Diagenesis of Upper Carboniferous sandstones: southern North Sea Basin

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  • Geological Society, London, Special Publications

    doi: 10.1144/GSL.SP.1989.041.01.05p57-73.

    1989, v.41;Geological Society, London, Special Publications G. Cowan southern North Sea BasinDiagenesis of Upper Carboniferous sandstones:

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  • Diagenesis of Upper Carboniferous sandstones: southern North Sea Basin

    G. Cowan

    S U M MARY: Upper Carboniferous sediments from the central part of the southern North Sea Basin were deposited on a broad fluvio-deltaic plain. Potential reservoir rocks occur within distributary channel facies sandstones, but porosity and permeability have been reduced by authigenic mineral growth and improved by at least two separate phases of porosity enhancement: firstly by the infiltration of meteoric waters during post-Carboniferous uplift, and secondly by the action of acidic fluids generated during burial. The depth of invasion of meteoric fluids is highly variable, ranging from a few metres to hundreds of metres, and the best porosity is preserved in sediments which have undergone both phases of porosity enhancement. Virtually all the observed porosity in Carboniferous sandstones can be interpreted as having a secondary origin. Integrating the deduced diagenetic history with a burial and temperature history for specific wells allows the timing of diagenetic events to be estimated.

    An outline of the stratigraphy of the Carbonifer- ous is shown in Fig. 1.

    It has long been recognized that the source of gas discovered in Rotliegendes and Bunter sand- stones in the southern North Sea Basin is in deeply buried organic-rich mudrocks and coals

    Major Subdivisions of the Carboniferous

    SUB SYSTEM SERIES STAGES

    SILESIAN

    DINANTIAN

    STEPHANIAN

    WESTPHALIAN

    NAMURIAN

    VlSEAN

    TOURNASIAN

    C B STEPHANIAN A

    CANTABRIAN

    WESTPHALIAN D

    WESTPHALIAN C

    WESTPHALIAN B

    WESTPHALIAN A

    YEADONIAN

    MARSDENIAN

    KINDERSCOUTIAN

    ALPORTIAN

    CHOKIERIAN

    ARNSBERGIAN

    PENDLEIAN

    BRIGANTIAN

    ASBRIAN

    HOLKERIAN

    ARUNDIAN

    CHADIAN

    COURCEYAN

    FIG. 1. Carboniferous stratigraphy.

    of Carboniferous age, and gas shows have frequently been recorded from Carboniferous sandstones. Although oil has been produced from the Namurian and Westphalian fluvio-deltaic sediments of the E Midlands since the end of the First World War, rocks of similar age from the southern North Sea Basin have poor reservoir qualities in comparison with the overlying Rotlie- gendes sandstones (Glennie 1986). Consequently, the Carboniferous has been considered to be economic basement in the basin, and most exploration wells halt drilling when rocks of Carboniferous age are encountered. Rotliegendes sandstones are absent from the central and northern part of the basin (Marie 1975). This fact, combined with the recognition of wide- spread secondary porosity development (Schmidt & McDonald 1977) and the advances in under- standing of basin evolution and thermal matura- tion modelling (MacKenzie 1978; Waples 1980), has led to a re-evaluation of the reservoir potential of the Upper Carboniferous.

    The work presented here is based upon 'in- house' work carried out to determine the controls on reservoir potential of Upper Carboniferous fluvio-deltaic sediments in an attempt to produce a working diagenetic model for central southern North Sea hydrocarbon exploration.

    Data base

    A total of 580 m of core from six wells was logged at a 1:50 scale to determine the depositional facies, and 142 samples were taken for thin- section analysis. Each thin section was stained for carbonates with alizarin red S and potassium ferricyanide, and for K-feldspar with sodium

    From WHATELEY, M. K. G. & PICKERING, K. T. (eds), 1989, Deltas. Sites and Traps for Fossil Fuels, Geological Society Special Publication No. 41, pp. 57-73.

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  • 58 G. Cowan

    cobaltinitrite, prior to point counting (200 points) to determine the modal percentages of the detrital and authigenic phases present as well as the visible porosity. Selected samples were studied using a scanning electron microscope equipped with a semiquantitative energy dispersive analy- ser in both the secondary electron and back- scattered electron imaging modes to allow deter- mination of the clay and authigenic mineral morphologies. Whole-rock and clay mineral X- ray diffraction (XRD) was carried out on selected sandstone samples and five mudstone samples to determine the clay mineralogy.

    Fifteen thin sections were studied using a cold- cathode luminescence system.

    Depositional environments

    During the Westphalian, sedimentation occurred on a broad upper delta plain between the Grampian-Fenno-Scandian high to the N and the newly uplifted Rheno-Hercynian zone to the S. The bulk of the sediment was derived from the N, although sediment was supplied locally from the London-Brabant and Welsh Massifs (Fig. 2).

    A summary log showing the aggregate deposi- tional facies of Westphalian rocks encountered in the study area is shown in Fig. 3. In general, the sediments are similar to those described in the Durham Coalfield by Fielding (1984) and on the Northumberland coast by Haszeldine & Anderton (1980). Fielding's depositional model can be applied to the bulk of the Westphalian sediments encountered in the central part of the southern North Sea Basin with the addition of a pebbly braided distributary channel facies.

    During the Westphalian, deposition was pre- dominantly by overbank flooding and crevassing into interdistributary bays and lakes, producing argillaceous sequences which often coarsen up- wards. Thin sands interbedded with mudstones were deposited in distal crevasse-splay and overbank settings, and proximal crevasse splays (minor mouth bars) produce upward-coarsening fine- to medium-grained sand bodies, often cut into and overlain by distributary channel sand- stones. The channel sandstones have distinctive log motifs, forming blocky or upward-fining sequences, and were deposited in shoestring and sheet-like geometries. The distributary channel sand bodies have the best reservoir potential. These sands show variable thickness and internal structures but can form stacked multistorey channel bodies over 30 m thick. Grain size is highly variable, ranging from fine grained to microconglomeratic (Fig. 3).

    Present-day porosities are highly variable, ranging from zero to 15~, with correspondingly low permeabilities, ranging from zero to a few hundred millidarcies.

    Detrital mineralogy

    Representative point-count data for Carbonifer- ous distributary channel sandstones from the study area are presented in Table 1. Table 2 shows quantitative whole-rock XRD data from five representative shale samples.

    Quartz

    Detrital quartz grains are predominantly mono- crystalline and show weak to strong strain extinction, with Boehm lamellae in some samples. Polycrystalline grains account for 10~-30~ of the detrital quartz population. Where it is possible to observe the shape of the detrital grains, they are usually well rounded, but the shape is masked by later overgrowths and replacive cementation. Inclusions of rutile and zircon(?) are common. Cathodoluminescence (CL) photomicrographs show violet, dull grey and grey-brown emission colours, suggestive of derivation from a mixed igneous-metamorphic terrain.

    Feldspar

    Untwinned albite was the dominant feldspar type observed in the samples with minor amounts (10~ of the feldspar population) of twinned plagioclase. K-feldspar was recorded in trace amounts in a few samples only. Feldspar shows replacement by siderite and dolomite and its abundance is severely reduced in rocks which have undergone flushing by meteoric waters during the pre-Permian uplift phase, regardless of the stratigraphic age of the sediments. It is therefore probable that detrital feldspar suffered dissolution by meteoric fluids introduced at this time. Twinned plagioclase grains often show fracturing due to compaction but appear rela- tively fresh. Owing to its limited abundance, no attempt was made to estimate twinned plagio- clase composition. Untwinned feldspars are often inclusion rich and show incipient (hydrother- mal ?) alteration to illite. The limited composition of the feldspar grains is typical of an authigenic origin. Hawkins (1978) and Huggett (1984) have shown that the dominant feldspar type in the less deeply buried Carboniferous sandstones of the E Midlands is alkali feldspar. Walker (1984) has recorded the diagenetic alteration of alkali feld-

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  • Diagenesis of Upper Carboniferous sandstones 59

    KEY

    ~ LAND OIRECT,ON OF CLASTIC INFLUX

    PRESENT-DAY I" I- EROSIONAL

    LIMIT (SNS ONLY)