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Denver, CO, United States

Shanley K.W.,Anadarko Petroleum Co. | Cluff R.M.,Discovery Group
AAPG Bulletin | Year: 2015

In many tight-gas basins of the western United States distinguishing between productive and non-productive low-permeability sandstones, and predicting relative amounts of gas and water production is difficult. Comparison of gas shows, calculated water saturations, and saturation-height profiles between gas-productive and non-productive sandstones of equal reservoir quality all appear similar. Capillary pressure derived height functions are difficult to apply, and classic rock-typing procedures lack the predictive capability that is common to more traditional reservoirs. Basin reconstructions suggest the timing of petroleum charge and migration preceded maximum burial and uplift. This initial charge was likely a primary drainage displacement with reservoir porosity greater by a factor of 2-3 relative to values found today and permeability greater by 1-3 orders of magnitude. These reservoir systems became low-permeability following initial charge reflecting continued diagenesis throughout burial, subsequent uplift and erosion. With burial, decreasing pore volume caused water saturations and gas columns to increase. During uplift and erosion gas columns adjusted to changing structural configuration. In some cases this led to gas accumulations being leaked and spilled. In other cases, structural readjustment resulted in capillary imbibition and, in some cases, secondary (or higher order) drainage and imbibition. Within trapped accumulations, gas expansion upon uplift further increased gas columns. In cases where gas columns were spilled or within migration pathways imbibition led to residual or near-residual water saturations. Conventional formation evaluation is fundamentally rooted in concepts associated with primary drainage displacement. Tight-gas reservoirs that have experienced late uplift following an earlier phase of charge are unlikely to be characterized by primary drainage and are much more likely to be characterized by imbibition or secondary (or higher order) drainage and possibly imbibition. The hysteresis between primary drainage and imbibition or secondary (or higher order) drainage and imbibition in tight-gas reservoirs is significant and unlike many more traditional reservoirs does not tend to converge on a narrow range of values. Estimates of water saturation are scalar values and do not contain information that allows the saturation history and displacement direction to be deciphered. Recognition that reservoirs are unlikely to be in primary drainage equilibrium is a fundamental paradigm shift that impacts petroleum evaluation at all scales ranging from basin potential to completion decisions within a given well. Although this paper is written from the perspective of tight-gas petroleum systems, the principles are equally applicable to low-permeability oil reservoirs. © 2015. The American Association of Petroleum Geologists. All rights reserved. Source

Shanley K.,Discovery Group
Leading Edge (Tulsa, OK) | Year: 2010

Unconventional gas resources con-tinue to be a growing part of the total gas production in the United States and have captured the interest of energy firms around the world. Unconventional resources comprise three major categories; tight-gas sand-stones (low-permeability), shale gas, and coal-bed methane. Tight-gas reservoirs currently comprise approx-imately 35% of the uncon-ventional gas production in the U.S. Lower 48 (Wood Mackenzie's Unconventional Gas Service). © 2010 Society of Exploration Geophysicists. Source

Catuneanu O.,University of Alberta | Bhattacharya J.P.,University of Houston | Blum M.D.,ExxonMobil | Dalrymple R.W.,Queens University | And 19 more authors.
First Break | Year: 2010

Sequence stratigraphy emphasizes changes in stratal stacking patterns in response to varying accommodation and sediment supply through time. Certain surfaces are designated as sequence or systems tract boundaries to facilitate the construction of realistic and meaningful palaeogeographic interpretations, which, in turn, allows for the prediction of facies and lithologies away from control points. Precisely which surfaces are selected as sequence boundaries varies from one sequence strati-graphic approach to another. In practice, the selection is often a function of which surfaces are best expressed, and mapped, within the context of each case study. This high degree of variability in the expression of sequence stratigraphic units and bounding surfaces requires the adoption of a methodology that is sufficiently flexible to accommodate the wide range of possible scenarios in the rock record. We advocate a model-independent methodology that requires the identification of all sequence stratigraphic units and bounding surfaces, which can be delineated on the basis of facies relationships and stratal stacking patterns using the available data. Construction of this framework ensures the success of the method in terms of its objectives to provide a process-based understanding of the stratigraphic architecture and predict the distribution of reservoir, source-rock, and seal facies. © 2010 EAGE. Source

Whittaker S.D.,Discovery Group | Sharma R.,Discovery Group | Hallau D.,Discovery Group | Lewis J.P.,Intrepid Potash | Cluff R.M.,Discovery Group
Proceedings of the Symposium on the Application of Geophyics to Engineering and Environmental Problems, SAGEEP | Year: 2010

Prior studies of gamma-ray tools have all been focused around instruments that typically evaluate sandstone, shale, and limestone formations that are encountered in oilfield logging. For this study, a set of experiments were conducted to characterize the lateral and vertical response functions for a slim-hole gammaray sonde used in the mining industry to locate potash mineral deposits. The experiments were conducted in an indoor warehouse environment utilizing a set of large plastic tanks that were filled with light evaporite minerals (granular halite and sylvite) in differing arrangements to simulate various possible formation configurations. Measurements were taken while using a centralized slim-hole gamma ray tool in an air filled 4.5 inch plastic borehole. Sequential tests were run to establish the linearity of tool response, the radial depth of investigation, the vertical response function, and the repeatability of the measurement. Radial depth of investigation was measured using concentric radioactive rings of increasing diameter with two possible intermediary substances, air and halite. To test vertical response a column structure was built using halite as the bottom & quot;bed & quot; and a sylvite layer was systematically added in known quantities to acquire a response function of increasing thicknesses. Repeatability of the measurements was verified by logging several points with multiple tools of the same model for all the various experimental setups. Results were corrected for background radiation to predict the response in solid subsurface conditions without incident surface radiation seen in the experiments. The findings were also corrected for the differences between low porosity subsurface conditions and the unconsolidated granular products that were used in the simulated formation. The experimental results were surprisingly close to theoretical tool response for an oil field sonde, as well as to published specifications of major oilfield logging vendor's tools. Consequently, gamma-ray logs collected with a slim-hole tool in shallow mineral core holes should be directly comparable to oilfield gamma-ray tools run in open-hole wellbores, once corrections for borehole size and fluid content using best practices are applied. Source

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