Reservior characterization of West Waha and Worsham-Bayer Fields

Table of Contents

Chapter ONE
1.0 Introduction
1.1 General Statement
1.2 Study Location
1.3 Statement of the problem
1.4 Objectives of study
1.5 Physiographic Setting
1.5.1 Topography
1.5.2 Climate
1.6 Scope and Methodology
1.7 Previous work

CHAPTER TWO
2.0 Regional Geology of Texas
2.1 Generalized Geology of Texas
2.1.1 Geology of West Texas
2.2 Geologic history of West Texas
2.3 Regional Geology of Delaware Basin
2.3.1 General Statement
2.3.2 Geology of Delaware Basin
2.4 Geologic Setting of West Waha and Worsham-Bayer Fields
2.4.1 Ellenburger Group Reservoir Geology
2.4.1.1 Ellenburger Group Depositional Facies Assemblages
2.4.1.2 Karst Facies of Ellenburger Group
2.4.2 Fusselman Formation Reservoir Geology
2.4.3 Thirtyone Formation Reservoir Geology
2.4.4 Undifferentiated Mississippian Reservoir Geology

CHAPTER THREE
3.0 Methodology
3.1 Data Volumes
3.2 Use of the Data
3.3 Summary of the Data
3.4 Place of Data Interpretation
3.5 Procedure for Interpretation
3.5.1 Well log correlation
3.5.2 Calculation of Petrophysical parameters
3.5.3 Reservoir attributes
3.6 Seismic-to-well ties
3.7 3D Seismic Interpretation
3.7.1 Mapping of reservoirs
3.8 Structural Framework/Fault Network Mapping
3.9 Time-Depth Structure maps (Depth Conversion)
3.10 Recoverable reserve estimation/Volumetric
3.11 Production record interpretation
3.12 Limitation of study

CHAPTER FOUR
4.0 Results and Discussion
4.1Well log correlation
4.1.1 Petrophysical analysis
4.1.2 Reservoir Attribute Analysis Result
4.2 Seismic-to-well ties using check survey
4.3 Structural Framework/Fault Network Mapping
4.4 3-D Seismic Mapping
4.5 Time-Depth Structure Maps
4.6. Recoverable reserve estimation/volumetric result
4.7 Production record interpretation

CHAPTER FIVE
5.0 Conclusion and Recommendation References

FIGURES

1.1 Location map for the area of data collection

1.2 Map showing the location of Delaware Basin

1.3 Texas Elevation Map

1.4 Drainage map of Texas

1.5 Locations of West Waha and Worsham-Bayer Fields (Hardage et al, 1998)

2.1 Geologic map of Texas (Bureau of Economic Geology, the University of Texas, 1992)

2.2 Schematic Basin evolution of the Delaware Basin

2.3 Structures of Delaware Basin

2.4 Stratigraphic column of Delaware basin showing the geologic setting of West Waha and Worsham-Bayer Fields

2.5 (A) Cross section of facies assemblages of Ellenburger Group showing both onlap and truncation in a northwesterly direction. (B) Schematic cross section illustrating the major facies assemblages (from Kerans, 1990).

2.6 Three major Ellenburger Group reservoir types in West Texas (from Holtz and Kerans, 1992).

2.7 Thickness map showing distribution of dolomite and limestone in the Fusselman Formation in West Texas (from Ruppel and Holtz, 1994).

2.8 Block diagrams illustrating the distribution of facies (a) During Early Devonian relative sea-level rise, deep-water siliceous sediments dominated the area and platform carbonates were restricted to the north. (b) Highstand progradation of the carbonate platform (from Ruppel and Holtz, 1994).

2.9 Map illustrating the location of the Thirtyone Deep-Water Chert play. (Ruppel and Holtz, 1994).

2.10 Isopach and facies map of Mississippian rocks of West Texas. (after Wright, 1979).

3.1 Isopach map of Ellenburger Group showing the gradual thinning of the reservoir according to the base map towards North-West of Texas Arch.

3.2 Log example showing, SP, CAL, ILD, ILM and MSFL curve signatures.

3.3 Porosity Log type showing DT log signature.

3.4 Log examples, showing PHID, NPHI and RHOB curve signatures

3.5 Log examples, Showing Log signatures for LLD, LLS and LL8

4.1 Basemap showing well locations

4.2 Type log (well no. 29, located in fig. 4.1) for the area of the 3-D seismic volume

4.3 Correlated Stratigraphic Xsection of the four reservoirs [R1, R2, R3 and R4].

4.4 Showing the R1, R2, R3 and R4 reservoir tops in seismic-to-well ties using well 37 velocity check shot survey.

4.5 Interpreted 3-D section showing mapped R1, R2, R3 and R4 reservoirs, structural and stratigraphic features, seismic stratigraphic reflection configuration and termination.

4.6 Arrows indicating reservoir compartmentalization in 3-D Seismic section.

4.7 3-D Seismic section at Inline and Xline showing Well 41 sited away from a structural feature and a proposed well.

4.8 Ellenbuger Group [R1] reservoir Depth Structure Map

4.9 Fusselman Formation [R2] reservoir Depth Structure Map.

4.10 Thirtyone [R3] Depth Structure Map

4.11 Undifferentiated Limestone [R4] Depth Structure Map.

4.12 Date versus Gas per Month Mscf of Ellenburger at well 47

4.13 Date versus Gas per Month Mscf of Ellenburger at well 29

4.14 Date versus Gas Per Month Mscf of Fusselman at Well 47

4.15 Date versus Gas Per Month Mscf of Thirtyone [R3] Reservoir at Well 47

4.16 Date versus Cumulative Gas MMscf of Ellenburger reservoir at Well 41

4.17 Date versus Cumulative Gas MMscf of Fusselman Formation reservoir at Well 47

4.18 Date versus Cumulative Gas MMscf of Thirtyone Formation reservoir at Well 47

TABLES

3.1 Available well log types from the well log data set

3.2 Values for Sonic Matrix (DTMa) of Wyllie and Raymer-Hunt-Gardener Equations for different formations (Formations & values of interest painted yellow) [Krygowski and Asquith, 2004].

3.3 Density Matrix values (RhoMa). (The formations and values of interest dotted red). (Krygowski and Asquith, 2004).

3.4 Matrix values for Limestones and Dolomites for Porosity calculation using Neutron log (indicated with red dot) (Krygowski and Asquith, 2004).

4.1 Petrophysical Analysis Result

4.2 Reservoir Attribute Analysis Result for R1 (Ellenburger Group) reservoir

4.3 Reservoir Attribute Analysis Result for R2 (Fusselman Formation) reservoir

4.4 Reservoir Attribute Analysis Result for R3 (Thirtyone Formation) reservoir

4.5 Reservoir Attribute Analysis Result for R4 (Undifferentiated Mississippian Limestone) reservoir.

4.6 Recoverable reserve calculation result

4.7 Wells and Reservoirs with their corresponding Cumulative Gas (Bscf) [Extracted from the production record].

CHAPTER ONE INTRODUCTION

1.1 General Statement

Reservoir characterization and subsurface geological maps is perhaps the most important vehicle used to explore for undiscovered hydrocarbons and to develop proven hydrocarbon reserves. However, the subject of reservoir characterization and subsurface mapping is probably the least discussed, yet most important, aspect of petroleum exploration and development. As a field is developed from its initial discovery, a large volume of well logs, seismic, and production data are obtained. With the integration of these data, the accuracy of the subsurface interpretation is improved through time (Tearpock and Biscke, 2003).

A decade ago, approximately 800 trillion cubic feet (Tcf) of natural gas existed or was estimated to exist in conventional reservoirs in United States, yet only 538 Tcf of this gas is economically recoverable at prices of less than $3 per thousand cubic feet (Mcf) in 1987 dollars (Finley et al., 1988). More recently, considering only the largest 580 gas reservoirs on Texas State Lands, only half of an original 20 Tcf of natural gas in place has been recovered (Holtz and Garrett, 1997). One of the most promising new technologies for imaging gas reservoirs for reserve-growth studies is three-dimensional (3-D) seismic reflection data. The recent rapid increase in the use of 3-D seismic data in the oil and gas industry has vastly improved the level of detailed resolution of subsurface reservoir parameters such as petrophysical features (porosity, permeability, water saturation and so on), structural features interpretation (faults and folds), stratigraphic features (erosion and truncation features, karsting etc), and in some cases, even direct hydrocarbon

indicators (so-called "bright spots"). The data set used was acquired in the West Waha and Worsham-Bayer fields area of the southeastern Delaware Basin, West Texas (Fig. 1.1), comprising of three categories : (1) approximately 20 square miles (51.8 Km2) of 3-D seismic data, (2) well data, including digitized well logs from 11 wells within the 3-D seismic data volume, and a check-shot survey for well 37, and (3) production record from the wells.

These three data sets were thoroughly analyzed, studied, delineated and integrated to give a detailed reservoir characterization of the fields. Thus, detailed characterization of gas reservoirs is the key to increasing production efficiency in the mature gas-producing provinces of the world.

1.2 Study location

The study area which is approximately 20-square miles data volume from the southeastern Delaware Basin of West Texas, United States of America, in the vicinity of West Waha and Worsham-Bayer fields, contains 11 digitized wells (29, 36, 37, 38, 39, 41, 42, 43, 46, 47 and 98) (Figs. 1.1 and 4.1). The basin is

centered on Latitude (lat) 390 36' N and Longitude (long) 750 41' W (fig 1.2).

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Figure 1.1: Location map for the area of data collection.

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Figure 1.2: Map showing the location of Delaware Basin

1.3 Statement of the problem

One of the major petroleum exploration and exploitation difficulties of West Waha and Worsham Bayer fields is the issue of low rate recovery of the estimated natural gas in trillion cubic feet (Tcf) from the producing reservoirs. This means that the fields contain large estimated amount of natural gas (Tcf), but unable to yield at least a good estimated economic amount during natural gas recovery. This work highly focused on the estimation of the recoverable reserve of the West Waha and Worsham-Bayer fields to infer their hydrocarbon prospectivity through complete reservoir characterization of Ellenburger Group, Fusselman formation, Thirtyone formation and Undifferentiated Mississippian Limestone reservoirs.

1.4 Objectives of study

a. To infer the reserve impact on the estimated recoverable reserve of the evaluated four reservoirs
b. To determine the effect of thin beds (by-pass beds) in reservoir petrophysical analysis and if it impacts reserve.
c. Impact of trapping system based on structural evolution of the basin.
d. How the structures tie to the stratigraphy of the area.
e. To detect the possible causes of these fields’ low rate of natural gas recovery during petroleum exploration.

1.5 Physiographic Setting

The Delaware Basin in West Texas and southern New Mexico is famous for holding large oil fields and for exposing a fossilized reef. Guadalupe Mountains National Park and Carlsbad Caverns National Park protect part of the basin. It is part of the larger Permian Basin, itself part of the Mid-continent Oil Field. The period of sediment deposition left a thickness of 1600 to 2200 feet (490 to 670

m) of limestone interbedded with dark-colored shale. Structurally the Delaware

basin was one of the foreland basins created when the Ouachita Mountains were

uplifted as the southern continent Gondwana collided with Laurasia, forming the supercontinent Pangaea in the Late Carboniferous (Pennsylvanian). The Ouachita Mountains formed a rain shadow over the basins, and a warm shallow sea flooded the surrounding area. On the other side of the equator, the Ancestral Rocky Mountains formed a large mountainous island.

1.5.1 Topography

The Delaware Basin is an intracratonic basin in the western United States characterized by the gradual withdrawal of shorelines and the progressive increase in eolian (wind-transported) sands, red beds, and evaporites. The Basin is associated with Karst topography, which was developed as groundwater circulated in the buried limestone formations, dissolving away caves and underground caverns were later destroyed by infill and erosion. The erosion of the softer sediments of the basin by streams lowered the ground level to its current position. The Basin is also prone to mass wasting such as landslides that further reduced the topographic relief.

It is an ovoid shaped subsiding Basin that extended over 10,000 square miles (26,000 km²) in what is now western Texas. Delaware Basin was filled at least to the top of Capitan Reef and mostly covered by dry land before the end of the Ochoan epoch. Rivers migrated over its surface and deposited the red silt and sand that now constitute the siltstone and sandstone of the Rustler and Dewey Lake formations.

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Figure 1.3: Texas Elevation Map.

1.5.2 Climate

The Delaware basin is associated with semi-arid type of climate. Consequently, the karst topography that was created lacks the characteristic depressions, and this led to sink holes, pits, and solution fissures on the surface. Since almost all of Delaware (in general including the state) is a part of the Atlantic Coastal Plain, the climate is moderated by the effects of the ocean. The state is somewhat of a transitional zone between a humid subtropical climate and a continental climate. Despite its small size (roughly 100 miles (160 km) from its northernmost to southernmost points), there is significant variation in mean temperature and amount of snowfall between Sussex County and New Castle County. The southern portion of the state has a somewhat milder climate and a longer growing season than the northern portion of the State. The transitional climate of Delaware supports a surprising variety of vegetation. All parts of Delaware have relatively hot, humid summers, while Sussex and Kent Counties are considered to fall in the humid subtropical climate zone. Fig 1.4 below shows dendritic pattern of drainage.

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Figure 1.4: Drainage map of Texas

1.6 Scope and methodology

This research involves the proper application of geological and geophysical principles in delineating the fields’ subsurface geology. It principally involved data collection under the permission of the Director Bureau of Economic Geology, University of Texas at Austin, and the Project Manager-Exploration and Production, Gas Research Institute, 8600 West Bryn Mawr Avenue, Chicago, Illinois 60631. The data set used for this research is a publication of the University of Texas at Austin (UTA) developed under research funded by the Gas Research Institute (GRI) and the U.S. Department of Energy (DOE). The 3-D seismic data contained on the CD-ROM is derived from seismic data owned by GRI. Components of this data set are considered a single product for use on one computer by one user.

The research work involved the examination of the well logs, seismic sections and production data using the 2007 version of the Landmark GeoGraphix software available at the Department of Geology, University of Ibadan. Hydrocarbon (natural gas) bearing zones were chosen and correlated from the X section module on the GeoGraphix using the gamma ray and resistivity logs for ten wells (29, 36, 37, 38, 39, 41, 42, 43, 46 and 47). Well 98 was not among the correlated wells because it did not penetrate any of the reservoirs of interest. The reservoir

characteristics which include petrophysical analysis of these wells were calculated and analyzed using Surfer 8TM and LogCalculation spreadsheet softwares. Structural features (faults and folds), stratigraphic features (erosion and truncation features, karsting), direct hydrocarbon and the reservoirs of interest in the field were identified, interpreted and mapped in the seismic sections in Seisvision module. Time-Depth structural maps and Amplitude maps of the hydrocarbon bearing zones were produced from the GeoAtlas module revealing the trapping

system of West Waha and Worsham-Bayer fields. . The recoverable reserve (volumetric) estimation for the interpreted reservoirs was calculated to rate fields’ reserves. Finally, the production records for the wells were also plotted and analyzed using Microsoft Excel.

1.7 Previous work

Some works have been done in the fields though much are yet to be proved and discovered about the intracratonic complex fault basin. A study funded by the

U.S. Department of Energy, the Gas Research Institute, Mobil Exploration and Producing U.S., and Altura Energy by Hardage et al (1998) was done to determine Geologic controls on deep, prolific Ellenburger gas reservoirs at Lockridge, Waha, West Waha, and Worsham-Bayer fields in Pecos, Reeves, and Ward Counties in West Texas seismic. A major component of the data base amassed for the study was a 176-square mile 3-D survey extending across these fields. Ellenburger (Ordovician) reservoirs occurred at depths of 17,000 to 21,000 ft (5200 to 6400 m) over most of the study area. The eastern half of the 3-D seismic survey was covered with a thick (500 to 2000 ft [150 to 600 m]) variable layer of low-velocity Tertiary fill underlain by a varying thickness of high-velocity salt/ anhydrite. These near-surface conditions attenuated seismic reflection signals from deep targets and made static corrections of the data difficult.

The 3-D seismic data acquired in this study are thought to be some of the best quality data produced over these fields, yet the 3-D image of the deep pre- Pennsylvanian targets was of limited quality because of the combined effects of complex, attenuating, near-surface layers and weak reflection signals from deep seismic targets. The principal interpretation objective was to construct the complicated tectonic structure related to these fields to determine genetic relationships between faults and deep gas production. The challenge was to construct this structural picture from a 3-D seismic image that did not provide a

clear, unambiguous picture of the fault systems. Petrophysical analyses of logs from wells drilled at key structural locations were invaluable in interpreting fault geometry by identifying overturned beds and repeated sections. Once 3-D seismic horizon and fault interpretations were done across the complete 176-square mile area, depth maps of key pre-Pennsylvanian horizons were made, and vertical depth sections were constructed across critical structural areas. The seismic interpreted structures were then restored to pre-deformation conditions to verify that the pre- and post-deformation lengths of these key horizons were consistent, and thereby determined if the seismic fault interpretations were structurally valid.

The Worsham-Waha area of Reeves and Pecos Counties, Texas has seen a real boom drilling horizontal wells for the elusive Devonian gas reservoir. Worsham-Bayer and Waha Fields had great Ellenburger production with nearly 400 bcfg produced, but only 30 bcfg had been produced from the Devonian. The Worsham-Bayer Field is an east-west, large faulted anticline with two domes. The east dome had four wells where all the commercial Devonian production was found.

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Figure1.5: Locations of West Waha and Worsham-Bayer Fields (Hardage et al, 1998).

Another work was done in the area by Entzminger et al (2005) that since early in 2000, with the horizontal boom of the Montoya in Waha and Block 16 fields, operators started to utilize the similar techniques for the Devonian. Since that time a total of 22 wells have been drilled with a wide range of results from uneconomic wells (Colt #1) to wells that IPF- over 15 mmcfgpd (Horry-Pitts 49#1). The down-dip Lloyd Estate #1 well suggests a gas column of over 3000’ on a structural feature about 10 miles long and 7 miles wide. Volumetrics suggest a tremendous amount of gas-in-place. However, with drilling costs of $4-7 million per well, the well reserves need to be significant to substantiate this expense. It is not a play for those with a weak stomach or small pocketbook. The main players have been Tom Brown, PURE Resources, Finley, and Oxy Permian.

Tom Brown Inc. recognized that a better predictive reservoir model was needed to continue developing this play because of the high costs and dramatic variability in well results. In late 2003 a reservoir study was implemented to examine key-Devonian well samples in an effort to relate them to logs and production. Image logs in both the vertical and some of the horizontal wells confirmed fractures were present in the field. The erratic production throughout the field also suggested a fracture component affected the reservoir deliverability. Seismic attributes seem to have promise identifying fracture zones. Fractures can be a two-edge sword, either enhancing gas deliverability or bringing in too much water. Most wells had been drilled longitudinal along maximum horizontal stress. The Wahlenmaier #1 was drilled in 2004 to transverse the fracture system attempting to cross the major fracture system in hopes to enhance production. The well did intersect the seismic predicted fracture system but the higher water production 200-300 bwpd has hampered the gas production of 2000-4000 mcfgpd. Drilling has essentially ceased in the

area with operators licking their wounds. The two, recent prolific wells (Horry- Pitt 49#1 and Monsanto-McKellar #1) may one day entice the next generation of oil & gas finders to extract the remaining gas in this large structure.

Scott (1997) paper presentation on the study area also points at the reservoir characteristics of Ellenburger. They were held in Midland and Houston. The workshops reviewed findings of a multidisciplinary study of Ellenburger natural gas reservoirs in the Lockridge, Waha, and Waha West fields and parts of the Worsham-Bayer and Cayanosa fields of the southern Delaware Basin, West Texas.

According to Bruce (1996) on the topic; Results of 3-D Seismic Carbonate Project, West Texas Overview, Reservoir Characteristics, and Geophysics; a large cooperative field study began in January 1996 and is focused on Lower Paleozoic gas reservoirs in the Permian Basin, West Texas. The purpose was to evaluate development options for prolific, mature, carbonate gas reservoirs in a structurally-complex geologic province. New economic gas reserves likely exist in untapped fault blocks or poorly drained reservoir compartments within the Delaware and Val Verde Basins. Geologic structure and reservoir development is being evaluated utilizing 3-D seismic techniques and a multidisciplinary team approach. The cooperative research is supported by

U.S. Department of Energy, GRI, Texas Bureau of Economic Geology, Shell Western E&P Inc., Mobil E&P U.S. Inc., Landmark Graphics, Schlumberger, Petroleum Information Inc., and Tobin Data Graphics.

Hammes (1996) made a research on the topic Borehole Electrical Images from Microresistivity Logs of the Fractured, Karsted, and Brecciated Ellenburger Group. He discovered that the Electrical images in boreholes are becoming increasingly important in interpreting the rock record. In addition to identifying fractures and faults, borehole imaging tools are used for a variety of other applications such as horizontal drilling, environmental studies, stress

orientation measurement studies, sequence stratigraphy, paleotransport, facies, and diagenesis analyses. These tools produce electrical microconductivity images of the wellbore, which are interpreted using an interactive graphics workstation. High-resolution (~2.5mm) and nearly complete borehole coverage can greatly increase the detail and precision of geological interpretations. Yet, to be fully useful, borehole images should be calibrated with core. This study provides the first comprehensive comparison of carbonate features in cores with a suite of all currently available electrical imaging logs. The Lower Ordovician Ellenburger Group, West Texas, serves as a model for a dolomitized, fractured, karsted, and brecciated carbonate reservoir. Characteristic reservoir features, including fracture breccia, chaotic breccia, laminated mudstones, grainstones, and bioturbation are identified both on electrical imaging logs and in cores. Electrical images provide more complete information than cores in cavernous and highly fractured zones because cores commonly show no recovery or occur as rubble in these zones, which are the most productive zones in the Ellenburger reservoir. Borehole imaging, therefore, provides in-situ visualization of cavernous porosity, chaotic breccias or conglomerates, and highly fractured intervals, as well as other key insights into karst stratigraphy.

The research works of Bruce and Hammes (2000) were sponsored by the

U.S. Department of Energy’s Federal Energy Technology Center, under contract DEFG21- 88MC25031 with the Bureau of Economic Geology, The University of Texas at Austin, Box X, Austin, Texas 78713; phone: (512) 471- 1534, fax: (512) 471-9800. Additional support is from GRI, Shell Western E&P Inc., Mobil E&P U.S. Inc., Landmark Graphics, Schlumberger, Petroleum Information Inc., and Tobin Data Graphics. Publication authorized by the Director, Bureau of Economic Geology.

Hardage (2009) in his work on Well Logs Invaluable to Interpreting Limited-Quality Seismic of Complex Structures; was also carried out in the study area. The methodology described in the work may benefit those who are confronted with the problem of interpreting complex structures from limited- quality 3-D seismic images. The objective of the study was to characterize deep (20,000 feet/6,000 meters) Ellenburger gas reservoirs in West Texas. In addition to the Ellenburger reflection signals being weak because of the great depth of the target, the top of the Ellenburger across the area was a gentle, ramp-like increase in impedance that did not produce a robust reflection event. A further negative influence on data quality was that the area was covered by a variable surface layer of low-velocity Tertiary fill that was underlain by a varying thickness of high-velocity salt/anhydrite. These complicated near- surface conditions made static corrections of the data difficult; in fact, the combination of all of these factors has caused some explorationists to consider the region to be a no-record seismic area for imaging deep drilling targets. He concludes that the principal point is that although overturned strata cannot be interpreted from this limited-quality seismic image, the recognition of overturned beds on log data allows the proper structure to be imposed on the seismic data. Petrophysical analyses and interpretations of logs can be invaluable when interpreting complicated structure with any seismic data, regardless of seismic data quality – and particularly so when strata are overturned in the dramatic manner illustrated by this example.

Hardage et al (1996) researched on 3-D Seismic Evidence of the Effects of Carbonate Karst Collapse on Overlying Clastic Stratigraphy and Reservoir Compartmentalization concluded that there are approximately 200 wells inside the 26-mi2 (67 km2) area we studied, which is rather good subsurface control. Yet even with this drilling density, few, if any, operators in the area were aware that the Ellenburger-related karst phenomena described here existed in

Boonsville field. We are convinced that if we had relied strictly on well control, we too would not have recognized how seriously deep karsting affected shallower Pennsylvanian clastic stratigraphy and Boonsville gas production. Our experience tells us that operators in similar carbonate-prone basins must acquire 3-D seismic data to fully evaluate karst effects. However, once 3-D seismic data are acquired, we believe (and strongly recommend) that extensive geologic and reservoir engineering data bases be created to properly interpret the 3-D seismic images and that good-quality VSP data be acquired to allow these geologic and engineering controls to be inserted into the 3-D seismic image at the correct two-way time coordinates.

Because we know that Ellenburger solution-collapse phenomena span a distance of at least 500 mi (800 km) from our study area in the Fort Worth Basin to the Ellenburger outcrops in the Franklin Mountains at El Paso, Texas, karst phenomena must affect stratigraphy and reservoir compartmentalization over a vast area of rich hydrocarbon reservoirs in the Permian and Delaware Basins of West Texas. Private discussions with numerous companies who are industry research partners with the Bureau of Economic Geology in worldwide reservoir studies have convinced us that similar karst effects exist in many carbonate-prone basins around the world.

Several fundamental research questions remain to be answered, with some of the obvious issues that need to be addressed being the following:

a. Should wells be positioned inside or outside karst collapse zones?
b. How does a karst extend through an extensive clastic section such as the Bend Conglomerate? Does the collapse occur as episodic events or as a single, catastrophic event?
c. What is the genetic relationship between karsts and faults and what causes the collapse features we observe to be almost perfectly vertical?

Currently, we have only speculative answers to these questions. Both the drill bit and the coring bit will continue to provide valuable information on these intriguing karst phenomena, and we are convinced that 3-D seismic data will be critical in any such future investigations. Nassir (2006), in his master’s degree thesis concludes that the Permian basin (Delaware basin and Midland basin) is characterized by a progressively shallowing upward character. The depositional profile of the basin reflects this shallowing trend with the change from the initially open distally steepened ramp, into a restricted sigmoid- progradational ramp, an exposed platform, transitional ramp to rimmed shelf, and finally into mixed clastic-carbonate reef-rimmed shelf. This follows an increasing steepening of the shelf-margin directly resulting from basin starvation and the initial basin topography coupled with the shallowing trend. The latter caused the shelf-margin deposits to keep-up with rising sea-level, while basin starvation caused the margin to steepen as it prograded basin-ward. Superimposed on the shoaling upward cycles are the effects of glacial eustasy common to the Upper Permian.

James and Harry (2003), researched on the Permian Basin Petroleum Systems, Simpson-Ellenburger and Woodford-San Andres--New insights for Exploring a Mature Petroleum Province. He cited that two major petroleum systems evolved in the Permian Basin of west Texas and southeastern New Mexico: the Simpson-Ellenburger and the Woodford-San Andres.

The Simpson-Ellenburger petroleum system consists of over 90 oil fields and over 50 gas fields. Oil fields are on the Central Basin Platform and Eastern Shelf and produce from the Early Ordovician Ellenburger Formation at an average depth of 11,000 and 8,500 ft, respectively. The source rock for this petroleum system is the Middle-Late Ordovician Simpson Group. Reservoir rocks for these fields are karsted and fractured dolostone that contain oil with an average API gravity of 45° and a sulfur content of 0.2 wt. %. The most

prolific Ellenburger gas fields are in the adjacent Delaware and Val Verde basins where these same reservoir rocks are at average depth of 17,000 ft. Gas in these fields was originally oil expelled from the Simpson Group at a shallower depth which, after greater burial, was thermally cracked to gas.

The Woodford-San Andres petroleum system consists predominately of oil fields whose reservoir rocks span Mississippian-Late Permian. The Late Permian San Andres Formation is the dominant reservoir rock containing greater than 50% of the original in-place oil. Source rock for this petroleum system is the Late Devonian-Early Mississippian Woodford Shale. Most of the reservoir rocks are complex, restricted-platform carbonates that contain oil with an average API gravity of 37° and a sulfur content of 0.8 wt. %. Mapping these petroleum systems in space and time provides a more robust and predictive framework for exploration and development.

Shumaker (1992) made a research on the topic: Paleozoic Structure of the Central Basin Uplift and Adjacent Delaware Basin, West Texas. He discovered that the structural cross sections and a regional tectonic map show that the Central Basin uplift consists of upthrust blocks of alternating vergence. The uplift is separated from the adjacent Delaware basin by a major fault that changes throw and structural style along strike, accompanying changes in block vergence found along the uplift. The west-facing basement blocks were thrust westward over the Delaware basin, whereas the adjacent blocks face eastward. The distribution and style of structures suggest that small amounts of left-lateral movement occurred along west-trending and northwest-trending faults that separated rising blocks of the uplift. Transfer of lateral movement from vertical, west-trending faults on the Central Basin uplift and the Ozona arch to the west-trending Mid-basin fault in the Delaware basin rotated blocks of the uplift in a clockwise sense. Faults and folds within the study area have trends and slip directions that are generally similar to those within the Reagan

and Washita Valley fault zones in the Arbuckle uplift adjacent to the Anadarko basin. The difference in the style of deformation between the two uplifts relates to varied angular relationships between preexisting structure and the direction of late Paleozoic stress. The identity of structural trends within these two uplifts suggests that other late Paleozoic basins and uplifts of the Ancestral Rocky Mountains may have formed in a similar stress field and that crustal rotation was pervasive along the southwestern margin of the continent during the late Paleozoic Ouachita-Marathon deformation.

Owolabi (2009) also worked on the study area: The field is being explored for secondary gas recovery. The 3 D seismic data was modeled with the information from the seismic data and logs from well number 29. Formations from this area have been structurally identified, based on the well log data that was converted to time depth to match the seismic map. All available wells have been plotted based on their coordinates and depth. A cross section map was created with identified formations and a base map was also formed showing the location of available wells from the data. The Ellenburger group has been identified as the major gas producing formation. Three other formations, (Simpson group, Thirtyone formation, and Undifferentiated Mississippian limestone) are also possible producers.

Surface horizons were selected and ascribed to formation tops including two horizons from the topmost formation. A strata grid was constructed in three dimensions to show the true thickness of the identified formations. A time slice with transparency on the surface of the Ellenburger group was generated in black and white and multicolor resolution. All available well logs from well number 29 were correlated with the synthetic seismic trace. This geological model will allow better visualization and understanding of this area as well as the prospect for future hydrocarbon exploration. Despite the various research

carried out in the study area with respect to the discussed researches above, more research still need to be carried out and this work continued from where they stopped.

CHAPTER TWO REGIONAL GEOLOGY OF TEXAS

2.1 Generalized Geology of Texas

Texas contains a great variety of geologic settings. The state's stratigraphy has been largely influenced by marine trangressive-regressive cycles during the Phanerozoic, with a lesser but still significant contribution from late Cenozoic tectonic activity, as well as the remnants of a Paleozoic mountain range. Texas is approximately bisected by a series of faults that trend southwest to northeast across the state, from the area of Uvalde to Texarkana. South and east of these faults, the surface exposures consist mostly of Cenozoic sandstone and shale strata that grow progressively younger toward the coast, indicative of a regression that has continued from the late Mesozoic to the present. The coastal plain is under laid by salt domes that are responsible for many of the oil traps in the region. (See fig. 2.1 enclosed in an envelope at the back of the project).

North and west of the faults are the Stockton, Edwards, and Comanche plateaux; these define a crustal block that was upthrown during the Neogene. This large region of central Texas, which extends from Brewster County east to Bexar, and northeast to the Red River features extensive Cretaceous shale and limestone outcrops. The limestone in particular is important, both economically for its use in cement manufacture and as a building material, as well as practically; a porous limestone formation in the Texas Hill Country is the reservoir of the Edwards Aquifer, a vital water source to millions. Almost in the center of these Cretaceous rocks is the Llano Uplift, a geologic dome of Precambrian gneiss, schist, and granite, surrounded by Paleozoic sedimentary rocks. The granite here is quarried for construction, but it is perhaps best

known to Texans through its manifestation as Enchanted Rock. From San Saba north to Childress, and from Wichita Falls in the east to Big Spring in the west, the surface consists of late Paleozoic (Pennsylvanian) to early Mesozoic (Triassic) marine sediments. These strata grow younger from east to west, until they are overlain unconformably by terrigenous Ogallala sediments of Miocene and Pliocene age. These late Cenozoic deposits dominate the Texas Panhandle.

2.1.1 eology of West Texas

The geology of west Texas is arguably the state's most complex, with a mix of exposed Cretaceous and Pennsylvanian strata, overlain by Quaternary conglomerates. A series of faults trend southeast to northwest across the region, from Big Bend to El Paso; there are also extensive volcanic deposits. The Marathon Mountains northeast of Big Bend National Park have long been of special interest to geologists; they are the folded and eroded remains of an ancient mountain range, created in the same orogeny that formed the Ouachita and Appalachian Mountains.

2.2 Geologic history of West Texas

The Precambrian metamorphic and igneous rocks of the Llano Uplift probably formed during the Mesoproterozoic Grenville orogeny, which was part of the assembly of the supercontinent Rodinia. Over time, the mountains of the Grenville orogeny were eroded flat, and later covered by Paleozoic and Mesozoic sediments that were not uplifted and eroded in their present manner until the late Cenozoic. Early to middle Paleozoic rocks in Texas are typically carbonate deposited in epeiric seas. Exceptions include a significant area of Cambrian sandstone in west Texas, and some shale strata from the Devonian and Mississippian periods. The Ouachita Mountains were uplifted across the state during the succeeding Pennsylvanian period; this provided a nearby

source of sediment for shale and sandstone, along with more marine limestone deposition.

Permian rocks are the best-known of the Texas Paleozoic. They are widespread in north Texas, where their characteristic red beds are spectacularly exposed in Palo Duro Canyon. The strata are also oil-rich where buried in west Texas, such as in the Midland and Odessa region. This crude oil rich area is known as the Permian Basin. Permian Texas was covered by shallow seas to the west, with evaporation flats to the east and north in the Panhandle. Outstanding exposures of Permian strata are located in and around Guadalupe Mountains National Park, the geology of which resulted in the definition of several Permian stratotypes; the region is one of the world's best for studies of the period.

Early and middle Mesozoic strata are, on the whole, poorly represented in Texas. Triassic rocks are limited to sandstone and shale in the Panhandle, while the Jurassic record is almost nonexistent at the surface. This was far from a monotonous time, though, as it featured the creation of the Gulf of Mexico, from a rift southeast of the Ouachita Mountains. Deeply-buried salt deposits and marine limestones under the coastal plain date from the Jurassic, when the first shallow seas formed. The late Mesozoic record is much richer. Cretaceous rocks--particularly those of the lower Cretaceous--are widespread at the surface, with yet more buried under the coastal plain. The strata consist of massive limestone sequences deposited when the entire region was submerged under the Western Interior Seaway, during the last great marine transgression.

The Western Interior Seaway had withdrawn by the beginning of the Cenozoic, the era that put the finishing touch on Texas's current geology. The modern coastal plain formed during this time; it comprises increasingly thick sediments (perhaps 15 km deep at the coastline) deposited southeastward into

the downwarping Gulf of Mexico. West Texas was rent by volcanism during the Eocene and Oligocene epochs, activity which formed most of the modern topography of the area. Later crustal extension created a series of alternating horsts and grabens similar to those in the Basin and Range province of the western U.S. A late Cenozoic uplift of the Rocky Mountains led to the deposition of a vast fan of eroded sediment to their east, forming the Ogallala Formation that covers much of the Panhandle. Most of the state's current stream valleys and canyons date from the Pleistocene to the present, as the final geologic shaping of the state.

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Figure 2.1: Geologic map of Texas (Bureau of Economic Geology, the University of Texas, 1992)

2.3 Regional Geology of Delaware Basin

2.3.1 General statement

The Delaware basin of West Texas and southeast New Mexico covers an area about 200 miles long and 100 miles wide. It is a prime example of a large sedimentary basin in which vast amounts of rock have been deposited. As sedimentary basins in general are important to the petroleum industry of the world, so is the Delaware basin important to the industry of the West Texas-New Mexico area. This prominent geological feature can be expected to attain even greater importance in the future as the quest for oil and gas continues. The growing importance of this significant area is emphasized frequently, especially when the industry notes the ever-increasing number of deep tests announced for the basin and vicinity. The Midland basin on the east and the Delaware basin on the west are separated by the Central basin platform. Both of these basins were areas of deep water during most of Permian time; because of this, they were filled with shales and sandstones deposited in a marine environment. The Central basin platform is an area of Permian reefing that grew over an older structural feature - the West Texas structural platform. South of the Central basin platform is an elongated east-west sedimentary feature called the Sheffield Channel.

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