The origin and timing of quartz cementation in reservoir sandstones : evidence from in-situ microanalysis of oxygen isotopes

Abstract

High precision oxygen isotope analyses of quartz cements, in combination with quantitative petrography and Quantitative X-Ray Diffraction (QXRD) as well as timetemperature history modelling, were employed to constrain the timing and source of porosity reducing quartz cement in the Jurassic Ness Formation (North Sea) and the Palaeocene Wilcox Group (Texas). Temperature history modelling indicates maximum burial temperatures in the chosen Ness samples to be 109 to 165oC and 27 to 210oC in the Wilcox samples. In both sample areas the volume of quartz cement is controlled by temperature and quartz surface area. The volume of quartz cement recorded ranges from 1.3 to 22.5 %bv in the Ness Formation and from 4.4 to 18.8 %bv in the Wilcox sandstone. With the notable exception of the hottest Wilcox samples (>143oC), cement volume increases with temperature in both basins. In situ Secondary Ion Mass Spectrometry (SIMS) was performed at a spatial resolution of 12 μm and 2 μm on quartz overgrowths. δ18O(cement) measurements ranged from +27.7‰ to +19.3‰ in the Ness and +28.5‰ to +18.3‰ in the Wilcox. Precision for the 12 μm and 2 μm analysis was better than ±0.27‰ (2SD) and ±0.67‰ (2SD) respectively. δ18O(cement) measurements indicate that the initial 8% of cement in both the Ness and Wilcox sandstones precipitated below ~50oC. The remaining 90% of cement observed in both basins precipitated above 60-80oC in diagenetically evolving waters where δ18O becomes heavier during burial. In all cases cementation appears to continue through to maximum burial temperatures, from 60 to 143oC and can be accurately predicted using current cementation models. However, δ18O(cement) measurements in Wilcox sandstones sampled above 143oC indicate that cementation may be inhibited at high temperatures as a response to the late precipitation of diagenetic minerals such as illite and ankerite which act to reduce available quartz surface area. Within the quartz cementation window (50-143oC), silica producing reactions including the illitisation of kaolinite and smectite as well as the dissolution and albitisation of feldspars have been recorded in shale units adjoining both the Ness and the Wilcox sandstones. However, mass balance calculations indicate that insufficient silica is produced within these shale units to account for the total volume of cement quantified in the sandstone. These findings indicate that the majority of quartz precipitates above 60oC and is sourced internally within the sandstone body.