--> Abstract: Basin-Centered Gas Accumulations: Revisiting the Type Areas with Integrated Datasets, by Bruce Hart; #90101 (2010)

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Basin-Centered Gas Accumulations: Revisiting the Type Areas with Integrated Datasets

Bruce Hart
ConocoPhillips, Houston, Texas

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.

Chert-rich shoreface and foreshore sandstones and conglomerates lack quartz overgrowths and produce gas, whereas stratigraphically contiguous quartzose deposits have abundant quartz overgrowths and are tight. Natural fractures are generally not well enough developed in the Deep Basin to allow the tight sands to produce. In the Deep Basin, gas appears to be trapped downdip from water because of both stratigraphic discontinuities and a “permeability jail” in the tight quartzose sandstones. The mechanism that traps gas downdip from water in the San Juan Basin has yet to be defined publicly but, like the Deep Basin, there is no conventional structural trapping mechanism.

 

AAPG Search and Discovery Article #90101 © 2010 AAPG Foundation Distinguished Lecturer Series 2009-2010