Pore water evolution in oilfield sandstones: constraints from oxygen isotope microanalyses of quartz cement

摘要

Oxygen isotope microanalyses of authigenic quartz, in combination with temperatures of quartz precipitation constrained. by fluid inclusion microthermometry and burial history modelling, are employed to trace the origin and evolution of pore waters in three distinct reservoirs of the Brae Formation in the Miller and Kingfisher Fields (North Sea). Oxygen isotope ratios of quartz cements were measured in situ in nine sandstone thin sections with a Cameca ims-4f ion microprobe. In conjunction with quartz cement paragenesis in the reservoirs, constrained from textural and cathodoluminescence (CL) microscopy studies, pore water evolution was reconstructed from the time of deposition of the sandstones in the Upper Jurassic until the present. CL photomicrographs of quartz overgrowths in the Brae Formation sandstones show three cement zones (A, B and C) which can be related to different oxygen isotope compositions: (1) the earliest, and thinnest, zone A (homogeneous CL pattern with probable delta(18)O values between +23 parts per thousand and +26 parts per thousand -direct measurements were not possible) precipitated in the sandstones at temperatures < 60 degreesC; (2) the second zone B (complex CL pattern and directly measured delta(18)O values between +15 parts per thousand and + 18 parts per thousand) precipitated in the sandstones most likely between 70 and 90 degreesC; (3) the third zone C (homogeneous CL pattern and directly measured delta(18)O values between + 16 parts per thousand and +22 parts per thousand) precipitated in the sandstones most likely at temperatures > 90 degreesC. Calculated oxygen isotope compositions of pore waters show that zone A quartz cements, and enclosing concretionary calcite, precipitated from a meteoric-type fluid ( similar to -7 parts per thousand) during shallow burial (< 1.5 km). Zone B quartz cements precipitated from fluids which evolved in composition from a meteoric-type fluid (delta(18)O -7 parts per thousand) to a more O-18-enriched fluid (delta(18)O - 4 parts per thousand) as burial continued to similar to 3.0 km. Data from zone C quartz cements are consistent with further fluid evolution from 8180 - 4 parts per thousand to basinal-type fluids with delta(18)O similar to the present-day formation water oxygen isotope composition (+0.6parts per thousand at 4.0 km burial). A similar pore water evolution can be derived for all three reservoirs studied, indicating that hydrogeologic evolution was similar across sandstones of the whole Brae Formation. The quartz cement zones observed in the Brae Formation sandstones, and the pore water history derived for the area studied, is analogous to published petrographic and pore water evolution data from the nearby Brent Group reservoirs and from reservoirs located in the Haltenbanken area on the Atlantic margin offshore Norway. Considering quartz cement is a major porosity-occluding phase in many reservoir sandstones, and because pore waters both dissolve quartz and carry the dissolved silica to cementation sites, the data presented are valuable for improving the understanding and prediction of reservoir quality development in sandstones globally. (C) 2002 Elsevier Science B.V All rights reserved. [References: 45]