Scott P. Cooper

Fracture and fault patterns associated with basement-cored anticlines: The example of Teapot Dome, Wyoming

Teapot Dome is an asymmetric, doubly plunging, basement-cored, Laramide-age anticline. Most of the fractures, deformation bands, and faults at Teapot Dome are interpreted to have formed during contemporaneous longitudinal and transverse stretching of the sedimentary cover over a basement-involved thrust. Strain was accommodated by fractures, deformation bands, and normal and normal-oblique faults that strike both parallel and perpendicular to the fold hinge. The fracture and fault patterns at Teapot Dome are distinctly different from those formed within anticlines associated with thin-skinned thrust systems. The inferred fracture-influenced permeability anisotropy of thick-skinned systems is therefore distinct from that of thin-skinned systems. We propose that Teapot Dome is a good analog for similar basement-involved, thrust-generated anticlines.

A mechanical model for multiply-oriented conjugate deformation bands

A unique suite of three pairs of conjugate deformation band sets is present in Jurassic sandstones in the southeastern corner of the San Juan basin, northwestern New Mexico. In order of sequential development, these conjugate pairs are oriented to form each of the three principal conjugate attitudes: (1) upright Xs, (2) plan-view Xs, and (3) recumbent Xs. The symmetry axes of the three different X-geometries at this location are parallel, suggesting that the three systems are genetically related. A relatively simple stress history, with the horizontal stresses striking northeast and southeast and varying in magnitude but not in orientation, plausibly explains this suite of sequentially developed conjugate structures. In this model, the upright Xs formed with dip-slip, normal offset under the initial conditions where the overburden was the maximum stress and the northeast-striking horizontal stress was the intermediate stress. Plan-view Xs with strike-slip offset formed next, as the northeast-striking stress increased (due to northeastward Laramide translation of the Colorado Plateau and interaction with the local basement-cored Nacimiento Uplift) to become the maximum stress, leaving the overburden stress as the intermediate stress although unchanged in magnitude. As the northeast-directed horizontal compressive stress continued to increase, it eventually created a condition where the horizontal stresses equaled or exceeded the overburden stress, resulting in small-scale thrusting along the recumbent Xs. The proposed mechanical model shows that shear stress levels dropped temporarily after the formation of the upright Xs, providing a hiatus in deformation and making the deformation at the next stage distinct, without overlap. The three systems of intersecting conjugate deformation bands that resulted have severely compartmentalized potential reservoirs in this unit, and illustrate why similar high-porosity, deformation-banded units can have low hydrocarbon production rates despite otherwise good reservoir potential.

Natural fracture distributions in sinuous, channel-fill sandstones of the Cedar Mountain Formation, Utah

A set of regional natural fractures, present in the sandy to conglomeratic, fluvial, channel-fill deposits of the Cedar Mountain Formation (east-central Utah) has a consistent west-northwest strike regardless of the local axial orientations of the sinuous channels. The fracture-producing stresses were not significantly refracted by the mechanical-property contrast between the channel-fill sandstones and the encasing overbank mudstones. In addition, fracture spacing along the sinuous channel axes is relatively constant between one-half and one-third of the bed thickness for both large fractures that cut the full thickness of the channel deposits and for smaller fractures in the thinner, component beds. Fracture spacing was apparently not affected by the variations in stress amplification that commonly result from differently oriented stiff inclusions in a ductile matrix. Therefore, in the absence of other structures, fracture intensity and the orientation of fracture-related maximum horizontal permeability in sinuous elongated reservoirs will be relatively constant regardless of the orientation of the long axis of the reservoir. Whether maximum permeability trends along, oblique, or across such reservoirs, and the relative drainage efficiency of horizontal versus vertical wellbores drilled into them, will vary only with the local trend of the channel axis.

Lithologic and Structural Controls on Natural Fracture Characteristics Teapot Dome, Wyoming

Teapot Dome is an asymmetric, doubly plunging, basement-cored, Laramide-age anticline. A systematic study of natural fractures within the Cretaceous Mesaverde Formation at Teapot Dome, Wyoming indicates that lithology and structural position control outcrop fracture patterns. Lithology controls fracture, deformation band and fault patterns in the following ways: 1) fracture intensity increases with increased cementation, 2) fracture spacing increases proportionally with bed thickness within two sandstone facies, but not in carbonaceous shales where fracture spacing is inversely proportional to bed thickness, 3) coal cleats are generally oblique, by up to 20 degrees, to fractures in sandstones, 4) most fractures in sandstone units terminate at contact with shale layers, 5) deformation bands occur almost exclusively in a poorly cemented, high porosity, beach-sand facies, 6) normal faults within well cemented sandstones are generally expressed as fracture zones, whereas the same faults within poorly cemented sandstones are diffuse zones of subparallel deformation bands. Three primary throughgoing fracture sets were documented within this context. The oldest fracture set is oblique to the hinge of the anticlinal fold. The vast majority of these fractures strike NW to WNW. A small number of these oblique fractures strike roughly NNE. Fractures that strike oblique to the fold hinge appear to predate folding. The other two fracture sets are related to folding. The most common of these fractures, which are found throughout the fold, are bed-normal extension fractures striking subparallel to the fold hinge. A third set consists of bed-normal extension fractures striking perpendicular to the fold hinge. In many areas this fracture set is spatially related and subparallel to NE-striking, normal oblique-slip faults. The normal oblique-slip faults are common along the eastern limb, but more than 90% of these faults terminate before intersecting the western limb. Conjugate fractures, deformation bands and faults, oriented such that they have a vertical bisector to the acute angle and striking subparallel to the axis of the anticline, are common in the southwestern limb and southern arc of

Tectonic Setting and Characteristics of Natural Fractures in Mesaverde and Dakota Reservoirs of the San Juan Basin, New Mexico and Colorado

A set of vertical extension fractures, striking N-S to NNE-SSW but with local variations, is present in both the outcrop and subsurface in both Mesaverde and Dakota sandstones. Additional sets of conjugate shear fractures have been recognized in outcrops of Dakota strata and may be present in the subsurface. However, the deformation bands prevalent locally in outcrops in parts of the basin as yet have no documented subsurface equivalent. The immature Mesaverde sandstones typically contain relatively long, irregular extension fractures, whereas the quartzitic Dakota sandstones contain short, sub-parallel, closely spaced, extension fractures, and locally conjugate shear planes as well. Outcrops typically display secondary cross fractures which are rare in the subsurface, although oblique fractures associated with local structures such as the Hogback monocline may be present in similar subsurface structures. Spacings of the bed-normal extension fractures are approximately equal to or less than the thicknesses of the beds in which they formed, in both outcrop and subsurface. Fracture intensities increase in association with faults, where there is a gradation from intense fracturing into fault breccia. Bioturbation and minimal cementation locally inhibited fracture development in both formations, and the vertical limits of fracture growth are typically at bedding/lithology contrasts. Fracture mineralizations have been largely dissolved or replaced in outcrops, but local examples of preserved mineralization show that the quartz and calcite common to subsurface fractures were originally present in outcrop fractures. North-south trending compressive stresses created by southward indentation of the San Juan dome area (where Precambrian rocks are exposed at an elevation of 14,000 ft) and northward indentation of the Zuni uplift, controlled Laramide-age fracturing. Contemporaneous right-lateral transpressive wrench motion due to northeastward translation of the basin was both concentrated at the basin margins (Nacimiento uplift and Hogback monocline on east and west edges respectively) and distributed across the strata depth.

Natural gas production problems : solutions, methodologies, and modeling.

Natural gas is a clean fuel that will be the most important domestic energy resource for the first half the 21st centtuy. Ensuring a stable supply is essential for our national energy security. The research we have undertaken will maximize the extractable volume of gas while minimizing the environmental impact of surface disturbances associated with drilling and production. This report describes a methodology for comprehensive evaluation and modeling of the total gas system within a basin focusing on problematic horizontal fluid flow variability. This has been accomplished through extensive use of geophysical, core (rock sample) and outcrop data to interpret and predict directional flow and production trends. Side benefits include reduced environmental impact of drilling due to reduced number of required wells for resource extraction. These results have been accomplished through a cooperative and integrated systems approach involving industry, government, academia and a multi-organizational team within Sandia National Laboratories. Industry has provided essential in-kind support to this project in the forms of extensive core data, production data, maps, seismic data, production analyses, engineering studies, plus equipment and staff for obtaining geophysical data. This approach provides innovative ideas and technologies to bring new resources to market and to reduce the overall environmental impact of drilling. More importantly, the products of this research are not be location specific but can be extended to other areas of gas production throughout the Rocky Mountain area. Thus this project is designed to solve problems associated with natural gas production at developing sites, or at old sites under redevelopment.

West Pearl Queen CO2 sequestration pilot test and modeling project 2006-2008.

The West Pearl Queen is a depleted oil reservoir that has produced approximately 250,000 bbl of oil since 1984. Production had slowed prior to CO{sub 2} injection, but no previous secondary or tertiary recovery methods had been applied. The initial project involved reservoir characterization and field response to injection of CO{sub 2}; the field experiment consisted of injection, soak, and venting. For fifty days (December 20, 2002, to February 11, 2003) 2090 tons of CO{sub 2} were injected into the Shattuck Sandstone Member of the Queen Formation at the West Pearl Queen site. This technical report highlights the test results of the numerous research participants and technical areas from 2006-2008. This work included determination of lateral extents of the permeability units using outcrop observations, core results, and well logs. Pre- and post-injection 3D seismic data were acquired. To aid in interpreting seismic data, we performed numerical simulations of the effects of CO{sub 2} replacement of brine where the reservoir model was based upon correlation lengths established by the permeability studies. These numerical simulations are not intended to replicate field data, but to provide insight of the effects of CO{sub 2}.

ABSTRACT: Reservoirs in the Abo Formation, Southeastern NM: A Fractured Play within the Perturbed Stress Field at the Termination of Wrench Faults

Production of natural gas from the low-pemreability arkoses and sandstones of the Permian Abo Formation, southeastern New Mexico exceeds matrix deliverability capacity, suggesting that the reservoirs are fractured. This suggestion is corroborated by the fact that the Abo Formation is highly fractured in the relatively undeformed outcrops, far distant from the reservoirs but indicating that the strata are susceptible to fracturing. Natural fractures are also present in the few available cores, thus the presence of significant fracturing is inferred in the subsurface. Although the Abo Formation was deposited over much of southeastern New Mexico, Abo sandstones are most productive where the strata are associated with a series of sub-parallel, northeast-trending, deep-seated, right-lateral wrench faults called the Pecos Buckles. The surface strata along these buckles consist of poorly exposed evaporite and carbonate facies, complicating structural interpretations. However, well-exposed sandstones along similar wrench faults in the central Sahara have numerous irregular fractures that are commonly formed in the vicinity of such wrench faults. Strata are fractured due to volume constraints at irregularities during lateral offset along the faults. By analogy, reservoirs in the Abo Formation were fractured during lateral offset along the Pecos Buckles. Most of the Abo reservoirs are located near the northeastern termination of these wrench faults, therefore fracturing may also have been a response to stresses associated with offsets along fault splays at the ends of the Pecos Buckles. ItI troduction