60
Lower Paleocene shaly limestones and calcareous turbidites
50 Unit 3: Ejecta-rich layer 40
Unit 2: Fine-grained carbonate breccia
30 20
Unit 1: Coarse-grained calcareous breccia
10
Negative Impact The previous examples have shown how impact of an extraterrestrial mass may bring about con- ditions conducive to the formation of hydrocar- bon reservoirs. Perhaps equally important, a direct hit by an asteroid can also cause the demise of a hydrocarbon accumulation. The Avak structure, in Alaska, shows evidence of such destruction. In 1949 gas was discovered on the flank of a
0
Upper Maastrichtian pelagic limestones with chert nodules
seismic and gravity anomaly near the village of Barrow, Alaska. Subsequent exploration revealed a number of small gas accumulations in struc- tural highs encircling the feature. As early as 1967, investigators at the US Geological Survey (USGS) suggested an impact origin for the circu- lar structure, citing the ring-like morphology and the disrupted stratigraphy encountered by bore- holes penetrating the anomaly.45
Microscopic
analysis identified shock-metamorphic PDFs in quartz grains from a well in the central uplift, confirming the impact origin.46
Time of impact
> Outcrop analog of the K-T boundary carbonate breccia succession at Bochil, Tabasco, southeastern Mexico. Although the impact-related deposit is thinner here than in the Cantarell field, this outcrop exhibits the same stratigraphy, including the fining-upward trend in Unit 1 and the fine-grained ejecta of Unit 3. Additionally, an Ir anomaly has been documented in the uppermost layer of Unit 3 here. The length of the pencil in the top four photographs (right) is 13 cm [5 in.]. The length of the rock hammer in the bottom photograph is 46 cm [18 in.].
layers can also be correlated between offshore wells and onshore outcrops (above). Interpretation of the sedimentary succession
supports the following sequence of events within the minutes and hours of the Chicxulub impact: The carbonate platform collapsed, resulting in deposition of the lower breccias. Impact ejecta arrived and were reworked and mixed with coarser material by surges of impact-generated tsunamis that reverberated across the Gulf of Mexico. The final ejecta layer blanketed these deposits, sealing in diagenetic fluids. Folding and compression in the early Miocene to Pliocene thrust up a large block of Cretaceous and Upper
42. Grajales-Nishimura et al, reference 40.
Aquino JAL, Ruis JM, Flores MAF and García JH: “The Sihil Field: Another Giant Below Cantarell, Offshore Campeche, Mexico,” in Halbouty MT (ed): Giant Oil and Gas Fields of the Decade 1990–1999. Tulsa: The American Association of Petroleum Geologists, AAPG Memoir 78 (2003): 141–150.
43. Magoon et al, reference 40. 44. Aquino et al, reference 42.
45. Collins FR and Robinson FM: “Subsurface Stratigraphic, Structural and Economic Geology, Northern Alaska,” USGS Open-File Report 287, US Geological Survey, 1967.
46. Therriault AM and Grantz A: “Planar Deformation Features in Quartz Grains from Mixed Breccias of the Avak Structure, Alaska,” Abstract 1702 in Lunar and Planetary Science XXVI, Abstracts of the 26th Lunar and Planetary Science Conference (1995): 1403–1404,
Jurassic rocks, forming the giant Cantarell trap.42 Hydrocarbons migrated into the breccias in the Miocene period from high-quality Upper Jurassic source rock.43 In 1998, using improved seismic imaging,
Oilfield Review Autumn 09
PEMEX discovered another giant accumula- tion—the Sihil field—beneath the Cantarell res- ervoirs.44
Impact Fig. NEW 19 ORAUT09-Impact Fig. NEW 19
reserves of 1.136 billion bbl [180 million m3], are also dolomitized carbonates formed from the detritus of the Chicxulub impact. The block con- taining the Cantarell field was thrust up and over that of the Sihil field, forming the trap that now contains the Sihil reserves.
http://www.lpi.usra.edu/meetings/lpsc1995/pdf/1702.pdf (accessed October 8, 2009).
47. Kirschner CE, Grantz A and Mullen MW: “Impact Origin of the Avak Structure, Arctic Alaska, and Genesis of the Barrow Gas Fields,” AAPG Bulletin 76, no. 5 (May 1992): 651–679.
48. Herd CDK, Froese DG, Walton EL, Kofman RS, Herd EPK and Duke MJM: “Anatomy of a Young Impact Event in Central Alberta, Canada: Prospects for the Missing Holocene Impact Record,” Geology 36, no. 12 (December 2008): 955–958.
49. Pilkington M and Grieve RAF: “The Geophysical Signature of Terrestrial Impact Craters,” Reviews of Geophysics 30, no. 2 (May 1992): 161–181.
Mazur MJ, Stewart RR and Hildebrand AR: “The Seismic Signature of Meteorite Impact Craters,” CSEG Recorder 25, no. 6 (June 2000): 10–16.
The Sihil reservoirs, with hydrocarbon
was estimated at 90 to 100 Ma. The Avak impact structure is situated on the
same regional feature—the Barrow Arch—as the nearby Prudhoe Bay field that contains 25 bil- lion bbl [4 billion m3] of oil. Yet the Avak struc- ture contains only small gas reservoirs, which are located on its flanks. To explain the lack of reserves, scientists proposed that a hydrocarbon accumulation as extensive areally as but volu- metrically smaller than that at Prudhoe Bay was trapped in this area before being disrupted by the impact and flushed to the surface.47 As part of an Alaska North Slope (ANS) 3D
multiclient petroleum system study, geologists at Schlumberger and the USGS tested this hypothe- sis by modeling the geologic events and processes that led to hydrocarbon generation, migration and accumulation in this area. Simulation results show an extremely large oil accumulation in the Barrow Peninsula at 97 Ma (next page). Results from running the simulation to the present day with no intervening extra terrestrial bombard- ment show preservation of this accumulation. Incorporating the impact effects in the simu-
lation required modification of several parame- ters of the petroleum system model. The impact itself was not modeled, but its overall effects on the target rock were estimated and used to update the model. Permeabilities of the rocks within the 1,200-m [4,000-ft] damaged zone were increased. Temperature was increased to 3,000°C within the entire structure, and vertical faults extending from the 97 Ma surface to the base- ment were introduced.
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