John C. Lorenz

Lithologic and Structural Controls on Natural Fracture Distribution and Behavior Within the Lisburne Group, Northeastern Brooks Range and North Slope Subsurface, Alaska

The Carboniferous Lisburne Group of northern Alaska has been deformed into a variety of map-scale structures in both compressional and extensional structural settings, thus providing a series of natural experiments for observing the formation, distribution, and behavior of fractures in this thick carbonate unit. Two fracture sets dominate the Lisburne Group carbonates of the North Slope subsurface and the nearby northeastern Brooks Range fold and thrust belt. North-northwest-striking regional extension fractures probably formed in front of the northeastern Brooks Range fold and thrust belt. In the North Slope subsurface, this fracture set overprints east-northeast-striking fractures related to earlier extensional deformation; in contrast, in the fold and thrust belt, the north-northwest-striking fracture set is overprinted by younger east-northeast-striking fractures related to subsequent contractional deformation. Lithology is the primary control on the fracture density of both sets. In mildly deformed Lisburne ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received February 20, 1996; revised manuscript received November 25, 1996; final acceptance June 5, 1997. 2Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska 99775. 3Sandia National Laboratories, MS 0705, Albuquerque, New Mexico, 87185. 4Department of Petroleum & Natural Gas Engineering, New Mexico Institute of Mining & Technology, Socorro, New Mexico 87801, and Sandla National Laboratories, MS 0705, Albuquerque, New Mexico, 87185. This study was supported by a Department of Energy subcontract administered by Sandia National Laboratories. Additional support was provided by ARCO Alaska, BP Alaska, Chevron, Exxon, Mobil, and Japan National Oil Company. We would like to thank ARCO Alaska and BP Alaska for giving us permission to view selected Lisburne Group cores, W. Wallace for helpful discussions on deformational styles of detachment folds, A. J. Mansure for help in interpreting the interference tests, and W. Wallace, K. Biddle, W. Belfield, N. Hurley, and J. Kelley for helpful reviews of the manuscript.

Natural fractures in the Spraberry Formation, Midland basin, Texas: The effects of mechanical stratigraphy on fracture variability and reservoir behavior

Horizontal cores from sandstone-siltstone reservoirs in the Spraberry Formation (Midland basin, west Texas) have documented two systems of dramatically different yet dynamically compatible natural fractures, in reservoirs separated vertically by only 145 ft (44 m). Each system is capable of producing a different degree of the northeast-trending permeability anisotropy recognized in Spraberry reservoirs. One fracture system consists of two vertical fracture sets with an apparent conjugate geometry (striking north-northeast and east-northeast). The other system consists of evenly spaced, northeast-striking vertical fractures, nearly bisecting the acute angle of the first system. Although lithologically similar, differences in quartz-overgrowth and clay content in the layers resulted in a yield strength of the lower bed that is only half of that of the upper layer, producing different fracture systems in the two reservoirs despite their proximity. Such differences in the mechanical properties, due to variations in diagenetic and depositional histories of the strata, are probably widespread within the formation. They have the potential to cause significant vertical and lateral variation in the Spraberry fracture system across the basin. Low present-day in-situ stresses in the reservoirs allow the fractures to open, to become more conductive, and even to propagate, under very low injection pressures.

Regional Fractures II: Fracturing of Mesaverde Reservoirs in the Piceance Basin, Colorado (1)

Cretaceous strata in the east-central Piceance basin of northwestern Colorado were subjected to several phases of stress due to horizontal tectonic compression, burial, and uplift. These rocks contain a regional set of west-northwest extension fractures, observed in abundant vertical and deviated core from the U.S. Department of Energys Multiwell Experiment and Slant Hole Completion Test wells. Time-depth relationships, fracture orientation, and fluid inclusion analyses indicate that the fractures formed at about 36-40 Ma, during a phase of increased Laramide west-northwest compression, and in a pressure-temperature regime compatible with the geologically reconstructed maximum burial depths. This regional fracturing is an example of load-parallel extension fracturing and basinwide dilatancy at depth, under conditions of high pore pressure and anisotropic, tectonically created, horizontal stress.

Determination of Widths of Meander-Belt Sandstone Reservoirs from Vertical Downhole Data, Mesaverde Group, Piceance Creek Basin, Colorado

Paleohydrologic reconstructions have been applied to downhole measurements made in the U.S. Department of Energys Multi-Well Experiment (MWX) wells in order to derive sand-body widths for the nonmarine part of the Mesaverde Group (Cretaceous) in the east-central part of the Piceance Creek basin of northwestern Colorado. These sediments were deposited in a predominantly meandering fluvial system. Paleochannel depths, recognized in the wells from the heights of fining-upward trends in core, can be converted to channel widths,using a published formula. The resulting channel widths are used to calculate meander-belt amplitudes (sand-body widths) from relationships derived from published empirical data. The estimate of width derived from this technique can be compared with sa d-body widths derived by three other methods. These various techniques give compatible results, indicating the validity of the downhole technique.

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.

Examination of a Cored Hydraulic Fracture in a Deep Gas Well (includes associated papers 26302 and 26946 )

A hydraulic fracture stimulation conducted during 1983--1984 in non-marine, deltaic, Mesaverde strata at a depth of 7100 ft (2164 m) was cored in a deviated well in 1990. The observed fracture consists of two fracture intervals, both containing multiple fracture strands (30 and 8, respectively). While the core had separated across many of the fracture strands during coring, the rock remained intact across 20 of the strands, preserving materials within the fractures. Nine of the remaining intact strands were split open, revealing abundant gel residue on the surfaces of every fracture examined. Of 7 strands associated with major bedding planes, 4 displayed offsets of 1--3 mm at the planes and 3 strands had their growth terminated at the planes, showing the importance of bedding (petrophysical heterogeneities) on fracture propagation. Implications of all these findings for hydraulic fracture design and analysis are also addressed. 55 refs., 9 figs.

Fracture characteristics and reservoir behavior of stress-sensitive fracture systems in flat-lying lenticular formations

A model is presented that suggests that regional fracture systems commonly control permeability in flat-lying reservoirs. Such fractures are distributed in a continuum of sizes and occur in subparallel, en echelon patterns. Few high-angle, orthogonal fractures exist because this system is created by high pore pressures and relatively low differential horizontal (tectonic) stresses rather than by significant structural deformation. Interfracture communication occurs primarily at infrequent, low-angle intersections of fractures. Vertical continuity of such fractures through a reservoir commonly is limited to the numerous lithologic discontinuities inherent in nonmarine sandstones. This type of fracture system has been documented in Mesaverede rocks in the Rulison field of the Piceance Creek basin, northwestern Colorado, by studies of 4,300 ft (1310 m) of core from the U.S. DOEs three Multiwell Experiment (MWX) wells and by studies of the excellent nearby outcrops. Well test results and geologic data from core and outcrop support the model. The described natural fracture system has a significant effect on production and stimulation.

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.

Stress-Sensitive Reservoirs

Changes in reservoir fluids during production (fluid expansion, dissolution of gas, among others) have long been recognized, but reservoir strata themselves, except for compaction-drive reservoirs, typically have been considered to be static systems. However, a growing number of increasingly sophisticated measurements have demonstrated that some variations in reservoir deliverability are related to interactions between changing fluid pressures, reservoir stresses, and natural-fracture permeability during production and/or injection.

Triassic Sediments and Basin Structure of the Kerrouchen Basin, Central Morrocco

ABSTRACT The Kerrouchen basin, on the southern end of the Middle Atlas Mountains of central Morocco, is an elongated asymmetric basin underlain by metamorphosed Paleozoic basement. With the onset of rifting of Africa from North America in Triassic time, the basin was downfaulted in the southeast against Hercynian granites, which shed 180 meters of alluvial fan conglomerates and sandstones into the basin. Forty kilometers across the basin to the northeast, a Paleozoic meta-sedimentary source terrain contributed sediments to a smaller and separate fluvial sequence, filling in that gently downwarped limb of the basin. Between these two coarser sequences, a mudflat existed which increased in size as tectonic activity diminished. The mudflat deposits eventually transgressed over the coarse sediment and the granites, covering them with red mudstones and localized evaporites. Thickest observable Triassic deposits, in the center of the basin, total approximately 600 meters of section; 275 meters of mudstone covered by 225 meters of basalt, and underlain by 80 meters of fluvial sandstone resting on basement. The basalts, averaging 100 meters thick, are intercalated with the uppermost mudstones. They covered the entire area as a series of flows of varying thickness, samples of which have been radiometrically dated as Triassic. The sequence is conformably covered by Jurassic carbonates consisting of limestones and dolomites deposited with the transgression of Jurassic seas.