University of Utah, United States

Biostratigraphy has been an integral part of hydrocarbon exploration for much of the last century. The basic concepts and methods of using fossils to characterize coeval strata remain unchanged from the pioneer studies of the late 18th century. This paper describes major developments in biostratigraphic techniques that have occurred during the last twenty years. Advances in deep-water drilling and exploitation technology have promoted a shift from deltaic and shelf reservoirs to slope and basin-floor-fan reservoir targets. This has resulted in a greater emphasis on open marine, planktonic fossils, such as nannofossils and planktonic foraminifera, and lesser emphasis on terrestrial and shallow-water fossils such as benthic foraminifera, pollen, and spores. The development of deviated and horizontal production wells has stimulated the need to monitor the path of wells in real time at the wellsite. The resulting "biosteering" is the quantitative bed-by-bed study of a vertical pilot well, and the subsequent monitoring of the fossil content of the horizontal well, which enables the drill path to stay in the target strata. The Ocean Drilling Program and its predecessor the Deep Sea Drilling Project led to the construction of more refined geologic time scales that integrate biostratigraphy with magnetostratigraphy, isotope stratigraphy, and astronomical cycles. These time scales allow the allocation of numeric ages to increasing amounts of biostratigraphic datums, which aids in the automation of correlation using computers. The concepts and methods of sequence stratigraphy refined in the last twenty years placed biostratigraphy at the heart of geological models, closely integrating it with well-log and seismic stratigraphy. The concepts of sequence stratigraphy stimulated further biostratigraphic research and changed the way biostratigraphic data are used in correlations. Computational power has made it possible to rapidly record, manipulate, and display biostratigraphic data. This has led to the development of programs that automate the process of correlation, paleobathymetric interpretation, construction of expert systems for the identification of fossils, and compilation of databases of text and digital images. These advances have led to more information being extracted from the raw biostratigraphic data.