New Constraints from Quartz Cementation Histories with Oxygen Isotope Stratigraphies Derived from High Precision Sims

Harwood, Joseph ¹; Aplin, Andrew ¹; Fialips, Claire ¹; Iliffe, James ²; Kozdon, Reinhard ³; Valley, John ^3
1 School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, United Kingdom.
² BP Exploration and Production, Sunbury-on-Thames, United Kingdom.
³ Department of Geology and Geophysics, University of Wisconsin, Madison, WI.

Quartz cement is the most abundant diagenetic mineral in reservoir sands of the Northern North Sea and in many other basins worldwide. Constraining cementation rates and mechanisms is therefore central to the prediction of reservoir quality. Current cementation models generally predict that the extent of quartz cementation increases as a function of temperature, due to increasing rates of quartz precipitation. Other models use the silica released from the transformation of smectite to illite as the driver for cementation. In principle, these hypotheses are testable in two ways: firstly, one expects to see more quartz cement in hotter sandstones; secondly, since the extent to which oxygen isotopes fractionate from water to mineral varies as a known function of temperature, one would expect to see predictable variations in δ18O values within a quartz overgrowth. Until now, however, it has been impossible to determine an isotopic history across the cement stratigraphy. We have used the analytical capabilities of the Wisc-SIMS CAMECA 1280 ion microprobe, which has a spatial resolution of 10-12µm, to determine a unique, high precision isotope stratigraphy across quartz overgrowths in seven samples from the Ness formation of the Northern North Sea, presently lying at 2.6 - 4.6km subsea (106-156oC). The amount of quartz cement was quantified using BSEM and SEM-CL microscopy. In these samples, cement abundance appears to be controlled primarily by grainsize, clay content and rock fabric. Only a small amount of CL zonation was observed. δ18O of quartz overgrowths from the 7 samples range from +27.7‰ to +19.2‰ (0.38‰ 2SD), becoming in all cases less positive towards the outer edge of the overgrowth. The data are thus consistent with continuing cementation at higher temperatures. Modeled time-temperature histories were then combined with the isotope data to provide unique constraints on quartz cementation histories. The key unknown is the extent to which δ18Owater has changed through the period of cementation history. Nevertheless, the isotope stratigraphies provide significant constraints on quartz cementation histories and will in the future be used to quantify, for example, the impact of oil emplacement on quartz cementation.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009