LATE TO MIDDLE JURASSIC SOURCE FACIES AND QUALITY VARIATION, SOUTH VIKING GRABEN, NORTH SEA

H. Justwan*¹, B. Dahl¹, G.H. Isaksen² and I. Meisingset³

¹ Department of Earth Science, University of Bergen, Allegaten 41, 5007 Bergen, Norway.

² ExxonMobil Exploration Company, 233 Benmar, Houston, Texas 77210, USA.

³ Aker Kvaerner Geo AS, PO Box 242, Lilleaker, 0216 Oslo, Norway.

*corresponding author, email: holger.justwan@geo.uib.no

Source facies and quality of the Late to Middle Jurassic source rock system in the South Viking Graben between 58ºN and 60º15’N are highly variable both regionally and stratigraphically. In order to assess the degree of variability and to create a model of source rock quality and potential, isochore maps of the syn- and post-rift sections of the Upper Jurassic Draupne Formation and underlying Heather Formation were generated from seismic and well data, and maturity-corrected Rock-Eval data were used to generate quantitative maps of oil and gas potential. The thin post-rift section at the top of the Draupne Formation is a rich oil-prone source, while the up to 1,600 m thick syn-rift section contains a mixture of Type III and Type II material with substantial amounts of gas-prone and inert organic matter. The Heather Formation, which reaches modelled thicknesses of up to 930 m, is a lean source and is generally gas-prone.

Detailed analyses and interpretations of biomarker and isotopic characteristics support this upward increase in oil-prone Type II material. The analytical parameters include increasing relative amounts of C₂₇ regular steranes; decreasing ratios of C₃₀ moretane relative to C₃₀ hopane; and an increasing predominance of short chain n-alkanes and progressively lighter isotopic values for saturate and aromatic fractions of source rock extracts. In addition, increasing amounts of 17α(H),21β(H)-28,30-bisnorhopane and decreasing amounts of C₃₄ homohopanes relative to C₃₅ homohopanes, as well as decreasing Pr/Ph ratios, suggest a general decrease in oxygenation upwards. Maps of average Pr/Ph ratios for the syn- and post-rift Draupne Formation and for the Heather Formation are consistent with permanent water column stratification and gradual ascent of the O₂:H₂S interface from the Callovian to the Ryazanian.

Interpretation of oil and gas potential maps, molecular parameters and estimates of sediment accumulation rates in combination suggest that the source facies of the upper, post-rift Draupne Formation is controlled by widespread anoxia, reduced siliciclastic dilution and reduced input of gas-prone organic and inert material; by contrast, the potential of the lower, syn-rift Draupne Formation is strongly controlled by dilution by gas-prone and inert organic matter resulting from mass flows and also by varying degrees of oxygenation. The oil and gas potential of the Heather Formation is mainly controlled by the degree of oxygenation and siliciclastic dilution.

JPG Home (opens in this window)