Seismic and Resistivity Imaging for Quaternary Channels, Rainbow Lake Northwest Alberta, Canada Jawwad Ahmad and Douglas R

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Seismic and Resistivity Imaging for Quaternary Channels, Rainbow Lake Northwest Alberta, Canada Jawwad Ahmad and Douglas R. Schmitt Institute for Geophysical Research, Department of Physics, University of Alberta, Edmonton, Alberta, Canada Introduction To better understand the buried channels, the University of Alberta (U of A) in conjunction with Geological Survey of Buried channels are former valleys that were filled in as a Canada (GSC) and the Alberta Geological Survey (AGS) initi- result of recent glaciation. Such features contain high ated a project to acquire high resolution seismic data close to porosity sands and gravels often saturated with water or existing shallow Quaternary gas wells near between Rainbow even natural gas, both of which are important resources. Lake and High Level, Northern Alberta (Figure 1). The objec- With regards to gas production, although the reservoirs tives of the seismic survey were to: 1) to image the surface of found to date are quite shallow (50-100 m), these deposits the Sub-Cretaceous unconformity upon which a maximum of have produced as much as 4-5 Bcf gas per well in as little as about 300 meter of Tertiary and Quaternary sediments are 4-5 years. Obviously the capital costs associated with deposited; 2) to image shallow Quaternary or Tertiary chan- producing such near surface deposits are small, and as a nels resting on and above unconformity; and 3) to create a result they have attracted a good deal of attention (e.g. public geoscience information database. An electrical resis- Pawlowicz, 2005) with much exploration for this resource tivity tomography (ERT) profile was also recorded along the carried out using DC electrical resistivity tomography same in order to delineate Quaternary Channels and find (Bauman, 2005). It must also be noted that such shallow and highly resistive anomalies which can be used to detect gas overpressured gas deposits are a drilling hazard. As recently and fresh water saturated zones. as this last winter two drilling rigs were destroyed by shallow blowout and fire in; 1) Northern Alberta (Safety Bulletin, Our initial results have 2005); and 2) Northeastern British Columbia. recently been pre s e n t e d (Ahmad et al., 2005a, ACQUISITION PARAMETERS 2005b) and more detailed reports and a Seismic M.Sc. thesis on the S o u r c e geology and geophys- Energy Source Mini-Vibroseis(6000 Ib) peak force ical results on this work Sweep Frequency 20-250 Hz. a re forthcoming. In Sweep Length 7 sec. this brief contribution, No. of Sweeps 5-8 Sweeps we focus only on some Sweep Type Linear of the unique aspects of Source Point Interval 24 meter the seismic and electrical data acquisition Instrument Pa r a m e t e r s and show portions of Receding Instrument Geometrics (Geode); Multiple the results. F i g u re 1. Map showing location of study are a Geode operating system (MGOS) Recording Format SEG-2 Sample Rate 0.5 msec. Record Length 1.19 sec No. of Channels 192-240

C a b l e Spread Geometry Split Spread and end on Group Interval 4 meter Geophone Frequency 40 Hz Fold ~40 No. of Geophone per group Single

Electrical Resistivity To m o g r a p hy (ERT ) Instrument ABEM, Terrameter SAS 1000 Electrode Spacing 15 meter and 30 meter Booster No F i g u re 2. Main panel: Geode Box (Analog to digital (AD) converter)) along Table 1. Seismic and Electrical resistivity Tomography with battery for power supply. Top left: Mini-Vi b roseis generating sweep. To p ( E RT) acquisition Parameters. c e n t re: 40 Hz single geophone planted in the field. This paper was presented as an oral presentation at the 2005 CSEG Convention, and adjudged as an ‘Honourable Mention’ item in the student category. Continued on Page 41

40 CSEG RECORDER September 2005 Article Cont’d Seismic and Resistivity Imaging for Quaternary... Continued from Page 40 Acquisition such areas the loss of bandwidth is more than compensated by the boost in the relative strength of the near surface reflections. The The field area was selected on the basis of analysis of existing well g round roll was further reduced by using a vibrator sweep of 20 logs that suggested the existence of a deep channel. The availability to 250 Hz (Table 1). The U of Alberta high frequency minivibe unit of proximate ERT soundings and earlier seismic cut lines were also was employed in this survey (Figure 2). taken into consideration when designing the program. As the goal of the survey was to image the stru c t u re above the pre - C re t a c e o u s High spatial resolution was maintained by using geophone unconformity that lies at depths of approximately 300 m and as singles at a 4-m station spacing. Shot spacing of 24-m (6 stations) such a non-standard acquisition program employing both high was a compromise between the maximum number of shots (up spatial group spacing and high frequencies was employed. to 150) that could be carried out in during daylight hours and the desire to maximize mid-point fold. An average fold of 40 was G round roll is always a problem with land seismic acquisition but obtained for the survey. can be severely so for near surface imaging because the weak reflections are overwhelmed by the surface waves. Often, much of The seismic survey was acquired in late March, 2004. Conditions the energy of the ground roll is contained at lower frequencies; for for acquisition at this time were ideal for the vibrator system. this reason (and also because of the potential for additional off - l i n e Near perfect coupling, as determined by monitoring of the force g ro u n d - roll produced by highway traffic) high frequency (40-Hz) versus time during the sweep, on the snow covered frozen geophones were selected. The use of such high frequency re c e i v e r s ground was obtained for almost all the shot points. The field has the disadvantage of reducing the overall bandwidth and crew worked hard but had a good time during the 10 day shoot hence also the resolution, but based on our field experiences in (Figure 3). Initial analysis of the seismic refractions and the preliminary Figure 3.Work and fun in the field. reflection profile showed promise in locating the buried channel. Based on these results, an ERT profile was acquired in January 2005 in order to add additional information particularly with regards to the correlation of nearby resistivity anomalies. This work was carried out commercially in January 2005 with members of the scientific team assisting. Conditions in January were not so favourable with extreme cold causing some prob- lems with placement of electrodes and equipment operation (Figure 4).

Results and Discussion Seismic data was processed at the U of A using a conventional p rocessing flow. The shallow nature of the targets made pro p e r velocity analysis difficult; a standard semblance analysis was essentially confirmed by close analysis of the first break re f r a c- tions, however. Both indicated low velocities to the top of the pre - C retaceous unconformity, consistent with weakly consolidated channel fill materials, to the east that evolved to higher velocity F i g u re 3a. Coffee time. Doug in the recording dog house. Alain on the phones.

F i g u re 3b. Tim and John with high precision GPS used for establishing geodetic F i g u re 3c. Jawwad and Fabian having lunch on the line. c o n t rol points in the are a .

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F i g u re 6. Portion of the final, unsmoother (block parameterization), p rocessed/inverted electrical resistivity tomography profile (15 meter electro d e spacing) showing high resistivity anomaly at location 8640, depth 275m and low resistivity feature at location 7200, depth 300m. Datum for this profile is 400m a.s.l. and ground elevation ranges from 385m to 365m. Note creek crossing between 5760 F i g u re 4a. Very hard to pull 1 m steel electrode from frozen gro u n d . and 6240 with highly resistive futures at the surface. C retaceous sediments to the west. Only a portion of the seismic p rofile is provided here. Preliminary interpretation of seismic p rofile reveals the Pre - C retaceous unconformity and shows the p resence of stacked Quaternary channel margins (Fig. 5). The p rocessing results are presented in detail in Ahmad et. el., (2005).

The refraction interpretation and velocity analysis clearly reveals the presence of the Sub-Cretaceous unconformity (Mississippian), characterized by materials with seismic velocities of > 5000 m/sec; this is in good agreement with the lime- stone/dolomote lithology below the unconformity (Ahmad et. el., 2005). Above the unconformity, the refraction analysis yields velocities ~2600 m/s and ~1800 m/s, respectively. These velocities are consistent with the expected and more competent Cretaceous sediments on the west of the line and loosely consolidated laterally migrating Quaternary paleochannel fill sediments to the east (Ahmad et. el., 2005). The detailed Geological interpretation is presented in two different poster presentations of Pawlowicz, et. el., (2005) and in Project Poster, (2005).

F i g u re 4b. Warming up ERT field instrument on one frosty morning. The ERT surveys can highlight potential gas zones based on their high resistivity relative to the surrounding formations. A number of attempts were made to invert this data. A portion of one such inversion shows the existence of one high resisitivity anomaly within the paleovalley filled Quaternary drift (Fig. 6).

Quaternary sediments are deposited almost all over Canada during the periods of last glaciations. Bedrock topography map helps us identifying incised valleys, paleo highs and lows and can be used in a regional context for identification of potential gas plays. Similar features i.e. incised valleys, meltwater channels, glacial lake deposits and preglacial fluvial river systems may be used to find Quaternary gas in many regions.

Such shallow Quaternary gas produces safety hazards which are not unlike those caused by gas chimneys in the offshore envi- ronment for which a geohazord survey prior to the drilling is normal practice (Barthold et. el., 2003). High resolution seismic especially processed for detecting ultra shallow high amplitude F i g u re 5. Portion of the seismic section showing Top Devonian (Wabamun L.st.) at anomalies, followed by an ERT profile on the same place, will 0.4 sec. Dipping event at cmp 580, 0.3 sec is the base of the Quaternary sediments, confirm the presence of shallow gas prior to the drilling. Shallow above that high amplitude events are believed to be from glacial till, gravel beds and meltwater channels interbedded in layers of fluvial and glacial lake clay deposits. high resolution seismic is a cost effective tool and widely used in Seismic datum is 400 a.s.l and ground elevation ranges from 370m to 385m. hydrogeological and engineering studies. Shallow seismic data

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42 CSEG RECORDER September 2005 Article Cont’d Seismic and Resistivity Imaging for Quaternary... Continued from Page 42 processing requires experienced personnel and modification in revealed by bedrock topography and drift thickness mapping show potential for shallow gas, Northwestern Alberta, Canada, Poster presentation: AAPG annual Convention Calgary: conventional industry processing software’s that are mainly [pdf]. Available from http://www. a g s . g o v. a b . c a / a c t i v i t i e s / c o n f e r e n c e _ p re s e n t a- optimized for deeper targets. tions.html [Cited August 6, 2005] URLS: Electrical Resistivity Tomography is a DC resistivity technique Nabors 29 Blowout (28 January 2005), A c c o rdi ng to Rig finder: CNRL H Z that is relatively new in the oil and gas industry although it has MIDWINTER B-93-L-94-P-10: [html]. Available from http://www.canadian-well- been used for decades in detecting aquifers. Advancement in site.com/Gallery/gallery.htm [Cited August 7, 2005] modern computers and inversion algorithms made ERT more Project Poster, 2005, Northern Mapping, Alberta Geological Survey: [pdf]. Available effective in imaging shallow geological features. Details of ERT f rom http://www. a g s . g o v. a b . c a / a c t i v i t i e s / m i n e r a l s / n o r t h e r n _ m a p p i n g . h t m l [Cited August 6, 2005] surveying can be found from the RECORDER article by Paul Safety Bulletin 2005-01, 2005, Sweet Well Blowout, Energy Utility Board (EUB): Bauman, (2005). [html, pdf]. Available from http://www.eub.gov.ab.ca/BBS/requirements/safety- bulletins/2005-01.html [Cited August 18, 2005] Acknowledgements: The lead author is thankful to the financial support by Geological Survey of Canada through ‘Targeted Geoscience Initiative-2’ (TGI-2). Thank to the field crew: Marek Welz, Len Tober, Gabrial Solano, Tiewei He, Dean Rokosh and Doug Schmitt (University of Alberta); John Pawlowciz, (Alberta Geological S u r v e y, Edmonton); and A l a i n P l o u ffe (Geological Survey of Canada, Ottawa). Marek Welz also assisted greatly with org a n i z a t i o n , reconnaissance, and data header p rep aration. The ERT data was a c q u i red by Komex, Calgary by Chris, Christeen and other employees. John Pawlowciz and Jawwad Ahmad also assisted in the ERT field data acquisition. R

References: Ahmad, Jawwad, Douglas R. Schmitt, Dean Rokosh, John G. Pawlowicz, Mark M. Fenton, and Alain Plouffe, 2005, Seismic imaging of Quaternary channels, Rainbow Lake, Northern Alberta, Canada: Expanded Abstract, Presented at CSEG A n n u a l Convention 2005, [pdf], Available fro m h t t p : / / w w w . c s e g c o n v e n t i o n . o r g / 2 0 0 5 a b s t r a c t s / 0 0 8 S 0 1 2 1 - paper_cseg_uofa_final.pdf, [Cited August 7, 2005] Ahmad, Jawwad., Douglas R. Schmitt, Dean Rokosh, John G. Pawlowicz, Mark M. Fenton, and Alain Plouffe, 2005, Seismic imaging of Quaternary channels for shallow gas Rainbow Lake, Northwest Alberta, Canada: Expanded Abstract and Poster Presentation, AAPG annual Convention Calgary Barthold M. Schroot, 2003; Surface and subsurface expressions of shallow gas accumulations in the southern North Sea: Search and Discovery Article #40090, Presented at AAPG Headberg Conference Vancouver April 7-10, 2002 Bauman, Paul, 2005, 2-D Resistivity Surveying for Hydrocarbons –A Primer, RECORDER, vol. 30, no. 4, 25-33 Pawlowicz John, 2005, Shallow gas Northwestern Alberta, Geological Setting: AAPG annual Convention Calgary Poster presentation: [pdf]. Available fro m h t t p : / / w w w. a g s . g o v. a b . c a / a c t i v i t i e s / c o n f e r - ence_presentations.html [Cited August 6, 2005] Pawlowicz John, Tami J, Nicoll, Mark M. Fenton, Jawwad Ahmad, Douglas R. Schmitt, Dean Rokosh, and Alain Plouffe, 2005, P a l e o v a l l e y

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