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Structural Characterization of the Eagle Ford Formation, Val Verde County, Texas.

A Thesis Proposal Submitted to the Faculty

of the Department of Geology The University of Louisiana at Lafayette

by

Shanna L. Mason

February 16, 2015

Ms. Cathy Bishop

Dr. David Borrok

Ms. Kristie Cornell

Dr. Timothy Duex (TCM)

Dr. Raphael Gottardi (TA)

Dr. Gary Kinsland

Dr. Brian E. Lock (TCM)

Dr. Durga Poudel

Dr. Carl Richter

Dr. Brian Schubert

Dr. Rui Zhang

After circulation, please return to Dr. Gottardi.

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I. Summary

The Upper Cretaceous Eagle Ford formation (Texas, Fig. 1) has attracted

considerable industry attention in recent years. It is one of the largest single economic

developments in the history of the state of Texas and ranks as the largest oil and gas

development in the world based on capital invested (according to

http://eaglefordshale.com/). High initial production rates are being reported. Both gas and

oil are being produced from horizontally completed multilateral wells. Although the main

targeted area of the play is located in South Texas and produces from various depths

between 4,000 and 14,000 feet (Fig. 1), deep roadcuts through the Eagle Ford formation

along U.S. Highway 90 north of Del Rio (Val Verde County) provide an accessible

window for improved understanding of the play (Fig. 2).

The Eagle Ford formation has been extensively studied through well logs and

geophysical investigations. However, little is known about the structure and structural

evolution of the play. The goal of this research project is to fill this gap by (1) measuring

fracture systems orientation, frequency, and spacing (density) in relation to lithology

variations, to reconstruct paleo- and contemporary stress fields; (2) recording fracture

intensity and length to determine the number and nature of mechanical units present in

order to establish a mechanical stratigraphy; and (3) characterizing calcite-filled veins in

order to determine their origin, spatial distribution, and age relationship with the fracture

systems.

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II. Literature Review

1. Introduction

The Cretaceous Eagle Ford

Formation (Cenomanian-

Turonian), that encompasses

Texas from the southwest to

the northeast (Fig. 1), has

attracted considerable industry

attention in the recent years,

with substantial high initial production rates of both gas and oil produced from

horizontally completed multilateral wells (Lock et al., 2010). The Eagle Ford Formation

(EFF) is both an unconventional shale reservoir and the source rock for the overlying

Austin Chalk (Corbett et al., 1991). The formation carries between 40% - 90% carbonate

content in some areas of the play, with the shale proportion increasing towards the

northeast (Tian et al., 2012). This special characteristic of the EFF, a high carbonate to

clay ratio, makes the formation more brittle and more susceptible to hydraulic fracturing

(Dawson and Almon, 2010). The stress field of the EFF has been studied through well

logs and seismic interpretation. However, the structural integrity of the EFF has yet to be

fully characterized.

Figure  1:  Geographic  reference  map  showing  Upper  Cretaceous  outcrop  (shaded),  subsurface  structural  contours  on  the  top  of  the  Austin  Chalk  (dashed  lines).  Adapted  from  Corbett  et  al.  (1989,  1991).    

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This special characteristic of the EFF, a high carbonate to clay ratio, makes the formation

more brittle and more susceptible to hydraulic fracturing (Dawson and Almon, 2010).

The stress field of the EFF has been studied through well logs and seismic interpretation.

However, the structural integrity of the EFF has yet to be fully

characterized.

2. Stratigraphy of the Eagle Ford Formation

The EFF changes regionally (Eagle Ford or Boquillas), and has

been described differently by various authors. For simplicity, we will

use the terminology established by Lock et al. (2010) that

distinguishes a “lower”, “middle”, and “upper” member (Fig. 3).

The lower member of the EFF (Fig. 3) is interpreted as a lowstand

systems tract, and is dominated by intermixed mass flow unstable

slope facies, including debrites, slump folded beds, possible ash

turbidite or subaqueous volcanic ash-flows, and possible contourites

(see Lock and Peschier, 2006, for detailed description). The whole

Val Verde Co

Val Verde Co

Terrell Verde Co

MEXICO

Del Rio

LozierCanyon

OsmanCanyon

Langtry

Comstock

•

•

90

90

90

10 Miles

Study Area

Figure  3:  Map  of  the  field  area,  which  extends  along  US  90,  from  Del  Rio  to  Lozier  Canyon  (USGS  base  map)  

Figure  2:  Stratigraphic  framework  of  the  Boquillas  (Eagle  Ford)  Formation  in  Val  Verde  County,  Texas.  

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formation displays abundant evidence of diagenetic differentiation (migration of

carbonate into the areas already richest in carbonate) (Lock and Peschier, 2006; Lock et

al., 2010).

The middle member of the EFF (Fig. 3) is the most commonly exposed along US 90,

with the greatest potential as a source rock or “shale” reservoir. Although commonly

described as shale, these rocks have high carbonate content and are better described as

intermixed marlstones, argillaceous lime mudstones and more resistant limestones,

largely recrystallized (Lock and Peschier, 2006; Lock et al., 2010). The middle member

is divided into several parasequences, each of which commences with a resistant

limestone 1 to 10 inches thick, followed by several feet of more “shaly” (fissile)

lithologies with a pronounced internal cyclicity (Lock and Peschier, 2006; Lock et al.,

2010). These particular beds expose two sets fractures, as well as extensive sets of veins,

and will be the target of this study.

The upper member of the EFF (Fig. 3) is arbitrarily defined as starting with three beds

of thicker limestone, relatively light in color, that are physically prominent (Lock and

Peschier, 2006; Lock et al., 2010). There is no sign of an unconformity between the Eagle

Ford and the overlying basal Austin chalks (Fig. 3).

3. Tectonic Evolution of the Eagle Ford Formation

On the large scale, the EFF is a homocline that gently dips towards the SE (Ferril et

al., 2014), but its structure is very diverse, with thick deposits in the SW and NE, and

thinning in the middle of the San Marcos Arch of central Texas (Fig. 1). In the SW, the

EFF overlies the Buda Limestone and is overlain by the Austin Chalk, with no evidence

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of an unconformity between the EFF and the Austin Chalk. Over the San Marcos Arch,

only a condensed section of the lower facies occurs and thickens again on the NE flank of

the arch. Past the arch, the Maness Shale and Woodbine Group occur between the Buda

and EFF. In the NE, the EFF pinches out against the Sabine Uplift (Hentz and Ruppel,

2010). Important SW regional structural features include the Maverick and Val Verde

basins and the Devils River Uplift. The Balcones fault zone also extends along the SW.

The structural diversity of the EFF comes from a geologic history rich in tectonic

activity. The tectonic evolution of the Texas Cretaceous section is dominated by

divergent-margin development related to the opening of the Gulf of Mexico and

compression from the Laramide Orogeny (Goldhammer and Johnson, 1999). The uplifts

were caused by plutonic intrusions of the Cretaceous period and the basins from

geosynclinal subsidence. This regional subsidence also created local anticlinal and

synclinal folds (Bornhauser, 1958).

III. Nature and Significance of Problem

The global demand for oil and gas is ever rising, and facing the depletion of

conventional plays geologists have been tasked with assessing unconventional reservoirs.

With the recent advent of horizontal drilling, shale reservoirs have become a primary

focus for geologists in search of producible hydrocarbon reservoirs within the United

States. Hydraulic fracturing techniques have been instrumental in the successful

harvesting of oil and natural gas from these mature, low-permeability reservoirs

(Mokhtari et al., 2013). Optimum success in both exploration and production depends on

the adequate characterization of fracture systems and associated rock mass permeability

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as a function of lithology (bed thickness, clay content, percent calcite), rock mechanical

properties (compressive strength, porosity, permeability), structural position, tectonic

history, and in-situ stress conditions (Hubbert and Willis, 1972; Corbett et al., 1991;

Wiltschko et al., 1991; Mokhtari et al., 2013). This information is used to locate of zones

of high fracture intensity and density of fracture intersections (Corbett et al., 1991;

Engelder, 2009). Although lithology and rock properties can be extracted from well and

geophysical data, fracture zones and connectivity, as well as regional and local joint

systems are not detectable from seismic surveys. As a result, conceptual models of

fracture and joint systems are extrapolated from surface and subsurface data.

Areas of high fracture/joint density are attractive to hydraulic fracturing because they

require less stimulation to move hydrocarbons from the reservoir to the well bore. For

fractured reservoirs, the best producing rates come from wells that intercept the most

hydraulically conductive fractures, all other factors affecting porosity, permeability, and

fluid saturation being equal (Nolen-Hoeksema and Howard, 1987). The best direction to

drill is perpendicular to the plane of the fracture faces, assuming open-hole or similar

completion and only one set of fractures present (Nolen-Hoeksema and Howard, 1987).

Moreover, fractures formed during hydraulic fracturing are oriented in the plane

perpendicular to the minimum principal stress axis. It is therefore critical to understand

the regional stress field, to (1) drill wells at a preferred orientation to take advantage of

the pre-existing fractures and joint sets present in the rock, and (2) obtain optimum

hydraulic stimulation.

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IV. Objectives of the Project

Although much research has been done on the overlying Austin Chalk (Corbett et al.

1987, 1991, 2004), outcrop fracture investigations on the EFF have only concentrated on

one outcrop, with comparisons to other subsurface methods (Ferrill et al., 2014). The

purpose of this project will be to characterize the fracture network amongst all readily

accessible outcrops along U.S. Highway 90 north of Del Rio (Val Verde County) and

potentially in the LeHigh Cement Quarry (Waco, Texas). This method will give the most

detailed and accurate analysis of the fractures exhibited in outcrop of the EFF.

The aim of this project is to conduct a thorough structural characterization of the EFF

by (1) measuring fracture systems orientation, frequency, and spacing (density) in

relation to lithology variations, to reconstruct paleo- and contemporary stress fields; (2)

recording fracture intensity and length to determine the number and nature of mechanical

units present in order to establish a mechanical stratigraphy; and (3) characterizing

calcite-filled veins in order to determine their origin, spatial distribution, and age

relationship with the fracture systems.

The results of this project can be used to extrapolate new targets to investigate

through hydraulic fracturing techniques, and will yield important information on

hydrocarbon migration pathways. The data gathered will also provide information

regarding the connectivity of the fractured beds. Crosscutting relationship between joint

systems and veins will reveal the chronologic history of fracture development, as well as

joint relations to tectonic or neotectonic forces (Engelder et al., 1997, Price, 1966).

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V. Research methods

1. Data to be collected

To assess the structural integrity of the EFF, multiple structural characteristics of the

formation must be examined. The following fracture data will be gathered: fracture

systems orientation, length, frequency, spacing and density, in relation to lithology

variations. The data will be divided by outcrop location. The gathered data will be plotted

on rose diagrams and stereonets, in order to identify populations, and master/minor joint

systems. Further stereographic fracture set analysis will reveal the three principal stress

direction solution of the observed fracture population distribution.

Vertical fracture propagation (i.e., fracture length) measurements will reveal if

fractures are bed restricted or transect multiple beds. Combined with fracture density and

frequency data, a mechanical layering will be established and compared to lithological

variations.

2. Preliminary Field Work

During the summer of 2014, three outcrop locations along U.S. Highway 90 north of

Del Rio (Val Verde County) were examined to see if they were suitable for this project.

Fracture orientations were measured and different fracture sets were identified (see

Appendix I). The data reveal the presence of two joint sets, with one more dominant than

the other. The prominent joint set Joint Set 1 in Appendix I, strikes NE-SW, Joint Set 2

strikes NW-SE. I also managed to gain access to the LeHigh Cement Quarry in Waco,

Texas, where the contact between the EFF and the Austin Chalk outcrops at different

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locations in the quarry. I will be able to compare the measurements in the EFF with pre-

existing, extensive work, published on the Austin Chalk.

VI. Timeline of Completion

After the approval of this proposal, two field trips are planned this spring, and all

fieldwork will be completed by May of 2015. Analysis of all data taken will be analyzed

into stereonets, distribution diagrams and plots, by July of 2015. The defense of this

project will occur in October of 2015.

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VII. References Cited

Bornhauser, M., 1958, Gulf Coast Tectonics: American Association of Petroleum Geologists, v. 42, n. 2, p. 339-370.

Corbett, K. P., M. Friedman, and J. Spang, 1987, Fracture development and mechanical stratigraphy of Austin Chalk, Texas: American Association of Petroleum Geologists Bulletin, v. 71, p. 17-28. Corbett, K. P., M. Friedman, D. V. Wiltschko, and J. Hung, 1991, Controls on fracture development, spacing, and geometry in the Austin chalk formation, Central Texas: Considerations for exploration and production: Dallas Geological Society, p. 1-49.

Dawson, W. C., and W. R. Almon, 2010, Eagle Ford Shale variability: Sedimentologic influences on source and reservoir character in an unconventional resource unit: Gulf Coast Association of Geological Societies Transactions, v. 60, p. 181-190. Engelder, T., and A. Lacazette, 1990, Natural hydraulic fracturing in rock joints: Rotterdam, AA Balkema, 35-44. Engelder, T., M. R. Gross, and P. Pinkerton, 1997, An analysis of joint development in thick sandstone beds of the Elk Basin anticline, Montana-Wyoming, in T. Hoak, A. Klawitter, and P. Bloomquist, eds., Fractured reservoirs: Characterization and modeling guidebook: Rocky Mountain Association of Geologists, p. 1–18. Ferrill, D. A., et al., 2014, Control of mechanical stratigraphy on bed-restricted jointing and normal faulting: Eagle Ford Formation, south-central Texas: American Association of Petroleum Geologists Bulletin, v. 98, n. 11, p. 2477-2506.

Goldhammer, R. K., and C. A. Johnson, 1999, Middle Jurassic-Upper Cretaceous paleogeographic evolution and sequence-stratigraphic framework of the northwest Gulf of Mexico rim, in C. Bartolini, R. T. Buffler, and A. Cantu-Chapa, eds., The western Gulf of Mexico Basin: Tectonics, sedimentary basins, and petroleum systems: American Association of Petroleum Geologists Memoir 75, p. 45-81. Hentz, T. F., and S. C. Ruppel, 2010, Regional lithostratigraphy of the Eagle Ford Shale: Maverick Basin to East Texas Basin: Gulf Coast Association of Geological Societies Transactions, v. 60, p. 325-337.

Howard, J. H., and R. C. Nolen-Hoeksema, 1990, Description of Natural Fracture Systems for Quantitative Use in Petroleum Geology: American Association of Petroleum Geologists Bulletin, v. 74, n. 2, p. 151-162. Hubbert, M. K., and D. G. Willis, 1957, Mechanics of hydraulic fracturing: Transactions of the American Institute of Mining, Metallurgical, and Petroleum Engineers 210, p. 153–68.

Lock, B. E., and L. Peschier, 2006, Boquillas (Eagle Ford) upper slope sediments, West Texas: Outcrop analogs for potential shale reservoirs: Gulf Coast Association of Geological Societies Transactions, v. 56, p. 491-508.

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Lock, B. E., L. Peschier, and N. Whitcomb, 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, v. 60, p. 419-434. Mokhtari, M., A. A. Alqahtani, A. N. Tutuncu, and X. Yin, 2013, Stress-Dependent Permeability Anisotropy and Wettability of Shale Resources: Unconventional Resources Technology Conference, Denver, Colorado, USA, 12-14 August 2013.

Nolen-Hoeksema, R. C., and J. H. Howard, 1987, Estimating drilling direction for optimum production in a fractured reservoir. American Association of Petroleum Geologists Bulletin, v. 71, n. 8, p. 958-966. Price, N. J., 1966, Fault and joint development in brittle and semi-brittle rock , v. 1, Oxford: Pergamon Press. Tian, Y., W. B. Ayers, and D. McCain, 2012, Regional Analysis of stratigraphy, reservoir characteristics, and fluid phases in the Eagle Ford Shale, south Texas: Gulf Coast Association of Geological Societies Transactions, v. 62, p. 471-483.

Wiltschko, D. V., K. P. Corbett, M. Friedman and J. H. Hung, 1991, Predicting fracture connectivity and intensity within the Austin Chalk from outcrop fracture maps and scanline data.

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VIII. Appendix 1

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