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3D Seismic Expression of a Cryptoexplosion Structure CANADUN JOURNAL OF EXPLORATION GEOPH”SICS “OLD 29. Pm 2 ,tECEMBER 19931, P 429~439 3-D SEISMIC EXPRESSION OF A CRYPTOEXPLOSION STRUCTURE’ J. HELEN ISAAC~AND ROEERTR. SSEWART~ Mclosh, 1989: Sharpton and Ward, 1990). Impact craters can AWTRACT he termed either simple or complex, the main difference being the presence of multiple ring structures and central An enigmaticcircular itr”Ct”ce is observedon three-*immsionai uplift in a complex crater. The morphological change takes (3-D, ,eimlic d&mfrom Jilmcs River. Alberta It hasan uuter diammr uf 4.8 km anda raisedcentral uplift mrruundrdby a ring ,ynform. place at an excavation cavity diameter of ahout 2 km in sedi- The Centraluplift hasB diameter“1 2.4 km and if6 crestappears to mentary rocks and 4 km in crystalline rocks (Dence. 1972). he aho”, 400 m abovercgkml levels.The top of ihe StmClllr~ii ill B The principal feature of a complex impact crater is a central depth“f aho”, 4500,” and is belowtile LOW“f previousrc”n”“lic interest.Consequently. the featurehas ll”L beenpcnctrated by any peak, or group of peaks. surrounded by a llat tloor inside a wc,,s in the surveyarea. me seismic&lra inteqmation indicatr, terraced rim (Dencr, 1965). Complex crater central peaks are that the dimrbed scdimmtrare Cambrian in age.tt ii cstimilledIhat composed of def(mned and fractured rocks which arc uftcn the structure was formed during the Late CambrianIu Middle older than the country rock surrounding the structure. Partial Devonianlime petid andsuffered SeYcre W”bi,,” Mrw ,llCdrpositim “1 ,hCwcrlying Middle andUpper “ev”“ian carbonares.Rim fml,5, collapse of the central peak may he the source of some of the prohahiycaused by slumpingOf mmria, iill the dqmsion. are hreccia filling the surrounding rim nynform. observrd “” rhc ““tel slope of the synforrn. thversc faults are Without geological evidence it is difficult to establish the rrvealed by the seismicdata undernrathIhe feuture.The crnlral uplift appears1” havecoherent interm, rr,,rc,ionr andthe .illl”““l ot origin of a structure observed on seismic data alone. since uplift is *em to decreasewith increasingdepth. The whulr featurehas similarly shaped features can result from different causes. ,ile “wphologicalcharacteristics “fii ,“CIe”n,empact hlruclure. However, observed morphological and tectonic features such as central uplift. changes in structural uplift with depth or fault patterns. may provide compelling evidence for the origin of the structure. INTRODUCTlON Several enigmatic circular structures have been described Economic aspects of impact craters from observations on seismic data from the Western Canadian Mineral deposits such as metals and hydrocarbons are Sedimentary Basin (WCSB) (Sawatrky, 1976: Isaac and associated with about twenty percent of all known impact Stewart, 1993) and other parts of Canada (Scott and Hajnal, craters and in wme cases the economic implications are sig- 1988; Jensa et al., 1989). These structures are often imaged nificant (Masaytis, lY89). In some structures, interpreted to well on seismic data. Cryptoexplosion structures have a char- be impact craters, commercial hydrocarbon accumulations acteristic circular tu polygonal morphological outline and have been found (Table I: Sawatzky, 1972; Brenan et al., display evidence of violent disruption during their formation 1975: Cnrpenter and Carlson, 1992). The significance for the hut lack evidence of volcanic material to confirm a volcanic petroleum industry of impact craters as hydrocarbon-hearing origin. Cryptoexplosion structures can result from meteorite was discussed in detail by Donofrio (198 I ). impact or diatreme intrusion (Nicolaysen and Ferguson, The geophysical responses of impact structures were sum- 1990). Diatreme structures are thought to he caused by marised by Pilkington and Gricvc (1992). Several probable abrupt and violent exsolution of deep gasses and fluids hut impact craters have heen imaged with seismic data, use of arc nor well documented, geophysically. In contrast, the which allows good estimates of a structure’s size and age. 3- mechanics and morphology of impect craters have been studied D seismic data, in particular, afford three-dimensional and documented in great detail (e.g.. Roddy cf al., 1977: images unohtainahle by other methods. 1Presented at the 19th Annual National Mming. Canadian Ocophysical Union. Ranff, Alhena. May 9, 1991. Manuscript received by the Editor Augusl 10, IO%: ,CYi\Cd lnanurcnpt received November 1”. 1993. 2Depannient of Geology and Geophysics. The University of Calgary. Caigary. Alberta T2N IN4 The authon would like 10 ackn”wlrdfe Husky Oil “perations Ltd.. Calgary, for their generous donation Of the 3~D *risrnic &?,a, horehole (lilta and “SC Of computer facilities. In panicular, we would like to thank Mr. Murray Watt\. Mr. Lawrence Mewho* and Mr. David Emery of Husky Oil. Mr. Raymon*, Probat and Mr. Dean Simon. whu prcviouly interprcW* this intrresring feature on the 3~D data. shared thysir knuwlcdge with us. Vfe also thayk Ms. Susan Miller of the l?UWES Prujecr a! The University 01 Calgary fur data managemenr. The several anonymous rerlrrers are thanked for lhelr Eunst~UCtIve ideas. Tills WOrk was pantally Sup- ported hy the CREWES Project. CEC 429 DrcEmher,991 J.H. lSAAC imt, R,R. STFNAKT Table 1. Tabulation of some commercialhydrocarbon accwn~lations associated with probableimpact craters. ‘iam 4ce Hydrucsrbuns * Jiewfield, SasknKhewar ,4 rriassic/ Commercinl oil field discovered in Iumssic ,968. Production of up 1” 6S &id (400 hhlid) from Mississippian carbonate hrecci:, and In-silu Mississippian in the ruised rim Pay thicknesses from 4 m to SO “1. Estimzed reserves: 3.2 x 10” mB120 MMhbl) recoverable. 0 = 14% k = 400 md. ted Wing Creek, Nortt :1 friassici Commercial oil field discwercd in Iakota limssic lY7?. X70 m of pay in Mississippian carhonxtc hrcccia rrom u 1.6 km diameter arra within ihc 6.5 km diameter centml Uplifl. Kcscrvoir rocks arc stesply dipping and intensely faulted. Estimated rescues: 6.4 x IOh- I x IO” nG(40 ~70MMhhl) ---.-rccwcrahlc. hvpone, North D;tkou , :nd Oil shows fwnd in I977 ill Camhro Zamhrian Ordovician sands draped over lhc raised rim. Some production from highly-fracrured Prrcxnhri;in gncisi- -- schist. \mes, Oklahoma Lower Oil and gas production from dolomite 3rdovician on crater rim and from hrecciated granite and dolornitc on crater floor. Estimatedpotential T~SCIYCS: ...-“-. - “-over7 x 106n+ (SOMMhhl). .“..“- iteen River. Alherr:! 2 pre Lntc Precambrian hasemcn, complex C~~IX~““~ uplifted 7h0 m above regional levels. Producing 95 m’id (fiXI bhlid) oil. - Also gas presentin the Slave Point. Within the WCSB there xc two widely known str”ct”res impact could have caused the breakup of Gondwanaland and believed to be impact crz~lers: Steen River (Winzer, 1972) demonstrates the geological importance of this subject. and Eagle Butte (Sawat/ky, lY76: Lawton et al.. 1993). I” addition, several other str”ct~res in Alberta can be identified GEOI.OGICAI. SETTING on proprietary seismic data, including thawat James River. A 45.km’ 3-D seismic survey (Figure I) was acquired in The subject of impact cratering has significance beyond the James River area of Albert” (Twp. 34, Rge. 7. W5) in current economic implications. The well-know” extinctions 19X6 for Canterra Energy Ltd. (now Husky Oil Operations at a “umber of geological boundaries (e.g., Permian/Triassic. Ltd.). The survey was designed to image beneath the Cremceous~ertiary) may be the result of meteorite impacts disturbed Cretaceous section and to delineate an Upper (Alvarez cf al., 1980; Lampto”, 1986; Raup. 1991). The Devonian Lcduc carbonate structure observed on 2-D meteorite impact theory for mass extinction has rccrntly seismic data. The Leduc structure is at a depth of about gained momentum wilh the possible impact c~~tcr discovered -2800 m subsea (4050 m below ground level). The main at Chicxulub, Mexico (Hildebrand et al., 1991). A proposal hydrocarbon reservoir at James River is the Upper by Ohrrbeck et al. (1993) suggests that a giant asteroid Cretaceous Cardium Formation with the Leduc Formation ClMi 430 3-D SElSMlC EXPRESSl”N OF A CKYl”lOtXPLOSrON 2 2 PASKAPOO \ 1 mite - \ 16irn \ \ EDMONTON GP 3~0seismic ILlrw wme 0 seismicEeCIions A-A Tl”x slice 0 z Cryptoerpms~“”s!wuct”rB I \ 6 9 5 tj $ Ek Fig. 1. Outline 0, James River 3-D seismic survey with Iocations 0f BELLY RIVER seismic iines and wells. -f2 2 2 =- LEA PARK t; being an additional target. In this area, structural and strati- COLORADO GP graphic traps exist and major reserves of oil and sour gas 5 have been found nearby (e.g., Caroline. Harmattanj. A 2500 CARDIUL generalised stratigraphic chart for this part of southwestern Alberta is shown in Figure 2. Most of the wells in the area covered by the 3-D survey are shallow Cardium tests but three were drilled deeper. Mobil et al. James 8.14.34.07WS was drilled in thr project 3000 a: area in 1969 and penetrated 14 m of the Middlr Devonian MANNVILLE GP Elk Poinr Formation before T.D. at -3017 m subsea (4240 m 8 log depth). Drillstem testing of the Ireton-Elk Point interval produced gas to surface from the Leduc Formation, which MlSSlSSlPPlbN- was subsequently cored. The gas discovery was not commer- 3500 BANFF cial. Canadian Hunter et al. Ricinus 8-19.34.07W5 was 2 drilled in IYX2 10 test the Leduc Formation. It penetrated 2Y WABAMUN m of the Elk Point Formation before T.D. at -3164 m subsea (4444 m log depth). The 8-l’) well production tested gas WINTERBURN GP 4000 from the Mississippian Pekisko Formation and oil from the WTO Cardium Formation but was plugged and abandoned. Husky WOODBEND GP Caroline III-lh-34-07WS was drilled in 1990 to lest a strut- ture interpreted to be a Leduc reef. The well drilled 28 m into the Swan Hills (Beaverhill Lake) Formation before T.D.
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