Katalin Juhász-Bodnár¹, Calum I. Macaulay¹, Karl Ramseyer², Anthony E. Fallick³, R. Stuart Haszeldine¹, Colin M. Graham¹

(1) University of Edinburgh, Edinburgh, Scotland
(2) Universität Bern, Bern, Switzerland
(3) Scottish Universities Research and Reactor Centre, East Kilbride, Scotland

ABSTRACT: Poikilotopic Quartz Cementation during Basin Inversion, Rotliegend Leman Gas Reservoir, Southern North Sea

Quartz cement is unevenly distributed in the Rotliegend aeolian sandstones of the Leman gas field. Most authigenic quartz post-dates pressure solution, has an unusual poikilotopic morphology, and shows complex zonation under cathodoluminescence. Quartz cement is abundant (0-20%) in "clean" foreset layers and is sourced locally by pressure solution from bottomset and "clay-rich" foreset layers. Quantitative petrography reveals that quartz cement mass-balances with pressure solution on a reservoir scale. Silicon isotopic compositions (d30Si) of detrital and authigenic quartz are indistinguishable, supporting a silica source from grain-to-grain pressure dissolution.

Primary fluid-inclusions show quartz cement precipitated from a NaCl±CaCl2 fluid, with decreasing total salinity (26 to 17wt%) during cooling (111 to <50°C). The hotter and more saline inclusions contain a more CaCl2-rich brine, similar in composition to pore-fluid in the overlying present-day Zechstein. In contrast, the less saline and cooler inclusions contain a more NaCl-rich brine, resembling present-day pore-fluids of the Rotliegend and underlying Carboniferous. The more dilute pore-fluids might be due to meteoric influence as the conventional and SIMS oxygen isotope measurements of quartz cement (d18O~20o/oo) suggest precipitation from an isotopically light pore-fluid (d18O ~-3o/oo) at elevated temperature (~85°C).

The combination of: (i) conventional and SIMS d18O; (ii) fluid inclusion microthermometry; (iii) burial and temperature history modelling; all indicate that the poikilotopic quartz, of uniform d18O, precipitated during uplift and cooling, following basin inversion at ~60Ma, with progressive mixing of different pore-fluids. Gas emplacement post-dated the retrograde poikilotopic quartz. An analogous model may also apply to the nearby V-fields.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado