Distinguished Lecturer, Fall 2009

The introduction of seismic stratigraphic techniques in the 1970s gave sedimentary geologists in the petroleum industry and Academia new tools for predicting lithology and analyzing the depositional history of sedimentary basins. Seismic stratigraphy originally focused on large-scale exploration problems and was based on analyses of 2-D seismic data in areas that were relatively "data poor" (i.e., few logs, core or production data). Although these conventional seismic stratigraphic analyses are still used fruitfully, new challenges and opportunities confront the petroleum industry as it faces the need to improve recovery from mature fields.

These areas are commonly data rich (lots of log, core and production data), and covered by relatively small 3-D seismic surveys that do not image all of the sequences or systems tracts that the reservoir rocks are part of. As such, a new mindset is needed, here termed "reservoir-scale seismic stratigraphy," to help geoscientists maximize the stratigraphic information they can extract from seismic data. Integration of geologic and geophysical concepts and data is critical.

Techniques employed by geophysicists for at least the past decade (inversion, seismic attribute studies, seismic facies analysis, etc.) need to become routine parts of the sedimentary geologist's toolkit, whereas seismic interpreters need to study outcrops, core and modern analogs in order to anticipate the presence of depositional features that cannot be resolved seismically. This cross-disciplinary interaction will undoubtedly spawn new breakthroughs in sedimentary geology, reflection seismology, petroleum geology and related fields (e.g., hydrogeology). These are exciting times.

The basin-centered gas concept has evolved considerably in the 30 years since the publication of Master's AAPG Bulletin paper that compared the Deep Basin of Alberta to the San Juan Basin of New Mexico and Colorado. In both areas, Masters noted that gas-charged, low-permeability Cretaceous sandstones are present over broad areas in the deeper part of the basin. Furthermore, the gas-charged sandstones appeared to be in stratigraphic continuity with wet sands around the margins of the basin. The basin-centered gas concept was widely adopted, but has since come under attack.

It is time to revisit the type areas, summarize existing knowledge, and integrate 3-D seismic, log, core, outcrop, and production data to study the controls on gas production. Commercial production from Cretaceous sandstones of the San Juan Basin is made possible by extensive regional fractures, and is enhanced by fracture stimulations. These tight sands are gas charged everywhere in the deepest, central portion of the basin. Integration of 3-D seismic and other data show that production "sweet spots" in this basin are generated by fracture swarms that are related to subtle structural features. On the other hand, production sweet spots in the Deep Basin are clearly stratigraphic in origin.