KOSTERS, ELIZABETH C., FRANS J. VANDERZWAAN, and J. GIJSBERT, University of Utrecht, Utrecht, Netherlands

ABSTRACT: Organic Facies and Systems Tracts: Implications for Source Rock Preservation and Prediction

Sequence stratigraphy is concerned with making predictions about reservoirs ahead of the drill. Whereas much progress has been made in this field, little attention has been paid to the configuration of organic-rich facies of source rock quality. We suggest that preservation of source rock type facies in clastic systems, both continental and marine, is mutually exclusive and time successive.

The main database is a collection of cores and other samples through the Holocene Rhone delta. The early Holocene Transgressive Systems Tract (TST) contains five levels of channelization; channels are encased in delta plain muds and thin peats. The most significant peat bed is located immediately landward of the shoreline of maximum transgression (SMT). The Highstand Systems Tract (HST) consists of two parasequences, containing mostly laterally continuous strandplain complexes without peat.

Benthic foraminiferal patterns indicate a more or less continuous shallowing of the system throughout the sequence. Their signature is influenced strongly by the predominant shore-parallel current system as well as by clastic input. Eutrophic conditions, resulting in organic-rich shelf muds, are probably predominant in HST. Additional analyses of Holocene sequences in the Mississippi, Orinoco, and Po deltaic systems suggest very similar relationships between occurrence of continental and marine organic facies.

In addition to sufficient accommodation space, an important control on formation of fresh-water peats and organic-rich shelf muds in such a system is availability of river-induced nutrients. Peat quality, however, is best without riverine clastics. In a delta plain, a balance between these two controls may be reached when river-fed nutrients are trapped there indirectly, i. e., by groundwater recharge. The potential for such a condition arises in a TST setting.

On the shelf, eutrophication of marine habitats is controlled also by river-fed nutrients, but excess river clastics are detrimental to marine source rock quality. A balance between these two controls may be reached in HST settings, where fine-grained riverine clastics are forced onto the shelf rather than in the delta plain. In this situation, nutrient supply to the shelf results in large quantities of marine biomass. This biomass becomes sufficiently concentrated due to moderate fine-grained riverine sedimentation, which, at the same time, guarantees burial and preservation.

Thus, varying river-water and nutrient supply in TST and HST settings seems to control large-scale preservation patterns of both continental and marine organics. This hypothesis suggests further potential for using sequence stratigraphy for source rock occurrence.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.