Use of Well Logs and Core Data to Assess the Sequence Stratigraphic Distribution of Organic-Rich Rocks

Abstract

Application of a well log technique for routine profiling of total organic carbon (TOC) in source rocks has revealed that patterns in the vertical distribution of TOC commonly occur. Marine source rocks frequently are composed of one or more units with the highest TOC values near the base. These units coalesce in a seaward direction and form a relatively smooth TOC profile with increasing TOC upward to a central maximum or series of maxima with a gradual decrease in TOC above this maximum. A model for marine source rock accumulation is presented that invokes shelfal accommodation of sediment as the primary control on the magnitude of TOC. In the presence of an anoxic sediment/water interface, deposited organic carbon will be preserved. The amount of organic matter will vary inversely as the influx of terrigenous clastics varies. If the amount of shelfal accommodation is high, the influx of clastics is greatly reduced in basinward locations, and the resulting weight % TOC in the sediment is increased. The maximum TOC in a vertical profile through a single marine sequence probably correlates to the time of maximum flooding. In an idealized lowstand to transgressive through highstand prograding cycle, the trend in organic mattermore » type would be predominantly type 3/4 to type 2, back to type 3/4 kerogen as controlled by the interplay of sedimentation rate and the presence or absence of anoxia. Lacustrine oil-prone TOC vertical profiles show strong similarities to marine source rocks, suggesting that sedimentation rate may be a controlling mechanism in this environment also. Coals and organic-rich muds within delta plan/coastal environments are readily identified using well logs.« less