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Mechanical Stratigraphic Controls on Fracturing (Jointing) and Normal Faulting in the Eagle Ford Formation, South-Central Texas, U.S.A. David A. Ferrill, Ronald N. McGinnis, Alan P. Morris, Kevin J. Smart, Zachary T. Sickmann, Michael Bentz Southwest Research Institute, San Antonio, Texas Daniel Lehrmann Trinity University, San Antonio, Texas Mark A. Evans Central Connecticut State University, New Britain, Connecticut 3
Introduction Natural fracture systems influence production in unconventional reservoirs Mechanical layering and in situ stress strongly influence natural and induced fracturing Eagle Ford Formation is mechanically layered and naturally deformed Eagle Ford productive trend includes contractional and extensional structural styles Improved understanding of these factors is leading to better well planning and stimulation design 4
Tectonic Setting Ferrill et al. (2014), AAPG Bulletin 5
Tectonic Setting Ferrill et al. (2014), AAPG Bulletin 6
Tectonic Setting Ferrill et al. (2014), AAPG Bulletin 7
Tectonic Setting Ferrill et al. (2014), AAPG Bulletin 8
Tectonic Setting Ferrill et al. (2014), AAPG Bulletin 9
Extensional Deformation in Eagle Ford Gulf of Mexico related extensional deformation Activity in Cretaceous (syndepositional) Continued in Tertiary Structures Seismic-scale faults Subseismic faults Extension fractures 10
Normal Faults NW SE Some active before or during Eagle Ford deposition Others post-date Eagle Ford deposition Wells avoid seismic scale faults Faults are largest fractures in the Eagle Ford fracture network Stratigraphic picks after Treadgold et al. (2010) Olmos Eagle Ford Buda/DR/ED/GR Pearsall Salt 11
Normal Faults NW SE Some active before or during Eagle Ford deposition Others post-date Eagle Ford deposition Wells avoid seismic scale faults Faults are largest fractures in the Eagle Ford fracture network Stratigraphic picks after Treadgold et al. (2010) Olmos Eagle Ford Buda/DR/ED/GR Pearsall Salt 12
Fault Zone Deformation NW Olmos SE Fault zone architecture varies with mechanical stratigraphy formation scale Eagle Ford Buda/DR/ED/GR Pearsall Block diagrams after Ferrill & Morris (2008), AAPG Bulletin 13
Eagle Ford Formation Lower, middle, and upper units designated based on outcrop character Equivalent of Boquillas Formation west of Devils River Lithologies include: Mudrock Chalk Limestone Volcanic ash Buda Limestone and Lower Eagle Ford After (A) Donovan & Staerker (2010) and (B) Lock et al. (2010) 14
Eagle Ford Formation Lower, middle, and upper units designated based on outcrop character Equivalent of Boquillas Formation west of Devils River Lithologies include: Mudrock Chalk Limestone Volcanic ash Middle Eagle Ford After (A) Donovan & Staerker (2010) and (B) Lock et al. (2010) 15
Eagle Ford Formation Lower, middle, and upper units designated based on outcrop character Equivalent of Boquillas Formation west of Devils River Lithologies include: Mudrock Chalk Limestone Volcanic ash Upper Eagle Ford & Austin Chalk After (A) Donovan & Staerker (2010) and (B) Lock et al. (2010) 16
Mechanical Stratigraphy Mechanical stratigraphy represents the mechanical properties of the rock, thickness of the mechanical layers, and frictional properties of the boundaries between mechanical layers within a section. 17
Mechanical Stratigraphy Thinly bedded Heterolithic Mechanical layers defined by individual beds or packages of beds Volcanic Ash Limestone Mudrock Chalk 18
Compositional Controls on Strength in Carbonate Rocks Compositional factors Strong fraction Calcite Dolomite Quartz Weak fraction Clay Organic carbon Void space Schmidt rebound (R) can be used as a proxy for Young s Modulus and Unconfined Compressive Strength Ferrill et al. (2011), AAPG Bulletin 19
Characterization of Mechanical Stratigraphy After (A) Donovan & Staerker (2010) and (B) Lock et al. (2010) 53 m Sample interval < 0.15 m (0.5 ft) Morris et al. (2013) 20
Location of Sycamore Creek Pavements Ferrill et al. (2014), AAPG Bulletin 21
Eagle Ford Section Sycamore Creek Pavements Middle Eagle Ford Formation Mudrock and chalk Mechanically layered Ferrill et al. (2014), AAPG Bulletin 22
Subseismic Scale Extensional Structures Joints and faults at Sycamore Creek pavements Ferrill et al. (2014), AAPG Bulletin 23
Before Makeover 24
After Makeover 25
Joints 2 dominant joint sets at Sycamore Creek pavements NE striking (1st) NW striking (2nd) Joints are bed restricted terminate in mudrock Ferrill et al. (2014), AAPG Bulletin 26
Normal Faults Not bed restricted Joints abut against (postdate) faults 27
Subseismic Scale Extensional Structures Ferrill et al. (2014), AAPG Bulletin 28
Vertical Penetration of Faults and Joints Faults cut multiple mechanical layers Joints are bed restricted and terminate in mudrock Schmidt Rebound Profile Ferrill et al. (2014), AAPG Bulletin 29
Failure Modes From Ferrill & Morris (2003), Journal of Structural Geology 30
Hybrid and Shear Failure Hybrid failure (mode 1.5) in chalk beds Shear (mode 2) failure in mudrock beds Crack seal texture indicates multitude of reactivation events each producing porosity Ferrill et al. (2014), AAPG Bulletin 31
Hybrid Failure Ferrill et al. (2014), AAPG Bulletin 32
Hybrid Failure Ferrill et al. (2014), AAPG Bulletin 33
Hybrid Failure Failure surface shows evidence of hybrid failure (mode 1.5) in chalk beds Calcite fibers grow in direction of displacement on shear (mode 2) surfaces in adjacent mudrock beds Ferrill et al. (2014), AAPG Bulletin 34
Fault Refraction Mode switching Mode 1 to mode 2 Mode 1.5 to mode 2 Change in shear failure angle (mode 2) From Ferrill et al. (2012), Journal of Structural Geology 35
Fault Slip and Dilation Shear failure in mudrock Hybrid failure in chalk Crack-seal textures indicate incremental opening in response to multitude of slip (reactivation) events Dilational zones are self-propping Shear Hybrid Shear Ferrill et al. (2014), AAPG Bulletin 36
Stress History Stress inversion based on fault measurements Ferrill et al. (2014), AAPG Bulletin 37
Fluid History from Calcite Veins Fault zone veins from Sycamore Creek Pavements Fluid inclusion microthermometry, fluid inclusion UV fluorescence, and O and C stable isotope analysis Fault zone calcite precipitated by several isotopically different fluids Smaller displacement faults more likely to have single fluid source Crack-seal texture indicates numerous (10s to 100s) events Ferrill et al. (2014), AAPG Bulletin 38
Fluid History A Source fluids for veins was not meteoric water but formation water Low salinity inclusions suggest meteoric mixing to dilute the fluids Abundance of hydrocarbon inclusions in the veins indicates that liquid hydrocarbons were ubiquitous B Ferrill et al. (2014), AAPG Bulletin 39
Fluid History Stable isotope values of vein calcite are significantly different from host rock indicating an open fluid system Elevated homogenization temperatures suggest trapping of aqueous fluids 2 km deep Faults likely formed at depths equivalent to some present-day oil and gas production from the Eagle Ford A B Ferrill et al. (2014), AAPG Bulletin 40
Conclusions Mechanical layering strongly influenced fracturing Faults cut multiple layers refract through mechanical layers dip more steeply in competent layers formed as hybrid fractures in chalk formed as shear fractures in mudrock dilated along steep segments with each slip event 41
Conclusions Joints bed restricted best developed in chalk beds terminate vertically in mudrock layers have much greater lateral extent than vertical penetration 42
Conclusions Abutting relationships indicate the following sequence 1 st Normal faults 2 nd NE-striking joints 3 rd NW-striking joints Faults at Sycamore Creek pavements formed at 2 km depth Fault zone calcite veins indicate precipitation from formation water, fluid mixing, and movement of liquid hydrocarbons Joints formed during subsequent erosional unloading 43
Conclusions Natural fracture systems, mechanical stratigraphy, and in situ stress conditions are the context for hydraulic stimulation Natural fractures are pre-existing weakness likely to reactivate before conditions for failure of intact rock are reached Open or mineral filled faults and extension fractures have contrasting porosity and permeability with respect to host rock layers, and may dilate, slip or compartmentalize fluid pressure increase during stimulation Outcrop structures serve as analogs for induced hydraulic fractures 44
Finite Element Model of Induced Hydraulic Fracturing Shows Complex Damage Horizontal Layering with Normal-Faulting Stress State at depth of 3 km (~9,800 ft): 1 = V = 60 MPa; 2 = H = 50 MPa; 3 = h = 40 MPa; P = 30 MPa Pore Pressure Contours & Principal Stress Axes Shear Damage & Max. Extension Directions Model domain is vertical section parallel to lateral wellbore at base. Mechanical stratigraphic layering strongly controls (i) stress state and pore pressure evolution, and (ii) damage and strain patterns Smart et al. 2014, AAPG Bulletin 45
Smart et al. 2014, AAPG Bulletin 46
Acknowledgements Financial support for this work was provided by Southwest Research Institute s Eagle Ford joint industry project, funded by Anadarko Petroleum Corporation BHP Billiton Chesapeake Energy Corporation ConocoPhillips Eagle Ford TX LP EP Energy Hess Corporation Marathon Oil Corporation Murphy Exploration & Production Company Newfield Exploration Company Pioneer Natural Resources Shell 47
References Cited Donovan, A.D., Staerker, T.S., 2010. Sequence stratigraphy of the Eagle Ford (Boquillas) Formation in the subsurface of South Texas an outcrops of West Texas. Gulf Coast Association of Geological Societies Transactions, 60, 861-899. Ferrill, D.A., McGinnis, R.N., Morris, A.P., Smart, K.J., 2012. Hybrid failure: Field evidence and influence on fault refraction. Journal of Structural Geology 42, 140-150. Ferrill, D.A., McGinnis, R.N., Morris, A.P., Smart, K.J., Sickmann, Z.T., Bentz, M., Lehrmann, D., and Evans, M.A., 2014, Control of mechanical stratigraphy on bed-restricted jointing and normal faulting: Eagle Ford Formation, south-central Texas, U.S.A. American Association of Petroleum Geologists Bulletin v. 98, 2477-2506. Ferrill, D.A., Morris, A.P., 2003. Dilational normal faults. Journal of Structural Geology 25, 183 196. Ferrill, D.A., and Morris, A.P., 2008. Fault zone deformation controlled by carbonate mechanical stratigraphy, Balcones fault system, Texas. American Association of Petroleum Geologists Bulletin 92, 359-380. Ferrill, D.A., Morris, A.P., McGinnis, R.N., Smart, K.J., Ward, W.C., 2011. Fault zone deformation and displacement partitioning in mechanically layered carbonates: The Hidden Valley fault, central Texas. American Association of Petroleum Geologists Bulletin 95, p. 1383-1397. Lock, B.E., Peschier, L., Whitcomb, N., 2010. The Eagle Ford (Boquillas Formation) of Val Verde County, Texas A window on the south Texas play. Gulf Coast Association of Geological Societies Transactions, 60, 419-434. Morris, A.P., Smart, K.J., McGinnis, R.N., Ferrill, D.A., 2013. Integrating outcrop analogs and geomechanical modeling insights into induced hydraulic fractures. Applied Geoscience for Mudrocks System Characterization to Improve Exploitation of Unconventional Oil and Gas Reservoirs. Extended abstract, February 18-19, 2013, Houston, Texas. Smart, K.J., Ofoegbu, G.I., Morris, A.P., McGinnis, R.N., Ferrill, D.A., 2014. Geomechanical modeling of hydraulic fracturing: Why mechanical stratigraphy, stress state, and pre-existing structure matter. American Association of Petroleum Geologists Bulletin, v. 98, p. 2237-2261. Treadgold, G., McLain, B., Sinclair, S.,and Nicklin, D., 2010. Eagle Ford Shale prospecting with 3D seismic data within a tectonic and depositional system framework. Bulletin of the South Texas Geological Society LI(1), 19 28. 48
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