Sample Ltd. Carbonates Cambridge

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CONFIDENTIAL
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Multiclient Report for
2011
Peter Gutteridge
Late Palaeozoic Sedimentology of 7128/4-1
Contents
1.
INTRODUCTION ...................................................................................................... 6
Scope and objectives of study ......................................................................... 6
1.2.
Methodology ................................................................................................... 6
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2.
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1.1.
CONCLUSIONS ........................................................................................................ 8
Billefjorden Group ........................................................................................... 8
2.2.
Cyclic carbonate unit ....................................................................................... 8
2.3.
Bioclastic limestone unit ................................................................................. 9
2.4.
"Late Permian" siliciclastic unit ....................................................................... 9
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3.
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2.1.
DEPOSITIONAL ENVIRONMENTS: CORED INTERVALS ......................................... 10
Core 5 ............................................................................................................ 10
3.2.
Core 4 ............................................................................................................ 10
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3.1.
Depositional setting ............................................................................... 10
3.2.2.
Provenance ............................................................................................ 11
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3.2.1.
Cores 2 and 3 ................................................................................................. 12
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3.3.
3.3.1.
Deep ramp facies association ................................................................ 12
3.3.2.
Middle ramp facies association ............................................................. 12
3.3.3.
Shallow ramp facies association ............................................................ 13
3.3.4.
Peritidal facies association ..................................................................... 13
3.3.5.
Emergent facies association .................................................................. 13
Late Palaeozoic sedimentology: 7128/4-1
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3.3.6.
3.4.
4.
Microfacies types ................................................................................... 14
Core 1 ............................................................................................................ 14
DEPOSITIONAL ENVIRONMENTS: NON-CORED INTERVAL .................................. 15
Lithostratigraphic sub-division ...................................................................... 15
4.2.
Billefjorden Group 2033.0 -2510m ............................................................... 15
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4.1.
Lower sand-rich sub-unit 2395.0 -2510.0m (base not seen) ................. 15
4.2.2.
Upper sand-poor sub-unit 2033.0 -2395.0m ......................................... 15
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4.2.1.
Cyclic carbonate unit 1839.25 -2033.0m ...................................................... 16
4.4.
Bioclastic limestone unit 1705.0 -1839.25m................................................. 16
4.5.
"Late Permian" siliciclastic unit 1578.0 -1705.0m ........................................ 17
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4.3.
Basal sandy sub-unit 1705.0 -1655.0m .................................................. 17
4.5.2.
Middle bioclastic sub-unit 1655.0 -1592m ............................................ 18
4.5.3.
Upper spiculitic sub-unit 1592.0 -1568.0m ........................................... 18
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4.5.1.
HIGH RESOLUTION SEQUENCE STRATIGRAPHY ................................................... 19
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5.
Sequence stratigraphic sub-division ............................................................. 19
5.2.
Cyclic carbonate unit ..................................................................................... 19
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5.1.
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5.3.
6.
Bioclastic limestone unit ............................................................................... 20
5.3.1.
“Late Permian” siliciclastic unit.............................................................. 21
DIAGENESIS AND RESERVOIR PROPERTIES .......................................................... 22
6.1.
Billefjorden Group ......................................................................................... 22
6.1.1.
Lower sand-rich sub-unit ....................................................................... 22
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6.1.2.
Upper sand-poor sub-unit ..................................................................... 22
Cyclic carbonate unit ..................................................................................... 23
6.3.
Bioclastic limestone unit ............................................................................... 24
6.4.
"Late Permian" siliciclastic unit ..................................................................... 25
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6.2.
Basal sandy sub-unit .............................................................................. 25
6.4.2.
Middle bioclastic sub-unit...................................................................... 25
6.4.3.
Upper spiculitic sub-unit ........................................................................ 25
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6.4.1.
REFERENCES ......................................................................................................... 27
8.
APPENDIX A. SEDIMENTOLOGY OF CONVENTIONAL CORES ............................... 28
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7.
? Billefjorden or pre-Billefjorden sedimentary sequence ............................. 28
8.2.
Heterolithic facies ......................................................................................... 29
8.3.
Composite sandstone facies ......................................................................... 31
8.4.
Channel sandstone facies .............................................................................. 32
8.5.
Coral bafflestone facies ................................................................................. 33
8.6.
Anhydritic facies ............................................................................................ 34
8.7.
Laminated carbonate mudstone facies......................................................... 35
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8.1.
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8.8.
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8.9.
Palaeoaplysina boundstone facies................................................................ 36
Phylloid algal bafflestone facies .................................................................... 38
8.10.
Oncoid bioclast packstone facies .............................................................. 39
8.11.
Bioclastic sandstone facies ........................................................................ 40
8.12.
Bioclastic shale facies ................................................................................ 41
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Graded bioclastic packstone facies ........................................................... 41
8.14.
Bioclast grainstone /packstone facies ....................................................... 43
8.15.
Bioclast wackestone / packstone facies .................................................... 45
8.16.
Spiculite facies ........................................................................................... 47
APPENDIX B: SIDEWALL CORES ............................................................................ 49
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9.
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8.13.
Terrestrial sandstone / siltstone ................................................................... 49
9.2.
Marine sandstone ......................................................................................... 49
9.3.
Spiculite ......................................................................................................... 50
9.4.
Bioclast packstone / grainstone .................................................................... 50
9.5.
Bioclast wackestone ...................................................................................... 51
9.6.
Bioclast grainstone ........................................................................................ 51
9.7.
Spiculite bindstone ........................................................................................ 52
9.8.
Ooid terrigenous grainstone / packstone ..................................................... 52
9.9.
Dolomite ........................................................................................................ 53
APPENDIX C: PALAEOHYDRAULIC FORMULAE .................................................. 55
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Occurrence of microfacies in sidewall cores ............................................. 54
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9.1.
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11.
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12.
APPENDIX D: MICROFACIES METHODS AND DATA .......................................... 56
APPENDIX E: POINT COUNTING METHODS AND DATA .................................... 57
13.
FIGURES............................................................................................................. 66
14.
PLATES ............................................................................................................... 80
15.
CORE LOGS ...................................................................................................... 103
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4. DEPOSITIONAL ENVIRONMENTS: NON-CORED INTERVAL
4.1.
Lithostratigraphic sub-division
The interpretation of the depositional environments of the non-cored late
lithological interpretation of wire line logs in the interval 1500-2530m. The
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Palaeozoic succession is based on a microfacies study of sidewall cores and the
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stratigraphic succession is summarized by figure 4. Detailed descriptions of the
microfacies are given in Appendix A.9. The following four informal lithostratigraphic
Billefjorden Group 2033.0 -2510m
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4.2.
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units have been identified from the bottom upwards:
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The base of this unit is interpreted at a gamma log break at 2510.0m MD. The
position of the top has been constrained by sidewall cores. This unit has been
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divided into two sub-units on the basis of sand content.
4.2.1. Lower sand-rich sub-unit 2395.0 -2510.0m (base not seen)
Sidewall cores from this interval are mainly coarse, poor to moderately-sorted
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sandstone and occasional siltstones of the terrestrial sandstone/siltstone
microfacies. These were deposited in channel, crevasse splay and overbank settings
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in a fluvial system. The log signature suggests that this interval consists of thickly-
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bedded sandstone with some thin shale and siltstone beds. This suggests that this
sub-unit is dominated by channel and proximal crevasse splay sands with minor
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overbank sandstone and siltstone. This sub-unit includes core 5 that is interpreted as
4.2.2. Upper sand-poor sub-unit 2033.0 -2395.0m
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a rotational collapse of overbank sediment adjacent to a major channel.
The base of this unit is picked at the top of the uppermost thick sandstone which
marks the boundary between the sand-dominated succession (below) and the sandpoor succession (above). Sidewall cores from this interval are mainly medium to fine,
moderately-sorted micaceous sandstone and occasional siltstone of the terrestrial
Late Palaeozoic sedimentology: 7128/4-1
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Reservoir potential in the bioclastic unit is moderate to poor. The reservoir facies are
restricted to shallow ramp setting where they form layered sheet-like bodies which
thin up dip and down dip. This passes up dip into porous marginal facies which are
likely to form a poor up dip seal.
"Late Permian" siliciclastic unit
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6.4.
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6.4.1. Basal sandy sub-unit
This sub-unit has minimal reservoir potential. Depositional porosity in bioclast
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grainstone and glauconitic sandstone is generally good but has been eliminated by
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compaction and cementation (Plate 38). Depositional porosity in the spiculitic
bindstone is very poor and has been reduced by compaction and cementation by
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6.4.2. Middle bioclastic sub-unit
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silica (Plate 42). Sponge spicules have been dissolved and infilled by silica cement.
This sub-unit has minimal reservoir potential. Depositional porosity in bioclast
packstone / grainstone is poor to moderate and has been reduced by progressive
compaction and cementation by non-ferroan and ferroan calcite cement in the form
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of a blocky mosaic and as overgrowths on echinoderms.
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6.4.3. Upper spiculitic sub-unit
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Figure 12 shows the distribution of pore types through the cored part of the
spiculitic facies. Spicule moulds form the most abundant pore types. Fracture
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porosity has mainly been produced by compaction of dissolved sponge spicules and
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minor intergranular porosity is present throughout.
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The intergranular pores are lined by early microcrystalline silica cement that
preserves the spicule moulds. This was followed by dissolution of sponge spicules
producing abundant microvuggy porosity with low connectivity (Plate 36). The
connectivity was locally increased by compactive fracturing of spicule moulds. A
phase of pervasive cementation by microquartz and chalcedony followed which
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base of the anhydrite is marked by 1cm thick stylolitised shale parting; the anhydrite
passes transitionally upwards into the laminated dolomicrite facies.
Microfacies: Anhydrite nodules are made up of a felted mass of tabular to acicular
anhydrite nodules are made up of a densely peloidal dolomicrite (Plate 19).
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Interpretation: The nodular anhydrite formed by displacive growth of finely
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pseudomorphs after gypsum. The stringers and wisps of sediment between the
crystalline gypsum within peloidal micritic or dolomicritic sediment. The lack of
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bioclasts and the early growth of gypsum indicate hypersaIine conditions. The
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depositional setting is interpreted as a evaporitic tidal flat or sabkha.
Diagenesis: This facies contains no porosity. The gypsum was later replaced by
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anhydrite. There is a later partial replacement of the anhydrite by euhedral and
subhedral megaquartz. The host sediment may have been an original dolomite mud
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or may have been dolomitised subsequently.
Occurrence: Core 3 1840.44-1839.25m
8.7.
Laminated carbonate mudstone facies
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Core: This is a carbonate mudstone which comprises mm to 0.5cm thick laminae
separated by wispy partings. Nodular anhydrite is present as a partial replacement in
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the lower part of this facies (Plate 20). The laminae show a low relief domal
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structure, the crest of which is penetrated by a sub-vertical fracture infilled by calcite
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and anhydrite spar. No bioclasts are present.
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Microfacies: Not sampled.
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Interpretation: The absence of biota suggests that this facies was deposited in a
hypersaIine setting. The laminations may be algal in origin. The domical structure
may represent an original growth form of the algal mats or may represent a teepee
that formed by alternating desiccation and wetting. This facies was probably
deposited in an intertidal setting.
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Occurrence: Core 2 1814.8-1816.2m, 1819.6-1824.0m, 1828.35-1828.93m and
1831.35-1832.3m. Core 3 1846.5-1847.3m, 1850.0-1852.3m, 1854.9-1856.1m,
1858.0-1861.9m and 1864.95-1865.7m
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8.15. Bioclast wackestone / packstone facies
Core: This is a bioclast wackestone which contains layers and lenses of reworked
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bioclasts. Occasional whole and fragmented solitary rugose corals, calcitised
gastropods, disarticulated bivalves and articulated brachiopods, crinoids, fusulinids
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and beresellids are abundant in some intervals. Solitary and colonial rugose and
tabulate corals and sponges are occasionally preserved in situ. Allochems are
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occasionally partly coated by Tubiphytes. This facies occasionally contains an
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argillaceous matrix and terrigenous grains. The limestones are heavily bioturbated
which is picked out by the disorganized orientation of bioclasts and also by mottling
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defined by the distribution of clay and micrite (Plate 32). Bedding planes sometimes
show intensive bioturbation which have been penetrated by clay-lined burrows.
Beds of this facies are between 0.15 to 1.0m in thickness and are separated by
occasional shale partings, stylolites and occasional beds of the bioclast grainstone /
packstone facies. The bases of these beds are sharp and sometimes show evidence
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of scour.
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This facies is partly dolomitised; the boundaries between dolomite and limestone are
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formed by a bedding plane, stylolite or shale parting, or are transitional over a
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vertical distance of up to 0.2m (Plate 32).
Microfacies: This facies has been divided into two microfacies types (not
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distinguishable in core).
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Non-micritised bioclast packstone / wackestone which contain disarticulated,
fragmented and abraded bioclasts including brachiopods, crinoids, fusulinid and
other foraminifera and fenestrate bryozoans. Whole bioclasts include
gastropods, rugose and tabulate corals and articulated brachiopods. There is no
micritisation or boring of allochems and occasional coating of bioclasts by
Tubiphytes (Plate 33). There are occasional silt to very fine-grains quartz grains.
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10. APPENDIX C: PALAEOHYDRAULIC FORMULAE
The following empirical relationships have been used to estimate fluvial channel and
meander belt widths from the thickness of the channel sandstone body in core 4.
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h = Channel depth
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w = 6.8h1.54
w = Channel width
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Wm = 64.6h1.54
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Relationships from CoIlinson (1986).
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Wm = Meander belt width
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