GEOLOGICAL SURVEY CIRCULAR 767 

Current Oi I and Gas 

Production from 

North American 

Upper Cretaceous Chalks 

Current Oil and Gas 

Production from 

North American 

Upper Cretaceous Chalks 

By Peter A. Scholle 

G E 0 L 0 G I C A L S U R V E Y C I R C U L A R 767 

19n 
United States Department of the Interior 

CECIL D. ANDRUS, Secretary 

Geological Survey 

V. E. McKelvey, Director 

Free on application to Branch of Distribution, U.S. Geologica/Survey,1200 South Eads Street,Arlington, VA 22202 
CONTENTS 

Abstract----------------------------------------------------------------------------------

Introduction------------------------------------------------------------------------------

Acknowledgments---------------------------------------------------------------------------

Depositional and diagenetic patterns of chalk---------------------------------------------

Regional geologic studies of chalk--------------------------------------------------------

Gulf Coast---------------------------------------------------------------------------

Stratigraphy and sedimentology--------------------------------------------------

Petrophysical properties and diagenesis-----------------------------------------

Hydrocarbon production----------------------------------------------------------

U.S. Western Interior----------------------------------------------------------------

Stratigraphy and sedimentology--------------------------------------------------

Petrophysical properties and diagenesis-----------------------------------------

Hydrocarbon production-----------------~----------------------------------------

Scotian Shelf------------------------------------------------------------------------

Stratigraphy and sedimentology--------------------------------------------------

Petrophysical properties and diagenesis-----------------------------------------

Hydrocarbon production----------------------------------------------------------

Comparisons with the North Sea------------------------------------------------------------

Summary and exploration models------------------------------------------------------------

References cited--------------------------------------------------------------------------

Figure 1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11-14. 

15-17. 

18. 

ILLUSTRATIONS 

Graph showing relationships between depth of burial, and matrix permeability---

Chart correlating Gulf Coast Upper Cretaceous chalks---------------------------

Index map of Upper Cretaceous paleogeographic features of the Gulf Coast-------

Map showing percent HCl-insoluble material in chalks of Texas------------------

SEM photomicrograph of chalk from the Atco Formation in Texas------------------

SEM photomicrograph of chalk from the Bluffport }1ember in Alabama--------------

Map showing porosity of outcropping Gulf Coast chalks--------------------------

Map showing oxygen isotopic analyses of outcropping Gulf Coast chalks----------

Graph showing oxygen isotopic analyses of subsurface chalk from the Austin 

Group, central and southcentral Texas-----------------------------------------

SEM photomicrographs of progressive diagenetic alteration of chalk of the 

Austin Group------------------------------------------------------------------

Maps: 

11. Regional distribution of cumulative oil production from Gulf Coast 

chalks-----~----------------------------------------------------------

12. Regional distribution of oil fields producing from Gulf Coast chalks---

13. Regional distribution of cumulative gas production from Gulf Coast 

chalks----------------------------------------------------------------

14. Regional distribution of gas fields producing from Gulf Coast chalks---

Graphs: 

15. Number of oil and (or) gas fields producing from the Austin Group as 

a function of depth----------------------------------------------~----

16. Cumulative oil production from the Austin Group as a function of 

depth-----------------------------------------------------------------

17. Cumulative gas production from the Austin Group as a function of 

depth-----------------------------------------------------------------

Stratigraphic section of the Colorado Group of Kansas and eastern Colorado-----

III 

Page 

1 

1 

2 

2 

4 

4 

4 

7 

10 

26 

26 

30 

30 

43 

43 

44 

44 

44 

48 

48 

Page 

4 

5 

5 

6 

6 

7 

7 

7 

8 

9 

11 

25 

26 

27 

28 

29 

29 

30 
Figure 19. 

20. 

21-24. 

25. 

26-29. 

Map showing thickness and regional distribution of Greenhorn Limestone facies--

}1ap showing thickness and regional distribution of Niobrara Formation facies---

SEM Photomicrographs: 

21. Chalk in the Hartland Shale Member (Greenhorn Limestone----------------

22. Chalk in the Fairport Chalky }1ember (Carlile Shale)--------------------

23. Chalk in the Fort Hays Limestone Member (Niobrara Formation)-----------

24. Chalk in the Smoky Hill Chalk Member (Niobrara Formation)--------------

Regional isopach map of the Pierre Shale---------------------------------------

Haps: 

26. Regional distribution of cumulative oil production from Western 

Interior chalks-------------------------------------------------------

27. Regional distribution of oil fields producing from Western 

Page 

31 

32 

33 

33 

33 

33 

39 

39 

Interior chalks------------------------------------------------------- 39 

28. Regional distribution of cumulative gas production from Western 

Interior chalks------------------------------------------------------- 42 

29. Regional distribution of gas fields producing from Western 

Interior chalks------------------------------------------------------- 42 

30. Graph showing typical FDC-CNL log response in gas-bearing zone at top of 

Niobrara---------------------------------------------------------------------- 43 

31. Locality map of Scotian Shelf structural features and exploration wells-------- 43 

32. SEM photomicrograph of chalk from the Wyandot Formation------------------------ 44 

33. Isopach map of Tertiary sediments of the North Sea in relation to chalk oil 

and gas reservoirs------------------------------------------------------------ 46 

34. SE~1 photomicrograph of chalk from the Ekofisk field of the North Sea----------- 67 

Table 1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

TABLES 

Oil and gas production from chalk in Texas--------------------------------------

Oil and gas production from chalk in Louisiana----------------------------------

Oil and gas production from chalk in Mississippi--------------------------------

Oil and gas production from chalk in Alabama------------------------------------

Oil and gas production from chalk in Colorado-----------------------------------

Oil and gas production from chalk in Wyoming------------------------------------

Oil and gas porduction from chalk in New Mexico---------------------------------

Recent gas-producing wells in Niobrara Formation of eastern Colorado and 

western Kansas-----------------------------------------------------------------

Gas shows from the Wyandot Formation on the Scotian Shelf, Canada---------------

Estimated recoverable reserves of oil and gas in chalk of the North Sea area----

IV 

Page 

12 

22 

23 

24 

34 

37 

38 

40 

45 

46 
Current Oi I and Gas Production from 

North American Upper Cretaceous Chalks 

By Peter A. Scholle 

ABSTRACT 

Production of oil and natural gas from 

North American chalks has increased significantly during the past five years, spurred by 

the prolific production from North Sea chalks, 

as well as by higher prices and improved production technology. Chalk reservoirs have been 

discovered in the Gulf Coast in the Austin 

Group, Saratoga and Annona Chalks, Ozan Formation, Selma Group, Monroe gas rock (an informal 

unit of Navarro age), and other Upper 

Cretaceous units. In the Western Interior, 

production has been obtained from the Cretaceous 

Niobrara and Greenhorn Formations. Significant, 

though subcommercial, discoveries of natural 

gas and gas condensate also have been made in 

the Upper Cretaceous Wyandot Formation on the 

Scotian Shelf of eastern Canada. 

All North American chalk units share a 

similar depositional and diagenetic history. 

The chalks consist primarily of whole and 

fragmented coccoliths with subordinate planktonic and benthonic Foraminifera, inoceramid 

prisms, oysters, and other skeletal grains. 

Most have between 10 and 35 percent HClinsoluble residue, predominantly clay. Deposition was principally below wave base in tens to 

hundreds of meters of water. 

The diagenetic history of a chalk is 

critical in determining its reservoir potential. 

All chalk has a stable composition (low-Mg 

calcite) and very high primary porosity. With 

subsequent burial, mechanical and chemical 

(solution-transfer) compaction can reduce or 

completely eliminate pore space. The degree of 

loss of primary porosity in chalk sections is 

normally a direct function of the maximum depth 

to which it has been buried. Pore-water 

chemistry, pore-fluid pressures, and tectonic 

stresses also influence rates of cementation. 

Oil or gas reservoirs of North American 

chalk fall into three main groups: 

1. Areas with thin overburden and significant primary porosity retention (for 

example, Niobrara Formation of Kansas 

and eastern Colorado). 

1 

2. Areas with thicker overburden but 

considerable fracturing. Here primary 

porosity has been largely lost but 

secondary (fracture) porosity provides 

some storage capacity and greatly 

improves permeability (for exanple, 

Austin Group of the Pearsall field, 

Texas). 

3. Areas with thick overburden in which 

marine pore fluids have been retained 

or where hydrocarbons (including ' 

biogenically generated methane) were 

introduced early in the diagenetic 

history. In these settings, primary 

porosity is reduced to a lesser 

degree than in group two, and adequate 

reservoir properties can be maintained 

to depths approaching 2,000 m (6,600 

ft) (for example, Scotian Shelf of 

Canada). 

Continued small-scale oil and gas discoveries can be expected from these type8 of 

reservoirs in North America. The prolific 

production of oil and gas from North Sea chalk 

reservoirs will not be matched in North America 

unless deeply buried, overpressured chalks can 

be located. It is the early formation of 

overpressures and (or) early oil input into 

North Sea chalks that have preserved porosities 

as high as 40 percent at 3,000- to 3,500-m 

(9,800- to 11,500-ft) depths and provide1 the 

outstanding reservoir capacity of those chalks. 

INTRODUCTION 

Oil and gas have been produced from chalk 

reservoirs for at least four decades. The past 

5 years, however, have seen a remarkable 

upsurge in exploration for, and discovery of, 

hydrocarbons in chalk. Study of North s~a 

fields such as Ekofisk, West Ekofisk, Tor, 

Albuskjell, Dan, and others have shown that, 

although the reservoir properties of chalk are 

generally poor, excellent production can be 

attained with sufficient fracturing and a 

sufficiently thick pay section. Production 
rates of 10,000 BOPD (barrels of oil per day) 

per well are common in these areas, and total 

reserves in North Sea chalk exceed 2.5 billion 

bbl (barrels) of oil and 8 trillion ft3 

(cubic feet) of gas (Tiratsoo, 1976). 

Partly as a result of such exploration 

successes in North Sea and Middle Eastern chalk 

reservoirs and partly as a result of the rapid 

rise in crude oil prices over the past few 

years, exploration has increased dramatically 

in North American chalks as well. Significant 

oil and gas discoveries, for example, have been 

made in the Upper Cretaceous Austin Group of 

Texas. Indeed, the Austin drilling was the 

third most important exploration play in the 

United States in 1976 in terms of the number of 

exploration wells drilled (Petroleum Information 

Corporation, 1977). Commercial gas discoveries 

have been made from chalk reservoirs in the 

Niobrara Formation in several areas of Kansas 

and Colorado. In Louisiana, Mississippi, and 

Alabama, increased oil and gas production have 

been obtained from chalk facies of the Annona, 

Selma, Saratoga, and Ozan Formations, Monroe 

gas rock, and other Upper Cretaceous units. 

Finally, in the Scotian Shelf area of the 

Canadian Atlantic continental margin, several 

wells had promising, if subcommercial, shows of 

hydrocarbons. 

This upsurge in exploration activity and 

the high potential for further hydrocarbon 

discoveries in North American chalks make it 

imperative that we gain a better understanding 

of the factors that control the reservoir 

properties of chalk. This paper will outline 

the production histories of chalks of North 

America and Europe and try to isolate those 

factors that improve or reduce the hydrocarbon 

potential of those units. It will also attempt 

to generalize from these relationships in order 

to provide a model for the prediction of 

subsurface reservoir properties of chalks. 

Those who are interested in greater detail 

about the depositional and diagenetic aspects 

of chalk are referred to other recent papers 

including Hancock (1976) and Scholle (1977), 

and to the extensive references contained in 

those papers. 

ACKNOWLEDGMENTS 

I would like to thank the following companies and individuals who kindly supplied samples 

and (or) data for this study: Shell Canada, 

Shell Development Co., Mobil Oil Canada, Exxon 

Co., Mountain Petroleum Corporation, KansasNebraska Natural Gas Co., and T. S.M. Ranneft. 

Discussions with J. M. Hancock, Kelton Cloud, 

and D. E. Hattin were instrumental in formulating many of the ideas in this paper. R. B. 

Halley, G. L. Dolton, Kelton Cloud, C. W. 

Spencer, and J. P. Lockridge reviewed early 

versions of this paper and provided numerous 

suggestions and improvements for which I am 

grateful. 

2 

DEPOSITIONAL AND DIAGENETIC PATTERNS OF CHALK 

The term "chalk" is restricted in this 

paper to include only those fine-grained 

carbonate sediments composed primarily of 

calcareous nannofossils (especially coccoliths) 

and calcareous microfossils (such as Foraminifera and calcispheres). Other compJnents such 

as coarser calcareous skeletal grain2, detrital 

quartz, glauconite, chert, phosphate pellets, 

and clay may be important in some units but are 

subordinate constituents overall. NJ limitation 

is placed on the degree of induration of a 

chalk, because this is an entirely artificial 

concept. Chalk can be found in all stages of 

lithification, ranging from a watery slurry (or 

"ooze") to very hard limestone. 

The restricted, biologically defined 

meaning of the term "chalk" has several important implications. First, true chalk has 

existed only since the Jurassic or Early Cretaceous when coccoliths and planktonic Foraminifera first achieved widespread distribution. 

Second, chalk forms only where calcareous 

nannofossils can live in the overlying water 

column. The accumulation rate of chalk is limited by the rate of production of the constituent organisms. Thus chalk normally forms only 

in areas where clastic terrigenous input is 

low enough that it does not overwheln and mask 

the microfossil and nannofossil production. 

Coccoliths, in particular, have a renarkably 

broad range of salinity tolerance (one species 

is known to live in waters of as low as 11 ppt 

salinity) and can exist in abundance in very 

shallow inland seas or coastal zones. Therefore, the exclusion of diluting constituents 

such as clastic terrigenous debris, siliceous 

microfossils, or coarser carbonate steletal 

grains is normally the critical factor in chalk 

deposition. Given the exclusion of dilutants, 

chalk can form in any water depth from tens to 

thousands of meters. In water depth~ greater 

than about 4,000 m (13,100 ft), exter.sive 

dissolution of CaC03 (compensation depth) 

reduces the probability of accumulating chalk. 

Because of its pelagic origin, chalk 

deposition often marks the times of rraximum 

transgression on continental shelves or interior 

seas. For example, during the Upper Cretaceous, 

a time interval of very high eustatic sea level 

stand, chalk deposition occurred across major 

portions of Europe, North America, and other 

areas. This type of widespread chalk sedimentation results from a combination of factors 

including the reduction of the expose1 area of 

continental sources of terrigenous clastic 

sediment, raising of erosional base levels, and 

enlargement of areas of shelf sedimentation far 

removed from shorelines. These factors, in 

addition to depositional sites generally below 

wave base, also contribute to the lateral 

uniformity of chalk facies. 

During times of lower eustatic sea level 

stand, chalk deposition does not cease; rather 
it retreats from the continental interiors and 

shelves to the continental slopes and deeper 

ocean basins. Chalk deposition was extremely 

widespread in the deep sea during the relatively 

low sea level stands of the Tertiary, as shown 

by the Deep Sea Drilling Project (DSDP) coring. 

Indeed, chalk accounts for about 67 percent of 

the total limestone deposited worldwide over 

the past 100 m.y. (million years) (Hay and 

others, 1976). Most of this chalk is in the 

ocean basins rather than on the continents. 

A third consequence of the biological 

origin of chalk is reflected in its grain size 

and petrophysical properties. Coccoliths (and 

related groups of nannofossils) range in size 

from about 1 to 20 ~m (micrometers) (Bukry, 

1969). These organisms, in turn, are made up 

of individual crystal elements which are between 

0.2 and 1 ~min size (Neugebauer, 1975a). 

Typical chalk, therefore, often has a polymodal 

grain-size distribution, with maxima in the 

0.2-1, 1-20, and 62 ~m ranges (Hakansson and 

others, 1974). 

The very small average grain size of chalk 

has a major effect on its reservoir properties. 

Chalk may be deposited with as much as 70 or 80 

percent porosity, but nearly half of this pore 

space is lost by dewatering during the first 

few tens to hundreds of meters of burial. 

Chalk that has never been buried deeper than a 

few hundred meters generally has between 35 and 

45 percent porosity (Scholle, 1974, 1977). In 

areas where chalk has been deeply buried, 

porosity may be reduced to less than 1 pereent 

by compaction and cementation. Yet even with 40 

percent porosity, a pure chalk will have only 

between 2 and 10 md (millidarcies) permeability 

(Scholle, 1977). When porosity is reduced to 

20 percent, matrix permeability ranges between 

0.1 and 0.5 md. At values representing less 

than 10-percent matrix porosity, permeabilities 

are between 0.05 md and unmeasurably low values. 

Thus, it is apparent that any diagenetic reduction of porosity in chalk carries with it a 

commensurately large reduction in matrix 

permeability. 

The extremely low permeability of chalk, a 

function of intergranular pore diameters 

primarily in the range of 0.1 to 1 ~m (Price 

and others, 1976), implies very slow movement 

of fluids through the rock matrix. Because 

most of the fluid in matrix pores is held by 

capillary and molecular forces, gravity-induced 

drainage of fluids from chalks is a very slow 

and incomplete process except under unusual 

circumstances. Indeed, chalks often act as 

seals rather than as reservoirs where they are 

unfractured. 

A fourth consequence of the nannofossilmicrofossil origin of chalk is apparent during 

diagenesis. Coccoliths (and most other nannofossil groups) as well as planktonic Foraminifera are composed of low-Mg calcite. This 

is the most stable polymorph of calcium carbonate under near-surface temperature and pressure 

3 

conditions. Even the coarser skeletal contributions to chalk (especially inoceramids and 

oysters) are often completely or partly composed 

of low-Mg calcite. In this respect, chalk 

differs radically from most shallow-water 

limestone which originally contains predominantly aragonite and high-Mg calcite. 

This primary chemical stability is a very 

important factor in understanding the reservoir 

properties of chalk. It implies that, because 

generally few unstable components are susceptible to leaching, primary porosity is the only 

porosity likely in chalk except for fractures. 

The very low matrix permeability of chalk also 

contributes to this diagenetic stability, 

especially in deep subsurface settings. Largescale water flow is largely confined to fractures, and the water that moves by means of 

matrix porosity normally is in equilibrium with 

calcite. Secondary porosity development, even 

at near-surface locations where chalk may be 

exposed to extensive freshwater input, is not 

an important factor in most chalk reservoirs. 

Fracturing is the only major exception to this 

rule. Indurated chalk is a brittle rock and 

secondary fracture porosity often plays a 

significant role in reservoir formation, especially through enhancement of overall permeability. Enlargement of fractures by subsurface 

dissolution may have had an effect on permeability relations in a few reservoirs (Woods, 

1963), but this has not been fully demonstrated. 

Primary chemical stability also implies 

that freshwater exposure alone, at near-surface 

conditions, will have little effect on the 

diagenesis of chalk. This situation is very 

different from that of most shallow-water 

limestones such as reefs, oolites, or lagoonal 

carbonate muds, which are highly susceptible to 

wholesale alteration by freshwater in both 

vadose and phreatic environments. Chalks can 

be found on outcrop today (in both North America 

and Europe) that have been exposed continuously 

to freshwater since the Late Cretaceous (approximately 70 m.y.). Yet, where these sediments 

have not been deeply buried, they show porosities of between 35 and 45 percent, with little 

or no evidence of significant secondary porosity 

formation by leaching. 

The main factor controlling the degree of 

diagenetic porosity reduction in chalk is 

burial depth. The principles of this depthrelated diagenesis have been extensively described by Matter (1974), Neugebauer (1973, 

1974, 1975a), Schlanger and Douglas (1974), 

Scholle (1974, 1977, in press), and others and 

will not be elaborated on in detail her£. 

Briefly, all chalk undergoes both mecharical 

and chemical compaction (pressure solution and 

reprecipitation) when subjected to sufficient 

differential stress. This stress is generally 

induced by addition of overburden but can also 

be influenced by tectonic effects and porefluid pressures. Differential intergrarular 

stresses lead to dewatering and grain reorienta-
tion during early stages of diagenesis and then 

induce the formation of solution seams, embayed 

grains, and finally stylolites during later 

stages of compaction. Calcium carbonate dissolved at grain contacts and along solution 

seams is reprecipitated locally at sites of 

lower differential stress (such as open pores). 

In this manner a progressive, depth-related 

loss of porosity is effected. 

The rate of porosity loss during burial is 

affected by a large number of secondary factors: 

1. Pore-water chemistry--Although freshwater does not strongly influence 

porosity of chalk in outcrop or very 

shallowly buried sections, Mg-poor 

fluids (and most freshwater is Mgpoor) will greatly accelerate the 

rate of porosity loss when overburden 

is added (Neugebauer, 1973, 1974, 

1975a). 

2. Primary composition--Variations in 

grain size, clay content, or faunal 

composition can make individual beds 

more or less susceptible to diagenetic 

alteration. Beds having abundant 

coccoliths, for example, often are 

less cemented than those containing 

primarily Foraminifera. This can 

lead to significant bed to bed 

variation in porosity and permeability. 

3. Tectonic stress--Tectonic deformation 

can be as effective as overburden 

load in generating intergranular 

stresses. Thus, in areas of intense 

faulting or folding the rates of 

porosity loss in chalks may be 

accelerated, as shown in the Isle of 

Wight area of England by Mimran 

(1975). Tectonic deformation can 

also lead to an increase in porosity 

and permeability through fracture 

development. 

4. Presence of hydrocarbons--The early 

introduction of hydrocarbons into 

chalk can prevent porosity loss 

during subsequent burial, especially 

complete or nearly complete oil 

saturation is reached (Scholle, 1976; 

Wilson, 1977). These hydrocarbons 

may include biogenically generated 

natural gas formed within the chalk 

or adjacent units. Alternatively, 

the hydrocarbons may have formed in 

much deeper units with subsequent 

upward migration into the porous 

chalk reservoirs. In either case, 

prevention of porosity loss is largely 

accomplished by reduction of water 

flow through (or complete exclusion 

of water from) the reservoir. 

5. Pore-fluid pressure--Compaction 

(either mechanical or chemical) 

depends on the expulsion of pore 

4 

fluids and the generation of strong 

intergranular stresses. When porefluid escape is retarded and higher 

than normal pore-fluid pressures are 

developed, differential intergranular 

stresses are reduced and pressure 

solution and cementation are inhibited. This can have a major effect on 

chalk-reservoir properties (Harper 

and Shaw, 1974; Scholle, 1977). 

The rates of chalk porosity loss as a 

function of burial depth are shown in figure 1. 

"Normal" rates are those produced with marine 

pore fluid, no major tectonic deformation, and 

normal pore-fluid pressure. The "max~_mum" rates 

apply to chalk that has had freshwater input 

before or during burial, has been exposed to 

major tectonic deformation, or has eyoerienced 

both of these factors. No minimum rate is 

shown in figure 1, but with early introduction 

of hydrocarbons or early generation of abnormal 

pore-fluid pressure, porosities as high as 40 

to 50 percent can be maintained to burial 

depths of at least 3,000 to 4,000 m (9,800 to 

13,100 ft). This is illustrated by the North 

Sea chalk reservoirs plotted in figure 1. 

NORTH SEA 

OIL REDVOIRS 

10 

5 

0.5 

0.1 r 

0.05 =i 

:< 

DEPTH, IN METERS 

Figure !.--Relationships between depth of 

burial, porosity, and matrix permeability in 

chalk. "Normal curve" represents units 

buried with marine (or Mg-rich) pore fluids; 

"maximum curve" represents units buried with 

freshwater (or Mg-poor) pore fluids or 

subjected to significant tectonic deformation. 

Approximate present-day porosity-depth data 

for overpressured North Sea chalk reservoirs 

are shown for comparison. Permeability 

relations taken from Scholle (1977). 

REGIONAL GEOLOGIC STUDIES OF CHALK 

Gulf Coast 

Stratigraphy and sedimentology 

Chalk is widely distributed in both outcrop 

and subsurface sections in the Gulf Coast area. 

Coniacian to lower Campanian chalk of the 
Austin Group and its equivalents extends from 

northern Coahuilla in Mexico acro ss Texas and 

into southwest Arkansas (Pessagno, 1969; Powell, 

1970). Equivalent or slightly younger chalk is 

also present within the Selma Group, the 

Saratoga and Annona Chalks, the Monroe gas 

rock, and other units of Louisiana, Mississippi, 

and Alabama . A review of the stratigraphic 

relationships of these units is beyond the 

scope of this paper, but interested readers are 

referred to Cloud (1975), Durham (1957), Freeman 

(1961), Murray (1961), Pessagno (1969), Seewald 

(1967), and Smith (1973) for details of Austin 

stratigraphy, and to Bollman (1968), Braunstein 

(1959), Mellen (1958), Scott (1968), and 

Shreveport Geological Society (1968) for 

information on chalks of Louisiana, Mississippi, 

and Alabama. Generalized correlations of Upper 

Cretaceous chalks of the Gulf Coast are shown 

TEXAS 

,-

' NOITH~"' floii.BAY~NT ~T TEXAf 

' ' ' BH T;:;._ HIGH 

EXPLANATION 

--- Appro•i..,ol• ~pdip li mit of Uppe r 

(reloc e ouo c holh or equi¥olento 

' \ 

Al 

in figure 2. 

In Texas, the Austin Group has been 

-reported to be about 183 m (600 ft) thick in 

the Dallas area (Dallas Geological Society, 

1965). It thins to less than 46 m (150 ft) 

over the Belton high (locality shown in fig. 

3); thickens to as much as 168m (550ft) in 

the type area of the Austin in Travis County, 

thins again to the south to a minimum of about 

Figure 3.--Index map showing location of major 

paleogeographic features influencing the 

deposition and diagenesis of Upper Cretaceous 

chalk of the Gulf Coast. 

30 m (100 ft) across the San Marcos arch, and 

finally thickens again to as much as 183 m (600 

ft) in the Rio Grande embayment (Seewald, 1967; 

EUROPEAN STAGES PROVINCIAL 

STAGES lJ CENTRAL TEXAS ARKANSAS AND / OR LOUISIANA WESTERN ALABAMA 

AND / OR MISSISSIPPI 

I I II I I I I I I I I I I I I I I I I I I I I I I I ?I I I I I I II I II I 

MAESTRICHTIAN 

CAMPANIAN 

SANTONIAN 

CONIACIAN 

TURONIAN 

CENOMANIAN 

0 w CORSICANA MARL ARKADELPIA MARL 0 NAVARRO AN cr:o. cr: CI::::> 

<(0 z >cr: NACATOCH SAND NACATOC H SAND 0 

~(.') NEYLANDVILLE MARL SARA TOGA CHALK ~::! 

BERGSTROM FM. MARLBROOK MARL 

;. a: ~I PECAN GAP CHALK ANNONA CHALK TAYLORAN ;:s(.'JI WOLFE CITY SAND 

SPRINKLE FM . OZAN FORMATION 

a. BURDITT MARL :::l 

0 BROWNSTOWN MARL 

cr: DESSAU FM. ? AUSTINIAN (.') BRUCEVILLE FM . z 

11111 ;::: ATCO FM . TOKIO FORMATION (/) 

:::l 

<( RAPIDES ? SHALE_l/ 

SOUTH BOSQUE FM !-'--'--'-?'-'-'?~?'-'-'? 0 

cr: AKE WA M. Oo. 

LL.:::J EAGLEFORDIAN wo 

-'cr: EAGLE FORD SHALE 1/ ~(.') w LAKE WACO FM . 

PEPPER SHALE MEMBER 

WOODBINIAN OF WOODBINE FORMATION WOODBINE FORMATION 

_1/ FOLLOWING USAGE OF SHREVEPORT GEOLOGICAL SOCIETY ( 1968) 

3J AN INFORMAL NAME 

0 "' PRAIRIE BLUFF CHALK 0 

cr: 

(/) 

<( RIPLEY FM . (.') a. 

=---~~ :::l 

0 

cr: BLUFFPORT MARL MBR (.') 

<( DEMOPOLIS CHALK 

::! 

-' w 

(/) 

MOOREVILLE CHALK 

EUTAW FORMATION 

? 

FORMATION M~~ :L 

?lTTT a. 0:::> 

-'0 GORDO FM . <(cr: 

~(.') ? 

:::l<( 

f--(/) COKER FM . 0 

I IIIII II 

Figure 2.--Stratigraphic correlations of the chalk-bearing Upper Cretaceous rocks of the Gulf 

Coast. Data modified from Hazel. and others (in press), Owens aqd others (1970), Pessagno 

(1969), Shreveport Geological Society (1965). 

5 

I 
Smith, 1973). Much of the Austin Group consists 

of chalk facies, especially the basal Atco 

Formation which constitutes about one-third the 

total thickness of the Austin Group. 

In Louisiana, the Annona Chalk is about 30 

m (100 ft) thick, the Marlbrook Marl (consisting 

of chalk and calcareous shale) is about 53 m 

(175 ft) thick, and the Saratoga Chalk is about 

6 m (20 ft) thick (Shreveport Geological Society , 1965). In western Alabama, the Demopolis, 

Mooreville, and Prairie Bluff Chalks of the 

Selma Group consist predominantly of chalk. 

The Demopolis is 128-151 m (420-495 ft) thick, 

the Mooreville is 79-183 m (260-600 ft) thick, 

and the Prairie Bluff is 3-21 m (10-70 ft) 

thick (Scott, 1968). All three units interfinger with sand and shale to the east. 

Most of the chalk in the Austin, Annona, 

and Selma is massive-bedded, with regular 

alternations of thin marls or calcareous shale 

seams and thicker, less clayey chalks. Similar 

cyclic deposition of chalk has been noted in 

England (Kennedy, 1967), France (Cayeux, 1935) 

and Italy (Arthur and Fischer, 1977). Using 

average sedimentation rates, these cycles 

appear to reflect 20,000- to 100,000-year 

fluctuations in the input of the carbonate 

components (Arthur and Roggenthen, 1976; Arthur 

and Fischer, 1977). 

Figure 4 shows averages of insoluble 

residues for chalk beds in the Atco Formation. 

The data indicate that clay and silt-sized 

terrigenous material were contributed to the 

Austin sea mainly from the north and that the 

Atco Formation in the south Texas area is a 

relatively pure limestone. Chalk and marl of 

the Selma Group in western Alabama, although 

not plotted on figure 4, generally have insoluble residues of 20 to 50 percent. 

0 KILOMETERS 400 

Figure 4.--Percent HCl-insoluble residue in 

outcropping chalk of the lower part of the 

Austin Group in Texas and equivalent units 

in northern Mexico. Each data point represents 

an average of a number of analyses for that 

locality. 

6 

Few primary sedimentary structures are 

seen in the Austin and other Gulf Coast chalks 

because most of the sediment is intensely 

bioturbated. However, the rarity of wave or 

current-generated, large-scale structures (such 

as channels or cross bed sets), the uniform 

preservation of the meter-scale bedding across 

wide areas, and the general absence of bioherms 

indicate deposition below wave base. Exact 

water depths cannot be determined, but deposition of most Gulf Coast chalk probably occurred 

in 30-300 m (100-1,000 ft) of water. Exceptions 

occur locally, as on the San Marcos arch and in 

Figure 5.--SEM photomicrograph of Coniacian 

chalk in the basal part of the Atco Formation 

of the Austin Group from the Atlas Cement 

Co. quarry, southwest of Waco (McLennan 

County), Tex. Coccolith plates and rhabdolith 

spines constitute the major portion of the 

sediment. Sediment has 32 percent insoluble 

residue and about 25 percent porosity. 

Scale bar is 3.3 ~m. 

association with small volcanoes within the 

Austin sea. In these areas, calcarenitic 

limestone, significant unconformities within 

the Austin, and biohermal accumulations are 

indicative of shallower water deposition 

(McNulty, 1971; White, 1960). 

Although faunal diversity is not as great 

as in English chalk of equivalent age, it is 

sufficient to indicate probable normal marine 

salinity throughout the depositional area. 

Gulf Coast chalks are generally rich in coccoliths and planktonic Foraminifera (Clark and 

Bird, 1966; Pessagno, 1969; Smith, 1973) and, 

the comments of Scott (1977, p. 201) notwithstanding, these pelagic chalks are very similar 

in overall composition and texture to those of 

England and the No rth Sea. SEM (scanning 

electron micrography) (figs. 5 and 6) often 

shows a sediment composed almost exclusively of 

coccolith plates and fragments. 
Figure 6.--SEM photomicrograph of chalk from 

the Bluffport Marl Member of the Demopolis 

Chalk of the Selma Group in Marengo County, 

Ala. Sample has abundant coccoliths and 

rhabdoliths, 29 percent insoluble residue, 

and 33 percent porosity. Scale bar is 2 ~m. 

Petrophysical properties and diagenesis 

There is considerable variation in porosity 

and permeability of Gulf Coast chalks, both 

along outcrop and into the subsurface. Figure 

7 illustrates outcrop porosity values for 

chalks of the Austin and Selma Groups. The 

significant loss of porosity south and west of 

13% 

9%. 

0 KILOMETERS 400 

Figure 7.--Porosity in percent, of outcropping 

chalks of the Gulf Coast area. Samples are 

from the San Vicente Formation in northern 

Mexico, the lower half of the Austin Group 

in Texas, and the Selma Group in Alabama. 

Each data point represents an average of a 

number of analyses for that locality. 

Samples were collected in deep roadcuts or 

quarries to minimize weathering effects. 

7 

the San Marcos arch is presumably r elated to a 

number of factors including possible deeper 

burial, higher paleogeothermal gradients, and 

greater tectonic deformation in the Rio Grande 

embayment in conjunction with development of 

the Sierra Madre structural front of northern 

Mexico. The high porosities reported for chalk 

from the Northeast Texas embayment may be due, 

in part, to the very small number of samples 

analyzed from that area. Porosity of Gulf 

Coast chalk also consistently decreases in a 

downdip direction as a direct consequence of 

increased overburden. 

The loss of porosity noted above involves 

solution transfer of calcium carbonate from 

zones of high stress to zones of l ower stress 

(generally open pores). This process is marked 

in the Austin Group by the appearance of 

corroded and embayed grains and stylolites 

(Cloud, 1975). It can also be traced by a 

progressive shift in bulk oxygen isotopic 

values of the chalk as cementation takes place. 

(For an explanation of this process and the 

isotopic data, see Scholle, 1977 and references 

provided therein.) Figure 8 shows oxygen 

isotopic data for outcropping Gulf Coast chalks. 

The least negative values (indicating minimum 

alteration) are located on the San Marcos arch, 

on the Sabine up l ift, and in western Alabama. 

Areas of deeper burial or more tectonism, such 

as the Rio Grande embayment or the Northeast 

Texas embayment, have more negative oxygen 

isotopic values, indicative of greater diagenetic alteration. 

·6.2 . 

· 7 o. 0 KILOMETERS 400 

Figure 8.--Stable oxygen isotopic analyses of 

outcrop samples of Gulf Coast chalks. 

Samples are from the San Vicente Formation 

in northern Mexico, the lower half of the 

Austin Group in Texas, the Annona and 

Saratoga Chalks of Arkansas, and the Selma 

Group in Alabama. Samples were collected 

from deep roadcuts or quarries to minimize 

weathering effects. Data are reported as 

permil deviation from the PDB standard (oo 18 

PDB). Each data point represents an average 

of a number of analyses for that locality. 
Similar results a re seen in figur e 9, 

which shows analyses of cuttings f r om the 

Austin Group in nine wel ls in south-centr al 

Texas . The str ong corr elation between depth of 

bur ial and oxygen isotopic values implies 

progressive cementation and reequilibration of 

the chalk as burial dep th inc r eases. The shift 

in oxygen isotopic values probably is due to 

elevated subsurface temperatures a t which t he 

solut ion- reprecipit a t ion processes t ake place, 

but it may also reflect isotopicall y light 

interstitial waters in the chalk. The depthoxygen iso t opic correlation also impli.es , as 

was shown in figure l, t hat Gulf Coast chalks 

will have ver y low matrix porositi es and permeabilities at dep t hs below about 1,500- 2,000 m 

(4,900- 6 ,600 ft ). 

Although oxygen isot opic and pe t rophysical 

analyses provide quantit ative data on the 

1000 2000 

diagenetic alteration of chalks, the same 

effects can be seen qualitative l y by SEM. 

Figure lOA- C illustra t es some t ypica l fabrics 

of the Austin at various stages of diagenesis. 

Cementation is noted to have been accomplished 

largely by overgrowth of primary grains. 

Different types of gr ains have different susceptibil ities to overgrowth cementation; plankt onic 

Foraminifera readily receive over growths whereas 

many types of coccoliths do not (Ma tter, 1974; 

Neugebauer, l975a ; Wise , 197 3) . Yet even 

coccol iths are eventuall y incorporated into an 

interlocking framework of angular, euhedral 

crystal overgrowths which i ncrease average 

grain size , reduce overall por e space, and 

drastically reduce permeability by narrowing 

pore throa ts. The process is a continuous one 

that can proceed to the stage of virtually 

complete e limination of porosity. 

DEPTH IN METERS 

3000 4000 .5000 1~--------L-------~---------L ________ J_ ________ L-______ _J ________ _L ________ ~------~L-______ _L_ 

- 2 

- 3 

-4 . . 

CD 

0 

"- -5 ::;; 

0 

cr 

u_ 

~ 

0 

<() -6 

-7 

.. . -8 

. . -9 

1000 2000 3000 4000 5000 6000 7000 8000 9000 10,000 11,000 12,000 13,000 14,000 15,000 16,000 

DEPTH IN FEET 

Figure 9. --Trend of 34 stable oxygen isotopic 

analyses of cuttings of chalks of the 

Austin Group from nine wells in central and 

south- central Texas as permil devia tion from 

the PDB standard (ool 8 PDB) versus depth. 

8 

Trend line is a least squares fit to the 

data. Some of t he sample scatter may be 

related to caving of cuttings from overlying 

units or to analysis of calcite from sources 

other than the chalk matrix. 
Figure 10.--SEM photomicrographs of chalk of the Austin Group in various stages of diagenetic 

alteration. A, Sample from the basal part of the Atco Formation, Austin Group from the 

Atlas Cement Co. quarry, southwest of Waco (McLennan County), Tex. Note presence of 

coccoliths, coccosphere, and rhabdolith spines with virtually no cement (porosity 27 

percent). Most grains have primary biogenic outlines. Scale bar is 3.3 ~m. B, Sample 

from Atco Formation, Austin Group at Austin (Travis County), Tex. Sample shows intermediate stage of alteration with coccoliths still visible, but most grains have euhedral 

outlines produced by overgrowth cementation. Crystals are only partly interlocking and 

sample retains 19 percent porosity. Scale bar is 1 m. c, Sample from the Austin Group 

in Shell Oil Co. # 1 Jackson well (Washington County, Tex.) at 4,465 m (14,650 ft) 

depth. Most traces of original biogenic grains have been obliterated by cementation. 

Matrix crystals are completely interlocking and porosity is estimated to be less than 5 

percent. Scale bar is 3 ~m . 

9 
Hydrocarbon production 

The patterns of diagenetic alteration of 

Gulf Coast chalks has had a major effect on th 

distribution of oil and gas reservoirs in thos 

units. Production of hydrocarbons from chalk 

in the Gulf Coast was established nearly 70 

years ago. In Texas, Alta Vista field (Bexar 

Co.) was discovered in 1912, Luling-Branyon 

(Guadalupe Co.) in 1922, and Darst Creek 

(Guadalupe Co.) in 1929. The Pearsall Field 

(Frio Co.), the largest chalk reservoir in 

Texas, was discovered in April 1936 by the 

Amerada and Rycade 1, Tract 6 McWilliams well 

(Scott and Prestridge, 1977). In northern 

Louisiana, commercial production began in the 

Zwolle Field (Sabine Parish) in November 1928 

(Woods, 1963). During the seven decades of 

production, at least 30 million bbls of oil an 

5 billion ft 3 of gas have been produced from 

chalk in Texas; more than 50 million bbls of 

oil have been extracted to date from chalk in 

Louisiana. Available data on lithology of 

producing horizons in Alabama and Mississippi 

are too poor to provide reliable production 

totals. 

Oil and gas production data from chalk of 

Texas, Louisiana, Mississippi, and Alabama are 

shown in tables 1-4. The data are shown by 

county (or parish) and by field. Fields that 

overlap two or more county boundaries are 

listed only under the first county alphabetically. The data are incomplete for a number 

of reasons. In many fields, especially those 

in Louisiana or those discovered before 1940 i 

Texas, no data have been kept on the amount of 

production from each producing unit in a 

multiple-pay field. Thus, for example, in the 

Luling-Branyon field of Texas, although most o 

the hydrocarbon production is from the Lower 

Cretaceous Edwards Limestone, significant 

production is also obtained from the Austin 

Group. No data are available on how much of 

the total field production is from the Austin, 

however. 

In the case of the Pearsall field and 

other areas of intense current exploration 

interest, the data do not necessarily reflect 

the increases in production subsequent to late 

1976. Although the Austin exploration in Texa 

is often economically marginal (with average 

completed well costs of about $250,000), 347 

wells were drilled in the Pearsall area alone 

during 1976 (Petroleum Information Corporation, 

1977). These wells are expected to have an 

average ultimate recovery of about 80,000 bbl 

per well where drilled on 80-acre spacing 

(Stewart-Gordon, 1976), and thus eventually 

will add several million barrels to the total 

production figures. 

In other cases, production from chalks rna 

have been overestimated. Some fields in the 

Austin are associated with volcanic plugs and 

surrounding contemporaneous calcarenitic 

limestone facies. Some of the hydrocarbon 

10 

production listed from the Selma, Saratoga, 

Monroe, Annona, or Ozan may be from sand, 

shale, or other limestone facies rather than 

from chalk. The Eutaw Formation of Alabama and 

Mississippi, in particular, contains both sand 

and chalk facies. Little information is 

generally available about the subsurface facies 

distribution within these units in many producing fields. 

Even with these constraints on the quality 

of the data, some interesting relationships are 

brought out by plotting the data by region. 

Figures 11-14 illustrate the area distribution 

of oil and gas in Gulf Coast chalks by number 

of fields and by cumulative production. It is 

apparent that oil and gas production is concentrated in the areas of the San Marcos arch, the 

Sabine uplift, the Monroe uplift, and the 

Jackson dome. Oil reservoirs, both in terms of 

number of fields and of cumulative production, 

are located mainly at shallow depths (2,000 m; 

6,600 ft or less). Gas production, while 

concentrated at a depth of about 2,000 m (6,600 

ft), extends to depths as great as 4,500 m 

(14,800 ft). These relationships between depth 

of burial, total production, and type of production are shown more clearly in figures 15-17. 

These regional and depth relationships of 

hydrocarbon production fit the patterns predicted from an understanding of the principles 

of chalk diagenesis. Production from paleotopographically positive areas may be related to 

both depositional and diagenetic factors. 

Paleohighs may have been sites of winnowing and 

deposition of more calcarenitic chalk. These 

areas also were subjected to thinner accumulations of overburden and thus underwent less 

diagenetic porosity loss. In these uplift 

areas optimal reservoir characteristics are 

found. Fields at depths of less than 1,000 m 

(3,300 ft) often have porosities as great as 

25-30 percent. For example, along the LulingMexia trend in central Texas, matrix porosities 

of as much as 24 percent have been reported, 

along with as much as 40 percent oil saturation 

of matrix pore space (Doyle, 1955). In fields 

located at greater depth, porosity is much 

lower. In the Pearsall field, at about 1,800 m 

(5,900 ft) of depth, matrix porosity averages 

6-8 percent with an average permeability of 0.0 

to 0.4 md (Doyle, 1955). In most of these 

areas, present burial depths closely approximate 

maximum depths. 

Fracturing is the second major factor 

involved in the location of hydrocarbon reservoirs in chalks. Production from chalk reservoirs of the.Austin Group, as well as other 

Gulf Coast chalks, would be entirely uneconomic 

at depths below about 1,500 m (4,900 ft) were 

it not for imporved permeability related to 

fracturing. In many fractured chalk-reservoir 

fields, initial production rates are quite 

high, commonly as much as 370 BOPD in modern 

wells that have had massive hydraulic fracture 

treatment using sand-oil-gel mixtures (Long, 
1976a, 1976b; Stewart-Gordon, 1976). This 

initial production declines rapidly, in most 

cases, to average values of 60-70 BOPD, and the 

wells need to be pumped after about 6 months 

(Long, 1976a and b). The rapid decline rates, 

coupled with the long-term, low-level productivity of many wells, indicate that while initial 

production is largely from fracture porosity, 

slow drainage of oil from the rock matrix into 

the open fractures often occurs. The initial 

discovery well in the Pearsall field, for 

example, still produced 239 bbl of oil during 

May 1976, over 40 years after the well was 

completed (Scott and Prestridge, 1977). Similarly, farther east, many fields such as 

Pendleton-Many and Fort Jessup in Sabine parish, 

Louisiana, have had long histories of production, chiefly from fracture porosity. 

Fracturing is important in production at 

all depths. In fields at shallow depths, 

fracturing enhances the effective permeability 

of the chalk and efficiently channels oil or 

gas from the matrix pores to the well bore. 

Thus, many of the most productive shallow 

fields are associated with major fracture 

zones. The Dunlap, Fentress, Salt Flat, Tenney 

Creek, Staples, Luling-Branyon, Darst Creek, 

Day, Jayeddie, Jayeddie South, United, Wolf 

Creek, Zoboroski, LaVernia, Saspamco, Philtop, 

and other fields are all associated with the 

Luling-Mexia fault system (Doyle, 1955). In 

deeper fields, fracturing not only enhances 

production but often provides the major porosity 

for hydrocarbon storage. Where matrix p'Jrosity 

is less than about 20 to 25 percent, fra~turing 

is essential for hydrocarbon production. The 

Pearsall field and its extensions, the largest 

single producing field in the Austin Gro•1p, is 

productive only because of development of 

fractures on the crest of the Pearsall anticline. Artificial fracturing of the chalk in 

modern wells in this area improves both longand short-term production, because it allows 

connection of more open fractures with the well 

bore and because it provides a larger surface 

area of fractures for very slow drainage of oil 

from the rock matrix. 

c ~~ c _,,..,._._., • • ' ,,, -

T E 

I 

I 

I 

7 

I 

, 

J 

I 

IO 

, I 

I 

I 

I 

() 

I () (), 

,I 

I 

/. 8 0 

/ "'. 8 () -.: ..~- ..... -:- - () 8~ 

-() 

0 

() 

CUMULATIVE OIL PRODUCTION 

(in barrels) 

0 1000-10,000 

E9 10,000-100,000 

() 100,000-1,000,000 

8 1,000,000-10,000,000 

• 10,000,000-and greater 

--- Approximate updip limit of Upper 

Cretaceous chalks or equivalent~ 

0 KILOMETERS 300 

Figure 11.--Regional distribution of cumulative oil production from Gulf Coast chalks, plotted 

by county or parish. Data are taken from tables 1-4. 

11 
1-' 

N 

Table 1.--0il and gas production, by county, from Upper Cretaceous chalk reservoirs in Texas 

[Data taken from International Oil Scouts Association (1976), University of Oklahoma Petroleum Data System (M. Atkins, 

written commun., 1977), and Texas Railroad Commission records. Leader (---) indicates information not available; 

N.A., data column not applicable; A.C., Austin Chalk; Seg, segment; Mcf, 1,000 ft3] 

Field or pool 

Charlotte A.C. 

Charlotte 

A.C. Seg A. 

Coughran A.C. 

Coughran 

South A.C. 

Coughran 

West A. C. 

Horn A.C. 

Horn Seg B 

Austin. 

Horn Seg C 

Jourdanton 

A. C. 

Pleasanton 

A. C. 

Poteet 

W.P. Holloway 

A. C. 

Stratigraphic 

unit 

Austin Group 

---do------

---do------

---do------

---do------

---do------

---do------

---do------

---do------

---do------

---do------

---do------

Average top 

of producing 

formation 

Feet Meters 

6,435 

6,345 

6,330 

6,385 

6,265 

6,203 

6,450 

6,703 

6,506 

7' 726 

3,567 

6,100 

1,961 

1,934 

1,929 

1,946 

1,910 

1,891 

1,966 

2,043 

1,983 

2,355 

1,087 

1,859 

Average 

API 1 

gravity 

(degrees) 

Total 

wells 

Atascosa County 

38 

37 

23 

27 

23 

34 

37 

39 

32 

N.A. 

41 

3 

1 

10 

2 

1 

5 

1 

3 

1 

1 

3 

2 

Cumulative 

oil (bbl) 

(through 

1976 8) 

141,700 

1,599 

248,511 

13,267 

2,236 

128,458 

92,047 

192,919 

1,309 

0 

99,983 

644,186 

Tota1--------------------------------------------------------------------------qhh,21~ 

Cumulative 

gas (Mcf) 

(through 

1974) 

4,008 

38 

151,918 

3,526 

2,190 

56 

1111, ?)f) 

Current 

oil 

(bbl) 

(19768) 

0 

0 

58,194 

0 

0 

480 

0 

0 

0 

0 

1,466 

72,oos 

12 'l'f5 

Current 

gas 

(Mcf) 

(1976) 

0 

0 

33 

0 

0 

45 

0 

0 

0 

0 

513 

12 

103 
Bastrop County 

Bastrop A.C. Austin Group 2,496 761 28 1 4,236 

Bateman A.C. ---do------ 2,139 652 39 3 19,617 

Elgin ---do------ 2,855 870 36 --- 3,565 

Paige Austin ---do------ 6,792 2,070 33 1 3,043 

Paige NE A.C. ---do------ 6,143 1,872 --- 1 --- Peg A.C. ---do------ 1,549 472 37 3 16,060 

Peg N A.C. ---do------ 1,559 475 38 3 58,696 

Red Rock W ---do------ 2,208 673 42 1 1,406 

A. C. 

Total--------------------------------------------------------------------------106,623 

Bexar County 

Alta Vista Chalk 1,180 360 --- --- 56,494 f-' Leon Creek Austin Group 1,100 335 18 1 0 L.oJ 

Losoya A.C. ---do------ 1,855 565 17 4 32,797 

Phil top ---do------ 1,100 335 23 --- 8,171 

Rinehart A.C. ---do------ 1,163 354 20 67 625,754 

Total--------------------------------------------------------------------------723,216 

Brazos County 

Ferguson Austin Group 11,016 3,358 47 1 15,403 

Crossing A.C. 

Burleson County 

Northeast Clay Austin Group 11,436 3,486 N.A. --- ---

0 

0 

0 

0 

0 

0 

685 

0 

0 

636 

0 

1,321 

0 

0 

6,479 

6,479 

0 

24 

0 

0 

0 

11 

0 

35 

0 

11 

0 

31 

42 

0 

0 
Table 1.--0il and gas production, by county, from Upper Cretaceous chalk reservoirs in Texas--Continued 

Average top 

of producing Average Cumulative Cumulative Current 

Field or pool Stratigraphic formation API 1 Total oil (bbl) gas (Mcf) oil 

unit gravity wells (through (through (bbl) 

Feet Meters (degrees) 1976 8 ) 1974) (1976 8 ) 

--

Caldwell County 

Bob Rose Austin Group 1,975 602 39 22 48,859 --- 0 

Drunnnond ---do------ 1,382 421 36 6 32,496 15,692 1,945 

1400 Austin. 

Dunlap ---do------ 2,123 647 36 292 4,660,040 --- 135,850 

Fentress ---do------ 1,608 490 

Fentress East ---do------ 1,615 492 36 1 

Joliet 2000 ---do------ 1,901 579 --- 1 353,817 --- ---

Lane A.C. ---do------ 1,609 490 41 1 1,427 --- ---

Luling- Austin Group and 1,834 559 36 1,752 1-' Branyon . Edwards Limestone. .1:--

O'Leary A.C. Austin Group 1,932 589 37 3 10,546 --- ---

Salt Flat Austin Group and 2,250 686 36 61 

Edwards Limestone. 

Staples Austin Group 695 2i2 23 23 30,496 --- 441 

Tenney Creek ---do------ 2,335 712 36 75 1,608,124 --- 37,308 

-- Total------------------------------------------------------------------------6,745,805 15,692 175,544 

Dinnnit County 

Espantosa Austin Group --- --- --- 1 34,234 34,234 

Lake 

Evergreen ---do------ --- --- --- 1 6,322 6,322 

Farms. 

Fitz A.C. ---do------ 5,374 1,638 44 1 720 

Thirteen ---do------ 5,808 1. 770 34 1 77,219 999 

Austin. 

Total--------------------------------------------------------------------------118,495 41,555 

Current 

gas 

(Mcf) 

(1976) 

0 

340 

5,905 

0 

0 

0 

18 

363 

6,626 

7 

9,658 

0 

54 

9,719 
Fayette County 

Monument Hill Austin Group 10,633 3,241 47 1 3,402 0 

Austin. 

Frio County 

Dilley Austin Group --- --- --- 9 86,437 86,437 5,932 

Divot ---do------ 5,533 1,686 --- --- 6,003 --- Doering Ranch ---do------ 5,500 1,676 --- --- 4,522 --- East Derby ---do------ --- --- --- 8 45' 776 45,776 2,047 

Frio Town ---do------ 2,600 792 --- --- 666 0 

Hindes A.C. ---do------ 7,106 2,166 25 1 2,475 0 

Pearsall A.C. ---do------ 5,350 1,631 35 7232 11,756,510 4,759,850 1,701,635 

Pearsall ---do------ --- --- --- 9 107,841 107,841 58,286 

t-' South. V1 Sand Hollow ---do------ 4,934 1,504 --- --- 2,381 -- 3700 SD. 

Schattel ---do------ 5,300 1,615 --- --- 2633 3o 0 

Stacy ---do------ --- --- --- 3 38,655 38,655 8 

Total-----------------------------------------------------------------------12,051,899 5,038,559 1,767,908 

Gonzales County 

Austin Pierce Austin Group 10,838 3,303 43 2 119,176 3,341 83 

Cheap side ---do------ 10,108 3,081 39 1 15,979 7,455 0 

NW A.C. 

Cosmopolitan ---do------ - 6,824 2,080 --- --- 2,306 2,306 2 

Ottine ---do------ 3,790 1,155 32 --- 12,000 0 

Mag ---do------ --- --- --- 1 67,354 67,354 27,109 

Pilgrim ---do------ --- --- --- 1 17,060 17,060 3,270 

Total--------------------------------------------------------------------------233,875 7,455 90,061 30,464 
Table 1.--0il and gas production, by county, from Upper Cretaceous chalk reservoirs in Texas--Continued 

Average top 

of producing Average Cumulative Cumulative Current Current 

Field or pool Stratigraphic formation API 1 Total oil (bbl) gas (Mcf) oil gas 

unit gravity wells (through (through (bbl) (Mcf) 

Feet Meters (degrees) 1976 8 ) 1974) (1976 8 ) (1976) 

-

Guadalupe County 

Alderman Austin Group 1,748 533 23 1 1,360 --- 135 12 

Berlin New ---do------ 1,650 503 --- --- 2387 

Carver- ---do------ 2,450 747 34 1 57,169 --- 47 0 

Kallison. 

Chas. N. Jones ---do------ 2,402 732 14 2 4,045 34 --- 1 

Clark ---do------ 2,360 719 34 --- 424,471 --- --- 0 

Darst Creek ---do------ 2,240 683 33 769 --- --- --- 0 

Day A.C. ---do------ 2,554 778 38 4 1,834 --- --- 0 

Geise-Springs Austin Group and 1,912 583 137 1 1,040 --- 0 0 1--' 

0'\ 1900. Buda Limestone. 

Jayeddie Austin Group 2,015 614 29 7 72,670 --- --- 0 

Jayeddie ---do------ 1,970 600 25 3 89,891 --- --- 0 

South. 

John Deering ---do------ --- --- --- --- 17,684 --- 33 2 

La Vernia ---do------ 1,716 523 17 3 65,792 --- 2,878 24 

A. C. 

Luling- ---do------ 1,713 522 --- 3 --- --- --- 0 

Branyon 1700 

Luling- ---do------ 1,608 490 1 1 --- --- --- 0 

Branyon 1600 

Manford ---do------ 2,290 698 37 2 434,825 --- --- 0 

Maple Hughes ---do------ 1,552 473 16 2 3,715 72 --- 0 

1500 Burton 

Mill Creek ---do------ 2,182 665 33 3 35,956 --- --- 0 

Sal Mar ---do------ 2,040 622 --- --- 211,427 

United ---do------ 2,582 787 39 --- 17,364 --- --- 0 

Wolf Creek ---do------ 2,041 622 36 8 35,497 --- --- 0 

Zoboroski ---do------ 2,200 671 35 30 387,460 --- --- 0 

Total------------------------------------------------------------------------1,662,587 106 3,093 39 
...... 

-....! 

Schubert A.C. 

Mt. Calm 

Turpentine 

10390 Lime. 

Stratton A.C. 

West Lower. 

Austin Group 

Austin Group 

Austin Group 

Austin Group 

Stratton A.C. ·---do------

West Middle. 

Stratton A.C. ---do------

West Upper. 

741 

607 

10,390 

6,595 

226 

185 

3,167 

2,010 

Hays County 

-

25 1 

Hill County 

--

31 

--

Jasper County 

49 

Jim Wells County 

62 

6 

1 

1 

5 

6 

Total---------------------------------------------------------------------------

Falls City 

A. C. 

Labus Austin 

Austin Group 

---do------

10,258 

10,053 

Karnes County 

3,127 40 2 

3,064 41 1 

1,084 

1,760 

16,795 

1, 729 

Total---------------------------------------------------------------------------18,524 

Stratton 

A.C. Lower. 

Austin Group 6,880 2,097 

Kleberg County 

57 1 2o 

10,712 

330,909 

330,909 

0 

4,299 

4,299 0 

1,390,699 0 

0 

0 

0 

0 

231,739 

862,660 

1,094,399 

0 

0 

0 

152,058 
,_.. 

00 

Table 1.--0il and gas production, by county, from Upper Cretaceous chalk reservoirs in Texas--Continued 

Field or pool 

Millet West 

Ronnie SW 

A. C. 

Stratigraphic 

unit 

Austin Group 

---do------

Average top 

of producing 

formation 

Feet Meters 

8,218 2,505 

Average 

API 1 Total 

gravity wells 

(degrees) 

La Salle County 

30 

1 

1 

Cumulative 

oil (bbl) 

(through 

1976 8 ) 

1,146 

676 

Total----------------------------------------------------------------------------1,822 

Giddings A.C. 

Giddings A.C. 

East. 

Austin Group 

---do------

7,480 

8,190 

2,280 

2,496 

Lee County 

40 

37 

3 

1 

610,186 

78,242 

Total--------------------------------------------------------------------------688,428 

Madison County 

Halliday A.C. Austin Group 8,174 2,491 39 2 5,500 

Marion County 

Caddo Saratoga Chalk 1,400 427 

Potter Austin and 2,470 753 40 100 3,660,139 

Navarro Groups. 

Total------------------------------------------------------------------------3,660,139 

Cumulative 

gas (Me£) 

(through 

1974) 

640 

640 

63 

63 

16,358 

Current 

oil 

(bbl) 

(1976 8 ) 

1,146 

1,146 

158,072 

12,035 

170,107 

15,869 

15,869 

Current 

gas 

(Me£) 

(1976) 

7 

0 

7 

250,914 

24 

250,938 

0 

1,328 

1,328 
I-' 

\0 

Maverick County 

Burr, North Austin Group --- --- --- 1 14,138 

Chittim A.C. ---do------ 1,440 439 17 5 149,707 

Four Aces A.C. ---do------ 4,040 1,231 37 1 39,968 

Jay-Boy ---do------ ca. 4,300 1,310 --- 1 1,688 

Robert Cage ---do------ --- --- --- 3 2,791 

Sacatosa ---do------ 2,780 847 31 1 4,873 

2780 Austin. 

Total--------------------------------------------------------------------------213,165 

Cherokee 

A.C. Lower. 

Lower pt., 

Austin Group. 

Agua Dulce 

Austin L. 

Agua Dulce 

Austin L. 

Agua Dulce 

Austin U. 

Austin Group 

6850. 

---do------

---do------

5,652 1,723 

6,850 2,088 

Milam County 

33 6 159,525 

Nueces County 

N.A. 2 

6 

N.A. 3 5,184 

Total----------------------------------------------------------------------------5,184 

Maggie Austin Austin Group 

Macedonia 

Chalk. 

Austin Group 

6,814 2,077 

6,380 1,945 

Robertson County 

34 1 5,229 

San Augustine County 

38 2 2,435 

10,710 12 

0 

40 0 

1,688 1,241 

2,791 13 

1,676 --- 0 

1,676 15,229 1,266 

8,613 48,991 83 

0 0 

473,808 

5,184 235,014 

5,184 708,822 

41,339 2,561 1,339 

0 
Table 1.--0il and gas production, by county, from Upper Cretaceous chalk reservoirs in Texas--Continued 

Average top 

of producing Average Cumulative Cumulative Current Current 

Field or pool Stratigraphic formation API 1 Total oil (bbl) gas (Mcf) oil gas 

unit gravity wells (through (through (bbl) (Mcf) 

Feet Meters (degrees) 19768 ) 1974) (1976 8 ) (1976) 

--

Shelby County 

Center Saratoga Chalk 2,204 672 42 2 136,349 437,148 85,083 37,148 

Saratoga. 

Van Zandt County 

Van A.C. Austin Group 2,662 811 34 6 203,574 26,057 29,739 6,053 

N 

0 Washington County 

Brenham Austin Group 14,800 4,511 N.A. --- --- 495,373 --- 44,974 

Webb County 

Melinda-Mag Austin Group 9,752 2,972 --- 1 2o 27,08? 3o 0 

Wilson County 

Dismukes A.C. Austin Group 4,913 1,497 32 1 45,551 --- --- 0 

Donoho Ch. Upper Chalk 4,341 1,323 32 1 1,016 22 0 0 

Upper. 

Floresville Austin Group 4,885 1,489 32 2 56,751 --- 0 0 

E A. C. 

Higgins ---do------ 5,612 1,711 40 6 178,078 --- --- 0 

La Vernia ---do------ 2,812 857 33 6 137,244 --- --- 0 

A.C. Crevice. 
N 

McLain ---do------ --- --- --- 1 68,798 

Pawlelek ---do------ --- --- --- 1 18,887 

Path N A.C. ---do------ 6,157 1,877 34 1 39,414 

Path E A.C. ---do------ 7,255 2,211 30 1 14,884 

Saspamco ---do------ 2,833 863 24 50 1,217,796 

Stockdale A.C. ---do------ 4,882 1,488 31 4 285,985 

Stockdale ---do------ 4,826 1,471 34 5 200,913 

E Chk L. 

Stockdale ---do------ 4,755 1,449 33 1 93,399 

E Chk U. 

Stockdale ---do------ 4,848 1,479 36 1 43,300 

N Chk U. 

Stockdale ---do------ 4,768 1,453 34 2 41,248 

W Chk U. 

Sutil ---do------ --- --- --- 1 545 

Total------------------------------------------------------------------------2,383,809 

Goodwell 

Jeune 

Leta AC-A 

Leta AC-B 

Lorna Vista 

Austin Group 

---do------

---do------

---do------

---do------

Zavala County 

1 

2 

2 

5,162 

9,115 

129,944 

3,883 

8,022 

Total--------------------------------------------------------------------------156,126 

Texas total-----------------------------------------------------------------30,270,173 

1API, American Petroleum Institute. 

2Cumulative production to December 31, 1974. 

3Production from January 1, 1974 to January 1, 1975. 

4cumulative production to December 31, 1976. 

5Production from January 1, 1976 to January 1, 1977. 

6Cumulative production to June 1, 1976. 

7Production from June 1, 1975 to June 1, 1976. 

8rncludes condensate. 

5,963 

1,319 

7,304 

640,570 

3,627 

644,197 

2,187,458 

78,798 

18,887 

0 

328 

1,477 

545 

30,035 

5,162 

9,115 

86,481 

1,489 

8,022 

110,269 

5,882,970 

3 

7,576 

0 

0 

6 

0 

0 

0 

0 

47 

8 

7,640 

8 

3,645 

49,042 

673 

6,486 

59,854 

4,180,845 
N 

N 

Table 2.--0il and gas production, by parish, from Upper Cretaceous chalk reservoirs in Louisiana 

[Data taken from International Oil Scouts Association (1976). 

Leader (---) indicates information not available; Mcf, 1,000 ft 3 .] 

Parish 

BossierCaddo. 

Caddo 

E & W 

Carroll. 

Desoto 

MorehouseOuachitaUnion. 

Sabine 

Field or pool 

Millers Bluff 

Caddo-Pine 

Island. 

Epps 

Logansport 

Oxford 

Red RiverBull Bayou 

Spider 

Monroe 

Pendleton 

Many. 

---do----

---do----

Blue Lake 

Converse 

---do---

Fort Jessup 

Many New 

---do---

Pleasant Hill 

Rattan 

---do...,--

Zwolle 

---do---

and Ozan Fm. 

Stratigr<lphic 

unit 

Annona Chalk 

---do-------

Monroe gas rock 

Annona Chalk 

Saratoga Chalk 

Saratoga and 

Annona Chalks. 

Annona Chalk 

Monroe gas rock 

Saratoga Chalk 

---do---------

Annona Chalk 

Chalk 

Annona Chalk 

and Ozan Fm. 

Saratoga Chalk 

---do---------

Annona Chalk 

Saratoga Chalk 

Annona Chalk 

Austin 

equivalent. 

Saratoga Chalk 

---do---------

Annona Chalk 

Average top 

of producing 

formation 

Feet Heters 

2,300 

1,400 

2,300 

1,545 

1,807 

850 

1,626 

2,145 

2,840 

2, 771 

3,074 

2,100 

1,500 

1,716 

3,017 

3,074 

2,771 

1,974 

6,890 

5,566 

2,230 

2,100 

701 

427 

701 

471 

551 

259 

496 

654 

866 

845 

937 

640 

457 

523 

920 

937 

845 

602 

2,100 

1,697 

680 

640 

Average 

API 1 

gravity 

(degrees) 

42 

42 

41 

41 

42 

42 

45 

23 

47 

42 

47 

42 

43 

62 

60 

42 

Total 

wells 

11 

29 

4,100 

803 

18 

512 

200 

3 

431 

Cumulative 

oil (bbl) 

(through 

1975 2 ) 

19,774,705 

161,510 

5,612,981 

7,571,179 

533 

564,150 

16,325,485 

Louisiana Total-------------------------------------------------------------------50,010,543 

lAPI, American Petroleum Institute. 

2Includes condensate. 

Cumulative 

gas (Hcf) 

(through 

1974) 

3,489,342 

814,794 

2,243,453 

89,393 

6,636,982 

Current 

oil 

(bbl) 

(1975 2 ) 

0 

94,662 

0 

55,947 

36,851 

0 

93,339 

280,799 

Current 

gas 

(Hcf) 

(1974) 

0 

170,540 

0 

2,632 

173,172 
N 

w 

Table 3.--0il and gas production, by county, from Upper Cretaceous chalk reservoirs in Mississippi 

[Data taken from International Oil Scouts Association (1976). Leader (---) 

indicates information not available; Mcf, 1,000 ft3.] 

Average top 

of producing Average Cumulative Cumulative 

County Field or pool Stratigraphic formation API 1 Total oil (bbl) gas (Mcf) 

unit gravity wells (through (through 

Feet Meters (degrees) 19752 ) 1974) 

Clark Junction City Selma Group 2,950 899 18 --- 119,108 21' 779 

Langsdale ---do------ 3,421 1,043 19 14 368,160 119,473 

West Selma 

Chalk Oil. 

Total--------------------------------------------------------------------------------487,268 141,252 

Forest- Piston Ridge Selma Group 6,340 1,932 45 1 20,200 4 

Pearl 

River. 

Hinds- Jackson ---do------ 2,420 738 --- 157 --- 117' 777,195 

Rankin. 

Jasper Heidelberg ---do------ 3,921 1,195 --- --- 353 440,669 

Jones- Sharon ---do------ 4,430 1,350 59 --- 137 123,034 

Jasper. 

Jefferson Gwinville ---do------ 6,700 2,042 52 2 362 94,610 

DavisSimpson. 

Lamar- Baxterville ---do------ 6,560 1,999 54 --- 6,672 1,931,441 

Marion. 

Madison Flora ---do------ 4,357 1,328 26 33 5,113,652 6,340 

Selma Gas 

Rock Oil. 

Rankin Pearl Selma ---do------ 2,480 756 16 1 --- 91,227 

Gas Rock. 

Sharkey Cary Selma Gas ---do------ 3,270 997 25 7 153,458 --- Rock Oil. 

Yazoo- Pickens ---do------ 4,434 1,351 38 --- 1,432,894 293,655 

Madison. 

Yazoo Tinsley ---do------ 4,400 1,341 --- --- 189,199,274 93,225 

Mississippi Total----------------------------------------------------------------1Q~,414,270 120,Q92,~"i2 

1API, American Petroleum Institute. 

2 Includes condensate. 

Current 

oil 

(bbl) 

(1975 2 ) 

4,575 

0 

4,575 

3,460 

---

12 

0 

111 

432 

269,551 

---

0 

17,116 

1,208,287 

l,"i01,'144 

Current 

gas 

(Mcf) 

(1974) 

0 

0 

0 

0 

23,907 

90,224 

0 

22,469 

151,242 

0 

2,860 

9,765 

0 

100,4n7 
N 

.p. 

Table 4.--0il and gas production, by county, from Upper Cretaceous chalk reservoirs in Alabama 

[Data taken from International Oil Scouts Association (1976). Leader (---) 

indicates information not available; Me£, 1,000 £t 3 .] 

Field or pool 

Gilbertown 

---do-----

Gilbertown 

South. 

Langsdale 

Langsdale East 

Stratigraphic 

unit 

Selma Group 

Eutaw Formation 

---do----------

---do----------

---do----------

Average top 

of producing 

formation 

Feet Meters 

2,578 786 

3,480 1,061 

3,423 1,043 

3,742 1,141 

3,794 1,156 

Average 

APil 

gravity 

(d,egrees) 

Total 

wells 

Choctaw County 

20 118 

20 

17 4 

19 3 

19 1 

Cumulative 

oil (bbl) 

(through 

19752 ) 

10,627,933 

55,887 

151,988 

204,159 

Tota1-----------------------------------------------------------------------11,039,967 

Alabama tota1---------------------------------------------------------------11,039,967 

lAPI, American Petroleum Institute. 

2 Includes condensate. 

Cumulative 

gas (Me£) 

(through 

1974) 

Current 

oil 

(bbl) 

(1975 2 ) 

157,530 

8,190 

2,975 

168,695 

168,695 

Current 

gas 

(Me£) 

(1974) 
E9 1-4 

~ 5-8 

() 9-12 

8 13-16 0 KILOMETERS 300 

• 17-20 

Approximate updip limit of Upper 

Cretaceous chalks or equivalents 

Figure 12.--Regional distribution of oil fields producing from Gulf Coast chalks, plotted by 

county or parish. Data are taken from tables 1-4 and are based, in part, on state 

classification of fields as oil or gas producers. 

Gas production can be extended deeper than 

oil production because of the greater ease of 

getting gas to move through the extremely small 

pore throats and overall low permeability of 

deeply buried sediments. Yet, even here, 

fracturing is critical for the economic viability of any given well. 

Fracture or fault systems may also be 

important as conduits for migrating hydrocarbons 

into chalks. Much of the oil in the LulingBranyon, Darst Creek, and other associated 

fields may have migrated up from the underlying 

Edwards Limestone through faults within the 

Luling-Mexia system (McCallum, 1933). In other 

areas, oil may have been generated in the Eagle 

Ford Group which underlies the Austin Group. 

Upward migration of hydrocarbons would have 

been maximized in areas of intense fracturing 

of the Austin. In Louisiana as well, upward 

migration of oil through fractures from the 

Ozan and other units into overlying chalks has 

occurred in a number of important fields (Woods, 

1963). 

25 

Fracture systems can be enlarged through 

solution (Woods, 1963), so that in some areas, 

despite very low matrix permeabilities, chalk 

sections are major zones of fluid movem~nt. 

Indeed, in England the chalk is a major aquifer 

supplying about 15 percent of all the p~table 

water in the country (Price and others, 1976). 

Water-flow rates as much as 2 km/day have been 

measured in this unit (Atkinson and Smith, 

1974), and exchange of fluids between the 

matrix pores and fractures is considerei to be 

insignificant as a result of the extrem~ly 

small pore sizes (Price and others, 1976). 

On the other hand, fracture system~ can 

undergo cementation with calcite and other 

minerals and can therefore also be rend~red 

ineffective as fluid conduits. Thus, s~me 

areas that might be expected to have re~ervoir 

potential because of structural deformation and 

fracturing may, in fact, be virtually impermeable owing to later cementation of the open 

fractures. Prediction of these conditions 

prior to drilling is very difficult. 
CUMULATIVE GAS PRODUCTION 

(Mcf) 

0 1000-10,000 

ffi 10,00-100,000 

() 100,000-1,000,000 0 KILOMETERS 

8 1,000,000-10,000,000 

• 10,000,000-and greater 

---Approximate up dip limit of Upper 

Cretaceous chalks or equivalents 

Figure 13.--Regional distribution of cumulative gas production from Gulf Coast chalks, plotted 

by county or parish. Data are taken from tables 1-4. 

Thus, oil and gas production from Gulf 

Coast chalks to date has depended on the presence of a number of factors. Shallow burial 

(and paleoburial) allow retention of significant 

matrix porosity and hydrocarbon storage capacity 

of the formation. Fracturing greatly increases 

the effective permeability of the sediment and 

often provides pathways both for migration of 

the hydrocarbons into the chalk and later for 

withdrawal of them during production. Effective 

exploration in Gulf Coast chalks maximizes one 

or both of these factors. 

U.S. Western Interior 

Stratigraphy and sedimentology 

Chalx deposition was widespread, both in 

time and space, in the Upper Cretaceous seaway 

of the U.S. Western Interior. Chalk facies 

were particularly extensive during transgressive 

intervals of the late Cenomanian-middle Turonian 

and Coniacian-Campanian (Kauffman, 1967, 1973; 

26 

Hancock, 1975). Although the discussion in 

this section will focus mainly on the Niobrara 

Formation, significant thicknesses of chalk are 

present, both in outcrop and in the subsurface, 

in the Greenhorn Limestone (especially the 

Hartland Shale and Jetmore Chalk Members) and 

in the Carlile Shale (particularly the Fairport 

Chalky Member) (stratigraphy shown in fig. 18). 

The regional extent and thickness of chalk and 

associated carbonate facies of the Greenhorn 

and Niobrara Formations are shown in figures 19 

and 20. Toward the west, both units grade 

progressively into calcareous shales, noncalcareous shales, and sandstones; to the east, 

facies equivalents have been removed l'Y erosion. 

Details of the stratigraphy and facies 

distribution of the Upper Cretaceous strata of 

the Western Interior have been described by 

many authors and will be only briefly reviewed 

here. For further information, readers are 

referred to papers by Cobban and Scot1i; (1972), 

Hancock (1975), Hattin (1962, l965, 1975a), 
Kauffman (1967, Reeside (1944): Scott and 

Cobban (1964), and Weimer (1960). 

The Lincoln Limestone Member of the Greenhorn Limestone is composed predominantly of 

shaly chalk in beds ranging in thickness from 3 

em to 2m (0.1-6.7 ft). The unit reaches a 

maximum thickness of 10 m (33 ft) in Kansas 

(Hattin, 1975a) and 11 m (37 ft) in southcentral Colorado (Cobban and Scott, 1972). The 

Hartland Shale Member of the Greenhorn Limestone 

has been reported to range from 3-21 m (9-68 

ft) in thickness in Kansas (Hattin, 1975a) and 

is 18 m (59 ft) near Pueblo, Colo. (Cobban and 

Scott, 1972). Although primarily a calcareous 

or chalky shale, the Hartland has several thick 

beds of burrowed, lithified chalk (Hattin, 

1971, 1975a). The overlying Jetmore Chalk 

Member of the Greenhorn Limestone contains 

numerous beds of chalk that have been traced 

for hundreds of kilometers. The Jetmore varies 

from 4 to 7 m (13-23 ft) in thickness in Kansas 

(Hattin, 1975a), while in Colorado it is part 

2-4 

of the more inclusive Bridge Creek Limestone 

Member of the Greenhorn. Chalk and lithified 

chalk also form a significant part of the 

Pfeifer Member of the Greenhorn, some in the 

form of concretionary or lenticular beds. The 

Pfeifer ranges from 5 to 8 m (15-25 ft) in 

thickness in central and western Kansas (Hattin, 

1975a). Equivalents of this unit are also part 

of the Bridge Creek Member in south-central 

Colorado, where the total Bridge Creek is 17 m 

(57ft) thick (Cobban and Scott, 1972). The 

total Bridge Creek section (which inclu1es the 

upper part of the Hartland Shale Member and all 

of the Jetmore Chalk and Pfeifer Shale Members) 

thins to only 10 m (34 ft) at Huerfano Park, 

Colo. (Kauffman and others, 1969); thickens 

rapidly to 40 m (130 ft) near Littleton, Colo. 

(Scott, 1962) and thins again northward to 25 m 

(83 ft) near Lyons, Colo. (Cobban and Scott, 

1972). 

The Fairport Chalky Member of the Carlile 

Shale contains a significant volume of chalk 

Approximate updip limit of Upper 

Cretaceous chalks or equivalents 

0 KILOMETERS 300 

Figure 14.--Regional distribution of gas fields producing from Gulf Coast chalks, plotted by 

county or parish. Data are taken from tables 1-4 and are based, in part, on state 

classifications of fields as oil or gas producers. 

27 
DEPTH IN METERS 

30 

28 

26 

CJ) 24 0 

_J 

w 

u::: 22 CJ) 

<: 

(!) 

cc 20 

Q 

0 

z 18 <: 

_J 

6 16 u.. 

0 

a: 

w 

CD 14 :::E 

:J 

z 12 

10 

6 

4 

0 

0 0 !ii! 8 !ii! ~ ~ 0 !ii! ~ ~ 0 ~ ~ ~ 8 !ii! 0 ~ 0 ~ 0 ~ 8 ~ ~ 0 N 0 N 0 ...... ~ N 8 ...... ~ N 0 ...... N ~ ...... ~ ...... 0 ...... vi N M II') -o ...... co 0. 0. o' ....: N' N' M' ··i vi -o' ...... - ~ 

DEPTH IN FEET 

Figure 15.--Histogram showing number of oil and (or) gas fields producing from the Austin 

Group in Texas as a function of burial depth. Data are taken from table 1. 

and shaly chalk and reaches a thickness of 18-

24 m (60-80 ft) in Nebraska (Condra and Reed, 

1959), 17-21 m (55-70ft) in central Colorado 

(Mann, 1958; Dane and others, 1937), and 23-37 

m (90-122 ft) in central and western Kansas 

(Hattin, 1962). 

The basal member of the Niobrara Formation, 

the Fort Hays Limestone, contains some of the 

purest chalk in the Western Interior. It has 

virtually no shale interbeds and is 12 m (40 

ft) thick in the Pueblo, Colo. area (Scott and 

Cobban, 1964) and 17-23 m (55-75 ft) thick in 

central and western Kansas (Hattin, 1965). The 

overlying Smoky Hill Chalk Member of the 

Niobrara contains shaly chalk in most areas and 

is 171-189 m (560-620 ft) thick where present 

in Kansas (Hattin, 1965) and 213 m (700 ft) 

thick near Pueblo, Colo. (Scott and Cobban, 

1964). 

28 

Although many different chalk units are 

found in the Western Interior, they show a 

number of unifying elements. Many are uniformly 

bedded with bedding thickness in the 10-cm to 

1-m (4- to 39-in.) range. In fresh outcrops 

many are dark gray, but they weather to a 

light-gray, tan, or cream color. Chalk of the 

Smoky Hill Chalk Member of the Niobrara Formation and parts of the Greenhorn Limestone shows 

well-preserved lamination consisting of millimeter-scale alternations of clay-rich and claypoor chalk. Many of these laminated sediments 

also contain small white specks which have been 

shown by Hattin (1975b) to be fecal pellets 

containing abundant coccoliths. In other 

units, however, extensive burrowing has obliterated any traces of primary depositional 

fabrics (Frey, 1972). 

Most chalk of the Western Interior area 
DEPTH IN METERS 

8.5 

7.5 

6.5 

5.5 

4.5 

3.5 

2.5 

DEPTH IN FEET 

Figure 16.--Histogram showing cumulative oil 

production in millions of barrels from the 

Austin Group in Texas as a function of 

burial depth. Data are taken from table 

1, but only include production from 1974 

and earlier years. 

contains appreciable quantities of HClinsoluble detrital terrigenous material. 

Insoluble residues of the Greenhorn Limestone 

in Kansas range from 9.5 to 56.6 percent (by 

weight) and average 31.9 percent (Hattin, 

1975a, p. 73). Insoluble residues of five 

samples of Greenhorn chalk from the Pueblo, 

Colo. area, analyzed during the present study, 

average 15 percent and range as high as 45 

percent. Runn~ls and Dubins (1949, p. 17) 

reported 2 to 12 percent insoluble residue in 

the Fort Hays Limestone Member of Kansas. 

Three samples of the same unit from Pueblo, 

Colo. examined in the present study, average 18 

percent insoluble material. Samples from the 

Smoky Hill Member of the Niobrara Formation 

average 30 percent insoluble residue in southcentral Colorado and 28 percent in central and 

western Kansas. The vast bulk of this HClinsoluble material has been shown by X-ray 

analysis to consist of clay minerals, detrital 

quartz, pyrite, hematite, and gypsum. Phosphate 

is important in some local areas. 

Coccoliths are the predominant constituent 

of Western Interior chalks. Hattin and Darko 

(1971) discovered as many as 6xl09 coccoliths 

per cm3 in shaly chalk of the Greenhorn Lime29 

600,000 

575,000 

550,000 

525,000 

500,000 

475,000 

450,000 

425,000 

400,000 

375,000 

350,000 

325,000 

300,000 

275,000 

250,000 

225,000 

200,000 

175,000 

150,000 

125,000 

100,000 

75,000 

50,000 

25,000 

Figure 17.--Histogram showing cumulativ?. gas 

production in millions of cubic fe?.t (Mcf) 

from the Austin Group in Texas as a function 

of burial depth. Data are taken from table 

1. 

stone. SEM photographs of Western Interior 

chalks (figs. 21-24) show that coccoliths are 

abundant in other units as well. Subordinate 

to coccoliths, but still important rock-forming 

constituents in many Western Interior chalk 

samples, are planktonic Foraminifera, inoceramid 

prisms, and oyster shell fragments. 

Reconstruction of depositional environments 

of Western Interior chalks is a complex problem. 

Estimates of water depths have ranged widely 

from tens of meters to nearly 1,000 m (3,300 

ft) (Eicher, 1969). The problems of comparing 

faunas from warm interior seas with modern 

species makes paleoecological determinations of 

water depths very difficult. The preservation 

of thin lamination, the common absence of 

burrowing, restricted benthonic faunas (Eicher 

and Worstell, 1970), and specialized adaptations 

of organisms to poorly oxygenated conditions 

(Hattin, 1965, p. 21) all indicate that bottomwater oxygen content, salinity, or temperature 

conditions may often have been inhospitable for 

most organisms. In addition, isotopic and 

faunal studies (Kauffman and Scholle, 1977; 

Scholle and Kauffman, 1977) indicate that 
salinities lower than normal marine and temperatures far higher than the average for modern 

oceans may have prevailed at times in the 

restricted Western Interior seaway. All these 

factors make water-depth interpretations difficult. Deposition was apparently below wave 

base and below the photic zone, so that most 

depth interpretations fall in the 50- to 300-m 

(150- to 1,000-ft) range (Hancock, 1975). 

Detailed paleoecological interpretations are 

published in Frey (1972), Hancock (1975), 

Hattin (1965, 1975a), and Kauffman (1967). 

Petrophysical properties and diagenesis 

Despite significant bed-to-bed variations 

in porosity, which may be related to primary 

depositional differences (for example, grain 

size, faunal composition, or primary aragonite 

content), porosities in Western Interior chalks 

are primarily a function of burial depth. 

Figure 25 is an isopach map of the Upper 

Cretaceous Pierre Shale, in most areas the 

thickest single unit overlying the chalk 

section. 

In areas where thin Pierre and younger 

units were deposited, underlying chalks have 

high porosity. In Kansas, for example, outcrop 

AGE STRATIGRAPHIC UNIT 

SMOKY HILL CHALK MEMBER 

FORT HAYS LIMESTONE MEMBER 

BLUE HILL SHALE MEMBER 

FAIRPORT CHALKY MEMBER 

PFEIFER SHALE MEMBER 

JETMORE CHALK MEMBER 

HARTLAND SHALE MEMBER 

LINCOLN LIMESTONE MEMBER 

UPPER FOSSILIFEROUS UNIT OF 

COBBAN AND SCOTT (1972) 

THATCHER LIMESTONE MEMBER 

EQUIVALENT TO 

BRIDGE CREEK 

LIMESTONE 

MEMBER OF 

GREENHORN 

LIMESTONE OF 

HATTIN (1975) 

Figure lB.--Stratigraphic section of the Upper 

Cretaceous Colorado Group of Kansas and 

eastern Colorado. Modified from Hattin 

(1975a), Smith (1975), Cobban and Scott 

(1972), and Scott and Cobban (1964). 

30 

samples of the Smoky Hill have an average of 40 

percent porosity, Fort Hays samples have 39 

percent porosity, and Fairport samples have 37 

percent porosity. In wells in western Kansas 

and easternmost Colorado, where the s~oky Hill 

is buried to a present depth of less than 550 m 

(1,800 ft), the chalk retains 35-40 p~rosity 

(with 2.8 md permeability). 

In areas where the Pierre and yo·.mger 

sediments are (or were) thicker, poro~ities in 

underlying chalks are significantly reduced. 

Outcrop samples of the Niobrara near Pueblo, 

Colo., have an estimated 10-15 percent porosity. 

These samples, although at the surface now, 

probably were once buried by more than 1,400 m 

(4,500 ft) of overlying sediments (fig. 25). 

Outcrop samples of chalk from the Niobrara at 

Fort Collins, Colo. have an estimated 5 percent 

porosity or less and were once overlain by 

approximately 3,050 m (10,000 ft) of total 

overburden. Within the Denver Basin, subsurface 

samples of the upper part of the Smoky Hill 

from 1,655 to 1,667 m (5,429-5,470 ft) in depth 

have an average of 9.2 percent porosity (0.03 

md permeability) in the Excelsior A. G. Nay No. 

1 well, Weld County, Colo. (T. Smagala, written 

commun., 1977). In general, these porosity 

versus depth relationships hold for all chalk 

units examined in the Western Interior, and 

they agree with the predicted porosities shown 

in figure 1. Porosities of Greenhorn samples 

are generally significantly lower tha~ those of 

the Niobrara from the same area. This may be 

due to the significantly greater over~urden 

thickness over the Greenhorn; it may be related 

to primary compositional differences (especially 

in aragonite and clay content) betwee~ the two 

units; or it may be a product of sea-floor 

cementation during Greenhorn deposition. 

Isotopic analyses of Western Interior 

chalk samples are more difficult to interpret 

than those from the Gulf Coast becaus~ of the 

variable water temperatures and salinities in 

the Western Interior seaway (Kauffman and 

Scholle, 1977). Yet, even in this setting, 

progressive shifts in oxygen isotopic values, 

which can be related to the stage of burial 

diagenesis, are observable. In the Fort Hays 

Limestone Member of the Niobrara Formation, for 

example, average oxygen isotopic valu~s vary 

from -4.9 permil (relative to the PDB standard) 

in western Kansas; to -6.2 at Pueblo, Colo.; 

and -7.0 at Fort Collins, Colo. This shift to 

more negative oxygen isotopic values with 

increased burial can be demonstrated for most 

other units in the Western Interior as well, 

and it parallels the loss of porosity through 

progressive cementation which is seen in SEM. 

Hydrocarbon production 

Oil production was discovered as early as 

1916 in the Niobrara Formation of the Big Muddy 

field in Wyoming. Subsequently, a large number 

of additional fields have been discov,~red that 
I 

I 

--I 

I 

KILOMETERS 

Figure 19.--Thickness and regional distribution of facies in the Greenhorn Limestone and 

lateral equivalents. Map taken from Reeside (1944). 

have oil or gas production from the Niobrara or 

Greenhorn chalk. None of the fields is large, 

and cumulative oil production from chalk 

reservoirs of the Western Interior totals only 

about 7-8 million bbl. Production figures for 

the states of Colorado, Wyoming, and New Mexico, 

broken down by county and field, are presented 

in tables 5-7. The data are subject to the 

same qualifications as those listed in the 

discussion of the Gulf Coast data (tables 1-4). 

In some cases non-chalk reservoirs may have 

been included in the tables if they are part of 

a formation consisting dominantly of chalk. 

Likewise, some chalk reservoirs undoubtedly 

have been excluded when no detailed lithologic 

information was available and they were part of 

dominantly non-chalk formations. 

The regional distribution of production, 

shown in figures 26-29, indicates that produc31 

tion is not strongly concentrated in any one 

area. Furthermore, as shown by comparison with 

figures 19, 20, and 25, the production is 

largely from areas that have been burie1 below 

1,525 m (5,000 ft) and from areas where the 

chalk interfingers with thick marine sh~les. 

In these fields production is primarily from 

fracture porosity in calcareous shales or hard 

shaly chalks (McAuslan, 1959); it is ge~erally 

associated, therefore, with anticlines, arches, 

fault zones, or other areas of concentrated 

structural deformation and fracture dev~lopment. 

Cumulative production has been very low, largely 

because of the burial-related loss of rratrix 

porosity and the associated lack of large-scale 

hydrocarbon storage capacity of the rock in 

many of the explored areas. The wides~read 

distribution of production may reflect the fact 

that Niobrara oil and gas may be indigenous to 
-~---- ~--r---- _L __ -

I 

I 

, ~ I C) 

0 

I of 

II Shale tac1es 

'_;'/_ -- -- - i // 

., ~I Limestone and 

J/ chalk fac1es 

I 

KILOIIIIETERS 

/ 

/ 

I 

,~ 

I ~-

T 

\ 

) 

! 

Figure 20.--Thickness and regional distribution of facies in the Niobrara Formation and 

lateral equivalents. Map taken from Reeside (1944). 

that unit; assays reported by McAuslan (1959) 

indicate the presence of 10-12 gallons of oil 

per ton of rock in the Niobrara. 

The past 5 years have seen a resurgence of 

exploration in the Niobrara Formation in 

particular. This exploration has centered in 

the Beecher Island field of Yuma County, 

Colo., initially discovered in 1919, and in the 

Goodland area (Sherman Coun~y, Kans.), where 

natural gas has been produced for local consumption since 1939. More than 60 wells have been 

drilled over the past 5 years to test the 

Niobrara in western Kansas and eastern Colorado 

along a northwest-trend. Currently, exploration 

is extending into northern Colorado and western 

Nebraska. Approximately 40 wells, shown in 

table 8, have resulted in gas production. 

Although none of these wells has been a major 

discovery, increased prices for natural gas and 

32 

modern techniques of well stimulation by hydraulic fracturing have made continued exploration 

financially attractive for small, independent 

producers. Most of this recent exploration is 

in areas where the Niobrara is buried to about 

305-762 m (1,000-2,500 ft). At these depths, 

completed wells cost an average of about 

$50,000, and wells will presumably pa~T out in 

about 2 years. 

The bulk of the new gas production is from 

the top of the Smoky Hill Member of the Niobrara 

Formation. Figure 30 illustrates typical FDCCNL (Compensated Neutron Formation Density) log 

response indicative of a gas-bearing zone at 

the top of the Niobrara. The characteristic 

crossover of the neutron and density porosity 

curves is, however, seen only when logs are run 

within a short time interval after penetration 

of the chalk interval. Longer time intervals 
Figure 21.--SEM photomicrograph of a shaly 

chalk from the lower Turonian, middle part 

of Hartland Shale Member of the Greenhorn 

Limestone. Sample is from an outcrop about 

5 km (3 mi) north of Bunker Hill (Russell 

County), Kan. Numerous coccoliths are 

welded together in an interlocking calcitecement framework. Sample contains 27 percent 

insoluble residue. Scale bar is 2 ~m. 

Figure 22.--SEM photomicrograph of a chalk from 

the Turonian Fairport Chalky Member of the 

Carlile Shale. Sample is from an outcrop 6 

km (4 mi) west of Fairport (Ellis County), 

Kan. A typical coccolith- and rhabdolithrich chalk with 15 percent insoluble residue 

and an estimated 30 percent porosity. Scale 

bar is 2 ~m. 

33 

Figure 23.--SEM photomicrograph of a chalk from 

the Coniacian-Santonian Fort Hays Limestone 

Member of the Niobrara Formation about 10 m 

(33 ft) above the base of the unit. Sample 

is from bluffs along Smoky Hill River, 

southwest of WaKeeney (Trego County), Kan. 

Numerous coccoliths and early stages of 

euhedral overgrowth cementation are visible. 

Note nonuniform orientation of grains. 

Sample has 8 percent HCl- insoluble residue 

and an estimated 35-40 percent porosity. 

Scale bar is 2 ~m . 

Figure 24.--SEM photomicrograph of a shaly 

chalk in the Campanian Smoky Hill Chalk 

Member of the Niobrara Format ion. Sample is 

from within a gas-producing interval of 

Mountain Petroleum Ltd. No. 1-16 State well 

(515 m or 1,690 ft in depth), Beecher Island 

field, Yuma County, Colo. Note abundant 

coccoliths and rhabdoliths having random 

orientation and virtually no cementation. 

Estimated overall porosity of sediment is 40 

percent. Scale bar is 2 ~m. 
w 

~ 

Table 5.--0il and gas production, by county, from Upper Cretaceous chalk reservoirs in Colorado 

[Data taken from International Oil Scouts Association (1976). Leader (---) indicates 

information not available; Mcf, 1,000 ft 3 .] 

Field or pool 

Chromo 

Greenhorn. 

Stratigraphic 

unit 

Greenhorn 

Limestone. 

Canadian River Niobrara 

Niobrara. 

Carlstrom 

Coalmont 

Niobrara. 

Formation. 

---do-----

---do-----

Average top 

of producing 

formation 

Feet Meters 

408 124 

6,457 1,968 

Average 

API 1 

gravity 

(degrees) 

Total 

wells 

Archuleta County 

Jackson County 

1 

1 

1 

3 

Cumulative 

oil (bbl) 

(through 

19752 ) 

1,083 

3,147 

4,026 

39,166 

Total--------------------------------------------------------------------------46,339 

Larimer County 

Berthoud Niobrara 240 73 --- --- --- Niobrara. Formation. 

Kelim ---do----- 7,000 2,134 38 1 653 

Niobrara. 

Loveland ---do----- --- --- --- 1 5,555 

DakotaNiobrara. 

Loveland ---do----- 4,578 1,395 --- --- 377' 958 

Niobrara. 

Loveland Fort Hays --- --- --- 3 63,607 

Niobrara- Limestone Member, 

Timpas. Niobrara 

Formation. 

Total-------------------------------------------------------------------------447,773 

Cumulative 

gas (Mcf) 

(through 

1974) 

176 

2,293 

23,061 

25,530 

1,164,459 

---

---

658,546 

140,862 

1,963,867 

Current 

oil 

(bbl) 

(1975 2 ) 

0 

0 

1,161 

24,169 

25,330 

---

0 

0 

17,516 

1,123 

18,639 

Current 

gas 

(Mcf) 

(1974) 

0 

661 

20,641 

21,302 

0 

4,786 

0 

4,786 
Moffat County 

Buck Peak Niobrara 6,868 2,093 37 --- 32,607,167 32,039,606 3220' 324 3277 '960 

Niobrara. Formation. 

Moffat ---do----- 2,405 733 --- --- 86,786 14,031 954 0 

Niobrara. 

Waddle Creek ---do----- 2,076 633 41 6 314,062 --- 8,121 

Niobrara. 

Total-----------------------------------------------------------------------3,008,015 2,053,637 229,399 277 '960 

Morgan County 

Bijou Greenhorn 5,875 1,791 --- 1 1,261 900 0 0 

Greenhorn. Limestone. 

Empire ---do----- 6,114 1,864 --- --- 5,037 --- 0 

Total---------------------------------------------------------------------------6,298 900 0 0 

w 

Vl 

Routt County 

---

Bear River Niobrara --- --- 36 --- 14,779 

Formation. 

Curtis ---do----- 3,795 1,157 36 3 210,734 90,376 3,074 576 

Niobrara. 

Dill Gulch ---do----- --- --- --- 1 1,467 --- 1,467 

Niobrara. 

Fish Creek ---do----- --- --- --- --- 9,313 2,791 2,958 885 

Niobrara. 

Grassey Creek ---do----- 4,750 1,448 --- 10 458,982 50,845 28,364 0 

Niobrara. 

Hidden Valley ---do----- 3,691 1,125 39 1 18,986 5,650 0 0 

Niobrara. 

Sage Creek ---do----- 2,418 737 39 4 73,830 --- 0 

Niobrara. 

Sage Creek ---do----- 4,944 1,507 --- 3 414,388 --- 14,822 

North 

Niobrara. 

Tow Creek ---do----- 2,600 792 35 42 2,908,936 338,899 7,935 0 

Niobrara. 

Total-----------------------------------------------------------------------4,111,415 488,561 58,620 1,461 
w 

0'1 

Table 5.--0il and gas production, by county, from Upper Cretaceous chalk reservoirs in Colorado--Continued 

Field or pool 

Baxter Lake 

Niobrara. 

Lost Creek 

Niobrara. 

Stratigraphic 

unit 

Niobrara 

Formation. 

---do-----

Average top 

of producing 

formation 

Feet Meters 

5,249 1,600 

Average Cumulative 

API 1 Total oil (bbl) 

gravity wells (through 

(degrees) 19752 ) 

36 1 963 

1 119 

Total---------------------------------------------------------------------------1,082 

Beecher 

Island 

Niobrara. 

Niobrara 

Formation. 

Yuma County 

Colorado total---------------------------------------------------------------7,622,005 

1American Petroleum Institute. 

2Includes condensate. 

3As of December 31, 1976 (J. P. Lockridge, written commun., 1977). 

Cumulative 

gas (Mcf) 

(through 

1974) 

629 

629 

3598,000 

4,640,211 

Current 

oil 

(bbl) 

(1975 2 ) 

0 

0 

0 

331,988 

Current 

gas 

(Mcf) 

(1974) 

0 

0 

74,329 

379,838 
w 

~ 

Table 6.--0il and gas production, by county, from Upper Cretaceous chalk reservoirs in Wyoming 

[Data taken from International Oil Scouts Association (1976). Leader (---) 

indicates information not available; Me£, 1,000 ft 3 .] 

County Field or pool Stratigraphic 

unit 

Carbon Espy Niobrara 

Formation. 

G P Dome ---do-----

Overland ---do-----

Average top 

of producing 

formation 

Feet Meters 

4,013 1,223 

3,000 914 

3,819 1,164 

Average 

API 1 

gravity 

(degrees) 

43 

Total 

wells 

3 

1 

Cumulative 

oil (bbl) 

(through 

1975 2 ) 

497,377 

203,031 

11,916 

Total-------------------------------------------------------------------------------712,324 

Converse Big Muddy and Niobrara 2,100 640 

Big Muddy Formation. 

East. 

Crook Thornton Carlile Shale 600 183 --- 21 38,516 

Wakeman Flats ---do-------- 505 154 --- 5 987,965 

rotal-----------------------------------------------------------------------------1,026,481 

Natrona Smokey Gap Niobrara 705 215 38 21 487,889 

Formation. 

Teapot ---do------ 2,440 744 --- 20 179,784 

Outside 

Reserve. 

Total-------------------------------------------------------------------------------667,673 

Sweetwater Buster Basin 

Teepef 

Niobrara 

Formation. 

---do------

13,216 4,028 

848 254 1 0 

Wy~~ing T~t2l---------------------------------------------------------------------2,406,478 

1API, American Petroleum Institute. 

2 Includes condensate. 

Cumulative 

gas (Me£) 

(through 

1974) 

Current 

oil 

(bbl) 

(1975 2 ) 

13,720 

1,162 

1,988 

16,870 

705 

810 

1,515 

25,570 

0 

25,570 

0 

43,955 

Current 

gas 

(Me£) 

(1974) 
w 

00 

Table 7.--0il and gas production data, by county, from Upper Cretaceous chalk reservoirs in New Mexico 

[Data taken from International Oil Scouts Association (1976). Leader (---) 

indicates information not available; Mcf, 1,000 ft 3 .] 

Average top 

of producing Average Cumulative 

County Field or pool Stratigraphic formation API 1 Total oil (bbl) 

unit gravity wells (through 

Feet Meters (degrees) 1975 2 ) 

Rio Arriba Puerto Greenhorn 3,419 1,042 38 

Chi quito Limestone. 

West Mancos. 

Stephenson (T) ---do----- 2,931 893 --- 1 366 

Not Named ---do----- --- --- --- 1 2,496 

Greenhorn Gas. 

Total---------------------------------------------------------------------------------2,862 

New Mexico Total----------------------------------------------------------------------2,862 

1API, American Petroleum Institute. 

2Includes condensate. 

Cumulative 

gas (Mcf) 

(through 

1974) 

--- 255,652 

255,652 

255,652 

Current 

oil 

(bbl) 

(1975 2 ) 

0 

0 

0 

0 

Current 

gas 

(Mcf) 

(1974) 

0 

0 

0 
Figure 25.--Isopach map of the Pierre Shale and 

its lateral equivalents (contour interval in 

thousand feet). In most areas of chalk 

deposition the Pierre forms the major part 

of the total overburden. Adapted from 

Reeside (1944). 

110° 

44" 

42" 

WYOMING 

() 

() 

() 8 

COLORADO 

8 so 

NE 

0 

0 

CUMULATIVE OIL PRODUCTION 

11n barrels) 

0 0 

r----.,.---i---~---'''''-'' ___ ,_~-- Etl 

1000-10,000 

10,000-100,000 

100,000-1,000,000 () 

ARJZONA 

NEW MEXICO 8 1,000,000-10,000,000 

0 

0 KILOMETERS 200 l____---,L __ ---,--1 

Figure 26.--Regional distribution of cumulative 

oil production from Western Interior chalks, 

plotted by county. Data are taken from 

tables 5-7. 

39 

40" 

'COLORADO 

ARIZONA 

NEW MEXICO 

NUMBER OF CliLFIELDS 

Etl 1-4 

~ 5-8 

KILOMETERS 200 

Figure 27.--Regional distribution of oil fields 

producing from Western Interior chalk, 

plotted by county. Data are taken from 

tables 5-7. 

between drilling and logging allow invasion of 

the formation by mud filtrate and make evaluation of gas-bearing zones more difficult. The 

gas saturated zones range from 8 to 18 m (25-60 

ft) in most productive wells. The porosities 

(as much as 40 percent) and permeabilities (as 

much as about 5 md) of the productive intervals 

are typical of much of the Niobrara Formation. 

Hydrocarbon entrapment at the top of the formation apparently is related to the presence of 

low relief structures, the fracturing of the 

Smoky Hill, and the presence of an unfractured, 

overlying Pierre Shale seal. The presence of 

fractures, here, as in the Gulf Coast, is 

instrumental in making these extremely low 

permeability chalk zones commercially productive. Fortunately, indurated chalk is a brittle 

sediment that fractures readily, even ~-~re 

affected only by minor deformation. 

The gas in these fields, which have never 

been deeply buried, is presumably derived from 

organic matter in the Niobrara Formation itself. 

Carbon isotopic analyses of natural gas samples 

from the Niobrara of eastern Colorado clearly 

indicate that the gas, more than 92 percent 

methane at Beecher Island field (Yuma County, 

Colo.), is of biogenic (bacterial) origin (D. 

D. Rice, oral comrnun., 1977). Thus, burial 
.j::--

0 

Table 8.--Recent gas-producing wells in Niobrara Formation, eastern Colorado and western Kansas 

[Data from Kansas-Nebraska Natural Gas Company (Written commun., 1977); summarizes wells drilled up to February 22, 1977. 

Leader (---) indicates information not available; Mcfpd, 1,000 ft 3 per day; IP, initial production.] 

Operator 

Kansas-Nebraska 

Mountain Petroleum 

Kansas-Nebraska 

Kansas-Nebraska 

Kansas-Nebraska 

Mountain Petroleum 

Mountain Petroleum 

Kansas-Nebraska 

Mountain Petroleum 

Mountain Petroleum 

Mountain Petroleum 

Mountain Petroleum 

Kansas-Nebraska 

Mountain Petroleum 

Mountain Petroleum 

Mountain Petroleum 

Mountain Petroleum 

Kansas-Nebraska 

Kansas-Nebraska 

Kansas-Nebraska 

Kansas-Nebraska 

Kansas-Nebraska 

Kansas-Nebraska 

Kansas-Nebraska 

Kansas-Nebraska 

Location 

Well name Sec. T. R. 

Beecher Island field, Yuma County, Colorado 

Whomble 1-5 5 3S 43W 

Whomble 1-6 6 3S 43W 

Bruder 1-7 7 2S 43W 

Bruder 1-8 8 2S 43W 

Herbert 1-9 9 2S 43W 

State 1-16 16 2S 43W 

New 1-A 17 2S 43W 

Rose 1-18 18 2S 43W 

Rose 1-19 19 2S 43W 

Strangeways 1-A 20 2S 43W 

Beecher Island #1 21 2S 43W 

Eckberg 1-22 22 2S 43W 

Cranmer 1-27 27 2S 43W 

Gracie Chase Ill 28 2S 43W 

State 1-29 29 2S 43W 

State 2-30 30 2S 43W 

State 1-31 31 2S 43W 

Whomble 1-32 32 2S 43W 

Eckberg 1-33 33 2S 43\.J 

Simmons 1-24 24 2S 44W 

Republican field, Yuma County, Colorado 

Stone 1-26 

Fonte 1-27 

Toner 1-28 

20 

27 

28 

lN 

lN 

lN 

45W 

45W 

45W 

Shout field, Yum2 County, ColoYado 

Neuhaus 1-34 

Pioneer Farms 1-3 

34 

3 

3N 

2N 

47W 

47W 

Completion 

date 

9-10-76 

---

8-25-76 

8-24-76 

---

3-11-72 

1-11-75 

8-26-76 

2-20-75 

1-11-75 

3-31-72 

9-10-76 

2-21-75 

4-10-72 

4-03-74 

5-19-75 

5-19-75 

9-10-76 

2-18-75 

8-27-76 

2-10-76 

9-10-76 

4-07-76 

Status 

IP 4 73 Mcfpd. 

Incomplete. 

IP 455 Mcfpd. 

IP 445 Hcfpd. 

Complete. 

IP 435 Mcfpd. 

IP 716 Mcfpd. 

IP 980 Mcfpd. 

IP 839 Mcfpd. 

IP 519 Mcfpd. 

IP 68 Mcfpd. 

IP 863 Mcfpd. 

IP 818 Hcfpd. 

IP 120 Mcfpd. 

IP 675 Hcfpd. 

IP 1598 Mcfpd. 

IP 1203 Mcfpd. 

IP 587 Mcfpd. 

IP 1287 Mcfpd. 

IP 410 Mcfpd. 

Incomplete. 

IP 304 Mcfpd. 

IP 82 Mcfpd. 

IP 640 Mcfpd. 

Incomplete. 
.j::'-

Kansas-Nebraska 

Mountain Petroleum 

Kansas-Nebraska 

Mountain Petroleum 

Murfin 

Mountain Petroleum 

Mountain Petroleum 

Kansas-Nebraska 

Kansas-Nebraska 

Kansas-Nebraska 

Shakespear Oil 

Kansas-Nebraska 

Kansas-Nebraska 

Murfin 

Federer 

Allison 1-29 

Allison 1-32 

State 1-16 

Vernon field, Yuma County, Colorado 

29 

32 

lS 

2S 

44W 

44W 

Whisper field, Yuma County, Colorado 

16 3N 47W 

Cherry Creek field, Cheyenne County, Kansas 

Schauer 1-12 

Zyegardt 1-1 

12 

1 

3S 

3S 

41W 

41W 

Unnamed fields, Yuma-Phillips County, Colorado 

Chapman 1-10 

Ferguson 1-26 

Lippert 1-8 

Engel 1-10. 

O'Brian 1-31 

Walter #1 

State 1-16-436 

Stulp 1-1 

Walz #1 

Leach tfl 

10 

26 

SN 

6N 

Wildcat fields 

46W 

46W 

Yuma County, Colo. 

8 lS 44W 

Cheyenne County, Kansas 

10 3S 42W 

Cheyenne County, Kansas 

31 4S 41W 

Cheyenne County, Kansas 

9 3S 41W 

Yuma County, Colorado 

16 3N 46W 

Yuma County, Colorado 

1 lN 47W 

Yuma County, Colorado 

3 SS 42W 

Cheyenne County, Kansas 

29 4S 39W 

3-11-76 

5-08-76 

IP 260 Mcfpd. 

IP 344 Mcfpd. 

IP 700 Mcfpd. 

IP 260 Mcfpd. 

Tight-hole. 

Tight-hole. 

Tight-hole. 

Incomplete. 

IP 132 Mcfpd. 

Incomplete. 

IP 100 Mcfpd. 

Incomplete. 

IP 329 Mcfpd. 

Tight-hole. 

Tight-hole. 
WYOMING 

42" 

() 

COLORADO 

() 

NEW MEXICO 

CUMULATIVE GAS PROc,UCTION 

IMcf) 

EB 1o,ooo-1oo,ooo 

() 100,000-1,000,000 

f9 1,000,000-1G,OOO,:JOO 

Figure 28.--Regional distribution of cumulative 

gas production from Western Interior chalks, 

plotted by county. Data are taken from 

tables 5-7. 

depth is not a critical factor in the origin of 

the gas. 

With increasing gas prices, future increases in gas production can be expected from the 

Niobrara. The regional extent of chalk facies, 

the high porosity, and the susceptibility of 

the Niobrara to fracturing insure at least 

adequate reservoir properties over broad areas 

where the unit has not been deeply buried. 

Even where more extensive burial has taken 

place, such as in the Denver-Julesburg basin, 

fractured oil or gas reservoirs, similar to the 

Loveland or Tow Creek fields of Colorado and 

the Pearsall field of Texas, may be found. The 

moderately high kerogen content of much of the 

Niobrara and the biogenic production of gas 

even at shallow depths indicate that source 

beds and conditions suitable for hydrocarbon 

generation will normally not be limiting factors 

in exploration. Furthermore, the thick overlying section of Pierre Shale insures adequate 

seals in most regions. Although considerable 

additional production can be expected from the 

Niobrara, it is unlikely that the fields will 

be more prolific than the ones discovered to 

date. There is no indication that the conditions responsible for the remarkable chalk 

reservoirs of the North Sea (discussed later in 

42 

this paper) are present in the Western Interior. 

New fields will be small but numerous and will 

probably be of economic interest mainly to 

independent operators. 

Many areas of potential Niobrara production 

may already have been drilled. Historically, 

tens of thousands of wells have penetrated the 

Niobrara section but only rarely was the chalk 

interval cored, immediately logged, or tested. 

Undoubtedly considerable potential production 

has therefore been overlooked. Increased 

production from the Niobrara, as well as from 

Gulf Coast chalks, is augmenting awareness of 

the potential of chalk reservoirs and should 

insure more adequate evaluation in the future. 

It should be noted that, although the 

Niobrara is undergoing very active current 

exploration, a number of other potentially 

interesting chalk units continue to be unstudied 

and untested. The Fairport Chalky Member of 

the Carlile Shale, for example, has interesting 

features. It is quite porous on outcrop in 

central and western Kansas (as much as 38 

percent pore space), has a section thick enough 

to be of commercial interest in many areas, and 

is overlain by the thick Blue Hill Shale Member 

of the Carlile Shale, an excellent seal. 

Likewise, the Fort Hays Limestone Member, the 

44' 

38° 

ARIZONA 

36° c~- i L..! 

WYOMINC? 

EB 1-2 

~ 2-4 

KILOMETERS 200 

Figure 29.--Regional distribution of gas fields 

producing from Western Interior chalks, 

plotted by county. Data are taken from 

tables 5-7. 
1410 

1420 

1430 

1440 

f- 1450 w 

w 

LL 

~ 1460 

I 

f-

[l_ 

w 1470 0 

1480 

1490 

1500 

1510 

60 

DENS1TY POROSITY INDEX IN PERCENT 

NEUTRON POROSITY INDEX IN PERCENT 

(BOTH BASED ON SAND MATRIX) 

45 30 15 

Figure 30.--Redrawn Schlumberger FDC-CNL (Compensated Neutron-Formation Density) log 

showing typical response in a gas-saturated 

zone (shaded area) a t the top of the Niobrara 

Formation. Note characteristic crossover of 

neutron- and density-porosity curves in gas 

zone. Data from Mountain Petroleum Ltd. 

State No. 1-29 well, Yuma County, Colo., 

provided by J. P. Lockridge, Mo untain Petroleum Corp. 

lower part of the Smoky Hill Chalk Member of 

the Niobrara Formation, as well as the Hartland 

Shale, Jetmore Chalk, and Pfeifer Shale Members 

of the Greenhorn Limestone could be locally 

prospective sources and reservoirs. 

Scotian Shelf 

Stratigraphy and sedimentology 

A thick chalk section was described by 

Mciver (1972) from the Upper Cretaceous of the 

Scotian Basin of the Canadian Atlantic outer 

continental shelf. On the basis of extensive 

drilling in the area, it has been established 

that the entire Santonian-Campanian interval 

and part of the lov1er Maestrichtian consist ot 

chalk with interbedded calcareous mudstone and 

marl (Jansa and Wade, 1975). This unit was 

termed the Wyandot Forma tion by Mciver (1972). 

The Wyandot reaches a thickness of 397 m 

(1,302 ft) in the Dauntless D-35 well, thins 

westward to 104 m (340 f t) in the Naskapi N-30 

well (Williams and others, 1974), and is entirely absent on the southwestern part of the 

LaHave Platform (localities shmvn in fig. 31). 

The vlyandot is also absent in wells on the 

Avalon Uplift to the east and north, but equivalent chalks have been f ound at JOIDES site 111 

(Orphan Knoll) (Jansa and Wade, 1975; Ruffman 

and van Hinte, 1973). Contacts with overlying 

units are generally conformable (Williams and 

others, 1974). 

Figure 31. --Map of Scotian Shelf area showing 

major structural features, location of 

exploration wells mentioned in text and 200 

m isobath. 

The Wyandot, although predominantly 

composed of chalk, becomes progressively more 

calcarenitic toward the landward portions of 

the basin and progressively marlier toward the 

present-day shelf edge (Jansa and Wade, 1975). 

Wyandot chalk intervals are extremely rich in 

coccoliths (fig. 32), but can also contain 

significant quantities of inoceramid prisms and 

shell fragments, planktonic Foraminifera, 

sponge spicules, and other skeletal grains. 

Clay minerals and cristobalitic lepispheres are 

major constituents of the marls. 

43 

Primary sedimentary structures have not 

been described from the Wy andot Formation 

because it is rarely cored . Bioturbation is 

the most predominant feature in the chalk 

observed in conventional cores from the ShellMobil-Tetco Eagle D-21 we ll (Jansa and Wade, 

1975). 

Paleoecological data based on planktonicbenthonic foraminiferal ratios and other 

features lead to the determination of "outer 

neritic to upper epibathyal" (120-450 m; 

400-1,500 ft) water depths for Wyandot 

deposition (Jansa and Wade, 1975; Mciver, 1972; 

Ruffman and van Hinte, 1973). Wyandot equivalents which may be found beneath the presentday continental slope most likely were 

deposited in even deeper waters. 
Figure 32.--SEM photomicrograph of chalk from 

the Wyandot Formation at 1,676 m (5,500 ft) 

of depth in the Shell-Mobil-Tetco Eagle D-21 

wel l in the Scotian Basin of Canada. Sample 

is composed almost exclusively of whole and 

fragmented coccoliths. Porosity is estimated 

to be about 25 percent. Scale bar is 2.5 ~m . 

Petrophysical properties and diagenesis 

Information on the distribution of porosities and permeabilities of the Wyandot Formation 

is sparse. Porosities (determined from electric 

logs) are as high as 35-38 percent in the 946-

to 1,039-m (3,105- to 3,408-ft) interval of the 

Huron P-96 well, and 38-40 percent in the Mic 

Mac H-86 well at 713-832 m (2,340-2,730 ft) of 

depth. Where the unit is more deeply buried, 

porosities decrease significantly. In the 

Onondaga E- 84 well, porosity is about 34 

percent at 1,350-1,420 m (4,428- 4,658 ft), 

whereas in the Sauk A-57 it is about 23 percent 

at 1,476-1,578 m (4,842- 5 ,178 ft) . 

This porosit y-depth relationship fits that 

shown in figure 1 as the "normal curve." It 

implies that diagenetic alteration took place 

in the presence of marine pore fluids, a reasonable inference considering that the Wyandot is 

conformably overlain by a thick section (as much 

as 1,220 m or 4,000 ft) of marine shales of the 

Upper Cretaceous-Tertiary Banquereau Formation 

(Jansa and Wade, 1975). The data also imply 

that diagenetic porosity loss is produced by 

pressure solution due to overburden loading 

rather than by structural deformation. Horsetail solution seams, microstylolites, and other 

solution features were mentioned by Jansa and 

Wade (1975) as indicative of intrastratal 

dissolution. 

As with Gulf Coast and Western Interior 

chalk units, this progressive depth-related 

44 

cementation is visible in SEM photographs and 

evident from oxygen isotopic analyses . These 

data will not be presented here because of 

their similarity to those shown for the Gulf 

Coast and Western Interior. 

Hydrocarbon production 

No commercial production has been ob tained 

to date from the Wyandot Formation. A number 

of significant gas shows have been found, 

however, and these are summarized in table 9. 

Two of these well s, the Shell-Mobil-Tetco Eagle 

D-21 and the Shell Primrose la-41 were completed 

in the Wyandot interval (although presently 

shut-in), anticipating possible future production. 

Potential is high for future production 

from the Wyandot Formation. The absence of 

freshwater displacement of marine pore fluids 

accounts for the unit having lost porosity 

during burial at a significantly slower rate 

than that of Gulf Coast or Western Interior 

chalks. Even samples buried as deeply as 

1,525-1,825 m (5,000- 6,000 ft) retain primary 

porosities of 20-25 percent. With section 

thicknesses of 400 m (1,300 ft) or greater and 

fracture enhancement of permeability, significant fields could be discovered both on the 

present-day shelf and on the modern continental 

slope. 

COMPARISONS WITH THE NORTH SEA 

The total cumulative hydrocarbon production 

from all North American chalk reservoirs equals 

less than 10 percent of the estimated recoverable reserves from North Sea chalks, shown in 

table 10. Wells in the Ekofisk area produce an 

average of about 10,000 BOPD (Byrd, 1975; World 

Oil, 1971). What makes these fields so prolific? Are North Sea chalk facies similar to 

North American ones in terms of original lithology and diagenetic history? Does any potential exist fo r equivalent prolific production 

from North American chalks? The answers to 

these questions are neither obvious nor easily 

obtainable, but studies conduc ted to date have 

provided at least some partial answers. 

It is beyond the scope of this paper to 

fully cover the deposition, diagenesis, and 

petroleum production history of North Sea 

chalks. Byrd (1975), Childs and Reed (1975), 

Hancock and Scholle (1975), Harper and Shaw 

(1974), Mapstone (1975), Rickards (1974), and 

Scholle (1977) have provided extensive data on 

North Sea chalks; and Kent (1975) and Ziegler 

(1975) have discussed the overall geologic 

framework of the area. The North Sea chalk 

section spans the approximately 30-m. y . time 

interval from Cenomanian to Danian. Chalk 

deposition covered about 30 percent or more of 

Europe during that time interval, and over 

1,400 m (4,600 ft) of chalk was deposited in 

the central part of the North Sea (Hancock and 
Table 9.--Gas shows from the Wyandot Formation on the Scotian Shelf, Canada 

[Data compiled from open-filed well reports by D. J. Taylor (written commun., 1976). Leader (---) indicates 

data not available; Cfpd, ft 3 per day; Bopd, bbl of oil per day.] 

Depth 

Well name Location Feet Meters Shows Initial production 

Shell-Mobil- 43°50'00.000" N. --- --- Gas 1.58 million Cfpd. 

Tetco Eagle D-21 59°34'09.210" w. 

+:'- Shell Huron 44°35'47.035" N. 2,570- 783- Gas VI P-96 58°28'50.688" w. 3,400 1,036 

Shell Primrose 44°00'06.000" N. 4,960- 1,512- Gas Not tested. 

A-41 59°06'14.000" w. 5,020 1,530 

Shell Primrose 44°00'06.000" N. 4, 672- 1,424- Gas and 3.3 million Cfpd and 

la-41 59°06'14.000" w. 5,120 1,561 condensate 2.5 Bopd of condensate 

after acidizing. 

Shell Primrose 43°59'48.430" N. 4,500- 1,372- Gas 

N-50 59°06 I 51. 630" W • 4,594 1,400 

Mobil-Tetco 43°57'20.500" N. 3, 720- 1,134- Gas 

Sable Island E-48 60°07'23.900" w. 3,847 1,173 
..., 

. .., 

0 

"" 0 

tJ} 

QJ 

::0. 

~ tJ} 

QJ 

., 

QJ 

~ (\1 

., 

<lJ 

::0. 

8 

<lJ 

., 

'tl 

<lJ 

.j.j 

. .., 

~ 

.j.j 

tJ} 

ki 

I 

I 

0 

,..., 

<I! 

,..., 

..0 

CIJ 

H 

~ 

I 

I 

I 

~ 

0 

0 

,..., 

I 

"' "' 

0 

0 

Ul 

.._, 

CIJ 

H 

.,., 

H 

s 

0 

H 

..... 

~ 

"'"" CIJ .._, 

004-< 

"04-< 

<I! 0 .._, 

CIJ Ul 

.,., " u 0 0 .,., 

UlM 

UlM <r: .,., 

e 

~ 

,..., 

..0 

..0 

,..., """ .,., 0 

0 Ul 

"Cl <I! " 0 ;::l .,., 

HM 

UM .,., 

5 

~ 

H 

.._, 

§ 

0 

u 

0000000 

00tn00lf)\.O 

lf"'l tr) N /""'-.. N 0'\ 

OOlf'\OOOlf'\0 

1"'1')-..j"QNt.nOONO 

0("'),..., ,..., ("') 

,_..;-

0 

lf') 

0 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

,..., 

CIJ 

.._, 

0 

H 

46 

Scholle, 1975). Oil and gas production, to 

date, has been limited to the upper 

Maestrichtian and Danian part of the section. 

Production has also been limited to the area 

within that portion of the Central Graben of 

the North Sea which has the thickest overlying 

section of Tertiary sedimentary rocks (fig. 

33). In this area the top of the chalk section 

is buried at a depth between 2,000 and 3,500 m 

(6,600-11,500 ft). No production is known from 

shallower chalk sections within the North Sea 

or from chalks in surrounding onshore regions. 

This pattern is opposite from that seen in 

the Gulf Coast and Western Interior areas where 

oil and gas production generally decrease as a 

function of burial depth. However, in the 

North Sea area much higher porosity values are 

found in the chalks of the Central Graben than 

would be predicted by theoretical or observational studies. (See fig. 1.) Porosities as 

high as 40 percent (and averaging 30 percent) 

were reported for much of the hydrocarbon 

.a....u..... Northern North Sea grabens 

-2- Isopach of Tert1ary sedimentary rocks. Contour interval 

1 kilometer 

~ Approximate area of oil production from chalk 

Figure 33.--Isopach map of Tertiary sedimentary 

rocks of the North Sea in relation to chalk 

oil and gas reservoirs. Adapted from Scholle 

(1977). 
productive seceion in the Ekofisk field (Owen, 

1972; Rickards, 1974), despite burial depths in 

excess of 3,000 m (9,900 ft). SEM studies have 

indicated that the bulk of this pore space is 

primary and that the North Sea chalks are 

extremely similar petrographically to those in 

the Gulf Coast and Western Interior. Figure 34 

illustrates a typical porous chalk from Ekofisk 

and shows remarkable compositional and textural 

analogies between North Sea and North American 

chalks. What is unusual about North Sea chalk, 

then, is not its primary composition but its 

subsequent diagenetic history. 

The major factor that sets the North Sea 

chalk reservoirs apart from all other chalk 

fields is overpressuring. Harper and Shaw 

(1974) reported pore fluid pressures of 7,100 

psi at a depth of 3,050 m (10,000 ft) in the 

Ekofisk area (normal pressures at that depth 

would be about 4,300 psi), and T. D. Eames 

(oral commun., 1974) indicated that such overpressures are common in wells throughout the 

deep Central Graben area. Overpressures are 

restricted to the graben, however, and do not 

extend beyond the border faults. The overpresFigure 34.--SEM photomicrograph of Danian chalk 

from Phillips Petroleum Co. Ekofisk 2/4-2X 

well (3,127 m or 10,258 ft in depth) in 

Nonvegian sector of the North Sea. Abundant 

coccoliths show only the earliest stages of 

overgrowth cementation in spite of deep 

burial. Sample has 31 percent porosity. 

Scale bar is 2 ~m. 

suring is presumably related to the retardation 

of water expulsion due to extremely low permeabilities of the chalk and thick overlying 

shale. Overpressuring may also be related to 

early generation of biogenic gas which accumulated in structural highs, expelling original 

pore waters and preserving porosity. Areas 

laterally adjacent to these structures would 

have retained pore fluids and may have undergone 

considerable porosity loss during burial. This 

would provide a lateral seal for maintaining 

geopressures in the chalk reservoirs. Oil may 

have entered the reservoirs late in the burial 

history, moving downsection from the even more 

overpressured overlying Tertiary shales (Byrd, 

1975). Alternatively, oil derived from a 

deeper part of the section may have entered the 

structures early in their burial history and 

the oil itself may be responsible for much of 

the porosity preservation. 

Overpressuring has such a profound effect 

on chalk diagenesis because it reduces differential intergranular stresses (Scholle, 1977). 

The chalk of the Ekofisk area, for example, is 

subjected to an effective overburden stress 

equivalent to only about 1,000 m (3,300 ft) of 

burial, despite an actual burial depth of 3,050 

m (10,000 ft). The stress difference between 

these two val ues is borne by the pore fluids 

and therefore does not cause the pressure 

solution- reprecipitation or mechanicalcompaction phenomena responsible for chalk 

cemention. The early (pre- deep burial) establishment of abnormally high pore-fluid pressures 

or the early introduction of oil and (or) gas 

into the reservoir can significantly retard the 

cementation of chalk, and one or both of these 

mechanisms have clearly acted in the North Sea 

reservoirs. 

A number of other factors also are 

involved, however, in making the North Sea 

fields as prolific as they are. Fracturing 

improves overall permeability (calculated from 

oil flow rates) to about 12 md (World Oil, 

1972) as compared with measured chalk matrix 

permeabilities of less than 2 md. Very thick 

pay sections (as much as 223m or 731ft thick), 

high gas- to- oil ratios (ratios of 1,000-2,000 

were noted by Harper and Shaw, 1974), low 

viscosity oil (36° gravity), and oil saturation 

of 70-100 percent all contribute to the high 

production rates from sediments with apparently 

poor reservoir properties. The oil is thought 

by some to have been derived from overlying, 

thick Tertiary marine shales which are even 

more overpressured than the chalk (Byrd, 1975), 

or it may have been generated, in part at 

least, within the chalk section. It is quite 

possible, however, that the oil migrated from 

underlying Jurassic or Lower Cretaceous source 

beds very early in the burial history, before 

overpressures were initiated. How one achieves 

oil saturation of such extremely low permeability sediments is still a largely unsolved 

problem. 

47 
This type of production could potentially 

be found in North American chalk areas as well. 

Overpressuring depends largely on the rapid 

deposition ·of thick sections of lowpermeability sediments. These criteria could 

be met in a number of areas in the Gulf Coast, 1 

particularly in downdip areas not yet exten- ; 

sively tested by drilling. With the maintenan e 

of overpressures, burial depth is no longer th 

controlling factor in chalk diagenesis and 

excellent reservoirs could be found at very 

considerable depths. Chalk-density data 

presented by Scholle (1977) indicate that such 

deep overpressured reservoirs could be located 

by seismic techniques. 

SUMMARY AND EXPLORATION MODELS 

North American chalks have produced an 

estimated 100 million bbl of oil and significa*t 

quantities of gas. These discoveries all occu 

in one of three geological situations: 

1. 

2. 

3. 

Where chalks have never been deeply 

buried (less than 1,000 m or 3,300 

ft) and thus retain considerable 

primary porosity (for example, shall w 

Niobrara gas production from western, 

Kansas and eastern Colorado). ( 

Where chalk has been buried to intermediate depths (1,000-2,000 m or 

3,300-6,600 ft) with marine pore 

fluids. The Mg-content of the pore 

waters inhibits pressure solutionreprec.ipitation and thereby maintainr 

significant amounts of primary pore 

space (for example, Wyandot gas on 

the Scotian Shelf). 

Where chalk has been buried to depths 

of 1,000 m (3,300 ft) or more with 1 

Mg-poor pore waters and extensive ! 

loss of primary porosity has taken 

place. In these areas production is 

mainly from secondary fracture 

porosity (for example, the Pearsall 

field of Texas). 

Future discoveries of these types will undoubt~ 

edly continue to be made. All three situations 

can be commercially profitable as a result of · 

sharply increased prices for crude oil and 

natural gas and of improved well-stimulation 

techniques. 

None of these types of fields will achiev 

even a small fraction of the potential of the 

North Sea chalk reservoirs, which have reserve 

of 2.5 billion bbl of oil and 8 trillion ft3 o 

gas. The key to this prolific production is 

early overpressuring (and (or) early oil input 

into the reservoirs prior to deep burial. 

Overpressuring and oil saturation dramatically 

reduce diagenetic porosity loss and maintain I 

porosities of 30-40 percent at depths of 3,000r 

3,500 m (9,900-11,500 ft). F!acturing, lowviscosity oil, low water saturations, and a 

48 

high gas-oil ratio further improve production 

characteristics. 

Outstanding production will be obtained 

from North American chalks only if similar 

overpressured reservoirs can be found in downdip, deeply buried areas of the Gulf Coast or 

the Atlantic continental margin. Without early 

formation and subsequent maintenance of overpressures, however, production from chalks in 

such deeply buried sections will be minimal. 

REFERENCES CITED 

Arthur, M.A., and Fischer, A. G., 1977, Upper 

Cretaceous-Paleocene magnetic stratigraphy 

at Gubbio, Italy, 1--Lithostratigraphy and 

sedimentology: Geol. Soc. America Bull., 

v. 88, no. 3, p. 367-371. 

Arthur, M.A., and Roggenthen, W. M., 1976, 

Primary bedding rhythms in pelagic calcareous sediments [abs.]: Am. Assoc. Petroleum Geologists Bull., v. 60, p. 646. 

Atkinson, T. C., and Smith, D. I., 1974, Rapid 

groundwater flow in fissures in the chalk--

An example from South Hampshire: Quart. 

Jour. Eng. Geology, v. 7, p. 197-205. 

Bollman, J. F., 1968, A field guide to 

Cretaceous and Tertiary exposures in westcentral Alabama: Gulf Coast Assoc. Geol. 

Sacs., 18th Ann. Convention, Field Trip 

Guidebook, 24 p. 

Braunstein, Jules, 1959, Subsurface stratigraphy 

of the Upper Cretaceous in Mississippi, in 

Mississippi Geol. Soc. 14th Field Trip, 

Guidebook, Jackson, Miss.: p. 5-10. 

Bukry, David, 1969, Upper Cretaceous coccoliths 

from Texas and Europe: Kansas Univ. 

Paleont. Contr., Art. 51 (Protista 2), 79 

p. 

Byrd, W. D., 1975, Geology of the Ekofisk 

Field, offshore Norway, in Woodland, A. 

W., ed., Petroleum and the Continental 

Shelf of north-west Europe, Volume 1: New 

York, John Wiley and Sons, p. 439-445. 

Cayeux, L., 1935, Les roches sedimentaires de 

France--Roches carbonatees (calcaires et 

dolomies): Paris, Mason, 463 p; translated 

by Carozzi, A. V., 1970, Darien, Conn., 

Hafner Pub. Co., 506 p. 

Childs, F. B., and Reed, P. E. C., 1975, Geology 

of the Dan Field and the Danish North Sea, 

in Woodland, A. W., ed., Petroleum and the 

Continental Shelf of northwest Europe, 

Volume 1: New York, John Wiley and Sorts, 

p. 429-438. 

Clark, D. L., and Bird, K. J., 1966, Foraminifera'and paleoecology of the upper 

Austin and lower Taylor (Cretaceous) 

strata in north Texas: Jour. Paleontology, 

v. 40, no. 2, p. 315-327. 

Cloud, K. W., 1975, The diagenesis of the 

Austin Chalk: Dallas, Texas Univ. M.S. 

thesis, 71 p. 
Cobban, W. A., and Scott, G. R., 1972, Stratigraphy and ammonite fauna of the Graneros 

Shale and Greenhorn Limestone near Pueblo, 

Colorado: U.S. Geol. Survey Prof. Paper 

645, 108 p. 

Condra, G. E., and Reed, E. C., 1959, The 

geological section of Nebraska: Nebraska 

Geol. Survey Bull. 14A, 82 p. 

Dallas Geological Society, 1965, The geology of 

Dallas County: Symposium on surface and 

subsurface geology, gravity, physiography, 

underground water supply, economic geology 

and engineering geology of Dallas County, 

211 p. 

Dane, C. H., Pierce, W. G., and Reeside, J. B., 

Jr., 1937, The stratigraphy of the Upper 

Cretaceous rocks north of the Arkansas 

River in eastern Colorado: U.S. Geol. 

Survey Prof. Paper 186-K, p. 207-232. 

Doyle, W. M., Jr., 1955, Production and reservoir characteristics of the Austin Chalk in 

South Texas: Gulf Coast Assoc. Geol. 

Sacs. Trans., v. 5, p. 3-10. 

Durham, C. 0., Jr., 1957, The Austin Group in 

central Texas: New York, Columbia Univ. 

Ph. D. dissert., pages unknown. 

Eicher, D. L., 1969, Paleobathymetry of 

Cretaceous Greenhorn sea in eastern 

Colorado: Am. Assoc. Petroleum Geologists 

Bull., v. 53, no. 5, p. 1075-1090. 

Eicher, D. L., and Worstell, Paula, 1970, 

Cenomanian and Turonian foraminifera from 

the Great Plains, United States: Micropaleontology, v. 16, no. 3, p. 269-324. 

Freeman, V. L., 1961, Contact of Boquillas 

flags and Austin Chalk in Val Verde and 

Terrell Company, Texas: Am. Assoc. Petroleum Geologists Bull., v. 45, no. 1, p. 

105-107. 

Frey, R. W., 1972, Paleoecology and depositional 

environment of Fort Hays Limestone member, 

Niobrara Chalk (Upper Cretaceous), westcentral Kansas: Kansas Univ. Paleont. 

Contr., Art. 58, 72 p. 

Hakansson, E., Bromley, R., and Perch-Nielsen, 

K., 1974, Naastrichtian chalk of northwest Europe--A pelagic shelf sediment, in 

Hsu, K. J., and Jenkyns, H. C., eds., 

Pelagic sediments--On land and under the 

sea: Oxford, England, Blackwell Sci. 

Pubs. (Internat. Assoc. Sedimentology 

Spec. Pub. 1), p. 211-233. 

Hancock, J. M., 1975, The sequence of facies in 

the Upper Cretaceous of northern Europe 

compared with that in the Western Interior, 

in Caldwell, W. G. E., ed., The Cretaceous 

System in the Western Interior of North 

America: Geol. Assoc. Canada Spec. Paper 

13, p. 83-118. 

1976, The petrology of the Chalk: 

------Geologists' Assoc. (London) Proc., v. 86 

(1975), pt. 4, p. 499-535. 

Hancock, J. H., and Scholle, P. A., 1975, Chalk 

of the North Sea, in Woodland, A. W., ed., 

Petroleum and the Continental Shelf of 

northwest Europe, Volume 1: New York, 

John Wiley and Sons, p. 413-425. 

Harper, M. L., and Shaw, B. B., 1974, 

Cretaceous-Tertiary carbonate reservoirs 

in the North Sea: Offshore North Sea 

Technology Conference, Stavanger, Norway, 

Paper G IV/4, 20 p. 

Hattin, D. E., 1962, Stratigraphy of the Carlile 

Shale (Upper Cretaceous) in Kansas: 

Kansas Geol. Survey Bull. 156, 155 p. 

_____ 1965, Upper Cretaceous stratigraphy, 

paleontology, and paleoecology of western 

Kansas: Geol. Soc. America Ann. Htg. 

(Kansas City), Field Conf. Guideboo~, 69 

p. 

_____ 1971, Widespread, synchronously deposited, 

burrow-mottled limestone beds in Greenhorn 

Limestone (Upper Cretaceous) of KanFas and 

southeastern Colorado: Am. Assoc. Petroleum Geologists Bull., v. 55, no. 3, p. 

412-431. 

1975a, Stratigraphy and depositional 

-----environment of Greenhorn Limestone (Upper 

Cretaceous) of Kansas: Kansas Geol. -

Survey Bull. 209, 128 p. 

_____ 1975b, Petrology and origin of fecal 

pellets in Upper Cretaceous strata of 

Kansas and Saskatchewan: Jour Sed. Pet., 

v. 45, no. 3, p. 686-696. 

Hattin, D. E., and Darko, D. A., 1971, Te0.hnique 

for determining coccolith abundance in 

shaly chalk of Greenhorn Limestone (Upper 

Cretaceous) of Kansas: Kansas Geol. 

Survey Bull. 202, pt. 2, 11 p. 

Hay, W. W., Southam, J. R., and Noel, M. R., 

1976, Carbonate mass balance--Cycling and 

deposition on shelves and in deep sea 

[abs.]: Am. Assoc. Petroleum Geologists 

Bull., v. 60, no. 4, p. 678. 

Hazel, J. E., and others, in press, Biostratigraphy of the deep corehole (Clubhouse 

Crossroads corehole 1) near Charleston, 

South Carolina: U.S. Geol. Survey Prof. 

Paper 1028-F. 

International Oil Scouts Association, 1976, 

International oil and gas development 

yearbook: Internat. Oil Scouts Assoc., v. 

45, pt. 2, 335 p. 

Jansa, L. F., and Wade, J. A., 1975, Geology of 

the continental margin off Nova Scotia and 

Newfoundland, in van der Linden, W. J. M., 

and Wade, J. A., eds., Offshore geology of 

eastern Canada, Volume 2: Canada Ge~l. 

Survey Paper 74-30, p. 51-105. 

Kauffman, E. G., 1967, Coloradoan macroinvertebrate assemblages, central Western Interior, United States, in Paleoenvironm?.nts 

of the Cretaceous seaway--A symposiu~: 

Golden, Colorado School Mines, p. 67-143. 

_____ 1973, Cretaceous Bivalvia, in Hallam, A., 

ed., Atlas of palaeobiogeography: N?.w 

York, Elsevier, p. 353-383. 

Kauffman, E. G., Powell, J. D., and Hattin, D. 

E., 1969, Cenomanian-Turonian facies 

49 
across the Raton basin: Mtn. Geologist, 

v. 6, no. 3, p. 93-118. 

Kauffman, E. G., and Scholle, P. A., 1977, 

Abrupt biotic and environmental changes 

during peak Cretaceous transgressions in 

Euramerica, in North American Paleont. 

Convention II, part 3 of 3 [abs.]: Jour. 

Paleontology, v. 51, no. 2, supp., p. 16. 

Kennedy, W. J., 1967, Burrows and surface 

traces from the Lower Chalk of southern 

England: British Mus. (Nat. History) 

Bull., Geology, v. 15, no. 3, p. 125-167. 

Kent, P. E., 1975, Review of North Seq Basin 

development: Geol. Soc. London Quart. 

Jour., v. 131, pt. 5, p. 435-568. 

Long, M., 1976a, Austin chalk drilling surges 

in South Texas: Oil and Gas Jour., v. 74, 

no. 11, p. 62-63. 

______ 1976b, Austin Chalk play spreads far to 

northeast in Texas: Oil and Gas Jour., v. 

74, no. 36, p. 60-64. 

Mann, C. J., 1958, Geology of Chandler Syncline, 

Fremont County, Colorado, in Kansas Geol. 

Soc. Guidebook, 22nd Ann. Field Con£.: p. 

153-163. 

Mapstone, N. B., 1975, Diagenetic history of a 

North Sea chalk: Sedimentology, v. 22, 

no. 4, p. 601-613. 

Matter, Albert, 1974, Burial diagenesis of 

pelitic and carbonate deep-sea sediments 

from the Arabian Sea, in California Univ., 

Scripps Institution of Oceanography, La 

Jolla, Initial reports of the Deep Sea 

Drilling Project, Volume 23: Washington, 

D. C., Natl. Sci. Foundation, p. 421-469. 

McAuslan, E. R., 1959, In the Niobrara ... oil 

may yield to special methods: Oil and Gas 

Jour., v. 57, no. 38, p. 158-166. 

McCallum, H. D., 1933, Darst Creek oil field, 

Guadalupe County, Texas: Am. Assoc. 

Petroleum Geologists Bull., v. 17, no. 1, 

p. 16-37. 

Mciver, N. L., 1972, Cenozoic and Mesozoic 

stratigraphy of the Nova Scotia shelf: 

Canadian Jour. Earth Sci., v. 9, no. 1, p. 

54-70. 

McNulty, C. L., Jr., 1971, Causal relations 

bet~een the Austin Chalk (Upper Cretaceous) 

and the Edwards Limestone (Lower 

Cretaceous) in northeast Texas: Geol. 

Soc. America Abs. with Programs, v. 3, no. 

3, p. 242. 

Mellen, F. F., 1958, Cretaceous shelf sediments 

of Mississippi: Mississippi Survey Bull. 

85, 111 p. 

Mimran, Y., 1975, Fabric deformation induced in 

Cretaceous chalks by tectonic stresses: 

Tectonophysics, v. 26, no. 3/4, p. 309-

316. 

Murray, G. E., 1961, Geology of the Atlantic 

and Gulf coastal province of North America: 

New York, Harper and Bros., 692 p. 

Neugebauer, Joachim, 1973, The diagenetic 

problem of chalk--The role of pressure 

solution and pore fluid: Neues Jahrb. 

Geologie u. Palaontologie Abh., v. 143, 

no. 2, p. 223-245. 

1974, Some aspects of cementation in 

------chalk, in Hsu, K. J., and Jenkyns, H. C., 

eds., Pelagic sediments--On lard and under 

the sea: Oxford, England, Blackwell Sci. 

Pub. 1), p. 149-176. 

1975a, Fossil-diagenese in der 

------Schreibkreide--Coccolithen: Neues Jahrb. 

Geologie u. Palaontologie Monatsh., 1975, 

no. 8, p. 489-502. 

1975b, Foraminiferen-Diagenese in der 

------Schreibkreide: Neues Jahrb. Geologie u. 

Palaontologie Abh., v. 150, no. 2, p. 182-

206. 

Owen, J.D., 1972, A log analysis method for 

Ekofisk Field, Norway: Soc. Prof. Well 

Log Analysts 13th Ann. Logging Symposium, 

May 7-10, 1972, Proc., Paper lC, 22 p. 

Owens, J. P., Minard, J. P., Sohl, ~- F., and 

Mello, J. F., 1970, Stratigrapry of the 

outcropping post-Magothy Upper Cretaceous 

formations in southern New JerEey and 

northern Delmarva Peninsula, Delaware and 

Maryland: U.S. Geol. Survey Prof. Paper 

674, 60 p. 

Pessagno, E. A., Jr., 1969, Upper Cretaceous 

stratigraphy of the western Gulf Coast 

area of Mexico, Texas and Arkansas: Geol. 

Soc. America Mem. 111, 139 p. 

Petroleum Information Corporation, 1977, 

Development well drilling increases in top 

ten fields: "Energy Information, v. 6, no. 

4, 2 p. 

Powell, J. D., 1970, Field trip guidebook for 

the First Interamerican Micropaleontological Colloquium (July 19-30, 1970): 

Dallas, Texas Univ., 91 p. 

Price, M., Bird, M. J., and Foster, S. S.D., 

1976, Chalk pore-size measurements and 

their significance: Water Services, v. 

80, no. 968, p. 596-600. 

Reeside, J. B., Jr., 1944, Hap showing thickness 

and general character of ·the Cretaceous 

deposits in the Western Interior of the 

United States: U.S. Geol. Survey Oil and 

Gas Inv. Map OM-10. 

Rickards, L. M., 1974, The Ekofisk area, 

discovery to development: Offshore North 

Sea Technology Conference, Stavanger, 

Norway, Sept. 6, 1976, Paper G-IV/3, 16 p. 

Ruffman, A., and van Hinte, J. E., 1973, Orphan 

Knoll--A "chip" off the North Anerican 

"plate", in Hood, P. J., ed., Earth science 

symposium on offshore eastern Canada: 

Canada Geol. Survey Paper 71-23, p. 407-

449. 

Runnels, R. T., and Dubins, I. M., 1949, 

Chemical and petrographic studies of the 

Fort Hays Chalk in Kansas: Kansas Geol. 

Survey Bull. 82, pt. 1, p. 1-36. 

Schlanger, S. 0., and Douglas, R. G., 1974, The 

Pelagic ooze-chalk-limestone transition 

and its implications for marine stratigraphy, in HsG, K. J., and Jenkyns, H. C., 

50 
eds., Pelagic sediments--On land and under 

the sea: Oxford, England, Blackwell Sci. 

Pubs. (Internat. Assoc. Sedimentology 

Spec. Pub. 1), p. 117-148. 

Scholle, P. A., 1974, Diagenesis of Upper 

Cretaceous chalks from England, Northern 

Ireland and the North Sea, in Hst, K. J., 

and Jenkyns, H. C., eds., Pelagic sediments--On land and under the sea: Oxford, 

England, Blackwell Sci. Pubs. (Internat. 

Assoc. Sedimentology Spe~. Pub. 1), p. 

177-210. 

1976, Diagenetic patterns of chalks 

------(abs): Am. Assoc. Petroleum Geologists 

Bull., v. 60, no. 4, p. 719-720. 

______ 1977, Chalk diagenesis and its relation 

to petroleum exploration--Oil from chalks, 

a modern miracle?: Am. Assoc. Petroleum 

Geologists Bull., v. 61, no. 7, p. 982-

1009. 

______ in press, Sedimentology, diagenesis, and 

petroleum potential of deepwater limestones, in Klein, G. deV., ed., Predictive 

sedimentary models for earth resources: 

Geol. Soc. America Spec. Paper. 

Scholle, P. A., and Kauffman, E. G., 1977, 

Paleoecological implications of stable 

isotope data from Upper Cretaceous limestones and fossils from the U.S. Western 

Interior, in North American Paleont. 

Convention II, part 3 of 3, [abs.]: 

Jour. Paleontology, v. 51, no. 2 supp., p. 

24-25. 

Scott, G. R., 1962, Geology of the Littleton 

quadrangle, Jefferson, Douglas, and 

Arapahoe Counties, Colorado: U.S. Geol. 

Survey Bull. 1121-L, 53 p. 

Scott, G. R., and Cobban, W. A., 1964, Stratigraphy of the Niobrara Formation at 

Pueblo, Colorado: U.S. Geol. Survey Prof. 

Paper 454-L, 30 p. 

Scott, J. C., 1968, Facies changes in the Selma 

Group in central and eastern Alabama: 

Alabama Geol. Soc. 6th Ann. Field Trip 

Guidebook, 69 p. 

Scott, R. J., 1977, A 1974 Austin Cretaceous 

discovery opened new frontiers in south 

Texas: Oil and Gas Jour., v. 75, no. 20, 

p. 200-204. 

Scott, R. J., and Prestridge, J., 1977, South 

Texas Austin Chalk (Cretaceous) play 

(abs): Am. Assoc. Petroleum Geologists 

Bull., v. 61, no. 2, p. 296-297. 

Seewald, K. 0., 1967, Stratigraphy of the Upper 

Cretaceous Austin Group, central Texas 

[abs.]: Gulf Coast Assoc. Geol. Socs. 

Trans., v. 17, p. 151. 

Shreveport Geological Society, 1965, Cretaceous 

of southwest Arkansas: Shreveport Geol. 

51 

Soc. Guidebook for 1965 Field Trip, 34 p. 

Geological Society, 1968, Stratigraphy and 

selected gas-field studies of north 

Louisiana, in Beebe, B. W., and Curtis, B. 

F., eds., Natural gases of North America, 

Volume 1: Am. Assoc. Petroleum Geologists 

Mem. 9, p. 1099-1137. 

Smith, C. C., 1973, Calcareous nannoplankton of 

the upper Eagle Ford and lower Austin 

Formations: Dallas, Texas Univ. Ph. D. 

dissert., 336 p. 

Smith, C. C., 1975, Upper Cretaceous calcareous 

nannoplankton zonation and stage boundaries: Gulf Coast Assoc. Geol. Socs. 

Trans., v. 25, p. 263-278. 

Stewart-Gordon, T. J., 1976, High oil prices, 

technology support Austin Chalk boom: 

World Oil, v. 183, no. 5, p. 123-126. 

Tiratsoo, E. N., 1976, Oilfields of the l~rld, 

2d edition: Beaconsfield, England, 

Scientific Press, Ltd., 384 p. 

Weimer, R. J., 1960, Upper Cretaceous stratigraphy, Rocky Mountain area: Am. Assoc. 

Petroleum Geologists Bull., v. 44, no. 1, 

p. 1-20. 

White, R. H., Jr., 1960, Petrology and depositional pattern in the Upper Austin Group, 

Pilot Knob area, Travis County, Texas: 

Austin, Texas Univ. M.A. thesis, 133 p. 

Williams, G. L., Jansa, L. F., Clark, D. F., 

and Ascoli, P., 1974, Stratigraphy of the 

Shell Naskapi N-30 well, Scotian Shelf, 

eastern Canada: Canada Geol. Survey Paper 

74-50, 8 p. 

Wilson, H. H., 1977, Frozen-in hydrocarbon 

accumulations or diagenetic traps--

Exploration targets: Am. Assoc. Petroleum 

Geologists Bull., v. 61, no. 4, p. 683-

491. 

Wise, S. W., Jr., 1973, Calcareous nannofossils 

from cores recovered during Leg 18, Deep 

Sea Drilling Project--Biostratigrap~y and 

observations of diagenesis, in California 

Univ., Scripps Institution of Oceanc~raphy, 

La Jolla (Initial reports of the Deep Sea 

Drilling Project, Volume 18: Washirgton, 

D.C., Natl. Sci. Foundation, p. 569-615. 

Woods, D. J., 1963, Fractured chalk oil reservoirs, Sabine Parish, Louisiana: Gulf 

Coast Assoc. Geol. Socs. Trans., v. 13, 

p. 127-138. 

World Oil, 1971, Massive Danian limestone key 

to Ekofisk success: World Oil, v. 172, 

no. 6, p. 51-52. 

Ziegler, P. A., 1975, Geologic evolution of 

North Sea and its tectonic framework: Am. 

Assoc. Petroleum Geologists Bull., v. 59, 

no. 7, p. 1073-1097. 

<J U.S. GOVERNMENT PRINTING OFFICE: 19n-n7-102/14 REGION NO.8