Steve received a Ph.D. from the University of California at Los Angeles in 1978. He works on sandstone and carbonate reservoir studies for exploration and production projects.

A B S T R A C T: Replacement of kaolinite by dickite has been observed to occur with increasing depth of burial in sandstones from three different basins on the Norwegian continental shelf. In the Garn Formation (Middle Jurassic) of Haltenbanken, samples from 1.4-2-7 km below the sea floor (I10~ contain kaolinite, whereas deeper than 3.2 km (130~ mainly dickite is present. In the Statfjord Formation (Late Triassic-Early Jurassic) from Gullfaks and Gullfaks Sew Fields, transformation of kaolinite to dickite occurs at ~3.1 km below the sea floor (120~ From the St~ and Nordmela Formations (Lower to Middle Jurassic) to the Troms Area, kaolin polytypes have been identified in only two shallow and two deep samples, but the results are consistent with the transformation depth determined in two other areas studied. These occurrences are significant because they allow the temperature of the kaolinite/dickite transformation to be established with greater confidence than had been possible previously. Also the observation of this transformation in all three areas so far examined indicates that it may be a general and predictable feature of kaolinbearing sandstones worldwide and therefore a potentially reliable paleogeothermometer. In most cases, the kaolinite occurs as relatively large vermicular crystals, whereas dickite forms more euhedral, blockier crystals. This morphological difference, together with the nature of the structural difference in octahedral occupancy between the kaolinite and dickite, suggests that the transformation occurs by dissolution and reprecipitation, rather then in the solid state. KAOLIN POLYTYPESKaolinite and dickite are two polytypes of the kaolinite sub-group of clay minerals (Bailey, 1980a) which also includes the polytype nacrite. The AIPEA nomenclature thus recommended the name "kaolinite" for both the polytype and the sub-group, so that when one says that the clay present in a rock is "kaolinite", it remains unclear whether the polytype present is in fact dickite, kaolinite, both, or has not actually been determined. The alternative sub-group name "kandite" has been specifically disallowed (Bailey, 1980a), so probably the best solution is informal use of the term "kaolin" for the sub-group, as used by

The Asmari Formation has been studied in the subsurface at the Bibi Hakimeh, Marun and Ahwaz oilfields and in an outcrop section from the Khaviz anticline. It consists of approximately 400 m of cyclic platform limestones and dolostones with subordinate intervals of sandstone and shale. The method of Sr-isotope stratigraphy is well suited for dating these strata because of the rapid rate of change of marine 87 Sr/ 86 Sr during Asmari deposition (roughly 32-18 Ma) and the common presence of well-preserved macrofossils.Profiles of age against depth in the four areas show a decrease from higher stratigraphic accumulation rates in the lower Asmari to lower rates in the middle to upper part of the formation. There is also a trend towards less open-marine depositional conditions and increasing early dolomitization and anhydrite abundance above the lower part of the formation. These changes reflect the dynamics of platform progradation across the areas studied, from early deposition along relatively high accommodation margin to slope settings to later conditions of lower accommodation on the shelf top.Ages of sequence boundaries are estimated from the age-depth profiles at each locality, providing a framework for stratigraphic correlation. Asmari deposition began in early Rupelian time in the Bibi Hakimeh area, when the studied areas to the NW were accumulating basinal marl facies. Progradation of the platform across the Marun and Ahwaz areas took place in mid-Chattian time and somewhat later in the more basinward Khaviz area. Depositional sequences have durations of 1-3 Ma, whereas component cycles represent average time intervals of 100-300 Ky.Sr analyses of most dolomite, anhydrite and celestite samples plot close to or below the macrofossil age-depth trend for each locality, indicating formation from waters preserving seawater 87 Sr/ 86 Sr approximately contemporaneous with or slightly younger than the time of sediment deposition. Local deviations from this trend are interpreted as indicating episodes of seepagereflux and also a contribution of Sr from non-marine sources during formation of the Gachsaran cap rock anhydrite.

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: Subarkosic arenites of the Middle Jurassic Garn Formation are important hydrocarbon reservoirs in the Haltenbanken area. XRD and petrographic analyses of core samples from 11 wells show that a transition from incipient to extensive illitization in these sandstones is associated with a present burial depth of 3.7 km below the sea floor, corresponding to a formation temperature of 140~ Illite has formed by reaction between K-feldspar and earlier diagenetic kaolinite and probably also by alteration of mica. Although several of the cores are filled with hydrocarbons that probably were in place before base-Eocene (57 Ma), there are no indications that illitization was inhibited by the presence of hydrocarbons in pore spaces. It is therefore suggested that 20-30% residual water saturation is sufficient for extensive illitization to occur by short-range diffusive transport in a compositionally closed system. There is an unresolved problem regarding both the timing and cause of illitization. On the one hand, the extent of illitization in the 11 wells correlates with variations in both present formation temperature and thermal maturity, implying that the present burial depth of 3.7 km below the seafloor coincides with a critical thermal threshold for illitization. According to this interpretation, the times of illitization in the different wells should correlate with variations in burial history and should in several cases be as young as 3 Ma, when deposition of one kilometer of glacially-derived sediment sharply increased temperatures throughout the Haltenbanken area. On the other hand, conventional K-Ar analyses of iUite separates give dates ranging from 31-55 Ma. In general, these dates appear to be too old to fit the interpretation that the degree of illitization is a simple function of present temperature or thermal maturity. This inconsistency may reflect errors in the K-Ar dates due to a contamination problem.Formation of diagenetic illite is one of several processes causing severe degradation of reservoir quality in deeply buried sandstone reservoirs worldwide. This study was undertaken in order to characterize the factors controlling illitization in one such reservoir unit, as part of a program to develop a general predictive model for sandstone reservoir quality. The scope of this paper includes description of the petrologic features related to formation of diagenetic illite and discussion of the timing and cause of illitization.The unit we have studied is the Middle Jurassic Garn Formation (known as the Upper Tomma Formation previous to 1988), a regressive association of near-shore marine sandstones (foreshore and upper shoreface zones), possibly also including braided stream deposits. In order to account for the wide area covered by the Garn Formation as a blanketlike sand deposit, Gjelberg et al. (1987) proposed a model of 'back-stepping progradational * Present address: Acadia Resources,