Chapter 15
Measured Versus Predicted Vitrinite Reflectance from Scotian Basin Wells
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Implications f o r Predicting Hydrocarbon Generation—Migration 1
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P. Κ Mukhopadhyay , J. A. Wade , and M. A. Williamson
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Global Geoenergy Research Limited, 14 Crescent Plateau, Halifax, Nova Scotia B 3 M 2V6, Canada Geological Survey of Canada, Atlantic Geoscience Centre, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada Model predicted vitrinite reflectance data are calibrated with measured reflectance profiles for wells in the Scotian Basin, Eastern Canada. The predicted data were derived using a basin modeling package (BasinMod) which assumed a rifting heat flow at 200 Ma, a variable stretching factor (β), and present day heat flow of 42 mW/m . Using combined measured and predicted maturity parameters, measured kinetics on selected source rock kerogens (Kerogen Type IIΑ-IIΒ and IIB), and a kinetic model, the timing of hydrocarbon generation and migration from various source rocks (especially Type IIΑ-IIΒ from the Lower Cretaceous in N. Triumph G-43 and Type IIΒ from the Upper Jurassic in Venture B-52) are estimated. Variations in maturation profiles are correlated to abnormal heat transfer due to changes in thermal conductivity or the presence of older rocks (closer to rifting). Anomalies in maturation boundaries are attributed to high sedimentation rates and differences in thermal conductivities due to variations in lithology. Lower Cretaceous (Naskapi Member) source rocks generated and migrated oils during the last 25 Ma; Upper Jurassic-Kimmeridgian Type IIB source rocks migrated mainly condensates and gases between 85-50 Ma. 2
Present day geochemical basin modeling can illustrate the thermal history and hydrocarbon generation of a source rock through time using a chemical kinetic expression (1-6). Generally, it implies various first order simultaneous reactions with variable activation energies and reaction rates instead of a single reaction as envisaged by empirical basin modeling (7-9). A basin's thermal history will control the generation of hydrocarbons from suitable source rocks. The thermal history is assumed to be derived through convection and conduction (possibly by stretching and rifting of crust) from the upper mantle/lower crust to upper crust (10, 11). In the Scotian Basin of Eastern both empirical and kinetic basin modeling 0097-6156/94/0570-0230$08.00/0 Published 1994 American Chemical Society
Mukhopadhyay and Dow; Vitrinite Reflectance as a Maturity Parameter ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
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15. MUKHOPADHYAY ET AU
Vitrinite ReflectancefromScotian Basin WeUs
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concepts were used by earlier workers to define the level of organic maturation of various sequences through time (9, 12 and 13) and to define hydrocarbon generation rates. Those concepts also define the relationship between overpressuring and gas generation (14 and 15). Since 1989, extensive research on various source rocks, crude oils and condensates taken from several Scotian Basin formations have defined the specific kerogen types and maturation; their distribution in the basin; oil-source rock correlation; and possible hydrocarbon migration (16, 17, and 18). In this study, a commercial basin modeling package (BasinMod; 19) was used to illustrate the following: (a) the influence of various geological parameters affecting thermal modeling or maturation history through time on individual source rocks; (b) the causes of variable maturity in the basin; (c) the relationship between measured kinetics of individual source rocks and the boundary conditions of oil and gas generation; and (d) the possible timing of hydrocarbon migration in relation to their thermal maturity. Boreholes, Data Input and Limitations of the Data Eight boreholes (Alma F-67, Cohasset D-42, Evangeline H-98, N . Triumph G-43, S. Desbarres 0-76, S. W. Banquereau F-34, Thebaud C-74, and Venture B-52) were selected from a variety of depositional settings for one-dimensional basin modeling (Figure 1). A l l wells are located on the Scotian Shelf near Sable Island. Figure 2 illustrates the approximate position of the 8 wells within the stratigraphie column of the central Scotian Basin. This present paper focusses on three of the wells, Cohasset D-42, N . Triumph G-43 and Venture B-52, which encountered key source rocks for light oils or condensates. Cohasset D-42 penetrated the Abenaki carbonate platform; Venture B-52 encountered the Missisauga and Mic Mac formations in shallow marine to non-marine deltaic fades; and N . Triumph G-43 sampled Lower Cretaceous and Upper Jurassic distal shelf to basinal facies. A series of input data, such as geological parameters, kerogen type, total organic carbon content, compaction, etc., are required to perform the modeling analysis. The geological parameters used follow Wade and MacLean (20). The absolute age ascribed to each formation is referenced to Palmer (21) and Wade and MacLean (20). Lithology, top depth and thickness of each formation were based on CanStrat log analysis and seismic stratigraphy. The Breakup Unconformity or the time of rifting was placed at 200 M a (22). Dehler and Keen (22) also indicated three phases of rifting: one at 220 Ma, another at 200 M a and the third at 180 Ma. The data below total depth in the modeled wells were extrapolated from regional geological and geophysical studies. The model is calibrated to measured vitrinite reflectance data for each well. Various default parameters such as decompaction factor, expulsion efficiency, β, heat flow, etc., which were used for the calculation of maturity and hydrocarbon generation, are described in Appendix 1. Table 1 illustrates model calculated conductivity and heat capacity data for standard lithologies such as sandstone, shale etc., and for mixed lithologies. The kerogen type and TOC (wt %) values were derived partly from the measured analytical data and partly from an approximation based on lithologie variation, depositional environment and knowledge of Scotian Basin source rocks. Kinetics data for Kerogen Type I, Π (considered as Kerogen Type IIA) and ΠΙ are taken from the model default values. The model allows for mixed kerogen types and, Mukhopadhyay and Dow; Vitrinite Reflectance as a Maturity Parameter ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
Mukhopadhyay and Dow; Vitrinite Reflectance as a Maturity Parameter ACS Symposium Series; American Chemical Society: Washington, DC, 1994.
Figure 1. Index map of wells in the Sable Island area of the Scotian Shelf, Canada. Black dots show locations of exploratory and delineation wells. Named wells were modeled for this study.
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15. MUKHOPADHYAY ET AL.
Vitrinite Reflectance from Scotian Basin Wells
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