Illite dates record deep fluid movements in petroleum basins

Abstract

K-Ar age dates of authigenic illite from sandstones in the UK south Central Graben have a bimodal distribution. In contrast to established hypotheses of thermal triggers, this illite growth is explained by changes in hydrogeological history. Fluid motion during burial can hence be dated. Illite growth at 84-58 Ma on the graben margins was coincident with rapid subsidence of the Graben axis and consequent expulsion of pore fluids onto the margins. This event pre-dated both overpressure, and the secondary migration and accumulation of hydrocarbon in the region. Illite growth was probably caused by increased solute transport rates during pore fluid motion. Illite growth on an axial high (33-30 Ma) occurred during overpressured conditions. This was triggered either by: (1) downward fluid migration as a consequence of overpressure release. These fluids carried carboxylic acids or hydrocarbons from the highly overpressured Kimmeridge Clay Formation into the less pressured Fulmar Sandstone Formation; or (2) a decrease in water-rock ratios as fluid flow declined within the sandstones, as a consequence of overpressure build-up. Published experiments show that illite growth is favoured by low water-rock ratios.

Introduction: Approaches to dating and duration of fluid flow and fluid-rock interaction

“…The precipitation of illite in sandstones has been related to either porewater geochemistry (Liewig et al 1987) or to high rates of fluid flow (Hamilton et al 1992). Recent evidence from the North Sea led Darby et al (1997) to infer that the illite represents changes in hydrogeological history, which in some cases increased solute transport rates (i.e. supply of K) and in others decreased the water-rock ratio when overpressuring inhibited pore fluid flow rates.…”

“…In one sense, we may simply be considering how long a process continues to occur, such as up-dip fluid expulsion and associated diagenetic effects at the margin of a sedimentary basin. Estimates of the duration of diagenetic processes could come from the spread of values of, for example, K-Ar ages of illite (Scotchman et al 1989;Darby et al 1997) or fluid inclusion temperatures in cements converted to age of burial (Robinson & Gluyas 1992). Similarly, spread of data can be significant in palaeomagnetic analysis.…”

Oil charge preserves exceptional porosity in deeply buried, overpressured, sandstones: Central North Sea, UK

“…A proviso for the preservation of primary porosity is that the overpressure would have had to have formed and been maintained continuously during burial beyond the effective depth. The origin of the overpressure is not important; either disequilibrium compaction within the reservoir or lateral transfer from deeper buried sections as suggested by Darby et al (1997) would be predicted to have the same effects. The timing of the Fig.…”

“…Important diagenetic cements are early dolomite and grain-rimming micro-quartz, and later quartz overgrowths, illite and ankerite (Fig. 2;Saigal et al 1992;Wilkinson et al 1994Wilkinson et al , 1997Aase et al 1996;Wilkinson & Haszeldine 1996;Darby et al 1997;Osborne & Swarbrick 1999;Hendry et al 2000a,b). The Fulmar Formation is at present highly overpressured in all the deeper reservoirs (e.g.…”

Fibrous illite in oilfield sandstones – a nucleation kinetic theory of growth

“…Despite an extensive literature, the fundamental controls upon fibrous illite growth have not been determined, and the significance of the K–Ar ages remains open to debate. Attempts at understanding the significance of illite age dates have relied upon the calculated conditions of illite stability (Bjørlykke and Aagaard, 1992; Berger et al ., 1995) and by a consideration of processes likely to control the rate of growth such as: porefluid flow velocity (Darby et al ., 1997; Swarbrick, 1994); solute supply (Hamilton et al ., 1992) and temperature or reaction kinetics (Ehrenberg and Nadeau, 1989; Berger et al ., 1995; Lanson et al ., 1996). Although progress has been achieved using these approaches, they do have limitations.…”