(NTS 092N, O, 093B, C, F, G)

Transcription

1 Preliminary Images of the Conductivity Structure of the Nechako Basin, South-Central British Columbia (NTS 092N, O, 093B, C, F, G) from the Magnetotelluric Method J.E. Spratt, Geoscience BC, Vancouver, BC and Geological Survey of Canada, Ottawa, ON, J.A. Craven, Geological Survey of Canada, Ottawa, ON Spratt, J.E. and Cra ven, J.A. (2009): Pre lim i nary im ages of the con duc tiv ity struc ture of the Nechako Ba sin, south-cen tral Brit ish Co lum - bia (NTS 092N, O, 093B, C, F, G) from magnetotelluric meth ods; in Geoscience BC Sum mary of Ac tiv i ties 2008, Geoscience BC, Re port , p Introduction The rapid spread and de struc tive ef fects of the moun tain pine bee tle (MPB) in ar eas of Brit ish Co lum bia have prompted the need to de velop eco nomic di ver si fi ca tion op - por tu ni ties for the in te rior of the prov ince. As a re sult, Geoscience BC has ini ti ated a num ber of pro jects with the aim of as sess ing the min eral and pe tro leum po ten tial of the af fected re gion. Lim ited ex plo ra tion to date has in di cated some po ten tial for oil and gas res er voirs within the in te rior bas ins of cen tral BC, in clud ing the Nechako sed i men tary ba sin (Hannigan et al., 1994; Hamblin, 2008). In or der to gain a better un der stand ing of the po ten tial for hy dro car bon re sources in the re gion, a magnetotelluric sur vey, de signed to eval u ate the use ful ness of the method in oil and gas ex - plo ra tion and to char ac ter ize the con duc tiv ity struc ture of the Nechako Ba sin, was con ducted in the fall of 2007 (Spratt et al., 2008; Spratt and Craven, 2008; Figure 1). The Nechako Ba sin is an Up per Cre ta ceous to Oligocene sed i men tary ba sin lo cated in the Intermontane Belt of the Ca na dian Cor dil lera (Fig ure 1). The ba sin formed in re - sponse to terrane amal gam ation to the west ern edge of an - ces tral North Amer ica, and con sists of over lap ping sed i - men tary se quences (Mon ger et al., 1972; Mon ger and Price, 1979; Mon ger et al., 1982; Gabrielse and Yorath, 1991). Un der ly ing the Nechako sed i men tary rocks are the Stikine and Quesnel vol ca nic arc ter ranes, sep a rated by the oce anic Cache Creek Terrane (Struik and Mac In tyre, 2001). Transpressional tec tonic pro cesses were dom i nant un til the Eocene, with west ward-di rected thrust ing be - tween the Stikine and Cache Creek ter ranes prior to 165 Ma (Best, 2004). Re gional transcurrent fault ing and as so ci ated east-west ex ten sion, be gin ning in the Late Cre ta ceous, were ac com pa nied by the extrusion of basaltic lava in Eocene and Miocene times. Keywords: Nechako Ba sin, hy dro car bon ex plo ra tion, magnetotelluric, elec tro mag netic, con duc tiv ity struc ture This pub li ca tion is also avail able, free of charge, as col our dig i tal files in Adobe Ac ro bat PDF for mat from the Geoscience BC website: Ba saltic flows of the Neo gene Chil cotin Group, vol ca nic rocks of the Eocene Endako and Ootsa Lake groups, and Pleis to cene gla cial de pos its cover a large por tion of the Nechako Ba sin, com pli cat ing the in ter pre ta tion of mod ern subsurface im ag ing meth ods. As a re sult, much of the stra - tig ra phy and struc ture of the un der ly ing sed i men tary rocks re mains un cer tain. It has been sug gested that the Chil cotin Group can reach a thick ness of ~200 m and av er ages ~100 m (e.g., Mathews, 1989); how ever, new stud ies sug - gest that it is com par a tively thin (<50 m) across most of its dis tri bu tion and only thick (>100 m) in paleochannels (An - drews and Rus sell, 2008). The pres ence of the sur face ba - saltic flows and Ter tiary vol ca nic rocks cov er ing most of the re gion has, to date, pre vented uni form and con sis tent seis mic-en ergy pen e tra tion and has com pli cated the mag - netic in ter pre ta tions. It has been shown that the magnetotelluric (MT) method can be use ful in re solv ing geo log i cal struc tures that are less fa vour able for char ac ter - iza tion by seis mic meth ods (Unsworth, 2005; Spratt et al., 2007). As the method is sen si tive to but not im peded by the sur face vol ca nic rocks and can de tect vari a tions within the dif fer ent units, it can be useful in locating the boundaries of the Nechako Basin and defining its internal structure. Methodology The magnetotelluric (MT) method mea sures the nat u ral time-vary ing elec tri cal and mag netic fields at the sur face of the Earth to pro vide in for ma tion on the elec tri cal con duc - tiv ity of its subsurface (Cagniard, 1953; Wait, 1962; Jones, 1992). Sig nal for MT is gen er ated from in ter ac tions be - tween so lar winds and the ion o sphere at low fre quen cies, and from dis tant light ning storms at higher fre quen cies. The MT re sponse curves (phase lags and ap par ent resistivities) are cal cu lated from the mea sured fields at var - i ous fre quen cies for each site re corded. As lower fre quen - cies pen e trate deeper through re sis tive ma te ri als, an es ti - mate of con duc tiv ity vari a tion with depth can be made from the response curves beneath each site. Where the Earth is elec tri cally two-di men sional (2-D), the con duc tiv ity var ies lat er ally along a pro file and with depth. Geoscience BC Re port

2 Fig ure 1. Re gional ge ol ogy of the study area (Rid dell, 2006), show ing the lo ca tion of the magnetotelluric sta tions and the bore - hole wells. 176 Geoscience BC Sum mary of Ac tiv i ties 2008

3 In this case, ap par ent resistivities and phases dif fer along strike com pared to those in the per pen dic u lar di rec tion, and some form of directionality anal y sis is re quired to de ter - mine the pre ferred geoelectric strike di rec tion. The trans - verse-elec tric (TE) mode re fers to the along-strike di rec - tion and the trans verse-mag netic (TM) mode is perpendicular to strike. The MT method is sen si tive to con trasts in the re sis tiv ity of jux ta posed ma te ri als and can there fore dis tin guish be tween some lithological units. Ba salt and ig ne ous rocks, for ex - am ple, com monly have elec tri cal re sis tiv ity val ues >1000 ohm-m (Ù-m), whereas sed i men tary rocks are less re sis tive, with val ues rang ing be tween 1 and 1000 Ù-m. In the crust, other fac tors that can greatly in flu ence the over all con duc tiv ity of a spe cific unit in clude the pres ence of sa line flu ids, changes in po ros ity and the pres ence of graph ite films or in ter con nected me tal lic ores (Haak and Hutton, 1986; Jones, 1992). In ad di tion to de fin ing struc ture, the MT method may be able to pro vide some es ti mate of bulk prop er ties, such as po ros ity and per cent sa lin ity, that may give direct evidence for the presence of hydrocarbons (Unsworth, 2005). Data and Analysis Com bined high-fre quency au dio-magnetotelluric (AMT) and broad band (BBMT) data were col lected at a to tal of 734 sites through the south ern part of the Nechako Ba sin in the fall of 2007 (Fig ure 1). The data were ac quired by Geosystem Can ada us ing MTU-5A re cord ing in stru ments man u fac tured by Phoe nix Geo phys ics Ltd. of To ronto. The data were pro cessed by Geosystem us ing ro bust re mote ref - er ence tech niques re sult ing, in gen eral, in ex cel lent data qual ity cov er ing a pe riod range of nearly seven de cades ( sec onds). The dataset was di vided into eight sep a rate pro files for sub se quent data analysis and 2-D modelling (Figure 1). Sin gle-site and multisite Groom-Bailey de com po si tions were ap plied to each of the MT sites along pro files A, B, C, D and F, in or der to de ter mine the most ac cu rate geoelectric strike di rec tion and to an a lyze the data for dis tor tion ef fects (Groom and Bailey, 1989; McNeice and Jones, 2001). Fig - ure 2 il lus trates the re sults of sin gle-site strike anal y sis for each de cade pe riod band re corded at each site along the pro files. Nearly all of the sites show a max i mum phase dif - fer ence be tween the two modes of less than 10 at pe ri ods be low 0.1 sec ond (s), in di cat ing that the data are in de pend - ent of the geoelectric strike an gle and can be con sid ered 1- D. The max i mum phase splits are ob served be tween 0.1 and 10 s, where small changes in the se lected strike an - gle will most affect the data and associated errors. Pro files A and C are sim i lar, with strikes of 5 10 at the wes tern most edge of the pro files chang ing to a strike of ~35 to wards the east. Pro file B shows only mod er ate phase splits at only a few sites, sug gest ing that the ma jor ity of the data are 1-D. Pro file D shows much stron ger phase splits, with a pre ferred strike an gle of ~32 for most sites to pe ri - ods of 100 s; how ever, the mis fit val ues for the de com posed data, even when no con straints were placed on the data, were sig nif i cantly high. This is a strong in di ca tion of 3-D dis tor tion ef fects, so 2-D mod els may not ac cu rately rep re - sent the data. Re sults along pro file F in di cate strong phase dif fer ences at pe ri ods greater than 1 s with a fairly con sis - tent strike an gle of In gen eral, there is a roughly north east to south west trend ob served in the vari a tions in the geoelectric strike an gle. These changes may have re - sulted from dif fer ent tec tonic pulses, where the stress directions are preserved in the conductivity structure. Data Modelling and Preliminary Results One-Dimensional Models One-di men sional mod els have been gen er ated for all of the MT sites within the Nechako Ba sin. Lay ered Earth mod els were de rived from Occam in ver sions us ing the WingLink TM in ter pre ta tion soft ware pack age. Dimensionality and depth anal y sis in di cate that, in gen eral, the data can be con sid ered 1-D up to pe ri ods be tween 0.1 and 1 s, cor re spond ing to depths be low m. Fig ure 3 shows the re sults of the 1-D mod els for vary ing depth slices in the MT sur vey re gion. In gen eral, among the north ern set of sites, there ap pears to be a north east to south west trend in con duc tiv ity struc ture, con sis tent with the re sults from the de com po si tion anal y sis. How ever, more com plex struc ture is re vealed in the south ern set of sites, and ad di tional dimensionality anal y sis is nec es sary. At 50 m depth, the blue re gion at the wes tern most ex tent of the sur vey area most likely rep re sents the re sis tive vol ca nic cover. Con sis tent with the re sults from An drews and Rus - sell (2008), the lim ited lat eral ex tent of this re sis tor sug - gests that the vol ca nic cover is ei ther thin ner than 50 m or not as wide spread as ini tially pre sumed. There is a change in con duc tiv ity from ~200 to >700 Ù-m in the east ern half of the sur vey area be tween depths of 1000 and 2000 m. This likely rep re sents the change from con duc tive sed i men tary rocks to the un der ly ing re sis tive base ment units; how ever, this change is not ob served through the en tire re gion, in di - cat ing that the sed i men tary pack ages are thicker to wards the east ern edge of the Nechako Ba sin. An an oma lously con duc tive zone (<15 Ù-m) is ob served in the cen tre of the MT sur vey re gion at a depth of 500 m, along pro file D, within the sed i men tary units and appears to dip towards the east. These dramatic changes may be related to salinity of groundwater or changes in porosity. Two-Dimensional Modelling Two-di men sional mod els have been gen er ated along pro - files A, B, C, D and F us ing the WingLink in ter pre ta tion Geoscience BC Re port

4 Fig ure 2. Pre ferred geoelectric strike an gle at each de cade pe riod band re corded for each of the MT sites within in the Nechako Ba sin. The colours il lus trate the max i mum phase dif fer ence be tween the TM and TE modes, where the warmer colours rep re sent a higher phase split. 178 Geoscience BC Sum mary of Ac tiv i ties 2008

5 Fig ure 3. Re sults of one-di men sional mod el ling of all MT sites within the Nechako Ba sin at var i ous depths within the Earth. The warm colours rep re sent ar eas of high con duc tiv ity and the cooler colours rep re sent re sis tive re gions. Geoscience BC Re port

6 soft ware pack age (Fig ure 4). More than 100 it er a tions were ex e cuted and in cluded data from the TE mode, the TM mode and the ver ti cal field trans fer func tion, in the pe riod be tween and 1000 s. All of the mod els re veal a de - crease in con duc tiv ity at depths rang ing from 1000 to 3000 m, cor re spond ing to ap prox i mate depth es ti mates for the thick ness of the Nechako sed i men tary pack ages. This in di cates that the MT data are sen si tive to the base of the Nechako Ba sin and can delineate its structural boundaries. In ad di tion, along-pro file vari a tions in the con duc tiv ity struc ture are re vealed. Fea ture A (Fig ure 4b, d) is an an - oma lously con duc tive re gion (>10 Ù-m) that lies within the sed i men tary units. This fea ture is ob served at the north ern end of pro file B and along the west-cen tral part of pro file D, con sis tent with the north east to south west trend ob served in the strike anal y sis. Causes for sig nif i cantly high con duc tiv - ity may in clude the pres ence of sa line flu ids, graph ite sheets or sulphides, or may re sult from a sig nif i cant in - crease in the rel a tive po ros ity of the sed i men tary rocks. Fea ture B is an an oma lously re sis tive unit lo cated at shal - low depths along pro file C (Fig ure 4c). This fea ture cor re - lates spa tially with the mapped ex po sure of the vol ca nicarc as sem blages of the Hazelton Group of the Stikine Fig ure 4. Cross-sec tions il lus trat ing the two-di men sional mod els gen er ated along pro files A, B, C and D. The lo cal ge ol ogy is shown above each pro file and the dif fer ent units are de scribed in the leg end for Fig ure 1. The red squares mark the sec tions where de tailed fo cused in ver - sions have been gen er ated and shown in Fig ure 5. The black lines il lus trate struc tural bound aries in the lat eral con ti nu ity of the shal low con - duc tive layer. 180 Geoscience BC Sum mary of Ac tiv i ties 2008

7 Terrane. Ad di tional mod el ling is re quired to con - strain the struc tural na ture of this fea ture. The east ern most ex tent of pro file D il lus trates ma jor struc ture at mid-crustal depths, la belled as fea - ture C. These struc tures may be re lated to the lat - eral fault-bounded east ern ex tent of the Nechako Ba sin, and may be an in di ca tion for north-north - west-strik ing subvertical splays of the Fra ser fault. Sev eral breaks in the con ti nu ity of the up per con duc tor are ob served along many of the pro files (Fig ure 4a, c, d). These may rep re sent faulting that juxtaposes resistive material from deeper regions against the conductive sedimentary rocks. Pre lim i nary 2-D mod el ling has been ini ti ated along pro file F (not shown). Al though ad di tional test ing is re quired to as sess the va lid ity of the model, there is a gen eral deep en ing of the re sis tive lower layer from north east to south west. This is con sis tent with in ter pre ta tion of the seis mic data, which sug gests a thick en ing of the Cre ta ceous sed i men tary rocks to wards the south west (Hayward and Calvert, 2008). Pro files A, B, and D were di vided into shorter seg - ments, and each seg ment was mod elled sep a rately to ob tain higher res o lu tion of the shal low struc ture be neath the pro file and to gen er ate a model that rea son ably fits all the data. This is achieved by al - low ing a denser mesh and by re duc ing weighted av er ag ing be tween sites by fit ting the model to a smaller dataset. These fo cused in ver sions en able the res o lu tion of en hanced de tail, such as im ag ing the surficial vol ca nic rocks where they are thicker than ~50 m (Fig ure 5a), and re veal spe cific struc - ture that im poses con straints on the lat eral con ti - nu ity of the con duc tive sed i men tary rocks (Fig - ure 5b, d). In ad di tion, they al low for a com par i son be tween the mod els and geo log i cal ob ser va tions in the bore holes (Fig ure 5b). They also re sult in mod els that have a better fit to the data, reducing inaccurate modelling effects (Figure 5c). Conclusions and Future Work Con tin ued anal y sis of magnetotelluric data col - lected through out the Nechako Ba sin has, to date, yielded one-di men sional mod els for the en tire dataset and two-di men sional con duc tiv ity mod els along four of the seven ma jor pro files. A con duc - tiv ity con trast be tween the sur face vol ca nic rocks, the Nechako sed i men tary rocks and the un der ly - ing base ment rocks is ob served, in di cat ing that the method is ca pa ble of im ag ing the struc ture of the ba sin. The two-di men sional mod els im age the sur - face vol ca nic rocks in iso lated lo ca tions, sug gest - Fig ure 5. Ex am ples of the de tail ob tained with fo cused two-di men sional in ver - sions along sec tions of the main pro files. The red line marks the bound ary be - tween the con duc tive up per unit and a more re sis tive un der ly ing layer, which is in ter preted as the base of the Nechako sed i men tary rocks. The black line in - di cates the base of the sur face bas alts. Wells d-96-a and a-4-l (from Ferri and Rid dell, 2006) are shown in (b). Geoscience BC Re port

8 ing that they are ei ther too thin to be de tected us ing au diomagnetotelluric meth ods or they are not as wide spread as ini tially thought. The thick ness of the sed i men tary pack - ages var ies greatly along the dif fer ent pro files, and struc - tural con straints are placed on the lat eral con ti nu ity of the con duc tive sed i men tary rocks. Along-strike vari a tions in con duc tiv ity within the sed i men tary pack ages are ob - served, sug gest ing changes in min er al ogy, po ros ity or sa - lin ity. Char ac ter iz ing these changes will be important in assessing the potential for oil and gas within the region. Acknowledgments The au thors are grate ful to the Geo log i cal Sur vey of Can - ada, Geoscience BC and the BC Min is try of En ergy, Mines and Pe tro leum Re sources for pro vid ing fi nan cial sup port for this re search. They also thank M. Pilkington for prov ing in sight ful com ments and a con struc tive review of this paper. 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