wolfcampian development of the nose of the eastern shelf

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Wolfcampian Development of the Nose of the Eastern Shelf of the Wolfcampian Development of the Nose of the Eastern Shelf of the

Midland Basin, Glasscock, Sterling, and Reagan Counties, Texas Midland Basin, Glasscock, Sterling, and Reagan Counties, Texas

Douglas S. Flamm Brigham Young University - Provo

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BYU ScholarsArchive Citation BYU ScholarsArchive Citation Flamm, Douglas S., "Wolfcampian Development of the Nose of the Eastern Shelf of the Midland Basin, Glasscock, Sterling, and Reagan Counties, Texas" (2008). Theses and Dissertations. 1555. https://scholarsarchive.byu.edu/etd/1555

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WOLFCAMPIAN DEVELOPMENT OF THE NOSE OF THE EASTERN SHELF OF THE

MIDLAND BASIN, GLASSCOCK, STERLING, AND REAGAN COUNTIES, TEXAS

Title

By

Douglas Scott Flamm

A thesis submitted to the faculty of

Brigham Young University

in partial fulfillment of the requirements for the degree of

Master of Science

Department of Geological Sciences

Brigham Young University

December 2008

BRIGHAM YOUNG UNIVERSITY

GRADUATE COMMITTEE APPROVAL

of a thesis submitted by

Douglas Scott Flamm

This thesis has been read by each member of the following graduate committee and by majority vote has been found to be satisfactory.

__________________ _____________________ Date John H. McBride Chair, Graduate Committee __________________ _____________________ Date Scott M. Ritter Graduate Committee __________________ _____________________ Date Thomas H. Morris Graduate Committee __________________ _____________________ Date R. William Keach II Graduate Committee

BRIGHAM YOUNG UNIVERSITY As chair of the candidate’s graduate committee, I have read the thesis of Douglas Scott Flamm in its final form and have found that (1) its format, citations, and bibliographical styles are consistent and acceptable and fulfill university and department style requirements; (2) its illustrative materials including figures, tables, and charts are in place; and (3) the final manuscript is satisfactory to the graduate committee and is ready for submission to the university library. _______________________ _________________________________________ Date John H. McBride Chair, Graduate Committee Accepted for the Department _______________________ _________________________________________ Date Scott M. Ritter Department Chair Accepted for the College _______________________ _________________________________________ Date Thomas W. Sederberg Associate Dean College of Physical and Mathematical Sciences

ABSTRACT

WOLFCAMPIAN DEVELOPMENT OF THE NOSE OF THE EASTERN SHELF OF THE

MIDLAND BASIN, GLASSCOCK, STERLING, AND REAGAN COUNTIES, TEXAS

Douglas Scott Flamm

Department of Geological Sciences

Master of Science

The nose of the Eastern shelf of the Midland Basin is a prominent structural and

depositional feature present in Glasscock, Sterling, and Howard counties, Texas. This

feature has been expressed in many regional maps and mentioned in some literature, but

has not otherwise been studied significantly. This study looks at the viability of using an

acoustic impedance seismic inversion to interpret the 2nd and 3rd order sequence

stratigraphy of the southern portion of the nose of the Eastern shelf along with its shelf to

basin transition in Glasscock, Sterling, and Reagan counties during the Wolfcampian

(Asselian-Sakmarian) time (Early Permian).

The Wolfcamp Formation (Wolfcampian-Leonardian) was subdivided into six

units based on regionally mapped shale markers that correlate with 3rd order sequence

boundaries. These horizons were mapped throughout the study area utilizing 3D seismic

data and well logs.

Analysis of seismic amplitude and inversion (acoustic impedance) volumes, along

with well logs were then used to create a 2nd and 3rd order sequence stratigraphic

framework in the study area. Six 3rd order sequences and two 2nd order sequences were

identified in the study area during the Wolfcampian. From this framework a 2nd order

sea-level curve was developed.

The oldest Wolfcampian 3rd order sequence is marked by sediment bypass of the

shelf and slope into the basin during a 3rd order sea level fall. Shelfal deposition

resumed during subsequent sequences as sea-level rose and carbonate production

resumed. Carbonate production increased during sequences four through six as part of a

2nd order sea-level highstand. During this highstand the nose of the Eastern shelf grew

vertically increasing the gradient of the slope from less than 1° to 3.5°. The end of

Wolfcampian deposition is marked by a large number of gravity flows into the basin

resulting from subaerial exposure and erosion after a second order sea-level fall.

Acknowledgements

I would like to thank my wife Rachel for her support throughout my education

and especially during these last few months. Her encouragement and dedication have

helped immensely and kept me focused. I would like to thank my son Peter for

distracting me enough so that I did not go crazy during this endeavor.

I would also like to thank the members of my committee; John McBride for his

seismic and well log knowledge and also for helping me avoid potential pitfalls during

my education; Bill Keach for the time that he spent with me in the lab answering my

questions and teaching me about seismic interpretation; Scott Ritter for his help with

carbonate stratigraphy and deposition; and Tom Morris for his sequence stratigraphic

knowledge.

Special thanks to Chevron Corp. of North America for providing the 3D seismic

data, well logs, and core images. From Chevron I would like to thank the West Texas

Gas Asset Development Team from the Mid-Continent Alaska business unit. In particular

I would like to thank Chip Story for his mentorship and Gary Chapman for his help and

advice.

On behalf of Brigham Young University, I would like to gratefully acknowledge a

software grant from the Landmark (Halliburton) University Grant Program.

TABLE OF CONTENTS

Title ...................................................................................................................................... i

GRADUATE COMMITTEE APPROVAL ........................................................................ ii

ABSTRACT ....................................................................................................................... iv

Acknowledgements ............................................................................................................ vi

Introduction ......................................................................................................................... 1

Geologic Setting.................................................................................................................. 2

Structure and Tectonics ................................................................................................... 2

Stratigraphy ..................................................................................................................... 4

Depositional Environment .............................................................................................. 6

Methodology ....................................................................................................................... 7

Data Acquisition ............................................................................................................. 8

Well to Seismic Correlation ............................................................................................ 9

Data Interpretation ............................................................................................................ 10

Isochrons ....................................................................................................................... 11

Sequence Stratigraphic Framework .............................................................................. 13

Amplitude Volume Interpretation ............................................................................. 14

Inversion Volume Interpretation ............................................................................... 15

2nd Order Sea-level Curve ......................................................................................... 17

Wolfcampian Mass Wasting ......................................................................................... 19

Turbidites and Grainflows ........................................................................................ 20

Debris Flows ............................................................................................................. 21

Seismic Features ....................................................................................................... 21

Failure Mechanisms .................................................................................................. 24

Development of the Eastern Shelf ................................................................................ 25

Geologic Models ....................................................................................................... 26

Conclusions ....................................................................................................................... 27

References ......................................................................................................................... 29

Figures............................................................................................................................... 35

Figure 1 ......................................................................................................................... 35

Figure 2 ......................................................................................................................... 36

Figure 3 ......................................................................................................................... 37

Figure 4 ......................................................................................................................... 38

Figure 5 ......................................................................................................................... 39

Figure 6 ......................................................................................................................... 40

Figure 7 ......................................................................................................................... 41

Figure 8 ......................................................................................................................... 42

Figure 9 ......................................................................................................................... 43

Figure 11 ....................................................................................................................... 45

Figure 12 ....................................................................................................................... 46

Figure 13 ....................................................................................................................... 47

Figure 14 ....................................................................................................................... 48

Figure 15 ....................................................................................................................... 49

Figure 16 ....................................................................................................................... 50

Figure 17 ....................................................................................................................... 51

Figure 18 ....................................................................................................................... 52

Figure 19 ....................................................................................................................... 53

Appendix ........................................................................................................................... 54

Acquisition Data ........................................................................................................... 54

1

Introduction

The Permian basin in west Texas and southeast New Mexico has been producing oil for

over 80 years and is the largest onshore petroleum producing province in the United States.

When offshore drilling is taken into account it is the third largest behind the Gulf of Mexico and

offshore Alaska. From 1921 until 1995 it produced over 14 billion barrels of oil and continues to

produce (Ball, 1995). In 2002, the Permian basin accounted for 17% (327 million bbl) of US

production and 22% (5 billion bbl) of domestic reserves (Energy Information Administration,

2003; Dutton et al., 2005). Estimates of remaining oil in place are as high as 23 billion barrels

with another 3.5 billion barrels of undiscovered oil (Ball, 1995). Production in the Permian

Basin is primarily out of Pennsylvanian through Permian age strata with initial production in the

Wolfcampian section first established in 1954.

As time goes on however, economic accumulations of hydrocarbons are harder to find

and produce. Consequently, expanding our knowledge of the structural and depositional history

of the Permian basin and its sub-basins is becoming increasingly important. One advance in

technology that has led to an increase in our understanding of the Permian basin as well as other

basins throughout the world has been the widespread use of 3D seismic reflection data. 3D

seismic data has provided researchers with the ability to gain further insight into the formation of

the Permian basin and its depositional history.

The concepts of seismic and sequence stratigraphy have also helped to advance our

knowledge of the Permian basin. These concepts have been used in academia and industry over

the past 20 years to better understand the depositional settings and facies relationships of

sedimentary strata throughout the world. Sequence stratigraphy in conjunction with 3D seismic

data has led to the discovery of numerous new oil and gas fields. The use of 3D seismic data (and

2

sequence stratigraphy) in Glasscock County, Texas alone has increased oil well completion

success from less than 15% to more than 70% (Gibson, 1993).

Within the greater Permian basin (Figure 1), the Delaware basin and Central Basin

platform (CBP) have been studied by many researchers due to excellent exposures of the

Permian section in the Guadalupe Mountains. In contrast to the Delaware basin, there is an

absence of Wolfcampian-age outcrops in the Midland basin resulting in a smaller body of

research. Several studies within the Midland basin have looked at the seismic and sequence

stratigraphy of the Wolfcampian through Leonardian age strata (Silver and Todd, 1969;

Mazzullo and Reid, 1988; and others). However, these studies have generally been conducted in

the northern part of the Midland basin or along its western margin with the CBP. This study was

conducted further south in the basin along the Eastern shelf and focuses on development of the

nose of the Eastern shelf and its slope-to-basin transition in response to changes in sea level

during the Early Permian.

The study area is located in Glasscock, Sterling, and Reagan counties, Texas covering an

area of approximately 143 mi2 (370 km2) along the Eastern shelf of the Midland basin (Figure 1).

Geologic Setting

Structure and Tectonics

The Permian basin is an asymmetrical structural depression sitting on top of Precambrian

basement dated between 1350 and 1275 Ma (Ewing, 1995). It covers approximately 115,000

square miles (300,000 square kilometers) in west Texas and southeastern New Mexico

(Mazuingue-Desailly, 2004). During the Late Paleozoic, the Permian basin was segmented into

several smaller basins and uplifts as a result of the breakup of the Pangaea and late Paleozoic

3

continental reassembly (Hills, 1985), locally, the Marathon-Ouachita collision (Ross, 1986;

Ward et al., 1986; Yang and Dorobek, 1995). Since Early Permian time, there has been little

deformation of the Permian basin other than uplift and tilting along its edge during the Triassic

(Ward et al., 1986; Mazzullo, 1995).

The structural development of the basin can be divided into three stages; formation of the

Tobosa basin (predecessor to the Permian Basin) during Cambrian through Mississippian time;

fragmentation of the Tobosa basin into several smaller deep basins separated by uplifts due to the

Ouachita (Alleghanian) Orogeny during the Early Pennsylvanian through Early Permian Periods;

and structural/tectonic stability and infilling of the basin from the Middle Permian to the Early

Triassic.

During the Mid-to-Late Mississippian, the Tobosa basin was divided by the formation of

a central high, effectively initiating the formation of the Delaware basin, Central Basin Platform,

and Midland Basin. From the Late Mississippian until the end of the Pennsylvanian the Ouachita

Orogeny fragmented the Tobosa basin into a series of horsts and grabens along high-angle

reverse faults (Horak, 1985; Hills, 1984). At this time, carbonate shelves began to form along

the margins of the newly formed basins and were intermixed with siliclastic sediment throughout

the filling of the basins.

The Mid-to-Late Permian interval was characterized by relative tectonic quiescence and

rapid subsidence during which the majority of the sub-basins within the Permian basin were

filled. As the Delaware and Midland basins subsided, the CBP continued to rise several thousand

feet (as much as 8,000 ft) (Horak, 1985), creating more accommodation. Since the beginning of

the Triassic, the Permian basin has been tilted to the east as the western edge of the basin was

4

uplifted approximately 9,000 ft (2743 m), half of which is the result of uplifting during the

Laramide orogeny (Horak, 1985).

The Midland basin is located on the eastern side of the Permian basin and is surrounded

by the Ozona arch and Ouachita-Marathon orogenic belt to the south, the CBP to the west, the

Matador arch to the north, and the Eastern Shelf and Chadbourne fault zone to the east (Figure

1). The Eastern shelf does not have a defined eastward edge but transitions into the Concho

platform and Bend arch.

Stratigraphy

The first usage of the term Wolfcamp occurs in Udden et al. (1916) in reference to the

lowermost Permian strata of the Glass Mountains. The origin of neither the name nor the type

locality was discussed in the paper, but the term “Wolfcamp” was likely derived from the

Wolfcamp Hills. Through the first half of the 20th century, several more studies were conducted

in the Glass Mountains area, which adopted the name (Udden, 1917; Bose, 1917; Schuchert,

1927; King and King, 1928; Sellards, 1932; Wilmarth, 1938; Adams et al. 1939; Ross, 1963). In

1939 Adams et al. suggested that the “Wolfcamp Formation” be used as the first series marker of

the Permian System. This usage was accepted into the North American Stratigraphic Code

(NASC) and is in current use in industry today. In academia however, the International

Stratigraphic Code (ISC) has become more predominant. Wolfcampian age rocks of the NASC

are equivalent to the Asselian and Sakmarian of the ISC. NASC terminology will be used in this

study for consistency with previous studies.

Ross (1959) subdivided the Wolfcamp Formation into two formal formations; the Neal

Ranch Formation, and overlying Lennox Hills Formation. This separation of the Wolfcamp

5

worked well in the exposed Permian section of the Glass Mountains, but not in the subsurface.

Consequently, industry stratigraphers have continued to use the term “Wolfcamp” to refer to the

formation and the stage name.

Within the last 25 years, the top and base of the Wolfcampian stage have been redefined.

The base of the Wolfcampian stage was adjusted in the mid to late 1990’s based largely on the

correlation of conodonts from the Ural Mountains in Russia to those in Kansas (Davydov et al.,

1995; Ritter, 1995; Chernykh and Ritter, 1997). The lowest one-third of the lower Wolfcampian

was reassigned to the Upper Pennsylvanian.

The Wolfcampian-Leonardian time series boundary was redefined in the early 1980’s

(though still the subject of some debate). Based on biostratigraphic markers (fusilinids),

Mazzullo et al. (1987) determined that the Wolfcampian-Leonardian boundary lies below the

Dean Sandstone (Tubb equivalent) in the Midland basin at what is commonly referred to as the

“Wolfcamp shale marker”. The Wolfcamp shale marker as described by Silver and Todd (1969)

is representative of a regional unconformity separating the middle and upper Wolfcampian.

Mazzullo and Reid (1988) point out that such an unconformity would require a drop in sea level

of 800-1500 ft (250-450 m), well above the accepted 300 ft (100 m) of fluctuation during

icehouse conditions. Instead, they suggest that the Wolfcamp shale marker is the result of rapid

subsidence within the basin (Mazzullo, 1982). However, they do recognize the possibility of an

unconformity on the shelf due to subaerial erosion. It is probable that the Wolfcamp shale

marker is representative of both a drop in sea level and subsidence. In this paper the

Wolfcampian stage will be used as defined above (The Wolfcampian-Leonardian boundary lying

within the Wolfcamp shale marker and the Wolfcampian-Virgilian boundary at the Wolfcamp-

Cisco Formation boundary)(Figure 2).

6

Depositional Environment

Carbonate platforms in the Midland basin formed during Late Pennsylvanian through

Early Permian time. Icehouse conditions dominated this time with eustatic sea level changes

caused by the melting and growth of continental glaciers in the southern hemisphere (Heckel,

1986; Ross and Ross, 1987; Veevers and Powell, 1987; Crowley and Baum, 1992). These

eustatic sea-level changes were both high amplitude and high frequency in nature (Saller et al.,

1994).

The platforms were dominated by phylloid algal growth along with crinoids, sponges,

foraminifera, and other faunal elements. Sediment derived from these platforms shed in the

Midland basin during Late Paleozoic time (Tai, 2000). Reefal carbonate deposition during the

Early Pennsylvanian resulted in the formation of structures like the Horseshoe Atoll (in the north

central part of the basin), which developed into large hydrocarbon reservoirs (Adams et al.,

1951). At the end of Wolfcampian time, a significant drop in sea level subaerially exposed the

Wolfcampian shelf. Locally, this drop in sea level coincides with the Wolfcampian-Leonardian

boundary. Sea level subsequently rose during the early Leonardian, flooding the shelf. By the

end of Leonardian time, the Eastern shelf had prograded as much as 15 miles (24 km) into the

Midland basin (Mazzullo and Reid, 1989).

The Wolfcamp Formation is a two-lithology system dominated by shales and limestones.

Limestones represent shelf to proximal basin deposits while shales represent a more basinal

setting. This mixed carbonate-siliciclastic system in conjunction with the Permian basin’s

paleolatitude near the equator during the Early Permian (Walker et al., 1991) suggests that the

climate during this time was humid to sub-humid (Saller et al., 1994).

7

The carbonate factory was located at the shelf edge, with small patch reefs down the

slope but still within the photic zone. Buildups of shelf facies carbonates in the northern

Midland basin range from phylloid algae-foraminifera-Tubiphytes buildups during the

Wolfcampian to sponge-Tubiphytes-foraminifera buildups in the Leonardian (Mazzullo, 1984;

Mazzullo and Reid, 1989).

Limestones in the Wolfcamp Formation can be categorized into four facies groups-

lithoclastic facies, grain-supported facies, matrix-supported facies, and boundstone facies

(Merriam, 1999; Buenafama, 2004). The lithoclastic facies is primarily composed of

grainstones, packstones, wackestones, and boundstones. Grain-supported facies are composed of

grainstones and packstones and have been interpreted as shallow-water deposits near wave base.

Matrix-supported facies are composed of wackestones and lime mudstones and have been

interpreted as being deposited below wave base-but still in the photic zone. The boundstone

facies is the predominant reef-building facies and contains phylloid algae, crinoids, bryozoans,

and other skeletal elements have been interpreted as being derived from the platform edge

(Merriam, 1999; Buenafama, 2004).

Shales within the basin are predominantly black, suggesting a biogenic origin. Black

shales are representative of basinal depositional environment beyond most influences from the

shelf. The shales have been reported to contain skeletal limestone, radiolarians, and other clasts

(Merriam, 1999).

Methodology

A 3D seismic-amplitude volume was evaluated in conjunction with a 3D seismic

inversion (acoustic impedance) volume, and well logs to produce a seismic/sequence

8

stratigraphic interpretation of the data, to produce depositional models of the area, and a 2nd

order sea-level curve for the study area. A seismic interpretation (Figure 3) was carried out

using the amplitude and inversion volumes in which the Wolfcamp Formation was subdivided

into five units (A-E) based upon regionally correlative shales as picked from well logs (Figure

4), representing deepening events within the basin. The uppermost unit (A) is likely not

Wolfcampian, but Leonardian in age due to its location above the Wolfcamp shale marker. As

discussed earlier, it is commonly referred to as part of the Wolfcamp Formation even though it is

not part of the Wolfcampian stage. The underlying Pennsylvanian Cisco, Canyon, and Atoka

Formations were also interpreted in order to put changes in deposition during Wolfcampian time

into context with previous depositional systems. These interpreted horizons were used to create

a 3rd order sequence stratigraphic framework for the study area. After the sequence stratigraphic

framework was developed, it was correlated to well log interpretation (Figure 5) in a dip

direction (north-south) in an attempt to further refine the sequence stratigraphic framework.

During development of the sequence stratigraphic framework, the evolution of the Eastern shelf

was examined in the study area. This was accomplished by looking at the stratal geometries of

the amplitude and inversion volumes, well log evaluation, and isochron mapping of the study

area to evaluate the direction and rate of sedimentation.

Data Acquisition

Two 3D seismic surveys shot by Chevron in the late 1990’s were merged to form the

current 3D volume (Figure 6). Both surveys had a receiver spacing of 110 ft (33.5 m) and a bin

size of 55 ft (16.7 m), with the nominal fold of both surveys being 40. The two surveys were

shot using vibroseis trucks with a sweep frequency of 12-92 Hz. The merged volume is zero

9

phase, has a dominant frequency of approximately 45 Hz, and a nominal fold of 40 with the fold

decreasing toward the edges. The combined survey has a total area of approximately 143 square

miles (370 km2). The volume has 706 north-south in-lines and 833 east west cross-lines

resulting in 588,098 common cell gathers. The survey is non-rectangular resulting in variable

length of both the in-lines and cross-lines. The length of in-lines ranges from 9.5 to12 miles (15-

19 km) and the length of the cross-lines ranges from 3 to 14 (5-22.5 km) miles.

After the merge, an acoustic impedance seismic inversion was created using the new 3D

volume. The inversion is model based and assumes that the Seismic trace = Reflectivity *

Wavelet (Figure 7). A median wavelet was extracted at five well locations within the 3D

volume. This wavelet was then deconvolved from the seismic volume to produce an acoustic

impedance volume. The acoustic impedance volume was used to distinguish between lithologies

(carbonate/shale) in the Wolfcampian section.

Well to Seismic Correlation

Within the 3D seismic volume there are over 1500 wells, approximately half of which

have wireline logs. The majority of those that have logs, have only gamma ray and caliper logs.

There were a total of 16 wells that had both sonic and bulk density logs to create quality

synthetic seismograms (Figure 8). Fortunately those 16 wells were distributed throughout the

study area and not grouped together in one area. Even though the wells have a good distribution,

limiting the bias of the data, there is a slightly higher concentration of synthetic seismograms in

the southern portion (seven synthetics) of the study area as opposed to the central (four

synthetics) and northern (five synthetics) areas.

10

The synthetic seismograms were created by generating an acoustic impedance log from

the product of a bulk density (RHOB) log and sonic log (Δt). The acoustic impedance log was

then convolved with a Klauder wavelet, because it best matched the vibroseis source. The

synthetic seismograms were then matched against the actual seismic traces at the well locations.

The synthetics were stretched and squeezed accordingly to achieve as near a match as possible.

The synthetic seismograms matched with the seismic at the well locations achieved a high degree

of correlation in the zone of interest (Wolfcamp Formation) as well as just above and below it

(Figure 9). Wells in the central and southern part of the study area generally had a higher

correlation than those in the northern part of the study area. This is likely due to the fact that the

northern wells are situated higher up the slope and on the edge of the Eastern Shelf resulting in

more heterogeneous lithologies. Increased lithologic heterogeneity increases the difficulty of

creating accurate synthetic seismograms.

Once the synthetic seismograms were created, they were used to generate a velocity

model to tie the remaining wells in the study area to the seismic and inversion volumes. The

velocity model was accurate overall. However; the less accurate synthetics in conjunction with

thinning synthetic coverage to the north reduces the accuracy of the velocity model in that part of

the study area. After the well logs were tied to the seismic data using the velocity model, the

wells relevant to the interpretation of the data were edited to attain a more accurate match

between the formation tops as interpreted on the well logs and the seismic data.

Data Interpretation

The Dean Sandstone, Wolfcamp, Cisco, Canyon, and Atoka Formations were mapped

within in the study area. As previously stated, the Wolfcamp Formation was subdivided based

11

on five regional shale markers (Figure 4) within the formation. Each marker represents the top

of the formation or unit. The Dean Sandstone, Cisco, Canyon, and Atoka Formations were

manually interpreted every 10th in-line and cross-line. These mapped horizons were then

interpolated to fill in between the manually mapped lines. The Wolfcamp subunits with the

exception of the Wolfcamp B and C were also interpreted every 10th in-line and cross-line with

the remaining lines being interpolated. The Wolfcamp B and C subunits were mapped manually

every in-line and cross-line to enhance the accuracy of the interpretation. This was done because

of the higher degree of difficulty to map these horizons as well as the significance of these

horizons to the overall interpretation. The higher degree of difficulty is not due to structural

complexity but to stratigraphic complexity in a rapidly changing depositional environment.

Isochrons

Isochron maps were generated from the seismic amplitude volume for each of the

Wolfcamp subunits, the Cisco Formation, and the interval from the top of the Atoka Formation

to the top of the Canyon Formation. The isochron maps were then used to qualitatively

determine sedimentation rate, source, and direction for each isochron (Figure 10).

The isochron map from the top of the stratigraphic interval from the top of the Atoka

Formation to the top of the Canyon Formation (Figure 10, map A) is the largest time interval of

the isochron maps, and is meant only to show what had occurred prior to the Late Pennsylvanian.

The isochrons indicate deposition occurred primarily along the edge of the basin on the Eastern

Shelf as evidenced by the thicker isochrons along the shelf and shelf edge. Isochron thickness

ranges from approximately 250 ms in the east to 100 ms in the west with a fairly uniform

gradient to the isochrons.

12

The isochron map of the Cisco Formation (Figure 10, map B) was created by subtracting

the top of the Cisco Formation from the top of the Canyon Formation. During Cisco deposition

the sediment source began to rotate from the along strike of the basin margin as it had been prior,

to a northeast-southwest orientation. Isochron thickness is the greatest in the northeast at

approximately 90 ms and decreases to 20 ms in the southwest. Isochrons also indicate that

during this time there may have been a large debris flow that moved in a northeast-southwest

(i.e. from shelf to slope) into the basin. The debris flow appears to be oriented in the same

direction as general deposition.

The isochron map of the Wolfcamp E subunit (Figure 10, map C) was created by

subtracting the top of the Wolfcamp E from the top of the Cisco Formation. During Wolfcamp

E time, the depositional source reverted back to the Eastern shelf along strike, with the locus of

deposition shifting from the shelf to the basin. Basinward isochron thickness is as much as 70

ms compared to isochron thickness at the shelf which is as little as a few milliseconds thick.

This suggests a lowering of sea-level, which subaerially exposed the shelf. Sediment bypassed

the shelf and was deposited further into the basin resulting in high rates of deposition in the basin

and low rates on the shelf and slope.

The isochron map of the Wolfcamp D subunit (Figure 10, map D) was created by

subtracting the top of the Wolfcamp D from the top of the Wolfcamp E. During Wolfcamp D

deposition, the sediment source moved to the north with the majority of deposition taking place

in the northwest portion of the study area. Thickness in the northwest is a much as 100 ms and

thins to as little as 15 ms in the southeast. Wolfcamp D isochron patterns suggest that sea level

had risen relative to its level during the Wolfcamp E. The rise in sea level may have reinitiated

carbonate production on the nose of the Eastern shelf explaining the higher rates of deposition in

13

the northern part of the study area. The majority of the carbonate sediment was probably

produced in-situ with some of it being transported into the basin.

The isochron map of the Wolfcamp C subunit (Figure 10, map E) was created by

subtracting the top of the Wolfcamp C from the top of the Wolfcamp D. During deposition of

the Wolfcamp C, the sediment source remained focused in the northern part of the study area,

thinning to the south and southwest. Thicknesses in the northwest are as much as 180 ms and

thin to as little as 10 ms in the southwest. The increased thickness of the isochrons suggests that

sediment production during this time increased substantially over sediment production during the

Wolfcamp D.

The isochron map of the Wolfcamp B subunit (Figure 10, map F) was created by

subtracting the top of the Wolfcamp B from the top of the Wolfcamp C. During deposition of

the Wolfcamp B sediment continues to be sourced from the north. Deposition also continues to

be focused in the north with isochron thickness as high as 180 ms. Isochron thickness thins

rapidly southward to approximately 30 ms.

Sequence Stratigraphic Framework

The seismic amplitude and inversion volumes were used to interpret the 2nd and 3rd order

sequence stratigraphic framework of the study area during Wolfcampian time. Sequence

boundaries were defined by laterally continuous seismic reflectors that show onlap and downlap

onto them and toplap or erosional truncation below them. Each of the five shale markers used to

subdivide the Wolfcamp was analyzed due to initial indications from the wireline logs that they

were potential sequence boundaries. Other reflectors within the Wolfcamp were also evaluated

14

as possible sequence boundaries. Six third order sequences were interpreted during

Wolfcampian time and two second order sequences.

Amplitude Volume Interpretation

Ten north-south seismic cross sections spaced every 50 lines (approximately one mile

apart) were interpreted in the study area and compared to each other (Figure 11). The results are

presented herein with primary discussion of the interpretation based on cross section B-B’

(Figure 12).

Six 3rd order sequences were deposited during Wolfcampian time, four of which correlate

with the Wolfcamp B, C, D, and E markers picked on the wireline logs. The first 3rd order

sequence (1050-1100 ms) deposited during Wolfcampian time (dark green sequence in Figure

12) ends at the level of the Wolfcamp E marker. The Wolfcamp E horizon has fewer seismic

reflectors terminating against it than other sequence boundaries, but still shows the requisite

reflectors terminations to indicate that it is a sequence boundary. In cross-section B-B’ onlap

and downlap can be seen above the Wolfcamp E shale marker but toplap and erosional

truncation are not seen below it.

The second 3rd order sequence (1000-1050 ms) (light green sequence in Figure 12) ends

at the level of the Wolfcamp D marker. Reflection terminations against this sequence boundary

are similar to those of sequence one, predominantly showing downlap with some onlap, but

lacking strong evidence of top lap or erosional truncation. Other evidence for the Wolfcamp D

horizon as a sequence boundary is the amount of thinning that takes place between it and the top

of the Wolfcamp C marker as it transitions from the slope into the basin.

15

The interval from the top of the Wolfcamp D to the top of the Wolfcamp C contains three

third order sequences. These sequence boundaries associated with these sequences show better

defined reflection terminations against them than sequences one and two. The first of these

sequences (sequence 3) (930-1000 ms) shows well defined downlap with some onlap, toplap,

and erosional truncation onto the sequence boundary (purple sequence in Figure 12).

Sequence four (900-930 ms) (red sequence in Figure 12) is the thinnest of the sequences

and does not extend all the way into the basin but terminates at or near the toe of slope. The

sequence boundary for sequence four shows well developed reflector terminations against it

including onlap, downlap, toplap, and erosional truncation. During this sequence the gradient of

the slope increases slightly compared to previous sequences.

The fifth sequence (820-900 ms) (yellow sequence in Figure 12) extends further into the

basin than sequence four but does not extend all the way out like sequences one, two, and three.

The sequence boundary for this sequence corresponds to the Wolfcamp C marker. Like

sequences three and four the reflection terminations against the sequence boundary are well

defined and exhibit all four types of reflection terminations. Slope gradient continued to increase

during deposition of this sequence, but at a higher rate than during sequence four.

The sixth and last sequence deposited during Wolfcampian time (720-820 ms) (light blue

sequence in Figure 12) is the thickest of the sequences. The sequence boundary is defined by

onlap above and erosional truncations below and corresponds to the Wolfcamp B marker.

Inversion Volume Interpretation

The seismic inversion was used to look at the second order sequence stratigraphy of the

study area, and to a lesser degree the third order sequence stratigraphy. The inversion is an

16

acoustic impedance inversion and can be considered as a proxy for lithology. Acoustic

impedance is affected by porosity, permeability, and density which are related to lithology.

Because the Wolfcamp Formation is a two lithology system (carbonate/shale) changes in

acoustic impedance in the inversion can be interpreted to represent changes in lithology because

of the significant difference in the properties of shales and carbonates. The inversion was

correlated with gamma ray logs to verify that this relationship and found to hold true in the study

area (Figure 13). This allows the inversion to be used to differentiate basinal shale and mudstone

deposits from shelf and slope derived carbonates. Carbonates are represented by greens and

yellows in the Wolfcampian section of the inversion (Figure 14) while the blues are

representative of shales and mudstones. Looking at the geometries produced by these deposits, a

second order sequence stratigraphic framework was produced. Cross section D-D’ (Figure 14) is

shown to illustrate this framework.

Two 2nd order sequences were deposited during Wolfcampian time. The first 2nd order

sequence is better preserved than the other 2nd order sequence during the Wolfcampian. The first

2nd order sequence contains a transgressive systems tract (TST), highstand systems tract (HST)

and falling stage systems tract (FSST). The lowstand systems tract (LST) is the only systems

tract missing from this sequence. This second order sequence contains 3rd order sequences one

through five discussed earlier. The maximum flooding surface (MSF) is not readily identifiable

in between the TST and the HST so a maximum flooding zone (MFZ) has been mapped where

the MFS is likely to be found. The boundary between the HST and FSST of this sequence is also

not easily separated. The HST and FSST both contain prograding carbonate clinoforms, with the

amount of shale and mudstone decreasing as the HST transitions into the FSST. Directly above

the FSST a lowstand wedge can be seen in the basin. The sequence boundary is located between

17

the FSST and the lowstand wedge. The lowstand wedge comprises the LST of the second

Wolfcampian 2nd order sequence and represents the Mid-Wolfcamp unconformity of Silver and

Todd (1969). The TST and FSST are not preserved in this sequence, placing a thin HST directly

on top of the LST.

Another, larger, lowstand wedge lies above the HST of the second Wolfcampian 2nd

order sequence and onlaps onto the Wolfcamp B marker. This lowstand deposit represents the

Wolfcampian-Leonardian boundary. On the shelf and upper slope, the Wolfcampian-Leonardian

boundary is difficult to identify because the TST was not preserved in the first Leonardian 2nd

order sequence, placing the two HSTs directly on top of one another in an updip direction.

Moving in a downdip direction, the HSTs are separated by the previously mentioned lowstand

wedge allowing the Wolfcampian-Leonardian boundary to be more easily identified.

2nd Order Sea-level Curve

Using this 2nd order sequence stratigraphic framework a 2nd order sea-level curve was

developed (Figure 15). The 2nd order sea-level curve presented herein is compared to the curves

of Sarg et al., 1999 and Golonka and Kiessling, 2002. The sea-level curve from Sarg et al. is a

generalized interpretation of sea-level through time. The curve is relatively smooth, showing

broad changes in sea-level that can be used to correlate stratigraphy worldwide. The Curve from

Golonka and Kiessling shows more detail compared to the curve from Sarg et al., representing

some of the local variation from the data set that Golonka and Kiessling used.

Sarg et al. shows sea-level falling at the beginning of the Permian until Mid-

Wolfcampian time (Mid-Wolfcamp unconformity) from approximately 150 m above present day

sea-level to 50 m above present day sea-level. Sea-level then rose throughout the rest of the

18

Wolfcampian into the Leonardian achieving approximately the same level as at the beginning of

Wolfcampian time. In the Mid-Leonardian time sea-level began to fall again until just before the

Leonardian-Guadalupian boundary, falling to the approximate sea-level seen today. Sea-level

then rose quickly until the beginning of Guadalupian time.

In contrast to Sarg et al., Golonka and Kiessling show sea-level rising slightly at the

beginning of the Permian from just below to just above 100 m above present day sea-level. Sea-

level then fell quickly to approximately 50 m above present day sea-level at about the same time

as shown by Sarg et al. Sea-level then rose to about 120 m above present day sea-level in the

Early Leonardian but quickly fell to just below 100 m above present day sea-level in the Mid-

Leonardian. Sea-level then rose back to its previous level and stayed constant until the

beginning of Guadalupian time.

The sea-level curve presented in this paper more closely matches that of Golonka and

Kiessling than Sarg et al. Like the Golonka and Kiessling curve, sea level at the beginning of

Permian time was rising in the study area. Sea-level rose through the Early Wolfcampian until

the end of Middle Wolfcampian time when it fell approximately matching the timing of the

Middle Wolfcampian unconformity from Sarg et al., and Golonka and Kiessling. Sea-level in

the study area then rose through the end of the Wolfcampian into the Leonardian. In the Early

Leonardian (at approximately the Sakmarian-Artinskian boundary of the ISC) sea-level fell for a

short time and then rose quickly to its previous level. Unlike the curve from Golonka and

Kiessling this short sea-level fall and rise is depicted as a very abrupt change reflecting a gap in

the geologic record likely due to erosion and/or non-deposition. The only remnant of this event

is a lowstand deposit in the basin and a thin highstand deposit with no transgressive of falling

stage systems tract present. Sea-level in the study area then rose

19

Variations between these sea-level curves can be attributed to several factors. These

include but are not limited to local variation in subsidence rates, tectonism, sediment production,

and isostatic adjustment. Each of these factors can influence local fluctuation in sea-level.

Variation between the curves could also be the result of different gaps in the rock record in each

of the study areas. As mentioned previously the curve of Sarg et al. is a smoother curve showing

second order sea-level fluctuations on a more global scale than that of Golonka and Kiessling,

and the curve presented in this paper. Because of this much of the local variation that is included

in the other curves is not seen. However, the overall pattern between the curves tends to match

with the exception of the beginning of the Permian.

Wolfcampian Mass Wasting

Mass wasting deposits within the Wolfcampian section are typically gravity flows found

in the form of turbidites, grainflows, and debris flows forming large scale mass wasting events.

Mass wasting events are initiated at the shelf break or on the slope itself and are deposited

further down slope, at the toe of slope, and/or into the basin proper (Cook, 1983). The toe-of-

slope is defined as “the transition from a net-erosional slope to a net-depositional basin floor

setting” (Playton and Kearns, 2002). If the mass wasting event has enough energy it can move

large amounts of sediment well into the basin. A large debris flow can transport a volume of

sediment capable of covering more than one hundred square miles more than 60 miles (100 km)

from the source (Crevello and Schlager, 1980; Cook, 1983). One such example is in the Exuma

Sound in the Bahamas during the Pleistocene when a platform collapse of 20 miles (32 km)

created a debris flow that went more than 72 miles into the basin (Crevello and Schlager, 1980;

20

Hobson et al., 1985a). These large debris flow events are less common than turbidites and grain

flows but as illustrated can be volumetrically significant.

Turbidites and Grainflows

Turbidites and grainflows likely account for the majority of slope deposits during the

Wolfcampian. Individually they are not volumetrically significant but collectively they account

for the bulk of resedimented deposits. Turbidites on the Eastern shelf are thin-bedded calcarenite

turbidites that are generated fairly continuously at a platform margin.

Middleton and Hampton (1976) defined a grain flow as a gravity flow where the

sediment is supported by grain to grain interactions. Lowe (1976) makes a distinction between

“true” and “modified” grain flows. Lowe defines a true grainflow as a, “gravity flow of

cohesionless solids maintained in a dispersed state against the force of gravity by an

intergranular dispersive pressure arising from grain interactions within the shearing sediments."

A modified grain flow differs from a true grain flow in that there is some agent in the grainflow

that reduces friction and allows the grain flow to move down a gentler slope. Factors that can

create a modified grainflow include a dense interstitial fluid, downward shear from surface

currents, interstitial fluid turbulence, and fluidization of escaping pore fluids (Cook, 1983). This

is significant because true grain flows require slopes with gradients greater than 30° (Lowe,

1976b; Middleton and Hampton, 1976; Cook, 1983). Modified grain flows, however, can be

initiated and sustained on slopes whose gradients are much shallower (Lowe, 1976b; Cook,

1983). Grain flows on the nose of the Eastern Shelf are therefore likely to have occurred as

modified grain flows and not true grain flows.

21

Debris Flows

Debris flow deposits are commonly found on the lower slope but as previously

mentioned have also been found to extend onto the adjacent basin floor over 60 mi (100 km)

from their source (Crevello and Schlager, 1980). They are commonly associated with sea-level

lowstands (Vail and Mitchum, 1977) but can be emplaced at any time if conditions are right.

Debris flows are typically part of lowstand line source deposits that form a wedge of onlapping

sediment that thins basinward (Handford and Loucks, 1993). The thickness, areal extent, and

runout distance of the individual debris flows within the line source gives an indication of the

size of the initial failure if the head scarp cannot be located. Debris flows can form on slopes

with gradients of only a few degrees up to several tens of degrees such as a carbonate platform.

Seismically these large scale events are the most easily recognizable. The debris flows in the

study area are part of a line source apron deposit and not submarine fan deposits.

Seismic Features

During Wolfcampian time within the study area there are relatively few seismically

visible mass wasting events until the end of Wolfcamp B deposition. This does not suggest that

there were no mass wasting events prior to deposition of the Wolfcamp B, since they are below

the seismic resolution of the 3D volume. Using the Rayleigh criterion to determine the vertical

resolution of the seismic data at the top of the Wolfcamp B gives a maximum resolution of 80.99

ft (24.7 m) where Δz=6920ft (2110 m), t=.534 s (one-way travel time), and f=40 Hz.

𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣 𝑣𝑣𝑣𝑣𝑟𝑟𝑟𝑟𝑣𝑣𝑟𝑟𝑣𝑣𝑣𝑣𝑟𝑟𝑟𝑟 = 14

Δ𝑧𝑧𝑣𝑣�

𝑓𝑓= 1

4𝜆𝜆 = 80.99𝑓𝑓𝑣𝑣

However; the quality of the seismic data may not allow even this level of resolution.

Resolving features using ¼λ requires seismic data with low noise levels and low attenuation of

22

the waveform. Taner (1997) lists several factors that make imaging of carbonate rocks difficult

as a result of their rock properties and geometries. Some of these include (1) lower vertical and

spatial resolution relative to siliciclastic rocks due to high interval velocities; (2) a higher

probability of multiples; (3) high-velocity heterogeneity and anisotropy; (4) dipping beds below

the carbonate layer with incidence angles greater than the critical angle will be refracted instead

of reflected narrowing the bandwidth of seismic data (Sarg and Schuelke, 2003); and (5) steep

carbonate slopes are affected by dispersion and diffraction of energy like those caused by steeply

dipping faults. It is therefore common in seismic data, that is not acquired under ideal

conditions, to only be able to resolve features using 1/3 λ or even 1/2 λ which in this 3D volume

would result in values of 107.99 ft (33 m) and 161.98 ft (49 m) respectively. Even under ideal

conditions the limit of detectability is 1/8 λ (Widess, 1973) where a feature can be detected but

not completely resolved would require a feature to have a thickness of at least 40.49 ft (12.3 m).

Turbidites have a typical thickness of several inches up to a few feet, and most grain flow

deposits have a maximum thickness of 10 ft (3 m). Even when stacked most grainflow deposits

reach a maximum thickness that just approaches the limits of detectability.

Consequently only large debris flows are likely to be completely resolved at depths in the

range of the Wolfcampian section. Martin et al. (2002) suggests the use of cross-well

tomography in order to better identify deposits on the order of 10-15 ft (3-5 m) thick, but a cross-

well tomography study has not yet been conducted for the study area.

At the end of Wolfcamp B deposition, a series of major slope failures sent large debris

flows into the basin. There are four main flows that have a total area of approximately 16.5

square miles (43 km2) and an average run-out distance of 5 miles. The largest of these covers an

area of approximately 7.5 square miles (19.5 km2) and has a run-out distance of approximately 6

23

miles (9.5 km). There are many smaller debris flows that were deposited on the slope at or near

the same time (Figure 16). The debris flows originate from a line source that trends north-west

to south-east that is sub parallel to the nose of the Eastern Shelf. Evidence of a head scarp is

seismically visible for only one of these debris flows. There is evidence of repeated failure at

that location as evidenced by successively smaller debris flows stacking onto one another.

In approximately the middle of the study area, at the toe-of-slope, there is a depositional

high that runs parallel to the nose of the Eastern shelf that acts as a dam to sediment coming

down the slope. This is interpreted to be similar to the megabreccia dam described by Mazzullo

and Reid (1987). It is appears that this dam prevented the debris flows from moving further into

the basin. There is some sediment that has bypassed the sediment dam by moving around it to

the east and to some extent over top of it.

Seismic attributes based on amplitude, frequency, and phase were analyzed in order to

better delineate the debris flows (Figures 16 and 17). They were also used to identify any

potential mass wasting events just below the spatial and vertical resolution of the seismic

volume. A large variety of attributes were generated even though most attributes based on the

same underlying wavelet characteristic (amplitude, frequency, or phase) produce similar results.

Each is mathematically different from the next, giving subtle variations between them (Barnes,

2007). These variations made different parts of the attribute maps clearer, aiding our

interpretation. Not all of the attributes maps that were generated are presented in this paper, but

several that were of particular use will be discussed.

Anomalies in the amplitude attributes are represented by different shades of blue outlined

by green. These anomalies are interpreted to represent debris flows that have been deposited on

the slope and at the toe-of-slope. A possible head scarp for the western most debris flows is

24

visible on the Average Absolute Amplitude, and Average Peak Amplitude attribute maps (Figure

16, maps C and D). Dominant F1 and F2 (Figure 17, maps C and D) attribute maps show the

head scarp more clearly in the north western part of the study area. The sediment dam

mentioned previously is represented in the amplitude attribute maps as A break in the amplitude

anomalies approximately in the middle of the study area. The curved shape of the debris flows

terminating against the sediment dam in a downdip direction can be seen more clearly in the

Average Instantaneous Frequency and Zero Crossing Frequency attribute maps (Figure 17, maps

A and B). The westernmost debris flow can be seen moving around the edge of the sediment

dam in the Maximum Trough Amplitude and RMS Amplitude maps (Figure 16, maps A and B).

Using these attribute maps a series of geobodies were created around the anomalies

(Figure 18). Two sets of geobodies were created and then merged. The first was created on

negative amplitude anomalies and the second on positive amplitude anomalies. The geobodies

better delineate the debris flows, line source, location of the sediment dam than the individual

seismic attribute maps.

Failure Mechanisms

Mazzullo and Reid (1987) proposed 3 mechanisms for carbonate shelf failure in the

Midland basin. These include (1) shelf erosion during falling sea level or tectonic emergence;

(2) subaqueous failures triggered by tectonism; and (3) shelf failure resulting from shelf

accretion and oversteepening during highstands. However, Hanford and Loucks (1993) state that

slope failure can happen at any time if sediment conditions are right and a triggering mechanism

is available. The triggering mechanism must either increase stress or decrease sediment strength

(Coleman and Prior, 1988). Of the mechanisms proposed by Mazzullo and Reid (1987), shelf

25

erosion during falling sea level is interpreted to be the most likely triggering mechanism for mass

wasting at the end of Wolfcampian time. As previously stated a 2nd order sea-level fall occurred

at the end of Wolfcampian time (Figure 15). This drop in sea-level led to subaerial exposure and

erosion of the carbonate platform and slope. Some of the factors that help cause failure during

sea-level fall (Handford and Loucks, 1993) are (1) loss of buoying effect of seawater (Schwarz,

1982); (2) cyclic wave-loading and pulsating pore pressure; (3) erosion and undercutting of a

shelf edge; (4) oversteepening and loading during a previous highstand; (5) submarine erosion by

deep-sea currents (Pinet and Popenoe, 1985); and (6) dissolution of shelf edge in the mixing-

zone (Back et al., 1986). Of these, oversteepening and loading during a previous highstand are

the most likely factors in the slope failure. This is due to the fact that during the latest

Wolfcampian a second order highstand build up the shelf and slope. Evidence of this can be

seen in the seismic where small scale thrust faults propagate out lower down the slope relieving

stress being built up on the upper slope and shelf.

Development of the Eastern Shelf

From the Late Pennsylvanian to the end of the Early Permian the carbonate platform

evolved from a ramp to a distally steepened ramp (Wolfcampian) to a rimmed shelf (Leonardian)

(Hobson et al., 1985b; Mazzullo and Reid, 1989). During this transition the areal extent of the

carbonate platform decreased until the end of the Wolfcampian (Hobson et al., 1985) when the

platform stabilized and grew vertically during deposition of the Wolfcamp B and C. Initial

Wolfcampian shelf-to-basin relief in the Midland basin has been estimated to be greater than

1,000 ft with some putting it as much as 1,600 ft by the end of the Early Permian (Hobson et al.,

1985). Estimates of the slope gradient along the Eastern shelf during Wolfcampian are around 1°

26

(Wermund and Jenkins, 1969). Slope gradients in the study area based upon well log analysis

agree with this estimate at the onset of the Wolfcampian. The slope gradient of the nose of the

Eastern shelf increased from approximately 1° during Wolfcamp E deposition to 3.5° by the end

of the Wolfcamp. Most of the increase in gradient occurred during deposition of the Wolfcamp

B and C.

During the Late Pennsylvanian through Early Permian subsidence in the Midland Basin

created accommodation space for deposition of approximately 1500 ft (500 m) of strata (Saller et

al., 1994). According to Mazzullo and Reid (1989) sedimentation rates during this time period

were the highest during the Middle Wolfcampian with maximum sedimentation rates on the

order of 100 m/m.y. (300 ft/m.y.) (Wilson, 1975; Schlager, 1981; Mazzullo and Reid, 1989).

However, in our study area sedimentation rates appear to be the highest in the latest

Wolfcampian.

Geologic Models

We propose two models for development of the nose of the Eastern shelf. In the first

model (Figure 19) the Eastern shelf started out as a linear feature trending north-south with

carbonate sedimentation moving roughly from east to west. At some point in time during

Wolfcampian time, carbonate production increased at one location along the Eastern Shelf

relative to its surrounding. As time passed carbonate production continued at a higher rate than

surrounding areas causing a bulge in the profile of the Eastern Shelf. This bulge of carbonate

sediment continued to prograde out into the basin until it reached its most basinward extent. It is

certainly possible that carbonate production could have been high enough to keep pace with

rising sea level and cause the progradation of the nose of the Eastern Shelf into the basin. With

27

the available data for this study however it is hard to determine if the general dip of the

reflections below the Wolfcampian section is the result of a large scale regional dip from north to

south or if it is the result of a pre-existing structural high just to the north.

A second model is possible in which a pre-existing structural high was accentuated by

carbonate deposition. The initial dip on the high before Wolfcampian deposition would have

been less than 1° as indicated by well logs and seismic data. During Wolfcampian time

carbonate production was established on this high and began to build vertically and areally. By

the end of Wolfcampian time the nose of the eastern shelf had increased its gradient to 3.5°.

While both models are plausible there is not enough seismic coverage to make a

conclusive determination as to which model is correct. As previously mentioned the seismic

data available for this study only covers the southern portion of the nose of the Eastern Shelf.

The second model seems more plausible because the first model requires some unique conditions

(prolonged rates of high carbonate sedimentation and higher sedimentation rates than

surrounding areas) in order for the formation of the nose of the Eastern shelf to occur. The

second model is simpler and does not rely on any special circumstances other than basement

uplift along deep seated faults, many of which have been reported in other areas.

Conclusions

Use of an acoustic impedance seismic inversion has been integral to creating a 2nd and 3rd

order sequence stratigraphic framework for the southern portion of the nose of the Eastern shelf

during Wolfcampian and Early Leonardian time. The inversion enables 2nd order depositional

geometries to be seen in more clarity and detail. This reduces uncertainty, allowing a sequence

28

stratigraphic framework to be created much faster than with well logs and standard seismic data

alone.

Using this method Wolfcampian deposition in the study area has been interpreted to

contain two 2nd order sequences composed of six third order sequences. This sequence

stratigraphic framework makes it easier to identify the Wolfcampian-Leonardian boundary in

conjunction with wireline log information placing it at the level of the Wolfcamp B marker. In

Early Leonardian time another second order sequence was deposited.

The nose of the Eastern shelf was likely formed on a pre-existing structural or

stratigraphic high and accentuated as the result of carbonate production initiated on it. A large

percentage of this development took place during the Middle to Late Wolfcampian. During

Wolfcampian time the Eastern shelf grew vertically increasing the slope gradient from 1° to 3.5°

and transitioned from homoclinal ramp (Wolfcampian) to rimmed shelf (Leonardian). The

majority of this growth occurred during 3rd order sequences four through six. The nose of the

Eastern shelf also grew in areally due to progradation and shedding of carbonate materials into

the basin during highstand. At the end of Wolfcampian time a series of large debris flows were

deposited down the slope and into the basin. These large debris flows did not occur during

highstand but just after during sea-level lowstand caused by shelf and slope failure due to

subaerial exposure.

29

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Figures

Figure 1: Regional map of the Permian basin and its sub-basins modified after Atchley et al.,

1999.

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Figure 2: Midland Basin Stratigraphic Column

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Figure 3: Time structure maps from the Pennsylvanian through Early Permian; A-Top Atoka Formation; B-Top Cisco Formation; C-

Wolfcamp E Horizon; D- Wolfcamp D Horizon; E-Wolfcamp C Horizon; F-Wolfcamp B Horizon.

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Figure 4: Type Log for the Wolfcampian section showing the tops of the Cisco and Canyon Formations and the five shale markers

subdividing the Wolfcamp Formation.

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Figure 5: Well log Cross section showing the tops of the Cisco and Canyon Formations and the five shale markers subdividing the

Wolfcamp Formation.

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Figure 6: The areal extent of the two 3D seismic datasets merged to create the current 3D seismic amplitude and inversion volumes.

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Figure 7: In the forward modeling portion of the figure, an acoustic impedance log is made from a sonic and bulk density log. The acoustic impedance log is convolved with a wavelet to produce a synthetic seismogram. In the inversion portion of the figure the

wavelet is deconvolved from the seismic amplitude volume to create an acoustic impedance inversion volume.

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Figure 8: Location of wells in the study area with both density and sonic logs that were used to create the synthetic seismograms.

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Figure 9: Example of a synthetic seismogram generated in the Wolfcampian section (highlighted in orange).

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Figure 10: Isochron maps from the Pennsylvanian through Early Permian with arrows indicating the direction of sedimentation; A-Top of the Atoka Formation to the Top of the Canyon

Formation; B-Cisco Formation; C-Wolfcamp E; D-Wolfcamp D; E-Wolfcamp C; F-Wolfcamp B.

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Figure 11: Index Map of the cross-sections used to construct the 2nd and 3rd order sequence stratigraphic frameworks.

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Figure 12: Cross-section B-B’ showing the 3rd order sequence stratigraphic interpretation in the study area (Dl-Downlap; TL-Toplap;

OL-Onlap; ER-Erosional Truncation).

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Figure 13: Cross-section D-D’ showing the 2nd order sequence stratigraphic interpretation of the study area based on the seismic

inversion. During Wolfcampian time two second order sequences were deposited. Lowstand systems tracts (LST) are shown in blue, transgressive systems tracts (TST) are shown in orange and highstand systems tracts (HST) are shown in red.

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Figure 14: Well tie Inversion Well Tie

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Figure 15: Comparison of 2nd order sea-level curves from Sarg et al., 1999; Golonka and Kiessling, 2002; and this paper.

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Figure 16: Attribute maps based on seismic amplitude variations showing debris flows moving down the Wolfcampian slope into the

basin; A-Max Trough Amplitude, B-RMS Amplitude, C-Average Absolute Amplitude, D-Average Peak Amplitude.

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Figure 17: Attribute maps based on seismic frequency variations showing debris flows head scarps and terminations during the latest Wolfcampian in the study area; A-Average Instantaneous Frequency; B-Zero Crossing Frequency; C-Dominant F1; D-Dominant F2.

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Figure 18: Geobodies created on the Wolfcamp B horizon showing debris flows moving down the slope into the basin.

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Figure 19: Possible development of the Eastern shelf showing the transition from a line source running north-south (1) to the initiation

of a prograding bulge in the line source (2) to a fully developed nose (3-4).

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Appendix

Acquisition Data

wolfcampian development of the nose of the eastern shelf

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