WORDEN, RICHARD H.¹ and STUART A. BARCLAY^2
1Queen's University, Belfast, UK
²Robertson's Research, Llandudno, Gwynedd, UK.

Abstract: Internally-sourced Quartz Cementation Caused By Externally-derived CO₂ In Sub-arkosic Sandstones Of The Northern North Sea, UKCS

The Upper Jurassic sub-arkosic turbidite sandstones of the Magnus oil field, UK North Sea, are dominated by coeval quartz, dolomite and illite cements that all grew immediately prior to, and during, the early stages of oil-filling. The sandstones have no evidence of siliceous bioclasts or stylolite formation (or pressure dissolution) yet contain up to 15% quartz cement.

Petrographic analysis revealed that the sandstones have undergone major K-feldspar alteration. XRF whole-rock data show that sandstones have inversely correlated potassium with the degree of K-feldspar alteration indicating bulk export of this element. Carbon isotope data from the dolomite require an influx of organically-derived CO₂. Formation waters have unusually high K/Na ratios.

The local Upper Jurassic source rocks are mixed type 2/3 kerogen. Modelling shows that they will have generated very large quantities of CO₂ prior to, and during, petroleum expulsion:

Geochemical modelling of a CO₂-influx into an arkosic sandstone suggests that quartz, dolomite and illite cements are likely to be genetically-related. Reaction of K-feldspar with water and a proton will produce quartz and illite. Low pH was probably buffered through the CO₂-influx causing dolomite precipitation. The reaction leads to bulk loss of potassium (most likely to the surrounding mudrocks).

The ranges of petrographic, isotopic and bulk rock chemical data are fully explained by the integrated geochemical model. Quartz cement is thus most likely derived internally in the sandstone but is the result of an influx of source rock-derived CO₂. While there is no need to invoke large scale-import of silica-laden waters, quartz cementation is the result of fluid influx.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas