DIAGENETIC INFLUENCES ON RESERVOIR PROPERTIES OF THE SHERWOOD SANDSTONE (TR IASS IC) IN THE MARCHWOOD

show that the formation is made up of two units of differing primary lithological character: a lower unit with lithic sandstone and conglomerate, and an upper unit with arkosic sandstone. A combination of compaction and calcite cementation (early and late) has severely reduced porosity and permeability in the lower unit. In the upper unit compaction is again important, but several beds have largely escaped cementation, and their primary porosity has been enhanced by leaching of feldspars. The secondary porosity reaches 7 % in some samples. Oxygen and carbon isotope data for the early (calcrete) cements indicate isotopic equilibrium with typical freshwater compositions. The later cements yield more variable values, indicating precipitation under a wider ange of conditions. The Marchwood Borehole, near Southampton (Fig. 1), was drilled to investigate the geothermal potential of the Triassic sandstones (Sherwood Sandstone) of the Wessex Basin although an assessment of diagenetic influences on their reservoir properties is the chief concern of this paper. The cores, whose recovery was such that 62 % of the Sherwood Sandstone sequence was available for study, were subsampled at approximately 0.5-m intervals for determination of their reservq!r properties, and samples for petrographica