Nechako Basin Research Articles

Gravity Field Imaging by Continued Fraction Downward Continuation: A Case Study of the Nechako Basin (Canada)

Gravity Field Imaging by Continued Fraction Downward Continuation: A Case Study of the Nechako Basin (Canada)

Waveform tomography in 2.5D: Parameterization for crooked‐line acquisition geometry

AbstractA method for 2.5D viscoacoustic waveform tomography that can be applied to generate 2D models of velocity and attenuation from inversion of refraction waveforms on land seismic reflection data acquired along crooked roads is developed. It is particularly useful for typical crustal reflection surveys. First‐arrival travel time tomography is applied using a 3D method, but with constraints on the intermediate 3D velocity model; the result is the starting model for the next step. A 2.5D frequency‐domain full‐waveform inversion stage parameterizes 3D geometry in the seismic source and receiver arrays, with the assumption that the velocity and attenuation models are homogeneous in the out‐of‐plane direction. This approach results in superior results compared to a strictly 2D approach when the acquisition line is crooked, with a moderate increase in computational cost. A case study using data acquired in the Nechako Basin in south‐central British Columbia, Canada, exemplifies and validates the procedure. The velocity model derived from 2.5D waveform tomography is compared with that from a previous study in which 2D waveform tomography was applied to the same data set and with results from 3D travel time tomography. The resolution and accuracy of the velocity model from 2.5D waveform tomography are demonstrated to be greater than those from travel time or 2D waveform tomography. A model of viscoacoustic attenuation, which was not possible in the 2D case, is also generated. These models are interpreted jointly to highlight features of geological interest, such as a sedimentary basin, basement rocks, and faults, from surface to about 3 km depth.

Waveform tomography of field vibroseis data using an approximate 2D geometry leads to improved velocity models

Waveform tomography, a combination of traveltime tomography (or inversion) and waveform inversion, is applied to vibroseis first-arrival data to generate an interpretable model of P-wave velocity for a site in the Nechako Basin, south-central British Columbia, Canada. We use constrained 3D traveltime inversion followed by 2D full-waveform inversion to process long-offset (14.4 km) first-arrival refraction waveforms, resulting in a velocity model of significantly higher detail than a conventional refraction-statics model generated for a processing workflow. The crooked-line acquisition of the data set makes 2D full-waveform inversion difficult. Thus, a procedure that improves the tractability of waveform tomography processing of vibroseis data recorded on crooked roads is developed to generate a near-surface ([Formula: see text]) velocity model for the study area. The data waveforms are first static corrected using a time shift determined by 3D raytracing, which accounts for the crossline offsets produced by the crooked-line acquisition. The velocity model generated from waveform tomography exhibits substantial improvement when compared with a conventional refraction-statics model. It also shows improved resolution of sharp discontinuities and low-velocity regions when compared to the model from traveltime tomography alone, especially in regions where the geometry errors are moderate. Interpretation of the near-surface velocity model indicates possible subbasins in the Nechako Basin and delineates the Eocene volcanic rocks of the study area. This approach limits the ability of the full-waveform inversion to fit some propagation modes; however, the tractability of the inversion in the near-surface region is improved. This new development is especially useful in studies that do not warrant 3D seismic acquisition and processing.

Imaging the Nechako Basin, British Columbia, using ambient seismic noise1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

The Nechako Basin in British Columbia, Canada is suspected to have hydrocarbon potential. However, it has been a difficult basin to explore because of the presence of Tertiary volcanic outcrop. The volcanic outcrop makes the use of conventional seismic exploration methods difficult owing to a strong velocity inversion at its base. An alternative is the passive source method known as ambient noise surface wave tomography. The method, which examines the high-frequency surface wave field that is obtained from noise analysis, is sensitive to large-scale crustal structure and has been successfully applied to measuring the depths of sedimentary basins. Station-to-station Green’s functions within the basin were estimated by cross-correlating the vertical components of the seismic noise data recorded by 12 POLARIS (Portable Observatories for Lithosphere Analysis and Research Investigating Seismicity) and CNSN (Canadian National Seismgraph Network) seismic stations between September 2006 and November 2007. The resulting Green’s functions were dominated by Rayleigh waves. The dispersion characteristics of the Rayleigh waveforms were measured within the microseismic band. Inversion of the dispersion curves produced 1-D and 2-D thickness models and 2-D group velocity models for the Nechako Basin and its surrounding region. The velocity models indicate two low group velocity structures within the basin that might represent sedimentary packages, and some pockets of high-velocity zones that show the presence of volcanic rocks within and on the basin. The thickness models indicated the presence of about six different velocity layers, in which the average thickness of the basin and the crust are ∼4.8 and 30–34 km, respectively.

Crustal velocity structure of the southern Nechako basin, British Columbia, from wide-angle seismic traveltime inversion1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

In the BATHOLITHSonland seismic project, a refraction – wide-angle reflection survey was shot in 2009 across the Coast Mountains and Interior Plateau of central British Columbia. Part of the seismic profile crossed the Nechako Basin, a Jurassic–Cretaceous basin with potential for hydrocarbons within sedimentary strata that underlies widespread volcanic rocks. Along this 205 km-long line segment, eight large explosive shots were fired into 980 seismometers. Forward and inverse modelling of the traveltime data were conducted with two independent methods: ray-tracing based modelling of first and secondary arrivals, and a higher resolution wavefront-based first-arrival seismic tomography. Material with velocities less than 5.0 km/s is interpreted as sedimentary rocks of the Nechako Basin, while velocities from 5.0–6.0 km/s may correspond to interlayered sedimentary and volcanic rocks. The greatest thickness of sedimentary rocks in the basin is found in the central 110 km of the profile. Two sub-basins were identified in this region, with widths of 20–50 km and maximum sedimentary depths of 2.5 and 3.3 km. Such features are well-defined in the velocity model, since resolution tests indicate that features with widths greater than ∼13 km are reliable. Beneath the sedimentary rocks, seismic velocities increase more slowly with depth — from 6.0 km/s just below the basin to 6.3 km/s at ∼17 km in depth, and then to 6.8–7.0 km/s at the base of the crust. The Moho is found at a depth of 33.5–35 km beneath the profile, and mantle velocities are high at 8.05–8.10 km/s.

Interpretation of structures in the southeastern Nechako Basin, British Columbia, from seismic reflection, well log, and potential field data1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.2Geological Survey of Canada Contribution 20100002.

The structure and stratigraphy of the southeast Nechako Basin, which are poorly understood primarily because of substantial volcanic cover, are investigated in an analysis of seismic reflection, well, and potential field data. Formation and development of the SE Nechako Basin resulted in sub-basins containing Cretaceous and Eocene rocks. Interpretation reveals that dextral transtension in the Early to Middle Eocene created NNW-trending, en echelon, strike-slip faults linked by pull-apart basins, which locally contain a thickness of Eocene volcaniclastic rocks of >3 km. This structural pattern is consistent with regional observations that suggest the transfer of slip from the Yalakom fault to the north via a series of en echelon strike-slip faults. In the Middle to Late Eocene, faults associated with a change in the direction of stress, echoed by the north-trending right-lateral Fraser fault, reactivated and cut earlier structures. A simple model agrees with local observations, that northeast-directed compression was subparallel to the relic Cretaceous grain. Cretaceous rocks are discontinuous throughout the basin and may be remnants of a broader basin, or a number of contemporaneous basins, formed in a regional transpressional tectonic setting that caused northeast-directed thrusting along the eastern side of the Coast Plutonic Complex. Results suggest that thrusting affected most of the SE Nechako Basin, as observed across the Intermontane Belt to the northwest and southeast. The pattern of deposition of Neogene volcanic rocks of the Chilcotin Group was in part controlled by the Eocene structural grain, but we find no evidence of Neogene deformation.

Seismic reflection constraints on upper crustal structures in the volcanic-covered central Nechako basin, British Columbia1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

In 2008, a Vibroseis seismic reflection survey was acquired by Geoscience BC across the eastern part of the volcanic-covered Nechako basin in central British Columbia, where Cretaceous sedimentary rocks have been exhumed along a NNW trend. Good signal penetration through the volcanic cover is indicated by lower crustal reflections at 8–12 s, which were recorded by the entire seismic survey. Comparison of the 2008 seismic survey with data from a previous survey indicates that the lack of reflectivity in the earlier surveys is generally representative of the subsurface geology. The seismic data show that ∼1700 and ∼2900 m thick sub-basins are present at the northern and southern ends of this trend, but the intervening Cretaceous rocks are discontinuous and relatively thin. The creation of a passive-roof duplex by Campanian or later low-angle thrusting is inferred within the thickest Cretaceous strata, but elsewhere faulting is likely related to Eocene extension or transtension. Seismic reflections are also recorded from folded volcanic stratigraphy, the base of the surface volcanic rocks, an underlying volcaniclastic stratigraphy, and intrusions projecting into a Quaternary volcanic cone. Seismic interpretation is complemented by coincident audiofrequency magnetotelluric surveys, from which faulting is inferred at offsets in a regional conductor. No regionally extensive stratigraphy can be identified within the seismic data, and the central Nechako basin appears to be a complex network of small, deformed sub-basins, rather than a single large basin.

Near-surface and crustal-scale images of the Nechako basin, British Columbia, Canada, from magnetotelluric investigations1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.2Geological Survey of Canada, GSC Contribution No.: 20100129.

Two-dimensional (2-D) models of audio and broadband magnetotelluric (MT) data collected throughout the southern Nechako basin in south–central British Columbia, Canada, provide electrical resistivity images of the Cretaceous to Oligocene volcanic and sedimentary packages and underlying crustal-scale features. Analysis of distortion effects and structural dimensionality indicate that the MT responses are primarily one-dimensional (1-D) at periods less than 0.1 s. Departures from a 1-D response occur with maximum phase differences occurring between 0.1 and 10 s. The upper crustal resistivity models reveal a low resistivity layer at near surface depths, interpreted as Chilcotin basalts, that blankets portions of the region to depths less than 50 m, but locally thicken up to 200 m. Cretaceous sedimentary units (those showing the highest potential for hydrocarbons) are characterized by moderately low resistivities that are laterally variable. More uniform, lower resistivities appear to be associated with the Eocene volcaniclastic groups, suggesting that the MT method can distinguish between these units and may be useful in targeting areas that are more prospective for hydrocarbon exploration. In general midcrustal resistivity values are high, consistent with values of typical volcanic terranes; however, a low-resistivity zone is identified at 8–10 km depth that correlates closely with the location of seismic reflectors as well as recent microseismic activity. This low-resistivity zone is interpreted as a midcrustal reservoir of magma, the top of which marks the upper limit of fluids migrating from lower crust depths. Additionally, several crustal-scale faults are imaged.

Eocene and Neogene volcanic rocks in the southeastern Nechako Basin, British Columbia: interpretation of the Canadian Hunter seismic reflection surveys using first-arrival tomography

Geological investigation of the near-surface in the southeastern Nechako Basin is difficult. Shallow seismic reflection imaging is poor due in part to an extensive cover of Eocene and Neogene volcanic rocks. Outcrops of these volcanic rocks, and the primarily Cretaceous bedrock, are commonly obscured by Quaternary deposits and vegetation. Estimates of near-surface P-wave velocity are derived from the tomographic inversion of seismic first-arrivals, an effective tool when seismic imaging is poor. Tomographic model velocities are in agreement with sonic logs and laboratory samples, except for those from the Neogene Chilcotin Group. Cretaceous sedimentary rocks have velocities of ∼2800–4200 ms–1. The Eocene Endako and Ootsa Lake groups, which have velocities of ∼3000–4200 ms–1, are not distinguishable based on velocity. The velocity, the character (density, focus, and penetration depth) of rays, and ties with well and surface geology constrain the subsurface extent of the Endako Group adjacent to well b-82-C. The Chilcotin Group typically exhibits velocities (∼2400–3000 ms–1) lower than corresponding velocities from sonic logs (4500–5200 ms–1) and laboratory measurements (5000–5200 ms–1). These low model velocities may be due to the presence of high porosity, brecciated rocks near to the surface, in comparison with the other measurements that have focussed on lower porosity massive lavas. The lowest mean velocities, located to the southeast, are related to anomalously thick, high porosity, breccia-rich deposits of Chilcotin Group. This conclusion is consistent with the interpretation that the Chilcotin Group is thicker in paleo river valleys.

Cordilleran Intermontane thermotectonic history and implications for neotectonic structure and petroleum systems, British Columbia, Canada

Heat flow increases northward along Intermontane Belt in the western Canadian Cordillera, as shown by geothermal differences between Bowser and Nechako sedimentary basins, where geothermal gradients and heat flows are ∼30 mK/m and ∼90 mW/m2 compared to ∼32 mK/m and 70 –80 mW/m2, respectively. Sparse temperature profile data from these two sedimenatary basins are consistent with an isostatic model of elevation and crustal parameters, which indicate that Bowser basin heat flow should be ∼20 mW/m2 greater than Nechako basin heat flow. Paleothermometric indicators record a significant northward increasing Eocene or older erosional denudation, up to ∼7 km. None of the heat generation, tectonic reorganization at the plate margin, or erosional denudation produce thermal effects of the type or magnitude that explain the north–south heat flow differences between Nechako and Bowser basins. The more southerly Nechako basin, where heat flow is lower, has lower mean elevation, is less deeply eroded, and lies opposite the active plate margin. In contrast, Bowser basin, where heat flow is higher, has higher mean elevation, is more deeply eroded, and sits opposite a transform margin that succeeded the active margin ∼40 Ma. Differences between Bowser and Nechako basins contrast with the tectonic history and erosion impacts on thermal state. Tectonic history and eroded sedimentary thickness suggest that Bowser basin lithosphere is cooling and contracting relative to Nechako basin lithosphere. This effect has reduced Bowser basin heat flow by ∼10–20 mW/m2 since ∼40 Ma. Neither can heat generation differences explain the northerly increasing Intermontane Belt heat flow. A lack of extensional structures in the Bowser basin precludes basin and range-like extension. Therefore, another, yet an unspecified mechanism perhaps associated with the Northern Cordilleran Volcanic Province, contributes additional heat. Bowser basin’s paleogeothermal gradients were higher, ∼36 mK/m, before the Eocene and this might affect petroleum and metallogenic systems.

Oil and Gas Developments in Western Canada in 1981

In 1981, there was a marked slump in drilling activity in western Canada resulting from the combined effects of the National Energy Program (NEP), economic recession, and a worldwide oil surplus. The number of wells drilled dropped by 22% to a total of 6,904. Exploratory drilling decreased 19% to 3,074 wells, and development drilling decreased 25% to 3,830 wells. The exploratory success rate also decreased to 62% in 1981, based on 740 oil discoveries and 1,172 gas discoveries. The development success rate decreased slightly to 88%, with 1,334 oil discoveries and 2,034 gas discoveries. Average well depth increased in 4 of the 5 western areas, but total revenue from land sales plummeted to half of the previous years' record totals. Alberta land sale revenue totaled only $58 million, British Columbia $61 million, Saskatchewan $37 million, and Manitoba $2 million. Average price per hectare also dropped, especially in British Columbia where the average price was off by 80%. In relative terms, exploratory activity in Alberta and Saskatchewan dropped to 1978 and 1979 levels, while British Columbia activity was the lowest since 1977. Manitoba was the only province to go against the trend, with a record number of exploratory wells and discoveries. In Alberta, significant exploration activity was centered in the Shekilie, Golden-Evi, and West Pembina areas, while major gas discoveries were made at Whiskey and Elmworth, and oil discoveries at Tangent. No significant new discoveries were made in British Columbia or Saskatchewan, but the 1980 Manitoba oil discovery at Waskada continued to stimulate activity. Preliminary results from the Nechako basin drilling in central British Columbia proved to be disappointing with 3 successive dry holes. In the Arctic Islands, 2 major oil discoveries were made at Skate and Cisco, off Lougheed Island, and a gas discovery at MacLean. Beaufort Sea drilling was hampered by weather, but nonetheless resulted in major oil discoveries at Koakoak and at a Kopanoar follow-up well. The year 1981 will probably be viewed as a major turning point in exploration in western Canada, with interest shifting from the provinces to the frontiers, and the participants mainly Canadian-controlled companies. Cash flow and drilling activity were markedly reduced, mergers and takeovers common, and major heavy oil and tar sands projects cancelled. The expected downturn in exploration during 1981 took place as predicted, and no improvement is seen for 1982.