Source Quality Variations Tied to Sequence Development: Integration of Physical and Chemical Aspects, Lower to Middle Triassic, Western Barents Sea

BOHACS, K. M., and G. H. ISAKSEN, Exxon Production Research Company, Houston, TX

Triassic mudrocks from the Barents Sea area demonstrate the covariance of physical and chemical properties of mudrocks deposited in shelfal environments and the aspect of depositional sequences in distal settings. The tie of physical parameters to chemical character within a detailed sequence-stratigraphic framework enables the construc-tion of depositional-facies models to predict organic-matter content and quality. This allows the explorer to more closely constrain and predict the nature of potential source rocks using seismic and well-log data.

Changes in lithology, bedding geometry, sedimentary structures, body and trace-fossil assemblages, and inorganic, bulk-organic, and molecular geochemistry revealed the detailed depositional environments. Burrow types changed from diverse, inclined to horizontal grazing traces to small, monospecific, horizontal mining traces, representing decreasing oxygen content in the water column and sediment interstices. The sediment/water system appears to have never become completely anaerobic, but reasonable quality potential source rocks were deposited under a dysaerobic water column over dysaerobic to anaerobic sediment interstitial water. The depositional environments stack predictably, according to their position in the depositional sequence: from aerobic lower-shoreface--offshore transitio environments in lowstand systems tracts to dysaerobic-anaerobic distal open-marine-shelf environment in transgressive and early highstand systems tracts.

Quantitative molecular geochemistry also revealed variations within this distal setting and strong covariance with sequence position. Input of organic matter from terrigenous higher plants dominates the lowstands whereas marine-algal organic matter is most prevalent within transgressive and highstand systems tracts. Specifically, the abundance of C steranes, total steranes, and moretane reflected development of the sequences.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)