A 5985 m deep borehole, Gravberg 1, was sunk in 1986-1987 into the 65 to 75 km diameter Siljan impact structure in Sweden. The project endeavoured to investigate the possibility that commercial volumes of mantle-derived abiogenic methane gas could be trapped in the fractured and brecciated lower parts of the impact structure and could provide a sustainable source of energy for Sweden (Castaño 1993). This country derives much of its energy supply from hydrotechnological facilities. It was hoped that impact-induced fracturing of the lower crust could tap a large gas potential (Gold 1988). The possibility of the presence of mature oil had also been discussed by Vlierboom et al. (1985). However, the project failed, as no mantle-derived methane could be detected. A further result was that Siljan does not possess a suitable, hydrocarbon-trapping caprock stratum. Pore-pressure at depth was determined to be largely hydrostatic in contrast to the a priori assumption that it would be lithostatic. No free gas was detected.
In contrast to the negative outcome of this project at Siljan, massive volumes of hydrocarbons have been confirmed or are already exploited in other impact structures. In fact, it has been estimated that total commercial hydrocarbon production from North American impact structures has been between 5 to 16 billion US$ per annum - and this since many years already (e.g., Grieve and Masaitis 1994; Donofrio 1981, 1997, 1998). Of 19 confirmed impact structures in 1997 in North America (Koeberl and Anderson 1996), 9 were exploited for oil or gas at the time (Donofrio 1981, 1997). Ages of these productive impact structures range from Cambrian/Ordovician to late Tertiary. Production comes from impact-affected basement granites, carbonate rocks, and sandstones. Donofrio (1997) estimated that productions ranged from 30 to 2 million barrels per day, plus more than 1.4 billion cubic feet of gas per day. Various hydrocarbon reservoirs may exist in all parts of an impact structure, including central uplifts, rim structures, slump terraces, and ejecta. In the case of very large impact structures, such as Chicxulub in Mexico, even disrupted and fractured rocks in the wider environs of an impact structure may be favorable exploration targets. Donofrio (1997) reported that approximately 50 % of confirmed impact structures as well as other only suspected (i.e., no definitive evidence for impact available yet) impact sites in petroleum provinces are commercial oil and gas fields.
The Ames impact structure in Oklahoma boasts a 7200 barrel oil per day well test, Sierra Madera in Texas a 4.3 billion cubic feet of gas per day calculated well test, and a well with a 2850 ft oil column is known from the Red Wing Creek impact structure. Other impressive reserves exist at the 25 km diameter Steen River impact structure (Alberta, Canada), with an estimated 3 million barrels of oil in structural traps in the rim strata of this structure (Grieve 2003), and in the Chicxulub region off the Yucatán peninsula (Mexico) with 30 billion barrels of oil and 15 trillion cubic feet of gas (see, for example, Grajales-Nishimura et al. 2000). Grieve (2003) emphasized that Steen River is located in a very remote part of northern Alberta Province, where lack of infrastructure inhibits exploitation. However, this author comments that this structure may be "a sleeping giant from a hydrocarbon perspective" (ibid).
Grieve and Masaitis (1994) discussed in detail hydrocarbon resources at the Ames, Red Wing Creek and Avak (Alaska) structures. In addition, an entire monograph (Johnson and Campbell 1997) has been devoted to the Ames Structure. Ames is an approximately 14 km wide complex impact structure, which comprises a central uplift surrounded by an annular graben, and an outer, slightly uplifted rim section. The structure is buried by several kilometers of Ordovician and post-Ordovician sediments. Particularly important horizons are the Arbuckle Dolomite Formation that occurs regionally and that is itself overlain by middle Ordovician Oil Creek Shale that forms an effective trap for hydrocarbons and is also regarded as the source for them (Kuykendall et al. 1997). The age of the structure has been estimated on stratigraphic grounds (as summarized in Koeberl et al. 2001) - namely the absence of the Arbuckle dolomite within the area of the structure, and complete cover with Oil Creek Shale - at approximately 460 Ma. The first hydrocarbon discoveries in this structure were made in 1990 within a 500 m thick section of Lower Ordovician Arbuckle dolomite of the rim section. The economic importance of this impact crater section of a regionally not very productive dolomite layer stems from the impact structure-specific amount of fracturing and associated karst formation. Wells into central granite breccia have also been very productive: for example, the famous Gregory 1-20 well has been appraised as representing the most productive oil well from a single pay zone in all of Oklahoma - a state with a more than 100 year hydrocarbon exploration history. In 1994, about 100 wells had been drilled into the Ames structure - with 52 of them producing oil, and another producing gas.
In this case, the impact event resulted in fracturing and brecciation, leading to enhanced porosity and permeability in rocks of all parts of the structure. It also led to significant topography in the crater area that could accelerate erosion of granite and development of karst topography in the crater rim section - resulting in further enhanced porosity of reservoir rocks. The source of the Ames oil is the Oil Creek Shale, which is unique to the structure and has not been recognized outside of it (Castaño et al. 1997). The Ames impact apparently produced a unique environment to deposit the post-impact oil shale and, in addition, allowed the formation of the structural traps for hydrocarbon accumulation.
The Ames situation, according to Grieve and Masaitis (1994), resembles that at Newporte, another oil-producing impact structure (Koeberl and Reimold 1995) in North Dakota. In contrast, at Red Wing Creek (Koeberl et al. 1996b), also located in North Dakota, and like Newporte in the Williston Basin, hydrocarbons are also recovered from the brecciated basement rocks of the central uplift, but the impact structure does only represent a structural trap and is not responsible for the accumulation of hydrocarbons. The Red Wing Creek structure was discovered when a pronounced seismic anomaly was drilled in 1965. After non-productive drilling results on the flank of the central uplift and in the annular trough, the central uplift eventually proved productive. It was estimated in 1994 that reserves within the ca 3 km wide central uplift were over 130 million barrels of oil, with up to 70 million barrels possibly recoverable (Donofrio 1981; Pickard 1994). The reserves of natural gas were estimated at that time at some 100 billion cubic feet. Red Wing Creek is considered the most productive oil play in the USA, with a cumulative production of 12.7 million barrels of oil and reserves of 20 million barrels of oil and 25 billion cubic feet of gas (Grieve 2003).
Another structure in the Williston Basin still needs to be confirmed as an impact structure: Viewfield, a small structure of 2.5 km diameter, has, at 20 million barrels, quite substantial oil reserves. Grieve (2003) proposed - on the basis of the terrestrial cratering rate - that there could be as many as 12 ± 6 impact structures > 10 km in diameter in the region of the Williston Basin alone, all of which could be viable exploration targets for hydrocarbon deposits. Grieve estimated that if only 50% of these impact structures had reserves similar to those associated with Red Wing Creek, the impact-related reserves in the Williston basin alone could amount to 1 billion barrels of oil and 600 billion cubic feet of gas.
Another structure, for which an impact origin was proposed but still remains to be confirmed, is the 7-8 km wide Calvin structure in Michigan (Milstein 1988). It is estimated that, by 1994, more than 500 000 barrels of oil had been produced from this structure.
The Avak structure, located in the Arctic coastal plain of Alaska, was shown to be of impact origin by Kirschner et al. (1992), who described shatter cones and planar deformation features in quartz. The age of this structure is given by Kirschner et al. (1992) as 100 ± 5 Ma, based on stratigraphic information. Avak, at about 12 km diameter, is a complex impact structure with an annular trough and central uplift. The central uplift has been drilled at the Avak well that penetrated the regional Lower Cretaceous to Ordovician successions. This well also showed some oil, but not of commercial amounts. However, in the immediate vicinity of the impact structure (Fig. 11), three major gas fields - Sikulik and East and South Barrow - occur and straddle annular structures that have been related to the impact event. The idea is, as explained by Grieve and Masaitis (1994), that listric faults of the crater rim, which truncated Lower Cretaceous Barrow sand and juxtaposed it against Lower Cretaceous Torok shale, created an effective gas seal. Both the South and East Barrow gas fields have been exploited. Lantz (1981) estimated a primary recoverable gas reserve of 37 billion cubic feet for this structure.
Hydrocarbon deposits are also known from crater sediments of the Boltysh and Rotmistrovka impact structures, in the Ries crater, and in several other structures. At Boltysh, a 25 km wide and 100 Ma old impact structure, the Eocene crater sedimentary sequence contains oil shales that are 400-500 m thick. According to Masaitis (1989), several tens, > 0.5 m thick, exploitable layers have been identified. Some of the most persistent and thickest layers have an average thickness of 4.4 m. The total resource reserves were estimated by Masaitis at 4.5 billion tons (see also Bass et al. 1967). Oil shales have also been reported from Rotmistrovka and Obolon craters (Masaitis et al. 1980; Gurov and Gurova 1991). In the Ries crater (24 km diameter, 15.1 Ma age), up to 1 m thick allochthonous layers of clayey lignite occur in the upper part of the sedimentary crater fill column (Wolf 1977). Some 600 barrels of oil were produced in 1994 from two wells on the northern rim of the 25-km-diameter Steen River structure in Canada. Gas is produced from the 22 km diameter Marquez Dome structure in Texas. And Gorter et al. (1989) postulated that the large, 55 km diameter, Tookoonooka crater structure of Australia could have potential to yield hydrocarbons because of its vicinity to the hydrocarbon-rich Eromanga Basin. Finally, a recently proposed new impact structure, the 7 km diameter Cloud Creek crater in Wyoming (USA) of about 190 ± 20 Ma age (Stone and Therriault 2003), also has several oil producing wells (in the so-called Lost Dome oil field) associated with the fault zone of the crater rim. It appears that several boreholes drilled into the central uplift area of this structure (Fig. 2, ibid) proved dry - however, it is not known how deep these holes extended.
Large amounts of oil and gas are also recovered from the Lomas Triste breccia (thought to represent brecciation related to seismic disturbances after the gigantic Chicxulub impact event) deposit of the Campeche oil field in the Gulf of Mexico 300 km from the Chicxulub impact structure (Camargo Zanoguera and Quezada Muneton 1992; Limon et al. 1994). Production is estimated to be in excess of 2 million barrels of oil and 1.5 billion cubic feet of gas per day (Grieve 2003). The large Chicxulub impact structure that, with regard to the catastrophic events of global importance that took place at Cretaceous-Tertiary boundary times is most intriguing, was discovered and confirmed as the smoking gun for the K/T impact as a direct consequence of oil exploration by geophysical methods and drilling (e.g., Hildebrand et al. 1991; Grajales-Nishimura et al. 2000). These latter authors made a case for both the offshore oil-producing breccias and the sealing rocks from the oil fields (such as the Cantarell oil field) in the Campeche marine platform being probably related to the Chicxulub impact. Both the oil-producing carbonate breccias and the capping dolomitized layer contain impact products. Grajales-Nishimura et al. (2000) considered the dolomitized layer part of the impact ejecta layer. They emphasized that "the K-T breccia reservoir and seal ejecta layer of the Cantarell oil field, with a daily production of 1.3 million barrels of oil, are probably the most important known oil-producing units related to an impact event" (ibid). Grieve (2003) summarizes that the up to 300 m thick breccias of 10-20% porosity contain proven reserves of 30 billion barrels of oil and 15 trillion cubic feet of gas. These amounts, according to this author, exceed the entire on- and offshore reserves of the United States.
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