE MATERIAL The sandstones included in this study are from the Upper Jurassic Brae Formation of the Miller oil field in the South Viking Graben, North Sea. A total of 30 samples of core chips from the cored intervals in three wells were selected (Table 1). The oil–water contact (OWC) in the Miller Field is at 4090 m true vertical depth (TVD) and the samples were taken from both above (20) and below (10) the OWC. No core plugs for Petroleum Geoscience, Vol. 11 2005, pp. 215–223 1354-0793/05/$15.00  2005 EAGE/Geological Society of London