back-stripping

Back-stripping in petroleum system modeling applies the layer-removal and burial reconstruction technique specifically to reconstruct the thermal maturity history of source rock intervals in Western Canada Sedimentary Basin basin models, calculating how the temperature exposure of organic-rich shales such as the Devonian Duvernay, Triassic Montney, and Cretaceous Second White Specks evolved through geological time as successive overburden layers were deposited, compacted, and in some areas eroded, and using this burial-temperature history as the time-temperature integral input to kinetic models that predict when, where, and how much hydrocarbons were generated and expelled from the source rock into adjacent reservoir formations during the 370 million year geological history of the WCSB. The petroleum system modeling application of back-stripping differs from the basin subsidence analysis application (covered in the primary back-stripping entry) in its primary output and purpose: while subsidence-focused back-stripping reconstructs the tectonic driving mechanism (thermal subsidence, flexural loading, stretching) that controlled basin geometry, the petroleum system modeling application uses the same stratigraphic decompaction and depth reconstruction to generate a temperature-time history (TTI curve or burial history diagram) at each well or basin location that is then input to organic geochemical kinetic models (Easy%Ro for vitrinite reflectance prediction, Basin2 or Petromod cracking kinetic models for hydrocarbon generation) to predict the onset, peak, and termination of oil and gas generation from each source interval. In the WCSB Duvernay petroleum system, back-stripping-based burial history modeling has been essential to understanding the spatial variation in Duvernay maturity across the play fairway: the Duvernay in the west-central Alberta deep basin (Edson, Drayton Valley, Rocky Mountain House areas) reached peak oil generation temperature of 100 to 120 degrees C during the Laramide orogeny (60 to 70 Ma) when Cordilleran thrust loading created maximum burial depths of 5,000 to 7,000 m above the current Duvernay depth, while the same Duvernay in east-central Alberta (Stettler, Hanna areas) reached maximum burial of only 2,000 to 3,000 m and remains in the early oil window today at vitrinite reflectance of 0.6 to 0.8%Ro, a spatial maturity difference that back-stripping burial history models quantify from the compaction state and missing section estimates at well locations across the basin. Understanding how WCSB back-stripping petroleum system models incorporate decompaction algorithms (Athy's law or empirical porosity-depth curves for each lithology), erosional unconformity estimates from sonic log anomaly analysis and apatite fission track dating, heat flow history (Paleozoic through Laramide), and organic geochemical kinetic parameters (activation energy distribution for Type II kerogen) to produce maturity maps that guide WCSB unconventional play delineation gives exploration geologists, geochemists, and basin modelers the quantitative tools to predict where in the WCSB each petroleum system is in the oil versus gas versus condensate generation window and to rank exploration acreage by charge risk.

• Duvernay petroleum system back-stripping model and maturity calibration across the WCSB: The Duvernay Formation (Upper Devonian, Frasnian age, deposited approximately 375 Ma) was buried progressively from deposition to maximum burial during the Laramide Orogeny (55 to 70 Ma) when Cordilleran thrust sheets loaded the western edge of the WCSB craton. Back-stripping the Duvernay burial history at a well in the deep basin (e.g., Pembina area, current depth 3,500 m) requires: (1) identifying and measuring all stratigraphic intervals above the Duvernay from the well logs; (2) decompacting each interval to its original depositional thickness using lithology-specific porosity-depth curves; (3) estimating the thickness of eroded Mesozoic section above the sub-Cretaceous unconformity from sonic log velocity anomaly analysis (typically 1,000 to 2,500 m of missing section in the west-central Alberta deep basin); (4) reconstructing the depth-time trajectory for the Duvernay top and base from Devonian deposition through maximum Laramide burial to present. The resulting burial history curve combined with a heat flow history yields the Duvernay temperature-time path, which integrated with Type II kerogen kinetics gives a predicted Easy%Ro of 1.0 to 1.8%Ro in the deep basin condensate and wet gas window, calibrated against measured vitrinite reflectance from Duvernay core samples.
• Erosional unconformity estimation methods for WCSB back-stripping models: The largest source of uncertainty in WCSB petroleum system back-stripping models is the amount of section eroded at major unconformities, particularly the sub-Cretaceous (Mesozoic) unconformity that truncates Jurassic and Upper Triassic stratigraphy across much of the WCSB and the sub-Tertiary unconformity in the Alberta plains. Three methods are used in WCSB practice to estimate eroded section: sonic log velocity anomaly analysis compares the observed compaction trend of a formation at a well to the regional compaction trend for that lithology, with the velocity excess indicating overcompaction from deeper burial than the present depth (typical estimates: 500 to 3,000 m eroded section); apatite fission track analysis (AFTA) on core samples determines the maximum paleotemperature experienced by the sample from the annealing state of radiation damage tracks in apatite crystals, which combined with the paleo-geothermal gradient gives a maximum paleodepth; and shale sonic velocity versus depth crossplots comparing WCSB wells in different erosion areas to establish regional compaction baselines. Uncertainty in eroded section estimates of plus or minus 500 m translates to approximately 15 to 25 degrees C uncertainty in maximum paleotemperature, which propagates into a 0.2 to 0.4%Ro uncertainty in predicted maximum maturity for Duvernay and Montney source rocks.
• Heat flow history in WCSB petroleum system models and its effect on maturity prediction: The temperature-time history calculated by back-stripping depends not only on burial depth but also on the geothermal gradient (heat flow / thermal conductivity of the overburden) at each time step. WCSB heat flow history is not constant: Paleozoic extensional tectonics produced elevated heat flow of 70 to 90 mW/m2 during Devonian rifting (375 to 360 Ma); Mesozoic through early Cenozoic subsidence reduced heat flow to 50 to 65 mW/m2; and the modern WCSB geothermal gradient of 25 to 35 degrees C/km (heat flow 55 to 75 mW/m2) represents the post-Laramide thermal relaxation state. Petroleum system models calibrated with only the modern heat flow substantially underpredict Devonian and Mississippian source rock maturity in the western WCSB because they miss the elevated Devonian heat flow contribution; incorporating a two-stage heat flow history (elevated Paleozoic, reduced Mesozoic-Cenozoic) improves Easy%Ro prediction accuracy from plus or minus 0.4%Ro to plus or minus 0.15%Ro when calibrated against measured vitrinite reflectance from Devonian shale core samples.
• Montney petroleum system back-stripping and charge timing for WCSB northeast BC tight gas: The Triassic Montney Formation in northeast British Columbia is both the reservoir and, in its organic-rich silty facies, a self-sourced petroleum system where the organic matter within the Montney itself generated the hydrocarbons now trapped in Montney tight siltstone pore space. Back-stripping burial history models for the Montney in the Dawson Creek and Groundbirch areas show: deposition at approximately 240 Ma; burial to maximum depth of 4,500 to 6,000 m during Laramide thrusting (60 to 75 Ma) with associated maximum temperature of 150 to 185 degrees C (well into the dry gas generation window at Easy%Ro of 2.0 to 3.5%Ro); subsequent exhumation of 500 to 1,500 m to current depths of 3,500 to 4,800 m during Tertiary erosion. The back-stripping model confirms that Montney hydrocarbon generation peaked during Laramide burial and that the current Montney reservoir contains the residual gas charge from that generation episode, implying no ongoing active charge and that the current reservoir pressure (18 to 35 MPa) reflects post-generation pressure equilibration rather than active generation overpressure.
• Petroleum system risk assessment from back-stripping maturity maps in WCSB exploration: Regional back-stripping basin models produce maturity maps (vitrinite reflectance or transformation ratio contour maps) that define the oil, wet gas, condensate, and dry gas generation windows across the WCSB play fairway for each source interval. For WCSB Duvernay exploration, the 0.9 to 1.4%Ro oil window (defined by back-stripping models calibrated at over 200 wells) delineates a crescent-shaped belt from central Alberta southwest to the Edson area that corresponds to the highest Duvernay oil production rates; areas with back-stripping-predicted maturity below 0.7%Ro (immature, Stettler-Hanna area) are excluded from the oil-economic development window, and areas above 2.0%Ro (dry gas, Deep Basin) require different development economics. Charge risk in WCSB exploration wells is assessed by comparing the well location's predicted maturity from the back-stripping model against the calibration dataset; wells in locations where the model is not calibrated by nearby vitrinite reflectance data carry higher charge risk.

Back-Stripping Maturity Model Guiding Duvernay Condensate Window Delineation in West-Central Alberta

A WCSB exploration company building a Duvernay acreage position in west-central Alberta commissioned a back-stripping petroleum system model covering 25,000 km2 of the play fairway using 340 wells with available sonic logs and 28 wells with vitrinite reflectance measurements from Duvernay core or cuttings. The model reconstructed Duvernay burial history from Devonian deposition through Laramide maximum burial (estimated 1,800 to 2,600 m of eroded Mesozoic section from sonic velocity anomaly analysis) to present depth. The maturity map output identified a 6,200 km2 condensate window (predicted 1.4 to 1.8%Ro) straddling the boundary between two land sale areas where the company had bid opposite positions; the company identified that 3,100 km2 of their existing acreage lay within the condensate window while 2,800 km2 lay in the adjacent over-mature dry gas area (above 2.0%Ro). The back-stripping model output directed the company to sell the over-mature dry gas acreage and acquire an additional 1,400 km2 of under-held condensate window acreage at the next Crown sale, a land position decision that the subsequent first three Duvernay wells validated with condensate yields of 180 to 340 bbl/MMscf.

Related Terms

Back-stripping is the primary entry covering the subsidence analysis application, where sequential layer removal reconstructs the tectonic driving mechanism of WCSB basin formation; this companion entry covers the petroleum system modeling application, where burial reconstruction provides the temperature-time history for source rock maturity and hydrocarbon generation kinetic models. Vitrinite reflectance (%Ro) is the primary thermal maturity indicator used to calibrate WCSB back-stripping petroleum system models; measured vitrinite reflectance from Duvernay, Montney, and Second White Specks core samples constrains the modeled burial history and heat flow parameters so that the model predicts correct maturities between calibration well locations. Source rock is the organic-rich formation whose maturity history back-stripping petroleum system models reconstruct; the Devonian Duvernay, Triassic Montney, and Cretaceous Second White Specks are the principal WCSB source rocks where back-stripping maturity models assess charge risk and hydrocarbon phase in unconventional plays. Petroleum system is the complete framework of source, reservoir, seal, trap, and timing that back-stripping petroleum system models evaluate; the charge timing output identifying when peak generation occurred relative to trap formation is one of three components of petroleum system risk assessment used to rank WCSB exploration acreage. Apatite fission track analysis (AFTA) independently estimates maximum paleotemperature and eroded section thickness at WCSB calibration wells, constraining back-stripping model parameters that cannot be derived from well logs alone and reducing maturity prediction uncertainty across WCSB play fairways.