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.