A renewed look at calcite cement in marine-deltaic sandstones: the Brent Reservoir, Heather Field, northern North Sea, UK

Abstract

Wireline and seismic acoustic impedance imaging show that the marine part of the clastic Brent Group reservoir in the Heather Field, northern North Sea, contains much calcite cement in the flank parts of the structure. The non-marine Ness Formation and crest parts of the structure contain negligible calcite cement. This localized calcite cement has led to relatively poor reservoir performance since first oil in 1978, although a new suite of wells has boosted production with plans to keep the field active until 2030. Understanding the origin and distribution of calcite cement would help the development of more realistic reservoir models and boost production rates through optimum well location. We have thus used a suite of techniques, including standard point counting, SEM-EDS mineralogy, BSE microscopy, fluid inclusion thermometry and stable isotope analysis, to develop new and improved models of calcite distribution. Calcite seems to have attributes of both early and late diagenetic cement. A 30-40% intergranular volume in calcite cemented beds seems to support pre-compactional growth but high-temperature fluid inclusions and the presence of primary oil inclusions suggest late growth. Much calcite may have developed early but it seems to have recrystallized, and possibly undergone redistribution, at close to maximum burial or had a late growth event. Calcite cement probably originated as marine-derived micrite, bioclasts or early marine cement but adopted the isotopic characteristics of high-temperature growth as it recrystallized. Quartz grains have corroded outlines in calcite-cemented areas with one sample, with 79% calcite cement, displaying signs of nearly total replacement of quartz grains by calcite. The flank localization of calcite cement remains to be explained, although it could be due to primary depositional factors, early diagenetic loss of calcite from crestal regions or late diagenetic loss of calcite from crestal regions. Controversially, the growth of calcite seems to be associated with quartz dissolution, although the geochemical and petrophysical cause of this remains obscure. Diagenetic loss of quartz from sandstones cannot easily be explained by conventional modelling approaches and yet seems to be an important phenomenon in Heather sandstones.

Reservoir Quality of Upper Jurassic Corallian Sandstones, Weald Basin, UK

“…Early calcite cement is here interpreted to have formed from neomorphism of bivalve tests, starting soon after deposition and continuing during burial. Petrographic evidence shows bivalve shells with various degrees of neomorphism (Figures 7b, 8a and 11a) to calcite [42,59,60]. Neomorphism of aragonitic bivalve shells is emphasised in Figure 11a where pervasive calcite cement cannot be distinguished from bivalve shells except where micrite envelops outline the boundary of bivalve shells which neomorphosed into calcite.…”

“…The addition Corallian sandstones rich in bioclast-derived calcite presumably were deposited in shallow, limpid ocean waters with a great abundance of biological activity. The addition of a quartz-dominated sand to a site of active bioclast accumulation led to compromised reservoir quality in the subsurface due to the propensity for such bioclast-quartz sand hybrids to become calcite cemented [12,60,72]. Conversely, sandstones rich in matrix clay, presumably in turbid ocean waters, near to the site where rivers fed sediment into the ocean, have far fewer bioclasts and thus much less calcite cement [72].…”

Lower Cretaceous Rodby and Palaeocene Lista Shales: Characterisation and Comparison of Top-Seal Mudstones at Two Planned CCS Sites, Offshore UK

“…The Lista Shales tend to have higher quartz concentrations than most of the Lower Cretaceous shales, as illustrated in Figures 10 and 11. Some of the calcite-rich Rodby samples have negligible clay apart from smectite and have relatively high plagioclase/(plagioclase + K-feldspar) ratios; the calcite-rich layers in the Rodby Shale may have undergone a different diagenetic history than the calcite-poor rocks [44]. Diagrams prepared using R-studio-ggplot2 software [15].…”

Application of analytical techniques to petroleum systems: an introduction

“…In the Brent Group reservoir of the Heather Field, calcite cement has a variable distribution which has impeded reservoir production, with cement being formed early during burial (Glasmann et al 1989a;Glasmann et al 1989b). Worden et al (2019) utilize an array of approaches including oil field data, optical microscopy, cathodoluminescence microscopy, SEM, fluid inclusion analysis and stable isotope analysis to identify both early and late stages of calcite cement growth in the Ness Formation. The formation of calcite after quartz cement, the corrosion of quartz grains by calcite cement, high aqueous fluid inclusion homogenization temperatures and oilfilled inclusions within calcite all indicate the presence of an additional later phase of calcite cement, than was previously identified.…”