The U.S. Geological Survey has undertaken a research program to investigate the geological parameters controlling the distribution of deep natural gas in basins in the United States. Areas of study include the distribution of known deep natural gas resources, structural evolution of deep sedimentary basins, source-rock analysis and reservoir geochemistry, and petroleum assessment. These study areas were defined in collaboration with the Gas Research Institute in order to determine the most important research areas of mutual interest.

Papers in this bulletin address the major areas of geologic research funded by the U.S. Geological Survey Onshore Oil and Gas Program and the Gas Research Institute (Rice, 1989; Dyman, 1992). During the first phase of this work (Rice, 1989), deep well data were tabulated and summarized, preliminary reservoir properties and structural settings for deep natural gas accumulations were identified, porosity and source-rock geochemistry studies were conducted for selected deep sedimentary basins, and U.S. basins were evaluated for favorability of natural gas accumulations. During the second phase of this work (Dyman, 1992), general geologic controls governing the distribution of natural gas in deep sedimentary basins were determined, geologic and production data for large, significant reservoirs were tabulated and summarized, diagenetic controls for selected reservoirs were established, production-test and pressure data from deep wells were interpreted on both a regional and a national basis, geochemical controls and geologic settings of nonhydrocarbon gases were identified, the setting and controls of unusually high porosity in deeply buried rocks were defined, the source-rock potential of Precambrian sedimentary rocks was investigated, and the range of generating potential of kerogen at high levels of maturation was studied. The problem of assessing volumes of natural gas in deep sedimentary basins also was addressed during the second phase of work after geologic studies were well underway.

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Papers in this bulletin summarize major conclusions reached during both phases of our work on deep natural gas resources. Chapters B and C define the areal extent of deep drilling and known resources in the United States. In chapter C, geologic controls of deep natural gas resources are summarized by basin and region. Although not complete, this summary is meant to introduce and establish a framework for subsequent chapters. Chapters D and E present a plate-tectonic framework for deep natural-gas resources (chapter D) and a sequence of Laramide deformation for the Rocky Mountain region with respect to the emplacement of deep natural gas accumulations (chapter E). Chapters F through I discuss reservoir rocks including a summary of reservoir pressures in deep sedimentary basins (chapter F), microporosity trends in reservoirs using mercury-injection porosimetry (chapter G), and porosity in clastic reservoirs in relation to thermal maturity for Rocky Mountain basins and the Anadarko Basin (chapters H and I). Chapters J through M describe geochemical and source-rock studies on Precambrian source-rock potential (chapter J), source and controls of deep-basin natural gas (chapters K and L), and migration of hydrocarbon and nonhydrocarbon gases (chapter M). Chapter N discusses the potential of deep natural gas resources in the Gulf Coast Basin in terms of both source- and reservoir-rocks. In chapter O, assessment methodologies are evaluated, an assessment is presented for a hypothetical deep natural gas play, and play input parameters are modelled in order to show the range of results under different play conditions. Assessment methods were based on natural gas plays, which are defined by unique geologic characteristics and commonly are basinwide.

The papers presented herein are intended to introduce the petroleum community to a set of geologic tools that may be used to predict deep undiscovered natural gas accumulations. This set of tools is not meant to be complete but is a starting point from which to conduct future exploration and production studies. We hope that future studies will include new and expanded applications of the techniques presented here. For example, studies of porosity prediction using thermal maturity measurements must be tested in many deep sedimentary basins and compared with reservoir controls governing the distribution of natural gas. Our assessment models must be evaluated under a broader range of geologic environments and tested in areas of known deep natural gas accumulations. With continued scientific commitment, we hope that deep natural gas resources will become an even more significant and valued part of our Nation's petroleum endowment.