After adjustments to the drilling, bit life increased considerably. After 175 m with the same drill bit, we reached 700 m this evening and the drill core is still very nice and recovery complete. We continue to drill through the turbidite unit, which now shows gradual changes in lithology: The coarse grained fraction increased considerably, partly accounting for up to 90 % of the drill core while the thin shale units become darker, rich in organic matter, sometimes almost resembling anthracite. This is most likely a sign that we approach alum shale, whose first continuous horizons we expect in about 50 - 100 m (according to the combined interpretation of seismic and magnetotelluric data from the site investigations) at the top of an about 500 m thick imbricate unit that overlies the basal Caledonian décollement
Eventually, the bedrock is reached after cleaning out the cement in the inner casing. The microbiologists immediately began to test their sampling procedures to adapt to the lithology and test contamination control.
After drilling has been completed last month, the OSG team has now begun downhole logging in the COSC-2A borehole. After rig-up of the two logging winches, an oriented 4-arm caliper run was the first measurement over the entire length of the borehole. The borehole wall showed to be very smooth and without any bigger hole enlargements.
The drilling at the COSC-2 site is successfully concluded. A big thank you to the on-site science team and the technical/drilling team. After the main borehole (COSC-2A) to 2276 m, the short COSC-2B was cored within 2 days. It is located 70 cm from COSC-2A and reached 116 m. It covers the top interval of COSC-2A. Overall, drilling went well. After mobilisation the operations started on 14th April. Down to 1575 m, the the core diameter is 61 mm (H-size) and from 1575 to 2275 m it is 45 mm (N-size). The borehole is now open (uncased) below the 100 m surface casing. The geology of COSC-2A held some surprises for us, mainly the level of main Caledonian décollement, the stratigraphy of the "imbricated section" and the type of basement encountered. Upcoming: Sep 2020: Downhole logging; Winter 2020/2021: Core description; Early 2021: Data ready & sampling party.
COSC-2 finished drilling during the night from Friday to Saturday at 2276.05 m depth. Unfortunately, the last round trip to change the drill bit was in vain but no major geological changes were expected in the following tens of metres. Our present assessment is that all objectives of the drilling operations are fulfilled. The geology had some surprises for us, mainly the rocks in the imbricated zone and the type of basement that underlies the sedimentary units (see earlier posts). Below ~1200 m, we lost correlation with the seismics. This issue will be addressed by re-processing and re-evaluation of the data based on the information from the up-coming downhole logging campaign.
During Saturday early morning, the borehole was flushed with clean water. Then the HQ drill string that acted as temporary casing was slowly rotated free and tripped out. At present (Sunday evening), the last hundreds of metres of NQ drill pipe are being tripped out, leaving the hole open (uncased) below the100 m long surface casing. We expect to finish all work on the COSC-2 main hole during the night. Not time for party yet (but for a BBQ!) because tomorrow, the drill rig will be moved some metres to core a short hole that covers the upper 100 m (to the bottom of the surface casing, which was hammer drilled).
Today is the last weekly break for change of drillers. Some will not come back since we have only 3 days of drilling left before hole-completion work will start. It feels like the beginning of the end. Also the rig seems to feel it. Problems here and there and spare parts are on the way (and proper maintenance is only a few drilling days away, after demobilisation).
Present depth is 2224 m. We are still in the porphyries and their lower boundary seems to be out of sight. New drill bit, new luck. Let's hope this bit and the rig will last for the full remaining drilling time.
After the drill bit had been changed on Thursday, the core barrel got stuck on the second core run. At this point, we discovered that the wire of the main hoist was damaged and needed a replacement before operations could continue. Luckily, we could order the spare and found a transport company that was able to pick it up within the remaining 90 minutes of the working day. Delivery close to the site happened over night and thus, the wire could be replaced on Friday. This was followed by the round trip to retrieve the stuck core barrel. Since Saturday noon, we are drilling again, now below 2200 m.
Drilling continues, now past 2100 m. We are still drilling through the green porphyry, with some variations. Sometimes a fabric is developed, sometimes we see minor deformation zones, but we have not found a real reason for the deep seismic reflections yet.
After speculations on social media about drilling in the woodland outside Järpen, journalists became interested in the project, and today we made it on the first page (+ double page inside).
COSC-2 reaches 2000 m. Already last night we and the cake were ready for 2000 m. But, and some say because of this, the drill bit wore out at 1996 m. 20 hours and a round trip later: 2000 m! We are still drilling through the volcanic succession and are approaching the last prominent seismic reflector.
Drilling through a thick succession of mafic volcanic rocks has a big advantage: long bit life. Only now, after 220 m, the drill bit is worn out and has to be replaced. The drillers take the round trip as an occasion for an attempt to do something about the crooked drill pipes: sort out the bad ones and try to straighten them with a hydraulic press.
Drilling progress is acceptable. We now reached 1700 m drilling through softer rock, a dolerite with a deformed, but originally most likely intrusive contact to the overlying volcanic rocks. The mystery of seriously crooked drill pipes is haunting even this drilling (cf. problems at COSC-1). We quickly built a device for sorting out the worst scrap and hope that this will improve drilling performance. Outdoor work is of little joy these days, because the recent wet weather has produced an invasion of "svidknott" (the Swedish model of no-see-um's). However, a microbiologist hunting biofilms and being presented open fractures in a core doesn't mind (George).
Today in the early evening, COSC-2 passed 1500 m depth. After the change of drill bit at 1470 m drilling went smoothly and fast. We passed through an about 4 m thick metasomatised deformation zone and thought that we eventually will encounter a new rock formation. But we are still in the porphyry, probably another tectonic slice of it. Presently, the reason for the laterally extensive seismic reflector at 1450 m (± 5 %) is still a riddle. Either it is different from our imagination of how such a reflector "looks like" or it is deeper than we expected. Future downhole logging and/or further drilling will hopefully solve this mystery
CLIFF HANGER: After diverse problems that slowed down the drilling progress we eventually passed 1450 m, the depth (plus/minus ~5 %) where we expect to encounter the cause for a major seismic reflection. What happened? Still in the same lithological unit, the drill bit wore out at 1470.20 m and the last 45 cm piece of drill core shows signs of increased deformation. We are back to "The thrill to drill"! The drillers are tripping drill pipe during this hot and sunny day to change the drill bit and we expect drilling to commence during the night. What comes next? Maybe we will know tomorrow!
The spring flood arrived four weeks late and very concentrated, additionally supported by rain during the last days. The lake level rose already considerably and the water drowned our pump repeatedly. If it rises further, we have a risk for wet feet at the drill site.
10 days of silence from the COSC-2 project. Status: 1235 m depth (after some delay due to repairs and problems with the core barrel and drill bits that are solved by now) and the first short logging campaign is presently going on. Drilling will commence again tomorrow. We are still in the unit that we entered at about 825 m, at the bottom of the black shale. It seems that the entire "imbricate unit" defined by the site investigations consists of these sandstones, conglomerates and volcanic/volcanosedimentary rocks with distinct internal deformation zones, and not of imbricated alum shale with associated over- and underlying rocks. From what we have seen, we conclude that the black shale that we drilled between ~775 and 825 m is the alum shale and that it's absence in the "imbricate unit" suggests that is not a tectonic repetition, i.e. it is unlikely that we will encounter it again. The observations are also supported by the first results from downhole logging. In the next 100 m, we will encounter the horizon that we originally interpreted as the basal Caledonian décollement, hosted in alum shale at it's original stratigraphic level ... obviously not very likely anymore. There wouldn't be a need for scientific drilling if we'd know everything in advance. But we wonder what surprise will be next!"
COSC-2 reached 1000 m depth! The last tens of metres of drill core through various sandstones and conglomerates suggest strongly that we are drilling through a sliver of basement cover. Similar late Precambrian to early Cambrian sediments are known from small tectonic intercalations in the Lower Allochthon of the Alsen area, about 20-25 km east of the drill site. The exciting question is what we will find below, before we eventually will encounter the basal Caledonian décollement. The seismic site investigations suggest several hundreds of meters of imbricate units that most likely involve the lower part of the Lower Allochthon and possibly the sedimentary basement cover (like the rocks we just drill through). We will get an answer during the coming weeks.
Today, COSC-2 passed 800 m depth. Even more exciting is that we drilled into a different geological unit at c. 775 m. At about this depth, the seismic and magnetotelluric site investigations indicated the shallowest alum shale at the top of an about 500 m thick imbricate unit that overlies the basal Caledonian décollement. We encountered an until now continuous unit of penetratively deformed black shales with lenses of calcareous shale or limestone. The coming days will show what other lithologies the imbricate unit is composed of. With the entry into the black shales, on-site sampling and preservation of shale from above, in and below the décollement commenced for rock mechanic investigations on the nature of the deformation in this regional scale overthrust.
Yesterday, the gamma source for the MSCL arrived. We installed source and detector immediately and from now on, gamma density will be acquired on all cores. Still drilling the same geological unit at 750 m.
Drilling through the sedimentary succession from the margin of paleocontinent Baltica continues. Still turbidites that seem to be surprisingly little deformed. Today we passed 500 m depth. Now change of drill bit at 525 m.
Last core before midnight to 449.5 m. Geological logging is not a high priority at the drill site under present circumstances, but a first simple log suggests that the entire sequence is much less deformed than originally assumed. What on the first glance looks like tectonic deformation seems to be syn-sedimentary - slumping, de-watering. This could be good news for stratigraphers. The second group of highly motivated scientists began their work at the drill site today. Soon, we will drill on towards 500 m after flushing of the borehole to remove excess cuttings.
Drilling proceeds: 300 m on Friday evening. After 160 m of drilling, the drill bit wore out and was replaced, followed by 40 m of reaming. Then core drilling continued, now approaching 350 m. The lithology is still the same.
Today is the first day with drilling around the clock. Depth is approaching 200 m. Turbidites with some healed fracture zones and faults. The stratigraphic position is still unclear - either an equivalent to the Silurian formations known from farther east but from a more distal location on the margin of paleocontinent Baltics or already the Ordovician greywackes. Further drilling will tell us.
First core on deck for COSC-2! The first 1.5 m of rock below the conductor casing. Now we are waiting for the cement of the inner HWT casing to cure. Then assembly of the well head and preparations for the start of HQ coring and related scientific operations. Today in the morning, ICDP equipment (core scanner and MSCL) arrived at the site and we hope to have everything assembled, tested and calibrated in time.
Winter and spring alternate rapidly, leading to difficult road conditions, thawing substrate at and around the drill site and special measures to get at fresh water for the operations.
The drill rig is aligned to the borehole and preparations for installing the HWT-casing in the previously hammer-drilled upper 100 m are continuing. The microbiology lab is almost ready for operations. ICDP OSG equipment was shipped from Potsdam today (delayed due to Covid-19). If things run smoothly, we'll have all science set up when drilling operations commence.
Operations at the COSC-2 drill site commenced today. The first group of drillers arrived and preparations for positioning the drill rig are underway. Winter and spring alternate rapidly, leading to difficult road conditions, thawing substrate at and around the drill site and special measures to get at fresh water for the operations.
Establishment of the working facilities at the COSC-2 drill
site. Next will be the heavy drilling equipment and the storage tents.
Scientific and other valuable equipment will arrive and set up just before the start of the operations. With the recent development of Covid-19, we just hope that the scientists will be able to travel to the drill site for the start of the operations in about a month's time.
Today Riksriggen, the drill rig of the Swedish National Research Infrastructure for Scientific Drilling, arrived at the drill site. This is only the beginning of the mobilisation and it will take a couple of weeks until all equipment and the scientific facilities will be in place, the scientific instruments and drilling equipment set up and tested and the first motivated science and drilling teams ready for core drilling and processing.
First activities at the COSC-2 drill site! The snow was cleared from the access road and drill site and a local company is presently drilling the first 100 m with DTH air hammer. In this borehole, a conductor casing will be cemented to stabilise the rock close to the surface and protect the groundwater. Core drilling with a target depth of 2500 m will start at the bottom of hammer drilled borehole. Cuttings from hammer drilling were collected in 3 m interval to get a first idea about the bedrock in the upper part of the borehole.
Yesterday's beauty was an exceptio, but the porphyry seems to be more affected by penetrative veining and metasomatism. In addition, drilling speed increased considerably: only 3.5 - 4 h per 6 m core run. Those were the good news. At about 5 pm, drilling stopped and it soon became clear that the bit was worn down 37 h from the planned end of the drilling operations. What to do? 5 hours earlier or later, the decision would have been easy. Eventually, we decided to change the bit a last time and try to get a couple of core runs before we have to pull out for good.
Today's beauty: metasomatism @ ~2235 m. Obviously, the borehole is making fun of us. Hundreds of metres more or less monotonous porphyry, a small change here, another one there. Now the end of drilling is in sight and we get this. Let's see what we get from the next core runs ...
This borehole is a thriller. Small details often cause long delays. After the change to the 6 m core barrel (double tube), however, overall drilling performance has increased and we get 6 m of very nice drill core every 4 hours. The invention of the day (by the night shift): We now pump out the entire 6 m of core into HQ3 liners without breaking the core barrel in the middle. At ~1925 m, we are still drilling through the volcanic sequence that we assume is equivalent to the Dala Porphyries farther south. It is interrupted only by a 150 m thick dolerite intrusion. Highest priority is to sample the entire sequence and its contact to the Svecofennian age(?) basement. We reached the depth of the first of the lowermost two reflectors. Do they represent another dolerite or are they related to the base of the volcanic sequence?
Drilling stopped for the Swedish midsummer celebrations at 1575 m. We are still in porphyry, but now with mafic dykes of irregular size and distribution, and associated contact metamorphism. We will use the days without drilling for downhole logging and change dimension from HQ to NQ for the remaining part of the drilling.
After change of drill bit and some trouble with stuck core barrels that required an additional round trip, we are drilling on. Yesterday through a massive unit that is probably of volcanic or volcanosedimentary origin, today massive sandstones with only few and very minor shale layers. Depth of the borehole 940 m.
