Is There a Basin-Centered Gas Accumulation 

in Cotton Valley Group Sandstones, 

Gulf Coast Basin, U.S.A.? 

By Charles E. Bartberger, Thaddeus S. Dyman, and Steven M. Condon 

Geologic Studies of Basin-Centered Gas Systems 

Edited by Vito F. Nuccio and Thaddeus S. Dyman 

U.S. Geological Survey Bulletin 2184-D 

This work funded by the U.S. Department of Energy, National 

Energy Technology Laboratory, Morgantown, W. Va., under 

contracts DE-AT26-98FT40031 and DE-AT26-98FT40032, and by the 

U.S. Geological Survey Central Region Energy Resources Team 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

Gale A. Norton, Secretary 

U.S. Geological Survey 

Charles G. Groat, Director 

Posted online February 2002, version 1.0 

This publication is only available online at: 

http://geology.cr.usgs.gov/pub/bulletins/b2184-d/ 

Any use of trade, product, or firm names in this publication 

is for descriptive purposes only and does not 

imply endorsement by the U.S. Government 
Contents 

Abstract .......................................................................................................................................................... 1


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


Data Sources ........................................................................................................................................ 3


Acknowledgments ........................................................................................................................................ 3


Continuous-Type Gas Accumulations ....................................................................................................... 3


Basin-Centered Gas Accumulations ................................................................................................3


Method of Evaluating Potential of Basin-Centered Gas in Cotton Valley Group Sandstones ......... 4


Geologic Setting of Cotton Valley Group................................................................................................... 4


Cotton Valley Group Stratigraphic Nomenclature................................................................................... 6


Cotton Valley Group Depositional Systems .............................................................................................. 9


Regional Framework ........................................................................................................................... 9


Blanket Sandstones of Northern Louisiana .................................................................................. 11


Blanket- and Massive-Sandstone Productive Trends................................................................. 13


Diagenesis of Cotton Valley Group Sandstones .................................................................................... 13


Impact of Diagenetic Mineralogy on Wireline Logs ............................................................................. 15


Source Rocks .............................................................................................................................................. 16


Burial and Thermal History ....................................................................................................................... 16


Abnormal Pressures................................................................................................................................... 17


History of Cotton Valley Group Sandstone Exploration ........................................................................ 17


Comparison of Blanket-Sandstone and Massive-Sandstone Trends ................................................ 20


Blanket-Sandstone Trend................................................................................................................. 20


Massive-Sandstone Trend ............................................................................................................... 24


Limited Data in Published Literature ..................................................................................... 24


Analysis of Drill-Stem-Test and Production-Test Data....................................................... 24


Discussion of Evidence For and Against Basin-Centered Gas ........................................................... 30


Source Rocks and Burial and Thermal History............................................................................. 30


Porosity, Permeability, and Gas-Production Rates ...................................................................... 30


Abnormal Pressures.......................................................................................................................... 30


Gas-Water Contacts.......................................................................................................................... 32


Basin-Centered Gas Potential within Bossier Shale ............................................................................ 34


Conclusions ................................................................................................................................................. 34


References Cited ........................................................................................................................................ 36


Figures 

1. 	 Map of north-central Gulf Coast Basin showing outlines of three Cotton Valley Group 


plays identified by the U.S. Geological Survey in the 1995 National Assessment of 


United States oil and gas resources ......................................................................................... 2


III

IV


2. Index map of north-central Gulf Coast Basin showing major tectonic features ............... 5


3. Chronostratigraphic section of northern Louisiana................................................................ 7


4. 	 Generalized structure contours on top of Cotton Valley Group sandstone 


across northeastern Texas and northern Louisiana............................................................... 8


5. 	 Generalized stratigraphic nomenclature of Cotton Valley Group in 


northern Louisiana........................................................................................................................ 9


6. 	 North-south stratigraphic cross section of Cotton Valley Group across 


northern Louisiana based on data from 15 wells .................................................................. 10


7. 	 Regional paleogeographic map showing sedimentary environments of 


Cotton Valley Group during deposition of uppermost Cotton Valley sandstones ............ 12


8–13. Map of northeastern Texas and northwestern Louisiana showing: 

8. 	 Major fields that have produced hydrocarbons from Cotton Valley 


Group sandstones............................................................................................................ 14


9. 	 Fluid-pressure gradients calculated from shut-in pressures in Cotton 


Valley Group sandstone reservoirs............................................................................... 18


10. 	 Geographic distribution of abnormally high pressures in Cotton Valley 


Group sandstone reservoirs .......................................................................................... 19


11. 	 Measured values of porosity and permeability in Cotton Valley Group 


blanket sandstones ......................................................................................................... 21


12. 	 Initial rates of gas production from Cotton Valley Group 


blanket sandstones ......................................................................................................... 25


13. 	 Fields productive from Cotton Valley Group sandstones in which gas-


water contacts have been identified and reported in published literature ........... 26


14–15. 	 Map of Caspiana field in northwestern Louisiana in the tight, Cotton Valley 

Group massive-sandstone trend showing: 

14. Initial rate of gas production from Cotton Valley sandstone reservoirs................. 28


15. 	 Ratio of initial production rate of water to initial production rate of gas 


from Cotton Valley Group sandstones.......................................................................... 29


16. 	 Schematic diagram of gas-water transition zones in high- and 


low-permeability reservoirs...................................................................................................... 33


Tables 

1. 	 Comparison of two productive trends of Cotton Valley Group sandstones 


in eastern Texas and northern Louisiana ............................................................................... 20


2. Data on Cotton Valley Group sandstone fields in northern Louisiana ............................... 22

Is There a Basin-Centered Gas Accumulation 

in Cotton Valley Group Sandstones, 

Gulf Coast Basin, U.S.A.? 

By Charles E. Bartberger,

1

 Thaddeus S. Dyman,2 and Steven M. Condon2 

Abstract 

The U.S. Geological Survey (USGS), in cooperation with 

the U.S. Department of Energy, is reevaluating the resource 

potential of selected domestic basin-centered gas accumulations. Basin-centered gas accumulations are characterized by 

presence of gas in extensive low-permeability (tight) reservoirs 

in which conventional seals and trapping mechanisms are 

absent, abnormally high or low reservoir pressures exist, and 

gas-water contacts are absent. 

In 1995, the USGS assessed one basin-centered gas play 

and two conventional plays within the trend of Jurassic and 

Cretaceous Cotton Valley Group fluvial-deltaic and barrierisland/strandplain sandstones across the onshore northern Gulf 

of Mexico Basin. Detailed evaluation of geologic and production data provides new insights into these Cotton Valley plays. 

Two Cotton Valley sandstone trends are identified based 

on reservoir properties and gas-production characteristics. 

Transgressive blanket sandstones across northern Louisiana 

have relatively high porosity and permeability and do not 

require fracture stimulation to produce gas at commercial 

rates. South of this trend, and extending westward into eastern 

Texas, massive sandstones of the Cotton Valley trend exhibit 

low porosity and permeability and require fracture stimulation. 

The high permeability of Cotton Valley blanket sandstones is 

not conducive to the presence of basin-centered gas, but lowpermeability massive sandstones provide the type of reservoir 

in which basin-centered gas accumulations commonly occur. 

Data on source rocks, including burial and thermal history, are consistent with the interpretation of potential basincentered gas within Cotton Valley sandstones. However, pressure gradients throughout most of the blanket- and massivesandstone trends are normal or nearly normal, which is not 

characteristic of basin-centered gas accumulations. 

The presence of gas-water contacts in at least seven fields 

across the blanket-sandstone trend together with relatively 

high permeabilities and high gas-production rates without frac1

 Geologic consultant, Denver, Colo. 

2

 U.S. Geological Survey, Denver, Colo. 

ture stimulation indicate that fields in this trend are conventional. Within the tight massive-sandstone trend, permeability 

is sufficiently low that gas-water transition zones are vertically 

extensive and gas-water contacts either have not been encountered or are poorly defined. With increasing depth through 

these transition zones, gas saturation decreases and water saturation increases until eventually gas saturations become sufficiently low that, in terms of ultimate cumulative production, 

wells are noncommercial. Such progressive increase in water 

saturation with depth suggests that poorly defined gas-water 

contacts probably are present below the depth at which wells 

become noncommercial. The interpreted presence of gas-water 

contacts within the tight, Cotton Valley massive-sandstone 

trend suggests that gas accumulations in this trend, too, are 

conventional, and that a basin-centered gas accumulation does 

not exist within Cotton Valley sandstones in the northern Gulf 

Basin. 

Introduction 

The U.S. Geological Survey is reevaluating the potential 

for occurrence of continuous-type basin-centered gas accumulations in selected basins in the United States since completion 

of the USGS 1995 National Petroleum Assessment. This effort, 

which is partly funded by the U.S. Department of Energy, 

might result in identification of new continuous-type gas plays 

and petroleum systems or reevaluation of existing plays. 

As part of the 1995 National Assessment of United States 

Oil and Gas Resources by the USGS, Schenk and Viger (1996) 

identified one continuous-type basin-centered gas play and two 

conventional gas plays (fig. 1) within the Cotton Valley Group 

sandstone trend in eastern Texas and northern Louisiana. The 

purpose of this report is to reevaluate the 1995 USGS play 

definitions and parameters for establishing those plays through 

more extensive evaluation of data on reservoir properties, reservoir pressures, gas and water recoveries, gas-production rates, 

and gas-water contacts in Cotton Valley sandstones. Data favorable and unfavorable for the presence of continuous-type basincentered gas accumulations are summarized. No attempt is 

1

Figure 1. Map of north-central Gulf Coast Basin from Schenk and Viger (1996) showing outlines of three Cotton Valley Group plays identified by U.S. Ge1995 National Assessment of United States oil and gas resources. Shown are the Cotton Valley Blanket Sandstones gas play (4923) identified as a conti

Cotton Valley Salt Basins gas play (4922) and Cotton Valley Sabine Uplift gas play (4924) identified as conventional gas plays. 

0 0 

Mile

96°W 4°W 2°W 0°W 

34°N 32°N 30°N 28°N 

OKLAHOMA 

TEXAS 

ARKANSAS 

LOUISIANA 

MISSISSIPPI 

Gulf of Mexico 

4922 

4924 

4923 

5

9 9 9 8
Is There a Basin-Centered Gas Accumulation in Cotton Valley Group Sandstones, Gulf Coast Basin, U.S.A.? 3 

made, however, to identify new plays and petroleum systems or 

to assess gas resources for potential plays. 

From a regional perspective, two productive trends of 

Cotton Valley sandstones are recognized based on sandstonereservoir properties, gas-production rates, and necessity of 

hydraulic-fracturing treatments to achieve commercial production. Across northernmost Louisiana, so-called Cotton Valley 

blanket sandstones have sufficiently high porosity and permeability that commercial rates of gas production can be obtained 

without artificial well stimulation. South of this area, in northern Louisiana, and extending westward across the Sabine 

uplift into northeastern Texas, sandstones in the Cotton 

Valley massive-sandstone trend have poorer reservoir properties and require hydraulic-fracture treatments to achieve 

commercial rates of gas production. Because basin-centered, 

continuous gas accumulations characteristically occur within 

low-permeability reservoirs, the tight, Cotton Valley massivesandstone trend across northern Louisiana and northeastern 

Texas is an ideal setting in which to look for potential basincentered gas accumulations. 

Data Sources 

Interpretations and conclusions presented in this report 

are based on data from published literature and limited conversations with industry personnel, together with geologic and 

engineering data accessible in a publicly available CD-ROM 

database from IHS Energy Group (PI/Dwights Plus, a trademark of Petroleum Information/Dwights, d.b.a. IHS Energy 

Group). PI/Dwights Plus data evaluated for this report are 

current through February 2000. Primary data from PI/Dwights 

Plus pertinent to this study are results of drill-stem and production tests in Cotton Valley sandstones reported for individual 

wells. Because well-completion records depend on information provided by operators, well data in PI/Dwights Plus might 

be incomplete. 

Acknowledgments 

This work is funded by the U.S. Department of 

Energy, National Energy Technology Laboratory, Morgantown, W. Va. (contract nos. DE-AT26-98FT40031 and DEAT26-98FT40032), and by the U.S. Geological Survey Central 

Energy Team. Laura Biewick of the USGS provided instruction and assistance in using ArcView software to evaluate 

Cotton Valley well and test data. Gregory Zerrahn and 

Joseph Lott, geologists with Palmer Petroleum, Shreveport, 

La., assisted in obtaining reports, maps, and literature. We 

thank Al Brake, BP Amoco, and Al Taylor, Nomad Geosciences, for providing invaluable information on characteristics 

of Cotton Valley tight-gas accumulations. We acknowledge 

helpful reviews of this manuscript by Chuck Spencer, Curt 

Huffman, and Katharine Varnes of the USGS. 

Continuous-Type Gas Accumulations 

It is important to identify continuous-type gas accumulations because resource assessment for such gas accumulations 

is conducted using different methodology than that used for 

conventional fields. Continuous-type gas accumulations generally occur within an extensive volume of reservoir rock with 

spatial dimensions equal to or exceeding those of conventional 

hydrocarbon plays. The definition of continuous gas accumulations used here is based on geology rather than on government 

regulations defining low-permeability (tight) gas accumulations. Common geologic and production characteristics of continuous gas accumulations include their occurrence downdip 

from water-saturated rocks, lack of conventional traps or seals, 

reservoir rocks with low matrix permeability, presence of 

abnormal pressures, large in-place volumes of gas, and low 

recovery factors (Schmoker, 1996). 

Continuous gas plays were treated as a separate category 

in the U.S. Geological Survey 1995 National Petroleum 

Assessment and were assessed using a specialized methodology (Schmoker, 1996). These continuous plays are diverse 

geologically and fall into several categories including coal-bed 

gas, biogenic gas, fractured-shale gas, and basin-centered gas 

accumulations. This report focuses on the potential for basincentered gas within Cotton Valley sandstones. 

Basin-Centered Gas Accumulations 

From studies of hydrocarbon-productive basins in the 

Rocky Mountain region, Law and Dickinson (1985) and Spencer (1987) identified characteristics of basin-centered gas 

accumulations that distinguish them from conventional ones. 

Basin-centered gas accumulations: 

1. 	 Are geographically large, spanning tens to hundreds of 

square miles in aerial extent, typically occupying the 

central, deeper parts of sedimentary basins. 

2. 	 Occur in reservoirs with low permeability—generally 

less than 0.1 millidarcy (mD)—such that gas cannot 

migrate by buoyancy. 

3. 	 Lack downdip gas-water contacts because gas is not 

held in place by buoyancy of water. Consequently, 

water production is low or absent. If water is produced, 

it is not associated with a distinct gas-water contact. 

4. 	 Commonly occur in abnormally pressured reservoirs 

(generally overpressured, but occasionally underpressured). 

5. 	 Contain primarily thermogenic gas, and where overpressure is encountered, the overpressuring mechanism 

is thermal generation of gas. 

6. 	 Occur structurally downdip from water-bearing reservoirs that are normally pressured or occasionally 

underpressured. 

7. Lack traditional seals and trapping mechanisms. 

8. Have gas-prone source rocks proximal to the low-
4 Geologic Studies of Basin-Centered Gas Systems 

permeability reservoirs such that migration distances 

are short. 

9. 	 Occur in settings such that the tops of such gas accumulations occur within a narrow range of thermal 

maturity, usually between a vitrinite reflectance (Ro) of 

0.75 and 0.9 percent. 

What causes a basin-centered, continuous gas accumulation to form? The most common scenario involves lowpermeability reservoirs in which overpressure develops in 

response to thermal generation of gas. Gas-prone source 

rocks generally must be associated with, or proximal to, lowpermeability reservoirs, and this sequence of source and reservoir rock must be buried to a depth sufficient for the source 

rocks to generate gas. Overpressure develops because thermal 

generation of gas occurs at a rate that exceeds the rate at which 

gas is lost updip by migration through the low-permeability 

reservoir. As overpressure develops, any free water in pores of 

the tight reservoir is forced out updip into higher permeability, 

normally pressured, water-bearing strata. Only bound, irreducible water remains in the tight-gas reservoir. Permeability is 

sufficiently low within the tight reservoir that gas does not 

migrate through it by buoyancy as it does through conventional 

reservoirs with higher permeabilities (Gies, 1984; Spencer, 

1987; Law and Spencer, 1993). Instead, gas migrates slowly 

through the tight-gas reservoir with movement caused by the 

pressure differential between the region of high-pressure gas 

generation and the normally pressured, higher permeability, 

water-bearing rocks updip where gas does migrate upward 

rapidly by buoyancy. Thus, because of its inherent low permeability, a basin-centered gas reservoir itself retards the upward 

migration of gas, in effect forming its own leaky seal, and 

maintaining overpressured conditions. 

This scenario probably describes an ideal, end-member 

situation. In some cases, for example, basin-centered gas accumulations have subnormal reservoir pressures resulting from 

significant tectonic uplift of strata in the basin. For a basin that 

is tectonically active and in an intermediate stage of uplift, it 

might be possible to find a basin-centered gas accumulation 

that is normally pressured. It is also possible that particular 

gas accumulations might have only some of the characteristics 

for basin-centered gas described above and that differentiating 

between basin-centered and conventional accumulations could 

be difficult and subjective. It is with this understanding that the 

potential for basin-centered gas in Cotton Valley sandstones is 

evaluated. 

Method for Evaluating Potential of 

Basin-Centered Gas in Cotton 

Valley Group Sandstones 

One of the main requirements for occurrence of a basincentered, continuous gas accumulation is the presence of a 

regional seal to trap gas in a large volume of rock across 

a wide geographic area. Within that large volume of rock, 

discrete gas accumulations having conventional seals and gaswater contacts are absent, and occurrence of gas often cuts 

across stratigraphic units. In classic basin-centered gas accumulations (Law and Dickinson, 1985; Spencer, 1987; Law and 

Spencer, 1993), the regional seal is provided by the low permeability of the reservoir itself. To evaluate the potential for 

the presence of a continuous gas accumulation within Cotton 

Valley Group sandstones, therefore, it is necessary to examine 

reservoir properties of Cotton Valley sandstones across the 

northern Gulf Coast Basin. Because reservoir properties of 

Cotton Valley sandstones are governed by diagenetic characteristics, which are controlled primarily by depositional environment, it is helpful to understand Cotton Valley depositional 

systems and related diagenetic patterns. 

Although gas production from Cotton Valley sandstones 

seems to occur from discrete fields, it is necessary to determine 

if those fields are separate, conventional accumulations or socalled “sweet spots” within a regional, continuous gas accumulation. Thus, it is essential to understand what characterizes the 

apparent productive limits of existing Cotton Valley gas fields, 

including the presence or absence of gas-water contacts. For 

some fields in which gas-water contacts are not reported, an 

attempt was made to determine the presence or absence of gaswater contacts by examining fluid recoveries from drill-stem or 

production tests in wells on the flanks of those fields. The goal 

was to determine if certain Cotton Valley fields that produce 

from tight-gas sandstones are flanked by dry holes that tested 

water only, without gas, suggesting presence of a gas-water 

contact. To analyze test data spatially, data from PI/Dwights 

Plus were imported into ArcView 3.2, running on a desktop 

computer. While viewing the map display, test results from any 

particular well could be examined. 

Finally, because continuous gas accumulations often are 

characterized by overpressure associated with thermal generation of gas from source rocks in proximity to low-permeability 

reservoirs, it is important to evaluate the presence and quality 

of potential source rocks, burial and thermal history of those 

source rocks, and reservoir-pressure data. 

Geologic Setting of 

Cotton Valley Group 

The Cotton Valley Group is an Upper Jurassic to Lower 

Cretaceous sequence of sandstone, shale, and limestone that 

underlies much of the northern Gulf of Mexico coastal plain 

from eastern Texas to Alabama (figs. 2 and 3). Cotton Valley 

strata occur only in the subsurface and form a sedimentary 

wedge that thickens southward into the Gulf Basin from a 

zero edge in southern Arkansas and eastern Texas (fig. 2). 

The downdip limit of the Cotton Valley Group has not been 

delineated yet by drilling. The depth to top of the Cotton 

Valley ranges from about 4,000 ft below sea level near the 
Figure 2. Index map of north-central Gulf Coast Basin, modified from Dutton and others (1993), showing major tectonic features. Sabine and Monroe u

features during deposition of Cotton Valley Group sediments. Cotton Valley depocenters were located across the entire northern Gulf Basin from eastemovement in East Texas and North Louisiana Salt Basins was contemporaneous with deposition of Cotton Valley Group clastic sediments. The Cotton Vsubsurface sequence of strata with approximate updip limits shown here.

34°N32°N30°N

96°W 4°W 2°W 0°W

LOUISIANA

ARKANSASLOUISIANA

TEXAS

LOUISIANA

MISSISSIPPI

MISSISSIPPI

SALT

BASIN

NORTH

LOUISIANA

SALT

BASIN

MONROE

UPLIFT

SABINE

ARCH

COMANCHEAN SHELF EDGE

EAST

TEXAS

SALT

BASIN

MEXIA-TALCO FAULT ZONE

FAULT ZONE

GINGER

FAULT ZONE

MT ENTERPRISE

FAULT ARKANSAS ZONE SOUTH

FAULT

PICKENS

APPROX DOWNDIP LIMIT

OF COTTON VALLEY GROUP

SANDSTONE

0 0 40 60 80

MILES

GULF OF MEXICO

APPROX UPDIP LIMIT

OF COTTON VALLEY GROUP

(SWAIN, 1944)

(THOMAS & MANN, 1966)

9 9 9

2
6 Geologic Studies of Basin-Centered Gas Systems 

updip zero edge to more than 13,000 ft below sea level along 

the southern margins of the East Texas and Louisiana Salt 

Basins (figs. 2 and 4). In southeastern Mississippi, the top 

of the Cotton Valley occurs at nearly 20,000 ft below sea 

level. The greatest thickness of Cotton Valley rocks penetrated 

exceeds 5,000 ft in southeastern Mississippi (Moore, 1983). 

The Cotton Valley Group and overlying Travis Peak 

(Hosston) Formation represent the first major influx of terrigenous clastic sediments into the Gulf of Mexico Basin following its initial formation during continental rifting in Late Triassic time (Salvador, 1987; Worrall and Snelson, 1989). Earliest sedimentary deposits in East Texas and North Louisiana 

subbasins (figs. 2 and 3) include upper Triassic nonmarine 

red beds of the Eagle Mills Formation, the thick Middle and 

Upper Jurassic evaporite sequence known as Werner Formation (anhydrite) and Louann Salt, and the nonmarine Norphlet 

Formation. Following a major regional marine transgression 

across the Norphlet, Upper Jurassic Smackover Formation 

regressive carbonates were deposited, capped by red beds and 

evaporites of the Buckner Formation (fig. 3). A subsequent 

minor marine transgression is recorded by the Gilmer Limestone or Cotton Valley Formation in eastern Texas, although 

equivalent facies in northern Louisiana and Mississippi are 

terrigenous clastics known as the Haynesville Formation. The 

marine Bossier Shale, lowermost formation of the Cotton 

Valley Group (figs. 3 and 5) was deposited conformably on 

Gilmer-Haynesville strata. 

Louann Salt became mobile as a result of sediment loading and associated basinward tilting. Salt movement was initiated during Smackover carbonate deposition and became more 

extensive with influx of Cotton Valley clastics (McGowen and 

Harris, 1984). Many Cotton Valley and Travis Peak fields 

in eastern Texas, Louisiana, and Mississippi are structural or 

combination traps associated with Louann Salt structures. Salt 

structures range from small, low-relief salt pillows to large, 

piercement domes (McGowen and Harris, 1984; Kosters and 

others, 1989). 

The Sabine uplift is a broad, low-relief, basement-cored 

arch separating the East Texas and North Louisiana Salt Basins 

(figs. 2 and 4). With vertical relief of 2,000 ft, the Sabine uplift 

has a closed area exceeding 2,500 mi2 (Kosters and others, 

1989). Isopach data across the uplift indicate that it was a positive feature during deposition of Louann Salt in the Jurassic 

but that primary uplift occurred in late, mid-Cretaceous (101 

to 98 Ma) and early Tertiary time (58 to 46 Ma) (Laubach and 

Jackson, 1990; Jackson and Laubach, 1991). As a high area for 

the past 60 m.y., the Sabine uplift has been a focal area for 

hydrocarbon migration in the northern Gulf Basin during that 

time. Numerous smaller structural highs on the uplift in the 

form of domes, anticlines, and structural noses provide traps 

for hydrocarbon accumulations, including many gas fields 

in Cotton Valley sandstones. Origins of these smaller structures have been attributed to salt deformation and small igneous intrusions, as summarized by Kosters and others (1989). 

Because the Louann Salt is thin across the Sabine uplift, 

Kosters and others (1989) suggest that most of the smaller 

structures across the Sabine uplift developed in association 

with igneous activity. 

Cotton Valley Group 

Stratigraphic Nomenclature 

Since the first penetration of Cotton Valley strata in northern Louisiana in 1927, complex informal stratigraphic nomenclature developed as numerous Cotton Valley oil and gas fields 

were discovered across the region through the 1940’s. Nomenclature became complex because of local stratigraphic complexities within Cotton Valley strata in northern Louisiana 

and also because of regional variations in Cotton Valley 

depositional systems across the northern Gulf Basin. Terminology established by Swain (1944) was used until the complete revision of Cotton Valley stratigraphy by Thomas and 

Mann (1963) and Mann and Thomas (1964). Most subsequent 

reports, including the classic work of Collins (1980), have 

used Mann-Thomas terminology. Refinements to that terminology have been contributed by Coleman and Coleman 

(1981) and Eversull (1985). 

Cotton Valley lithofacies and associated stratigraphic 

nomenclature in northern Louisiana are shown in figures 5 

and 6. The basal formation of the Cotton Valley Group is the 

Bossier Shale, a dark, calcareous, fossiliferous, marine shale. 

In eastern Texas, isolated turbidite sandstones occur within the 

Bossier Shale (Collins, 1980). Overpressured gas currently is 

being produced from these sandstones in a rapidly developing 

new play (PI Dwights Drilling Wire, Jan. 3, 2000; Exploration 

Business Journal, 2nd quarter, 2000). Completely encased in 

marine shale, these gas-charged sandstones might represent a 

continuous gas accumulation. 

The Bossier Shale grades upward into Cotton Valley sandstones with interbedded shales. These sandstones consist of 

stacked barrier-island, offshore-bar, strandplain, and fluvialdeltaic sandstones and are referred to as the Terryville massive-sandstone complex in northern Louisiana by Coleman and 

Coleman (1981). In eastern Texas, the stratigraphically equivalent unit is called Cotton Valley Sandstone, and it consists 

of braided-stream, fan-delta, and wave-dominated-delta sandstones (Wescott, 1983; Coleman, 1985; Dutton and others, 

1993). Across the Cotton Valley hydrocarbon-productive trend 

in eastern Texas and northern Louisiana, the Terryville or 

Cotton Valley Sandstone averages about 1,000 to 1,400 ft in 

thickness (Finley, 1984; Presley and Reed, 1984). Sand deposition was interrupted in Early Cretaceous time by a regional 

transgressive event marked by deposition of the Knowles Limestone (figs. 5 and 6). In updip areas of eastern Texas and 

southern Arkansas, the Knowles Limestone pinches out, and 

clastic rocks of the Travis Peak, or equivalent Hosston, Formation directly overly Cotton Valley sandstones (figs. 3, 5, and 

6). Saucier (1985) interprets the Knowles Limestone as the 

uppermost formation of the Cotton Valley Group, but Coleman 
Is There a Basin-Centered Gas Accumulation in Cotton Valley Group Sandstones, Gulf Coast Basin, U.S.A.? 7


CHRONOSTRATAGRAPHIC SECTION OF NORTH LOUISIANA 

SYSTEM 

SERIES

STAGE GROUP FORMATION 

TERTIARY 

PALEOGENE 

PALEOGENE 

EOCENE 

AGE 

(Ma) 

GULFIAN 

UPPER CRETACEOUS 

COMANCHEAN 

LOWER CRETACEOUS 

COAHUILAN 

JURASSIC 

UPPER MIDDLE LOWER UPPER 

TRIASSIC 

YPRESIAN 

THANETIAN 

DANIAN 

MAASTRICHTIAN 

CAMPANIAN 

SANTONIAN 

CONIACIAN 

TURONIAN 

CENOMANIAN 

ALBIAN 

APTIAN 

BARREMIAN 

HAUTERIVIAN 

VALANGINIAN 

BERRIASIAN 

TITHONIAN 

KIMMERIDGIAN 

OXFORDIAN 

CALLOVIAN 

BATHONIAN 

BAJOCIAN 

AALENIAN 

TOARCIAN 

PLIENSBACHIAN 

SINEMURIAN 

HETTANGIAN 

RHAETIAN 

WILCOX WILCOX 

MIDWAY MIDWAY 

NAVARRO 

TAYLOR 

AUSTIN 

EAGLE FORD 

WOODBINE 

WASHITA 

TRINITY 

GLEN 

ROSE 

FREDRICKSBURG 

HIATUS 

HIATUS 

HIATUS 

ARKADELPHIA 

NACATOCH 

EAGLE FORD 

SARATOGA 

ANNONA 

OZAN 

TOKIO 

AUSTIN 

TUSCALOOSA 

PALUXY WASHITA-FREDERICKSBURG 

MOORINGSPORT 

FERRY LAKE ANHYDRITE 

RODESSA JAMES PINE ISLAND 

PETTET (SLIGO) MBR 

SLIGO 

HOSSTON 

(TRAVIS PEAK) 

HAYNESVILLE-BUCKNER 

SMACKOVER 

NORPHLET 

LOUANN 

WERNER 

EAGLE MILLS 

60 

70 

80 

90 

100 

110 

120 

130 

140 

150 

160 

170 

180 

190 

200 

210 

MILLIONS OF YEARS 

COTTON VALLEY (See figure 5) 

Figure 3. Chronostratigraphic section of northern Louisiana modified from Shreveport Geological Society (1987). Subdivisions 

of Cotton Valley Group shown in figure 5. 
Figure 4. Generalized structure contours on top of Cotton Valley Group sandstone across northeastern Texas and northern Louisiana, modified from Finle

sandstone has been designated as “tight-gas sandstone” in all counties shown on this map with exception of 15 gas fields in northern Louisiana (fig. 8).

TEXAS LA

0 10 20 30 40 50

Miles

Contour Interval: 1000 feet

OKLAHOMA

ARKANSAS

FANNIN

COLLIN

LAMAR

HUNT

HOPKINS

RAINS

WOOD

ROCKWALL

DELTA

DALLAS

KAUFMAN VAN ZANDT

ELLIS

NAVARRO

HENDERSON

LIMESTONE

FREESTONE

ANDERSON

LEON

HOUSTON

MADISON

TRINITY

BRAZOS

ROBERTSON

MILAM

FALLS ANGELINA

NACODOCHES

CHEROKEE

SMITH

UPSHUR

GREGG

RUSK

RED RIVER

FRANKLIN TITUS

CAMP

MORRIS

BOWIE

CASS

MARION

HARRISON

PANOLA

SHELBY

SAN

AGUSTINE

SABINE

SABINE

VERNON RAPIDES

DE SOTO RED RIVER

NATCHITOCHES

BIENVILLE

WINN

JACKSON

GRANT

CADDO

BOSSIER

WEBSTERCLAIBORNE LINCOLNUNION

MOREH

RICH

OUACHITA 

CALDWELL

LA SALLE

CATAHO

FRA

-8000

-4000

-5000

-6000

-7000

-8000 -8000

-9000

-10,000

-5000

-10,000

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-6000 -7000

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-10,000

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-7000

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-6000

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-4000

-13,000

-12,000

-11,000

-9000
Is There a Basin-Centered Gas Accumulation in Cotton Valley Group Sandstones, Gulf Coast Basin, U.S.A.? 9 

(SOUTH) (NORTH) 

DOWNDIP UPDIP 

CRETACEOUS JURASSIC 

COTTON VALLEY GROUP 

WINN LS 

MASSIVE 

SANDSTONE 

COMPLEX OF CALVIN SS. 

HOSSTON FORMATION 

UNNAMED SANDS & SHALES 

KNOWLES LIMESTONE 

BLANKET 

SANDSTONES 

MASSIVE SANDSTONE 

COMPLEX OF 

TERRYVILLE SS. 

HICO 

SHALE 

BOSSIER SHALE 

HAYNESVILLE, BUCKNER, AND 

SMACKOVER FORMATIONS 

SCHULER FM 

Figure 5. Generalized stratigraphic nomenclature of Cotton Valley Group (patterned units) in northern 

Louisiana, modified from Coleman and Coleman (1981). Schuler Formation and equivalents were assigned to 

Lower Cretaceous in mid 1980’s (Herrmann and others, 1991). Prior to that time, entire Cotton Valley Group was 

considered to be Late Jurassic in age. 

and Coleman (1981) include the stratigraphically higher Calvin 

Sandstone, Winn Limestone, and unnamed sands and shales 

within the Cotton Valley Group (figs. 5 and 6). 

Cotton Valley Group 

Depositional Systems 

Regional Framework 

From eastern Texas to Mississippi, stacked barrier-island, 

strandplain, and fluvial-deltaic sandstones, known as Cotton 

Valley or Terryville Sandstone, reflect influx of sands from 

a number of depocenters. Evolution of Cotton Valley depocenters and associated paleogeography across northern Louisiana 

are described and illustrated by Coleman and Coleman (1981), 

who subdivided the Terryville Sandstone into four depositional 

“events” (fig. 6) based on widespread shale breaks. Across 

south-central Mississippi, Moore (1983) shows three sequential paleogeographic reconstructions of Cotton Valley Group 

sandstone deposition. Although the setting was similar, concise 

paleogeographic reconstructions have not been published for 

the East Texas Basin; however, McGowen and Harris (1984) 

and Wescott (1985) provide data from which basic paleogeographic maps can be constructed. Figure 7 is a regional paleo-
10 Geologic Studies of Basin-Centered Gas Systems 

A 

SOUTH 

1 2 3 4 5 6 7 8 9 

WINN LIMESTONE 

CALVIN 

SANDSTONE TERRYVILLE 

KNOWLES LIMESTONE 

Figure 6 (above and facing page). North-south stratigraphic cross section of Cotton Valley Group across 

northern Louisiana based on data from 15 wells. Section is modified from Coleman and Coleman (1981) to 

show details of Cotton Valley blanket sandstones as identified and described by Eversull (1985) and Thomas 

and Mann (1963). Arrows indicate stratigraphic range of blanket sandstones. Line of cross section shown 

in figures 7 and 8. 

geographic map of upper Cotton Valley depositional systems 

(equivalent to Terryville IV of Coleman and Coleman, 1981) 

across the northern Gulf Basin from eastern Texas to Mississippi based on data integrated from these various workers. 

As shown in figure 7, Cotton Valley fluvial-deltaic depocenters were located in present-day northeastern Texas, 

south-central Mississippi, and along the Louisiana-Mississippi 

border. The system along the Louisiana-Mississippi border 

represents the ancestral Mississippi River and was a locus 

of major clastic influx. Large quantities of sand delivered 

to the marine environment by this system were transported 

westward by longshore currents producing an extensive eastwest barrier-island or strandplain complex (Thomas and 

Mann, 1966). Vertical stacking of barrier-island/strandplain 

sands through time resulted in accumulation of the Terryville 

massive-sandstone complex (figs. 6 and 7). The east-west 

barrier-island complex across northern Louisiana sheltered a 

lagoon to the north from open-marine waters to the south 

(Thomas and Mann, 1966). The Hico Shale accumulated in the 

lagoon while fluvial and coastal-plain sandstones and shales of 

the Schuler Formation were deposited in continental environments north of the lagoon (figs. 6 and 7). Development of 

a similar, but smaller, lagoon associated with barrier islands— 

formed from longshore-transported sands in south-central Mississippi—was documented by Moore (1983) (fig. 7). In eastern 

Texas, during the earliest phase of Cotton Valley Sandstone 

deposition, small fan deltas developed along the updip margin 

of the East Texas Basin (McGowen and Harris, 1984; Wescott, 

1985; Black and Berg, 1987). The drainage system was immature with small fan deltas formed by numerous small streams. 

According to McGowen and Harris (1984), fan-delta deposition persisted through Cotton Valley time along the western 

margin of the East Texas Basin where fan-delta deposits characterize most of the Cotton Valley Sandstone interval. Along 

the northern flank of the East Texas Basin, in the region 

of the present-day Sabine uplift, a mature drainage system 
Is There a Basin-Centered Gas Accumulation in Cotton Valley Group Sandstones, Gulf Coast Basin, U.S.A.? 11 

A' 

NORTH 

10 11 12 13 14 15 

1000 

500 

0 

Feet 

10 5 0 

Miles 

HOSSTON FM 

MASSIVE 

SANDSTONES I 

II 

III 

IV HICO SHALE 

SCHULER FORMATION 

(Alluvial, coastal plain, 

fluvial deltaic 

(Barrier Islands 

offshore bars) 

Cadeville 

B C 

D Bodcaw 

Vaughn Price 

McGreary

Bolinger 

Davis 

E 

Justiss 

Ardis 

(Lagoon/bay) 

Roseberry? 

Sexton 

Taylor 

Tucker 

BOSSIER SHALE 

developed as fan deltas prograded basinward and evolved into 

a wave-dominated delta system. Sandstones of the lower part 

of the Cotton Valley comprising this delta system are referred 

to informally as the Taylor sandstone, according to Wescott 

(1985). After Taylor sand deposition was terminated by a subregional transgressive event, delta progradation resumed with 

development of a more elongate, fluvial-dominated system in 

the upper part of the Cotton Valley (fig. 7), referred to as the 

Lone Oak delta by Kast (1983). 

Blanket Sandstones of Northern Louisiana 

In northern Louisiana, at least 20 distinct tongues of sandstone extend landward from barrier-island deposits of the Terryville massive-sandstone complex and become thinner northward before pinching out into shales of the Hico lagoon, as 

shown in figure 6. Some of these sandstones have limited 

geographic extent covering only part of the lagoon, whereas 

others extend across most or all of the lagoon and interfinger 

with continental deposits of the Schuler Formation on the 

landward side of the lagoon (Coleman and Coleman, 1981; 

Eversull, 1985). These sandstones have been interpreted as 

transgressive deposits with sand being derived from Terryville 

barrier islands and transported landward into the Hico lagoon 

during periods of relative sea-level rise and (or) diminished 

sediment supply (Coleman and Coleman, 1981; Eversull, 

1985). These transgressive sandstones have significantly better 

porosity and permeability than Terryville massive sandstones 

from which they were derived and have been prolific producers of oil and gas from structural, stratigraphic, and combination traps discovered in the 1940’s, 1950’s, and 1960’s 

across northern Louisiana (Collins, 1980; Bebout and others, 

1992). Referred to informally as “blanket” sandstones (Eversull, 1985), they can be correlated readily across northern 

Louisiana, and, as shown in figure 6, they were given informal 

names by operators during drilling in the 1940’s and 1950’s 

(Sloane, 1958; Thomas and Mann, 1963; and Eversull, 1985). 
Figure 7. Regional paleogeographic map showing sedimentary environments of Cotton Valley Group during deposition of uppermost Cotton Valley s

IV sandstone of Coleman and Coleman, 1981). Map synthesized from data of Thomas and Mann (1966), Coleman and Coleman (1981), Moore (1983), M(1984), Wescott (1985), and Eversull (1985). Location of north-south stratigraphic cross section of Cotton Valley Group across northern Louisiana in fig

by 15 numbered wells.

0 25 0 75 100

Miles Gulf of Mexico

TX

OK AR

DETAIL AL

AREA

LOCATION MAP

NM TN

LA

MS

ANCESTRAL M

RIVER COTTO

DEPOCE

LONE OAK DELTA

COTTON VALLEY

DEPOCENTER

ARKANSAS

LOUISIANA

TEXAS LOUISIANA

LOUISIANA

MISSISSIPPI

OKLAHOMA

TEXAS

MAXIMUM SOUTHERN

EXTENT

TERRYVILLE MASSIVE O SANDSTONES F BARRIER ISLAND BARRIER 

BAY/LAGO

HICO 

A

A' 15

14

13

12

11

10 9

8

7

5

4 6

2

3

1

UPPER COTTON VALLEY GROUP PALEOGEOGRAPHY

5

ISLAN

LAGOON
Is There a Basin-Centered Gas Accumulation in Cotton Valley Group Sandstones, Gulf Coast Basin, U.S.A.? 13 

Based on isopach map trends, Eversull (1985) identified 

two groups of blanket sandstones. Geographically more extensive sandstones of the first group span most of the Hico lagoon 

and often interfinger with continental deposits of the Schuler 

Formation. These sandstones generally are 30 to 70 ft thick 

and can reach a thickness of 140 ft toward the south where 

they merge with barrier-island sandstones of the Terryville 

massive-sandstone complex. Blanket sandstones of the second 

group generally are less than 30 ft thick, have limited geographic extent, and most commonly occur in the eastern part 

of the Hico lagoon proximal to the fluvial-deltaic source. 

These sandstones pinch out northward into shales of the 

Hico lagoon. Transgressive, blanket sandstones of both groups 

collectively have significantly higher porosity and permeability than barrier-island sandstones of the Terryville massivesandstone complex to the south (Collins, 1980; Bebout and 

others, 1992). 

Blanket- and Massive-Sandstone 

Productive Trends 

Significant differences in reservoir properties between 

transgressive, blanket sandstones to the north and massive, 

barrier-island sandstones to the south define two different 

hydrocarbon-productive trends of Cotton Valley sandstones 

(fig. 8). Blanket sandstones have higher porosity and permeability than Terryville massive sandstones to the south. Eversull (1985) reported that blanket sandstones are cleaner and 

better sorted. She attributed their superior reservoir properties 

to high-energy reworking during transgressive events. Coleman (1985), however, reported that blanket sandstones exhibit 

an increase in calcite cement and clay content northward 

toward their pinch-out edges, and that superior reservoir 

properties occur because (1) clays inhibited precipitation of 

quartz overgrowths and (2) secondary porosity was generated 

through widespread dissolution of calcite cement. Absence of 

detrital clay coatings on sand grains in high-energy barrierisland sandstones of the Terryville massive-sandstone complex to the south, however, permitted widespread precipitation of quartz cement as syntaxial overgrowths, resulting in 

nearly complete occlusion of porosity (Sloane, 1958; Coleman and Coleman, 1981). Whatever the cause of porosity differences, blanket sandstones generally have sufficient porosity 

and permeability to flow gas or liquids on open-hole drillstem tests (DST’s) and to produce gas without fracture-stimulation treatment (Collins, 1980; Bebout and others, 1992). 

Terryville massive sandstones to the south and west, however, 

have such poor reservoir properties that they generally do 

not flow gas or liquids during DST’s, and they require 

hydraulic-fracture treatments before commercial production 

can be achieved. 

Diagenesis of 


Cotton Valley Group Sandstones


Because understanding reservoir mineralogy is critical 

to successful wireline-log analysis and design of fracturestimulation treatments in Cotton Valley Group sandstones, 

considerable attention has been devoted to understanding diagenetic patterns of Cotton Valley sandstones, especially in 

the low-permeability, Cotton Valley massive-sandstone trend. 

Focusing on those sandstones in eastern Texas, Wescott (1983) 

reported that Cotton Valley sandstones are very fine grained, 

well-sorted quartz arenites and subarkoses with monocrystalline quartz and feldspar being the primary framework components. Principal cements include quartz, calcite, clays, and iron 

oxides. In unraveling the complex diagenetic history of these 

sandstones, Wescott (1983) interpreted two major diagenetic 

sequences. The most common sequence is (1) formation of 

clay coatings—primarily chlorite—on framework grains, usually covering grains partially, not completely, (2) precipitation 

of syntaxial quartz overgrowths on quartz grains, (3) dissolution of unstable grains, most commonly feldspars, (4) precipitation of clays, primarily illite and chlorite with minor kaolinite, (5) precipitation of calcite cement in both relict primary 

pores and secondary pores, and (6) large-scale replacement 

of grains and cements by calcite, resulting in poikilotopic 

texture in which a few relict quartz grains are “floating” in 

calcite. In the other, less-common diagenetic sequence, which 

occurs primarily in cleaner, coarser grained sandstones, calcite 

cementation commenced early and progressed to yield a fabric 

with widespread replacement of grains by calcite. 

Wescott (1983) classified Cotton Valley sandstones into 

three general groups on the basis of primary depositional 

texture and resulting diagenetic characteristics. In general, 

Wescott (1983) found that clean, well-sorted sands deposited 

in high-energy environments (type I) generally are nearly completely cemented by quartz and (or) calcite, have little or no 

porosity and permeability, and provide little reservoir potential. In some cases, these sandstones exhibit preservation of 

minor amounts of primary intergranular porosity from presence of authigenic chlorite coats (Hall and others, 1984). In 

sands deposited in lower energy environments where abundant 

detrital clays remained (type II), nucleation of quartz overgrowths generally was inhibited by clays. Most clay-bearing 

sandstones, however, contain significantly large amounts of 

clay, and although abundant microporosity is associated with 

these clays, permeability generally is low. Highest porosities, 

according to Wescott (1983), occur in type-III sandstones, 

which developed abundant secondary porosity from dissolution of unstable grains and calcite cement. Hall and 

others (1984), however, reported that dissolution of unstable 

grains often is incomplete, secondary pores generally are 

poorly interconnected, and these sandstones, too, have poor 
Figure 8. Map of northeastern Texas and northwestern Louisiana showing major fields that have produced hydrocarbons from Cotton Valley Group sandsto

productive trends are recognized based on reservoir properties and resulting producing capabilities of Cotton Valley sandstone reservoirs. Fifteen fields exc

designation by FERC in 1980 are shown in solid dark-gray shading. Map modified from Collins (1980) and White and others (1992). Location of north-south str

Cotton Valley Group across northern Louisiana in figure 6 is illustrated by 15 numbered wells.

0 

M

Cotton Valley sandston(designated tight gas bCotton Valley sandston(excluded from 1980 FECotton Valley tight, mas(low porosity & permeaCotton Valley blanket-s(good porosity & permeEast Texas & Louisiana 

Kaufman Gulf of Mexico

TX LA

OK AR

MS AL

LOCATION MAP

DETAIL

AREA

SABINE UPLIFT

NORTH

LOUSIANA

SALT BA

ARKANSAS

TEXAS LOUISIANA

EAST

TEXAS

SALT

BASIN

Woodlawn

Blocker

Oak Hill

Dirgin

Carthage

BethanyLongstreet

GreenwoodWaskom

Longwood

Dixie

Benton

Sligo

Sentell

Caspiana

Elm Grove

LakeBistineau

Plain

Dealing

Rocky

Mount

Ivan

S Sarepeta

Shongaloo

CottonValley

Haynesville

E Haynesville

Blackburn

Lisbon

W Lisbon

Homer

Minden

Leatherman

Creek

Ada

Sibley Bryceland

BearCreek

Athens

SugarCreek

Terryville

Rustin

Clay

Vernon

Tremont

Calhoun

Cadeville

Cheni

S D

CarlN Ca

S Downsville

Downsville

Choudrant

D'Arbonne

Unionville

Hico Knowes

NE Lisbon

A

A' 15

14

13

12

11 10

9

8

7

6

5

4 3

2

1

Frierson
Is There a Basin-Centered Gas Accumulation in Cotton Valley Group Sandstones, Gulf Coast Basin, U.S.A.? 15 

permeability and require fracture stimulation to produce gas 

commercially. 

In northern Louisiana, as interpreted by Russell and 

others (1984), the upper Cotton Valley Bodcaw Tongue of the 

Terryville Sandstone at Longwood field on the eastern flank 

of the Sabine uplift experienced a virtually identical diagenetic 

history to that described for Cotton Valley sandstones in eastern Texas by Wescott (1983). Like Wescott (1983), Russell and 

others (1984) reported that nucleation of quartz overgrowths 

was inhibited by presence of clays, but the quantity of porefilling clays generally is so large that permeability is low 

despite presence of high microporosity. Also, as in eastern 

Texas, the best reservoir sandstones are those that have 

low clay content and developed abundant secondary porosity 

through dissolution of unstable grains and cement. Similar 

diagenetic patterns in northern Louisiana also were reported 

for Cotton Valley sandstones at Frierson field by Sonnenberg 

(1976) and for the lowermost part of the Terryville Sandstone 

(the informal Taylor sandstone) at Terryville field by Trojan 

(1985). In addition to authigenic constituents reported in eastern Texas and northern Louisiana, Trojan (1985) also found 

small amounts of authigenic pyrite in Taylor sandstones at 

Terryville field. Pyrite occurs as small silt-size clusters (framboids). It is volumetrically the least abundant authigenic mineral reported by Trojan (1985), but its presence is significant 

because of its effect on wireline-log measurements of formation resistivity. 

Impact of Diagenetic Mineralogy 

on Wireline Logs 

The complex diagenetic mineralogy of tight Cotton Valley 

sandstones prohibits use of standard calculation procedures in 

reservoir evaluation with wireline logs. The main difficulty 

is that properties of certain diagenetic constituents result in 

abnormally low resistivity measurements, which lead to such 

high calculated water saturations that productive zones appear 

to be wet. Major factors contributing to abnormally low resistivities in tight Cotton Valley sandstones include bound water 

associated with pore-filling clays or clay coatings and conductive authigenic minerals such as pyrite and ankerite (Janks and 

others 1985; Turner, 1997). 

Pore-lining and pore-filling clays have exceptionally high 

ratios of surface area to volume. Large surface area and high 

cation-exchange capacity of clays result in formation of a 

double ionic layer on clay surfaces (Almon, 1979; Snedden, 

1984). This bound double layer can be significantly more 

conductive than pore waters, resulting in abnormally low 

measured resistivities, especially with induction logs (Almon, 

1979; Wescott, 1983). Highly conductive authigenic minerals, 

such as ankerite and pyrite, in Cotton Valley sandstones also 

cause abnormally low resistivities. Trojan (1985) found that 

pyrite concentrations as low as 1 percent in Cotton Valley 

sandstones had a dramatic effect on resistivity measurements 

and hence on calculated water saturations. Standard calculation methods showed that pyrite-bearing sandstones at Terryville field in northern Louisiana had water saturations in 

excess of 100 percent. Trojan (1985) showed that if these sandstones were pyrite-free, calculated water saturations would be 

closer to 50 percent. Although water saturations in productive 

Cotton Valley sandstones commonly are 25 to 30 percent, 

water-free gas production has been achieved from zones with 

calculated water saturations as high as 60 percent (Nangle and 

others, 1982; Wilson and Hensel, 1984; Dutton and others, 

1993). 

Porosity measurements from wireline logs also can be 

affected adversely from diagenetic mineral constituents in 

Cotton Valley sandstones. In a study of Taylor sandstones 

(fig. 6) at Terryville field in northern Louisiana, Ganer (1985) 

demonstrated the negative impact of authigenic carbonates on 

porosity measurements from wireline logs. Located within the 

porous, permeable blanket sandstone trend, Terryville field 

was discovered in 1954 with production from the Cotton 

Valley “D” sandstone (fig. 6), one of the blanket sandstones. 

The Taylor sandstone occurs in the lower part of the Cotton 

Valley Sandstone interval, and its productive potential at Terryville field was not discovered until 1978. Unlike the stratigraphically higher blanket sandstones, the Taylor sandstone 

has relatively poor reservoir properties similar to those of 

tight Cotton Valley massive sandstones to the south. Like 

Wescott (1983), Ganer (1985) found that, although the Taylor 

sandstone is predominantly a quartz sandstone, it contains 

authigenic carbonate cement and locally can be composed of 

more than 50 percent carbonate resulting in a poikilotopic 

texture. With abundant secondary porosity from carbonate dissolution, these carbonate-rich sandstones are the best gas producers within the Taylor sandstone interval at Terryville field. 

Ganer (1985) identified several different carbonate minerals 

in Taylor sandstones, including calcite, ankerite, and siderite. 

Grain densities of these minerals are 2.71, 3.00, and 3.96 

g/cm3, respectively. If porosity logs based on a sandstone 

matrix (grain density of 2.65 g/cm3) are run across an interval, 

such as the Taylor sandstone, containing abundant carbonate 

constituents with higher densities, measured porosity values 

will be pessimistic. Working with 420 ft of conventional core 

from four wells at Terryville field, Ganer (1985) reported sandstone intervals with abundant carbonate constituents where 

log-measured porosities were close to zero, but core-measured 

porosities exceeded 6 percent. With complex effects of diagenetic minerals on both porosity and resistivity measurements from wireline logs, Ganer (1985) showed that a single 

porosity/water saturation limit is not suitable for evaluating 

productive potential of Cotton Valley sandstones at Terryville 

field. Ganer’s conclusions probably are applicable to most, or 

all, of the tight, Cotton Valley massive-sandstone trend across 

northeastern Texas and northern Louisiana. 

In comparing core-derived reservoir properties with 

wireline-log measurements for Cotton Valley sandstones from 

Carthage field in eastern Texas, Wilson and Hensel (1984) 
16 Geologic Studies of Basin-Centered Gas Systems 

reported that no apparent relationship exists between porosity 

and permeability. From core analyses, they noted that it is not 

uncommon to find a sandstone interval with 10 percent porosity and 1 to 3 mD permeability adjacent to a zone with similar 

porosity but with permeability less than 0.05 mD. Similarly, 

Ganer (1985) reported that Taylor sandstones with 8 percent 

porosity at Terryville field in northern Louisiana have permeabilities ranging from 0.01 to 13 mD. For Carthage field, 

Wilson and Hensel (1984) also noted that empirically derived 

values of cementation factor (m) and saturation exponent (n), 

used in calculation of water saturation, vary significantly from 

zone to zone. Wilson and Hensel (1984) derived general empirical values of m and n for Carthage field area to achieve more 

accurate log-derived estimates of water saturation. Because of 

such difficulties in determining water saturations from wireline 

logs, Presley and Reed (1984) stress that gas-pay cutoff values 

should be based on experience by operators in a given area. 

A consequence of difficulties in accurate reservoir evaluation from conventional log analysis, of course, is that intervals 

capable of producing gas might be bypassed because of high 

calculated water saturations. For this study, the significance 

of these difficulties with wireline logs in tight Cotton Valley 

sandstones is that logs are of limited value in differentiating 

between gas-productive and wet intervals and therefore in 

identifying gas-water contacts on the flanks of Cotton Valley 

fields. 

Source Rocks 

Little information has been published on source rocks for 

hydrocarbons produced from Cotton Valley reservoirs in northern Louisiana and eastern Texas. In studying the overlying 

Travis Peak Formation in eastern Texas, Dutton (1987) showed 

that shales interbedded with Travis Peak sandstone reservoirs 

were deposited in fluvial-deltaic settings where organic matter 

commonly is oxidized and not preserved. With measured 

values of total organic carbon (TOC) in Travis Peak shales 

generally less than 0.5 percent, these shales are not considered 

as potential hydrocarbon source rocks (Tissot and Welte, 

1978). Dutton (1987) suggested that the most likely sources 

for hydrocarbons in Travis Peak reservoirs in eastern Texas 

are laminated, lime mudstones of the lower member of the 

Jurassic Smackover Formation and prodelta and marine shales 

of the Bossier Shale, basal formation of the Cotton Valley 

Group (fig. 3). Sassen and Moore (1988) demonstrated that 

Smackover carbonate mudstones are a significant hydrocarbon 

source rock, charging various reservoirs in Mississippi and 

Alabama, and Wescott and Hood (1991) documented the Bossier Shale as a significant source rock in eastern Texas. Presley 

and Reed (1984) suggested that gray to black shales interbedded with Cotton Valley sandstones, as well as the underlying 

Bossier Shale, probably are the source for gas in Cotton Valley 

Sandstone reservoirs. Coleman and Coleman (1981) agreed 

with this interpretation for Cotton Valley sandstone reservoirs 

in northern Louisiana, stating that “hydrocarbons were generated from neighboring source beds.” 

In summary, despite limited source-rock data, it seems 

likely that adequate hydrocarbon source rocks occur in the 

Bossier Shale immediately beneath Cotton Valley sandstones 

and also in stratigraphically lower Smackover carbonate mudstones (fig. 3). 

Burial and Thermal History 

In a study of diagenesis and burial history of the Travis 

Peak Formation in eastern Texas, Dutton (1987) reported that 

measured vitrinite reflectance (Ro) values for Travis Peak 

shales generally range from 1.0 to 1.2 percent. This indicates 

that these rocks have passed through the oil window (Ro = 0.6 

to 1.0 percent) and are approaching the level of onset of drygas generation (Ro = 1.2 percent) (Dow, 1978). A maximum 

Ro of 1.8 percent was measured in the deepest sample from 

a downdip well in Nacogdoches County, Texas. Despite thermal maturity levels reached by Travis Peak shales, the small 

amount, and gas-prone nature, of organic matter in these shales 

precludes generation of oil, although minor amounts of gas 

might have been generated (Dutton, 1987). 

In the absence of actual measurements of Ro, values 

of Ro can be estimated by plotting burial depth of a given 

source-rock interval versus time in conjunction with an 

estimated paleogeothermal gradient (Lopatin, 1971; Waples, 

1980). Dutton (1987) presented burial-history curves for tops 

of the Travis Peak Formation, Cotton Valley Group, Bossier 

Shale, and Smackover Formation for seven wells on the crest 

and western flank of the Sabine uplift. The burial-history 

curves show total overburden thickness through time and use 

present-day compacted thicknesses of stratigraphic units. Sediment compaction through time was considered insignificant 

because of absence of thick shale units in the stratigraphic 

section. Loss of sedimentary section associated with late, midCretaceous and mid-Eocene erosional events was accounted 

for in the burial-history curves. 

Dutton (1987) provided justification for using the average 

present-day geothermal gradient of 2.1°F/100 ft for the paleogeothermal gradient for the five northernmost wells. Paleogeothermal gradients in the two southern wells probably were 

elevated temporarily because of proximity to the area of initial 

continental rifting. Based on the crustal extension model of 

Royden and others (1980), Dutton (1987) estimated values 

for elevated paleogeothermal gradients for these two wells for 

80 m.y. following the onset of rifting before reverting to the 

present-day gradient for the past 100 m.y. 

Using estimated paleogeothermal gradients in conjunction with burial-history curves, Dutton (1987) found that calculated values of Ro for Travis Peak shales agree well with 

measured values. Because of this agreement, Dutton (1987) 

used the same method to calculate Ro values for tops of the 

Cotton Valley Group, Bossier Shale, and Smackover Forma-
Is There a Basin-Centered Gas Accumulation in Cotton Valley Group Sandstones, Gulf Coast Basin, U.S.A.? 17 

tion in eastern Texas. Estimated Ro values for the Bossier Shale 

and Smackover in seven wells range from 1.8 to 3.1 percent 

and 2.2 to 4.0 percent, respectively, suggesting that these rocks 

have reached a stage of thermal maturity in which dry gas was 

generated. Assuming that high-quality, gas-prone source rocks 

occur within these two formations, it is likely that one or both 

of these units generated gas found in overlying Cotton Valley 

and Travis Peak reservoirs. 

No such regional source-rock and thermal-maturity analysis is known for Travis Peak and Cotton Valley intervals in 

northern Louisiana. Scardina (1981) presented burial-history 

data for the Cotton Valley Group, but included no information 

on geothermal gradients and thermal history of rock units. 

Present-day reservoir temperatures in tight Cotton Valley sandstones of eastern Texas and the tight, massive Terryville Sandstone in northern Louisiana both are in the 250°F to 270°F 

range (Finley, 1986; White and Garrett, 1992). It is likely 

that Bossier and Smackover source rocks in northern Louisiana experienced relatively similar thermal history to their 

stratigraphic counterparts in eastern Texas and, therefore, are 

sources for Cotton Valley gas in northern Louisiana. Herrmann 

and others (1991) presented a burial-history plot for Ruston 

field in the Cotton Valley blanket-sandstone trend in northern 

Louisiana. They suggested that Smackover gas was derived 

locally from Smackover lime mudstones and Cotton Valley gas 

from Cotton Valley and Bossier shales. Their burial-history 

plot shows the onset of generation of gas from Smackover 

and Bossier source rocks at Ruston field occurred about 80 

Ma and 45 Ma, respectively. As noted above in this report, 

the Sabine uplift has been a positive feature for the past 60 

m.y. (Kosters and others, 1989; Jackson and Laubach, 1991). 

Therefore, it would have been a focal area for gas migrating 

from Smackover, Bossier, and upper Cotton Valley source 

rocks in eastern Texas and northern Louisiana. 

Abnormal Pressures 

Pore pressure or reservoir pressure commonly is reported 

as a fluid-pressure gradient (FPG) in pounds per square inch/ 

foot (psi/ft). Normal FPG is 0.43 psi/ft in freshwater reservoirs 

and 0.50 psi/ft in reservoirs with very saline waters (Spencer, 

1987). Abnormally high pore pressures as high as 0.86 psi/ft 

have been encountered in Cotton Valley reservoirs in northeastern Louisiana (fig. 9). Multiple FPG values for a particular 

gas field in figure 9 refer to gradients calculated for different, 

stacked blanket-sandstone reservoirs penetrated in that field. 

Across northern Louisiana, as shown in figure 9, the highest 

FPG’s of 0.84 and 0.86 psi/ft occur in the southeast, and 

gradients generally decrease to nearly normal values of 0.43 

to 0.50 psi/ft in the northwest. This pattern exhibits general 

agreement with reservoir-pressure data for northern Louisiana 

summarized by Coleman and Coleman (1981) (fig. 10). The 

dashed line in figure 10 shows a modification of Coleman 

and Coleman’s (1981) pressure boundary to include the 0.63 

psi/ft gradient in Hico-Knowles field and 0.67 psi/ft gradient 

in Tremont field (fig. 9). Most significant for this study, 

the boundary between overpressured and normally pressured 

Cotton Valley sandstones (fig. 10) shows no relationship to 

the two different productive Cotton Valley sandstone trends 

defined by differences in reservoir properties (fig. 8). Additionally, most Cotton Valley sandstone reservoirs, especially in the 

tight, massive-sandstone trend across northwestern Louisiana 

and eastern Texas are normally pressured, as shown in figure 9. 

History of Cotton Valley Group 

Sandstone Exploration 

Beginning in 1937 and continuing through the early 

1960’s, commercial gas production was established from 

porous and permeable Cotton Valley Group blanket-sandstone 

reservoirs across northern Louisiana. Blanket sandstones 

flowed gas at commercial rates without artificial stimulation. 

Initial discoveries were in anticlinal traps associated with salt 

structures. Subsequent discoveries came from more complex 

and subtle traps, including (1) combination traps with blanket 

sandstones pinching out across anticlines or structural noses, 

and (2) stratigraphic traps with blanket sandstones pinching 

out on regional dip (Pate, 1963; Coleman and Coleman, 1981). 

By the early 1960’s, the high-porosity blanket-sandstone play 

matured, and exploratory drilling waned. Low-porosity, lowpermeability, Cotton Valley massive sandstones to the south in 

Louisiana and to the west on the Sabine uplift in northwestern 

Louisiana and eastern Texas flowed gas at rates less than 1,000 

MCFD (thousand cubic feet of gas per day) and were not commercial with gas selling at $0.18/MCF in the 1960’s (Collins, 

1980). 

In the 1970’s, gas production from low-permeability, 

Cotton Valley massive sandstones became commercial as a 

result of technical advances in hydraulic-fracturing techniques 

together with significantly higher gas prices. At Bethany field 

on the Sabine uplift in eastern Texas in 1972, Texaco successfully increased the rate of production from tight Cotton 

Valley sandstones from 500 MCFD to a sustained rate of 2,500 

MCFD and 30 BCPD (barrels of condensate per day) through 

hydraulic fracturing (Jennings and Sprawls, 1977). In conjunction with development of improved stimulation technology, 

price deregulation through the Natural Gas Policy Act (NGPA) 

of 1978 spawned a dramatic increase in drilling for gas in 

low-permeability Cotton Valley sandstones (Bruce and others, 

1992). In 1980, the Federal Energy Regulatory Commission 

(FERC) officially classified low-permeability Cotton Valley 

sandstones as “tight gas sands,” qualifying them for additional 

price incentives. Production from tight Cotton Valley sandstones surged. At Carthage field in eastern Texas, for example, 

production from Cotton Valley sandstones increased from 2.2 

BCFG (billion cubic feet of gas) in 1976 to 70.9 BCFG 

in 1980 (Meehan and Pennington, 1982). The large area 
CALHOUN

0.860.47

Cotton Valley Group sandstone fieldFluid-pressure gradient (psi/ft) fordifferent Cotton Valley sandstones

LEGEND

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FLUID-PRESSURE GRADIENTS (PSI/FT)

COTTON VALLEY GROUP SANDSTONES

BENTON

0.470.44

S SAREPETA0.500.490.49

COTTON VALLEY0.550.510.490.43

HAYNESVILLE

0.43

ELM GROVE

0.53

BETHANY EAST

0.51

CARTHAGE

0.55

OAK HILL

0.32

SUGAR CREEK

0.450.34

HICO KNOWLES

0.630.470.42 RUSTON 0.47

CHENIERE

0.84

S DREW

0.530.45

TREMONT0.670.46

CALHOUN

0.860.47

Blanket-sands

trend

Figure 9. Map of northeastern Texas and northwestern Louisiana showing fluid-pressure gradients calculated from shut-in pressures in Cotton Valley GroMultiple pressure-gradient values for a particular gas field refer to gradients calculated for different, stacked blanket-sandstone reservoirs penetrated in t

data for Louisiana fields shown in table 2.

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DISTRIBUTION OF RESERVOIR PRESSURES 

COTTON VALLEY GROUP SANDSTONES 

NORMALLY 

OVERPRESSURED 

Blanket sandsto

trend 

PRESSURED 

5

Figure 10. Map of northeastern Texas and northwestern Louisiana modified from Coleman and Coleman (1981) showing geographic distribution of abnorm

Cotton Valley Group sandstone reservoirs. Dashed line shows modification of Coleman and Coleman’s (1981) pressure boundary to include the 0.63 psi/ft flu

Hico-Knowles field and 0.67 psi/ft gradient in Tremont field as shown in figure 9 and documented in table 2. Comparison of this map with map in figure 8 sho

overpressure and normal pressure cuts across two productive trends of Cotton Valley sandstones. 
20 Geologic Studies of Basin-Centered Gas Systems 

across northern Louisiana and northeastern Texas within which 

Cotton Valley sandstones have been designated tight-gas sandstones by FERC includes all the counties identified by name 

on figure 4 (Dutton and others, 1993). 

Comparison of Blanket-Sandstone 

and Massive-Sandstone Trends 

Two productive Cotton Valley sandstone trends are identified based on reservoir properties (fig. 8). As described above, 

Cotton Valley sandstone reservoir properties are a function 

of diagenetic characteristics, which are controlled primarily 

by variations in depositional environment. Reservoir properties, in turn, govern gas-production characteristics, including 

both initial rate of gas production and necessity of hydraulicfracture treatments to achieve commercial production rates. 

Table 1 summarizes these and other key parameters distinguishing blanket- and massive-sandstone Cotton Valley reservoir trends. Data presented in table 1 were derived from a 

variety of sources as indicated in the table headnote, with 

much of the information coming from a series of reports by 

the Shreveport Geological Society on oil and gas fields in 

northern Louisiana (Shreveport Geological Society Reference 

Reports, 1946, 1947, 1951, 1953, 1958, 1963, 1980, 1987). 

Detailed information obtained from those reports on more than 

20 Cotton Valley oil and gas fields in northern Louisiana, 

including data on porosity, permeability, initial production 

rates, gas-water contacts, and FPG’s, is presented in table 2. 

Most of the significant fields across northern Louisiana 

and northeastern Texas from which Cotton Valley sandstones 

produce gas are shown in figure 8. The area shown in figure 

8 is part of the larger region shown in figure 4 within which 

Cotton Valley sandstones were designated as tight-gas sandstones by FERC in 1980. As shown in figure 8, however, 

15 Cotton Valley fields were excluded from FERC’s tight-gas 

sandstone designation. All but one of these fields are located 

within the porous and permeable Cotton Valley blanketsandstone trend. 

Blanket-Sandstone Trend 

Transgressive, Cotton Valley blanket sandstones have 

porosities ranging from 10 to 19 percent and permeabilities 

from 1 to 280 mD (tables 1 and 2). Porosity and permeability 

data are not readily available for all productive blanket sandstones in Cotton Valley fields. However, sufficient data are 

available from several blanket-sandstone reservoirs within a 

dozen fields across northern Louisiana to observe the widespread distribution of relatively high quality reservoir sandstones across the Cotton Valley blanket-sandstone trend (fig. 

11). Data shown in figure 11 are derived primarily from field 

reports by the Shreveport Geological Society and from White 

and others (1992). Multiple values of porosity and permeability for a given field in figure 11 represent measured values 

for separate, stacked blanket-sandstone reservoirs within that 

field. Average porosity and permeability for Cotton Valley 

blanket sandstones, calculated from data in figure 11, are 15 

percent and 110 mD, respectively. 

The relatively high porosity and permeability of blanket 

sandstones is reflected in (1) the ability of these sandstones 

to flow gas and (or) liquids on open-hole DST’s, and (2) 

high initial flow rates of gas from these sandstones in production tests without hydraulic-fracture stimulation treatments, as 

shown in figure 12. Multiple values of initial flow rates for 

a given field shown in figure 12 indicate rates from different 

stacked blanket sandstones that produce in that field. Across 

Table 1. Comparison of two productive trends of Cotton Valley Group sandstones in eastern Texas and northern Louisiana. 

[Data from Shreveport Geological Society (1946, 1947, 1951, 1953, 1958, 1963, 1980, 1987); Collins (1980); Nangle and others (1982); Finley (1984, 1986); 

Bebout and others (1992); and Dutton and others (1993). TSTM, too small to measure] 

Parameter Blanket sandstone Massive sandstone 

Porosity (percent) 10 to 19 (average = 15) 6 to 10 

Permeability (mD) 1.0 to 280 (average = 110) 0.042 (E. Texas) 0.015 (N. Louisiana) 

Open-hole DST Wells flow gas and (or) liquids Wells generally do not flow gas or liquids 

Stimulation treatment No treatment necessary for commercial production Massive-hydraulic fracturing required to achieve 

commercial production 

Initial flow rates (MCFD) 500 to 25,000 (average 5,000) Pre-stimulation: TSTM to 300 

Post-stimulation: 500 to 2,500 

Sw in productive zones < 0.40 Can be as high as 0.60 

Gas/water contacts Short, well-defined transition zones and gas-water Long transition zones with poorly defined gas-water 

contacts contacts 

Formation damage Possible Commonly severe 
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TREMONT14 34 Porosity

(percent)

LEGEND

POROSITY AND PERMEABILITY

COTTON VALLEY GROUP BLANKET SANDST

BENTON

1814

135

85

S SAREPETA

171312

265

7550

COTTON VALLEY

15

18

1616

280150121

34

LISBON

1714

150

40

LEATHERMAN

CREEK

12 1

TERRYVILLE

10 125

191818

NE LISBON

17 196 HICO KNOWLES

181715

CHOUDRANT

0.19 250

RUSTON

1715

CALHOUN 150 100

80

S DREW

1210

20

8

TREMONT

14 34

Blanket-sandstone trend

Figure 11. Map of northeastern Texas and northwestern Louisiana showing measured values of porosity and permeability in Cotton Valley Group blanket sa

and permeability data documented in table 2. Multiple values of porosity and permeability for a given field represent measured values for separate, stacked 

reservoirs in that field.

5
Table 2. Data on Cotton Valley Group sandstone fields in northern Louisiana. [See end of table for explanation of abbreviations] 

Field FERC Trend Trap Disc CV disc date: ss Depth CV perfs IP (CV) GOR Por Perm (mD) BHT BHP FPG Sw GWC 

“tight” date (ft) (MCFD) (BOPD) (BCPD) (BWPD) (%) Avg Min Max (°F) (psi) (psi/ft) 

Ada-Sibley Yes M Comb (FA, FC) 1936 1954: CV 9,900 
Athens B Struct (FA) 1941 1948: Vaughn 8,500-8,544 10,500 254 41,338:1 

1949: "B" 8,464-8,494 12,000 156 76,923:1 1950:Bodcaw 8,148-8,186 694 2 347,000:1 1951: "D" 8,145-8,170 3,370 208 16,201:1 Bayou Middlefork Yes B Struct (A) 1953: Bodcaw 7,764 191 210 00,909:1 Bear Creek-Bryceland Yes M Struct (A) 1937 1966 CV 10,700 

Beekman No Comb (N, FC) 1942 1942: Cv 3,700-3,711 1,500 35 28 42,800:1 Benton Yes B Struct (A) 1944 1944: "D" 8,001-8,040 3,280 164 20,000:1 18 136 190 3,765 0.47 0.17 GWC -7,818 

1945: Bodcaw 8,137-8,148 1,306 127 10,286:1 14 85 190 3,725 0.44 OWC -7,876 Blackburn Yes B Comb (N, FC) 1953 1953: Bodcaw 8,717 1,301 54 24,092:1 E. Blackburn Yes B 1959 Cadeville Yes B 1955 9,700 Calhoun No B Struct (FA) 1948 1948: "D" 9,500 814 22 37,000:1 17 4,000 0.47 

Comb (FA, FC) 1957: Cadeville 9,121-9,124 4,779 1,148 4,162:1 15 2,132 8,201 0.86 0.05 

Carlton Yes B Struct (A) 1953 1953: Bodcaw N. Carlton No B Comb (A,FC) 1964: Purdy 8,950 

1965: CV ? 9,470 Cartwright Yes B 1960 

Caspiana Yes M Comb (N, FC) 1975: Cotton Valley 8,500 Cheniere No B Comb (N, FC) 1962 1962: Cadeville 9,682-9,697 4,401 528 8,335:1 8,188 0.84 

1963: CV "A" 9,603-9,609 1,230 8 153,750:1 

Choudrant Yes B Struct (A) 1946 1946: "D" 9,097--9129 4,732 211 21,000:1 19 250 Separate GWCs in 2 

Clay Yes M Struct (A) 1952: CV 9,700 Cotton Valley No B Struct (A) 1922 1937: Bodcaw 8,170 TD (OH) 5,323 455 11,700:1 16 121 775 231 4,000 0.49 0.15 GWC @ -8,420 

1937: Davis 8,521-8,551 (OH) 4,800 400 12,000:1 15 280 4,368 0.51 0.10 1938: "D" 8,502-8,532 1,020 1,200 00,850:1 18 150 3,926 0.43 1949: Justiss 9,050 16 34 4,700 0.55 0.22 

"C" Taylor 

D'Arbonne Yes B 1947 1947: Bodcaw 8,157 4,100 88 46,590:1 

Dixie Yes B 1929 Downsville Yes B 1948 S. Downsville No B Struct (A) 1961 1961: Vaughn 8,900 S. Drew Yes B Strat (FC) 1972 1972: "D" 9,061-9,069 2,560 85,000:1 12 20 200 4,850 0.53 0.40 3 GWCs in "D" ss 

1976: Vaughn 9,525-9,531 873 15 58,200:1 10 8 200 4,250 0.45 0.40 

Elm Grove Yes M Struct (FA) 1973 Cotton Valley 7,768 8 1 247 4,154 0.53 0.45 

Greenwood-Waskom No B 1924 Haynesville Yes B Struct (A) 1921 1944: Taylor 8,835-8,920 373 01068:1 3,870 0.43 

1945: Camp 7,980-8,004 264 00500:1 

E. Haynesville Yes B 1945 1949: Tucker 8,588-8,600 2,898 276 10,500:1 Hico-Knowles No B Hico No B Struct (FA) 1946 1946: Vaughn 8,525-8,556 8,240 121 68,100:1 17 3,686 0.42 Multiple GWCs 

1946: Bodcaw 8,287-8,345 892 41 21,649:1 18 4,061 0.47 1949: Feazel-McFearin 8,914-8,929 2,880 308 9,350:1 15 5,616 0.63 

Knowles No B Struct (A) 1945 1945: Vaughn 8,700-8,750 5,212 139 37,500:1 3,500 0.40 

1946: Bodcaw 8,630-8,670 8,516 158 53,900:1 1948: McCrary 8,912-8,924 6,762 761 8,886:1 

1953: Feasel-McFearin 8,996-9,008 7,000 275 25,450:1 
Table 2. Data on Cotton Valley Group sandstone fields in northern Louisiana—Continued. 

Field FERC Trend Trap Disc CV disc date: ss Depth CV perfs IP (CV) GOR Por Perm (mD) BHT BHP FPG Sw GWC 

“tight” date (ft) (MCFD) (BOPD) (BCPD) (BWPD) (%) Avg Min Max (°F) (psi) (psi/ft) 

Homer Yes M 1919 Ivan Yes B 1952 

Lake Bistineau Yes M 1916 Leatherman Creek Yes M Struct (FA) 1975 1975: Cotton Valley 10,400-10,800 12 1 0.30 Lisbon No B Struct (FA) 1936 1939: Vaughn 8,444-8,464 5,000 61 82,000:1 17 150 0.35 

1940: Burgess-Simmons 8,766-8,806 1,993 160 12,458:1 14 40 

N. Lisbon No B Comb (N, FC) 1941 1942: Burgess-Simmons 8,502-8,525 510 17 30,000:1 

1943: Bodcaw 7,790-7,816 19,000 17 196 

Longwood Yes M Strat (FC) 1927 1948: Bodcaw 8,350 Minden Yes M 1957 Plain Dealing Yes M 1946 Rocky Mount Yes B 1959 Ruston No B Comb (A,FC) 1943 1948: "D" 8,796-8,806 6,500 56 24,000:1 18 100 210 4,100 0.47 0.23 GWC in Vaughn ss 

1949: Bodcaw 8,707-8,730 4,263 195 21,860:1 19 150 1949: Vaughn 8,809-8,838 7,995 390 20,501:1 1949: "D" 8,674-8,706 8,250 1949: Bodcaw 8,760-8,810 15,500 1951: Feazel (Davis) 9,468-9,476 1,062 50 21,240:1 18 80 

S. Sarepta Yes B Comb (FA,FC) 1949 1949: Bodcaw 8,710 2,160 173 12,485:1 17 265 4300 0.49 0.14 

1949: Savis 9,000 13 75 4,500 0.50 0.15 1949: Ardis 9,150 12 50 4,525 0.49 0.14 

Sentell No B Comb (N, FC) 1951 1951: Bodcaw 8,320 25,500 455 56,043:1 16 50 

Shongaloo Yes B 1921 Sligo No M 1922 Sugar Creek Yes B Struct (FA) 1930 1957: Bodcaw 8,724-8,730 5,000 210 23,810:1 3,972 0.45 

1957: Vaughn 8,780-8,795 2,850 48.5 58,763:1 2,955 0.34 1958: "D" 7,917-7,925 21,000 896 23,437:1 1962: "D" 7,686-7,693 3,200 112 28,571:1 1962: McFearin 8,003-8,008 2,800 168 17,500:1 

1979: Price 9,462-9,474 520 

Terryville No B Comb (N, FC) 1954 1954: "D" 9,203-9,227 3,739 277 13,514:1 10 125 GWC in "D" ss 

1957: "C" 9,169-9,182 1,030 38 29 27,105:1 

1959: "C" 9,049-9,053 133 00,934:1 

1962: McGrary 9,354-9,362 4,000 300 13,333:1 

Tremont Yes B Comb (N, FC) 1944 1944: Bodcaw 9,060-9,080 2,235 97 23,000:1 4,200 0.46 

1971: Davis 9,633-9,706 1,145 144 7,951:1 14 34 217 6,519 0.67 0.15 

Unionville Yes B Strat (FC) 1950 1950: Vaughn 8,550 

1950: Davis 8,700 

Vernon Yes M Strat (FC) 1967: Cadeville 10,900 

Field Name of field producing from Cotton Valley Group (CV) sandstone 

FERC “tight” Did FERC designate the field “tight-gas sand” for Cotton Valley Group? 

Trend B, Blanket-sandstone trend 

M, Massive-sandstone trend 

Trap Trapping mechanism for field 

Struct, structural trap 

Strat, stratigraphic trap 

Comb, combination structural & stratigraphic trap A, anticline 

FA, faulted anticline 

FC, facies change (sandstone pinchout) 

N, structural nose 

Disc date Discovery date of field


CV disc date: ss Date of CV sandstone discovery and specific CV ss that was productive 


Depth CV perfs Depth in feet of CV perforations in discovery well for specific ss 


IP (CV) IP for specific CV ss 


GOR Gas:oil ratio 
Por Porosity (decimal) 
Perm Permeability (mD) [nonstressed] 
BHT Bottom hole temp (°F) 


BHP Bottom hole pressure (psi) 
FPG Fluid-pressure gradient (psi/ft) 


Sw Water saturation (decimal) 

GWC Information about gas-water contact 


Drive Drive mechanism


SG, solution gas 

PD, pressure depletion 

GC, Gas-cap expansion 


WD, water drive 

SGS ref report Shreveport Geological Society Referen
24 Geologic Studies of Basin-Centered Gas Systems 

the blanket-sandstone trend, as shown in figure 12, initial 

production rates range from 500 MCFD to 25,000 MCFD and 

average 5,000 MCFD. 

Gas-water contacts have been reported from seven fields 

across the blanket-sandstone trend as shown in figure 13. In 

Hico-Knowles, South Drew, and Choudrant fields, separate 

gas-water contacts for individual blanket-sandstone reservoirs 

have been identified (table 2). No gas-water contacts were 

encountered in Cheniere field as of 1963 or in Tremont field 

as of 1980 (table 2). In all other Cotton Valley fields described 

in Reference Reports by the Shreveport Geological Society 

(1946, 1947, 1951, 1953, 1956, 1963, 1987), no mention of 

fluid contacts was made. 

Massive-Sandstone Trend 

Cotton Valley sandstones in the massive-sandstone trend 

(fig. 8 and table 1) have significantly poorer reservoir properties than those in the blanket-sandstone trend. Massive Cotton 

Valley sandstones have sufficiently low permeability that they 

generally do not flow gas or liquids during open-hole DST’s, 

and they require fracture-stimulation treatment to obtain commercial rates of gas production (Collins, 1980). Commercial 

gas production from these sandstones was not achieved 

until technological advances in hydraulic fracturing occurred 

together with higher gas prices from deregulation in the 

1970’s. Consequently, development of Cotton Valley fields 

in the tight, Cotton Valley massive-sandstone trend did not 

occur until the late 1970’s and 1980’s. Cotton Valley development drilling in Elm Grove and Caspiana fields in northern 

Louisiana continues at the time this report is being written 

(Al Taylor, Nomad Geosciences, oral commun., April 2000). 

A consequence of such recent development of fields in the 

tight, Cotton Valley massive-sandstone trend is less published 

information on characteristics of these fields than on older 

fields in the blanket-sandstone trend. 

Limited Data in Published Literature 

Summary information presented by Dutton and others 

(1993) for the tight, Cotton Valley massive-sandstone trend 

across northeastern Texas and northern Louisiana indicates 

porosities in the 6- to 10-percent range. Based on measurements from cores in 11 wells in Carthage field, one of the 

largest Cotton Valley fields in northeastern Texas, Wilson and 

Hensel (1984) reported porosities ranging from 5.8 to 8.1 

percent, with an average of 6.6 percent. Associated permeabilities range from 0.02 to 0.33 mD, with an average of 

0.067 mD. From core data for 126 wells in Harrison and Rusk 

counties in northeastern Texas, Finley (1984) reported an average permeability of 0.042 mD for Cotton Valley sandstones. 

In northern Louisiana, average permeability was reported as 

0.015 mD based on data from Cotton Valley cores in 302 

wells. However, there are stratigraphic intervals within the 

tight, Cotton Valley massive-sandstone trend with significantly 

higher permeabilities. Locally, permeabilities approaching 100 

mD have been reported (Wilson and Hensel, 1984). 

Significantly lower porosity and permeability of tight, 

Cotton Valley massive sandstones relative to blanket sandstones is reflected in poorer production characteristics. The 

average flow rate prior to fracture-stimulation treatment is 

50 MCFD (Dutton and others, 1993). Post-stimulation rates 

generally are in the 500 to 2,500 MCFD range, although 

rates as high as 10,000 MCFD and 11,700 MCFD have been 

reported from Bethany field (Jennings and Sprawls, 1977) and 

Carthage field (Meehan and Pennington, 1982), respectively. 

There are few published data on presence of gas-water 

contacts or production of water without gas on the flanks 

of Cotton Valley fields in the tight, massive-sandstone trend. 

Summary data presented by Nangle and others (1982) 

described gas-water contacts as poorly defined with long transition zones in contrast to short, well-defined transition zones 

with sharp gas-water contacts in the blanket-sandstone trend. 

Dutton and others (1993) also suggested the presence of gaswater contacts with long transition zones by indicating that 

calculated water saturations should be less than 40 percent to 

achieve successful gas completions from Cotton Valley sandstone intervals 200 ft above the free-water level. 

In northeastern Texas, where most of the drilling for 

tight Cotton Valley sandstones has occurred, the best reservoir 

potential is reported to be in wave-dominated deltaic sandstones of the Taylor sandstone in the lower part of the Cotton 

Valley interval (Wescott, 1983, 1985). In Oak Hill field, production logs show that Taylor sandstones contribute more than 

80 percent of the gas production and that sandstones in the 

middle and upper Cotton Valley section contribute most of 

the water production, although they produce significant gas 

as well (Tindall and others, 1981). Presley and Reed (1984) 

and Dutton and others (1993) both report the presence of 

water-bearing sandstones in the upper Cotton Valley interval. 

To avoid production of water from these sandstones, fracturestimulation treatments in stratigraphically adjacent gas-bearing 

sandstones in the upper Cotton Valley must be significantly 

smaller than those in the Taylor sandstone. At Bethany field, 

several wells reportedly were plugged because of production 

of salt water from Cotton Valley sandstones (Jennings and 

Sprawls, 1997). 

Analysis of Drill-Stem-Test 

and Production-Test Data 

As mentioned above, general statements in published 

reports suggest the presence of gas-water contacts in fields 

that produce gas from tight Cotton Valley sandstones across 

northeastern Texas and northern Louisiana. Unlike data for the 

Cotton Valley blanket-sandstone trend, however, no documentation was found identifying specific gas-water contacts in Cotton 

Valley sandstones in any of the tight-gas-sandstone fields in 

Texas or Louisiana. In the absence of such published data, and 
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Cotton Valley sandstone fieldInitial rates of gas production (MCFD) from differentCotton Valley sandstones without fracture stimulation

BENTON

3,2801,306

LEGEND

INITIAL RATES OF GAS PRODUCTION

COTTON VALLEY GROUP BLANKET SANDSTON

S SAREPETA

2,160

BENTON

3,2801,306

SENTELL

25,500

COTTON

VALLEY

5,3234,9001,020

BLACKBURN

1,301

E HAYNESVILLE

2,898

LISBON

5,0001,993

ATHENS

12,00010,500

3,370

694

SUGAR

CREEK

21,000

5,0003,2002,8502,800

520

TERRYVILLE

4,0003,7391,030

TREMONT

2,2351,145

CALHOUN

4,779

814

CHENIERE

4,4011,230

NE LISBON

19,000

510

HICO KNOWLES

8,5167,0006,7625,2122,880

RUSTON

15,500

8,2507,9956,5004,2631,062

CHOUDRANT

4,732

S DREW

2,650

873

Blanket-sandstone

trend

Figure 12. Map of northeastern Texas and northwestern Louisiana showing initial rates of gas production from Cotton Valley Group blanket sandstones. Muflow rates for a given field represent rates from different stacked blanket-sandstone reservoirs that produce in that field. All rates are from blanket sandstofracture-stimulation treatment for commercial production. 

5
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KNOWN FIELDS WITH GAS-WATER CONTACT

IN COTTON VALLEY GROUP SANDSTONES 

BENTON 

COTTON 

VALLEY HICO 

KNOWLES 

TERRYVILLE 

RUSTON 

CHOUDRANT 

S DREW 

Blanket-sands

trend 

5

Figure 13. Map of northeastern Texas and northwestern Louisiana showing fields productive from Cotton Valley Group sandstones in which gas-water con

identified and reported in published literature. Presence of gas-water contacts in these fields suggests that they are conventional gas accumulations. Deta

contacts, including sources of information, are shown in table 2. 
Is There a Basin-Centered Gas Accumulation in Cotton Valley Group Sandstones, Gulf Coast Basin, U.S.A.? 27 

considering the difficulties of using wireline logs to evaluate 

water saturations in tight Cotton Valley sandstones, an attempt 

was made to document the presence or absence of gas-water 

contacts through analysis of data from DST’s and production 

tests. The goal was to determine if Cotton Valley fields that 

produce from tight-gas sandstones are flanked by dry holes that 

tested water without gas, suggesting presence of a gas-water 

contact. A data set of wells penetrating the Cotton Valley Group 

across most of northeastern Texas and northern Louisiana was 

extracted from a database (PI/Dwights Plus, a trademark of 

Petroleum Information/Dwights, d.b.a. IHS Energy Group) for 

analysis of DST and production-test data using ArcView software. Because tight Cotton Valley sandstones generally do not 

flow fluids on open-hole DST’s, it was anticipated that the 

most useful data would be derived from production tests made 

through perforations in casing following fracture-stimulation 

treatments. Well data were sorted and displayed in map view 

using ArcView software such that wells that produce from 

Cotton Valley sandstones could be distinguished from dry holes 

with tests. While viewing the map display, test results from any 

particular well could be examined. 

Reconnaissance analysis of data from Carthage, Bethany, 

Oak Hill, Waskom, and Woodlawn fields in northeastern Texas 

and from Bear Creek–Bryceland, Elm Grove, and Caspiana 

fields in northern Louisiana revealed few dry holes penetrating 

Cotton Valley strata on the flanks of these Cotton Valley fields. 

No flanking dry holes were found that tested only water. The 

few Cotton Valley dry holes present generally did not report 

tests, suggesting that no tests were performed in those wells 

and that, most likely, the wells were plugged based on evaluation of wireline logs. 

Test results from Cotton Valley sandstones in Oak Hill 

field in Texas and Elm Grove–Caspiana fields in Louisiana 

were evaluated more rigorously, revealing several general patterns. Initial rates of gas production generally are higher in 

crestal wells than in flank wells in these fields, as shown 

for Caspiana field in figure 14. At both Oak Hill and Elm 

Grove–Caspiana fields, initial rates of gas production from 

Cotton Valley sandstones range from 1,000 to more than 4,000 

MCFD in central parts of the fields and generally are less than 

1,000 MCFD in structurally lower wells on edges of the fields. 

Many flank wells exhibit initial rates of less than 500 MCFD, 

as shown in figure 14. This trend exhibits more variability at 

Oak Hill field, where a larger number of low-rate wells occur 

in the center of the field. Such low-rate wells in the central 

part of the field could be attributed to a number of factors, 

including reservoir variability, formation damage during drilling, and poor fracture-stimulation treatments. All these wells 

must be fracture stimulated, and significant variation in success of such stimulation treatments is not uncommon. Also, 

initial rates on the western flank of Oak Hill field are high 

and show an abrupt change to dry holes rather than a gradual 

decline toward the flank of the field. One well there flowed 

gas with an initial rate exceeding 4,000 MCFD and is flanked 

to the west by four Cotton Valley dry holes. In three of 

these dry holes, Cotton Valley sandstones apparently were 

not tested, and a test in the fourth well must have resulted 

in noncommercial production with only “one unit of gas” 

reported. 

Initial rates of water production in BWPD (barrels of 

water per day) also were mapped at Oak Hill and Elm Grove– 

Caspiana fields and show no obvious patterns across these 

fields. No attempt was made to contour water production data 

for several reasons. Not only is variability in initial rate of 

water production high and seemingly random, but also, data 

are incomplete. Whereas the IHS Energy database reports initial rate of gas production for most all wells in these fields, 

initial rate of water production is not reported for a significant 

percentage of wells. In wells at Oak Hill or Elm Grove– 

Caspiana fields, for which a value is entered in the appropriate 

position of the database for water production, a null value is 

never reported. Some volume of water production seems to 

occur along with gas in all these wells. Hence, it does not seem 

appropriate to interpret absence of water-production data for a 

given well as meaning zero production of water. Absence of 

data on initial water production is more prevalent for Oak Hill 

field, and that factor alone makes it difficult to analyze water 

production from that field. Data on initial water production 

at Elm Grove–Caspiana fields are more complete. Although 

initial rates of water production were considerably higher at 

Caspiana field, data were more complete for that field, and patterns of initial water production at Caspiana field were evaluated by plotting barrels of water produced per MMCFG (bbl 

water/MMCFG). As shown in figure 15, wells in the central 

part of Caspiana field commonly exhibit production of 100 or 

fewer bbl water/MMCFG. Progressing outward toward flanks 

of the field, rates of initial water production increase to 300 

bbl water/MMCFG and eventually to more than 600 bbl water/ 

MMCFG. The highest initial rate of water production occurs 

in a well on the western flank of the field where production 

of 1,477 bbl water/MMCFG is reported (fig. 15). That same 

well had an initial rate of gas production of only 325 MMCFD, 

as shown in figure 14. Nevertheless, no wells that tested water 

only without gas from Cotton Valley sandstones were identified on the flanks of Caspiana field to suggest the presence of 

a gas-water contact for the field. Twenty-one Cotton Valley dry 

holes were identified surrounding Elm Grove–Caspiana fields. 

Of these, 19 wells reported no tests in the Cotton Valley sandstone interval, presumably indicating that no Cotton Valley 

tests were run and that Cotton Valley completions were made 

on the basis of wireline-log evaluation. Production tests after 

fracture-stimulation were run in two other wells. One reported 

“one unit of gas and one unit of oil,” presumably indicating 

noncommercial rates. The other well reported only “one unit of 

water,” suggesting that the Cotton Valley sandstone might be 

below a gas-water contact at that location. On the southern and 

western flanks of Oak Hill field, six Cotton Valley dry holes 

without tests were identified, again suggesting abandonment 

of Cotton Valley potential based on wireline-log evaluation. 

Production tests were run in Cotton Valley sandstones in two 

wells on the western flank of Oak Hill field. One reported “one 

unit of gas,” the other “one unit of gas and one unit of water.” 
 

 

R 13 W 

T 15 NT 14 N

583

250

1081

666

200 1587

980 2100 264

1350

3100 730

1250 548

938 1736 905 400 700

605

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1512

1464

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1908

900 325

1450

1304

1200

1439

1250

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2618

2723

2110

2232

1 unit gas 1 unit gas

1850

1170 1020

2108

860 1454

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3230

2014 950

2500 1250

980

4850 2883

1591

1776

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1150

1750 1832

1687 1725

2376

2667 3050

1500

1500

956 185 1200

2006

2006

927

925

200 762

1700

1117

1035

1592

2256

1400

No Cotton V No

Cotton

Valley tests alley tests

2450 482

2500

2900

2100

2041

2022

500

150

785

583

250

1081

666

200 1587

980 2100 264

1350

3100 730

1250 548

938 1736 905 400 700

605

800 1 561

1512

1464

1800

1908

900 325

1450

1304

1200

1439

1250

1410

900

1300

2618

2723

2110

2232

1 unit gas

1850

1170 1020

2108

860 1454

2000

2061

3230

2014

950

2500 1250

980

4850 2883

1591

1776

1800

1150

1750 1832

1687 1725

2376

2667 3050

1500

1500

956 185 1200

2006

2006

927

925

200 762

1700

1117

1035

1592

2256

1400

TD Cotton Valley Gp.

No Cotton Valley Gp. tests

2450 482

2500

2900

2100

2041

2022

500

150

785

2000

1000

2000

4000 2000

1000

2000

1000

2000

1000

3000

1000

2000

1000

3000

1000

1000

1000

1000

2000

2000

3000

Figure 14. Map of Caspiana field in northwestern Louisiana in the tight, Cotton Valley Group massive-sandstone trend showing initial rate of gas productioValley sandstone reservoirs. Data from PI/Dwights Plus, a trademark of Petroleum Information/Dwights, d.b.a. IHS Energy Group. Contour interval is 1,000 Mdecrease in initial rates of gas production from center to flanks of field. 

R 12 W
Figure 15. Map of Caspiana field in northwestern Louisiana in the tight, Cotton Valley Group massive-sandstone trend showing ratio of initial production rat

production rate of gas (MMCFGD) from Cotton Valley Group sandstones. Data from PI/Dwights Plus, a trademark of Petroleum Information/Dwights, d.b.a. IH

interval is 100 BWPD/MMCFGD. Mapped data show progressive increase in ratio of initial production rates of water to gas from crest to flanks of field. This s

sandstones, gas saturation is decreasing and water saturation is increasing from crest to flanks of field and indicates presence of vertically extensive gas-w

R 13 W 

T 15 N

T 14 N

E

600

500 400

300 300

400

500

600

700

800

500400

600 500

600

1000

500 400

300 200

100

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200

200

600400

100

200

200 300

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400

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R 12 W
30 Geologic Studies of Basin-Centered Gas Systems 

On the southern flank of the field, production tests were run 

in Cotton Valley sandstones in two Cotton Valley dry holes, 

but no results were reported. Evaluation of test data from Oak 

Hill and Elm Grove–Caspiana fields, therefore, provides no 

definitive information regarding presence or absence of gaswater contacts in these Cotton Valley fields. 

Discussion of Evidence For 

and Against Basin-Centered Gas 

Source Rocks and Burial and Thermal History 

Source rocks responsible for generating gas in basincentered gas accumulations commonly are in stratigraphic 

proximity to low-permeability reservoirs that they are charging 

with gas. As described above, there are few published data on 

source rocks responsible for generating gas found in Cotton 

Valley sandstone reservoirs in both the blanket- and massivesandstone trends. However, the marine Bossier Shale, stratigraphically directly beneath Cotton Valley sandstones, and 

Smackover laminated lime mudstones, lying below the Bossier 

Shale, are considered to be source rocks capable of generating 

gas for Cotton Valley sandstone reservoirs. Gray to black 

marine shales interbedded with Cotton Valley sandstones also 

are considered to be potential source rocks. Also, as summarized above, burial- and thermal-history data for the northern 

Gulf Coast Basin suggest that burial depths of Bossier and 

Smackover source rocks, in conjunction with the regional geothermal gradient, have been sufficient to generate dry gas. Time 

of generation of much of the gas postdates development of both 

the Sabine uplift and structures in the East Texas and Louisiana 

Salt Basins. Hence, available data on presence of source rocks, 

burial and thermal history of source rocks, and timing of gas 

generation for Cotton Valley reservoirs would be consistent 

with interpretation of a potential continuous gas accumulation 

in massive sandstones of the Cotton Valley Group. 

Porosity, Permeability, 

and Gas-Production Rates 

Basin-centered gas accumulations commonly involve 

large volumes of gas-saturated rock in which gas cuts across 

stratigraphic units. Such gas accumulations require a regional 

seal to trap gas, and that seal characteristically is provided by 

inherent low permeability of reservoir rocks themselves. Thus, 

continuous gas reservoirs characteristically have low permeability, and when reservoirs are sandstones, they commonly are 

referred to as tight-gas sandstones. 

As described above, Cotton Valley sandstone reservoirs 

across the northern Gulf of Mexico Basin can be divided into 

two groups based on reservoir properties and associated rates 

of gas production. Sandstones in the Cotton Valley blanketsandstone trend across northernmost Louisiana have porosities 

in the 10 to 19 percent range and permeabilities from 1 to 280 

mD (table 1). These sandstones generally flow gas and (or) 

liquids during open-hole DST’s. Gas-productive sandstones 

flow at initial rates ranging from 500 to 25,000 MCFD without 

fracture-stimulation treatment. Consequently, these sandstones 

are not tight-gas reservoirs, and many fields producing from 

Cotton Valley sandstones in the blanket-sandstone trend were 

excluded from tight-gas status by FERC in 1980 (fig. 8). 

Therefore, in the absence of some other regional top seal that 

could allow development of basin-wide overpressure, sandstones in this trend would not be expected to host a basincentered gas accumulation. 

South of the blanket-sandstone trend in northern Louisiana lies the Cotton Valley massive-sandstone trend that extends 

westward across the Sabine uplift into northeastern Texas, as 

shown in figure 8. Cotton Valley massive sandstones generally 

have porosities in the 6 to 10 percent range with permeabilities 

commonly less than 0.1 mD. Most of these sandstones, therefore, would be defined as tight-gas sandstones, and most fields 

producing gas from these sandstones were designated as tightgas-sandstone fields by FERC in 1980. Tight, Cotton Valley 

massive sandstones generally do not flow gas and (or) liquids 

on open-hole DST’s, and they require hydraulic-fracture treatment to produce gas at commercial rates. As shown in table 

1, prestimulation initial-production rates generally range from 

too small to measure (TSTM) to 300 MCFD. Post-stimulation 

rates commonly are 500 to 2,500 MCFD. Although higher permeability intervals occur locally within the massive-sandstone 

trend, as noted by Wilson and Hensel (1984), the characteristic 

low permeability of sandstones throughout this trend suggests 

that they might have potential to provide their own seal for gas 

in a continuous gas accumulation. 

Abnormal Pressures 

In a study of abnormally high pressures in basin-centered 

gas accumulations in Rocky Mountain basins, Spencer (1987) 

considered reservoirs to be significantly overpressured if 

FPG’s exceed 0.50 psi/ft where waters are fresh to moderately 

saline, and 0.55 psi/ft where waters are very saline. With 

formation-water salinity of Cotton Valley sandstone reservoirs 

on the order of 170,000 ppm TDS (Dutton and others, 1993), 

salinity is considered high, and reservoirs should be considered 

to be overpressured if their FPG’s exceed 0.55 psi/ft. 

Based on Spencer’s (1987) cutoff value of 0.55 psi/ft, 

abnormally high reservoir pressures have been encountered in 

Cotton Valley sandstones in an area of northeastern Louisiana, 

shown in figure 10, where calculated pressure gradients of 0.63 

to 0.86 psi/ft occur. The boundary between areas of overpressure and normal pressure cuts across the permeable, blanketand tight, massive-sandstone trends such that overpressures 

occur within both reservoir trends. (figs. 8 and 10). Although 

overpressures associated with generation of gas might be antic-
Is There a Basin-Centered Gas Accumulation in Cotton Valley Group Sandstones, Gulf Coast Basin, U.S.A.? 31 

ipated in tight Cotton Valley sandstones, such overpressures 

would not be expected to develop in high-permeability blanket 

sandstones without a subregional top seal stratigraphically 

above the sandstones. As shown in figure 9 and table 2, some 

of the separate, stacked blanket sandstones within Hico, Tremont, and Calhoun fields are overpressured, whereas others are 

normally pressured. Examination of discovery dates of gas 

in individual sandstones shows that, in all cases for these 

three fields, normally pressured sandstone reservoirs were discovered prior to overpressured ones. Thus, pressure differences among individual blanket-sandstone reservoirs indicate 

presence of separate, compartmentalized reservoirs rather than 

pressure depletion from production of gas from different sandstones that are in pressure communication. Additionally, normally pressured Cotton Valley sandstones were encountered at 

South Drew field, whereas, at Cheniere field immediately to 

the west, Cotton Valley sandstones were significantly overpressured with an FPG of 0.86 psi/ft. Thus, for gas fields in 

the blanket-sandstone trend where data are abundant, reservoir 

pressures exhibit significant variation from normal to abnormally high among separate sandstone reservoirs within individual gas fields and also between adjacent fields. Such compartmentalization of overpressured reservoirs in proximity to 

normally pressured ones, rather than development of overpressure on a regional scale, is more indicative of conventional gas 

fields than basin-centered gas accumulations. 

Within the western half of the blanket-sandstone trend 

and spanning the vast majority of the tight, massive-sandstone 

trend across northwestern Louisiana and northeastern Texas, 

FPG’s range from 0.32 to 0.55 psi/ft and, therefore, would be 

considered normal, according to the methodology of Spencer 

(1987). However, two episodes of erosion have occurred in 

northeastern Texas, one in late mid-Cretaceous time and the 

second in early mid-Tertiary time (Dutton, 1987; Laubach and 

Jackson, 1990; Jackson and Laubach, 1991). During late midCretaceous time, maximum erosion occurred on the crest of 

the Sabine uplift where approximately 1,800 ft of sedimentary 

section were removed. Tertiary erosion resulted in removal of 

about 1,500 ft of section across much of northeastern Texas. 

Burial-history data for the Ruston field area in northern Louisiana on the boundary between overpressured and normally 

pressured regions, show about 1,500 and 500 ft of uplift and 

loss of section, respectively, in two erosional periods (Herrmann and others, 1991). It is possible, therefore, that, with 

deeper burial, reservoir pressures in much or all of the massive-sandstone trend could have been higher and that pressure 

reduction has occurred as a result of uplift and erosion. However, some of the gas found in Cotton Valley sandstone reservoirs could have been derived from Bossier Shale source 

rocks. Migration of most of that gas into Cotton Valley sandstones probably commenced between 57 and 45 Ma (Dutton, 

1987; Hermann and others, 1991). Therefore, if basin-wide 

overpressure in Cotton Valley sandstones were to have developed in response to thermal generation of gas from Bossier 

Shale source rocks, its development would have postdated both 

the Cretaceous and Tertiary erosional events. 

The sharp boundary between overpressured and normally 

pressured areas of Cotton Valley sandstones (fig. 10) and presence of overpressure in both permeable, blanket-sandstone 

and tight, massive-sandstone trends, suggest that abnormally 

high pressures encountered in Cotton Valley sandstones in 

northeastern Louisiana are not caused by thermal generation 

and migration of gas. Coleman and Coleman (1981) attributed 

development of overpressures in Cotton Valley sandstones 

across the region shown in figure 10 to a late stage of diagenesis in which extreme pressure, presumably overburden pressure, and temperature caused dissolution of silica at contact 

points of quartz-sand grains and precipitation of silica in adjacent pores. With pore waters apparently unable to escape, 

porosity reduction associated with this late-stage chemical 

compaction reportedly resulted in development of overpressure 

in Cotton Valley sandstones across the area shown in figure 

10. According to Coleman and Coleman (1981), a significant 

factor in preventing fluid loss from Cotton Valley sandstones 

during this late diagenetic episode was presence of a tight top 

seal provided by the Knowles Limestone and upper Cotton 

Valley–lower Hosston shales. 

If late-stage chemical compaction and cementation in 

conjunction with a top seal of tight limestone and shale are 

responsible for development of overpressure, it is not clear 

why the geographic distribution of overpressure exhibits the 

pattern shown in figure 10. Perhaps an alternative mechanism 

for generating the distribution of overpressures within Cotton 

Valley sandstones shown in figure 10 could be one reported 

by Parker (1972) as the cause for overpressures in Jurassic 

Smackover sandstone and carbonate reservoirs to the east in 

Mississippi. Parker (1972) noted that much of the Smackover 

gas is sour and has a relatively high content of CO2 and 

(or) N2. He suggested that migration of gases derived from 

Late Cretaceous emplacement of the Jackson (igneous) dome 

(fig. 2) might be responsible for “inflation” of pressures in 

well-sealed Smackover reservoirs. Specifically, Jones (1977) 

suggested that H2S and CO2 present in Smackover gas 

in Mississippi were derived from Smackover anhydrite and 

limestone/dolomite, respectively, which had been subjected 

to magmatic intrusion. The mapped pattern of overpressured 

Cotton Valley sandstones (fig. 10) extends east-southeastward 

into Mississippi directly toward the location of Jackson dome 

(fig. 2) (Studlick and others, 1990). Evidence supporting such 

a mechanism of overpressure development in Cotton Valley 

sandstones of northeastern Louisiana would be documentation 

of elevated levels of CO2 and (or) N2 in overpressured Cotton 

Valley sandstone reservoirs. 

In summary, within most of the tight, Cotton Valley 

massive-sandstone trend across northwestern Louisiana and 

northeastern Texas, Cotton Valley reservoirs are slightly, but 

not significantly, overpressured. Based on the methodology and 

terminology of Spencer (1987), these reservoirs would be characterized as normally pressured. Basin-centered, continuous 

gas accumulations commonly are significantly overpressured. 

Although pressure data for the tight, Cotton Valley massivesandstone trend are not definitive, they tend to suggest that a 
32 Geologic Studies of Basin-Centered Gas Systems 

basin-centered gas accumulation characterized by abnormally 

high pressures from geologically recent to present-day thermal 

generation of gas does not occur within Cotton Valley sandstone 

reservoirs. 

Gas-Water Contacts 

Perhaps the most definitive criterion for establishing the 

presence of a continuous gas accumulation is absence of gaswater contacts. Gas-water contacts are distinctive features of 

conventional gas accumulations. Presence of a gas-water contact indicates a change from gas-saturated to water-saturated 

porosity within a particular reservoir unit. This implies that a 

well drilled into that reservoir structurally below the gas-water 

contact should encounter only water, thereby demonstrating 

the absence of a continuous gas accumulation in that immediate area. 

Within the blanket-sandstone trend across northernmost 

Louisiana, gas-water contacts have been reported in seven 

fields, as shown in figure 13. Because of relatively high 

porosity and permeability in blanket sandstones, gas-water 

contacts are sharp and often are reported as depth below 

sea level to the nearest foot. Separate gas-water contacts for 

individual, stacked blanket sandstones have been identified 

in Hico-Knowles, South Drew, and Choudrant fields (table 

2). The seven fields in which gas-water contacts have been 

described are distributed across the blanket-sandstone trend 

(figure 13). Because of the relatively uniform distribution 

of high-permeability Cotton Valley sandstone reservoirs with 

conventional shale seals in fields across the blanket-sandstone 

trend, it is likely that all Cotton Valley fields in this trend have 

well-defined gas-water contacts similar to those documented in 

the seven fields shown in figure 13. The Cotton Valley blanketsandstone trend was defined as a continuous gas accumulation 

in the 1995 National Assessment of United States Oil and Gas 

Resources by the USGS (Schenk and Viger, 1996). However, 

the presence of abundant gas-water contacts across this area 

suggests that the blanket-sandstone trend should be redefined 

as a conventional gas play. 

Evaluating the presence or absence of gas-water contacts 

in the tight, Cotton Valley massive-sandstone trend is more 

difficult. No reference to specific gas-water contacts for Cotton 

Valley massive sandstones in any Cotton Valley gas field has 

been found in the published literature. Nangle and others 

(1982) and Dutton and others (1993), however, make general 

statements indicating that gas-water contacts are present in 

Cotton Valley fields across the tight, Cotton Valley massivesandstone trend. 

Although Taylor sandstones in the lower part of the 

Cotton Valley section produce gas in all significant Cotton 

Valley fields in the tight, massive-sandstone trend, waterbearing sandstones have been reported along with gas-charged 

sandstones in the middle and upper Cotton Valley interval in 

some fields. The seal for gas in wave-dominated deltaic Taylor 

sandstones at Waskom field reportedly is provided by marsh 

and lagoonal shales (CER Corporation and S.A. Holditch & 

Associates, 1991). This seal would be considered conventional 

rather than one provided by the low permeability of the reservoir sandstones. Along with Taylor sandstones, most of the 

upper Cotton Valley sandstone interval produces gas at some 

fields, such as Carthage field, according to Al Brake (BP, 

oral commun., 2000). At other fields, such as Woodlawn and 

Blocker, however, gas is produced only from lower Cotton 

Valley Taylor sandstones and from a few sandstones in the 

uppermost Cotton Valley section. Intervening middle and 

upper Cotton Valley sandstones are reportedly water-bearing. 

The presence of individual gas-bearing and water-bearing 

sandstone intervals, separated by conventional shale seals, suggests the presence of gas-water contacts and is more indicative 

of conventional gas accumulations than of continuous gas 

accumulations. 

The complex diagenetic mineralogy of tight Cotton Valley 

sandstones probably precludes use of wireline logs to identify 

gas-water contacts. As reported above, complex diagenetic 

mineralogy of tight Cotton Valley sandstones dramatically 

affects values of resistivity and porosity measured by wireline 

logs and, hence, the determination of water saturation by standard calculation techniques. Because of vertical and lateral 

diagenetic variations, accurate determination of water saturation is difficult without accompanying lithologic data from 

cores or cuttings to calibrate wireline logs. Examination of 

production-test data from wells flanking many Cotton Valley 

gas fields in the tight-gas-sandstone trend reveals no dry holes 

that tested water without gas. Therefore, even if wireline 

logs provided accurate estimates of water saturations in tight 

Cotton Valley sandstones, few wells apparently exist in which 

logs could be used to identify gas-water contacts. 

Reconnaissance evaluation of DST and production-test 

data from Cotton Valley sandstones in a number of fields in 

the tight-gas-sandstone trend revealed few dry holes penetrating Cotton Valley sandstones on flanks of those fields. No 

dry holes were found that tested water without gas, thereby 

implying existence of a gas-water contact for a particular field. 

Likewise, detailed examination of test data from all wells 

within and flanking Oak Hill and Elm Grove–Caspiana fields 

in the tight-gas-sandstone trend revealed no flank dry holes 

that tested water without gas. 

Initial rates of gas production from wells on the flanks 

of Cotton Valley fields in the tight-gas, massive-sandstone 

trend, however, generally are lower than from crestal wells, as 

illustrated for Caspiana field in figure 14. Also, as shown for 

Caspiana field in figure 15, the ratio of initial rate of water production to initial rate of gas production in terms of bbl water/ 

MMCFG is significantly higher in flank wells. Initial rates of 

gas production from crestal wells commonly range from 1,000 

to more than 4,000 MCFD and the ratio of initial rate of water 

to gas generally is less than 200 bbl water/MMCFG and often 

below 100 bbl water/MMCFG (figs. 14 and 15). Initial rates 

of gas production from flank wells generally are less than 

1,000 MCFD, and water production initially is significantly 

higher, usually in the 300 to 600 bbl water/MMCFG range, 
Is There a Basin-Centered Gas Accumulation in Cotton Valley Group Sandstones, Gulf Coast Basin, U.S.A.? 33 

SCHEMATIC DIAGRAM OF TRANSITION ZONES IN


HIGH- AND LOW-PERMEABILITY SANSTONE RESERVOIRS


CROSS SECTION FLUID SATURATION 

OF STRUCTURE 

HIGH PERMEABILITY LOW PERMEABILITY 

(Cotton Valley blanket sandstones) (Cotton Valley massive sandstones) 

WATER 

GAS 

0 50 100 

% Sw 

TRANSITION ZONE 

HEIGHT 

WATER 

GAS 

GAS-WATER CONTACT 

WATER 

GAS 

(AND WATER) 

0 50 100 

% Sw 

Figure 16. Schematic diagram of gas-water transition zones in high- and low-permeability reservoirs. Modified from Levorsen (1967). HEIGHT


but sometimes exceeding 1,000 bbl water/MMCFG (figs. 14 

and 15). These data suggest a decrease in gas saturation and 

an accompanying increase in water saturation in Cotton Valley 

sandstones from crestal wells to flank wells and that a commercial limit to gas production has been reached, although 

gas-water contacts have not been encountered. 

Corroborating these suggestions from figures 14 and 15 

and data reported by Nangle and others (1982) is the experience 

of Al Taylor (Nomad Geosciences, oral commun., 2000) who 

reports the presence of vertically extensive gas-water transition 

zones in Cotton Valley sandstone fields in the massive-sandstone 

trend. In moving structurally lower from the crest of one of 

these tight-gas Cotton Valley fields through the long gas-water 

transition zone toward the presumed gas-water contact, gas saturation of sandstone reservoirs continually decreases while water 

saturation simultaneously increases (fig. 16). Wells that are low 

in the transition zone on the edges of Cotton Valley fields in the 

tight-gas-sandstone trend exhibit low initial rates of gas production and high initial rates of water production, as shown by some 

flank wells at Caspiana field in figures 14 and 15. Hyperbolic 

decline rates of gas production in conjunction with lower gas 

saturations of reservoir sandstones in these transition-zone wells 

result in such low cumulative production of gas that these wells 

are marginally commercial to noncommercial and, in effect, are 

dry holes (Al Taylor, Nomad Geosciences, oral commun., 2000). 

Hence, commercial limits of gas production are reached before 

gas-water contacts can be encountered by development drilling. 

This probably could be documented by mapping cumulative gas 

production or estimated ultimate gas recovery from wells in 

these Cotton Valley fields. 

Knowing gas saturations of Cotton Valley reservoir sandstones from log calculations and capillary properties of those 

sandstones from core analyses at Caspiana field in northwestern Louisiana, Al Taylor (Nomad Geosciences, oral commun., 

2000) estimated gas-column heights required to produce those 

gas saturations. From column-height data, he determined the 

position of gas-water contacts below sea level. Estimates made 

in this fashion for the structural level of the gas-water contact 

at Caspiana field using data from a number of wells cluster 

within a zone about 75 ft thick, suggesting the presence of 

a single gas-water contact for the field. A Cotton Valley well 

situated structurally below this estimated gas-water contact 

reportedly tested water without gas from Cotton Valley sandstones. Because of the vertically extensive gas-water transition 

zones present in fields producing gas from tight Cotton Valley 

sandstones in the massive-sandstone trend, Al Taylor (Nomad 

Geosciences, oral commun., 2000) suggests that structural or 

stratigraphic traps with less than 150 ft of vertical closure will 

not have sufficient gas saturation to produce gas at commercial 

rates. 
34 Geologic Studies of Basin-Centered Gas Systems 

In summary, Cotton Valley blanket sandstones across 

northernmost Louisiana have sufficiently high porosity and 

permeability that gas accumulations exhibit short transition 

zones and have sharp gas-water contacts. Gas fields in this 

trend have clearly defined productive limits, beyond which 

wells produce water only. However, low-permeability Cotton 

Valley sandstones in the tight-gas massive-sandstone trend 

across northern Louisiana, the Sabine uplift, and East Texas 

Basin display long gas-water transition zones with poorly 

defined gas-water contacts. Productive limits of fields in this 

trend are difficult to define based on data from production tests 

or wireline logs. In conjunction with long gas-water transition 

zones, structural dips are gentle on the flanks of these gas 

accumulations. As development drilling progresses down the 

flank of one of these fields through the long gas-water transition zone, gas saturations in the sandstone reservoir decrease 

and water saturations increase. Eventually gas saturations 

become sufficiently low that, in terms of ultimate cumulative 

gas production, wells become marginally commercial to noncommercial at a structural position still within the transition 

zone above the gas-water contact. Consequently, development 

wells on the flanks of these gas accumulations generally have 

not encountered gas-water contacts. If drilling and completion 

costs hypothetically were reduced to zero, causing even the 

smallest amount of gas recovery to be commercial, development drilling could progress down the full length of transition 

zones, and gas-water contacts probably would be encountered. 

The progressive increase in water saturation with depth within 

these tight-gas Cotton Valley fields, therefore, suggests that 

poorly defined gas-water contacts are present below the depth 

at which wells become noncommercial. The presence of gaswater contacts in both Cotton Valley blanket- and massivesandstone trends suggests that gas accumulations in these 

trends are conventional and that a basin-centered gas accumulation does not exist within Cotton Valley sandstones in the 

northern Gulf of Mexico Basin. 

Basin-Centered Gas Potential 

within Bossier Shale 

A basin-centered, continuous-gas accumulation might 

have been discovered recently in sandstones within the Bossier 

Shale, the lower formation of the Cotton Valley Group. In a 

currently developing play on the western flank of the East 

Texas Basin, gas is being produced from turbidite sandstones 

within the Bossier Shale. These turbidite sandstones probably 

are downdip time-equivalent deposits of deltaic sandstones in 

the lower portion of the Cotton Valley and reportedly were 

deposited seaward of the underlying Haynesville Formation 

carbonate platform edge in a slope or lowstand-fan setting. 

Accommodation space was provided by salt withdrawal such 

that updip and lateral traps currently are formed by pinch out 

of sandstone into shale. Two stacked, stratigraphically separate 

Bossier turbidite-fan systems reportedly occur at depths of 

13,000 to 14,000 ft. Two fields, Dew and Mimms Creek, with 

combined estimated recoverable reserves of more than one 

TCFG, currently are being developed by Anadarko Petroleum, 

one of the main operators. As of January 2000 (PI-Dwights 

Drilling Wire, Jan 3, 2000; Jan 12, 2000), Anadarko had drilled 

more than 100 wells with only one dry hole in this Bossier sandstone play. Gas-charged sandstones reportedly are overpressured, and no water has been encountered in the system (Exploration Business Journal, 2nd quarter, 2000). Within the upper 

turbidite-fan interval, porosity ranges from 6 to 15 percent and 

permeability from 0.01 to 1.0 mD. Initial production rates from 

wells average 3 to 4 MMCFGD after fracture stimulation and 

decline exponentially with estimated per-well recoveries of 1 to 

5 BCFG. In the lower sandstone interval, porosity ranges from 

9 to 20 percent, permeability from 1 to 10 mD, pressures are 

higher, and initial production rates of as much as 30 MMCFGD 

have been obtained. This play does not seem to involve the 

classic type of basin-centered gas accumulation with the trap 

produced by inherent low permeability of reservoir sandstones. 

Instead, the trap seems to be provided by marine shales that 

completely encase these turbidite sandstones, but the sandstone 

reservoirs are overpressured, seem to lack water and gas-water 

contacts, and are gas-charged over an extensive area, as demonstrated by only one dry hole in more than 100 wells drilled. 

Conclusions 

1. 	 The Cotton Valley Group represents the first major 

influx of clastic sediment into the Gulf of Mexico 

Basin. Major depocenters were located in south-central 

Mississippi, along the Louisiana-Mississippi border, 

and in northeastern Texas. Sands supplied by the ancestral Mississippi River drainage along the LouisianaMississippi border were swept westward by longshore 

currents, creating an east-west barrier-island or strandplain system across northern Louisiana that isolated 

a lagoon to the north. More than 1,000 ft of 

stacked barrier-island sands accumulated as the Terryville massive-sandstone complex. Periodic transgressive events reworked barrier-island sands, transporting 

them northward into the lagoon. These transgressive 

sandstones pinch out into lagoonal shales, can be correlated across northern Louisiana, and are referred to 

informally as blanket sandstones. 

2. 	 Two major Cotton Valley sandstone-reservoir trends 

are identified based on reservoir properties and associated characteristics of gas production. Transgressive, 

blanket sandstones across northernmost Louisiana have 

porosities ranging from 10 to 19 percent and permeabilities from 1 to 280 mD. These sandstones flow 

gas and (or) liquids during open-hole DST’s and do 

not require fracture-stimulation treatment to produce 

gas at commercial rates. Fields producing from these 
Is There a Basin-Centered Gas Accumulation in Cotton Valley Group Sandstones, Gulf Coast Basin, U.S.A.? 35 

sandstone reservoirs were developed during the 1940’s 

through 1960’s. Cotton Valley massive sandstones to 

the south, and extending westward across the Sabine 

uplift into eastern Texas, exhibit porosities from 6 to 10 

percent and permeabilities generally less than 0.1 mD. 

Designated as tight-gas sandstones by FERC, these 

reservoirs commonly do not flow gas or liquids during 

DST’s, and they require fracture-stimulation treatments 

to achieve commercial rates of production. Gas production from these sandstones in eastern Texas and 

northern Louisiana was not established until the mid 

1970’s when advances in hydraulic-fracture techniques 

occurred in conjunction with a significant increase in 

gas prices as a result of price deregulation. 

3. 	 Porosity and permeability of Cotton Valley sandstones 

are controlled by diagenetic properties, which in turn 

are governed by depositional environment. Although 

diagenetic patterns and mineralogy are complex, highenergy, clean sandstones generally are cemented by 

authigenic quartz and (or) calcite and have poor reservoir properties. In lower energy sandstones, clay 

coatings on quartz grains inhibited development of 

quartz overgrowths, resulting in preservation of primary porosity. High clay content, however, generally 

imparts poor permeability to these sandstones. The best 

reservoir sandstones are those which have experienced 

development of significant secondary porosity from 

dissolution of calcite cement and unstable framework 

grains. 

4. 	 The complex diagenetic mineralogy of tight Cotton 

Valley sandstones precludes use of standard calculation 

methods in reservoir evaluation with wireline logs. 

Bound water associated with pore-filling clays or clay 

coatings and conductive minerals such as pyrite result 

in abnormally low resistivity measurements leading to 

such high calculated water saturations that productive 

zones often appear wet. Also resulting in erroneous 

reservoir evaluations are pessimistic measurements of 

porosity with wireline logs caused by the presence 

of high-density carbonate minerals such as ankerite 

and siderite. Therefore, without lithologic data from 

cores or drill cuttings to calibrate wireline logs, such 

logs are of limited value in differentiating between 

gas-productive and wet intervals and therefore are of 

limited value in identifying gas-water contacts on the 

flanks of Cotton Valley fields. 

5. 	 Abnormally high reservoir pressures with fluidpressure gradients exceeding 0.55 psi/ft occur in 

Cotton Valley sandstones in northeastern Louisiana. 

The boundary between the overpressured area on the 

east and normally pressured region to the west cuts 

across the permeable, blanket- and tight, massive-sandstone trends such that overpressures occur within both 

reservoir trends. Within the blanket-sandstone trend, 

where pressure data are more abundant, some Cotton 

Valley fields are overpressured whereas adjacent fields 

are normally pressured. Also, within certain fields, 

some of the stacked blanket sandstones are overpressured whereas others are normally pressured. Such 

compartmentalization of overpressured reservoirs in 

proximity to normally pressured ones, rather than 

development of overpressure on a regional scale, suggests that these blanket-sandstone fields are conventional gas accumulations and not part of a basin-centered gas accumulation. Also, the occurrence of normally pressured reservoirs across the majority of the 

tight, Cotton Valley massive-sandstone trend is not 

indicative of presence of a basin-centered, continuous 

gas accumulation. Geographic distribution of overpressures in Cotton Valley sandstones suggests that overpressuring was caused by “inflation” of existing pressures in tightly sealed reservoirs by gases derived from 

emplacement of the nearby Jackson (igneous) dome. 

6. 	 Gas found in Cotton Valley sandstone reservoirs is 

believed to be derived from (1) interbedded Cotton 

Valley marine shales, (2) underlying marine shales of 

the Bossier Shale, and (or) (3) stratigraphically lower, 

Jurassic Smackover laminated, lime mudstones. These 

source rocks are believed to have been buried to sufficient depths relative to the regional geothermal gradient to have generated dry gas during the past 60 m.y. 

Timing of gas generation and migration is favorable 

because it postdates development of the Sabine uplift, 

smaller structures on and flanking the uplift, and salt 

structures in the East Texas and Northern Louisiana 

Salt Basins. Stratigraphic proximity of source rocks 

to Cotton Valley sandstone reservoirs and appropriate 

thermal maturity and time of generation and migration 

would be consistent with interpretation of a potential 

basin-centered gas accumulation. 

7. 	 Presence of a gas-water contact is perhaps the most 

definitive criterion suggesting that a gas accumulation 

is conventional rather than a “sweetspot” within a 

basin-centered, continuous gas accumulation. Within 

the Cotton Valley blanket-sandstone trend across northernmost Louisiana, short gas-water transition zones 

and well-defined gas-water contacts have been reported 

in seven gas fields. Relatively high porosity and permeability of blanket sandstones and associated high gasproduction rates achieved without fracture stimulation 

throughout the trend suggest that all gas fields within 

the blanket-sandstone trend probably have well-defined 

gas-water contacts; therefore, these gas accumulations 

are conventional. 

8. 	 Within the tight, massive-sandstone trend, porosity and 

permeability are sufficiently low that gas-water transition zones are long and gas-water contacts poorly 

defined. Productive limits of these tight-gas-sandstone 

Cotton Valley fields are not defined by wells that 

encounter a gas-water contact or test water without 

gas from a zone below a gas-water contact, as in the 

blanket-sandstone trend. With increasing depth through 
36 Geologic Studies of Basin-Centered Gas Systems 

vertically extensive gas-water transition zones, gas saturation in reservoir sandstones decreases and water 

saturation increases. Eventually gas saturations become 

sufficiently low that, in terms of cumulative gas production, wells become marginally commercial to noncommercial at a structural position still within the transition zone above the gas-water contact. Therefore, 

development wells on the flanks of gas accumulations 

in the tight, Cotton Valley massive-sandstone trend 

rarely encounter gas-water contacts. If even the smallest amount of gas recovery were commercial, development drilling probably would progress down the 

full length of transition zones, and gas-water contacts 

would be encountered in these gas accumulations. The 

presence of gas-water contacts in gas accumulations 

within the tight, Cotton Valley massive-sandstone trend 

suggests that accumulations in this trend, too, are conventional and that a basin-centered gas accumulation 

does not exist within Cotton Valley sandstones in the 

northern Gulf of Mexico Basin. 

9. 	 A basin-centered, continuous gas accumulation might 

occur in turbidite sandstones within the Bossier Shale, 

the lower formation of the Cotton Valley Group. In a 

currently developing play on the western flank of East 

Texas Basin, gas production with estimated recoverable 

reserves exceeding one TCFG is being obtained from 

sandstone reservoirs, interpreted as slope or lowstandfan deposits, that are completely encased in marine 

shales. Reservoirs are significantly overpressured and 

no water has been encountered in the system. More 

than 100 successful wells and only one dry hole have 

been drilled. 

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Manuscript approved for publication October 26, 2001


Published in the Central Region, Denver, Colorado


Editing, layout, photocomposition—Richard W. Scott, Jr.


Cover design, graphics consultation—Carol A. Quesenberry


Graphics by the authors