Dale R. Issler

A New Approach to Shale Compaction and Stratigraphic Restoration, Beaufort-Mackenzie Basin and Mackenzie Corridor, Northern Canada (1)

Despite a long history of study, porosity reduction in shales and mudstones is still poorly understood. Many published shale compaction curves from different sedimentary basins exist, but it is difficult to make comparisons because of variations in sediment composition, geologic age, tectonic history, and experimental technique. The Beaufort-Mackenzie basin provides a unique opportunity to study shale compaction because it contains shales with rather uniform physical and chemical properties, deposited under highly variable sedimentation rates. The sonic log provides a sensitive measure of porosity change with depth in the Beaufort-Mackenzie basin. A new porosity-sonic transit-time equation, calibrated using shale and mudstone core porosity measurements from this basin, yields an acoustic formation-factor exponent of 2.19 and a matrix transit time of 220 microseconds/m, which are believed to be valid for low TOC (<2%), noncalcareous shales. Log-determined shale compaction curves define distinctive compaction zones that correlate with sedimentation rate, pore pressure, and erosion. Normal shale compaction is linear below 500 m at 1%/100 m decrease in porosity for hydrostatic pressure conditions. In eroded areas, the amount of missing strata can be estimated by reference to the normal compaction curve. In compositionally heterogeneous shale sequences, a knowledge of shale composition and its effect on log response is essential to avoid introducing significant error into calculated compaction curves. Preliminary results from the Brackett basin imply that maximum burial is more important than maximum paleotemperature in reducing shale porosity. This suggests that shale porosity could be a useful temperature-insensitive paleodepth indicator for eroded sedimentary sequences.

Calculation of Vitrinite Reflectance from Thermal Histories: A Comparison of Some Methods

Vitrinite reflectance values (%Ro) calculated from commonly used methods are compared with respect to time invariant temperatures and constant heating rates. Two monofunctional methods, one involving a time-temperature index to vitrinite reflectance correlation (TTI-%Ro) and the other involving a log(%Ro) to depth correlation, yield vitrinite reflectance values that are similar to those calculated by recently published Arrhenius-based methods, such as EASY%Ro. The approximate agreement between these methods supports the perception that the EASY%Ro algorithm is the most accurate method for the prediction of vitrinite reflectances throughout the range of organic maturity normally encountered. However, calibration of the e methods against vitrinite reflectance data from two basin sequences with well-documented geologic histories indicates that, although the EASY%Ro method has wide applicability, it slightly overestimates vitrinite reflectances in strata of low to medium maturity up to a %Ro value of 0.9%. The two monofunctional methods may be more accurate for prediction of vitrinite reflectances in similar sequences of low maturity. An older, but previously widely accepted TTI-%Ro correlation consistently overestimates vitrinite reflectances with respect to other methods. Underestimation of paleogeothermal gradients in the original calibration of time-temperature history to vitrinite reflectance may have introduced a systematic bias to the TTI-%Ro correlation used in this method. Also, incorporation of TAI (thermal alteration index) data and its conversion to %Ro-equivalent values may have introduced inaccuracies.

Thermal and isostatic consequences of simple shear extension of the continental lithosphere

Abstract Quantitative results are presented for a model involving large-scale simple shear of continental lithosphere. The locally compensated, finite element model includes the effects of sedimentation, radiogenic heat production in the crust and sediments, and finite rates of extension. Flexural effects during firting are examined using a simple shear model with a uniform initial elastic thickness. The locally compensated model predicts that a breakup unconformity develops on the upper plate when extension occurs at typical plate tectonic velocities. Subsidence occurs as the upper and lower lithospheric plates separate. Extension creates the potential for large accumulations of sediment ( ∼ 20 km), most of which is syn-rift if sedimentation keeps pace with subsidence. The effect of flexure is to modify the shape of the detachment surface and the uplift patterns on the flanks of the upper and lower plate sedimentary basins. A model which incorporates flexure predicts significant uplift of the lower plate as it is unloaded by the upper plate during extension. The Wernicke simple shear model can be distinguished from its opposite end member, the McKenzie pure shear model, by the predicted asymmetry in thermal evolution (higher predicted heat flux within the upper plate), uplift/subsidence histories and stratigraphic development of the upper and lower plates.

Optimizing time step size for apatite fission track annealing models

Empirical isothermal apatite fission track (AFT) annealing models are used to extract variable temperature histories from measured AFT parameters by forward and inverse modeling techniques. Using one such published annealing model based on Durango apatite data, a method has been developed for optimally discretizing thermal histories into isothermal steps for the evaluation of the annealing model. The equation S = (√log(TaTm)A1 + A2e−(Tm − T)A3)RA4 predicts isothermal time step size (S) as a function of maximum interval temperature (Tm in kelvins) and rate of temperature change (R in K m.y.−1) with constants A1 = 0.00596, A2 = 0.043, A3 = 34.8, and A4 = 0.978, and with the total annealing temperature (Ta) given as a power function of R, Ta = 398.15(R0.0157). This equation offers improved computational efficiency and accuracy for the full range of track length reduction in comparison with other methods published. Improved model performance is important particularly for inversion type models which may require the generation of thousands of model temperature histories with large variations in heating and cooling rates. For similar amounts of annealing, integration step sizes differ by two orders of magnitude for heating/cooling rates that range between 0.1 and 10 K m.y.−1, a range that encompasses most sedimentary basins. As an added advantage, users can specify the approximate degree of accuracy for track length calculations by multiplying S by the scaling factor, √10E, where E is the approximate percent error on calculated track lengths. For E values of 0.1 and 1.0, computed thermal histories are within ⩽0.2 °C and ⩽1.0 °C, respectively, of the true numerical solution.

Post-Early Devonian thermal constraints on hydrocarbon source rock maturation in the Keele Tectonic Zone, Tulita area, NWT, Canada, from multi-kinetic apatite fission track thermochronology, vitrinite reflectance and shale compaction

Abstract New thermal maturity (%Ro, Rock-Eval® pyrolysis), shale compaction and apatite fission track (FT) data were integrated into thermal history models for the East MacKay I-77 petroleum exploration well located approximately 80 km southeast of Norman Wells, Northwest Territories. The study well is in the Keele Tectonic Zone where multiple phases of anomalous Phanerozoic subsidence and uplift have resulted in an Upper Cretaceous–Cenozoic foreland succession resting unconformably on Devonian strata. This major unconformity, which developed during pre- and post-Albian time, displays a thermal maturity discontinuity (0.55-0.75%Ro) and represents approximately 270 m.y. of missing geological record. Linear shale compaction across this unconformity suggests that maximum burial occurred during the Cenozoic, whereas thermal maturity data imply that maximum temperatures were reached sometime between the Devonian and Early Cretaceous. Detrital apatite grains from a single sample from the Upper Devonian Imperial Formation of the I-77 well yielded two FT age populations (90.4±6.1 Ma and 222.2±22.5 Ma; ± one standard deviation) with different thermal annealing properties based on their chlorine content. An inverse multi-kinetic FT annealing model was developed and used to determine thermal histories that are consistent with the FT parameters and other geological constraints. Model results suggest that hydrocarbon generation from Devonian rocks at the I-77 well location occurred during the early Mesozoic prior to the development of Late Cretaceous-Cenozoic structures. Peak FT model temperatures are 124±10°C within the Early Triassic to Middle Jurassic (250–170 Ma), <75°C during the Albian (112–100 Ma) and 97±9°C during the Paleocene-Early Eocene (65–50 Ma). The Cretaceous-Cenozoic thermal history was modelled using a simple burial history with the present geothermal gradient (32°C/km) held constant; a range of geothermal gradients (31–42°C/km) and maximum burial depths for the pre-Aptian thermal history fit Devonian maturity data. If maximum burial was during the Cenozoic, then Mesozoic peak maturity was achieved under a higher geothermal gradient than present. Although hydrocarbon generation pre-dates structural trap development near the I-77 well, Devonian source rocks retain significant hydrocarbon potential. Given the complicated geological history of the central Mackenzie Valley, deeper Cenozoic burial elsewhere in the region may have generated hydrocarbons from Cretaceous and reactivated Devonian source rocks.

A Finite Element Model of the Subsidence and Thermal Evolution of Extensional Basins: Application to the Labrador Continental Margin

A one-dimensional finite element model has been developed to study postrift thermal and subsidence histories of basins formed by lithospheric stretching. The model includes depth-dependent extension, radiogenic heat production, and variations in the sediment thermal properties. Inputs to the model are a rifting age, a sediment budget, and the stretching parameters β and δ, for the crust and subcrustal lithosphere, derived from back-stripping analysis.

Hydrocarbon migration detected by regional temperature field variations, Beaufort-Mackenzie Basin, Canada

The regional Beaufort-Mackenzie Basin temperature field is characterized using data collected from drill-stem tests and bottom-hole temperature logs. We recognize two thermal anomalies, each of which is associated with a specific geological setting. Elevated temperatures are observed in (1) the western Beaufort Sea, where post-Eocene erosion removed Cenozoic strata and folding is common in a contractional tectonic regime, and (2) along fault zones where upward flow transports heat by advection. Depressed temperatures are observed in Eocene and post-Eocene rapidly subsiding depocenters, with overpressure developed below 3000 m (9843 ft). Older strata along the southeast rifted margin are characterized by a more normal thermal regime. Evidence from anomalously high temperatures in both map and cross-sectional views suggests that fault zones and major regional aquifers accommodate the upward expulsion of fluids from deep overpressured zones. Many significant petroleum discoveries occur in areas where anomalously high temperatures are observed, suggesting that petroleum migration occurs along the same flow networks. Identifying anomalies in the temperature field may therefore be a useful exploration technique.

A New Method for Recognizing Subsurface Hydrocarbon Seepage and Migration Using Altered Foraminifera from a Gas Chimney in the Beaufort-Mackenzie Basin

A new method for recognizing hydrocarbon seepage and migration in exploration wells is documented from the Immiugak A-06 exploration well that drilled through a hydrocarbon-related diagenetic zone (HRDZ). The HRDZ is seismically conspicuous as part of a gas chimney on a shale-cored anticline in the Tertiary of the Beaufort-Mackenzie Basin, Arctic Canada. The HRDZ contains classic diagenetic minerals, notably greigite (Fe3S4) and calcite with 34S and 13C values diagnostic of hydrocarbon-related, sulfate-reducing, microbial activity. The HRDZ also contains exceptionally preserved calcareous benthic foraminifera with conspicuous bitumen-filled chambers and agglutinated foraminifera with bitumen and diagenetic silica with bound particles. Silica was highly mobile within the seepage or migration system and was precipitated and dissolved extensively in the agglutinated foraminifera. Seismic profiles, resistivity anomalies, diagenetic minerals, and altered foraminifera all suggest that significant hydrocarbons migrated or seeped through sandy Oligocene and Miocene strata at the crest of a shale-cored anticline in response to late Miocene tectonism. Hydrocarbon-related diagenesis can be distinguished from standard burial diagenesis using the foraminiferal coloration index (FCI). Foraminiferal coloration within the HRDZ was controlled by silicification in a bitumen-rich environment. The FCI values in the HRDZ are much higher than predicted for normal burial and show abnormal variance caused by variable dissolution of foraminiferal silica. The FCI values from agglutinated foraminifera outside the HRDZ show a uniform linear trend increasing with depth. The extent of hydrocarbon-related diagenesis observed in foraminifera can be used to assess the relative magnitude of hydrocarbon seepage in the Beaufort-Mackenzie Basin and potentially other petroleum basins.

Application of Low-Temperature Thermochronology to Hydrocarbon Exploration

The maturation of organic material into petroleum in a sedimentary basin is controlled by the maximum temperatures attained by the source rock and the thermal history of the basin. A cycle of continuous deposition into the basin (burial) and regional basin inversions represented by unconformities (unroofing) may complicate the simple thermal development of the basin. Applications of low-temperature thermochronology via fission-track (FT) and (U–Th)/He dating coupled with independent measurements (vitrinite reflectance, Rock-Eval) resolving the paleothermal maximum are the ideal approach to illuminate the relationship between time and temperature. In this contribution, we review the basics of low-temperature thermochronology in the context of a project workflow, from sampling to modeling, for resolving the thermal evolution of a hydrocarbon-bearing sedimentary basin. We specifically highlight the application of multi-kinetic apatite FT dating, emphasizing the usefulness of the rmr0 parameter for interpreting complex apatite age populations that are often present in sedimentary rocks. Still a rapidly advancing science, thermochronology can yield a rich and effective dataset when the minerals are carefully and properly characterized, particularly with regard to mineral chemistry and radiation damage.