Tight sandstones of Williams Fork Formation,                          Southern Piceance Basin, Colorado:            Key re...
distribution, intensity, and orientation of this fracture network may help in designing the drilling,evaluation, completio...
Currently, total daily gas production all over the Piceance basin has increased from under 200MMCFD in the year 2000 to ov...
the prospect unattractive and disapproving response from the communities of the surrounding area.Rulison field, as the mai...
southern Piceance Basin, as large volumes of gas were generated from this 850ft thick coal as theyachieved high thermal ma...
Some of the work and analyses include complete core analysis program, extensive well log andwell testing program, well to ...
Microscopic mineralogical analysis points out that diagenetic cements commonly present arequartz, calcite and ubiquitous i...
Diagenetic processes commonly occurred in the sandstones; include K-Feldspar dissolution andillite pore filling. This K-Fe...
Figure 7. Piceance Basin gas model – present day.20There are two favourable conditions support the generation of highly ga...
Figure 8. Key reservoir features and fluid distribution control in Piceance Basin.15,28Natural Fracture characterisationCo...
•   High-resolution aeromagnetic data (2500 m2) to delineate the geometry of the basement    structure.•   2-D seismic and...
The 3D seismic survey later on was conducted on a 4.5 mi2 area, aimed at targeting naturalfractures especially located at ...
fracture zones associated with basement structural features as applied in the tight sandstones of theWilliams Fork formati...
14. Johnson, R.C.: Geologic history and hydrocarbon potential of Late Cretaceous age, low    permeability reservoirs Picea...
30. Lorenz, J.C. and Finley, S.J.” Significance of drilling- and coring-induced fractures in    Mesaverde core, northweste...
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Piceance yulini paper glgy699

  1. 1. Tight sandstones of Williams Fork Formation, Southern Piceance Basin, Colorado: Key reservoir features and natural fracture detection techniques By : Yulini ArediningsihAbstractKey reservoir features of tight sandstones of the Williams Fork formation of the Mesaverde Group,in Southern Piceance Basin that include fluid distribution controls and natural fracturecharacteristics are reviewed. Integrated techniques on delineating subsurface natural fracturesrelated to basement features in the reservoirs are also outlined.Gas production from a large deep basin-centered gas accumulation in this formation hassignificantly improved due to enhanced permeability by naturally occurring fractures in thereservoir. The gas production is mainly from continuously gas saturated of about 900feet intervalof the highly lenticular nature of the fluvial sandstone reservoirs. Key reservoir features in theSouthern Piceance basin include thick, matured Cameo coals, presence of high heat flow at depth,naturally occurring fractures due to over pressured condition and gas-saturated reservoir with littlemovable water. The naturally occurring fractures control the mobility and distribution of the fluidwithin the reservoir. Application of low cost aerial geophysical surveys integrated with an RTMbased basin modeling enables to detect anomaly fracture prone zones associated with basementstructures. The areas are then selected for 3D seismic survey to validate the presence of thesubsurface fracture zones. The 3D seismic survey is proven to provide a powerful technology foridentifying fracture zones associated with basement structural features to locate futuredevelopment area.IntroductionUnconventional tight gas reservoirs have become popular targets in petroleum exploration inrecent years, due largely to its large volume, the increasing market demand for gas and totechnology advances used in stimulation treatment. The latter moreover has increased chance ofreopening previously uneconomical gas accumulation. Significant milestone of tight gasexploration was marked by highly productive tight gas sand development from the Jonah andPinedale fields in Western Wyoming back in 1990s. The fields were discovered in 1957, but notproduced in next four decades due to advanced stimulation technology was not available yet at thattime to recover over-pressured gas, locked in tight reservoir rock. Finding ways to unlock the tightgas fields so that the gas can be tapped off to the surface is a paramount goal in developing thefields. In some tight gas fields, natural fracturing mechanism that may improve overall reservoirpermeability commonly occurs. In some extent, its occurrence requires favourable geologiccircumstances that should be fully understood. Optimization in detecting, and delineating its 1
  2. 2. distribution, intensity, and orientation of this fracture network may help in designing the drilling,evaluation, completion and stimulation programs which in turn may increase the gas recovery.This paper overviews key reservoir features of tight sandstones of the Williams Fork formation ofthe Mesaverde Group, particularly in Southern Piceance Basin where most of tight gas fields arelocated. It covers fluid distribution controls and natural fracture characteristics. The paper alsooutlines integrated techniques on delineating subsurface natural fractures related to basementfeatures in the reservoirs.Current Status of the Piceance BasinPiceance Basin is one of major unconventional oil and gas accumulations in Lower 48 States in theUnited States, located at the North West corner of Colorado (Figure 1). The basin is well knownfor extremely prolific of world class oil shale deposit, estimated to contain 1.525 trillion barrels ofin place oil shale resources.1 Moreover, the basin also contains approximately 311 trillion cubicfeet (TCF) of original gas in-place, with 106 TCF of this gas volume is existent in four gas fieldslocated in the southern part of the basin,2 including Rulison, Mamm Creek, Parachute and GrandValley (Figure 2). Figure 1. Map of Major Tight gas Plays in 48 Lower States, the United States3 2
  3. 3. Currently, total daily gas production all over the Piceance basin has increased from under 200MMCFD in the year 2000 to over 1 BCFD.4 The gas is primarily produced from the continuouslygas saturated sandstones of the Williams Fork Formation of the Mesaverde Group. Application of10-acre well density has proven successful, particularly in some gas fields of the southernPiceance Basin.4 Their EURs typically range from 1 to 2 BCF per well, resulting in reserves ofabout 60-120 BCF per section.2 Figure 2. A contour map of Piceance Basin showing location of the gas fields in southern part of the basin.5OverviewA wide range of age rock formations from Cambrian to Holocene in general makes up thePiceance basin. Stratigraphic unit of the main gas producer in the basin is Cretaceous WilliamsFork Formation of Mesaverde Group, consisting of shale, sandstones and coal deposited in acoastal plain environment. In the past, this sandstone reservoir was bypassed due to very lowpermeability in nano darcy range causing the prospect unappealing. Initial attempt to stimulatehydraulic fracturing was performed in a well drilled into the Williams Fork Formation southwestof Rifle Town, Colorado in September 1969, so called Project Rulison. The Project Rulison usedsubsurface nuclear explosion to determine the potential of the technique for commercialdevelopment of the tight Mesaverde sandstone.6,7 Utilisation of the well bore nuclear explosion left 3
  4. 4. the prospect unattractive and disapproving response from the communities of the surrounding area.Rulison field, as the main field in the southern Piceance, is situated near Rifle town, along theColorado River in the southern part of the Piceance Basin, Garfield County (Figure 1). The fieldwas discovered in 1956 with the drilling of the Juhan #1 well. Within three decades, in fact, only36 more wells were drilled, because of poor quality wells and numerous dry holes. Then, theRulison field was judged to be an uneconomic gas play. The field was estimated to haveapproximately 10 Tcf of gas in place in the Mesaverde tight sands.8 During the period 1980s and1990s, integrated work of Intensive resources Development and Multi-Well Experiments (MWX)on the southern gas fields, supported by Department of Energy (DOE)/National EnergyTechnology Laboratory (NETL) and several industry-sponsored resources, had been carried out tocharacterize the reservoir and its natural fracture systems. It results improvement on its producibleflow rate.9 In 2009, the Piceance basin contained five of the top 50 US gas fields in provedreserves including the Rulison field that posts rank of 24th.10Geological setting and StratigraphyThe Piceance basin is an intermountain structural basin formed by Laramide orogeny during LateCretaceous through Paleocene. The basin is located in the Rocky Mountain foreland,morphologically characterised by a gently dipping western and southwestern flanks and a sharplyupturned eastern flank.11 Structurally, the Piceance basin has asymmetric north-western trendingaxis with sedimentary rock that have thickness exceeding 20,000ft.12 The basin is bounded by theDouglas Creek Arch, separating it from the Uinta Coal Basin (Utah) to the west. Grand HogbackMonocline separates the basin from the White River uplift to the east (Figure 3). Fold structuresare commonly present, including northwestern trending anticlines within the basin such as such asthe Crystal Creek and Rangley Syncline, the Grand Hogback Monocline, and thrust westward.Faults are present in some areas at the northern margin.Tectonic setting associated with periods of uplift and erosion occurred during the Late Tertiaryhave contributed the formation of vertical extension natural fractures.13 These typical fractures arecommonly found in the William Fork sandstones. The natural fractures also have formed inresponse to high pore-fluid pressures that developed during hydrocarbon generation.4Stratigraphically, Piceance basin is primarily made up by Mesaverde Group of the Late CretaceousAge, about 100 to 65 million years in age.14 Figure 4 shows generalized stratigraphic column ofthe northern and southern part of the basin.The oldest formation of the Mesaverde Group, Iles Formation, primarily comprises of interbeddedblanket sandstone units deposited in shallow marine to meandering stream to shorelineenvironments with regressive cycles. The sandstone units include Corcoran, Cozzette, and RollinsSandstones, interfingering with deep water Mancos shale tongues. Williams Fork Formationconformably overlying the marine Rollins member of the Iles Formation, can be divided into twomain units, namely Cameo-Wheller Coal Zone and Williams Fork Formation.16 The Cameo-Wheller Coal zone with a total thickness up to 850ft, is made up by about 30 – 100ft thicklenticular sandstone, interbedded with marshy coals and shale. The Cameo-Wheller Coals areexposed in Coal Canyon on the western edge of the basin, and are buried as deep as 9,000ft in thecentral part of the basin.18 This coal is thought to be the main source of natural gas charge in the 4
  5. 5. southern Piceance Basin, as large volumes of gas were generated from this 850ft thick coal as theyachieved high thermal maturity.17 The main part of the Williams Fork Formation can be describedas a 1500-2400ft thick stacked unit, primarily composed of lenticular sand bodies in shale matrix.The sand bodies are deposited in distal braided stream, fluvial plain environment. The sand lensesare laterally discontinuous with irregular distribution.19 Overlying the Williams Fork is Ohio CreekConglomerate, defined as proximal braided stream deposit with significant occurrence of a 20ftthick laterally extensive shale known as the upper Williams Fork shale marker.20 Figure 3. Tectonic Map of the Piceance basin.12Williams Fork Sandstone CharacterisationReservoir characterisation of the William Fork sandstones of Mesaverde Group in the SouthernPiceance Basin is benefited by the availability of the well and core data from four MWX wells.The wells were drilled vertically and horizontally in the vicinity of southern Piceance gas fields forthe purpose of on site laboratory. Various work and analyses that have been done based on thosedata in order to get a better understanding on the lenticular sandstone characteristics,19,21,22 thenatural fractures2,8 and evaluation on stimulation performance on the low permeable sandstones.12 5
  6. 6. Some of the work and analyses include complete core analysis program, extensive well log andwell testing program, well to well profiling, and the use of seismic surveys and sedimentologicalanalyses.Figure 4. Simplified stratigraphic column representing sequences and depositional environments of the formations in the Piceance Basin.15The Williams Fork sandstones are compositionally grouped as lithic arkose to feldspatic litharenitewith porosity and permeability measured from the cores at 1200psi ranging from 4.6 – 7.7 % and6.1 – 7.1 microdarcy, respectively.22 Bulk density porosity determined from the logs ranged from7.4 to 8.8%. An average corrected porosity of 8.2% was determined for the lenticular WilliamsFork section on the MWX site and inputted in the modeled porosity, giving a gas filled porosity of3.7%. Based on MWX core analysis, average matrix permeability to gas is 250 nanodarcy, whilenatural matrix permeability is 36 to 600 milidarcy. The well testing program provides maximumpermeability whose direction is approximately N780W, consistent with direction of the naturalfractures and maximum principal stress. Reservoir pressures measured at MWX site were 3,410psi.12 6
  7. 7. Microscopic mineralogical analysis points out that diagenetic cements commonly present arequartz, calcite and ubiquitous illite.21,22 Common pore geometry present in the William Forksandstones is dual porosity type of narrow intergranular slots that connect secondary solutionpores.21 (Figure 5). Other types are present minor as intergranular primary porosity and secondaryporosity resulted by dissolved feldspar (Figure 6). A B Figure 5. Photomicrograph A22 and scanning microscope images B23 portraying slot type pores that is common in the sandstones Figure 6. Primary and secondary pore types present.22 7
  8. 8. Diagenetic processes commonly occurred in the sandstones; include K-Feldspar dissolution andillite pore filling. This K-Feldspar dissolution may contribute constituents required in alteringlower grade of mixed illite-smectite to higher-grade fibrous illite. Illite occurrence can provide adiagnostic evidence of diagenetic succession associated with the burial history and change in porefluid.24 In the lower part of the Williams Fork formation, pore-filling clays are ubiquitous but itsquantity is less than in the upper interval. The upper part contains more variable and moreconcentrations of pore-filling authigenic clays; however, less amount of K-feldspar has dissolved.Pore-filling clays are typically process of reducing porosity and permeability leading to loweringthe reservoir quality,25 in some case, illite may still have microrposity.26 However, the fibrousnature of the pore-filling clays makes them prone to breakage and pore-throat clogging duringstimulation and production.27 Mechanisms that likely to cause this are over pressured condition inthe lower section. The condition is prevailed as gas produced that drives the movement of porefluids from K-feldspar dissolutions to the upper section through natural fractures. Then, the fluidsprecipitated illite as pore filling at some intervals.22Key features of William Fork ReservoirA massive gas in Piceance Basin is produced from a deep basin centered gas accumulation, inparticular from thick discontinuous lenticular fluvial sandstones of the Williams Fork Formation,Mesaverde Group. A schematic cross section illustrating the key features of the reservoirs is shownin Figure 7 below.There should be mechanisms and specific geologic conditions prevailed in the Williams Forksandstone formations that have set off huge gas accumulation in the southern gas fields of Piceancebasin. First, significant presence of Cameo-Wheller Coal zone underlying the Williams Forformation has achieved high thermal maturity.17 As these coal beds generate abundant amount ofgas, over pressuring conditions prevailed as gas can not migrate out of basin center, inhibited bylow permeability and discontinuous lenticular nature of the Williams Fork sandstones. Extensiveover pressuring condition prevailed because of the gas phase pressure in the pore system exceededthe capillary pressure of the water-wet pores, and water was expelled from the pore system.4 Thiscondition resulted in pervasive natural fracturing especially within the Iles formation and upperpart of the William Fork formation.15 This natural fracture network primarily provides conduits forgas to move up to the Williams Fork lenticular sandstones. It appears that the natural fractureshave mostly controlled long-distance movement of gas and water within the reservoirs. However,the fluid mobility is quite limited, because of the fracture characteristics are lower spacing andlimited in vertical and lateral extent and discontinuous lenticular geometry of the low permeabilitysandstone reservoir.15 As a result, the Williams Fork sandstones well act as gas storage making itas continuously gas charged interval of about 1500’ to 2400 ft. The abundant natural fractures thatare more distributed in the Iles formation and upper part of the William Fork formation arepathways for gas expelled out of the Cameo coal zone and recharge of water into the subsurface,respectively. This close association is validated by test data from across the basin.15 While thesereservoirs are locally good gas producers, they are most likely to produce large volumes of waterwith gas. Test data also suggest a risk of high production water rates particularly for the upperWilliams Fork Formation and the Rollin Member of the Iles Formation. 8
  9. 9. Figure 7. Piceance Basin gas model – present day.20There are two favourable conditions support the generation of highly gas charged accumulations inthe tight Williams Fork formation.28 First, as discussed previously gas generation from CameoCoals and gas migration up through major fault or fracture zones from highly pressured organic-rich underlying units. Second condition is the presence of high heat flow that may also beessential in generating considerable amount of gas accumulations. An example, high heat flowappears to have significantly contributed to extensive amount gas generation in Wattenberg gasfield in the Denver Julesburg Basin.23Similar to what happened in Piceance Basin, different magnitude of high heat flow betweennorthern and southern part of the basin shows its variation how gas accumulated in the WilliamsFork reservoirs in both areas (Figure 8).In the southern Piceance, where the heat flow and the coal zone are higher and thicker than in thenorthern part of the basin, respectively, the gas accumulation has relatively thick gas-saturatedinterval with low water saturation. In contrast, in the northern Piceance Basin, the gasaccumulation is lower with variable water saturation and higher water production. The mechanismthat may have created the variation, is higher heat flow that exists in the southern Piceance. Theheat flow may have contributed to elevated over pressuring condition in the southern part, andcreated an extensive natural fracturing which resulted in the overlying sandstones become fully gascharged with thick interval. In the northern Piceance, areas near the fault zones have higher gassaturations with thicker interval. These suggest that gas has migrated primarily along main fracturezones. 28 9
  10. 10. Figure 8. Key reservoir features and fluid distribution control in Piceance Basin.15,28Natural Fracture characterisationCommercial production in a deep basin centered gas accumulations is generally related to sweetspots in the reservoirs in which their overall permeability has been enhanced by the presence ofextensive natural fractures.23 The continuously gas-saturated reservoir intervals in the WilliamsFork are significantly typified by natural fracturing and lenticular depositional geometry.15 Thenatural fractures are mainly present as joints of extension fractures and small faults of shearfractures. Their orientation may relate to tectonic stresses. However their distribution and intensityare very much controlled by over pressuring process during gas charging.4 Fracture distribution isalso lithologically bounded by rock layering, sand lenses or against other fractures.30An integrated program to characterize more detailed the natural fractures in the Williams Forkformation is required. As part of Intensive resources Development and Multi-Well Experiments(MWX) program in the southern gas fields, a related work has been set up. The program describedhere was designed to delineate location, natural fracture intensity and diversity of orientation.8,29The detection techniques applied are to identify the reservoir fractures associated with basementfeatures. The techniques involve reasonably low-cost large scale geophysical data surveys to focuson basin areas that are likely to contain fractures. Typical geophysical data required for theanalysis include8 : 10
  11. 11. • High-resolution aeromagnetic data (2500 m2) to delineate the geometry of the basement structure.• 2-D seismic and remote sensing imagery analysis were also used to provide consistency to the interpretation.These geophysical imagery data are correlated with fracture core data and integrated with aforward numerical basin model, called BasinRTM. The Basin RTM is one dimensional simulationpackage which accounts for compaction, fracturing, hydrocarbon generation, and effect of gasgeneration on the dynamics of the system.31 Subsequently, the simulation will produce aprognostic fracture mapping model that will provide a detailed fracture origins throughout thebasin evolution. The fracture mapping model is calibrated at the MWX laboratory field andcompared to match the present-day characteristics as observed at the MWX site. The model is alsocompared with regional structural basement faults and other fracture zones as interpreted from theimagery geophysical data previously mentioned. The results of the work are potential areas for 3-Dseismic location. A workflow of the techniques can be outlined as below : Figure 9. A workflow of integrated detection techniques of the natural fractures in Williams Fork formation, Southern Piceance Basin 11
  12. 12. The 3D seismic survey later on was conducted on a 4.5 mi2 area, aimed at targeting naturalfractures especially located at interval depth of 4000 to 7000 ft, in southern part of the Piceancebasin. The 3D seismic survey was multi-azimuth P-wave reflection survey, full-offset to evaluatethe P-wave azimuthal anisotropy and determine relative fracture density and orientation. The P-wave anisotropy interpretation from the 3D seismic survey suggests anomaly areas with presenceof open fracture sets. These anomaly areas are confined and validated by well production historyand a well test which is allocated on a seismic anomaly. Overall results from the program include :• Fractured production zones of gas reservoirs in the Mesaverde group overlie deep basement faults. The fractured zones have northwest-southeast trends, parallel to the basement faults.• Identification of basement faults is based on interpretation of the set of geophysical data of remote imagery, and high resolution aeromagnetic surveys.• Numerous basement faults have been located indicating many undrilled fracture prone areas are present in the basin.8ConclusionA massive gas accumulation in the tight lenticular sandstones of the Williams Fork formation insouthern Piceance basin is mainly associated with certain favourable geological conditions. Thekey parameters of the conditions include :• Presence of Cameo Coals with high thermal maturity as gas source• Natural fractures as conduits for gas generated moving up from the Coal zones to the Williams Fork formation.• Presence of abundant natural fractures in sandstones of the Iles formation and upper part of the Williams Fork formation. These fractures largely control fluid migration within the formation.• The tight sandstones of the William Fork formation is continuously gas saturated with 1500ft- 2400ft interval. A transition zone of mixed gas – water saturated sandstones above the continuously gas-saturated interval.Other relevant mechanisms that resulted in the enormous gas accumulation :• Pervasive natural fracturing results from extensive over pressuring conditions in Cameo Coals due to massive volume of gas.• High heat flow and thick coal beds are proven essential to creation of pervasive highly gas charged accumulations. The magnitude of the heat flow also determines the relative quantity of gas produced as what happened in both Northern and Southern Piceance Basin.• Due to high heat flow in the Southern Piceance basin, the gas accumulation has uniformly low water saturations, low water production, and a gas-saturated interval that gradually thickens into the deeper part of the basin, but the top of the gas interval shows little variation locally.• The higher heat flow in the southern Piceance may have created a pervasive fracture system that allowed all sandstones within the gas-saturated interval to be charged to high gas saturations.Integrated techniques involving low cost aerial geophysical survey data have been applied to getanomaly fracture prone zones. The anomaly areas then are selected for 3-D seismic surveylocation. This 3D seismic survey is proven to provide a powerful technology for identifying 12
  13. 13. fracture zones associated with basement structural features as applied in the tight sandstones of theWilliams Fork formation. The features can enhance overall reservoir permeability which providesignificant conduits required for commercial production from low permeability reservoirs.Selected Reference1. USGS : Oil Shale Assessment Project Fact Sheet Assessment of In-Place Oil Shale Resources of the Green River Formation, Piceance Basin, Western Colorado, Fact Sheet 2009-3012, U.S. Department of the Interior and U.S. Geological Survey, 2009.2. Kuuskraa, V.A., and Prestridge, A.L. : Advanced Technologies for Producing Massively Stacked Lenticular Sands, SPE 35630. This paper wav prepared for presentation at the Gas Technology Conference held in Calgary, Alberta, Canada 28 April – 1 May 1996.3. EIA : Map of Tight gas plays, Lower 48 States, updated June 2010.4. Cumella, S. and Scheevel, J. : Geology and Mechanics of the Basin-Centered Gas Accumulation, Piceance Basin, Colorado, An extended abstract, adapted from AAPG Hedberg Conference, April 24-29, 2005, Vail, Colorado.5. Koepsell, R., Cumella, S.P. and Uhl, D. : Applications of Borehole Images in the Piceance Basin, in Peterson, K.M., Terrilyn, M.O., and Anderson, D.S., eds., Piceance Basin 2003 Guidebook: Denver, CO, Rocky Mountain Association of Geologists, p. 233-251.6. Coffer, H.F., Frank, G.W., Bray, B.G. : Project Rulison and the Economic Potential of Nuclear Gas Stimulation, SPE paper 2876, presented in Gas Industry Symposium in Omaha Nebraska, 1970.7. Reynolds, M., Bray, B.G., Mann, R.L. : Project Rulison: A Status Report, SPE paper 3191, presented in SPE Eastern Regional Meeting, Pittsburgh, Pennsylvania, 1970.8. Kuuskraa, V., Decker, D., Squires, S., Lynn H : Naturally fractured tight gas reservoir detection optimization : Piceance Basin, The Leading Edge, August 1996, 947-948.9. Kuuskraa, V., and Ammer, J. : How to Dramatically Improve Recovery Efficiency, Gas TIPS, Winter 2004, 15-2010. EIA : Top 100 U.S. Oil & Gas Fields By 2009 Proved Reserves, US Energy Information Administration.11. Tremain, Carol M. and Tyler, R. : Cleat, fracture, and stress patterns in the Piceance Basin, Colorado: Controls on coalbed methane producibility. Rocky Mountain Association of Geologists, Fractured Reservoirs: Characterizations and Modeling Guidebook, 1997.12. Duda, J.R. and Hancock J.S. : Tight-Sand Development Potential in the Southern Rulison Area, Garfield County, Colorado, Journal of Petroleum Technology, 551-557, (1989).13. Janet K. Pitman, Eve S. Sprunt Origin and Distribution of Fractures in Lower Tertiary and Upper Cretaceous Rocks, Piceance Basin, Colorado, and Their Relation to the Occurrence of Hydrocarbons AAPG Special Volumes SG 24: Geology of Tight Gas Reservoirs, Pages 221 - 233 (1986) 13
  14. 14. 14. Johnson, R.C.: Geologic history and hydrocarbon potential of Late Cretaceous age, low permeability reservoirs Piceance basin, western Colorado: U.S., Geological Survey Bulletin (1989) v. 1787-E, 51 p15. Yurewicz, D.A.: Controls on gas and water distribution, Mesaverde basin center gas play, Piceance Basin, Colorado (extended abstract): AAPG Conference in Hedberg, (2005).16. Cole, R. D., and Cumella, S. P.: Sand body architecture in the lower Williams Fork Formation (Upper Cretaceous), Coal Canyon, Colorado, with comparison to the Piceance Basin subsurface: The Mountain Geologist, v. 42 (2005), p. 85– 107.17. Cumella, S. : Mesaverde Gas Accumulation - Geology of the Piceance Basin (extended abstract) Adapted from oral presentation at AAPG Annual Convention, Denver, Colorado, June 7-10, 2009.18. Johnson, R.C., and Flores, R.M.: History of the Piceance Basin From Latest Cretaceous Through Earliest Eocene and the Characterization of the Lower Tertiary Sandstone Reservoirs, Piceance Basin 2003 Guidebook: Rocky Mountain Association of Geologists, Denver, Colorado, p. 21-61.19. Lorenz, J.C. : Prediction of Size and Orientation of Lenticular Reservoirs in the Mesaverde group, Northwestern Colorado, paper SPE 13851 presented at the 1985 SPE/DOE Symposium on Low Permeability Reservoirs, Denver, May 19-22.20. Cumella, S., and Ostby, D. : Geology of the Basin-Centered Gas Accumulation, Piceance Basin, Colorado: Rocky Mountain Association of Geologists, Chapter 10, 171-193 (2003).21. Soeder, D.J. and Randolph, P.L : Porosity, Permeability and Pore structure of the tight Mesaverde Sandstone, Piceance Basin, Colorado, SPE Formation Evaluation 1987.22. Stroker, T., and Harris, N., : K-Ar Dating of Authigenic Illites: Integrating Diagenetic History of the Mesaverde Group, Piceance Basin, NW Colorado. Adapted from oral presentation at AAPG Annual Convention, Denver, Colorado, June 7-10, 200923. Shanley, K.W., Cluff, R.M.and Robinson, J.W. : Factors controlling prolific gas production from low-permeability sandstone reservoirs: Implications for resource assessment, prospect development, and risk analysis, AAPG Bulletin, v. 88, no. 8 (August 2004), pp. 1083–112124. Burtner, R.L. and Hathon, L., : K-Ar dating of authigenic illite constrains the time of diagenesis and brine migration in the Weber Sandstone of the Uinta-Piceance Basin, Colorado and Utah: AAPG Meeting Abs Vol. 80, Issue 13 (Annual Meeting 1996).25. Pallatt, N., Wilson, M. J. and McHardy, W. J. : The relationship between permeability and the morphology of diagenetic illite in reservoir rocks: Journal of Petroleum Technology, v. 36 (1984), p. 2225–2227.26. Rushing, J. A., K. E. Newsham, and T. A. Blasingame, 2008, Rock typing—Keys to understanding productivity in tight gas sands: SPE paper 114164.27. Almon, W. R., and Davies, D. K. : Clay technology and well stimulation: Transactions of the Gulf Coast Association of Geological Societies, 1978,v. 28, p. 1–6.28. Cumella, S.P. :Important Characteristics of Rocky Mountain Tight Gas Accumulations, The Geology of Unconevntional Gas Plays, AAPG Conference 5-6 October 2010, in Burlington.29. Kuuskraa, V., Decker, S., Lynn H. : Optimizing Technologies for Detecting Natural Fractures vin the Tight Sands of the Rulison Field, Piceance Basin, DOE/NETL report IADI1953, 1996. 14
  15. 15. 30. Lorenz, J.C. and Finley, S.J.” Significance of drilling- and coring-induced fractures in Mesaverde core, northwestern Colorado, Sandia Report SAND88-1623 UC-92, June 1988, prepared for US DOE.31. Dorothy F. P. and Kagan, T. : A Reaction-Transport-Mechanical Approach to Modeling the Interrelationships Among Gas Generation, Overpressuring, and Fracturing: Implications for the Upper Cretaceous Natural Gas Reservoirs of the Piceance Basin, Colorado AAPG Bulletin Volume 84 (2000) 15

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