Diagenesis of Upper Carboniferous sandstones: southern North Sea Basin

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<ul><li><p>Geological Society, London, Special Publications</p><p>doi: 10.1144/GSL.SP.1989.041.01.05p57-73.</p><p> 1989, v.41;Geological Society, London, Special Publications G. Cowan southern North Sea BasinDiagenesis of Upper Carboniferous sandstones:</p><p>serviceEmail alerting</p><p>new articles cite this article to receive free e-mail alerts whenhereclick </p><p>requestPermission</p><p>part of this article to seek permission to re-use all orhereclick </p><p>Subscribe</p><p>Collection London, Special Publications or the Lyell </p><p> to subscribe to Geological Society,hereclick </p><p>Notes</p><p> The Geological Society of London 2014</p><p> at University of Chicago on October 16, 2014http://sp.lyellcollection.org/Downloaded from at University of Chicago on October 16, 2014http://sp.lyellcollection.org/Downloaded from </p><p>http://www.lyellcollection.org/cgi/alertshttp://www.geolsoc.org.uk/permissionshttp://www.lyellcollection.org/site/subscriptionshttp://sp.lyellcollection.org/http://sp.lyellcollection.org/</p></li><li><p>Diagenesis of Upper Carboniferous sandstones: southern North Sea Basin </p><p>G. Cowan </p><p>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. </p><p>An outline of the stratigraphy of the Carbonifer- ous is shown in Fig. 1. </p><p>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 </p><p>Major Subdivisions of the Carboniferous </p><p>SUB SYSTEM SERIES STAGES </p><p>SILESIAN </p><p>DINANTIAN </p><p>STEPHANIAN </p><p>WESTPHALIAN </p><p>NAMURIAN </p><p>VlSEAN </p><p>TOURNASIAN </p><p>C B STEPHANIAN A </p><p>CANTABRIAN </p><p>WESTPHALIAN D </p><p>WESTPHALIAN C </p><p>WESTPHALIAN B </p><p>WESTPHALIAN A </p><p>YEADONIAN </p><p>MARSDENIAN </p><p>KINDERSCOUTIAN </p><p>ALPORTIAN </p><p>CHOKIERIAN </p><p>ARNSBERGIAN </p><p>PENDLEIAN </p><p>BRIGANTIAN </p><p>ASBRIAN </p><p>HOLKERIAN </p><p>ARUNDIAN </p><p>CHADIAN </p><p>COURCEYAN </p><p>FIG. 1. Carboniferous stratigraphy. </p><p>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 &amp; 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. </p><p>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. </p><p>Data base </p><p>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 </p><p>From WHATELEY, M. K. G. &amp; PICKERING, K. T. (eds), 1989, Deltas. Sites and Traps for Fossil Fuels, Geological Society Special Publication No. 41, pp. 57-73. </p><p>57 </p><p> at University of Chicago on October 16, 2014http://sp.lyellcollection.org/Downloaded from </p><p>http://sp.lyellcollection.org/</p></li><li><p>58 G. Cowan </p><p>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. </p><p>Fifteen thin sections were studied using a cold- cathode luminescence system. </p><p>Depositional environments </p><p>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). </p><p>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 &amp; 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. </p><p>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). </p><p>Present-day porosities are highly variable, ranging from zero to 15~, with correspondingly low permeabilities, ranging from zero to a few hundred millidarcies. </p><p>Detrital mineralogy </p><p>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. </p><p>Quartz </p><p>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. </p><p>Feldspar </p><p>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- </p><p> at University of Chicago on October 16, 2014http://sp.lyellcollection.org/Downloaded from </p><p>http://sp.lyellcollection.org/</p></li><li><p>Diagenesis of Upper Carboniferous sandstones 59 </p><p>KEY </p><p>~ LAND OIRECT,ON OF CLASTIC INFLUX </p><p>PRESENT-DAY I" I- EROSIONAL </p><p>LIMIT (SNS ONLY) : </p><p>"//'///,/.~ STUDY AREA </p><p>!ilii! 84 i : i </p><p>: :i 71 84 :~ i !:;i i i:i 84 !~!i!~i:ili~!!i~i~ ~ ~ ~ ~ : , . 7 ~ ( 4 c ~ ~ </p><p>ii?i!ii il : :iiiiiil ! i!C i i:!ili ii! ;i: 84 </p><p>i iiiiiiill !:~i~ i~ ~ i i~i~iiii:ii ii iliTiiiii:~i:!i~iiiii iii~i~ili!~i! iii!ii!7~i!ii~i!~7~!~!i! 8 4 ,!,~ :~, :~ ~ ~ ;;, i iiiii; !@?::i:;! 17:; i: i i , ; :7i~i!~!i!~!~!i~:~:~!:~i~i!:!~i~i!Ii~i:~iL~:/:i~i:!~!~:~!~!~/!~!~:~i:ii!~i~ ~ :i:~!!~iiii~ii~!!:~i!i~!i;iiiii~!iii!ii~:ii!ii:~i~!i:!i/i:~i:iii:~:i~/ /~:i~i tOi~ii::k!M::i!:~iiii&gt;~i i~!:~:i~ii!!!:i:l,~ ! ~ #~17~ii '~!L ~7/~I~!L~:~ ~iii:! ~~ ~ : : - : :~i~!~::~ii J~ ~ i ~ : / ~ ~ ~ : - </p><p>!~i!iii:!iiii!i:i~i!!iCiiiii!:ii!!!iiiQ:iiii}i!li~!iL:ii:ii;:!: iii:L: ~iiiii!ii!ii!iiiii!:ii!i%;!ii!ii!i!ii!iiii!!ii!~!!ii!iiTii!iii~i ii:ii:!i'iii!i! ii il/i!:~i~!:iii~!!:iiii~i !i !~i!i::; C!:!:::I!:I: </p><p>Will 4 MAJOR </p><p>::.: ~ ..- / / 9 i , </p><p>t \ ~ / / / / / </p><p>ve.'~ow~ ~#G--- .. </p><p>co;~g/i/&amp;.~, o \ )Sl I'ION \ )OEO SAAI~PSTONE ~ D f f IES a MUD~TONcs" ~ </p><p>vg._-, uc c-" " . , ' / ~ . . . . f ZONE </p><p>FIG. 2. Westphalian palaeogeography and location map. (From Johnson 1982; Ziegler 1982.) </p><p>spar to albite in Tertiary sandstones from the Gulf Coast of the USA. Such alteration is thought to have occurred through an intermediate phase of replacement by carbonate or evaporite ce- ments, and the result of alteration can produce grain textures similar to those found in microcline grains. Walker (1984) noted a progressive in- crease in albite modal percentages at tempera- tures in excess of 100 ~ It is probable that the dominance of K-feldspar in the Carboniferous rocks of the E Midlands is a reflection of the shallower burial of the E Midlands sediments. </p><p>Mica </p><p>Fresh muscovite is the dominant mica type with traces of highly altered biotite. Muscovite ex- hibits a variety of stages of alteration and deformation, ranging from fresh straight blades to buckled grains showing alteration to kaolinite. In some samples very large (30 btm) kaolinite crystals, pseudomorphing muscovite, can be seen. Splitting and fanning of grains also occurs, mainly in sediments which have undergone deep Carbon- iferous burial. In argillaceous sandstones, where </p><p> at University of Chicago on October 16, 2014http://sp.lyellcollection.org/Downloaded from </p><p>http://sp.lyellcollection.org/</p></li><li><p>GR API GRAIN S I Z E LITH 1 5 0 _-| ~ . _ </p><p>. ~ .o __~_~ ~i-_~:_: </p><p>10 ~ ~ ~ " I ~ " o '.: </p><p>- _ - _ ~ u </p><p>o ~ . 4 0 9 - </p><p>v </p><p>; ,,r , </p><p>@ </p><p>-50 "~ c:::::= </p><p>% r,..__,~ 7 6 0 ~ z_._ , </p><p>~ </p><p>0 ~ 7 0 %. </p><p>DEPOSITIONAL ENVIRONMENT </p><p>OVERBANK SILTSTONE AND SANDSTONE </p><p>MAJOR DISTRIBUTARY CHANNEL PEBBLY TO MEDIUM'GRAINED </p><p>SANDSTONES, DEPOSITED AS STACKED MULTISTOREY BRAIDED </p><p>CHANNEL BARS </p><p>ORIGINAL DEPOSITIONAL POROSITIES UP TO 35% </p><p>I (MAY BE OVER 3OM THICK) </p><p>EROSIVE BASE COAL </p><p>INTERDISTRIBUTARY SWAMP </p><p>OVERBANK </p><p>MAJOR DISTRIBUTARY CHANNEL MEDIUM-GRAINED MICACEOUS SANDSTONES, DEPOSITED AS </p><p>SAND BARS </p><p>ORIGINAL DEPOSITIONAL POROSITIES UP TO 35% </p><p>(MAY BE UP TO 30M THICK) </p><p>EROSIVE BASE </p><p>MINOR MOUTH BAR / CREVASSE DELTA COARSENING'UPWARDS UNIT </p><p>INTERDISTRIBUTARY BAY FACIES </p><p>MINOR (CREVASSE) CHANNEL </p><p>STACKED CREVASSE UNITS </p><p>INTERDISTRIBUTARY BAY </p><p>~OAL </p><p>INTERDISTRIBUTARY SWAMP </p><p>INTERDISTRIBUTARY BAY WITH THINLY INTERBEDDED CREVASSE </p><p>UNITS </p><p>FIG. 3. T y p i c a l U p p e r C a r b o n i f e r o u s f ac i e s a s s o c i a t i o n s e n c o u n t e r e d in the s o u t h e r n N o r t h S e a Bas in , b a s e d o n W e s t p h a l i a n A to C c o r e d sec t ions . </p><p> at University of Chicago on October 16, 2014http://sp.lyellcollection.org/Downloaded from </p><p>http://sp.lyellcollection.org/</p></li><li><p>Diagenes&amp; of Upper Carboniferous sandstones 61 </p><p>SANDSTONE ~ PARALLEL LAMINATION </p><p>SILTSTONE ~ FAINT LAMINATION </p><p>[ ] MUDSTONE ~ SHARP BED BOUNDARIES </p><p>m COAL ~ GRADATIONAL BOUNDARIES PEBBLES GRANULES ~ EROSIVE BOUNDARIES </p><p>[ ~ MUDCLASTsINTRAFORMATIONAL ~ LOAD CAST BOUNDARIES ] CLAYEY SILTY [ ~ BIOTURBATION </p><p>[ ] MASSIVE BEDDING [ ~ ROOTED HORIZONS </p><p>[ ] PLANAR CROSS-BEDDING [ - ~ SIDERITE </p><p>TROUGH CROSS-BEDDING [ ~ SIDERITE NODULES </p><p>CONVOLUTE LAMINATION ~ PLANT FRAGMENTS </p><p>[ ~ WAVY BEDDING ~ MICA </p><p>[ ~ FLASERS [ ~ PYRITE </p><p>[ ~ LENSES ~ DEWATERING STRUCTURES </p><p>KEY TO FIG. 3 </p><p>TABLE 1. Point-count data </p><p>Sample </p><p>A B C D E F G H I </p><p>Quartz 67.0 52.0 71.5 67.0 59.0 63.5 75.5 43.5 68.0 Feldspar* 0.5 5.0 3.0 3.5 4.0 3.5 Mica 1.5 10.5 2.0 0.5 6.0 1.5 2.5 5.7 2.5 Lithics 2.0 4.0 2.0 3.0 15.5 3.5 2.0 1.0 2.0 Opaques 0.5 0.5 Organic 2.0 1.0 </p><p>matter Matrix 15.0 </p><p>clays Authigenic quartz 9.5 N/C N/C N/C N/C N/C N/C N/C N/C Haematite 1.0 16.5 1.5 7.0 Anhydrite- 2.0 </p><p>barite Ferroan 3.5 0.5 1.0 11.0 6.0 18.0 1.5 </p><p>dolomite Dolomite 5.0 2.0 1.5 0.5 1.5 Siderite 2.0 2.0 2.0 1.0 20.0 4.0 Kaolinite 7.0 9.5 5.5 6 8.0 4.0 5.0 17.0 Illite 1.0 2.5 1.0 3 2.0 0.5 3.0 2.0 Others 0.5t Primary 1.0 </p><p>porosity Secondary 9.5 7.0 12.0 1.0 9 3.0 - - 0.5 </p><p>porosity Grain size Coarse Fine Medium Medium Medium Medium Coarse Very fine Medium Sorting Moderate Very poor Moderate Moderate Poor Moderate Poor Good Moderate </p><p>Samples A, B, C and D come from the zone which has been affected by meteoric flushing during post-Carboniferous uplift. N/C, not counted. *Dominantly albite. tZircon clay. </p><p> at University of Chicago on October 16, 2014http://sp.lyellcollection.org/Downloaded from </p><p>http://sp.lyellcollection.org/</p></li><li><p>62 G. Cowan </p><p>TABLE 2. Semiquantitative whole-rock X-ray diffraction of five representative shale samples </p><p>Sample Mica- Kaolin Anhydrite Albite Siderite Quartz Total Illite Crystallinity illite- (Kubler) (We...</p></li></ul>

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