Mechanisms for Natural Fracturing in the Rockies: Diverse modes, mechanisms and ages

Transcription

1 Mechanisms for Natural Fracturing in the Rockies: Diverse modes, mechanisms and ages Eric Erslev, University of Wyoming Alvis Lisenbee, South Dakota School of Mines and Technology Reality: Viable, low permeability reservoirs commonly require some, but not too many, natural fractures Problem: Predicting fractures Solution: Understand them! Thanks to: many CSU, U Wyo and SDSMT students, NSF -EarthScope, PRF, industry (Chevron, ConocoPhillips, EnCana, Comet Ridge, Noble Energy) and many colleagues Petroleum bleaching? Dallas Dome, WY

2 I. Pre-Laramide Mesozoic faulting and jointing (= fracturing)? Local gravity sliding, regional mechanisms? II. Laramide faulting and jointing: Localized fold-related fracturing Regional splitting and slip due to ENE-WSW shortening Late-stage gravitational collapse of arches III. Post-Laramide faulting and jointing: Regional stretching during post-laramide extension Local extension in front of back-sliding thrust faults IV. Plus, a major additional mechanism for tight, oily rocks: Fracturing due to reduced effective stress during over- pressuring (e.g. changes from solid to liquid to gas)

3 I. Pre-Laramide Some think that NW-trending, Eric Many fractures are sealed Cut pre-paleocene strata

4 Proposed pre-laramide Mesozoic Faulting and Jointing Facture diversity due to pre-existing fractures causing non-ideal fracture patterns (Bergbauer & Pollard, 2004): Interpretations: 1) Bed-perpendicular joints (A) form in horizontal strata 2) NW joints were first, 3) Faults (C) strike parallel to pre- existing joints

5 Pre-Laramide fractures filled with syn-sedimentary clastic dikes in the Frontier Fm., Bighorn Basin Neptunian clastic dikes cut across stratigraphy and contain thrust faults w/

6 II. Laramide Fractures cut pre-oligocene strata Consist of joints and faults Have regional patterns

7 Fractures consist of two types, neither of which need be large: ----Joints (tension cracks) ----Shears (faults) Example: Sheep Mountain, WY: Paleocene rocks cut at 120 o and 160 o post-laramide? Tension crack Fault Minimum Compression Maximum Compression

8 Syn-Laramide faulting and jointing: A key test of Laramide hypotheses Big Horns Major Laramide uplifts of Wyoming (Brown, 1983)

9 Southeastern Wind River Basin faults (Hamlin, in prep.) 1 for all faults ENE-WSW 1 regardless of arch trend Casper

10 Laramide minor faults: uni-modal ENE-WSW shortening ~8000 minor faults from CO Front Range (Erslev & Larson (2006) 21,000 Rocky Mountain faults (Erslev & Koenig (2009)

11 Application: syn-laramide fractures may parallel regional Laramide compression, not fold dip directions Directional permeability (Haws and Hurley 1992, red arrows) and fractures are oblique to fold axes and parallel to Laramide compression

12 NE Front Range fault kinematics and geometries NE Front Range, CO from Karl Mueller, CU Rose diagrams of ENE-WSW minor fault conjugate bisectors

13 Front Range faults predate NE J 1 and NW J 2 joints... NE Hematite seal Thrust fault Post-faulting joints, NE Front Range

14 Niobrara Formation: 6 Mile Fold, Boulder, CO Joints

15 Niobrara joints (Allen, 2010) also parallel regional joints Joints (n=1168) Calcite-filled fractures (n=209) Station (n=61) average joints Stylolites (n=97) J 2 J 1

16 Niobrara minor faults (Allen, 2010) parallel regional faults (Erslev and Larson, 2006) Trends of slickenlines and compression Minor S-S and thrust faults (n=127) Minor normal faults (n=40)

17 Speculation: Are NW-SE horizontal wells targeting NE and E-W Laramide faults and NNE joints?

18 Previous fracture studies in the Casper region (Cooley, 2008): Oil Mountain (Hennings et al., 2000) Casper Teapot Dome (Cooper et al., 2006) Study Area Douglas Emigrant Gap (Bergbauer and Pollard, 2004)

19 Sheep Mtn. (No. 4 or 5) Joint Strikes

20 Fracture strikes by formation: more NW fractures in younger rocks, more NE fractures in older rocks

21 Fracture timing: NE first, then NW... J 1 /NE-SW J 2 /NW-SE

22 Recent observations consistent w/ mechanical stratigraphy Laura Hamlin, U Wyo NW joints in syn-laramide units Mesozoic units also cut by NE joints

23 III. Post-Laramide Cut all strata northern Rocky Mountains

24 Wellbore Frontier Mowry Tensleep Madison FMI fracture data interpreted by Schlumberger correlates with Vfast P-wave azimuth observed at well

25 GPS model (Berglund, 2010) How much of this is recent deformation? Wyoming Bore-hole breakouts, Thompson, 2010 New GPS data suggest distributed regional extension throughout the Rockies as the crust stretches on top of a hot,

26 Post-Oligocene, 90 o -striking, normal fault, White River Badlands, SD

27 NW-striking joints found in syn- and post- Laramide (Oligocene White River Fm) rocks (Hamlin, 2011 M.S. thesis) All Joints Casper

28 Post-Eocene, 110 o striking normal fault, Wind River Basin

29 Applications to oil fields? Shoshoni Mesa Wind River Canyon Madden & Frenchie Draw? Could back sliding explain the normal faults at Jonah Field?

30 Conclusions: Regional systematic patterns of joints and small-scale faults are ubiquitous in the northern Rocky Mountains and likely in the adjacent basins and prairies: 1) Suggested pre-laramide Mesozoic regional fractures not proven? 2) Laramide regional minor faults and splitting joints indicate: (a) ENE-shortening (Erslev and Nicole Koenig, 2009) (b) Subsequent localized, radial spreading of arch highs. Most joints post-date conjugate thrust and strike-slip faults (Jason Ruf, 2000; Scott Larson, 2008; Cody Allen, 2010)..

31 Conclusions: Regional systematic patterns of joints and small-scale faults are ubiquitous in the northern Rocky Mountains and likely in 3) Post-Laramide joints are controlled by: (a) distributed post-laramide N-S to NE-SW extension, (b) back-sliding of Laramide thrusts (Duke Cooley, 2008; Cyrus Gillett, 2008; Ryan Thompson, 2010; Laura Hamlin, in prep.). Fluid-enhanced fracturing and detachment are common for the Niobrara Formation in the basins (Cody Allen, 2010 Know the fracture mechanism & timing in your play!

32 Important admissions: Greater details and data on Rocky Mtn. fracturing are closely held by many in industry than those of academics: Today we will present fracture case studies, however, which we hope you can apply to your fields & prospects. Our insights come from our graduate students: Bjorn Selvig, Phil Molzer, Steve Holdaway, Brani Jurista, Stephanie Nicole Koenig, Melissa Copfer, Seth Fankhauser, Cyrus Gillett, John Detring, Duke Cooley, Sara Smaltz, Scott Larson, Ryan Thompson, C. Allen, Laura Hamlin, Karen Aydinian and numerous similarly deluded undergraduate students * Names in green have contributed directly to this talk

33 Bonus material: 2010 Bighorn Project BASE Experiment: Superimposed active source seismic tomography and faults W E L. Worthington Note the coincidence between predicted zones of faulting and zones of low seismic velocity