Temporal and Spatial Variation in Diagenesis and Reservoir Quality: Brent Sandstone, Heather Field, North Sea: ABSTRACT

Abstract

Brent Group sandstones in the Heather field show extreme inter- and intrafacies heterogeneity in reservoir quality as a result of diagenetic variation. Diagenetic patterns varied spatially and temporally as a result of variations in paleofluid chemistry, the time of hydrocarbon accumulation, and detrital grain composition. Important diagenetic cements are poikilotopic calcite, kaolinite, quartz, and illite. Geochemical, petrographic, and structural evidence indicate that calcite precipitated in the Late Jurassic (approximately 150 Ma) at a low temperature (40/degrees/-50/degrees/C), from reducing water of partial meteoric derivation (/delta//sup 18/O water = /minus/4 to /minus/6 /per thousand/ SMOW) that contained highly radiogenic strontium (/sup 87/Sr//sup 86/Sr > 0.71). Calcite distribution was partially controlled by local erosion of the Brent immediately following its deposition. Subsequently, a major period of kaolinite precipitation and feldspar dissolution occurred. Isotopic and tectonic/thermal history data suggest that these events were caused by thorough meteoric flushing (/delta//sup 18/O water = /minus/6 to /minus 8/ /per thousand/ SMOW) during the mid-Cimmerian sea level low (ca. 140 Ma), but not via recharge at the mid-Cimmerian unconformity immediately above the structure. Quartz precipitated as a result of feldspar dissolution, pressure solution, and fluid movement up fault zones over a long period of geologic time. Inmore » the vicinity of major faults, quartz fluid inclusions indicate invasion of hot, saline brines.« less