North American Impact Structures Hold Giant Field Potential

FOCUS: EXPLORATION

Fig. 1 CROSS-SECTION OF COMPLEX-TYPE METEORITE-IMPACT STRUCTURE

Shows composite (ideally symmetrical) reservoir features*

Central Talus uplift deposits Rim Rim flank pinchout Ejecta Breccia lens pinchout Slump terraces (listric faults)

lts fau erse Normal and rev Subcrater fracture zone

*Radial faults not included. Source: Modified from Donofrio, 1981, and Brenan and others, 1975. OGJ North American impact structures hold giant field potential

Richard R. Donofrio Consulting Geologist Oklahoma City

eventeen confirmed races, and ejecta and proba- overall success rates for wells with Chicxulub. The current impact structures bly in subcrater fracture drilled during the past 5 estimated gross income of oil S occur in petroliferous zones. Disrupted rocks in years. Roughly 50% of con- and gas production from S firmed astroblemes and as- confirmed impact events in areas of North America, nine proximity to impact struc- of which are being exploited tures, such as Chicxulub in trobleme anomalies in petro- North America is $16 bil- for commercial hydrocar- the Gulf of Mexico off Yu- leum provinces are commer- lion/year. To determine the bons. Production comes catan, also contain hydrocar- cial oil and gas fields. hypothetical hydrocarbon from impact-affected gran- bon deposits. In some cases Outstanding statistics in- potential from undiscovered ites, carbonate rocks, and hydrothermal activity at- clude a 7,200 b/d drillstem or unrecognized astroblemes sandstones yielding from 30 tending impact events can test in the Ames impact in U.S. petroleum basins, the b/d to over 2 million b/d of diminish reservoir quality, structure, a 4.3 bcfd calculat- exercise of overlaying the oil plus over 1.4 bcfd of gas. and talus deposits resulting ed absolute open flow poten- distribution pattern of Cana- Reservoirs are found in cen- from erosion of the central tial test at the Sierra Madera dian Shield astroblemes onto tral uplifts, rims, slump ter- uplift and rim afford alterna- astrobleme, and a well hav- the lower 48 states was per- tive drilling targets. The ing a 2,850 ft oil column formed. The results suggest, drilling success rates for new (1,600 ft of net pay) in the intuitively at least, the pres- This is modified from one of 41 full field wildcats and total wells Red Wing Creek impact ence in basement rocks of im- papers, expanded abstracts, and posters published in Oklahoma Geo- into confirmed astroblemes structure. Reserves are also pact structures with potential logical Survey Circular 100, Ames are about 21% and 77%, re- impressive, ranging from 3 reserves ranging from 5 bil- Structure in Northwest Oklahoma and Similar Features: Origin and Pe- spectively. These figures ap- million bbl of oil at Steen lion bbl to over 105 billion troleum Production (1995 Sympo- proximate the industry’s River to 30 billion bbl of oil bbl. sium), updated through 1997. and 15 tcf of gas associated

May 11, 1998 • Oil & Gas Journal 69 FOCUS: EXPLORATION

Introduction Fig. 2

Twenty five years have DISTRIBUTION OF ASTROBLEMES ON EXPOSED PRECAMBRIAN CANADIAN SHIELD passed since the Red Wing Creek discovery revealed the 2 km prolific hydrocarbon poten- 5 tial of meteorite impact craters (astroblemes and im- pact structures). 1 km Commercial oil and gas 2 discoveries in other impact structures, as well as recogni- tion that certain existing fields resulted from impact, 3 km have provided a small but in- 1 km teresting data base for such Williston esoteric structures. basin

Included in the data base 1 km 3 p e e are astrobleme anomalies. d e 5 km 2 km r o These are curious circular f Michigan 2 km y n basin e structures that lack evidence h g e Illinois l l of shock metamorphism but basin A ns 2 km ia may be of impact origin. In h Anadarko 3 c la a this article, these anomalies basin p p A may include buried struc- 1 k m 1 km 2 Arkoma tures that mimic impact Black 5 Warrior s craters, such as calderas. ita 1 km basin ach To bring organization and Ou 5 km

2 currency to information on Llano 10 km producing impact structures, 5 uplift 5 it was apparent that the data 0 1,000 Km needed to be compiled and 0 621 Miles updated. Accordingly, this Contours show the approximate depth below sea level of the unconformity between Precambrian basement rock and sedimentary cover. The contours article reduces drilling re- in the Gulf Coast and Atlantic coastal margin show the depth to Paleozoic basement rocks. sults of producing impact Source: Base map modified from Suppe, 1986, and King and Edmonston, 1972. OGJ structures and other related anomalies into table form, cluded as confirmed on the to release reserve figures, es- beds were deposited in large which should provide useful basis of studies by Milstein;1 timates for these fields are areas of the North American information for the profes- Chicxulub, a confirmed as- provided. craton. sion. The review of drilling trobleme,2 is considered to be Over 1,000 wells, includ- Although Red Wing results includes a discussion a producing impact event on ing stratigraphic tests, dry Creek produces from dis- of astrobleme features, the basis of the nature of the holes, injection wells, and rupted Mississippian beds, drilling odds, hydrothermal areal oil fields that lie be- producers, have been drilled the impact event there is not considerations, and impact yond the crater’s outer rim. to date to delineate the struc- Mississippian in age but oc- probability rates and con- Also included for reference tures in Table 1. Some of the curred later—in the Jurassic- cludes with an attempt to es- are two producing astrob- more salient results of these Triassic in this case. Impact timate the potential reserves leme anomalies: Viewfield drilling efforts are briefly craters formed in middle and in impact craters in the base- and Lyles Ranch. Of these, noted below. late Paleozoic time have been ment. the origin of Lyles Ranch found elsewhere but are ap- (also known as Bee Bluff or Impact ages parently absent in petrolifer- Producing astroblemes the Uvalde structure) is the Producing impact struc- ous regions. North American onshore most controversial.3 tures range in impact age It could be argued that and offshore petroleum The structures in Table 1 from Cambrian-Ordovician certain oil and gas fields provinces include nine con- are shown with locations, di- for Newporte4 to late Ter- have not yet been recognized firmed impact structures that ameters, impact ages, discov- tiary for Lyles Ranch.5 as having an impact origin are commercial oil and gas ery year of hydrocarbons, Within this time span, the and that undiscovered hy- fields: Ames, Avak, Calvin, various well counts, reser- ages of the structures are drocarbon-bearing impact Chicxulub, Marquez, New- voir rock, producing depths, concentrated in the early Pa- craters are also present.6 porte, Red Wing Creek, Sier- daily production, and prima- leozoic, Mesozoic, and Ceno- ra Madera, and Steen River ry (recoverable) reserves. zoic; as Table 1 shows, there Discovery year (Table 1). Data were obtained from op- is an absence of producing None of the structures in Marquez and Sierra erators, participants, govern- impact structures in the mid- Table 1 was interpreted as an Madera produce from below ment and private organiza- dle and late Paleozoic a gap astrobleme when it was ini- the base of their structures tions, and consultants with of more than 200 million tially drilled. Of the 11 struc- but are included here be- direct access to well informa- years during which hydro- tures listed, hydrocarbons cause of possible impact ef- tion. Although some opera- carbon-prolific Pennsylvan- were discovered in 6 be- fects at depth. Calvin is in- tors were unable or reluctant ian and Mississippian source tween 1972-78. The year 1972

70 Oil & Gas Journal • May 11, 1998 FOCUS: EXPLORATION is significant because it Fig. 3 marked the phenomenal new ° field oil discovery at Red CANADIAN SHIELD ASTROBLEMES SHIFTED 15 SOUTH Wing Creek. The 1977 discovery of 2 km Sierra Madera hydrocarbons 5 was in Sierra Madera gas field below the structure’s central uplift. Hydrocarbons 1 km had been discovered earlier 2 in fields partially encom- passing Sierra Madera, and the dates are provided in

Table 1 notes. 3 km

Hydrocarbon discoveries 1 km and astrobleme confirmation Williston lags have narrowed signifi- basin cantly because geologists 1 km 3 p e e now know what to look for. d e 5 km r 2 km o Avak, for example, was dis- f Michigan 2 km y n basin e covered in 1948 and is Alas- h g e 7 Illinois l l ka’s oldest producing field. basin A ns 2 km ia Over 40 years later, shock h Anadarko 3 c la a metamorphism was recog- basin p p nized in its cores.8 Avak re- 1 km 1 km A 2 Arkoma mains the first gas-produc- Black 5 Warrior s ing confirmed impact struc- ita 1 km basin ach ture. Its counterpart for oil is Ou 5 km

2 Steen River in Northwest Al- Llano 10 km berta, where oil was discov- 5 uplift 5 0 ered in 1968. This discovery 0 1,000 Km was unrelated to shock- 0 621 Miles metamorphic studies in 1966, which were published in Note the impact structures within petroleum provinces. Any shift or rotation of this astrobleme cluster into U.S. petroleum 9 basins results in potential reserves ranging from 5 billion-105 billion bbl. 1968. OGJ Initial discovery of (mar- ginal) commercial hydrocar- gional trend that includes comparison, the average gas no apparent drop in pressure bons at Ames in 1990 preced- serpentine intrusions. If well in the Texas-Oklahoma (7,200 b/d of oil). ed evidence for confirmed Lyles Ranch were excluded area produces about 200 Though of short duration, astrobleme status by about 2 from Table 1, then the Calvin Mcfd, whereas the average this is the highest DST rate years. Oil and gas fields structure in Michigan, with gas consumption of a typical from a granite reservoir in older than Avak and those commercial oil at 800 ft, be- southwestern community North America and may be more recent than Ames may comes the shallowest-pro- consisting of 10,000 the highest worldwide. Com- also be recognized as impact ducing confirmed astrob- dwellings is about 1 bcf/ parable flow rates in similar structures. leme. year. lithology could not be found Also, records are held by in the literature. Estimated Producing depths Flow rates, reserves Red Wing Creek in North reserves for this well are over The producing depths of Sierra Madera also has a Dakota and Ames in Okla- 5 million bbl, assuming that astroblemes range from 200 distinct surface expression10 homa. its 500 b/d allowable is a ft at Lyles Ranch to over that is frequently referred to Red Wing Creek’s 2,850 ft prudent production rate. 17,000 ft at Chicxulub (Table as the Sierra Madera distur- oil column (1,600 ft of net With the exception of 1). bance. pay) in True Oil Co.’s 22-27 Chicxulub, Ames is the most Excluding Chicxulub, Geologic complexities re- Burlington Northern well13 is prolific astrobleme at 2,600 Sierra Madera has wells ap- sulting from the impact unmatched in North Dakota b/d plus 3.1 MMcfd. Of the proaching 13,500 ft and is the event and regional tectonics and apparently elsewhere in 40 producing wells, howev- deepest. Both Sierra Madera appear to have affected gas the U.S. An estimated 120 er, 6 completed in brecciated and Lyles Ranch are unique reservoirs at depth. A gas million bbl of oil are in place granite account for over half gas fields in Texas. A search well below the crater’s south within Red Wing Creek’s < 2 the daily oil production. of available data showed that rim area, the Texas Pacific 6 mile diameter reservoir area. The most recent studies Lyles Ranch, which has a dis- Montgomery-Fulk drilled in P. Page,14 wellsite geolo- by V. Sharpton at NASA15 tinct surface expression, is 1975, held the state record for gist on the D&J 1-20 Gregory put the maximum diameter the shallowest commercial many years for calculated ab- well in the Ames impact of the Chicxulub structure in gas field in Texas. It may be solute open-flow potential.11 structure, observed an unre- Mexico’s Yucatan Peninsula the shallowest overall, but Actual flow is generally ported open-flow drillstem at about 180 miles. this conclusion requires fur- about one third calculated test in the upper 60 ft of a 204 The reservoir rock is pri- ther study. LeVie5 noted that flow. PI12 records indicate a ft pay zone that yielded over marily dolomitized breccia,16 the field is anomalous in a re- calculated rate of 4.3 bcfd. In 100 bbl of oil in 20 min with which is believed to have

72 Oil & Gas Journal • May 11, 1998 FOCUS: EXPLORATION

PRODUCING IMPACT STRUCTURES IN NORTH AMERICA

Hydrocarbon Wells Active Diameter, Impact discovery required for producing Dry Total Success Reservoir Name* Location Miles Km age year discovery wells holes wells rate, % rock (1) Ames Major County, Okla. 8 13 E. Ordovician 1990 1 40 49 98 50 Granite, carbonates (2) Avak Point Barrow, Alas. 7.5 12 Cretaceous-Tertiary 1949 2 10 7 18 61 Sandstones (3) Calvin Cass County, Mich. 3.8 6.1 L. Ordovician 1978 2 30 25 91 73 Carbonates (4) Chicxulub Yucatan Peninsula, Mexico 180 300 Cretaceous-Tertiary 1974 6 453 93 658 86 Carbonates (5) Marquez Leon County, Tex. 7.9 12.7 E. Tertiary 1977 5? 6 4 10 60 Sandstones (6) Newporte Renville County, N.D. 2 3.2 Cambrian-Ordovician 1977 1 2 3 7 57 Granite, sandstones (7) Red Wing Creek McKenzie County, N.D. 5.6 9 Jurassic-Triassic 1972 3 14 14 34 58 Carbonates (8) Sierra Madera Pecos County, Tex. 8 13 L. Cretaceous 1977 4 20 10 65 84 Carbonates (9) Steen River N.W. Alta., Canada 15.5 25 M. Cretaceous 1968 7 2 25 29 14 Carbonates (A) Lyles Ranch Zavala County, Tex. 2.5 4 L. Tertiary 1979 2 4 5 14? 64 Sandstones (B) Viewfield S.E. Sask., Canada 2 3.2 E. Jurassic 1969 2 50 24 137 82 Carbonates *(1)-(9) are confirmed impact structures. (A) Lyles Ranch and (B) Viewfield lack diagnostic shock metamorphism and are classified as astrobleme anomalies. Structures disrupting Precambrian basement rock are in bold. Success rates usually are determined by more than one operator. Refer to text and references for details of structures and reservoir rocks. †Production and well-count figures are through March 1995 unless stated otherwise. Sources and notes correspond to listed order of structures: (1) Continental Resources, Inc.; Petroleum Information Corp. Production and well count figures are through August 1996. The strong natural water drive is affecting optimum production and recovery. (2) Alaska Oil and Gas Conservation Commission. (3) Michi- gan Geological Survey; Center Junction Corp.; Western Michigan University (4) Pemex. Early well records are unclear relative to pentration of key Lomas Tristes breccia or fractured interval. Apparently five deep wells were required for onshore discovery prior to exploration in adjacent offshore Bay/Gulf of Campeche, where hydrocarbons were discovered on the first well. Data shown, including discovery year, are for Bay of Campeche fields. Well count is through December 1994. Production figures are through November 1995. Production is from outside of outer rim within a distance of two crater radii to the southwest. (5) Texas Railroad Commission; Marathon Oil Co. Production is from below base of crater. Reserves not available; estimated. Diameter is from recent gravity studies by Wong originated in part by interac- duction associated with and sedimentary rock. At 3 in. wide.22 The configura- tion of impact-generated North American impact Ames, the central uplift pro- tion of the structure follow- subsea seismic waves with structures. duces primarily from brec- ing impact influenced the de- platform carbonates.17 Shoe- ciated Precambrian granite; positional environment for maker18 notes that re- Astrobleme features the rim and ejecta produce reefal-type development. searchers have now con- from Ordovician dolomites. Subcrater fracture-zone firmed the impact origin of Fig. 1 shows the astrob- At Newporte, the rim rocks production has not been con- the Lomas Tristes breccia. leme features where reser- of Cambrian sandstones and firmed, but Marquez and This producing breccia oc- voirs can develop, and Table brecciated Precambrian Sierra Madera appear to be curs in (proposed) postim- 2 lists producing examples. granite provide the reser- suitable candidates. The best pact structural traps about 90 Four such producing fea- voirs. gas production in both areas miles (within two crater tures have been confirmed to Chicxulub, Calvin, and is either directly below the radii) southwest of the outer date: central uplift, rim, Steen River have central up- crater (Marquez) or below rim in the offshore Bay of slump terraces and/or listric lifts of basement rock under- the peripheral rim area (Sier- Campeche. faults, and ejecta. lying sedimentary units but ra Madera). Marquez pro- As of January 1996, re- Listric faults are com- do not produce from Pre- duces from Early Cretaceous maining (proved) reserves bined with slump terraces cambrian. Steen River pro- fluvial sandstones and for this area were estimated because this type of faulting, duces from overturned and shales, and Sierra Madera by Pemex at 27.2 billion bbl which is curvilinear and con- fractured Devonian produces from Early Paleo- of oil (including condensate) cave upward, usually pro- dolomites in the rim,21 and zoic fractured carbonates. and 11.3 tcf of gas.19 duces slump terraces. Indi- Chicxulub produces from Reservoir properties of im- Of interest is that, about rectly, sedimentary units Cretaceous dolomitized brec- pact-affected rocks can be en- the same date, U.S. reserves draped over certain astrob- cia affected outside of the hanced at depth without no- were 22.9 billion bbl and leme features, such as the crater. ticeable seismic effects. 162.4 tcf. Thus, the oil re- central uplift, may also form Avak has a central uplift Briefly, some of the other serves believed to be associ- reservoirs. of metamorphic basement impact features include rim- ated with the Chicxulub im- In addition to Calvin, a pos- rock—a Paleozoic argillite— flank pinchout resulting from pact event exceed those of sible drapeover example need- underlying sedimentary eroded rim rocks and marine the entire onshore and off- ing further study is the pro- rocks. However, the struc- transgressive deposits on a shore U.S. reserves, includ- ducing Heidt anomaly (Crater ture produces from Jurassic rim or rim arc’s basinward ing Alaska’s Prudhoe Bay. field) in Stark County, N.D.20 sandstones in the rim area flank, breccia lens pinchout Current production from the Five of the producing as- that were displaced by listric created where the breccia lens Bay of Campeche is about 2.1 trobleme examples in Table 2 faults.8 abuts the crater wall updip, million b/d of oil (including disrupted Precambrian rock. Another example of this radial faults resulting from condensate) and about 1.4 Of these, Ames, Calvin, Steen type of faulting is found at radial tension fractures (anal- bcfd. This area constitutes River, and Chicxulub are the Calvin impact structure, ogous to spokes on a wheel), the major part of Mexico’s 2.8 complex-type structures where listric faults resulted simultaneous and overlap- million b/d production. with central uplifts, and in slump terraces. The De- ping craters resulting from Chicxulub also accounts for Newporte is a simple bowl- vonian reservoir rock in- multiple impacts or impact mostly all of the estimated shaped structure. cludes dolomitized algal overprinting, and elongate or $16 billion/year gross in- Ames and Newporte pro- mats, some areas of which “butterfly”-shaped craters come from hydrocarbon pro- duce from both crystalline have open flow channels 2 to formed by low-angle impacts.

74 Oil & Gas Journal • May 11, 1998 FOCUS: EXPLORATION

Table 1 structures to establish which were commercial. These numbers give a new field wildcat success ——— Primary reserves ——— y Total Success Reservoir Producing ———— Production ———— Oil, Gas, rate of about 21% (9 hits out es wells rate, % rock depth, ft Oil, b/d Gas, MMcfd million bbl bcf of 42 wells). Other wells into 9 98 50 Granite, carbonates 8,400-9,500 2,600 3.1 25 15 the 9 commercial astrob- 7 18 61 Sandstones 2,600-2,800 1.3 39 lemes can be difficult to clas- 5 91 73 Carbonates 800-900 110 5+ 3 658 86 Carbonates 8,300-17,000+ 2.1 million 1,400 30,000 15,000 sify as exploratory or devel- (11,500 avg.) opment. For example, most 4 10 60 Sandstones 9,000 avg. 30 1.8 0.10-0.15 5-7 operators consider that 3 7 57 Granite, sandstones 9,150-9,600 280 15 4 34 58 Carbonates 8,000-9,700 960 2.3 20 25 drilling in the Ames impact 0 65 84 Carbonates 12,000-13,500 7.7 270 structure is primarily explo- 5 29 14 Carbonates 4,265-4,500 550 3-5+ 5 14? 64 Sandstones 200-500 0.068 2 ration regardless of the sepa- 4 137 82 Carbonates 4,160-4,300 575 0.26 10.5 4.5 ration distances among (1994). (6) North Dakota Geological Survey; Eagle Operating Co. Production and well count figures are through November wells. 1996. One recompleted well in Cambrian sandstones yields mostly all production. (7) North Dakota Geological Survey; True Therefore, the success Oil Co. Secondary recovery using miscible hydrocarbon flooding was begun in 1982, prior to depletion of primary reserves. Total reserves not available. Using 120 million bbl of oil in place, the field may recover 35-60 million bbl of oil and 45-80 bcf rates in Table 1 are given for of gas. (8) University of Texas of the Permian Basin; Melzer Exploration Co. Reserves are from four fields possibly linked each structure as a percent- in part to the impact event. Names and discovery years of fields are Elsinore, 1958; Pikes Peak East, 1972; GMW, 1976; age of all wells for that par- Sierra Madera, 1977. Production is from below base of crater and rim area. (9) Mercantile Canada Energy, Inc. Structure is oblate, actual dimensions are 14.5 x 15.5 miles (23 x 26.5 km). Significant potential; needs more exploration. (A) Texas Rail- ticular structure. For exam- road Commission. Production figures are through December 1994. (B) Saskatchewan Energy and Mines. 700,000 bbl of oil ple, a total of 7 wells were remaining for primary recovery. Secondary recovery, if initiated, is limited and may yield an additional 1-3 million bbl. The two pay zones already have a partial natural water drive. drilled at Newporte, of which 3 were dry holes. Thus, 4 wells out of 7 (re- As with conventional eight confirmed impact ic and geophysical criteria gardless of the categories) reservoirs, these features re- structures where commercial for astroblemes. were successful, which gives quire source, seal, and trap. hydrocarbons have not been Some examples are a success rate of about 57%. Unlike conventional reser- reported. These are Montag- Haswell hole, Colo.;27 Pan- Of the 4 completions, 2 are voirs, geochemical studies by nais, Scotian Shelf;25 Eagle ther Mountain, N.Y.;28 Chim- currently active. Castano and others23 suggest Butte, Alta; Flynn Creek and ney prospect, Mont.;29 Hart- The drilling success rates that meteorite impacts can Wells Creek, Tenn.; Kent- ney, Man.; Elbow and for producing astroblemes create closed basins that are land, Ind.; Middlesboro, Ky.; Dumas, Sask.;30 and Lamont range from 14% at the remote favorable for the deposition Serpent Mound, Ohio; and and Selman, Okla. Steen River structure to 86% of hydrocarbon source rocks. Kilmichael, Miss. The latter Byler31 showed numerous at Chicxulub. The average Producing examples appear is tentatively included here intact and discontinuous cir- success rate for total wells to be Ames and Newporte. as a confirmed astrobleme cular anomalies in North into all producing astrob- It is proposed that almost because, subsequent to stud- America, some of which may lemes is about 77%. Exclud- all of the features in Table 2 ies by Robertson and Butler,26 be remnant impact scars. ing Chicxulub, which ac- are capable of producing hy- diagnostic shock-metamor- Overviews and/or refer- counts for two thirds of the drocarbons from basement phic features in quartz have ences for numerous astrob- total wells into producing as- rock. The exception is the reportedly been found. Veri- lemes and astrobleme anom- trobleme areas, the average subcrater fracture zone fication is currently under alies can also be found in success rate is about 58%. below the base of the crater. way. Grieve and Masaitis,32 Grieve In most cases, the success This feature is limited to One of these structures and others,33 and Koeberl rates would have been high- whatever sedimentary col- may prove to have commer- and Anderson.34 er if operators had been umn may be present because cial potential. According to An intriguing and contro- aware of what they were conventional source beds are the Kentucky Geological Sur- versial paper on the mete- drilling from the outset. In not present beneath other- vey, the 3.6 mile diameter orite-impact theory as a vi- comparison, the success rates wise undisturbed Precambri- Middlesboro structure in Bell able alternative to plate tec- for 127,253 U.S. wells drilled an crystalline rock.24 County, Ky., has a 15 Mcfd tonics theory can be found in from 1990 through 1994 were shut-in well completed in Butler.35 The list of astrob- about 20% for new field Nonproducing Mississippian siltstones at leme anomalies continues to wildcats and 74% for all astroblemes 3,200 ft. Two other wells expand, and candidate struc- wells.36 were dry, but additional tures will be confirmed as in- As previously noted, 53% Other impact events are drilling may be forthcoming. formation warrants. of confirmed astroblemes in known to have occurred in The remaining astroblemes petroleum provinces are oil and gas prone areas. identified above were either Drilling odds commercial oil and gas However, the timing for hy- dry, had shows, or may not fields. Of the 9 commercial drocarbon sourcing, sealing, have been adequately ex- The tentative and limited impact structures, Table 1 re- and trapping has not been plored. information on confirmed veals that 5 required between conducive to formation of impact structures in petrole- 1 and 3 wells for hydrocar- commercial deposits in some Other anomalies um provinces indicates that 9 bon discovery. The other 4 structures. of 17 astroblemes (53%) are craters needed between 4 Within North American Other structures exist that commercial oil and gas and 7 wells. offshore and onshore petro- lack shock-metamorphic evi- fields. However, it required Considering the hydro- leum provinces there are dence but meet other geolog- about 42 wells into the 17 carbon potential of astrob-

May 11, 1998 • Oil & Gas Journal 75 FOCUS: EXPLORATION lemes, the drilling success Table 2 general rule, hydrocarbon rates are quite favorable. reservoirs are unproductive However, all producing as- IMPACT STRUCTURE RESERVOIRS (or uneconomic) below a trobleme discoveries to date porosity of about 5%. have been by accident, and Impact feature Producing example The permeability thresh- the potential rewards of Central uplift Ames, Calvin, Red Wing Creek old for gas reservoirs is wildcat drilling come with a Rim Ames, Avak, Calvin, Newporte, about 0.1 md and about 0.5 Steen River, Lyles Ranch,* “Catch-22.” Viewfield md for oil reservoirs. Values The drilling success rates Ejecta Ames, Chicxulub(?) near the threshold, however, Slump terraces and/or listric faults Avak, Calvin are for confirmed astroblemes Radial faults Ames(?) can be offset by large pay in petroleum provinces, not Drapeover Heidt/Crater field*(?), Calvin zones. Subcrater fracture zone Marquez (?), Sierra Madera(?) for astrobleme anomalies, Marine impact Chicxulub breccia(?), North Sea Both porosity and perme- which may or may not be turbidites(?) ability are affected by hy- bona fide astroblemes (e.g., Brecca lens pinchout Not yet recognized drothermal activity, but the Rim-flank pinchout Not yet recognized solution collapse features, Simultaneous and/or overlapping reductions in porosity are calderas). Unless the objective craters Not yet recognized not as consequential as the is confirmed as an astrobleme Elongate or “butterfly” craters Not yet recognized reductions in permeability. prior to or during drilling, the *Impact origin not yet confirmed. All Calvin reservoirs are drapeover. An example of hydrothermal well may be headed into an effects across a basement as- anomaly having a lower (or trobleme is given in zero) chance of success. ature hydrothermal activity feldspar. Donofrio43 for the 8 mile di- Impact craters having sur- can enhance reservoir quali- Extrapolation of data ameter Deep Bay structure face expressions may display ty by rupturing overlying from fractured samples sub- on the Canadian Shield. The shock metamorphism and rocks. jected to temperatures of 300 rim core had a porosity of can be studied prior to An example of hydrother- to 500° C. showed that per- 8.5% and a permeability of drilling, but buried struc- mal rupture is Blackburn oil meability reductions of al- 0.01 md; breccia off the flank tures require drilling, and field in Nevada, where car- most three orders of magni- of the central uplift (possibly the exploration may find that bonates overlying an ancient tude (1,000 times less) can talus) had 21.4% and 13.7 the objective is not a con- magmatic heat source were occur within a few years in md; and the central uplift firmed astrobleme. possibly fractured and brec- crystalline basement areas had 14.9% and 0.05 md. All What are the chances of ciated by explosive hy- (such as the Canadian three areas had adequate finding commercial hydro- drothermal action, i.e., Shield). These permeability porosities, but only one area carbons in astrobleme anom- ground water contacting a decreases are most likely had adequate permeability. alies? pluton became superheated caused by hydrothermal-re- Hydrothermal effects thus Based on a preliminary and overpressured owing to lated solution-transfer place constraints on drilling check of published material, the overburden.38 This poros- processes that redistribute locations in impact struc- 6 of 12 curious circular struc- ity-creating mechnism, how- minerals in rock and can re- tures, particularly those in tures in North American pe- ever, is quite different from sult in negligible fluid flow basement rock. Moderate troleum provinces are com- conditions at meteorite-im- approaching that of intact crater erosion prior to burial mercial oil and gas fields. All pact sites. granite. can enhance reservoir quali- but one, a circular depression Following an impact At the Ries crater in Ger- ty and form talus deposits on in Texas County, Okla.,37 event, hydrothermal circula- many, Pohl41 calculated that the flanks of central highs were previously mentioned tion is initiated within the some of the suevite took and peripheral rims, thus off- in this article. target rocks. A boiling water about 2,000 years to cool setting permeability reduc- This tentative list suggests table forms below the im- from 600 to 100° C. Hy- tions due to hydrothermal that 50% of astrobleme pact-melt sheet, and steam drothermal activity mea- activity. anomalies have been drilled and water escape mainly at sured there has been docu- The most productive successfully. The chances of the rim in simple craters and mented at other terrestrial reservoir at Ames, for exam- hitting pay on the first well in both the rim and central impact structures, with esti- ple, is a relithified granodior- calculate at about 25% or bet- uplift of complex craters.39 mated postimpact tempera- ite that is frequently referred ter for anomalies of decreas- There is negligible pressure tures ranging from 100 to to as brecciated granite or a ing diameters. Combining buildup, however, and the 700° C.42 rubble pile. This rubble con- confirmed astroblemes and net effect of circulation activ- Hydrothermal alteration dition may have resulted astrobleme anomalies gives a ity is to plug fractures with of target rocks can occur in a from exposure of the central total of (at least) 29 circular mineral deposits. fraction of the time it takes uplift to subaerial weather- structures in North Ameri- Hydrothermal effects fol- for temperatures at impact ing and erosion.44 Similar can onshore and offshore pe- lowing impact events have sites to reach ambient levels, material (probably talus) troleum provinces. Of these, not been duplicated in the and this alteration is invari- may form the carbonate rub- 15 are commercial oil and gas laboratory, but experiments ably detrimental to reservoir ble of the productive rim at fields (about 52%). These fig- directed at understanding quality. the Viewfield structure.43 ures could differ if relevant fault-zone sealing may af- Hydrothermal activity at The central uplift at the unpublished drilling ford insight into hydrother- impact sites can produce eco- Steen River basement astrob- prospects were included. mal processes. Moore and nomic deposits of zinc min- leme was penetrated near the others40 studied the hy- erals, for example,32 but for center and found to be of Hydrothermal factors drothermal effects on a typi- potential hydrocarbon reser- competent, tight igneous cal granite composed of pla- voirs, the fractured rock rocks initially misidentified In rare cases high-temper- gioclase, quartz, and needs to remain open. As a as volcanic rocks.45 The cen-

76 Oil & Gas Journal • May 11, 1998 FOCUS: EXPLORATION tral-uplift flanks at Steen of bolides are influenced by The impact structures in ing exploration geologist River also need to be ex- gravitational variations with- Fig. 2 have been studied in may reason that such fanciful plored, and hydrothermal in the Earth. detail by the Geological Sur- superpositions are meaning- studies, including fluid-in- Of interest is that the vey of Canada. From geolog- less in the real world of ex- clusion analysis, need to be crater-age overlap forming ic and geophysical data, ploration, but one cannot ig- undertaken at both it and the node for North America which include core studies, nore impact densities of Ames. embraces mostly all petrole- the volume of brecciated and large astroblemes or the geo- Determining the hy- um provinces. If Shaw is cor- fractured rock can be esti- graphic distribution of petro- drothermal effects in sedi- rect, an abundance of astrob- mated and adjustments leum provinces. mentary astroblemes and as- lemes unpredicted by crater- made for hydrothermal ef- The only real challenge trobleme anomalies such as ing estimates may exist in oil fects. may come from optimistic Red Wing Creek and the and gas prone areas. If it is assumed that the geologists who claim that im- Chimney prospect, respec- Canadian Shield astrobleme pact structures remain undis- tively, would also be infor- Potential reserves of distribution is a fair repre- covered and that they occur mative. Until more informa- basement astroblemes sentation of impact density in greater numbers than are tion is forthcoming, geolo- for the larger, more resistant shown here. Of interest is gists should use the Ames While the random vs. structures, the question to be that no matter to what de- experience when planning nonrandom issue is debated, answered is, “What would gree or how far and wide the an exploration program for an intuitive approach might the potential reserves of Canadian Shield astrobleme similar structures in base- be used to estimate the re- these craters be if similar im- cluster is shifted or rotated ment rock.46 serves in undiscovered or pacts had occurred in petro- into the area of U.S. petrole- unrecognized U.S. impact leum basins?” um basins, the potential re- Impacts random? structures in basement rocks. The following parameters serves range from a low of 5 Fig. 2 shows the location of are used: a 50% crater ero- billion bbl to a high of over Impact events are as- astroblemes on the Canadian sional level before preserva- 105 billion bbl. If only a frac- sumed to be random in time Shield and several of the tion by overlying sedimenta- tion of this potential exists, and space. larger petroleum basins in ry deposits; a threshold sedi- giant fields remain to be Impact probability rates the U.S. For illustrative pur- mentary cover of about 7,000 found. of Cannon,47 for example, poses, the petroliferous areas ft to provide the geothermal The conditions leading to using the density of Earth- are shown in their present- conditions for hydrocarbon giant hydrocarbon accumu- crossing asteroids and crater- day configuration. generation; proper timing of lations in conventional reser- ing rates on other planetary Crater names and dimen- impact event, source rock, voir rocks such as sandstones bodies, suggest that at least sions can be found in Grieve and seal; and an oil recovery and carbonates appear to be 500 astroblemes the size of and Robertson.49 All of these factor of 100 bbl/acre-ft fortuitous, but they do occur. Ames should have been cre- astroblemes are developed in (Ames granite reservoir is According to one estimate, ated in the conterminous crystalline basement rock 130 bbl/acre-ft). worldwide exploration has states since the beginning of and range in impact age from To evaluate the potential revealed that giants consti- the Cambrian. The number Precambrian to Tertiary, reserves, the Canadian tute only 0.6% of significant preserved to the present will with the majority dated as Shield astrobleme cluster oil fields, yet they contain be considerably less, howev- Paleozoic. Diameters range was shifted about 15° south 84% of the reserves.51 er. from 1.2 to over 84 miles, (Fig. 3). Initially, this shift Giants within the U.S. Shaw48 argued against the with half approaching or ex- was selected because it is the usually are defined as having random nature of impact ceeding the 14 mile diameter point at which the most at least 100 million bbl of re- events and sampling bias of the Ries impact structure northerly crater of the cluster coverable oil or 1 tcf of recov- and called attention to the or- in Germany. contacts the requisite over- erable gas. Before excluding dered age grouping and po- The Ries is mentioned be- burden in a U.S. petrolifer- the lower 48 states from such sitioning of craters on North cause it illustrates the pene- ous area, the Michigan basin. potential, geologists should America, Eurasia, and Aus- tration effects of large Likewise, Fig. 3 shows consider Van Der Loop’s re- tralia. Shaw noted three spa- bolides. Pohl and others50 that such a shift displaces nu- source base study52 where tial nodes that have persisted noted that the Ries crater was merous astroblemes into on- she has noted, “although the since the late Precambrian. created by the impact of a shore and offshore petrolifer- biggest fields in any trend These nodes represent the 3,300 ft diameter stony mete- ous areas. Potential reserves are usually the first ones loci of mutual overlap of all orite that penetrated about for this scenario are about 50 found . . . if you do find age groups of known impact 2,000 ft of sedimentary rocks billion bbl of oil, an extraor- something in a lower 48 fron- events and mark the intersec- and continued for another dinary figure more than dou- tier area, the chances that it tion points of cratering 2,100 ft into crystalline base- ble current U.S. reserves. will be big enough to keep swaths that encircle the ment. If we control (or subtract) are better than if you had Earth. Seismic surveys have re- the reserves in all basement found something in a mature For example, the cratering vealed that basement rock at rock, including granite wash- trend.” nodes of North America, the crater center has been es, the drop in potential re- For astrobleme anomalies, Eurasia, and Australia can be brecciated and fractured serves is less than 1%. This those chances could be much connected by a nearly circu- down to about 20,000 ft. Im- result suggests that the vol- higher than the 4% or less lar swath during the pact events forming craters ume of undiscovered base- success rate for conventional Phanerozoic, suggesting that of this diameter or larger ment hydrocarbons may be prospects in lower 48 frontier bolides are impacting limited could penetrate deeply into significantly higher than areas. As I stated over 15 areas. A possible explanation petroleum basins and affect known basement reserves. years ago,43 “suspicious grav- is that the orbital parameters basement rock. At first glance the practic- ity, seismic, and magnetic

78 Oil & Gas Journal • May 11, 1998 FOCUS: EXPLORATION

anomalies in basement in, below, above, and beyond 6. Donofrio, R.R., The challenges of THE AUTHOR exploration: impact craters, Ul- should be penetrated and the structure; and tramar Quarterly, No. 3, 1976, p. tested where drilling depth 4. Reservoirs that include 8. permits. These anomalies in- crystalline basement rock. Richard R. Donofrio is a 7. Lantz, R.J., 1981, Barrow gas consulting geologist for Parwest fields—North Slope, Alaska, clude elevated areas of base- The drilling success rates Land Exploration Inc., Okla- OGJ, Vol. 79, 1981, pp. 197-200. ment as well as synclines . . . in confirmed astroblemes for homa City. He was an interna- 8. Kirschner, C.E., Grantz, A., and Detection of astroblemes by new field wildcats and total tional E&D geologist with Mullen, M.W., Impact origin of geophysical or geologic wells are 21% and 77%, re- American Ultramar Ltd. prior the Avak structure, Arctic Alas- to forming Astro Geological Re- ka, and genesis of the Barrow methods means that frac- spectively. These rates ap- sources Inc., Ridgefield, Conn., gas fields, AAPG Bull., Vol. 76, tured reservoirs have been proximate the industry’s av- where he received a Small Busi- 1992, pp. 651-679. located . . . Unquestionably erage of 20% and 74% for the ness Innovative Research pro- 9. Carrigy, M., and Short, N. M., Ev- with some deep-basement same categories. gram award from the U.S. De- idence of shock metamorphism partment of Energy. He began in rocks from the Steen River impacts the capital expendi- The similarity in figures is research on tektites and mete- structure, Alberta, in French, tures will be considerable but not surprising considering orite impact craters at the Uni- B.M., and Short, N.M. (eds.), the possible rewards can be that all producing astrob- versity of Oklahoma and re- Shock metamorphism of natural enormous.” Clearly such lemes to date were found by ceived a PhD degree in geology. materials, Mono Book Corp., He is a research associate in pe- Baltimore, 1968, pp. 135-158. structures are of strategic im- accident with conventional troleum and astrogeology at the 10. Wilshire, H.G., Offield, T.W., 52 portance to the U.S. exploration models and Institute for Exploration and Howard, K.A., and Cummings, The Canadian Shield clus- practices. After hydrocar- Development Geosciences at D., Geology of the Sierra ter displacement exercise bons were discovered came OU. Madera cryptoexplosion struc- ture, Pecos County, Tex., U.S. (Fig. 3) suggests that the po- the realization that the struc- Geological Survey Professional tential for hydrocarbon re- tures had an impact origin. Paper 599-H, 1972, 42 p. serves in impact craters in To use astroblemes as an program. Recognition of a 11. Melzer, S., personal communica- basement rocks may be sig- exploration concept, geolo- feature such as a rim arc is a tion, 1995. 12. Petroleum Information Corp., nificant and that, like con- gists need to reverse the se- challenge to explorationists, Scout card for Texas Pacific Oil ventional oil and gas fields, quence. and the basinward flank of a Co. Inc. No. 6 Montgomery- most of the reserves will be About half the confirmed large-scale rim segment is Fulk, Sec. 87, Block A, Pecos found in relatively few large astroblemes and astrobleme one of the crater areas where County, West Texas, 1975. 13. Brenan, R.L., Peterson, B.L., and structures. anomalies in petroleum a giant field awaits discovery. Smith, H.J., The origin of the Thus far, the largest pro- provinces are commercial oil Red Wing Creek structure, ducing confirmed astrob- and gas fields and, on aver- Acknowledgments McKenzie County, N.D., leme in the U.S. is Ames with age, these structures re- I offer my appreciation to Wyoming Geological Associa- tion Earth Science Bulletin, Vol. a diameter of about 8 miles. quired about 2-4 wildcats to D. Groff for his helpful sug- 8, 1975, 41p. Nonproducing confirmed find the pay. The number of gestions and to K. Mauk for 14. Page, P., personal communica- impact craters larger than wildcats appears to reflect his computer assistance with tion, 1994. Ames have not yet been rec- the dimensions of included Table 1. 15. Sharpton, V.L., personal commu- nication, 1997. ognized in U.S. petroleum anomalies in this initial 16. Santiago, J., and Baro, A., Mexi- provinces. In U.S. areas out- study. Like confirmed as- References co’s giant fields, in Halbouty, side petroleum provinces, troblemes, the database for 1. Milstein, R., The Calvin impact M. T. (ed.), Giant oil and gas the largest confirmed impact astrobleme anomalies is crater, Cass County, Michigan: fields of the decade 1978-1988: identification and analysis of a AAPG Memoir 54, 1990, pp. 73- structure is the 54 mile diam- small and inconclusive. This subsurface Ordovician astrob- 99. eter Chesapeake Bay crater study was confined to North leme: Oregon State University 17. Rosenfeld, J., and Penfield, G., in Virginia. America, but these esoteric unpublished PhD dissertation, personal communication, 1995. Where are the other large- structures are productive 1994, 114 p. 18. Shoemaker, E.M., Association 2. Koeberl, C., Sharpton, V.L., Schu- round table: AAPG Bull., Vol. scale impact structures com- elsewhere in the world and raytz, B.C., Shirley, S.B., Blum, 81, 1997, p. 849. parable to those on the Cana- have similar implications for J.D., and Marin, L.E., Evidence 19. Gonzalez, E., written communi- dian Shield? giant field potential. for a meteoritic component in cation, 1995. The least explored hori- impact melt rocks from the 20. OGJ, Apr. 15, 1996, p. 81. Conclusions Chicxulub structure, Geochimi- 21. Robertson, G., personal commu- zon and final frontier for the ca et Cosmochimica Acta, Vol. nication, 1995. The drilling record to date petroleum geologist is crys- 58, 1994, pp. 1,679-84. 22. Kuhns, T., personal communica- shows that, although pro- talline basement. Two pro- 3. Sharpton, V.L., and Nielson, tion, 1998. ducing impact structures are ducing impact structures D.C., Is the Bee Bluff structure 23. Castano, J.R., Clement, J.H., in south Texas an impact Kuykendall, M.D., and Sharp- few in number, they have a have already proven that this crater?, in Ryder, G., and Sharp- ton, V.L., Source rock potential disproportionate share of lithology can be a viable ton, V.L. (eds.), Proceedings of of impact craters (abs.), AAPG significant characteristics. reservoir. Undiscovered the 19th Lunar and Planetary annual convention program, These include: basement (and sedimentary Science Conference, Cambridge 1994, p. 118. University Press, Cambridge, 24. Donofrio, R.R., Olsen, K.H., Vlier- 1. The ability to form (or rock) astroblemes certainly and the Lunar and Planetary In- boom, F.W., Witschard, F., and enhance) structure, reservoir exist and are capable of host- stitute, Houston, Vol. 19, 1988, Petersson, G., 1984, Deep gas, rock, and possibly source ing giant oil and gas fields. pp. 1,065-66. Swedish premises: the Siljan rock independent of the re- The known and potential 4. Forsman, N.F., Gerlach, T.R., and Ring project—independent ex- Anderson, N.L., Impact origin pert evaluation, Vattenfall, gional geology; dimensions of impact events of the Newporte structure, Stockholm, 1984, 63 p. 2. Exceptional reservoir need to be realized. Large- Williston basin, North Dakota: 25. Jansa, L.F., Pe-Piper, G., Robert- thickness, high yields, and scale impact structures (or AAPG Bull., Vol. 80, 1996, pp. son, P.B., and Friedenreich, O., flow rates; their remnants) approach lin- 721-730. Montagnais: a submarine im- 5. LeVie, D.S., Lyles Ranch field, pact structure on the Scotian 3. Production (and poten- earity relative to regional geo- south Texas: production from shelf, eastern Canada, GSA tial production) from numer- logic and geophysical cover- an astrobleme?, GCAGS Trans- Bull., Vol. 101, 1989, pp. 450- ous types of reservoirs with- age used in an exploration actions, Vol. 35, 1985, pp. 179- 463. 187.

80 Oil & Gas Journal • May 11, 1998 FOCUS: EXPLORATION

26. Robertson, P.B., and Butler, M.D., Ries Crater, Geologica Bavarica, The Kilmichael structure, Mis- Vol. 75, 1977, pp. 329-438. sissippi: evidence for a mete- 42. Koeberl, C., Fredrickson, K., orite impact origin, Journal of Gotzinger, M., and Reimold, Geology, Vol. 90, 1982, pp. 589- W.U., 1989, Anomalous quartz 601. from the Roter Kamm impact 27. Gay, S.P., Jr., “Haswell hole,” a crater, Namibia: evidence for previously unknown impact post-impact hydrothermal ac- structure in southeast Colorado, tivity?, Geochimica et Cos- in Johnson, K.S., and Campbell, mochimica Acta, Vol. 53, 1989, J.A. (eds.), Ames structure in pp. 2,113-18. northwest Oklahoma and simi- 43. Donofrio, R.R., Impact craters: lar features: origin and petrole- implications for basement hy- um production, 1995 Sympo- drocarbon production, Journal sium, Oklahoma Geological of Petroleum Geology, Vol. 3, Survey Circular 100, 1997, pp. 1981, pp. 279-302. 272-276. 44. Roberts, C., and Sandridge, B., 28. Isachsen, Y.W., Wright, S.F., and The Ames hole, Shale Shaker, Revetta, F.A., The Panther Vol. 42, No. 5, 1992, pp. 118-121. Mountain feature possibly 45. Winzer, S.R., 1972, The Steen hides a buried impact crater, River astrobleme, Alberta, Northeastern Geology, Vol. 16, Canada, 24th International Geo- 1994, pp. 123-136. logical Congress, Montreal, Sec. 29. Plawman, T.L., and Hagar, P.I., 14, 1972, pp. 148-156. Impact structure, in Bally, A.W. 46. Donofrio, R.R., Petroleum explo- (ed.), Seismic expression of ration strategies for impact fea- structural styles, AAPG Studies tures (abs.), AAPG annual con- in Geology Series No. 15, Vol. 1, vention program, 1994, p. 138. 1983, pp. 1.4.1-1.4.3 47. Cannon, J., The frequency and 30. Sawatsky, H.B., Buried impact distribution of terrestrial impact craters in the Williston basin craters (abs.), Society of Inde- and adjacent area, in Roddy, D. pendent Professional Earth Sci- J., Pepin, P.O., and Merrill, R.B. entists annual convention, 1995, (eds.), Impact and explosion p. 9. cratering, Pergamon Press, New 48. Shaw, H.R., Craters, cosmos, and York, 1977, pp. 461-480. chronicles: a new theory of 31. Byler, W.H., Evidence of large Earth, Stanford University horizontal earth movements, in Press, Stanford, California, Mason, K. (ed.), Basement tec- 1994, 688 p. tonics 7: Kluwer Academic Pub- 49. Grieve, R.A.F., and Robertson, lishers, Dordrecht, Netherlands, P.B., Terrestrial impact struc- 1992, pp. 33-48. tures (Geological Survey of 32. Grieve, R.A.F., and Masaitis, V.L., Canada map), in AAPG Explor- The economic potential of ter- er, April 1989, p. 16. restrial impact craters, Interna- 50. Pohl, J., Stoffler, D., Gall, H., and tional Geology Review, Vol. 36, Ernston, K., 1977, The Ries im- 1994, pp. 105-151. pact crater, in Roddy, D.J., 33. Grieve, R.A.F., Rupert, J., Smith, Pepin, P.O., and Merrill, R.B. J., and Therriault, A., The record (eds.), Impact and explosion of terrestrial impact cratering: cratering, Pergamon Press, New GSA Today, Vol. 5, 1995, pp. York, 1977, pp. 343-404. 189-196. 51. Hobson, G.D., and Tiratsoo, E.N., 34. Koeberl, C., and Anderson, R.R., Introduction to petroleum geol- Manson and company: impact ogy, Scientific Press Ltd., Bea- structures in the United States, consfield, England, 1981, 352 p. in Koeberl, C., and Anderson, 52. Donofrio, R.R., The strategic im- R.R. (eds.), The Manson impact portance of detecting subsur- structure, Iowa: anatomy of an face hydrocarbon-bearing as- impact crater: GSA Special troblemes (abs.), U.S. Depart- Paper 302, 1996, 476 p. ment of Energy Small Business 35. Butler, M.D., 1996, the MI Theory: Innovative Research Program, a reply and further comments: Abs. of Phase I Awards, DOE- The Leading Edge, Vol. 15, ER-0181, 1983, p. 18. 1996, pp. 383-391. 52. Van Der Loop, M., Is there a re- 36. Petroleum Information Corp., source base in lower 48 fron- 1995, Resume 1994, Sec. 2, p. 8. tiers?, Houston Geological Soci- 37. OGJ, July 19, 1993, p. 62. ety Bull., Vol. 38, 1996, pp. 18- 38. Hulen, J.B., Bereskin, S., and 21. Bortz, L.C., High-temperature hydrothermal origin for frac- Other references tured carbonate reservoirs in King, P.B., and Edmonston, G.J., Gen- the Blackburn oil field, Nevada, eralized tectonic map of North AAPG Bull., Vol. 74, 1990, pp. America, U.S. Geological Sur- 1,262-72. vey Map I-688, 1972, scale 39. Newsom, H.E., Hydrothermal al- 1:5,000,000, 1 sheet. teration of impact melt sheets Suppe, J., Principles of structural ge- with implications for Mars, ology, Prentice-Hall, Engle- Icarus, Vol. 44, 1980, pp. 207- wood Cliffs, N.J., 1986, 535 p. 216. Wong, A.M., The subsurface charac- 40. Moore, D.E., Lockner, D.A., and ter of Marquez dome impact Byerlee, J.D., Reduction of per- structure, Texas, as determined meability in granite at elevated by well log and gravity analysis, temperatures, Science, Vol. 265, University of Houston unpub- 1994, pp. 1,558-61. lished MS thesis, 1994, 103 p. 41. Pohl, J., Paleomagnetics of the

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