diagenetic influences on reservoir properties of the sherwood sandstone

Clay Minerals (1984) 19, 441-456

DIAGENETIC INFLUENCES ON RESERVOIR P R O P E R T I E S OF T H E S H E R W O O D S A N D S T O N E ( T R I A S S I C ) IN T H E M A R C H W O O D GEOTHERMAL BOREHOLE, SOUTHAMPTON, UK R. W . O ' B .

KNOX,

W. G. BURGESS*, A. H. BATH

K. S. W I L S O N *

AND

British Geological Survey, Keyworth, Nottingham NG12 5GG, and *Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OXIO 8BB (Received 26 August 1983; revised 25 March 1984)

A B ST R A C T: Petrological studies of the Sherwood Sandstone of the Marchwood Borehole 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 range of conditions. The Marchwood Borehole, near Southampton (Fig. 1), was drilled to investigate the geothermal potential o f 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 petrographical analysis were taken wherever possible from the ends of test plugs. In this way it was hoped that an objective account of the factors governing the reservoir properties of the aquifer could be described in as quantitative a manner as possible, while recognizing that there is an inherent weakness in attempting to describe regional effects on the basis o f material from one borehole. LITHOSTRATIGRAPHY A generalized stratigraphy of the Marchwood Borehole is shown in Fig. 2. The target formation, the Sherwood Sandstone, is 59 m thick, its top being ~1660 m below ground level. It is underlain by a thick sequence of indurated and fractured sedimentary strata, probably of Devonian age. These consist of alternating purple-red and grey mudstones, siltstones and sandstones, most o f which are strongly hematitic; they display incipient cleavage and high-angle calcite and anhydrite veins. The Sherwood Sandstone is overlain by nearly 200 m of Mercia Mudstone, which consists largely of anhydrite-bearing, slightly calcareous, argillaceous siltstones (equivalent to the 'Keuper Waterstones') at its base. 9 1984 The Mineralogical Society

442

R. W. O'B. K n o x et al. a9 9

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FI6. 1. Location map for the Marchwood No. 1 Borehole. Contours indicate heat flow in rnW m -2 (after Richardson & Oxbrugh, 1979). Stippled area denotes the Wessex basin.

DEVONIAN

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FIG. 2. Generalized stratigraphy of the Marchwood Borehole.

In the borehole, the Sherwood Sandstone consists of a series of upward-fining cycles, 1-5 m thick. Each cycle commonly has an erosional base, above which a conglomerate (sometimes intraformational) grades upwards through coarse cross-bedded sandstone into laminated siltstone and mudstone, often with concretionary calcite at the top. The concretionary calcite is interpreted as calcrete (or 'cornstone'). The sandstones vary from clean to muddy, many of the former being extensively cemented with calcite; they are reddish-brown to buff coloured. The general character of the sediments is suggestive of deposition by small streams draining an alluvial fan under arid climatic conditions. The Sherwood Sandstone as a whole may be regarded as constituting a single upward-fining sequence, in that it displays an upward decrease in the proportion of

Diagenesis of Sherwood Sandstone, Marchwood B H

443

conglomerate to sand. This superficial appearance is, however, deceptive, because on closer examination the sequence is seen to consist of two distinct units, as distinguished by the following features: (i) a sharp upward increase in downhole gamma-ray values (Fig. 3), (ii) a sharp upward increase in the proportion of feldspar grains, coupled with a decrease in the proportion of lithic grains, and (iii) the restriction of high porosity and permeability to the upper part of the sequence, as revealed by both laboratory measurements and downhole geophysical logs (see below). The boundary between the two units can be precisely identified in the core at 1694 m. On the basis of these observations, the Sherwood Sandstone of the Marchwood Borehole is here subdivided into an upper and a lower unit (Fig. 3). The significance of this subdivision in terms of reservoir properties will be brought out in the following sections.

GAMMA RAY

POROSITY/DENSITY

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cent

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FIG. 3. Stratigraphy and subdivision of the Sherwood Sandstone in the Marchwood Borehole. Stippling denotes sandstones, coarse stippling those with appreciable porosity. Diagonal shading indicates cored intervaJ.

444

R. W. O'B. K n o x et al.

GEOPHYSICAL

LOG INTERPRETATION

All of the geophysical logs clearly illustrate the two-fold division within the Sherwood Sandstone. The more porous and permeable nature of the upper unit is clearly shown by the compensated neutron (n-y), formation density (7-7), and spherically-focused resistivity logs, with the best aquifers being confined to well-defined zones (Fig. 3). The sonic log reflects the same trends, with the lower unit displaying higher and more consistent velocities than the upper. Of particular interest is the gamma-ray log, in which the upper unit shows a substantial positive shift relative to the lower (Fig. 3). Especially prominent are the high-gamma peaks which would normally be associated with argillaceous horizons, but which here correspond to zones of clean sand. The explanation for this anomalous relationship lies in the unusually high K-feldspar content of the sand in the upper unit, the radioactivity associated with the feldspar being considerably greater than that associated with the mudstones. Thus the highest radioactivity is found where the highest concentrations of feldspars occur, i.e. in the clean grainstones rather than in the matrix-packed and cemented argillaceous sandstones. SANDSTONE

PETROLOGY

AND RESERVOIR

PROPERTIES

The contrast in feldspar proportion (Fig. 4) is of particular importance in the recognition of the two-fold division of the sand sequence, although it is accompanied by differences in

FORMATION

DEPTH l [(m)

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FIG. 4. Quartz:feldspar ratios in the cored Sherwood Sandstone and underlying Devonian sandstones.

Diagenesis of Sherwood Sandstone, Marehwood B H

445

the proportion of lithic grains (Fig. 5) and in the amount of mud matrix. Because of the contrast in primary mineralogical and physical characters, the diagenetic histories of the two units show a degree of divergence, and in this way the primary petrological character of the sediment has exerted a strong control on the reservoir properties of the two sandstones. High permeabifity is restricted to a few bands of loosely cemented sandstone in the upper unit with a combined thickness of 6 m. Sandstones of the lower unit are in general well-cemented with only minor loosely cemented sandstone, whereas in the upper unit cementation of the sandstones is more patchy and in places cuts across sedimentary boundaries. Calcite cementation is the dominant control in reducing the porosity and permeability throughout. Compaction and authigenic overgrowths of quartz and feldspar on detritat grains are subordinate factors in reducing the porosity. Enhancement of porosity has occurred principally by leaching of detrital orthoclase so that in the feldspar-rich upper unit porosity is 25% in places, and permeability is locally as high as 5 darcys (D). X-ray diffraction traces of