NIRAS 2015 ON-MOL2B
Lambert 1972: X, Y = 207923, 215705
Diepte 650 m
Maaiveld 39 m
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uit project H2O
Stratigraphy ON-Mol 2 (Mol-Rauw Postel ) boreholes (version 4/9/15) including the position of the septaria layers in the Boom Clay.
(Laurent / Coordinates / wat is Mol A2 op één van de logs? )
(Laurent , nog integreren van de data van de mudlogging voor grain size , glauconite content , carbonate ?)
2D deep / 2B shallow / 2C piezometer boreholes in cross hole
0-300 m interpreted from GR and RES logs in ON-Mol-2B , and data from nearby wells (31E/323 ;32W/415(SCK 13); RUS04/03).
300 – 408m interpreted from GR and RES logs in ON-Mol-2D and from reference stratigraphical data based on boreholes Mol-Dessel area and the Dessel-1 GR-RES logs: checks for S level position on the Sonic data and on the Elan results.
408- TD interpreted from GR,RES and Elan data with reference to boreholes in the Mol-Dessel area.
All depths are from surface , as read from the geophysical logs.
0-60m almost pure fine to medium quartz sand of the Mol Sand Formation (Pliocene) , observed in the surface trenches at the site below a very thin Quaternary layer.
The Quaternary deposits are thinner than the surface trench (3m) and are sandy, gravelly or loamy.
The Mol Formation can be subdivided into ( boundaries not sharply defined , only GR log available till 80m depth):
0-14m slightly higher GR : probably clay, organic matter of the Russendorp Member
14 – approximately 33m fine to medium quartz sand (Maatheide Member)
33 - 44m slightly higher and variable GR, probably a lignite bearing zone, in nearby wells identified as Maatheide lignite at the top and clayey Donk sand Member below.
44m – 60m fine to medium quartz sand (Donk Member)
60-80m fine glauconitic sand of the Kasterlee Sand Formation (Messinian).
The slightly higher GR values (25-30 cps ) between 60 and 80 m are interpreted in the area as the fine glauconitic Kasterlee Fm; the appearance of a green colour in the drilling mud indicating glauconite in the sediment is traditionally used to mark the boundary between the Mol and Kasterlee Formations.
The Kasterlee sand unit is significantly thinner than in the boreholes nearby : 20m versus 45m in 32W/415(SCK 13) borehole about 3km to the NE and 35m in 31W/341 borehole about 1,5 km to the east.
80-210m glauconitic sand of the Diest Formation ( Tortonian to Messinian)
The section with glauconitic Diest Formation is defined as currently done in the area between 2 sharp increases in GR values, at 80m marking the top of the Formation and at 210m marking its contact with the underlying Berchem Formation.
Subdivisions can be made based on RES and GR curves :
80-105m especially marked in the high RES values and also the GR where 2 fining up intervals can be identified; the interval is also recognised with similar thickness in the boreholes 32W/415(SCK 13) and 31W/341 (and B323), in the latter it is described as ‘top Diest’ with presence of green (glauconitic) clay and glauconite containing sand (by P.Laga) and in the former it is indicated as X? on the profile Postel-Maaseik in Vandenberghe et al. (2005, Fig.10).
105-150m rather homogeneous, slightly fining upwards, glauconitic sand interval.
150-181 m lower GR and higher RES fining-up glauconitic sand interval.
181-210m at the base about 4m transitional zone to low GR values and very high (> 100 Ohmm) RES values , followed above by fairly constant GR values and a marked high RES lobe.
Regionally in the Diest Fm, only a lower finer-grained glauconitic Dessel Member is distinguished; it is generally reported to be 20 to 40m thick and can be calcareous, entirely or only at the base. In the borehole 31W/341, a 37,5 m thick basal part of the Diest Fm is interpreted (by P. Laga) as the Dessel Member and corresponds to a lower GR and high RES log interval. In the 32W/415(SCK 13) borehole (see profile Postel-Maaseik in Vandenberghe et al. , 2005, Fig.10) the same interval is recognised (here about 30m thick) on the logs but only about the lower 20m with a slightly upwards decreasing GR record is indicated as Dessel Sand Member.
Therefore the Dessel Sand Member could correspond either to both lower subdivisions (150-181m and 181-210m) or to only the lower subdivision (181-210m) identified above in the present borehole.
210- 255 m The Berchem Formation (Burdigalian) consisting of dark green medium-sized sand very rich in glauconite
The dark green glauconitic sand unit between the Diest Fm (base at 210m) and the top of the Eigenbilzen Fm, with typical log signature at 285m, consists of the Berchem Fm (Burdigalian) above and the Voort Sand (Chattian) below. The total thickness, 75m , is much lower than the stratigraphically equivalent thickness in the borehole 31W/341where 123,5 m are described as Berchem Fm (74,5m) and Voort Sand (49m) (interpretation by P. Laga). Therefore the log signature of the interval is more comparable with the Mol-Dessel area where the Voort Sand is slightly over 20m thick (ON-Mol-1 , ON-Dessel 1) and the Berchem Sand slightly less than 20m (core description Gulinck & Laga, Mol SCK 15). Therefore the section between 210 and 285m is subdivided in two parts at 255m, based on the recognisable Voort Sand GR pattern similar to this sand unit in the ON-Mol-1 and ON-Dessel 1 boreholes (Vandenberghe and Wouters, 2011 ncs website).The boundary between the Berchem Fm and Voort Sand unit in the ON- Mol-1 is confirmed by biostrat data (Van Simaeys, 2004, Fig.1.4).
The boundary at 255m is marked by a sudden shift to lower GR values.
The Berchem Fm pattern can be subdivided in an upper high GR-high RES part (210-225m) and a substantial lower GR part (225-255m). No known particular stratigraphic subdivision corresponds to this log subdivision.
255-285 m The Voort Sand unit (Chattian) consisting of medium-sized glauconitic sand.
The GR pattern is comparable to the pattern of the Voort Sand unit identified in the ON-Mol-1 and ON-Dessel 1 boreholes; the log pattern shows a similar development as in the overlying Berchem Fm : a low GR at the base evolving upwards in a marked high GR lobe while the RES pattern does not show a marked evolution.
285 – 313 m The Eigenbilzen Formation (Rupelian) consisting of very fine, clayey and slightly glauconitic sand .
In an upwards coarsening pattern, 2 GR low - RES high cycles can be recognised , a subdivision occurring regionally in the Mol-Dessel area (Vandenberghe and Wouters, 2011 ncs website).
313-418m The Boom Clay Formation (Rupelian)
The limits between the known lithostratigraphic members (Boeretang , Putte, Terhagen, Belsele-Waas) and the positions of the septaria levels in the clay are derived based on the lithostratigraphic scheme including the septaria levels established in the Mol-Dessel area (Vandenberghe and Wouters, 2011 , ncs website). The limits and levels are re-plotted on the Dessel-1 geophysical GR and RES logs; the particular log signatures at these levels are then identified at the GR-RES logs of ON-Mol 2D. The septaria levels can be slightly shifted on the ON-Mol 2D logs in order to coincide with marked low Neutron-POR and low bulk density values. As in Neutron-POR and bulk density logs, also Sonic log data show a corresponding anomaly only at certain levels (e.g. 322m, 329,5m, 343,5m,...). On the Elan petrophysical interpretation the majority of the S levels correspond with the presence of carbonate but carbonate is also interpreted at other levels or zones without known septaria.
313-341m Boeretang Member (Boom Clay Formation) consisting of an alternation of silt and clay layers, dominated by silt rich horizons upwards. The log expression shows 10 regionally recognised wiggles , numbered 0 to 9.
S level S200 at 322m
S level S190 at 329,5 m
S levels S185 and S180 at 339,8m and 340,2m ( is the problematic water-bearing level drilled from the underground laboratory).
341- 388,4m Putte Member (Boom Clay Formation) consisting of an alternation of heavy clay layers and silty clay layers expressed by the resistivity wiggles at the meter scale; intervals rich in organic matter give a black stain to the clay. A particularly silt-rich couple of layers can regionally be identified on the RES logs and occurs in ON-Mol 2D at 385-386m.
S level S170 at 343,5m
S level S160 at 345,8m
S level S150 at 347,4m
S level S140 at 351,8m
S level S130 at 358m
S level S120 at 360m
S level S115 at 361,3m
S level S110 at 361,9m
S level S100 at 364m
S level S90 at 367,6m
S level S80 at 368,7m
S level S70 at 372,4m
S level S61 at 377 m
S level S60 at 378,5m
S level S50 at 382,9m ( flat shaped septaria at a seismic reflection level in Kruibeke)
S level S41 at 385m
388,4- 406,8m Terhagen Member (Boom Clay Formation) consisting of an alternation of heavy clay layers and silty-clay layers expressed by the resistivity wiggles at the meter scale; the lower part contains diffuse carbonate. The R level at 394,5m is a regionally recognised horizon.
S level S40 at 389,3m
S level S30 at 397,5m
S level S20 at 399,4m
S level S10 at 403m
406,8-418m Belsele-Waas Member (Boom Clay Formation) consisting of layered very silty clay becoming even fine sandy clay at the very base of the unit.
418-426 m Ruisbroek Member (Rupelian) (Zelzate Fm) consisting of fine glauconitic sand
The lower boundary at 426m is especially based on the Elan analysis showing a sharp base of the sand unit.
426- 454m Maldegem Formation (Lutetian) consisting of alternating clay and fine sand units. The boundaries between the members as identified below are approximate, based on a combined evaluation of GR, RES and Elan analysis.
Subdivided in :
426-427,5m Zomergem clay Member (note high GR but res curves show large permeability ...analysed as clay in Elan )
427,5-435,3m Onderdale sand Member , sand with clayey intervals
435,3- 445,1m Asse-Ursel clay Members , the basal 443,1-445,1m is the more sandy and glauconite rich base (Asse Member) of the high GR-low permeability RES Ursel clay Member above between 435,3-443,1m.
445,1-454m Wemmel clayey sand Member , it is differentiated from the Asse Member above by low GR and permeable res curves and higher carbonate content. Nummulites are reported in the same unit from the borehole 31W/323 (P. Laga). It is differentiated from the Lede Formation below by the lower GR, less clay content, of the Lede Fm.
454- 488,1m Lede and Brussel Formations (Lutetian)
Both formations consist of sand and are characterised by the presence of sandstone concretions and layers. Generally the Lede Formation is finer-grained and more calcareous than the Brussels Formation. This was well observed in the geophysical well logs of the cored Mol-SCK 15 borehole; the boundary between both formations was interpreted in this borehole by M. Gulinck based on a coarse horizon with reworked elements underlying and distinguishing 21m of Lede sand Formations and overlying 9m of Brussels sand Formation. The interpretation in this cored borehole is taken as the stratigraphic reference in the area. The total thickness of both Formations in the Mol-SCK-15 (30m) borehole and slightly less in the ON-Mol 2D (34,1m) borehole. The associated lower GR/higher RES in the Brussels Formation compared to the Lede Formation as observed in the Mol-SCK-15 borehole is not well expressed in the ON-Mol2D borehole. However the RES and GR pattern in the ON-Dessel-1 borehole, consistent with the interpretation in the Mol-SCK-15 allows to determine the boundary between the Lede and Brussels Formations on the GR and RES logs of ON-Mol 2D . It appears that about 6,5 m additional Lede sand is present in the top of the Formation in ON-Mol2D compared to Mol-SCK 15 and ON-Dessel-1.
454-481m Lede Formation (Lutetian) of which the 454-461,5m interval with rather homogeneous fine–grained and clayey sand is not present in the Mol and Dessel area to the west.
The 460-472 interval is particularly rich in carbonates (Elan) and consists of carbonate sandstone layers as shown by the RES pattern.
481-488,1m Brussels Formation (Lutetian) , consisting of carbonate rich sand with stoen layers
488,1-573,5 m Ieper Group (Ypresian) consisting of packages of clay-rich and sand-rich intervals .
488,1-492,8m Hyon Formation , fine glauconitic sand interval occurring above the top of Aalbeke clay. The boundary with the Aalbeke Member(Kortrijk Fm) is put at 492,8m based on the Elan analysis which shows a sharp and marked lithological boundary and on the estimated thickness of the Aalbeke clay which is in the Mol SCK 15 borehole 9m thick and has a sharp lower boundary at 503min ON-Mol 2D (giving 10,8 m in this well).The interval between the top of the Aalbeke clay and the top of the Ieper Group in the Mol SCK 15 is 13m while in the present ON Mol 2D , it is only 4,7m thick. In the Mol SCK 15 this thicker interval has a more variable lithology and was interpreted by E.Steurbaut to consist of a few meter of Kortemark Member (Tielt Formation) overlain by Egem Member (Hyon Formation) sand.
492,8-503m Aalbeke Member (Kortrijk Formation) consisting of homogeneous clay, picking up more silt towards the top as shown by GR/RES and Elan data.Thickness is similar as in the Mol SCK 15 (respectively 10,2m and 9m).
503m-543,2m Mons-en-Pévèle Formation consisting of fine sand with some more clay-rich intervals. The thickness corresponds very well with corresponding snady interval in the Mol SCK15 and ON-Dessel 1 boreholes. The lower boundary is picked at the top of an underlying thicker clay unit as expressed by the RES pattern showing loss of permeability.
543,2- 573,5m Orchies Member (Kortrijk Fm) dominated by high clay contents.
No Mont-Héribu Member, a thin coarser level at the base, is present in the ON Mol 2D well.It was not observed in the Mol SCK 1 borehole neither (Geets 1988).
Subdivision in accordance with regional stratigraphic understanding is the identification of a lower Orchies (Orchies1) unit at the base expressed by a well marked increased GR reading (with a high RES reading ?!) , the top of which is situated at 563,5m.
The top of the Orchies Member, or base of the Roubaix Member, has been defined by E. Steurbaut (1991 ,1998) in 2 different ways; the old definition almost corresponds with the base of the Mons-en-Pévèle Fm at 543,2m.The new definition is about ten meter lower and can be systematically picked in GR and RES logs; in general it seems to coincide with the start of the presence of carbonate in the clay above. In the ON Mol 2D this level occurs at 549m(14,5m versus 18m in Rijkevorsel borehole) , leaving 5,8m of would according to this definition of the boundary correspond to the Roubaix Member.
573,5- 615m Grandglise Member (Hannut Fm) (Thanetian) consisting om homogeneous fine sand.
615- TDlog at 648m Halen Member (Hannut Fm) (Thanetian) consisting of somewhat more clayey and silty sediment compared to the Grandglise sand above. The boundary with the Grandglise sand unit is marked by a shift in GR and RES.
Based on the thickness of the Halen silt Member in the Mol SCK 15 borehole the top of the Waterschei klei Member occurs just below TDlog.
