Application of Re-Os Geochronology to Hydrocarbon Exploration: Case Study of Middle Triassic Black Shales from the Barents Sea and Svalbard
Application of Re-Os geochronology to organic-rich black shales provides the depositional age and the seawater 187Os/188Os ratio at the time of deposition. The Re-Os isochron ages calibrate the biostratigraphic ages while the seawater 187Os/188Os ratios fingerprint source rocks; together, these data provide a valuable tool for basin modeling and petroleum exploration. Here we apply Re-Os geochronology to Anisian (Middle Triassic) black shales from outcrop in western Svalbard and drill core from the submerged Svalis Dome, ~600 km to the SE in the Barents Sea.
Absolute age control throughout the Triassic is sparse, and ages of stage boundaries are controversial. Black shales of the Blanknuten Member of Botneheia Formation, from Botneheia, western Spitsbergen basin, have total organic carbon (TOC) contents of 2.6 to 6.0 wt%. Rock-Eval data suggest the shales are moderately mature (Tmax = 438-446°C). Re-Os data yield a Model 3 age of 241 Ma and initial 187Os/188Os (Osi) of 0.83 (n = 8). Black shales of the Steinkobbe Formation from IKU core 7323/07-U-09 (94-95 m) from the Svalis Dome(about 73°30’N, 23°15’E) have TOC contents of 2.2 to 3.6%. Rock-Eval data suggest the shales are immature (Tmax = 426-430°C). Re-Os data yield a Model 3 age of 240 Ma and Osi of 0.68 (n = 7).
The sampled section of Blanknuten Member shale at Svalbard underlies a distinctive Ptychites layer, and is inferred to be middle to upper Anisian. The Steinkobbe section was sampled at ~3 m above the late Anisian Frechites laqueatus Zone and thus was assumed to be near the Anisian-Ladinian transition (Vigran et al. 1998). The two isochron ages are consistent with the biostratigraphy and agree with the current ICS-approved stage boundary ages. These Re-Os ages support the correlation of the Botneheia and Steinkobbe Formations.
Our 187Os/188Os seawater ratios are the highest yet recorded in black shales spanning the Toarcian stage (181 Ma, Late Lower Jurassic) to the earliest Cambrian (542 Ma) and provide new points on the Os seawater curve. Such high 187Os/188Os seawater ratios are consistent with the sharp rise in the 87Sr/86Sr seawater curve from 0.7068 in the Late Permian to a peak of 0.7082 at the Olenekian-Anisian boundary (Korte et al., 2003). Large variations and sudden swings in seawater Os ratios show great potential for correlating hydrocarbons with their source rocks.
References: 1. Korte et al. 2003, GCA, 67:46-62; 2. Vigran et al. 1998, Palynology, 22:89-141
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009