Abstract

The texture and faunal content of bedded limestones may be used to outline various environments of deposition including those of shelf (shoal, inter-shoal and lagoon) and basin areas. Shoal deposits, represented in general by well sorted calcarenites of both bioclastic and precipitated origin, are commonly the most porous and permeable limestone types. Thus, where petroleum accumulation in bedded limestone units is limited at least in part by primary up-dip or lateral permeability variations, facies maps showing the probable distribution of permeable shoal areas are extremely valuable in outlining petroleum prospects. Such prospects include both field extensions (most critical during early development drilling) and prospects in strictly wildcat areas. In most cases, drilling locations to test the stratigraphic anomaly may be given without further geological or geophysical work. This method of exploration is extremely cheap and its effectiveness is limited only by the quality of the lithologic data available and the environmental interpretation.

An example of the application of lithofacies and biofacies techniques to petroleum exploration is shown by recent studies along the Mississippian Midale trend is southeastern Saskatchewan. Fields along this trend form part of a huge regional stratigraphic trap formed largely by truncation and sealing of the Midale members of the Charles formation. The seal is formed by secondary anhydrite infilling pores at the truncated erosion surface and overlying impermeable red beds. Also important to entrapment is the presence of a locally, developed primary anhydrite bed beneath the pay zone. This anhydrite unit forms a floor to the reservoir and prevents oil from migrating updip along the unconformity.

Regionally, oil accumulations along the Middle trend are limited by erosional truncation of the Midale member in an up-dip direction and occurrence of water down-dip. In detail, however, productive fields along the trend are limited by lateral and locally by up-dip decrease in permeability. The permeability variations are directly related to primary sedimentation patterns. Thus, detailed facies maps of the Midale member show the close relationship between productive areas and permeable limestone (shoal) facies. These maps are presently being used to outline drillable petroleum prospects.