Dillon Falls Astrobleme (The North Zanesville Crater)
Greg Mason (NGO Development Corporation, Newark, Ohio, USA)
Ohio Geological Society meeting (Columbus, Ohio, USA)
21 April 2003
[Note: there's a fair amount of oil geology abbreviations & jargon in the notes for this talk]
The Dillon Falls Astrobleme was first imaged in 1993. It has since been drilled by CGas (several wells), NGO Development Corporation, Oxford Oil Co., and Dick Poling. The feature has gone unnamed for a while. Some have suggested Petes Astrobleme [after Pete MacKenzie, who first identified it on a seismic line], but here it is being called the Dillon Falls Astrobleme. It is located in the subsurface of Falls Township of Muskingum County, just northwest of Zanesville, and just north of Dillon Falls on the Licking River.
Three seismic projects have been completed over the feature:
1) FM-1-92/93 extended - a total of 4.5 miles long from southwest to northeast.
2) FM-2-93 - two miles long from east to west, on the western end of the anomaly.
3) OH-DE 94 - four individual lines from north to south.
The Murray Unit # 2, the Murray Unit # 5, the Lett Unit # 1, the West Unit # 1, the Holbein Unit # 3, the Vandenbark # 3, another Murray # 2, and the Wilson Unit # 3 have all been drilled on the feature.
The main anomaly is 1.7 miles across on the FM-1-92/93 extended line. This line shows a central uplift. The anomaly appears to be developed on the top of the Trenton Limestone [upper Middle Ordovician, sensu traditio] as well, but this is due to differential compaction.
The original Murray Unit # 2 drilled by CGas, the 1st well drilled, was wet. The Murray Unit # 5 well, drilled by Poling, was identical (also wet). The West Unit # 1 well drilled by NGO was a dry hole, but it is producing from the Clinton Sandstone [Lower Silurian] & the # 3 sand of the Rose Run Sandstone [Upper Cambrian]. Another well drilled on the anomaly hit 90 of sand and produces gas in the upper part. The Lett and the Holbein wells, drilled by NGO, are both production wells. These all have disruption at the Knox Unconformity [~ Cambrian-Ordovician boundary]. The West Unit # 1 was drilled on what appeared to be remnant on the outside of the feature. The Lett well was successful - it was drilled inside the feature, in the trough, and had 100 million cfg of cumulative production before it went wet.
A composite of the first two seismic lines (4.3 miles long, from west to east to southwest) has been put together. It shows the basement is a mishmash - it has been suggested that this is a karst collapse feature, but Greg Mason says no, and suggests it is an impact crater/astrobleme.
The Wilson Unit # 3 was not actually on a remnant, but appears to be on a ripple, a structural effect from the crater, ~1 mile from the anomaly. It appears to be an oblate feature, which doesnt fit with what we know about astroblemes, but current knowledge about this crater is incomplete.
Eight wells have been drilled:
1) Murray Unit # 2-1875 - in October 1994 by CGas - wet.
2) Wilson Unit # 3 - in October 1994 - on the interior of the feature.
3) West Unit # 1 - in November 1999 by NGO - on the crater rim - the only well that strayed from vertical drilling.
4) Lett Unit # 1 - in November 1999 by NGO - produced 100 million cfg.
5) Vandenbark # 3 - in July 2000 - wet.
6) Murray # 2 - in July 2000 by Oxford - produced gas for 6-9 months (60-90 million cfg).
7) Holbein Unit # 3 - in February 2001 by NGO.
8) Murray Unit # 5 - in May 2002 by Poling - on the central uplift & proved there was no central uplift (stratigraphically flat to slightly lower (5 lower) than the Murray # 2) - wet.
See a maximum of 90-135 of Rose Run/crater center sand fill in the feature.
Greg Mason correlated e-logs hung on the top of the Gull River Limestone [Middle Ordovician].
The Holbein & the Lett & the Murray # 2 were drilled in the crater. The top-Glenwood Shale [Middle Ordovician] picks are all flat. The Glenwood thins over the crater rim and thickens in the crater. The Murray # 2 well was drilled in the center of crater development - got 130 of sand, but a dirty sand. The West well was drilled to the lower Copper Ridge Dolomite [Upper Cambrian], below a developed B Zone. The B Zone picks are not present in the Holbein and are very low & ratty in the Murray # 2.
The sand in the crater center isnt a real Rose Run Sandstone.
Holbein - 20 of clean sand.
Lett - 90 of clean sand, with 14 thick gas cap, and the rest full of water.
The wells drilled so far are characteristic of a water driven reservoir.
Mattingly well - used as a regional well, to the north of the feature.
Top Trenton structure map - shows an anomaly, but it is subtle - the Trenton is high to regional over the anomaly.
Knox Unconformity structure map - again, a subtle pullup, but it is there.
Copper Ridge Dolomite structure map - picks are present, but they are forced by picks for mapping, and are probably not useful for correlation.
Glenwood isopach map - shows the anomaly very well - get 80+ to 90+ feet of Glenwood. Get a thicker Glenwood over the anomaly.
The Middle Ordovician Knox Unconformity erosional surface - have 1700 of stratigraphy between the Knox and basement. The impact occurred during development of the Knox Unconformity. It fractured the Precambrian basement. Can see undifferentiated crater fill (1700 of it) under the Knox in this feature. A lake formed during Knox Unconformity time. Sand fills the lake (eroded from the surrounding plain of old Rose Run Sandstone deposits) - have concurrently eroding Rose Run Sandstone filling the crater lake, technically producing St. Peter Sandstone in the crater. Get 20 of crater rim uplift. Also get a ripple (ripples?) ~1 mile out around crater.
Finished talk with a short movie of a gas flare from the Holbein.
The Dillon Falls Astrobleme is ~same size as Meteor Crater in Arizona - its asteroid is estimated to be the size of a semi-truck.
Get 90 feet of 14-16% porosity sand in the Lett well.
No good stratigraphic markers below the Rose Run in the undifferentiated crater fill - have doubtful B Zone picks, and no Conasauga picks, etc.
What about encountering fractures during drilling? The Murray well went artesian, so pressures may be sufficient to keep wells together when drilling potentially fractured rock in the feature.
Another well close to the crater rim is planned on being drilled in fall 2003.
Wright State academic gravity & magnetic surveys over the feature were a wash - they showed no anomaly.
Smaller craters shouldnt have a central uplift (those smaller than ~3 miles across - this one is ~1.7 miles across). The original image of a central uplift is probably a fault, if it is anything at all. The seismic data is all being reprocessed to get a better look.
[Locality info. on the e-logs of six of the wells mentioned in this talk:
Murry Unit # 5 well (permit # 8077) - 644 NL, 242 EL, Lot 12, 1st Quarter Township, Falls Township, Muskingum County, Ohio, USA.
D. Murray # 2-1875 (permit # 7892) - 1130 NL, 595 EL, Lot 12, 1st Quarter Township, Falls Township, Muskingum County, Ohio, USA.
Holbein Unit # 3 (permit # 8306) - 4930 SL, 123 EL, 2nd Quarter Township, Falls Township, Muskingum County, Ohio, USA.
Lett Unit # 1 (permit # 8241) - 760 SL, 100 EL, NE Quarter, 2nd Quarter Township, Falls Township, Muskingum County, Ohio, USA.
C. Wilson Unit # 3 (permit # 7925) - 445 SL, 1940 EL, 2nd Quarter Township, Falls Township, Muskingum County, Ohio, USA.
West Unit # 1 (permit # 8235) - 5913 NL, 1403 EL, 2nd Quarter Township, Falls Township, Muskingum County, Ohio, USA.]