# Is There a Basin-Centered Gas Accumulation in Cotton V alley 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 V ito 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 T e chnology Laboratory , Morgantown, W

. V a., under contracts DE-A T26-98FT40031 and DE-A T26-98FT40032, and by the U.S. Geological Survey Central Region Energy Resources T eam



### 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 fi rm 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-T ype Gas Accumulations

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

............................................................................................... .3 Method of Evaluating Potential of Basin-Centered Gas in Cotton V alley Group Sandstones

......... 4 Geologic Setting of Cotton V alley Group

........................................................................................ ........... 4
Cotton V alley Group Stratigraphic Nomenclature

................................................................................. .. 6
Cotton V alley Group Depositional Systems

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

............................................................................................................. .............. 9
Blanket Sandstones of Northern Louisiana

.................................................................................. 11
Blanket- and Massive-Sandstone Productive T rends

................................................................. 13 Diagenesis of Cotton V alley Group Sandstones

................................................................................... . 1 3 Impact of Diagenetic Mineralogy on Wireline Logs

............................................................................. 15
Source Rocks

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

..................................................................................................... .................. 16
Abnormal Pressures

............................................................................................................. ...................... 17 History of Cotton V alley Group Sandstone Exploration

........................................................................ 17 Comparison of Blanket-Sandstone and Massive-Sandstone T rends

................................................ 20
Blanket-Sandstone T rend

........................................................................................................ ......... 20
Massive-Sandstone T rend

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

..................................................................................... 24 Analysis of Drill-Stem-T est and Production-T est 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-W ater 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 V alley Group plays identified by the U.S. Geological Survey in the 1995 National Assessment of United States oil and gas resources

......................................................................................... 2

IIIIV

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 V alley Group sandstone across northeastern T exas and northern Louisiana

............................................................... 8

5. Generalized stratigraphic nomenclature of Cotton V alley Group in northern Louisiana.............................................................................................................

........... 9

6. North-south stratigraphic cross section of Cotton V alley Group across northern Louisiana based on data from 15 wells

.................................................................. 10

7. Regional paleogeographic map showing sedimentary environments of Cotton V alley Group during deposition of uppermost Cotton V alley sandstones

............ 12

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

8. Major fields that have produced hydrocarbons from Cotton V alley Group sandstones

............................................................................................................ 14

9. Fluid-pressure gradients calculated from shut-in pressures in Cotton V a lley Group sandstone reservoirs

............................................................................... 18

10. Geographic distribution of abnormally high pressures in Cotton V alley Group sandstone reservoirs

.......................................................................................... 19

11. Measured values of porosity and permeability in Cotton V alley Group blanket sandstones

......................................................................................................... 21

12. Initial rates of gas production from Cotton V alley Group blanket sandstones

......................................................................................................... 25

13. Fields productive from Cotton V alley 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 V alley Group massive-sandstone trend showing:

14. Initial rate of gas production from Cotton V alley sandstone reservoirs

................. 28

15. Ratio of initial production rate of water to initial production rate of gas from Cotton V alley Group sandstones

.......................................................................... 29

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

.................................................................................................... .. 33



# T

ables

1. Comparison of two productive trends of Cotton V alley Group sandstones in eastern T exas and northern Louisiana

............................................................................... 20

2. Data on Cotton V alley Group sandstone fields in northern Louisiana

............................... 22

# Is There a Basin-Centered Gas Accumulation in Cotton V alley Group Sandstones, Gulf Coast Basin, U.S.A.?

By Charles E. Bartberger

, 1 

### Thaddeus S. Dyman, 2 

### and Steven M. Condon 2



# Abstract

The U.S. Geological Surv e y (USGS), in cooperation with the U.S. Department of Ener gy , is ree v aluating the resourcepotential of selected domestic basin-centered gas accumulations. Basin-centered gas accumulations are characterized by presence of gas in e xtensi v e lo w-permeability (tight) reserv oirs
in which con v e ntional seals and trapping mechanisms are absent, abnormally high or lo w reserv oir pressures e xist, and

gas-w ater contacts are absent. In 1995, the USGS assessed one basin-centered gas play
and tw o con v entional plays within the trend of Jurassic and
Cretaceous Cotton V alle y Group fl uvial-deltaic and barrier

island/strandplain sandstones across the onshore northern Gulf
of Me xico Basin. Detailed e v a luation of geologic and production data pro vides ne w insights into these Cotton V a lle y plays.
T

w o Cotton V alle y sandstone trends are identifi ed based

on reserv oir properties and g as-production characteristics.
T

ransgressi v e blank et sandstones across northern Louisiana
ha v e relati v ely high porosity and permeability and do not require fracture stimulation to produce gas at commercial rates. South of this trend, and e xtending westw ard into eastern
T

e xas, massi v e sandstones of the Cotton V a lle y trend e xhibit
lo w porosity and permeability and require fracture stimulation. The high permeability of Cotton V alle y blank et sandstones is
not conduci v e to the presence of basin-centered gas, b ut lo wpermeability massi v e sandstones pro vide the type of reserv oir in which basin-centered gas accumulations commonly occur

. Data on source rocks, including b urial and thermal history , are consistent with the interpretation of potential basincentered gas within Cotton V alle y sandstones. Ho we v e r , pressure gradients throughout most of the blank et- and massi v e

sandstone trends are normal or nearly normal, which is not characteristic of basin-centered gas accumulations. The presence of gas-w ater contacts in at least se v en fi elds
across the blank et-sandstone trend together with relati v ely high permeabilities and high g as-production rates without frac1 Geologic consultant, Denver , Colo.

2 U.S. Geological Survey , Denver , Colo. ture stimulation indicate that fi elds in this trend are con v entional. W ithin the tight massi v e-sandstone trend, permeability
is suffi ciently lo w that g as-w ater transition zones are v ertically

e xtensi v e and gas-w ater contacts either ha v e not been encountered or are poorly defi ned. W ith increasing depth through these transition zones, gas saturation decreases and w ater saturation increases until e v entually gas saturations become suffi ciently lo w that, in terms of ultimate cumulati v e production, wells are noncommercial. Such progressi v e increase in w ater saturation with depth suggests that poorly defi ned gas-w ater contacts probably are present belo w the depth at which wells
become noncommercial. The interpreted presence of gas-w ater contacts within the tight, Cotton V alle y massi v e -sandstone trend suggests that g as accumulations in this trend, too, are

con v entional, and that a basin-centered gas accumulation does

not e xist within Cotton V a lle y sandstones in the northern Gulf
Basin.



# Introduction

The U.S. Geological Surv e y is ree v aluating the potentialfor 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 ef fort, which is partly funded by the U.S. Department of Ener gy , might result in identifi cation of ne w continuous-type gas plays and petroleum systems or ree v aluation of e xisting plays. As part of the 1995 National Assessment of United States Oil and Gas Resources by the USGS, Schenk and V iger (1996)
identifi ed one continuous-type basin-centered g as play and tw o

con v entional gas plays (fi g. 1) within the Cotton V alle y Group
sandstone trend in eastern T e xas and northern Louisiana. The purpose of this report is to ree v aluate the 1995 USGS play
defi nitions and parameters for establishing those plays through

more e xtensi v e e v aluation of data on reserv oir properties, reser

v oir pressures, g as and w ater reco v eries, gas-production rates, and gas-w ater contacts in Cotton V a lle y sandstones. Data f a v o r

able and unf a v orable for the presence of continuous-type basincentered g as accumulations are summarized. No attempt is

12Figure 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. Geological Survey in the1995 National Assessment of United States oil and gas resources. Shown are the Cotton Valley Blanket Sandstones gas play (4923)identified as a continuous gas play, and theCotton Valley Salt Basins gas play (4922) and Cotton Valley Sabine Uplift gas play (4924) identified as conventional gas plays.00100Miles96°W4°W2°W0°W8°W34°N32°N30°N28°NOKLAHOMATEXASARKANSASLOUISIANAMISSISSIPPIALABAMAGulf of Mexico492249244923FL59998 Geologic Studies of Basin-Centered Gas SystemsIs There a Basin-Centered Gas Accumulation in Cotton V alley Group Sandstones, Gulf Coast Basin, U.S.A.? 3

made, ho we v e r , to identify ne w plays and petroleum systems or to assess g as resources for potential plays.
From a re gional perspecti v e, tw o producti v e trends of
Cotton V alle y sandstones are recognized based on sandstonereserv oir properties, g as-production rates, and necessity of hydraulic-fracturing treatments to achie v e commercial production. Across northernmost Louisiana, so-called Cotton V alle y blank et sandstones ha v e suffi ciently high porosity and permeability that commercial rates of gas production can be obtained
without artifi cial well stimulation. South of this area, in northern Louisiana, and e xtending westw ard across the Sabine uplift into northeastern T e xas, sandstones in the Cotton
V

alle y massi v e-sandstone trend ha v e poorer reserv oir properties and require hydraulic-fracture treatments to achie v e commercial rates of gas production. Because basin-centered, continuous gas accumulations characteristically occur within
lo w-permeability reserv oirs, the tight, Cotton V a lle y massi v e

sandstone trend across northern Louisiana and northeastern
T

e xas 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 con v e r

sations with industry personnel, together with geologic and engineering data accessible in a publicly a v ailable CD-R OM
database from IHS Ener gy Group (PI/Dwights Plus, a trademark of Petroleum Information/Dwights, d.b .a. IHS Ener gy Group). PI/Dwights Plus data e v aluated for this report are current through February 2000. Primary data from PI/DwightsPlus pertinent to this study are results of drill-stem and production tests in Cotton V alle y sandstones reported for indi vidualwells. Because well-completion records depend on information pro vided by operators, well data in PI/Dwights Plus might be incomplete.



# Acknowledgments

This w ork is funded by the U.S. Department of
Ener gy , National Ener gy T echnology Laboratory , Mor ganto wn, W

. V a. (contract nos. DE-AT26-98FT40031 and DEA

T26-98FT40032), and by the U.S. Geological Surv e y Central
Ener gy T eam. Laura Bie wick of the USGS pro vided instruction and assistance in using ArcV ie w softw are to e v a luate
Cotton V alle y well and test data. Gre gory Zerrahn and Joseph Lott, geologists with P almer Petroleum, Shre v eport, La., assisted in obtaining reports, maps, and literature. W e thank Al Brak e, BP Amoco, and Al T aylor , Nomad Geosciences, for pro viding in v aluable information on characteristics
of Cotton V alle y tight-gas accumulations. W e ackno wledge helpful re vie ws of this manuscript by Chuck Spencer , Curt
Huf fman, and Katharine V arnes of the USGS. 

# Continuous-T ype Gas Accumulations

It is important to identify continuous-type gas accumulations because resource assessment for such gas accumulations is conducted using dif ferent methodology than that used for

con v entional fi elds. Continuous-type gas accumulations gener

ally occur within an e xtensi v e v olume of reserv oir rock with spatial dimensions equal to or e xceeding those of con v entional hydrocarbon plays. The defi nition of continuous g as accumulations used here is based on geology rather than on go v ernment

re gulations defi ning lo w-permeability (tight) gas accumulations. Common geologic and production characteristics of continuous gas accumulations include their occurrence do wndip
from w ater -saturated rocks, lack of con v entional traps or seals,

reserv oir rocks with lo w matrix permeability , presence of abnormal pressures, lar ge in-place v olumes of g as, and lo w

reco v ery f actors (Schmok er , 1996). Continuous gas plays were treated as a separate cate gory in the U.S. Geological Surv e y 1995 National PetroleumAssessment and were assessed using a specialized methodology (Schmok er , 1996). These continuous plays are di v erse geologically and f all into se v eral cate gories 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 V alle y sandstones.



## Basin-Centered Gas Accumulations

From studies of hydrocarbon-producti v e basins in the
Rock y Mountain re gion, La w and Dickinson (1985) and Spencer (1987) identifi ed characteristics of basin-centered gas accumulations that distinguish them from con v e ntional ones. Basin-centered gas accumulations:
1. Are geographically lar ge, spanning tens to hundreds of square miles in aerial e xtent, typically occup ying the central, deeper parts of sedimentary basins.
2. Occur in reserv oirs with lo w permeability—generally
less than 0.1 millidarc y (mD)—such that gas cannot migrate by b u o yanc y

. 3. Lack do wndip gas-w ater contacts because g as is not held in place by b u o yanc y of w ater . Consequently

,
w ater production is lo w or absent. If w ater is produced, it is not associated with a distinct gas-w ater contact. 4. Commonly occur in abnormally pressured reserv oirs (generally o v erpressured, b ut occasionally underpressured).
5. Contain primarily thermogenic g as, and where o v er

pressure is encountered, the o v erpressuring mechanism is thermal generation of g as. 6. Occur structurally do wndip from w ater -bearing reserv oirs that are normally pressured or occasionally underpressured.
7. Lack traditional seals and trapping mechanisms.
8. Ha v e gas-prone source rocks proximal to the lo w-4 Geologic Studies of Basin-Centered Gas Systems

permeability reserv oirs such that migration distances
are short.
9. Occur in settings such that the tops of such g as accumulations occur within a narro w range of thermal maturity , usually between a vitrinite refl ectance (R o ) of 0.75 and 0.9 percent.What causes a basin-centered, continuous gas accumulation to form? The most common scenario in v olv es lo wpermeability reserv oirs in which o v erpressure de v elops in response to thermal generation of g as. Gas-prone source rocks generally must be associated with, or proximal to, lo wpermeability reserv oirs, and this sequence of source and reser

v oir rock must be b uried to a depth suffi cient for the source rocks to generate g as. Ov erpressure de v elops because thermal generation of gas occurs at a rate that e xceeds the rate at which gas is lost updip by migration through the lo w-permeability

reserv oir

. As o v erpressure de v e lops, an y free w ater in pores of the tight reserv oir is forced out updip into higher permeability

, normally pressured, w ater -bearing strata. Only bound, irreducible w ater remains in the tight-g as reserv oir . Permeability is
suffi ciently lo w within the tight reserv oir that gas does not migrate through it by b u o yanc y as it does through con v entional

reserv oirs with higher permeabilities (Gies, 1984; Spencer

, 1987; La w and Spencer , 1993). Instead, g as migrates slo wly through the tight-gas reserv oir with mo v ement caused by the pressure dif ferential between the re gion of high-pressure g as generation and the normally pressured, higher permeability

,
w ater -bearing rocks updip where g as does migrate upw ard rapidly by b u o yanc y

. Thus, because of its inherent lo w permeability , a basin-centered g as reserv oir itself retards the upw ard migration of gas, in ef fect forming its o wn leak y seal, and maintaining o v erpressured conditions. This scenario probably describes an ideal, end-member situation. In some cases, for e xample, basin-centered gas accumulations ha v e subnormal reserv oir pressures resulting from signifi cant tectonic uplift of strata in the basin. F or a basin that is tectonically acti v e and in an intermediate stage of uplift, it might be possible to fi nd a basin-centered gas accumulation that is normally pressured. It is also possible that particular gas accumulations might ha v e only some of the characteristics for basin-centered gas described abo v e and that dif ferentiating
between basin-centered and con v entional accumulations could
be diffi cult and subjecti v e. It is with this understanding that the potential for basin-centered gas in Cotton V alle y sandstones is

e v aluated.



# Method for Evaluating Potential of Basin-Centered Gas in Cotton V alley Group Sandstones

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

re gional seal to trap gas in a lar ge v olume of rock across a wide geographic area. W i thin that lar ge v olume of rock, discrete gas accumulations ha ving con v entional seals and gasw ater contacts are absent, and occurrence of gas often cuts across stratigraphic units. In classic basin-centered g as accumulations (La w and Dickinson, 1985; Spencer , 1987; La w and Spencer , 1993), the re gional seal is pro vided by the lo w permeability of the reserv oir itself. T o e v aluate the potential for the presence of a continuous gas accumulation within Cotton
V

alle y Group sandstones, therefore, it is necessary to e xamine

reserv oir properties of Cotton V alle y sandstones across the
northern Gulf Coast Basin. Because reserv oir properties of
Cotton V alle y sandstones are go v erned by diagenetic characteristics, which are controlled primarily by depositional environment, it is helpful to understand Cotton V alle y depositional systems and related diagenetic patterns. Although gas production from Cotton V alle y sandstones
seems to occur from discrete fi elds, it is necessary to determine
if those fi elds are separate, con v entional accumulations or socalled “sweet spots” within a re gional, continuous gas accumulation. Thus, it is essential to understand what characterizes the apparent producti v e limits of e xisting Cotton V alle y gas fi elds, including the presence or absence of g as-w ater contacts. F or some fi elds in which gas-w ater contacts are not reported, an attempt w as made to determine the presence or absence of g asw ater contacts by e xamining fl uid reco v e ries from drill-stem or production tests in wells on the fl anks of those fi elds. The goal

w as to determine if certain Cotton V a lle y fi elds that produce from tight-gas sandstones are fl ank ed by dry holes that tested

w ater only , without gas, suggesting presence of a g as-w ater

contact. T o analyze test data spatially , data from PI/Dwights Plus were imported into ArcV ie w 3.2, running on a desktop computer

. While vie wing the map display , test results from an y particular well could be e xamined. Finally , because continuous g as accumulations often are characterized by o v erpressure associated with thermal generation of gas from source rocks in proximity to lo w-permeability

reserv oirs, it is important to e v aluate the presence and quality of potential source rocks, b urial and thermal history of those source rocks, and reserv oir -pressure data.



# Geologic Setting of Cotton V alley Group

The Cotton V alle y Group is an Upper Jurassic to Lo wer Cretaceous sequence of sandstone, shale, and limestone that
underlies much of the northern Gulf of Me xico coastal plain
from eastern T e xas to Alabama (fi gs. 2 and 3). Cotton V alle y strata occur only in the subsurf ace and form a sedimentary wedge that thick ens southw ard into the Gulf Basin from a zero edge in southern Arkansas and eastern T e xas (fi g. 2).
The do wndip limit of the Cotton V alle y Group has not been delineated yet by drilling. The depth to top of the Cotton
V

alle y ranges from about 4,000 ft belo w sea le v el near theFigure 2.Index map of north-central Gulf Coast Basin, modified from Dutton and others (1993), showing major tectonic features. Sabineand Monroe uplifts were not positivefeatures during deposition of Cotton Valley Group sediments. Cotton Valley depocenters were located across the entire northernGulf Basin from eastern Texas to Alabama. Saltmovement in East Texas and North Louisiana Salt Basins was contemporaneous with deposition of Cotton Valley Group clastic sediments. The Cotton Valley Group is an entirelysubsurface sequence of strata with approximate updip limits shown here.34°N32°N30°N96°W4°W2°W0°W

LOUISIAN AARKANSASLOUISIANA

TEXAS LOUISIA NA MISSI SSIPPIMISSISSIPPISALTBASINJACKSONDOMENORTHLOUISIANASALTBASINMONROEUPLIFTSABINEARCH

CO MANCHEAN SHELF EDGEEASTTEXASSALTBASIN

MEXIA- TA L C O FA U L T ZONE

FA U L T ZON E

GINGERFAULT ZONE

M T ENTERPRISE FA U L T ZONE ARKANSAS

SOUTH F AUL

T ZONE

PICKE NSAPPROX DOWNDIP LIMITOF COTTON VALLEY GROUPSANDSTONE00406080MILES GULFOF MEXIC

OAPPROX UPDIP LIMITOF COTTON VALLEY GROUP

(SW AIN, 1944) (THOMAS & MANN, 1966) Is There a Basin-Centered Gas Accumulation in Cotton V a lley Group Sandstones, Gulf Coast Basin, U.S.A.? 599926 Geologic Studies of Basin-Centered Gas Systems

updip zero edge to more than 13,000 ft belo w sea le v el along
the southern mar gins of the East T e xas and Louisiana Salt Basins (fi gs. 2 and 4). In southeastern Mississippi, the top
of the Cotton V alle y occurs at nearly 20,000 ft belo w sea le v el. The greatest thickness of Cotton V a lle y rocks penetrated

e xceeds 5,000 ft in southeastern Mississippi (Moore, 1983).
The Cotton V alle y Group and o v erlying T r a vis Peak (Hosston) F ormation represent the fi rst major infl ux of terrigenous clastic sediments into the Gulf of Me xico Basin follo wing its initial formation during continental rifting in Late Triassic time (Salv ador , 1987; Worrall and Snelson, 1989). Earliest sedimentary deposits in East T e xas and North Louisiana subbasins (fi gs. 2 and 3) include upper T riassic nonmarine red beds of the Eagle Mills F ormation, the thick Middle and Upper Jurassic e v aporite sequence kno wn as W e rner F ormation (anhydrite) and Louann Salt, and the nonmarine Norphlet
F ormation. F ollo wing a major re gional marine transgression across the Norphlet, Upper Jurassic Smack o v er F ormation

re gressi v e carbonates were deposited, capped by red beds and

e v aporites of the Buckner F ormation (fi g. 3). A subsequentminor marine transgression is recorded by the Gilmer Limestone or Cotton V alle y F ormation in eastern T e xas, although equi v alent f acies in northern Louisiana and Mississippi are terrigenous clastics kno wn as the Haynesville F ormation. The marine Bossier Shale, lo wermost formation of the Cotton
V

alle y Group (fi gs. 3 and 5) w as deposited conformably on
Gilmer -Haynesville strata.
Louann Salt became mobile as a result of sediment loading and associated basinw ard tilting. Salt mo v ement w as initiated during Smack o v er carbonate deposition and became more

e xtensi v e with infl ux of Cotton V alle y clastics (McGo wen and Harris, 1984). Man y Cotton V alle y and T r a vis Peak fi elds
in eastern T e xas, Louisiana, and Mississippi are structural or combination traps associated with Louann Salt structures. Salt structures range from small, lo w-relief salt pillo ws to lar ge, piercement domes (McGo wen and Harris, 1984; K osters and others, 1989). The Sabine uplift is a broad, lo w-relief, basement-cored arch separating the East T e xas and North Louisiana Salt Basins (fi gs. 2 and 4). W ith v ertical relief of 2,000 ft, the Sabine uplift
has a closed area e xceeding 2,500 mi 2 (K osters and others, 1989). Isopach data across the uplift indicate that it w as a positi v e feature during deposition of Louann Salt in the Jurassic
b ut that primary uplift occurred in late, mid-Cretaceous (101 to 98 Ma) and early T ertiary 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 pro vide traps for hydrocarbon accumulations, including man y gas fi elds
in Cotton V alle y sandstones. Origins of these smaller structures ha v e been attrib uted to salt deformation and small igneous intrusions, as summarized by K osters and others (1989). Because the Louann Salt is thin across the Sabine uplift,
K

osters and others (1989) suggest that most of the smaller structures across the Sabine uplift de v eloped in association with igneous acti vity

.



# Cotton V alley Group Stratigraphic Nomenclature

Since the fi rst penetration of Cotton V alle y strata in northern Louisiana in 1927, comple x informal stratigraphic nomenclature de v eloped as numerous Cotton V a lle y oil and gas fi elds
were disco v ered across the re gion through the 1940’ s. Nomenclature became comple x because of local stratigraphic comple xities within Cotton V a lle y strata in northern Louisiana
and also because of re gional v ariations in Cotton V alle y depositional systems across the northern Gulf Basin. Terminology established by Sw ain (1944) w as used until the complete re vision of Cotton V alle y stratigraphy by Thomas and Mann (1963) and Mann and Thomas (1964). Most subsequent reports, including the classic w ork of Collins (1980), ha v e used Mann-Thomas terminology . Refi nements to that terminology ha v e been contrib uted by Coleman and Coleman (1981) and Ev ersull (1985).
Cotton V alle y lithof acies and associated stratigraphic
nomenclature in northern Louisiana are sho wn in fi gures 5
and 6. The basal formation of the Cotton V alle y Group is the Bossier Shale, a dark, calcareous, fossiliferous, marine shale.
In eastern T e xas, isolated turbidite sandstones occur within the Bossier Shale (Collins, 1980). Ov erpressured g as currently is being produced from these sandstones in a rapidly de v eloping

ne w play (PI Dwights Drilling W ire, Jan. 3, 2000; Exploration Business Journal, 2nd quarter , 2000). Completely encased in marine shale, these gas-char ged sandstones might represent a continuous gas accumulation. The Bossier Shale grades upw ard into Cotton V alle y sandstones with interbedded shales. These sandstones consist of
stack ed barrier -island, of fshore-bar , strandplain, and fl uvialdeltaic sandstones and are referred to as the Terryville massi v e-sandstone comple x in northern Louisiana by Coleman and Coleman (1981). In eastern T e xas, the stratigraphically equi v a

lent unit is called Cotton V alle y Sandstone, and it consists of braided-stream, f an-delta, and w a v e-dominated-delta sandstones (W escott, 1983; Coleman, 1985; Dutton and others, 1993). Across the Cotton V alle y hydrocarbon-producti v e trend
in eastern T e xas and northern Louisiana, the T erryville or
Cotton V alle y Sandstone a v erages about 1,000 to 1,400 ft in thickness (Finle y , 1984; Presle y and Reed, 1984). Sand deposition w as interrupted in Early Cretaceous time by a re gional transgressi v e e v ent mark ed by deposition of the Kno wles Limestone (fi gs. 5 and 6). In updip areas of eastern T e xas and
southern Arkansas, the Kno wles Limestone pinches out, and
clastic rocks of the T r a vis Peak, or equi v alent Hosston, F ormation directly o v e rly Cotton V alle y sandstones (fi gs. 3, 5, and 6). Saucier (1985) interprets the Kno wles Limestone as the uppermost formation of the Cotton V alle y Group, b ut ColemanIs There a Basin-Centered Gas Accumulation in Cotton V a lley Group Sandstones, Gulf Coast Basin, U.S.A.? 7



## CHRONOSTRA T

AGRAPHIC SECTION OF NORTH LOUISIANASYSTEMSERIES 

### ST AGE GROUP FORMA TIONTERTIARYPALEOGENEPALEOGENE EOCENE 

### AGE (Ma)GULFIANUPPER CRETACEOUSCOMANCHEANLOWER CRETACEOUSCOAHUILANJURASSICUPPERMIDDLELOWERUPPERTRIASSIC YPRESIAN

THANETIAN

DANIAN

MAASTRICHTIAN

CAMP ANIAN

SANTONIAN

CONIACIAN

TURONIAN

CENOMANIAN

ALBIAN

APTIAN

BARREMIAN

HAUTERIVIAN

V

ALANGINIAN

BERRIASIAN

TITHONIAN

KIMMERIDGIAN

OXFORDIAN

CALLOVIAN

BA

THONIAN

BAJOCIAN

AALENIAN

TOARCIAN

PLIENSBACHIAN

SINEMURIAN

HETT ANGIAN

RHAETIAN WILCOX WILCOX

MIDW A

Y MIDW A

Y

NA

V

ARRO

T A

YLOR

AUSTIN

EAGLE FORD

WOODBINE

W

ASHIT A

TRINITY GLEN ROSE FREDRICKSBURG

HIA TUS

HIA TUS

HIA TUS ARKADELPHIA NACA TOCH

EAGLE FORD SARA TOGA
ANNONA

OZAN

TOKIO
AUSTIN

TUSCALOOSA

PA L U X Y W ASHIT A-FREDERICKSBURG

MOORINGSPORT FERRY LAKE ANHYDRITE

RODESSA JAMES PINE ISLAND PETTET (SLIGO) MBR
SLIGO

HOSSTON (TRA VIS PEAK)

HA

YNESVILLE-BUCKNER

SMACKOVER

NORPHLET

LOUANN

WERNER

EAGLE MILLS 60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210MILLIONS OF YEARS

COTTON V ALLEY (See figure 5)

Figure 3. Chronostratigraphic section of northern Louisiana modifi ed from Shreveport Geological Society (1987). Subdivisions of Cotton V alley Group shown in fi gure 5.Figure 4.Generalized structure contours on top of Cotton Valley Group sandstone across northeastern Texas and northern Louisiana, modified from Finley (1984). Cotton Valleysandstone 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).TEXASLA01020 30 4050MilesContour Interval: 1000 feetOKLAHOMAARKANSASFANNINCOLLINLAMARHUNTHOPKINSRAINSWOODROCKWALLDELTADALLASKAUFMANVAN ZANDTELLISNAVARROHENDERSONLIMESTONEFREESTONEANDERSONLEONHOUSTONMADISONTRINITYBRAZOSROBERTSONMILAMFALLSANGELINANACODOCHESCHEROKEESMITHUPSHURGREGGRUSKRED RIVER

FRANKLINTITUSCAMP MORRISBOWIECASSMARIONHARRISONPANOLASHELBYSANAGUSTINESABINESABINEVERNONRAPIDESDE SOTORED RIVERNATCHITOCHESBIENVILLEWINNJACKSONGRANTCADDOBOSSIER WEBSTERCLAIBORNELINCOLNUNIONMOREHOUSERICHLANDMADISONOUACHITAMISSISSIPPICALDWELLLA SALLECATAHOULAFRANKLINTENSAS

CONCORDIA

-8000

-4000 -5000 -6000 -7000 -8000 -8000

-9000 -10,000

-5000 -10,000 -11,000 -12,000 -13,000-6000 -7000
-8000 -10,000 -14,000 -15 ,000 -11,000 -12,000 -13,000 -11,000 -7000 -9000 -10,000 -6000 -5000 -4000

-13,000 -12,000 -11,000

-9000 8 Geologic Studies of Basin-Centered Gas SystemsIs There a Basin-Centered Gas Accumulation in Cotton V alley Group Sandstones, Gulf Coast Basin, U.S.A.? 9



## (SOUTH) (NORTH)

DOWNDIP UPDIPCRETACEOUSJURASSICCOTTON VALLEY GROUP WINN LS

MASSIVE SANDSTONE COMPLEX OF CAL VIN SS. HOSSTON FORMA TION UNNAMED SANDS & SHALES

KNOWLES LIMESTONE

BLANKET SANDSTONES

MASSIVE SANDSTONE COMPLEX OF TERRYVILLE SS. HICO SHALE

BOSSIER SHALE

HA YNESVILLE, BUCKNER, AND SMACKOVER FORMA TIONS SCHULER FM

Figure 5. Generalized stratigraphic nomenclature of Cotton V alley Group (patterned units) in northern Louisiana, modifi ed 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 V alley Group was considered to be Late Jurassic in age.

and Coleman (1981) include the stratigraphically higher Calvin Sandstone, W inn Limestone, and unnamed sands and shales
within the Cotton V alle y Group (fi gs. 5 and 6).



# Cotton V alley Group Depositional Systems

Regional Framework

From eastern T e xas to Mississippi, stack ed barrier -island, strandplain, and fl uvial-deltaic sandstones, kno wn as Cotton V alle y or T erryville Sandstone, refl ect infl ux of sands from a number of depocenters. Ev olution of Cotton V alle y depocenters and associated paleogeography across northern Louisiana are described and illustrated by Coleman and Coleman (1981),
who subdi vided the T erryville Sandstone into four depositional
“e v ents” (fi g. 6) based on widespread shale breaks. Across south-central Mississippi, Moore (1983) sho ws three sequential paleogeographic reconstructions of Cotton V alle y Group sandstone deposition. Although the setting w as similar , concise paleogeographic reconstructions ha v e not been published for
the East T e xas Basin; ho we v e r , McGo wen and Harris (1984)
and W escott (1985) pro vide data from which basic paleogeographic maps can be constructed. Figure 7 is a re gional paleo-10 Geologic Studies of Basin-Centered Gas Systems



# A

SOUTH

1 2 3 4 5 6 7 8 9

WINN LIMESTONE

CAL V IN

SANDSTONE TERRYVILLE KNOWLES LIMESTONE

Figure 6 (above and facing page). North-south stratigraphic cross section of Cotton V alley Group across northern Louisiana based on data from 15 wells. Section is modifi ed from Coleman and Coleman (1981) to show details of Cotton V alley blanket sandstones as identifi ed and described by Eversull (1985) and Thomas and Mann (1963). Arrows indicate stratigraphic range of blanket sandstones. Line of cross section shown
in fi gures 7 and 8.

geographic map of upper Cotton V alle y depositional systems (equi v alent to T e rryville IV of Coleman and Coleman, 1981)
across the northern Gulf Basin from eastern T e xas to Mississippi based on data inte grated from these v arious w ork ers. As sho wn in fi gure 7, Cotton V alle y fl uvial-deltaic depocenters were located in present-day northeastern T e xas, south-central Mississippi, and along the Louisiana-Mississippi
border

. The system along the Louisiana-Mississippi border represents the ancestral Mississippi Ri v er and w as a locus of major clastic infl ux. Lar ge quantities of sand deli v ered
to the marine en vironment by this system were transported

westw ard by longshore currents producing an e xtensi v e eastwest barrier -island or strandplain comple x (Thomas and Mann, 1966). V ertical stacking of barrier -island/strandplain sands through time resulted in accumulation of the T erryville
massi v e-sandstone comple x (fi gs. 6 and 7). The east-west
barrier -island comple x across northern Louisiana sheltered a lagoon to the north from open-marine w aters to the south (Thomas and Mann, 1966). The Hico Shale accumulated in the lagoon while fl uvial and coastal-plain sandstones and shales of
the Schuler F ormation were deposited in continental environments north of the lagoon (fi gs. 6 and 7). De v elopment of
a similar , b ut smaller , lagoon associated with barrier islands—formed from longshore-transported sands in south-central Mississippi—w as documented by Moore (1983) (fi g. 7). In eastern
T

e xas, during the earliest phase of Cotton V alle y Sandstone deposition, small f an deltas de v eloped along the updip mar gin
of the East T e xas Basin (McGo wen and Harris, 1984; W escott, 1985; Black and Ber g, 1987). The drainage system w as immature with small f an deltas formed by numerous small streams. According to McGo wen and Harris (1984), fan-delta deposition persisted through Cotton V alle y time along the western

mar gin of the East T e xas Basin where f an-delta deposits char

acterize most of the Cotton V alle y Sandstone interv al. Along
the northern fl ank of the East T e xas Basin, in the re gion of the present-day Sabine uplift, a mature drainage systemIs There a Basin-Centered Gas Accumulation in Cotton V alley 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 FORMA TION (Alluvial, coastal plain, fluvial deltaic (Barrier Islands

offshore bars)

Cadeville B C D Bodcaw Va u g h n Price McGreary

Bolinger Davis E Justiss Ardis
(Lagoon/bay)

Roseberry?

Sexton T aylor T u cker

BOSSIER SHALE

de v eloped as f an deltas prograded basinw ard and e v olv ed into

a w a v e-dominated delta system. Sandstones of the lo wer part of the Cotton V alle y comprising this delta system are referred to informally as the T aylor sandstone, according to W escott (1985). After T a ylor sand deposition w as terminated by a subre gional transgressi v e e v ent, delta progradation resumed with
de v elopment of a more elongate, fl uvial-dominated system in the upper part of the Cotton V alle y (fi g. 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 e xtend landw ard from barrier -island deposits of the T e r

ryville massi v e -sandstone comple x and become thinner northw ard before pinching out into shales of the Hico lagoon, as
sho wn in fi gure 6. Some of these sandstones ha v e limited geographic e xtent co v ering only part of the lagoon, whereas others e xtend across most or all of the lagoon and interfi nger with continental deposits of the Schuler F ormation on the
landw ard side of the lagoon (Coleman and Coleman, 1981;
Ev ersull, 1985). These sandstones ha v e been interpreted as transgressi v e deposits with sand being deri v ed from T erryville barrier islands and transported landw ard into the Hico lagoon during periods of relati v e sea-le v e l rise and (or) diminished sediment supply (Coleman and Coleman, 1981; Ev ersull, 1985). These transgressi v e sandstones ha v e signifi cantly better porosity and permeability than T erryville massi v e sandstones
from which the y were deri v ed and ha v e been prolifi c producers of oil and gas from structural, stratigraphic, and combination traps disco v ered in the 1940’ s, 1950’ s, and 1960’ s across northern Louisiana (Collins, 1980; Bebout and others, 1992). Referred to informally as “blank et” sandstones (Ev e r

sull, 1985), the y can be correlated readily across northern Louisiana, and, as sho wn in fi gure 6, the y were gi v en informal names by operators during drilling in the 1940’ s and 1950’ s (Sloane, 1958; Thomas and Mann, 1963; and Ev ersull, 1985).Figure 7.Regional paleogeographic map showing sedimentary environments of Cotton Valley Group during deposition of uppermost Cotton Valley sandstones (TerryvilleIV sandstone of Coleman and Coleman, 1981). Map synthesized from data of Thomas and Mann (1966), Coleman and Coleman (1981), Moore (1983), McGowen and Harris(1984), Wescott (1985), and Eversull (1985). Location of north-south stratigraphic cross section of Cotton Valley Group acrossnorthern Louisiana in figure 6 is illustratedby 15 numbered wells.002575100MilesGulf of MexicoTXOKARALDETAILAREALOCATION MAPNMTNLAMSANCESTRAL MISSISSIPPIRIVER COTTON VALLEYDEPOCENTERLONE OAK DELTACOTTON VALLEYDEPOCENTER ARKANSAS LOUISIAN A

TEXAS LOUISIAN ALOUISIANAMISSISSIPPI

OKLAHOMA TEXAS MAXIMUM SOUTHERN EXTENT TERRYVILLE MASSIVE SANDSTONES

O F BARRIER ISLAN D BARR IER D

BA Y/LAGOONHICOAA'151413121110987564231UPPER COTTON VALLEY GROUP PALEOGEOGRAPHY 12 Geologic Studies of Basin-Centered Gas Systems5 ISLANLAGOONIs There a Basin-Centered Gas Accumulation in Cotton V alley Group Sandstones, Gulf Coast Basin, U.S.A.? 13

Based on isopach map trends, Ev ersull (1985) identifi ed

tw o groups of blank et sandstones. Geographically more e xtensi v e sandstones of the fi rst group span most of the Hico lagoon
and often interfi nger with continental deposits of the Schuler
F ormation. These sandstones generally are 30 to 70 ft thick
and can reach a thickness of 140 ft to w ard the south where
the y mer ge with barrier -island sandstones of the T erryville
massi v e-sandstone comple x. Blank et sandstones of the second group generally are less than 30 ft thick, ha v e limited geographic e xtent, and most commonly occur in the eastern part of the Hico lagoon proximal to the fl uvial-deltaic source. These sandstones pinch out northw ard into shales of the Hico lagoon. T r ansgressi v e, blank et sandstones of both groups
collecti v ely ha v e signifi cantly higher porosity and permeability than barrier -island sandstones of the T erryville massi v e

sandstone comple x to the south (Collins, 1980; Bebout and others, 1992).



## Blanket- and Massive-Sandstone
Productive T rends

Signifi cant dif ferences in reserv oir properties between transgressi v e, blank et sandstones to the north and massi v e, barrier -island sandstones to the south defi ne tw o dif ferent hydrocarbon-producti v e trends of Cotton V alle y sandstones (fi g. 8). Blank et sandstones ha v e higher porosity and permeability than T erryville massi v e sandstones to the south. Ev er

sull (1985) reported that blank et sandstones are cleaner and
better sorted. She attrib uted their superior reserv oir properties to high-ener gy re w orking during transgressi v e e v ents. Coleman (1985), ho we v e r , reported that blank et sandstones e xhibit an increase in calcite cement and clay content northw ard

to w ard their pinch-out edges, and that superior reserv oir properties occur because (1) clays inhibited precipitation of quartz o v e r g ro wths and (2) secondary porosity w as generated through widespread dissolution of calcite cement. Absence of detrital clay coatings on sand grains in high-ener gy barrier

island sandstones of the T erryville massi v e-sandstone comple x to the south, ho we v e r , permitted widespread precipitation of quartz cement as syntaxial o v er gro wths, resulting innearly complete occlusion of porosity (Sloane, 1958; Coleman and Coleman, 1981). Whate v er the cause of porosity differences, blank et sandstones generally ha v e suffi cient porosity and permeability to fl o w gas or liquids on open-hole drillstem tests (DST’ s) and to produce gas without fracture-stimulation treatment (Collins, 1980; Bebout and others, 1992).
T

erryville massi v e sandstones to the south and west, ho we v e r , ha v e such poor reserv oir properties that the y generally do
not fl o w gas or liquids during DST’ s, and the y require hydraulic-fracture treatments before commercial production
can be achie v ed. 

# Diagenesis of Cotton V alley Group Sandstones

Because understanding reserv oir mineralogy is criticalto successful wireline-log analysis and design of fracturestimulation treatments in Cotton V alle y Group sandstones,
considerable attention has been de v oted to understanding diagenetic patterns of Cotton V alle y sandstones, especially in
the lo w-permeability , Cotton V alle y massi v e -sandstone trend.
F ocusing on those sandstones in eastern T e xas, W escott (1983) reported that Cotton V alle y sandstones are v ery fi ne grained, well-sorted quartz arenites and subark oses with monocrystalline quartz and feldspar being the primary frame w ork components. Principal cements include quartz, calcite, clays, and iron
oxides. In unra v eling the comple x diagenetic history of these sandstones, W escott (1983) interpreted tw o major diagenetic

sequences. The most common sequence is (1) formation of clay coatings—primarily chlorite—on frame w ork grains, usually co v ering grains partially , not completely , (2) precipitation of syntaxial quartz o v e r gro wths 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) lar ge-scale replacement of grains and cements by calcite, resulting in poikilotopic

te xture in which a fe w relict quartz grains are “fl oating” 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 f abric with widespread replacement of grains by calcite.
W

escott (1983) classifi ed Cotton V alle y sandstones into three general groups on the basis of primary depositional

te xture and resulting diagenetic characteristics. In general,
W

escott (1983) found that clean, well-sorted sands deposited in high-ener gy environments (type I) generally are nearly completely cemented by quartz and (or) calcite, ha v e little or no porosity and permeability , and pro vide little reserv oir potential. In some cases, these sandstones e xhibit preserv ation of minor amounts of primary inter granular porosity from presence of authigenic chlorite coats (Hall and others, 1984). In sands deposited in lo wer ener gy en vironments where ab undant detrital clays remained (type II), nucleation of quartz o v e r

gro wths generally w as inhibited by clays. Most clay-bearing sandstones, ho we v e r , contain signifi cantly lar ge amounts of clay , and although ab undant microporosity is associated with these clays, permeability generally is lo w

. Highest porosities, according to W escott (1983), occur in type-III sandstones,
which de v eloped ab undant secondary porosity from dissolution of unstable grains and calcite cement. Hall and others (1984), ho we v e r , reported that dissolution of unstable grains often is incomplete, secondary pores generally are poorly interconnected, and these sandstones, too, ha v e poorFigure 8.Map of northeastern Texas and northwestern Louisiana showing major fields that have produced hydrocarbons from Cotton ValleyGroup sandstones. Two differentproductive trends are recognized based on reservoir properties and resulting producing capabilities of Cotton Valley sandstonereservoirs. Fifteen fields excluded from “tight-gas”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 stratigraphic cross section ofCotton Valley Group across northern Louisiana in figure 6 is illustrated by 15 numbered wells.0025MilesCotton Valley sandstone field(designated tight gas by FERC, 1980)Cotton Valley sandstone field(excluded from 1980 FERC tight-gas designation)Cotton Valley tight, massive sandstone trend(low porosity & permeability, frac required)Cotton Valley blanket-sandstone trend(good porosity & permeability, no frac required)East Texas & Louisiana Salt BasinsKaufmanGulf of MexicoTXLAOKARALMSLOCATION MAPDETAILAREASABINEUPLIFTNORTHLOUSIANASALT BASINARKANSASLOUISIANA

TEXASEASTTEXASSALTBASINWoodlawnBlockerOak HillDirginCarthageBethanyLongstreetGreenwoodWaskomLongwoodDixieBentonSligoSentellCaspianaElm GroveLakeBistineauPlainDealingRockyMountIvanS SarepetaShongalooCottonValleyHaynesvilleE HaynesvilleBlackburnLisbonW LisbonHomerMindenLeathermanCreekAdaSibleyBrycelandBearCreekAthensSugarCreekTerryvilleRustinClayVernonTremontCalhounCadevilleCheniereS DrewCarltonN CarltonS DownsvilleDownsvilleChoudrantD'ArbonneUnionvilleHico KnowesNE LisbonBeekmanAA'151413121110987654321Frierson 14 Geologic Studies of Basin-Centered Gas Systems5Is There a Basin-Centered Gas Accumulation in Cotton V alley 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 V alle y Bodca w T ongue of the
T

erryville Sandstone at Longw ood fi eld on the eastern fl ank of the Sabine uplift e xperienced a virtually identical diagenetic history to that described for Cotton V alle y sandstones in eastern T e xas by W e scott (1983). Lik e W escott (1983), Russell and others (1984) reported that nucleation of quartz o v er gro wths

w as inhibited by presence of clays, b ut the quantity of porefi lling clays generally is so lar ge that permeability is lo w despite presence of high microporosity

. Also, as in eastern
T

e xas, the best reserv oir sandstones are those that ha v e lo w clay content and de v eloped ab undant secondary porosity through dissolution of unstable grains and cement. Similar diagenetic patterns in northern Louisiana also were reported
for Cotton V alle y sandstones at Frierson fi eld by Sonnenber g (1976) and for the lo wermost part of the T e rryville Sandstone (the informal T a ylor sandstone) at T erryville fi eld by T rojan(1985). In addition to authigenic constituents reported in eastern T e xas and northern Louisiana, T rojan (1985) also found small amounts of authigenic p yrite in T aylor sandstones at
T

erryville fi eld. Pyrite occurs as small silt-size clusters (framboids). It is v olumetrically the least ab undant authigenic mineral reported by T rojan (1985), b ut its presence is signifi cant because of its effect on wireline-log measurements of formation resisti vity

.



# Impact of Diagenetic Mineralogy on Wireline Logs

The comple x diagenetic mineralogy of tight Cotton V alle y sandstones prohibits use of standard calculation procedures in

reserv oir e v aluation with wireline logs. The main diffi culty is that properties of certain diagenetic constituents result in abnormally lo w resisti vity measurements, which lead to such high calculated w ater saturations that producti v e zones appear to be wet. Major f actors contrib uting to abnormally lo w resisti vities in tight Cotton V alle y sandstones include bound w ater associated with pore-fi lling clays or clay coatings and conducti v e authigenic minerals such as p yrite and ank erite (Janks and others 1985; T urner , 1997). Pore-lining and pore-fi lling clays ha v e e x ceptionally high
ratios of surf ace area to v olume. Lar ge surf ace area and high
cation-e xchange capacity of clays result in formation of a double ionic layer on clay surf aces (Almon, 1979; Snedden, 1984). This bound double layer can be signifi cantly more conducti v e than pore w aters, resulting in abnormally lo w measured resisti vities, especially with induction logs (Almon, 1979; W escott, 1983). Highly conducti v e authigenic minerals,
such as ank erite and p yrite, in Cotton V alle y sandstones also cause abnormally lo w resisti vities. T rojan (1985) found that

p yrite concentrations as lo w as 1 percent in Cotton V alle y sandstones had a dramatic ef fect on resisti vity measurements
and hence on calculated w ater saturations. Standard calculation methods sho wed that p yrite-bearing sandstones at T e r

ryville fi eld in northern Louisiana had w ater saturations in

e xcess of 100 percent. T rojan (1985) sho wed that if these sandstones were p yrite-free, calculated w ater saturations w ould be closer to 50 percent. Although w ater saturations in producti v e Cotton V alle y sandstones commonly are 25 to 30 percent,

w ater -free gas production has been achie v ed from zones with
calculated w ater saturations as high as 60 percent (Nangle and others, 1982; W ilson and Hensel, 1984; Dutton and others, 1993). Porosity measurements from wireline logs also can be
af

fected adv ersely from diagenetic mineral constituents in
Cotton V alle y sandstones. In a study of T aylor sandstones (fi g. 6) at T erryville fi eld in northern Louisiana, Ganer (1985)
demonstrated the ne gati v e impact of authigenic carbonates on porosity measurements from wireline logs. Located within the porous, permeable blank et sandstone trend, T erryville fi eld

w as disco v ered in 1954 with production from the Cotton
V

alle y “D” sandstone (fi g. 6), one of the blank et sandstones.
The T aylor sandstone occurs in the lo wer part of the Cotton
V

alle y Sandstone interv al, and its producti v e potential at T e r

ryville fi eld w as not disco v ered until 1978. Unlik e the stratigraphically higher blank et sandstones, the T aylor sandstone
has relati v ely poor reserv oir properties similar to those of tight Cotton V alle y massi v e sandstones to the south. Lik e W

escott (1983), Ganer (1985) found that, although the T aylor 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

te xture. W ith ab undant secondary porosity from carbonate dissolution, these carbonate-rich sandstones are the best gas producers within the T aylor sandstone interv al at T erryville fi eld. Ganer (1985) identifi ed se v eral dif ferent carbonate minerals
in T aylor sandstones, including calcite, ank erite, and siderite. Grain densities of these minerals are 2.71, 3.00, and 3.96 g/cm 3 , respecti v ely . If porosity logs based on a sandstone matrix (grain density of 2.65 g/cm 3 ) are run across an interv al,
such as the T aylor sandstone, containing ab undant carbonate constituents with higher densities, measured porosity v alues will be pessimistic. W orking with 420 ft of con v entional core
from four wells at T erryville fi eld, Ganer (1985) reported sandstone interv als with ab undant carbonate constituents where log-measured porosities were close to zero, b ut core-measured porosities e xceeded 6 percent. W ith comple x effects of diagenetic minerals on both porosity and resisti vity measurements from wireline logs, Ganer (1985) sho wed that a single porosity/w ater saturation limit is not suitable for e v aluating producti v e potential of Cotton V alle y sandstones at T erryville
fi eld. Ganer’ s conclusions probably are applicable to most, or all, of the tight, Cotton V a lle y massi v e-sandstone trend across
northeastern T e xas and northern Louisiana. In comparing core-deri v ed reserv oir properties with wireline-log measurements for Cotton V alle y sandstones from Carthage fi eld in eastern T e xas, W ilson and Hensel (1984)16 Geologic Studies of Basin-Centered Gas Systems

reported that no apparent relationship e xists between porosity and permeability . From core analyses, the y noted that it is not
uncommon to fi nd a sandstone interv al with 10 percent porosity and 1 to 3 mD permeability adjacent to a zone with similar porosity b ut with permeability less than 0.05 mD. Similarly

, Ganer (1985) reported that T aylor sandstones with 8 percent porosity at T erryville fi eld in northern Louisiana ha v e permeabilities ranging from 0.01 to 13 mD. F or Carthage fi eld,
W

ilson and Hensel (1984) also noted that empirically deri v ed

v alues of cementation f actor ( m ) and saturation e xponent ( n ),
used in calculation of w ater saturation, v ary signifi cantly from

zone to zone. W ilson and Hensel (1984) deri v ed general empir

ical v alues of m and n for Carthage fi eld area to achie v e more accurate log-deri v ed estimates of w ater saturation. Because of
such diffi culties in determining w ater saturations from wireline logs, Presle y and Reed (1984) stress that gas-pay cutof f v alues
should be based on e xperience by operators in a gi v en area. A consequence of diffi culties in accurate reserv oir e v aluation from con v entional log analysis, of course, is that interv als capable of producing g as might be bypassed because of high
calculated w ater saturations. F or this study , the signifi cance of these diffi culties with wireline logs in tight Cotton V alle y sandstones is that logs are of limited v alue in dif ferentiating between gas-producti v e and wet interv als and therefore in identifying gas-w ater contacts on the fl anks of Cotton V alle y fi elds.



# Source Rocks

Little information has been published on source rocks for hydrocarbons produced from Cotton V alle y reserv oirs in northern Louisiana and eastern T e xas. In studying the o v erlying
T

r a vis Peak F ormation in eastern T e xas, Dutton (1987) sho wed
that shales interbedded with T r a vis Peak sandstone reserv oirs were deposited in fl uvial-deltaic settings where or ganic matter commonly is oxidized and not preserv ed. W ith measured

v alues of total or ganic carbon (T OC) in T r a vis Peak shales generally less than 0.5 percent, these shales are not considered as potential hydrocarbon source rocks (T issot and W elte, 1978). Dutton (1987) suggested that the most lik ely sources for hydrocarbons in T r a vis Peak reserv oirs in eastern T e xas are laminated, lime mudstones of the lo wer member of the
Jurassic Smack o v er F ormation and prodelta and marine shales of the Bossier Shale, basal formation of the Cotton V alle y Group (fi g. 3). Sassen and Moore (1988) demonstrated that
Smack o v er carbonate mudstones are a signifi cant hydrocarbon source rock, char ging v arious reserv oirs in Mississippi and Alabama, and Wescott and Hood (1991) documented the Bossier Shale as a signifi cant source rock in eastern T e xas. Presle yand Reed (1984) suggested that gray to black shales interbedded with Cotton V alle y sandstones, as well as the underlying Bossier Shale, probably are the source for gas in Cotton V alle y Sandstone reserv oirs. Coleman and Coleman (1981) agreed with this interpretation for Cotton V alle y sandstone reserv oirs in northern Louisiana, stating that “hydrocarbons were gener

ated from neighboring source beds. ” In summary , despite limited source-rock data, it seems
lik ely that adequate hydrocarbon source rocks occur in the Bossier Shale immediately beneath Cotton V alle y sandstones and also in stratigraphically lo wer Smack o v er carbonate mudstones (fi g. 3).



# Burial and Thermal History

In a study of diagenesis and b urial history of the T r a vis
Peak F ormation in eastern T e xas, Dutton (1987) reported that
measured vitrinite refl ectance (R o ) v alues for T r a vis Peak shales generally range from 1.0 to 1.2 percent. This indicates
that these rocks ha v e passed through the oil windo w (R o = 0.6 to 1.0 percent) and are approaching the le v el of onset of drygas generation (R o = 1.2 percent) (Do w , 1978). A maximum
R o of 1.8 percent w as measured in the deepest sample from
a do wndip well in Nacogdoches County

, T e xas. Despite ther

mal maturity le v els reached by T r a vis Peak shales, the small amount, and g as-prone nature, of or ganic matter in these shales precludes generation of oil, although minor amounts of g as might ha v e been generated (Dutton, 1987).
In the absence of actual measurements of R o , v alues
of R o can be estimated by plotting b urial depth of a gi v en source-rock interv al v ersus time in conjunction with an estimated paleogeothermal gradient (Lopatin, 1971; W aples, 1980). Dutton (1987) presented b urial-history curv es for tops
of the T r a vis Peak F ormation, Cotton V alle y Group, Bossier Shale, and Smack o v er F ormation for se v e n wells on the crest
and western fl ank of the Sabine uplift. The b urial-history

curv es sho w total o v erb urden thickness through time and usepresent-day compacted thicknesses of stratigraphic units. Sediment compaction through time w as considered insignifi cant because of absence of thick shale units in the stratigraphicsection. Loss of sedimentary section associated with late, midCretaceous and mid-Eocene erosional e v ents w as accounted
for in the b urial-history curv es. Dutton (1987) pro vided justifi cation for using the a v eragepresent-day geothermal gradient of 2.1°F/100 ft for the paleogeothermal gradient for the fi v e northernmost wells. P aleogeothermal gradients in the tw o southern wells probably were
ele v ated temporarily because of proximity to the area of initial continental rifting. Based on the crustal e xtension model of
Ro yden and others (1980), Dutton (1987) estimated v alues
for ele v ated paleogeothermal gradients for these tw o wells for 80 m.y . follo wing the onset of rifting before re v erting to the present-day gradient for the past 100 m.y

.Using estimated paleogeothermal gradients in conjunction with b urial-history curv es, Dutton (1987) found that calculated v alues of R o for T r a vis Peak shales agree well with
measured v alues. Because of this agreement, Dutton (1987)
used the same method to calculate R o v alues for tops of the
Cotton V alle y Group, Bossier Shale, and Smack o v er F orma-Is There a Basin-Centered Gas Accumulation in Cotton V alley Group Sandstones, Gulf Coast Basin, U.S.A.? 17

tion in eastern T e xas. Estimated R o v a lues for the Bossier Shale
and Smack o v e r in se v en wells range from 1.8 to 3.1 percent and 2.2 to 4.0 percent, respecti v ely , suggesting that these rocks
ha v e reached a stage of thermal maturity in which dry g as w as generated. Assuming that high-quality , gas-prone source rocks
occur within these tw o formations, it is lik ely that one or both of these units generated g as found in o v e rlying Cotton V alle y and T r a vis Peak reserv oirs.
No such re gional source-rock and thermal-maturity analysis is kno wn for T r a vis Peak and Cotton V alle y interv als in northern Louisiana. Scardina (1981) presented b urial-history
data for the Cotton V alle y Group, b ut included no information on geothermal gradients and thermal history of rock units. Present-day reserv oir temperatures in tight Cotton V alle y sandstones of eastern T e xas and the tight, massi v e Terryville Sandstone in northern Louisiana both are in the 250°F to 270°F range (Finle y , 1986; White and Garrett, 1992). It is lik ely
that Bossier and Smack o v er source rocks in northern Louisiana e xperienced relati v ely similar thermal history to their stratigraphic counterparts in eastern T e xas and, therefore, are
sources for Cotton V alle y gas in northern Louisiana. Herrmann and others (1991) presented a b urial-history plot for Ruston
fi eld in the Cotton V alle y blank et-sandstone trend in northern
Louisiana. The y suggested that Smack o v e r gas w as deri v ed locally from Smack o v er lime mudstones and Cotton V a lle y gas from Cotton V alle y and Bossier shales. Their b urial-history plot sho ws the onset of generation of g as from Smack o v er and Bossier source rocks at Ruston fi eld occurred about 80 Ma and 45 Ma, respecti v ely

. As noted abo v e in this report, the Sabine uplift has been a positi v e feature for the past 60

m.y . (K osters and others, 1989; Jackson and Laubach, 1991). Therefore, it w ould ha v e been a focal area for gas migrating
from Smack o v e r , Bossier , and upper Cotton V alle y source rocks in eastern T e xas and northern Louisiana.



# Abnormal Pressures

Pore pressure or reserv oir pressure commonly is reported
as a fl uid-pressure gradient (FPG) in pounds per square inch/ foot (psi/ft). Normal FPG is 0.43 psi/ft in freshw ater reserv oirs and 0.50 psi/ft in reserv oirs with v ery saline w aters (Spencer

, 1987). Abnormally high pore pressures as high as 0.86 psi/ft
ha v e been encountered in Cotton V alle y reserv oirs in northeastern Louisiana (fi g. 9). Multiple FPG v alues for a particular gas fi eld in fi gure 9 refer to gradients calculated for dif ferent,
stack ed blank et-sandstone reserv oirs penetrated in that fi eld. Across northern Louisiana, as sho wn in fi gure 9, the highest
FPG’ s of 0.84 and 0.86 psi/ft occur in the southeast, and gradients generally decrease to nearly normal v alues of 0.43 to 0.50 psi/ft in the northwest. This pattern e xhibits general agreement with reserv oir -pressure data for northern Louisiana summarized by Coleman and Coleman (1981) (fi g. 10). The
dashed line in fi gure 10 sho ws a modifi cation of Coleman
and Coleman’ s (1981) pressure boundary to include the 0.63 psi/ft gradient in Hico-Kno wles fi eld and 0.67 psi/ft gradient
in T remont fi eld (fi g. 9). Most signifi cant for this study

, the boundary between o v erpressured and normally pressured
Cotton V alle y sandstones (fi g. 10) sho ws no relationship to
the tw o dif ferent producti v e Cotton V alle y sandstone trends
defi ned by dif ferences in reserv oir properties (fi g. 8). Additionally , most Cotton V alle y sandstone reserv oirs, especially in the tight, massi v e-sandstone trend across northwestern Louisiana
and eastern T e xas are normally pressured, as sho wn in fi gure 9.



# History of Cotton V alley Group Sandstone Exploration

Be ginning in 1937 and continuing through the early
1960’ s, commercial gas production w as established from porous and permeable Cotton V alle y Group blank et-sandstone

reserv oirs across northern Louisiana. Blank et sandstones
fl o wed gas at commercial rates without artifi cial stimulation.
Initial disco v eries were in anticlinal traps associated with salt structures. Subsequent disco v eries came from more comple x and subtle traps, including (1) combination traps with blank et sandstones pinching out across anticlines or structural noses, and (2) stratigraphic traps with blank et sandstones pinching

out on re gional dip (P ate, 1963; Coleman and Coleman, 1981). By the early 1960’ s, the high-porosity blank et-sandstone play matured, and e xploratory drilling w aned. Lo w-porosity , lo wpermeability , Cotton V a lle y massi v e sandstones to the south in Louisiana and to the west on the Sabine uplift in northwestern
Louisiana and eastern T e xas fl o w ed gas at rates less than 1,000MCFD (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 lo w-permeability

, Cotton V alle y massi v e sandstones became commercial as a
result of technical adv ances in h ydraulic-fracturing techniques together with signifi cantly higher gas prices. At Bethan y fi eld on the Sabine uplift in eastern T e xas in 1972, T e xaco successfully increased the rate of production from tight Cotton
V

alle y 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 Spra wls, 1977). In conjunction with de v elopment of impro v ed stimulation technology

, price dere gulation through the Natural Gas Polic y Act (NGP A) of 1978 spa wned a dramatic increase in drilling for g as in
lo w-permeability Cotton V alle y sandstones (Bruce and others, 1992). In 1980, the Federal Ener gy Re gulatory Commission (FERC) offi cially classifi ed lo w-permeability Cotton V alle y sandstones as “tight gas sands, ” qualifying them for additional price incenti v es. Production from tight Cotton V alle y sandstones sur ged. At Carthage fi eld in eastern T e xas, for e xample, production from Cotton V a lle y sandstones increased from 2.2 BCFG (billion cubic feet of gas) in 1976 to 70.9 BCFG in 1980 (Meehan and Pennington, 1982). The lar ge areaCALHOUN0.860.47Cotton Valley Group sandstone fieldFluid-pressure gradient (psi/ft) fordifferent Cotton Valley sandstonesLEGEND ARKANSAS LOUI SIAN A

LOUISIAN A LOUI SIAN A

TEXASDallasKaufmanGulf of MexicoTXLAOKARALMSDETAILAREALOCATION MAP MISSISSIPPI0025MilesFLUID-PRESSURE GRADIENTS (PSI/FT)COTTON VALLEY GROUP SANDSTONESBENTON0.470.44S SAREPETA0.500.490.49COTTON VALLEY0.550.510.490.43HAYNESVILLE0.43ELM GROVE0.53BETHANY EAST0.51CARTHAGE0.55OAK HILL0.32SUGAR CREEK0.450.34HICO KNOWLES0.630.470.42RUSTON0.47CHENIERE0.84S DREW0.530.45TREMONT0.670.46CALHOUN0.860.47Blanket-sandstonetrendFigure 9.Map of northeastern Texas and northwestern Louisiana showing fluid-pressure gradients calculated from shut-in pressures in Cotton Valley Group sandstone reservoirs.Multiple pressure-gradient values for a particular gas field refer to gradients calculated for different, stacked blanket-sandstone reservoirs penetrated in that field. Shut-in-pressuredata for Louisiana fields shown in table 2. 18 Geologic Studies of Basin-Centered Gas Systems5ARKANSAS LOUI SIAN A

LOUISIAN A LOUI SIAN A

TEXASDallasKaufmanGulf of MexicoTXLAOKARALMSDETAILAREALOCATION MAP MISSISSIPPI0025MilesDISTRIBUTION OF RESERVOIR PRESSURESCOTTON VALLEY GROUP SANDSTONES

NORMALL Y OVERPRESSUREDBlanket sandstonetrend PRESSURED5 Is There a Basin-Centered Gas Accumulation in Cotton V a lley Group Sandstones, Gulf Coast Basin, U.S.A.?Figure 10.Map of northeastern Texas and northwestern Louisiana modified from Coleman and Coleman (1981) showing geographic distribution of abnormally high pressures inCotton Valley Group sandstone reservoirs. Dashed line shows modification of Coleman and Coleman’s (1981) pressure boundary to include the 0.63 psi/ft fluid-pressure gradient inHico-Knowles field and 0.67 psi/ft gradient in Tremont field as shown in figure 9 and documented in table 2. Comparison of thismap with map in figure 8 shows boundary betweenoverpressure and normal pressure cuts across two productive trends of Cotton Valley sandstones. 1920 Geologic Studies of Basin-Centered Gas Systems

across northern Louisiana and northeastern T e xas within which
Cotton V alle y sandstones ha v e been designated tight-g as sandstones by FERC includes all the counties identifi ed by name
on fi gure 4 (Dutton and others, 1993).



# Comparison of Blanket-Sandstone and Massive-Sandstone T rends

T

w o producti v e Cotton V a lle y sandstone trends are identifi ed based on reserv oir properties (fi g. 8). As described abo v e, Cotton V alle y sandstone reserv oir properties are a function of diagenetic characteristics, which are controlled primarily by v ariations in depositional en vironment. Reserv oir proper

ties, in turn, go v ern gas-production characteristics, includingboth initial rate of gas production and necessity of hydraulicfracture treatments to achie v e commercial production rates.
T

able 1 summarizes these and other k e y parameters distinguishing blank et- and massi v e-sandstone Cotton V a lle y reser

v oir trends. Data presented in table 1 were deri v ed from a

v ariety of sources as indicated in the table headnote, with much of the information coming from a series of reports by
the Shre v eport Geological Society on oil and gas fi elds in northern Louisiana (Shre v eport Geological Society Reference Reports, 1946, 1947, 1951, 1953, 1958, 1963, 1980, 1987). Detailed information obtained from those reports on more than
20 Cotton V alle y oil and gas fi elds in northern Louisiana, including data on porosity , permeability , initial production rates, gas-w ater contacts, and FPG’ s, is presented in table 2. Most of the signifi cant fi elds across northern Louisiana
and northeastern T e xas from which Cotton V alle y sandstones produce gas are sho wn in fi gure 8. The area sho wn in fi gure 8 is part of the lar ger re gion sho wn in fi gure 4 within which Cotton V alle y sandstones were designated as tight-g as sandstones by FERC in 1980. As sho wn in fi gure 8, ho we v e r , 15 Cotton V alle y fi elds were e xcluded from FERC’ s tight-gas sandstone designation. All b ut one of these fi elds are located within the porous and permeable Cotton V alle y blank etsandstone trend.



## Blanket-Sandstone T rend

T

ransgressi v e, Cotton V alle y blank et sandstones ha v e 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 a v ailable for all producti v e blank et sandstones in Cotton V alle y fi elds. Ho we v e r , suffi cient data are

a v ailable from se v eral blank et-sandstone reserv oirs within a
dozen fi elds across northern Louisiana to observ e the widespread distrib ution of relati v ely high quality reserv oir sandstones across the Cotton V alle y blank et-sandstone trend (fi g. 11). Data sho wn in fi gure 11 are deri v ed primarily from fi eld reports by the Shre v eport Geological Society and from White and others (1992). Multiple v alues of porosity and permeability for a gi v en fi eld in fi gure 11 represent measured v alues for separate, stack ed blank et-sandstone reserv oirs within that
fi eld. A v erage porosity and permeability for Cotton V alle y blank et sandstones, calculated from data in fi gure 11, are 15 percent and 110 mD, respecti v ely

. The relati v ely high porosity and permeability of blank et sandstones is refl ected in (1) the ability of these sandstones
to fl o w gas and (or) liquids on open-hole DST’ s, and (2) high initial fl o w rates of gas from these sandstones in production tests without hydraulic-fracture stimulation treatments, as
sho wn in fi gure 12. Multiple v alues of initial fl o w rates for a gi v en fi eld sho wn in fi gure 12 indicate rates from dif ferent
stack ed blank et sandstones that produce in that fi eld. Across

Table 1. Compa rison of two productive trends of Cotton Valley Gr oup sandstones i n eastern Texas and northern Louisiana.

[ D ata f r om Sh r e v e p o r t Geo log ical Society (1 946

, 19 47

, 1 951

, 19 53 , 195 8, 1 963

, 19 80 , 198 7) ; Co llin s (19 80 ) ; Nang le and o t her s ( 1 9 8 2 ) ; Fin ley (19 84, 198 6) ; Bebo ut an d ot hers (1 99 2) ; a n d D u tto n an d ot hers (1 9 9 3 )

. TST M, to o s m all t o meas ure ]

Par am et er Blanket s a ndst one Massiv e sandstone

Porosity (percen t) 10 to 19 (average = 15) 6 to 10 Permeability (mD) 1.0 to 280 (average = 110) 0.042 (E. Texas) 0.015 (N. Lou isiana) Open-hole DST Wells flow gas and (or) liquids Wells gener ally do not flow gas or liquids
Stimulation treat ment No treatmen t necessary for commerci al product i on Massive-hydraul ic fractur ing req uired to achi eve

commer cial production
Initial flow rates (MCFD) 500 to 25,000 ( a verag e 5,000) Pre-stimulation: TSTM to 300
Post-stimulatio n : 500 to 2,500 Sw in productive zones < 0.40 Can be as high as 0.60
Gas/water cont acts Short, well-defin e d transition zon es and gas-water Long transition zones with poorly defined gas-water

contacts contacts Formation damage Possible Commonly severeARKANSAS LOUI SIAN A

LOUISIAN A LOUI SIAN A

TEXAS MISSISSIPPI0025MilesCotton Valley Groupsandstone fieldPermeability (mD)TREMONT1434Porosity(percent)LEGENDPOROSITY AND PERMEABILITYCOTTON VALLEY GROUP BLANKET SANDSTONESBENTON181413585S SAREPETA1713122657550COTTON VALLEY1518161628015012134LISBON171415040LEATHERMANCREEK121TERRYVILLE10125191818NE LISBON17196HICO KNOWLES181715CHOUDRANT0.19250RUSTON1715CALHOUN15010080S DREW1210208TREMONT1434Blanket-sandstone trendFigure 11.Map of northeastern Texas and northwestern Louisiana showing measured values of porosity and permeability in Cotton Valley Group blanket sandstones. Porosityand permeability data documented in table 2. Multiple values of porosity and permeability for a given field represent measuredvalues for separate, stacked blanket-sandstonereservoirs in that field. Is There a Basin-Centered Gas Accumulation in Cotton V a lley Group Sandstones, Gulf Coast Basin, U.S.A.? 21522Table 2.Data on Cotton ValleyGroup sandstone fields in northern Louisiana.[See endoftable forexplanation of abbreviations]FieldFERCTrendTrapDiscCV disc date: ssDepth CVperfsIP (CV)GORPorPerm (mD)BHTBHPFPGSwGWCDriveSGS refrpt (vol:pge)“tight”date(ft)(MCFD)(BOPD)(BCPD)(BWPD)(%)AvgMinMax(°F)(psi)(psi/ft)Ada-SibleyYesMComb (FA, FC)19361954: CV9,900AthensBStruct (FA)19411948: Vaughn8,500-8,54410,50025441,338:1II: 385;III-2: 41;IV: 197I: 93; I: 189 Geologic Studies of Basin-Centered Gas Systems1949:"B"8,464-8,49412,00015676,923:11950:Bodcaw8,148-8,1866942347,000:11951:"D"8,145-8,1703,37020816,201:1Bayou MiddleforkYesBStruct (A)1953: Bodcaw7,76419121000,909:1Bear Creek-BrycelandYesMStruct (A)19371966 CV10,700I:97; V: 114BeekmanNoComb (N, FC)19421942: Cv3,700-3,7111,500352842,800:1II: 391BentonYesBStruct(A)19441944: "D"8,001-8,0403,28016420,000:1181361903,7650.470.17GWC -7,818II: 395;VII: 441945: Bodcaw8,137-8,1481,30612710,286:114851903,7250.44OWC -7,876BlackburnYesBComb (N, FC)19531953: Bodcaw8,7171,3015424,092:1E. BlackburnYesB1959CadevilleYesB19559,700CalhounNoBStruct (FA)19481948: "D"9,5008142237,000:1174,0000.47SG,PDNo SGS reportComb (FA, FC)1957: Cadeville9,121-9,1244,7791,1484,162:1152,1328,2010.860.05SG,PDPate & Goodwin, 1961CarltonYesBStruct (A)19531953: BodcawN. CarltonNoBComb (A,FC)1964: Purdy8,950No SGS report1965: CV?9,470CartwrightYesB1960CaspianaYesMComb(N,FC)1975:CottonValley8,500CheniereNoBComb (N, FC)19621962:Cadeville9,682-9,6974,4015288,335:18,1880.84v: 1201963:CV"A"9,603-9,6091,2308153,750:1ChoudrantYesBStruct (A)19461946: "D"9,097--91294,73221121,000:119250Separate GWCs in 2 "D" ssII: 409;III-2: 55ClayYesMStruct (A)1952: CV9,700No SGS reportCotton ValleyNoBStruct (A)19221937: Bodcaw8,170 TD (OH)5,32345511,700:1161217752314,0000.490.15GWC @ -8,420WDII:413;VI:631937: Davis8,521-8,551 (OH)4,80040012,000:1152804,3680.510.10GC1938: "D"8,502-8,5321,0201,20000,850:1181503,9260.431949: Justiss9,05016344,7000.550.22GC"C"TaylorD'ArbonneYesB19471947:Bodcaw8,1574,1008846,590:1DixieYesB1929DownsvilleYesB1948S. DownsvilleNoBStruct (A)19611961: Vaughn8,900No SGS reportS. DrewYesBStrat (FC)19721972: "D"9,061-9,0692,56085,000:112202004,8500.530.403 GWCs in "D" ssGCVI: 1161976: Vaughn9,525-9,5318731558,200:11082004,2500.450.40GCElm GroveYesMStruct (FA)1973CottonValley7,768812474,1540.530.45Greenwood-WaskomNoB1924HaynesvilleYesBStruct(A)19211944:Taylor8,835-8,92037301068:13,8700.43I: 119;III-1, 181945: Camp7,980-8,00426400500:1E. HaynesvilleYesB19451949: Tucker8,588-8,6002,89827610,500:1II: 435;III-2: 63Hico-KnowlesNoBHicoNoBStruct(FA)19461946:Vaughn8,525-8,5568,24012168,100:1173,6860.42MultipleGWCsPDI:125;III-2: 751946: Bodcaw8,287-8,3458924121,649:1184,0610.47PD1949: Feazel-McFearin8,914-8,9292,8803089,350:1155,616 0.63KnowlesNoBStruct (A)19451945:Vaughn8,700-8,7505,21213937,500:13,5000.40III-2: 821946:Bodcaw8,630-8,6708,51615853,900:11948:McCrary8,912-8,9246,7627618,886:11953:Feasel-McFearin8,996-9,0087,00027525,450:1Table 2.Data on Cotton ValleyGroup sandstone fields in northern Louisiana—Continued.FieldFERCTrendTrapDiscCV disc date: ssDepth CVperfsIP(CV)GORPorPerm (mD)BHTBHPFPGSwGWCDriveSGS refrpt (vol:pge)“tight”date(ft)(MCFD)(BOPD)(BCPD)(BWPD)(%)AvgMinMax(°F)(psi)(psi/ft)HomerYesM1919IvanYesB1952Lake BistineauYesM1916Leatherman CreekYesMStruct (FA)19751975: Cotton Valley10,400-10,8001210.30PDVI: 70LisbonNoBStruct(FA)19361939:Vaughn8,444-8,4645,0006182,000:1171500.35PDI:1431940: Burgess-Simmons8,766-8,8061,99316012,458:11440PDN. LisbonNoBComb (N, FC)19411942:Burgess-Simmons8,502-8,5255101730,000:1I:1691943: Bodcaw7,790-7,81619,00017196LongwoodYesMStrat(FC)19271948:Bodcaw8,350MindenYesM1957Plain DealingYesM1946Rocky MountYesB1959RustonNoBComb(A,FC)19431948: "D"8,796-8,8066,5005624,000:1181002104,1000.470.23GWC in Vaughn ssPDI: 185;III-2: 87; VI: 1081949: Bodcaw8,707-8,7304,26319521,860:119150PD1949:Vaughn8,809-8,8387,99539020,501:1PD1949:"D"8,674-8,7068,250PD1949:Bodcaw8,760-8,81015,500PD1951: Feazel (Davis)9,468-9,4761,0625021,240:11880PDS. SareptaYesBComb (FA,FC)19491949: Bodcaw8,7102,16017312,485:11726543000.490.14PD1949: Savis9,00013754,5000.500.15PD1949: Ardis9,15012504,5250.490.14GCSentellNoBComb (N, FC)19511951:Bodcaw8,32025,50045556,043:11650No SGS reportShongalooYesB1921SligoNoM1922I:193Sugar CreekYesBStruct(FA)19301957: Bodcaw8,724-8,7305,00021023,810:13,9720.45I: 213;VI: 1261957: Vaughn8,780-8,7952,85048.558,763:12,9550.341958:"D"7,917-7,92521,00089623,437:11962:"D"7,686-7,6933,20011228,571:11962: McFearin8,003-8,0082,80016817,500:11979:Price9,462-9,474520TerryvilleNoBComb (N, FC)19541954: "D"9,203-9,2273,73927713,514:110125GWCin"D"ssWDV: 1961957: "C"9,169-9,1821,030382927,105:11959:"C"9,049-9,05313300,934:11962:McGrary9,354-9,3624,00030013,333:1TremontYesBComb (N, FC)19441944: Bodcaw9,060-9,0802,2359723,000:14,2000.46I:219;VI:1331971: Davis9,633-9,7061,1451447,951:114342176,5190.670.15PDUnionvilleYesBStrat (FC)19501950:Vaughn8,5501950:Davis8,700VernonYesMStrat(FC)1967:Cadeville10,900FieldName of field producing fromCottonValley Group (CV) sandstoneFERC “tight”Did FERCdesignate the field“tight-gas sand”for Cotton ValleyGroup?TrendB,Blanket-sandstone trendM,Massive-sandstonetrendTrapTrappingmechanismfor fieldStruct, structural trapStrat,stratigraphictrapComb,combinationstructural& stratigraphic trapA,anticlineFA, faulted anticlineFC, facies change(sandstonepinchout)N, structuralnoseDisc dateDiscoverydate offieldCV discdate: ssDateof CVsandstonediscoveryandspecific CV ssthatwasproductiveDepth CVperfsDepth infeetofCVperforations indiscovery well forspecific ssIP (CV)IP forspecificCVssGORGas:oilratioPorPorosity(decimal)PermPermeability (mD)[nonstressed]BHTBottom holetemp (°F)BHPBottom hole pressure(psi)FPGFluid-pressuregradient (psi/ft)SwWatersaturation(decimal)GWCInformation aboutgas-watercontactDriveDrivemechanismSG,solutiongasPD,pressuredepletionGC, Gas-capexpansionWD,waterdriveSGS refreportShreveportGeological Society Reference Report(vol:page) Is There a Basin-Centered Gas Accumulation in Cotton V alley Group Sandstones, Gulf Coast Basin, U.S.A.? 2324 Geologic Studies of Basin-Centered Gas Systems

the blank et-sandstone trend, as sho wn in fi gure 12, initial production rates range from 500 MCFD to 25,000 MCFD and

a v erage 5,000 MCFD.
Gas-w ater contacts ha v e been reported from se v en fi elds
across the blank et-sandstone trend as sho wn in fi gure 13. In
Hico-Kno wles, South Dre w , and Choudrant fi elds, separate

gas-w ater contacts for indi vidual blank et-sandstone reserv oirs
ha v e been identifi ed (table 2). No gas-w ater contacts were encountered in Cheniere fi eld as of 1963 or in T remont fi eld as of 1980 (table 2). In all other Cotton V alle y fi elds described in Reference Reports by the Shre v eport Geological Society (1946, 1947, 1951, 1953, 1956, 1963, 1987), no mention of
fl uid contacts w as made.



## Massive-Sandstone T rend

Cotton V alle y sandstones in the massi v e-sandstone trend (fi g. 8 and table 1) ha v e signifi cantly poorer reserv oir proper

ties than those in the blank et-sandstone trend. Massi v e Cotton
V

alle y sandstones ha v e suffi ciently lo w permeability that the y generally do not fl o w gas or liquids during open-hole DST’ s, and the y require fracture-stimulation treatment to obtain commercial rates of gas production (Collins, 1980). Commercial gas production from these sandstones w as not achie v ed until technological adv ances in h ydraulic fracturing occurred together with higher g as prices from dere gulation in the
1970’ s. Consequently , de v e lopment of Cotton V alle y fi elds in the tight, Cotton V alle y massi v e-sandstone trend did not
occur until the late 1970’ s and 1980’ s. Cotton V alle y de v elopment drilling in Elm Gro v e and Caspiana fi elds in northern Louisiana continues at the time this report is being written (Al T aylor , Nomad Geosciences, oral commun., April 2000). A consequence of such recent de v elopment of fi elds in the tight, Cotton V alle y massi v e-sandstone trend is less published
information on characteristics of these fi elds than on older
fi elds in the blank et-sandstone trend.



### Limited Data in Published Literature

Summary information presented by Dutton and others (1993) for the tight, Cotton V a lle y massi v e-sandstone trend
across northeastern T e xas and northern Louisiana indicatesporosities in the 6- to 10-percent range. Based on measurements from cores in 11 wells in Carthage fi eld, one of the
lar gest Cotton V a lle y fi elds in northeastern T e xas, W ilson and Hensel (1984) reported porosities ranging from 5.8 to 8.1 percent, with an a v erage of 6.6 percent. Associated permeabilities range from 0.02 to 0.33 mD, with an a v erage of
0.067 mD. From core data for 126 wells in Harrison and Rusk
counties in northeastern T e xas, Finle y (1984) reported an a v er

age permeability of 0.042 mD for Cotton V alle y sandstones. In northern Louisiana, a v erage permeability w as reported as
0.015 mD based on data from Cotton V alle y cores in 302
wells. Ho we v e r , there are stratigraphic interv als within the tight, Cotton V alle y massi v e-sandstone trend with signifi cantly higher permeabilities. Locally , permeabilities approaching 100
mD ha v e been reported (W ilson and Hensel, 1984). Signifi cantly lo wer porosity and permeability of tight,
Cotton V alle y massi v e sandstones relati v e to blank et sandstones is refl ected in poorer production characteristics. The

a v erage fl o w 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 ha v e been reported from Bethan y fi eld (Jennings and Spra wls, 1977) and Carthage fi eld (Meehan and Pennington, 1982), respecti v ely

. There are fe w published data on presence of gas-w ater contacts or production of w ater without gas on the fl anks
of Cotton V alle y fi elds in the tight, massi v e-sandstone trend. Summary data presented by Nangle and others (1982) described gas-w ater contacts as poorly defi ned with long transition zones in contrast to short, well-defi ned transition zones with sharp gas-w ater contacts in the blank et-sandstone trend. Dutton and others (1993) also suggested the presence of g asw ater contacts with long transition zones by indicating that
calculated w ater saturations should be less than 40 percent to
achie v e successful gas completions from Cotton V alle y sandstone interv als 200 ft abo v e the free-w ater le v el.
In northeastern T e xas, where most of the drilling for tight Cotton V alle y sandstones has occurred, the best reserv oir potential is reported to be in w a v e-dominated deltaic sandstones of the T a ylor sandstone in the lo wer part of the Cotton
V

alle y interv al (W escott, 1983, 1985). In Oak Hill fi eld, production logs sho w that T a ylor sandstones contrib ute more than 80 percent of the gas production and that sandstones in the middle and upper Cotton V a lle y section contrib ute most of
the w ater production, although the y produce signifi cant gas as well (T indall and others, 1981). Presle y and Reed (1984) and Dutton and others (1993) both report the presence of

w ater -bearing sandstones in the upper Cotton V alle y interv al.
T

o a v oid production of w ater from these sandstones, fracturestimulation treatments in stratigraphically adjacent gas-bearing sandstones in the upper Cotton V alle y must be signifi cantly
smaller than those in the T aylor sandstone. At Bethan y fi eld,

se v eral wells reportedly were plugged because of production
of salt w ater from Cotton V alle y sandstones (Jennings and Spra wls, 1997).



### Analysis of Drill-Stem-T est and Production-T est Data

As mentioned abo v e, general statements in published reports suggest the presence of g as-w ater contacts in fi elds that produce gas from tight Cotton V alle y sandstones across
northeastern T e xas and northern Louisiana. Unlik e data for the
Cotton V alle y blank et-sandstone trend, ho we v e r , no documentation w as found identifying specifi c gas-w ater contacts in Cotton
V

alle y sandstones in an y of the tight-gas-sandstone fi elds in
T

e xas or Louisiana. In the absence of such published data, andARKA NSAS LOUI SIANA

LOUISIANA LOUI SIANA

TEXASDallasKaufmanGulf of MexicoTXLAOKARALMSDETAILAREALOCATION MAP M ISSISSIPPI0025MilesCotton Valley sandstone fieldInitial rates of gas production (MCFD) from differentCotton Valley sandstones without fracture stimulationBENTON3,2801,306LEGENDINITIAL RATES OF GAS PRODUCTIONCOTTON VALLEY GROUP BLANKET SANDSTONESS SAREPETA2,160BENTON3,2801,306SENTELL25,500COTTONVALLEY5,3234,9001,020BLACKBURN1,301E HAYNESVILLE2,898LISBON5,0001,993ATHENS12,00010,5003,370694SUGARCREEK21,0005,0003,2002,8502,800520TERRYVILLE4,0003,7391,030TREMONT2,2351,145CALHOUN4,779814CHENIERE4,4011,230NE LISBON19,000510HICO KNOWLES8,5167,0006,7625,2122,880RUSTON15,5008,2507,9956,5004,2631,062CHOUDRANT4,732S DREW2,650873Blanket-sandstonetrendFigure 12.Map of northeastern Texas and northwestern Louisiana showing initial rates of gas production from Cotton Valley Group blanket sandstones. Multiple values of initialflow rates for a given field represent rates from different stacked blanket-sandstone reservoirs that produce in that field. All rates are from blanket sandstones, which do not requirefracture-stimulation treatment for commercial production. Is There a Basin-Centered Gas Accumulation in Cotton V a lley Group Sandstones, Gulf Coast Basin, U.S.A.? 25526

ARKANSAS LOUI SIAN A

LOUISIAN A LOUI SIAN A

TEXASDallasKaufmanGulf of MexicoTXLAOKARALMSDETAILAREALOCATION MAP MISSISSIPPI0025MilesKNOWN FIELDS WITH GAS-WATER CONTACTSIN COTTON VALLEY GROUP SANDSTONESBENTONCOTTONVALLEYHICOKNOWLESTERRYVILLERUSTONCHOUDRANTS DREWBlanket-sandstonetrend5 Geologic Studies of Basin-Centered Gas SystemsFigure 13.Map of northeastern Texas and northwestern Louisiana showing fields productive from Cotton Valley Group sandstones in whichgas-water contacts have beenidentified and reported in published literature. Presence of gas-water contacts in these fields suggests that they are conventional gas accumulations. Details on gas-watercontacts, including sources of information, are shown in table 2.Is There a Basin-Centered Gas Accumulation in Cotton V alley Group Sandstones, Gulf Coast Basin, U.S.A.? 27

considering the diffi culties of using wireline logs to e v aluate

w ater saturations in tight Cotton V alle y sandstones, an attempt

w as made to document the presence or absence of g as-w ater contacts through analysis of data from DST’ s and production

tests. The goal w as to determine if Cotton V a lle y fi elds that produce from tight-gas sandstones are fl ank ed by dry holes that
tested w ater without gas, suggesting presence of a g as-w ater

contact. A data set of wells penetrating the Cotton V alle y Group
across most of northeastern T e xas and northern Louisiana w as

e xtracted from a database (PI/Dwights Plus, a trademark of Petroleum Information/Dwights, d.b .a. IHS Ener gy Group) for analysis of DST and production-test data using ArcV ie w softw are. Because tight Cotton V alle y sandstones generally do not
fl ow fl uids on open-hole DST’ s, it w as anticipated that the
most useful data w ould be deri v ed from production tests made through perforations in casing follo wing fracture-stimulation

treatments. W ell data were sorted and displayed in map vie w using ArcV ie w softw are such that wells that produce from
Cotton V alle y sandstones could be distinguished from dry holes
with tests. While vie wing the map display , test results from an y particular well could be e xamined. Reconnaissance analysis of data from Carthage, Bethan y , Oak Hill, W a sk om, and W oodla wn fi elds in northeastern T e xas and from Bear Creek–Bryceland, Elm Gro v e, and Caspiana
fi elds in northern Louisiana re v ealed fe w dry holes penetrating
Cotton V alle y strata on the fl anks of these Cotton V alle y fi elds.
No fl anking dry holes were found that tested only w ater

. The
fe w Cotton V a lle y dry holes present generally did not report tests, suggesting that no tests were performed in those wells and that, most lik ely , the wells were plugged based on e v aluation of wireline logs.
T

est results from Cotton V alle y sandstones in Oak Hill
fi eld in T e xas and Elm Gro v e–Caspiana fi elds in Louisiana

were e v aluated more rigorously , re v e aling se v eral general patterns. Initial rates of g as production generally are higher in
crestal wells than in fl ank wells in these fi elds, as sho wn for Caspiana fi eld in fi gure 14. At both Oak Hill and Elm
Gro v e–Caspiana fi elds, initial rates of g as production from
Cotton V alle y sandstones range from 1,000 to more than 4,000 MCFD in central parts of the fi elds and generally are less than 1,000 MCFD in structurally lo wer wells on edges of the fi elds.
Man y fl ank wells e xhibit initial rates of less than 500 MCFD,
as sho wn in fi gure 14. This trend e xhibits more v ariability at
Oak Hill fi eld, where a lar ger number of lo w-rate wells occur
in the center of the fi eld. Such lo w-rate wells in the central part of the fi eld could be attrib uted to a number of f actors, including reserv oir v ariability , formation damage during drilling, and poor fracture-stimulation treatments. All these wells must be fracture stimulated, and signifi cant v ariation in success of such stimulation treatments is not uncommon. Also,
initial rates on the western fl ank of Oak Hill fi eld are high
and sho w an abrupt change to dry holes rather than a gradual
decline to w a rd the fl ank of the fi eld. One well there fl o wed gas with an initial rate e xceeding 4,000 MCFD and is fl ank ed to the west by four Cotton V alle y dry holes. In three of these dry holes, Cotton V alle y sandstones apparently were not tested, and a test in the fourth well must ha v e resulted in noncommercial production with only “one unit of gas” reported.
Initial rates of w ater production in BWPD (barrels of

w ater per day) also were mapped at Oak Hill and Elm Gro v e – Caspiana fi elds and sho w no ob vious patterns across these
fi elds. No attempt w as made to contour w ater production data
for se v eral reasons. Not only is v ariability in initial rate of

w ater production high and seemingly random, b ut also, data are incomplete. Whereas the IHS Ener gy database reports initial rate of gas production for most all wells in these fi elds,
initial rate of w ater production is not reported for a signifi cant percentage of wells. In wells at Oak Hill or Elm Gro v e – Caspiana fi elds, for which a v alue is entered in the appropriate position of the database for w ater production, a null v alue is

ne v er reported. Some v olume of w ater production seems to occur along with gas in all these wells. Hence, it does not seem appropriate to interpret absence of w ater -production data for a gi v en well as meaning zero production of w ater

. Absence of
data on initial w ater production is more pre v alent for Oak Hill
fi eld, and that f actor alone mak es it diffi cult to analyze w ater production from that fi eld. Data on initial w ater production
at Elm Gro v e–Caspiana fi elds are more complete. Although
initial rates of w ater production were considerably higher at Caspiana fi eld, data were more complete for that fi eld, and patterns of initial w ater production at Caspiana fi eld were e v aluated by plotting barrels of w ater produced per MMCFG (bbl

w ater/MMCFG). As sho wn in fi gure 15, wells in the central part of Caspiana fi eld commonly e xhibit production of 100 or
fe wer bbl w ater/MMCFG. Progressing outw ard to w ard fl anks
of the fi eld, rates of initial w ater production increase to 300
bbl w ater/MMCFG and e v entually to more than 600 bbl w ater/
MMCFG. The highest initial rate of w ater production occurs
in a well on the western fl ank of the fi eld where production of 1,477 bbl w ater/MMCFG is reported (fi g. 15). That same well had an initial rate of g as production of only 325 MMCFD,
as sho wn in fi gure 14. Ne v ertheless, no wells that tested w ater only without gas from Cotton V a lle y sandstones were identifi ed on the fl anks of Caspiana fi eld to suggest the presence of

a gas-w ater contact for the fi eld. T wenty-one Cotton V alle y dry
holes were identifi ed surrounding Elm Gro v e–Caspiana fi elds. Of these, 19 wells reported no tests in the Cotton V alle y sandstone interv al, presumably indicating that no Cotton V a lle y tests were run and that Cotton V alle y completions were made on the basis of wireline-log e v aluation. Production tests after
fracture-stimulation were run in tw o 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

w ater

, ” suggesting that the Cotton V alle y sandstone might be
belo w a gas-w ater contact at that location. On the southern and
western fl anks of Oak Hill fi eld, six Cotton V alle y dry holes
without tests were identifi ed, again suggesting abandonment
of Cotton V alle y potential based on wireline-log e v aluation.
Production tests were run in Cotton V alle y sandstones in tw o wells on the western fl ank of Oak Hill fi eld. One reported “one unit of gas, ” the other “one unit of gas and one unit of w ater

. ”LOUSISANALOCATION MAPMapAreaR 13 WT 15 NT 14 NEXPLANATIONHosston Fm. producer(TD in Cotton Valley Gp.)583250108166620015872100980264135031007301250548173693890540070060580015611512146418001908900325145013041200143912501410900130026182723211022321 unit gas1unitgas18501170102021088601454200020613230201495025001250980485028831591177618001150175018321725168723762667305015001500956185120020062006927925200762170011171035159222561400No Cotton Valley testsNoCottonValleytests482245025002900210020412022500150785583250108166620015872100980264135031007301250548173693890540070060580015611512146418001908900325145013041200143912501410900130026182723211022321 unit gas18501170102021088601454200020613230201495025001250980485028831591177618001150175018321725168723762667305015001500956185120020062006927925200762170011171035159222561400TD Cotton Valley Gp.No Cotton Valley Gp. tests482245025002900210020412022500150785 2000

1000

2000 4000 2000 1000 2000

1000 20001000

3000 1000

2000

1000 30001000

1000 1000 1000 2000
2000 3000Figure 14.Map of Caspiana field in northwestern Louisiana in the tight, Cotton Valley Group massive-sandstone trend showing initial rate of gas production (MCFD) from CottonValley sandstone reservoirs. Data from PI/Dwights Plus, a trademark of Petroleum Information/Dwights, d.b.a. IHS Energy Group.Contour interval is 1,000 MCFD. Map shows generaldecrease in initial rates of gas production from center to flanks of field. 28 Geologic Studies of Basin-Centered Gas SystemsR 12 WFigure 15.Map of Caspiana field in northwestern Louisiana in the tight, Cotton Valley Group massive-sandstone trend showing ratio of initial production rate of water (BWPD) to initialproduction rate of gas (MMCFGD) from Cotton Valley Group sandstones. Data from PI/Dwights Plus, a trademark of Petroleum Information/Dwights, d.b.a. IHS Energy Group. Contourinterval 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 suggests that, within reservoirsandstones, gas saturation is decreasing and water saturation is increasing from crest to flanks of field and indicates presence of vertically extensive gas-water transition zone.LOUSISANALOCATION MAPMapAreaR 13 WT 15 NT 14 NEXPLANATION?No data for waterproductionHosston Fm. producer(TD in Cotton Valley Gp.)

600 500 400 300 300 400
500 600

700 800 400

500 600

500 600 1000 500 400 300 200100 100 200 200 600 400 100 200

200 300 300 200 100 400 400 300 300 200 100 100 200 300

400

500 600

500 400 300 200

500 200 300??????3672051778325??363240244270144187?609290278285695100????160105757429320522921473255199223?333895207377398178,000694344147720024123833436844336 units wtr1 unit gas187173??424303172114270470?11416221713828623657148372188155350260201334385627549378800432549267 Is There a Basin-Centered Gas Accumulation in Cotton V a lley Group Sandstones, Gulf Coast Basin, U.S.A.? 29R 12 W30 Geologic Studies of Basin-Centered Gas Systems

On the southern fl ank of the fi eld, production tests were run
in Cotton V alle y sandstones in tw o Cotton V alle y dry holes,
b ut no results were reported. Ev aluation of test data from Oak
Hill and Elm Gro v e–Caspiana fi elds, therefore, pro vides no
defi niti v e information re g arding presence or absence of g asw ater contacts in these Cotton V a lle y fi elds.



# Discussion of Evidence For and Against Basin-Centered Gas

Source Rocks and Burial and Thermal History

Source rocks responsible for generating g as in basincentered g as accumulations commonly are in stratigraphic proximity to lo w-permeability reserv oirs that the y are char ging with gas. As described abo v e, there are fe w published data on source rocks responsible for generating g as found in Cotton
V

alle y sandstone reserv oirs in both the blank et- and massi v e

sandstone trends. Ho we v e r , the marine Bossier Shale, stratigraphically directly beneath Cotton V a lle y sandstones, and
Smack o v er laminated lime mudstones, lying belo w the Bossier Shale, are considered to be source rocks capable of generating gas for Cotton V alle y sandstone reserv oirs. Gray to black
marine shales interbedded with Cotton V a lle y sandstones also are considered to be potential source rocks. Also, as summarized abo v e, b urial- and thermal-history data for the northern Gulf Coast Basin suggest that b urial depths of Bossier and
Smack o v er source rocks, in conjunction with the re gional geothermal gradient, ha v e been suffi cient to generate dry gas. T ime of generation of much of the g as postdates de v elopment of both the Sabine uplift and structures in the East T e xas and Louisiana Salt Basins. Hence, a v ailable data on presence of source rocks,
b urial and thermal history of source rocks, and timing of g as generation for Cotton V alle y reserv oirs w ould be consistent with interpretation of a potential continuous g as accumulation
in massi v e sandstones of the Cotton V alle y Group.



## Porosity , Permeability

, and Gas-Production Rates

Basin-centered gas accumulations commonly in v olv e lar ge v olumes of gas-saturated rock in which gas cuts across stratigraphic units. Such gas accumulations require a re gional seal to trap gas, and that seal characteristically is pro vided by
inherent lo w permeability of reserv oir rocks themselv es. Thus, continuous gas reserv oirs characteristically ha v e lo w permeability , and when reserv oirs are sandstones, the y commonly are referred to as tight-g as sandstones.
As described abo v e, Cotton V alle y sandstone reserv oirs
across the northern Gulf of Me xico Basin can be di vided into

tw o groups based on reserv oir properties and associated rates of gas production. Sandstones in the Cotton V alle y blank etsandstone trend across northernmost Louisiana ha v e porosities in the 10 to 19 percent range and permeabilities from 1 to 280 mD (table 1). These sandstones generally fl o w gas and (or) liquids during open-hole DST’ s. Gas-producti v e sandstones
fl o w at initial rates ranging from 500 to 25,000 MCFD without fracture-stimulation treatment. Consequently , these sandstones are not tight-gas reserv oirs, and man y fi elds producing from
Cotton V alle y sandstones in the blank et-sandstone trend were

e xcluded from tight-g as status by FERC in 1980 (fi g. 8). Therefore, in the absence of some other re gional top seal that
could allo w de v elopment of basin-wide o v erpressure, sandstones in this trend w ould not be e xpected to host a basincentered gas accumulation.
South of the blank et-sandstone trend in northern Louisiana lies the Cotton V a lle y massi v e-sandstone trend that e xtends

westw ard across the Sabine uplift into northeastern T e xas, as sho wn in fi gure 8. Cotton V a lle y massi v e sandstones generally
ha v e porosities in the 6 to 10 percent range with permeabilitiescommonly less than 0.1 mD. Most of these sandstones, therefore, w ould be defi ned as tight-gas sandstones, and most fi eldsproducing gas from these sandstones were designated as tightgas-sandstone fi elds by FERC in 1980. T ight, Cotton V alle y massi v e sandstones generally do not fl o w gas and (or) liquids on open-hole DST’ s, and the y require hydraulic-fracture treatment to produce gas at commercial rates. As sho wn 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 per

meability interv als occur locally within the massi v e-sandstone trend, as noted by W ilson and Hensel (1984), the characteristic
lo w permeability of sandstones throughout this trend suggests
that the y might ha v e potential to pro vide their o wn seal for gas in a continuous gas accumulation.



## Abnormal Pressures

In a study of abnormally high pressures in basin-centered gas accumulations in Rock y Mountain basins, Spencer (1987)
considered reserv oirs to be signifi cantly o v erpressured if
FPG’ s e xceed 0.50 psi/ft where w aters are fresh to moderately saline, and 0.55 psi/ft where w aters are v ery saline. W ith
formation-w ater salinity of Cotton V alle y sandstone reserv oirs on the order of 170,000 ppm TDS (Dutton and others, 1993), salinity is considered high, and reserv oirs should be considered
to be o v erpressured if their FPG’ s e xceed 0.55 psi/ft. Based on Spencer’ s (1987) cutof f v alue of 0.55 psi/ft, abnormally high reserv oir pressures ha v e been encountered in
Cotton V alle y sandstones in an area of northeastern Louisiana,
sho wn in fi gure 10, where calculated pressure gradients of 0.63 to 0.86 psi/ft occur

. The boundary between areas of o v erpressure and normal pressure cuts across the permeable, blank etand tight, massi v e -sandstone trends such that o v erpressures occur within both reserv oir trends. (fi gs. 8 and 10). Although

o v erpressures associated with generation of g as might be antic-Is There a Basin-Centered Gas Accumulation in Cotton V alley Group Sandstones, Gulf Coast Basin, U.S.A.? 31

ipated in tight Cotton V alle y sandstones, such o v erpressures

w ould not be e xpected to de v elop in high-permeability blank et sandstones without a subre gional top seal stratigraphically
abo v e the sandstones. As sho wn in fi gure 9 and table 2, some of the separate, stack ed blank et sandstones within Hico, Tremont, and Calhoun fi elds are o v erpressured, whereas others are normally pressured. Examination of disco v ery dates of gas
in indi vidual sandstones sho ws that, in all cases for these
three fi elds, normally pressured sandstone reserv oirs were disco v ered prior to o v erpressured ones. Thus, pressure dif fer

ences among indi vidual blank et-sandstone reserv oirs indicate presence of separate, compartmentalized reserv oirs rather than pressure depletion from production of gas from different sandstones that are in pressure communication. Additionally , nor

mally pressured Cotton V alle y sandstones were encountered at
South Dre w fi eld, whereas, at Cheniere fi eld immediately to the west, Cotton V a lle y sandstones were signifi cantly o v e r

pressured with an FPG of 0.86 psi/ft. Thus, for gas fi elds in
the blank et-sandstone trend where data are ab undant, reserv oir

pressures e xhibit signifi cant v ariation from normal to abnor

mally high among separate sandstone reserv oirs within individual gas fi elds and also between adjacent fi elds. Such compartmentalization of o v erpressured reserv oirs in proximity to normally pressured ones, rather than de v elopment of o v erpressure on a re gional scale, is more indicati v e of con v entional gas
fi elds than basin-centered gas accumulations.
W

ithin the western half of the blank et-sandstone trend and spanning the v ast majority of the tight, massi v e -sandstone
trend across northwestern Louisiana and northeastern T e xas, FPG’ s range from 0.32 to 0.55 psi/ft and, therefore, w ould be considered normal, according to the methodology of Spencer (1987). Ho we v e r , tw o episodes of erosion ha v e occurred in
northeastern T e xas, one in late mid-Cretaceous time and the second in early mid-T ertiary time (Dutton, 1987; Laubach andJackson, 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 remo v ed. T ertiary erosion resulted in remo v al of about 1,500 ft of section across much of northeastern T e xas. Burial-history data for the Ruston fi eld area in northern Louisiana on the boundary between o v erpressured and normally pressured re gions, sho w about 1,500 and 500 ft of uplift and loss of section, respecti v ely , in tw o erosional periods (Her

rmann and others, 1991). It is possible, therefore, that, with deeper b urial, reserv oir pressures in much or all of the massi v e-sandstone trend could ha v e been higher and that pressure reduction has occurred as a result of uplift and erosion. Ho we v er , some of the gas found in Cotton V alle y sandstone reserv oirs could ha v e been deri v ed from Bossier Shale source rocks. Migration of most of that g as into Cotton V alle y sandstones probably commenced between 57 and 45 Ma (Dutton, 1987; Hermann and others, 1991). Therefore, if basin-wide

o v erpressure in Cotton V alle y sandstones were to ha v e de v e loped in response to thermal generation of g as from Bossier Shale source rocks, its de v elopment w ould ha v e postdated both
the Cretaceous and T ertiary erosional e v ents. The sharp boundary between o v erpressured and normally pressured areas of Cotton V alle y sandstones (fi g. 10) and presence of o v erpressure in both permeable, blank et-sandstone and tight, massi v e -sandstone trends, suggest that abnormally high pressures encountered in Cotton V a lle y sandstones in northeastern Louisiana are not caused by thermal generation and migration of gas. Coleman and Coleman (1981) attrib uted
de v elopment of o v e rpressures in Cotton V a lle y sandstones
across the re gion sho wn in fi gure 10 to a late stage of diagenesis in which e xtreme pressure, presumably o v erb urden pressure, and temperature caused dissolution of silica at contactpoints of quartz-sand grains and precipitation of silica in adjacent pores. W ith pore w aters apparently unable to escape, porosity reduction associated with this late-stage chemical compaction reportedly resulted in de v elopment of o v erpressure in Cotton V alle y sandstones across the area sho wn in fi gure 10. According to Coleman and Coleman (1981), a signifi cant f actor in pre v enting fl uid loss from Cotton V alle y sandstones during this late diagenetic episode w as presence of a tight top seal pro vided by the Kno wles Limestone and upper Cotton
V

alle y–lo wer Hosston shales. If late-stage chemical compaction and cementation in conjunction with a top seal of tight limestone and shale are responsible for de v elopment of o v erpressure, it is not clear why the geographic distrib ution of o v erpressure e xhibits the pattern sho wn in fi gure 10. Perhaps an alternati v e mechanism for generating the distrib ution of o v erpressures within Cotton
V

alle y sandstones sho wn in fi gure 10 could be one reported by P ark er (1972) as the cause for o v erpressures in Jurassic
Smack o v er sandstone and carbonate reserv oirs to the east in Mississippi. P ark er (1972) noted that much of the Smack o v er gas is sour and has a relati v ely high content of CO 2 and (or) N 2 . He suggested that migration of g ases deri v ed from Late Cretaceous emplacement of the Jackson (igneous) dome (fi g. 2) might be responsible for “infl ation” of pressures in
well-sealed Smack o v er reserv oirs. Specifi cally , Jones (1977) suggested that H 2 S and CO 2 present in Smack o v er gas in Mississippi were deri v ed from Smack o v er anhydrite and limestone/dolomite, respecti v ely , which had been subjected to magmatic intrusion. The mapped pattern of o v e rpressured
Cotton V alle y sandstones (fi g. 10) e xtends east-southeastw ard into Mississippi directly to w ard the location of Jackson dome (fi g. 2) (Studlick and others, 1990). Evidence supporting such
a mechanism of o v erpressure de v elopment in Cotton V alle y sandstones of northeastern Louisiana w ould be documentation
of ele v ated le v els of CO 2 and (or) N 2 in o v erpressured Cotton
V

alle y sandstone reserv oirs. In summary , within most of the tight, Cotton V alle y massi v e-sandstone trend across northwestern Louisiana and
northeastern T e xas, Cotton V alle y reserv oirs are slightly , b ut not signifi cantly , o v erpressured. Based on the methodology and terminology of Spencer (1987), these reserv oirs w ould be char

acterized as normally pressured. Basin-centered, continuous gas accumulations commonly are signifi cantly o v erpressured. Although pressure data for the tight, Cotton V a lle y massi v e

sandstone trend are not defi niti v e, the y tend to suggest that a32 Geologic Studies of Basin-Centered Gas Systems

basin-centered g as accumulation characterized by abnormally high pressures from geologically recent to present-day thermal generation of gas does not occur within Cotton V a lle y sandstone

reserv oirs.



## Gas-W ater Contacts

Perhaps the most defi niti v e criterion for establishing thepresence of a continuous gas accumulation is absence of gasw ater contacts. Gas-w ater contacts are distincti v e features of

con v entional gas accumulations. Presence of a gas-w ater contact indicates a change from g as-saturated to w ater -saturated porosity within a particular reserv oir unit. This implies that a
well drilled into that reserv oir structurally belo w the gas-w ater contact should encounter only w ater , thereby demonstratingthe absence of a continuous gas accumulation in that immediate area. W

ithin the blank et-sandstone trend across northernmost Louisiana, gas-w ater contacts ha v e been reported in se v en fi elds, as sho wn in fi gure 13. Because of relati v ely high porosity and permeability in blank et sandstones, gas-w ater contacts are sharp and often are reported as depth belo w sea le v el to the nearest foot. Separate gas-w ater contacts for
indi vidual, stack ed blank et sandstones ha v e been identifi ed
in Hico-Kno wles, South Dre w , and Choudrant fi elds (table 2). The se v en fi elds in which gas-w ater contacts ha v e been
described are distrib uted across the blank et-sandstone trend (fi gure 13). Because of the relati v ely uniform distrib ution of high-permeability Cotton V alle y sandstone reserv oirs with

con v entional shale seals in fi elds across the blank et-sandstone trend, it is lik ely that all Cotton V alle y fi elds in this trend ha v e well-defi ned gas-w ater contacts similar to those documented in
the se v en fi elds sho wn in fi gure 13. The Cotton V alle y blank etsandstone trend w as defi ned as a continuous gas accumulation
in the 1995 National Assessment of United States Oil and Gas Resources by the USGS (Schenk and V iger , 1996). Ho we v e r , the presence of ab undant g as-w ater contacts across this area suggests that the blank et-sandstone trend should be redefi ned

as a con v entional gas play

. Ev aluating the presence or absence of g as-w ater contacts in the tight, Cotton V alle y massi v e-sandstone trend is more
diffi cult. No reference to specifi c gas-w ater contacts for Cotton
V

alle y massi v e sandstones in an y Cotton V a lle y gas fi eld has been found in the published literature. Nangle and others (1982) and Dutton and others (1993), ho we v e r , mak e general statements indicating that g as-w ater contacts are present in
Cotton V alle y fi elds across the tight, Cotton V alle y massi v e

sandstone trend. Although T aylor sandstones in the lo wer part of the
Cotton V alle y section produce gas in all signifi cant Cotton
V

alle y fi elds in the tight, massi v e-sandstone trend, w ater

bearing sandstones ha v e been reported along with g as-char ged sandstones in the middle and upper Cotton V alle y interv al in
some fi elds. The seal for gas in w a v e -dominated deltaic T aylor
sandstones at W ask om fi eld reportedly is pro vided by marsh and lagoonal shales (CER Corporation and S.A. Holditch & Associates, 1991). This seal w ould be considered con v entional rather than one pro vided by the lo w permeability of the reser

v oir sandstones. Along with T aylor sandstones, most of the upper Cotton V alle y sandstone interv al produces g as at some fi elds, such as Carthage fi eld, according to Al Brak e (BP

, oral commun., 2000). At other fi elds, such as W oodla wn and
Block er , ho we v e r , gas is produced only from lo wer Cotton
V

alle y T aylor sandstones and from a fe w sandstones in the uppermost Cotton V alle y section. Interv ening middle and upper Cotton V alle y sandstones are reportedly w ater -bearing. The presence of indi vidual gas-bearing and w ater -bearing
sandstone interv als, separated by con v entional shale seals, suggests the presence of g as-w ater contacts and is more indicati v e of con v entional gas accumulations than of continuous gas
accumulations. The comple x diagenetic mineralogy of tight Cotton V alle y sandstones probably precludes use of wireline logs to identify

gas-w ater contacts. As reported abo v e, comple x diagenetic mineralogy of tight Cotton V a lle y sandstones dramatically
af

fects v alues of resisti vity and porosity measured by wireline logs and, hence, the determination of w ater saturation by standard calculation techniques. Because of v ertical and lateral diagenetic v ariations, accurate determination of w ater saturation is diffi cult without accompan ying lithologic data from cores or cuttings to calibrate wireline logs. Examination of production-test data from wells fl anking man y Cotton V alle y gas fi elds in the tight-gas-sandstone trend re v eals no dry holes
that tested w ater without gas. Therefore, e v en if wireline logs pro vided accurate estimates of w ater saturations in tight
Cotton V alle y sandstones, fe w wells apparently e xist in which logs could be used to identify g as-w ater contacts. Reconnaissance e v aluation of DST and production-test
data from Cotton V a lle y sandstones in a number of fi elds in the tight-gas-sandstone trend re v e aled fe w dry holes penetrating Cotton V alle y sandstones on fl anks of those fi elds. No dry holes were found that tested w ater without gas, thereby implying e xistence of a gas-w ater contact for a particular fi eld.
Lik e wise, detailed e xamination of test data from all wells
within and fl anking Oak Hill and Elm Gro v e –Caspiana fi elds in the tight-gas-sandstone trend re v e aled no fl ank dry holes
that tested w ater without gas. Initial rates of gas production from wells on the fl anks
of Cotton V alle y fi elds in the tight-gas, massi v e-sandstone trend, ho we v e r , generally are lo wer than from crestal wells, as illustrated for Caspiana fi eld in fi gure 14. Also, as sho wn for Caspiana fi eld in fi gure 15, the ratio of initial rate of w ater production to initial rate of gas production in terms of bbl w ater/ MMCFG is signifi cantly higher in fl ank 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 w ater to gas generally is less than 200 bbl w ater/MMCFG and often
belo w 100 bbl w ater/MMCFG (fi gs. 14 and 15). Initial rates of gas production from fl ank wells generally are less than 1,000 MCFD, and w ater production initially is signifi cantly higher , usually in the 300 to 600 bbl w ater/MMCFG range,Is There a Basin-Centered Gas Accumulation in Cotton V a lley Group Sandstones, Gulf Coast Basin, U.S.A.? 33



## SCHEMA TIC DIAGRAM OF TRANSITION ZONES IN HIGH- AND LOW -PERMEABILITY SANSTONE RESERVOIRS

CROSS SECTION FLUID SA TURA TION OF STRUCTURE

HIGH PERMEABILITY LOW PERMEABILITY (Cotton V alley blanket sandstones) (Cotton V alley massive sandstones)



### W

A

TER GAS

0 100 50 % S w



### TRANSITION ZONEHEIGHT



### W A

TER

GAS

GAS-W A

T ER CONT ACT W

A

TER GAS (AND W A

TER)

0 50 100 % S w

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

b ut sometimes e xceeding 1,000 bbl w ater/MMCFG (fi gs. 14 and 15). These data suggest a decrease in g as saturation and

an accompan ying increase in w ater saturation in Cotton V a lle y sandstones from crestal wells to fl ank wells and that a commercial limit to gas production has been reached, although

gas-w ater contacts ha v e not been encountered. Corroborating these suggestions from fi gures 14 and 15 and data reported by Nangle and others (1982) is the e xperience
of Al T aylor (Nomad Geosciences, oral commun., 2000) who reports the presence of v ertically e xtensi v e gas-w ater transition
zones in Cotton V alle y sandstone fi elds in the massi v e -sandstone
trend. In mo ving structurally lo wer from the crest of one of these tight-gas Cotton V alle y fi elds through the long gas-w ater transition zone to w ard the presumed gas-w ater contact, gas saturation of sandstone reserv oirs continually decreases while w ater saturation simultaneously increases (fi g. 16). W ells that are lo w in the transition zone on the edges of Cotton V a lle y fi elds in the tight-gas-sandstone trend e xhibit lo w initial rates of g as production and high initial rates of w ater production, as sho wn by some
fl ank wells at Caspiana fi eld in fi gures 14 and 15. Hyperbolic decline rates of g as production in conjunction with lo wer gas saturations of reserv oir sandstones in these transition-zone wells
result in such lo w cumulati v e production of gas that these wells

are mar ginally commercial to noncommercial and, in ef fect, are dry holes (Al T aylor , Nomad Geosciences, oral commun., 2000). Hence, commercial limits of g as production are reached before

gas-w ater contacts can be encountered by de v e lopment drilling. This probably could be documented by mapping cumulati v e gas production or estimated ultimate g as reco v ery from wells in
these Cotton V a lle y fi elds.
Kno wing gas saturations of Cotton V alle y reserv oir sandstones from log calculations and capillary properties of those sandstones from core analyses at Caspiana fi eld in northwestern Louisiana, Al T aylor (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-w ater contacts belo w sea le v el. Estimates made
in this f ashion for the structural le v el of the gas-w ater contact at Caspiana fi eld using data from a number of wells cluster within a zone about 75 ft thick, suggesting the presence of a single gas-w ater contact for the fi eld. A Cotton V alle y well situated structurally belo w this estimated g as-w ater contact reportedly tested w ater without gas from Cotton V alle y sandstones. Because of the v ertically e xtensi v e gas-w ater transition zones present in fi elds producing gas from tight Cotton V alle y sandstones in the massi v e-sandstone trend, Al T aylor (Nomad Geosciences, oral commun., 2000) suggests that structural or stratigraphic traps with less than 150 ft of v ertical closure will
not ha v e suffi cient gas saturation to produce gas at commercial

rates.34 Geologic Studies of Basin-Centered Gas Systems

In summary , Cotton V alle y blank et sandstones across
northernmost Louisiana ha v e suffi ciently high porosity and permeability that gas accumulations e xhibit short transition
zones and ha v e sharp gas-w ater contacts. Gas fi elds in this
trend ha v e clearly defi ned producti v e limits, be yond which wells produce w ater only . Ho we v e r , lo w-permeability Cotton
V

alle y sandstones in the tight-gas massi v e-sandstone trend across northern Louisiana, the Sabine uplift, and East T e xas Basin display long gas-w ater transition zones with poorly
defi ned gas-w ater contacts. Producti v e limits of fi elds in this
trend are diffi cult to defi ne based on data from production tests or wireline logs. In conjunction with long g as-w ater transition zones, structural dips are gentle on the fl anks of these gas
accumulations. As de v elopment drilling progresses do wn the
fl ank of one of these fi elds through the long gas-w ater transition zone, gas saturations in the sandstone reserv oir decrease
and w ater saturations increase. Ev entually gas saturations
become suffi ciently lo w that, in terms of ultimate cumulati v e gas production, wells become mar ginally commercial to noncommercial at a structural position still within the transition
zone abo v e the gas-w ater contact. Consequently , de v elopment
wells on the fl anks of these gas accumulations generally ha v e not encountered gas-w ater contacts. If drilling and completion costs hypothetically were reduced to zero, causing e v en the smallest amount of gas reco v ery to be commercial, de v elopment drilling could progress do wn the full length of transition zones, and gas-w ater contacts probably w ould be encountered. The progressi v e increase in w ater saturation with depth within these tight-gas Cotton V alle y fi elds, therefore, suggests that poorly defi ned gas-w ater contacts are present belo w the depth
at which wells become noncommercial. The presence of gasw ater contacts in both Cotton V alle y blank et- and massi v e

sandstone trends suggests that g as accumulations in these
trends are con v entional and that a basin-centered gas accumulation does not e xist within Cotton V alle y sandstones in the
northern Gulf of Me xico Basin.



# Basin-Centered Gas Potential within Bossier Shale

A basin-centered, continuous-g as accumulation might
ha v e been disco v ered recently in sandstones within the Bossier Shale, the lo wer formation of the Cotton V alle y Group. In a currently de v e loping play on the western fl ank of the East
T

e xas Basin, g as is being produced from turbidite sandstones
within the Bossier Shale. These turbidite sandstones probably
are do wndip time-equi v alent deposits of deltaic sandstones in
the lo wer portion of the Cotton V alle y and reportedly were deposited sea w ard of the underlying Haynesville F ormation carbonate platform edge in a slope or lo wstand-f an setting. Accommodation space w as pro vided by salt withdra w a l such that updip and lateral traps currently are formed by pinch out
of sandstone into shale. T w o stack ed, stratigraphically separate Bossier turbidite-f an systems reportedly occur at depths of 13,000 to 14,000 ft. T w o fi elds, De w and Mimms Creek, with
combined estimated reco v erable reserv es of more than one TCFG, currently are being de v e loped by Anadark o Petroleum, one of the main operators. As of January 2000 (PI-Dwights Drilling W ire, Jan 3, 2000; Jan 12, 2000), Anadark o had drilledmore than 100 wells with only one dry hole in this Bossier sandstone play . Gas-char ged sandstones reportedly are o v erpressured, and no w ater has been encountered in the system (Exploration Business Journal, 2nd quarter , 2000). W ithin the upper
turbidite-f an interv al, porosity ranges from 6 to 15 percent and permeability from 0.01 to 1.0 mD. Initial production rates from
wells a v erage 3 to 4 MMCFGD after fracture stimulation and
decline e xponentially with estimated per -well reco v e ries of 1 to
5 BCFG. In the lo wer sandstone interv al, 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
ha v e been obtained. This play does not seem to in v o lv e the classic type of basin-centered g as accumulation with the trap produced by inherent lo w permeability of reserv oir sandstones. Instead, the trap seems to be pro vided by marine shales that completely encase these turbidite sandstones, b ut the sandstone

reserv oirs are o v erpressured, seem to lack w ater and gas-w ater contacts, and are g as-char ged o v er an e xtensi v e area, as demonstrated by only one dry hole in more than 100 wells drilled.



# Conclusions

1. The Cotton V alle y Group represents the fi rst major
infl ux of clastic sediment into the Gulf of Me xico Basin. Major depocenters were located in south-central Mississippi, along the Louisiana-Mississippi border

, and in northeastern T e xas. Sands supplied by the ancestral Mississippi Ri v er drainage along the LouisianaMississippi border were swept westw ard 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
stack ed barrier -island sands accumulated as the T e r

ryville massi v e -sandstone comple x. Periodic transgressi v e e v ents re w ork ed barrier -island sands, transporting
them northw ard into the lagoon. These transgressi v e sandstones pinch out into lagoonal shales, can be cor

related across northern Louisiana, and are referred to informally as blank et sandstones.
2. T w o major Cotton V alle y sandstone-reserv oir trends
are identifi ed based on reserv oir properties and associated characteristics of gas production. T ransgressi v e, blank et sandstones across northernmost Louisiana ha v eporosities ranging from 10 to 19 percent and permeabilities from 1 to 280 mD. These sandstones fl ow 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 theseIs There a Basin-Centered Gas Accumulation in Cotton V alley Group Sandstones, Gulf Coast Basin, U.S.A.? 35

sandstone reserv oirs were de v eloped during the 1940’ s through 1960’ s. Cotton V alle y massi v e sandstones to the south, and e xtending westw ard across the Sabine uplift into eastern T e xas, e xhibit porosities from 6 to 10 percent and permeabilities generally less than 0.1 mD. Designated as tight-g as sandstones by FERC, these

reserv oirs commonly do not fl o w gas or liquids during
DST’ s, and the y require fracture-stimulation treatments
to achie v e commercial rates of production. Gas production from these sandstones in eastern T e xas and
northern Louisiana w as not established until the mid
1970’ s when adv ances in hydraulic-fracture techniques occurred in conjunction with a signifi cant increase in gas prices as a result of price dere gulation.
3. Porosity and permeability of Cotton V alle y sandstones are controlled by diagenetic properties, which in turn

are go v erned by depositional en vironment. Although diagenetic patterns and mineralogy are comple x, highener gy , clean sandstones generally are cemented by authigenic quartz and (or) calcite and ha v e poor reserv oir properties. In lo wer ener gy sandstones, clay coatings on quartz grains inhibited de v elopment of quartz o v e r g ro wths, resulting in preserv ation of primary porosity . High clay content, ho we v e r , generally imparts poor permeability to these sandstones. The best

reserv oir sandstones are those which ha v e e xperienced
de v elopment of signifi cant secondary porosity from
dissolution of calcite cement and unstable frame w ork grains.
4. The comple x diagenetic mineralogy of tight Cotton
V

alle y sandstones precludes use of standard calculation
methods in reserv oir e v aluation with wireline logs.
Bound w ater associated with pore-fi lling clays or clay coatings and conducti v e minerals such as p yrite result in abnormally lo w resisti vity measurements leading to such high calculated w ater saturations that producti v e zones often appear wet. Also resulting in erroneous

reserv oir e v aluations are pessimistic measurements of porosity with wireline logs caused by the presence of high-density carbonate minerals such as ank erite
and siderite. Therefore, without lithologic data from cores or drill cuttings to calibrate wireline logs, such logs are of limited v alue in dif ferentiating between gas-producti v e and wet interv als and therefore are of
limited v alue in identifying g as-w ater contacts on the
fl anks of Cotton V alle y fi elds.
5. Abnormally high reserv oir pressures with fl uidpressure gradients e xceeding 0.55 psi/ft occur in
Cotton V alle y sandstones in northeastern Louisiana. The boundary between the o v erpressured area on the east and normally pressured re gion to the west cuts across the permeable, blank et- and tight, massi v e-sandstone trends such that o v erpressures occur within both

reserv oir trends. W ithin the blank et-sandstone trend, where pressure data are more ab undant, some Cotton
V

alle y fi elds are o v erpressured whereas adjacent fi elds are normally pressured. Also, within certain fi elds,
some of the stack ed blank et sandstones are o v erpressured whereas others are normally pressured. Such compartmentalization of o v erpressured reserv oirs in proximity to normally pressured ones, rather than
de v elopment of o v e rpressure on a re gional scale, suggests that these blank et-sandstone fi elds are con v entional gas accumulations and not part of a basin-centered gas accumulation. Also, the occurrence of nor

mally pressured reserv oirs across the majority of the tight, Cotton V alle y massi v e-sandstone trend is not
indicati v e of presence of a basin-centered, continuous gas accumulation. Geographic distrib ution of o v erpressures in Cotton V a lle y sandstones suggests that o v er

pressuring w as caused by “infl ation” of e xisting pressures in tightly sealed reserv oirs by g ases deri v ed from emplacement of the nearby Jackson (igneous) dome.
6. Gas found in Cotton V a lle y sandstone reserv oirs is
belie v ed to be deri v ed from (1) interbedded Cotton
V

alle y marine shales, (2) underlying marine shales of the Bossier Shale, and (or) (3) stratigraphically lo wer , Jurassic Smack o v er laminated, lime mudstones. These
source rocks are belie v e d to ha v e been b uried to suffi cient depths relati v e to the re gional geothermal gradient to ha v e generated dry g as during the past 60 m.y

. T

iming of gas generation and migration is f a v orable because it postdates de v elopment of the Sabine uplift,
smaller structures on and fl anking the uplift, and salt
structures in the East T e xas and Northern Louisiana Salt Basins. Stratigraphic proximity of source rocks
to Cotton V alle y sandstone reserv oirs and appropriate thermal maturity and time of generation and migration

w ould be consistent with interpretation of a potential basin-centered gas accumulation.
7. Presence of a gas-w ater contact is perhaps the most
defi niti v e criterion suggesting that a g as accumulation
is con v entional rather than a “sweetspot” within a basin-centered, continuous gas accumulation. W ithin
the Cotton V alle y blank et-sandstone trend across northernmost Louisiana, short gas-w ater transition zones
and well-defi ned gas-w ater contacts ha v e been reported
in se v en gas fi elds. Relati v ely high porosity and permeability of blank et sandstones and associated high g asproduction rates achie v ed without fracture stimulation throughout the trend suggest that all g as fi elds within
the blank et-sandstone trend probably ha v e well-defi ned

gas-w ater contacts; therefore, these gas accumulations

are con v entional.
8. W ithin the tight, massi v e -sandstone trend, porosity and permeability are suffi ciently lo w that g as-w ater transition zones are long and g as-w ater contacts poorly
defi ned. Producti v e limits of these tight-g as-sandstone
Cotton V alle y fi elds are not defi ned by wells that encounter a gas-w ater contact or test w ater without gas from a zone belo w a gas-w ater contact, as in the
blank et-sandstone trend. W ith increasing depth through36 Geologic Studies of Basin-Centered Gas Systems

v ertically e xtensi v e gas-w ater transition zones, gas saturation in reserv oir sandstones decreases and w ater saturation increases. Ev entually g as saturations become
suffi ciently lo w that, in terms of cumulati v e gas production, wells become mar ginally commercial to noncommercial at a structural position still within the transition zone abo v e the gas-w ater contact. Therefore,
de v elopment wells on the fl anks of gas accumulations in the tight, Cotton V alle y massi v e-sandstone trend rarely encounter gas-w ater contacts. If e v en the smallest amount of gas reco v ery were commercial, de v e lopment drilling probably w ould progress do wn the full length of transition zones, and g as-w ater contacts

w ould be encountered in these gas accumulations. The presence of gas-w ater contacts in gas accumulations within the tight, Cotton V alle y massi v e-sandstone trendsuggests that accumulations in this trend, too, are conv entional and that a basin-centered gas accumulation
does not e xist within Cotton V alle y sandstones in the
northern Gulf of Me xico Basin.
9. A basin-centered, continuous gas accumulation might occur in turbidite sandstones within the Bossier Shale,
the lo wer formation of the Cotton V alle y Group. In a currently de v eloping play on the western fl ank of East
T

e xas Basin, gas production with estimated reco v e rable

reserv es e xceeding one TCFG is being obtained from
sandstone reserv oirs, interpreted as slope or lo wstandf an deposits, that are completely encased in marine
shales. Reserv oirs are signifi cantly o v erpressured and

no w ater has been encountered in the system. More than 100 successful wells and only one dry hole ha v e 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