ABSTRACT The Jurassic Norphlet Formation of the Deep

ABSTRACT
The Jurassic Norphlet Formation of the Deep-Water Eastern Gulf of Mexico: A
Sedimentologic Investigation of Aeolian Facies, their Reservoir Characteristics, and their
Depositional History
Scott W. Douglas, M.S.
Thesis Chairperson: Stephen I. Dworkin, Ph.D.
This study defined the sedimentary facies, reservoir architecture, and depositional
history of the Jurassic aeolian Norphlet Formation in the deep-water Eastern Gulf of
Mexico. Seven depositional facies were identified and grouped into four facies
associations, three of which being primary aeolian facies: 1) Grainflow, exhibiting the
highest permeability and dip angles; 2) Wind-Ripple, exhibiting moderate permeability
and dip angles; and 3) Wet Interdune, exhibiting the lowest permeability and dip angles.
Associated extradunal fluvial facies were identified in the lower Norphlet interval. All
facies displayed heterogeneous attributes of porosity and permeability. Permeability has
been reduced by various diagenetic cements in the Norphlet.
Stratigraphically equivalent aeolian stabilization surfaces were identified within
the Norphlet. The reconstructed Norphlet depositional model demonstrates a relatively
dry, mature, northwestward migrating aeolian erg with the Middle Ground Arch highland
sediment source to the east, and presumably an ancestral Gulf of Mexico to the west.
The Jurassic Norphlet Formation of the Deep-Water Eastern Gulf of Mexico: A
Sedimentologic Investigation of Aeolian Facies, their Reservoir Characteristics, and their
Depositional History
by
Scott W. Douglas, B.S.
A Thesis
Approved by the Department of Geology
___________________________________
Steven G. Driese, Ph.D., Chairperson
Submitted to the Graduate Faculty of
Baylor University in Partial Fulfillment of the
Requirements for the Degree
of
Master of Science
Approved by the Thesis Committee
___________________________________
Stephen I. Dworkin, Ph.D., Chairperson
___________________________________
John B. Wagner, Ph.D.
___________________________________
Steven G. Driese, Ph.D.
___________________________________
Stacy C. Atchley, Ph.D.
___________________________________
Sascha Usenko, Ph.D.
Accepted by the Graduate School
December 2010
___________________________________
J. Larry Lyon, Ph.D., Dean
Page bearing signatures is kept on file in the Graduate School.
Copyright © 2010 Scott W. Douglas
All rights reserved
TABLE OF CONTENTS
LIST OF FIGURES
v
LIST OF TABLES
vii
ACKNOWLEDGMENTS
viii
CHAPTER ONE – Introduction
1
Overview and Objectives
1
Norphlet Geologic Setting
3
Dataset and Approach
9
CHAPTER TWO – Depositional Facies
11
Aeolian Facies Descriptions
13
Facies 1: Avalanche-Dominated Grainflow
13
Facies 2: Wind-Ripple-Interbedded Grainflow
14
Facies 3: Cross-laminated Wind-Ripple
14
Facies 4: Planar-laminated Wind-Ripple
16
Facies 5: Damp Interdune
16
Facies 6: Wet Interdune
16
Facies 7: Reworked Aeolian
18
Norphlet Facies Associations
19
Depositional Facies Summary
20
CHAPTER THREE – Analysis of Reservoir Characteristics
21
Facies-Controlled Porosity and Permeability
21
Norphlet Detrital Texture Analysis
24
iii
Grain Size
24
Grain Sorting
24
Grain Rounding
25
Textural Summary
25
Norphlet Diagenesis
27
Reservoir Characteristics Summary
31
CHAPTER FOUR – Norphlet Depositional History
33
Seismic Analysis
33
Core-Log Correlation
35
Sedimentary Provenance
37
Depositional History
39
Dune Morphology and Wind Regime
40
Norphlet Analogs
41
Depositional Models
43
CHAPTER FIVE – Conclusions
47
APPENDIX – Supplementary Figures
50
REFERENCES
54
iv
LIST OF FIGURES
1. Norphlet Formation study area map
2
2. Stratigraphic column of study interval
3
3. Late Jurassic paleogeographic map
4
4. Depositional model of Norphlet
5
5. Seismic isopach map of dunes in Mobile Bay
6
6. Norphlet paleogeographic reconstruction
7
7. Jurassic Oxfordian tectonic map of the Gulf of Mexico region
8
8. Schematic diagram of aeolian deposits
13
9. Navajo Sandstone Outcrop showing aeolian Facies 1-4
15
10. Modern Namibia dune showing aeolian facies formation
15
11. Modern Namibia interdune with crinkly beds of (Facies 5)
17
12. Modern Great Sand Dunes ephemeral flooding in interdune in (Facies 6)
17
13. Modern Namibia coastal region where seawater is reworking aeolian (Facies 7)
18
14. Norphlet porosity and permeability against facies
22
15. Facies controlled porosity and permeability at Mobile Bay
23
16. Permeability heterogeneity in Rotliegendes sandstone core
23
17. Relation of sandstone texture to porosity and permeability
26
18. Representative thin section photos of diagenetic cements
28
19. Norphlet diagenetic phase chart
30
20. Seismic profile through wells in study area
34
21. Thin section with detrital constituents in Norphlet
38
v
22. Norphlet sandstone classification ternary diagrams
38
23. Progression of Norphlet dune morphology change
41
24. Plot of grainflow thickness against dune slipface height
42
25. Namibia modern analog
44
26. Cedar Mesa ancient analog
45
27. Cedar Mesa analog to lateral field changes
45
28. Norphlet depositional model
46
A1.1 Aeolian facies summary table
51
A1.2 Aeolian processes of deposition diagram
52
A1.3 Schematic diagram of aeolian bounding surface
52
A1.4 Schematic diagram of modern dune morphologies
53
vi
LIST OF TABLES
1. Core-described facies table
12
vii
ACKNOWLEDGMENTS
First and foremost, I want to thank Nexen Petroleum USA, Inc. for providing me
with the project dataset used for my thesis. This project would not have been possible
without it. More specifically, I want to thank Dr. John Wagner, Chief Geologist at
Nexen, who personally set me up with this research and with an internship allowing me
access to all the data necessary for the project. Dr. Wagner has been a great mentor
throughout the entire project. Thanks to the many others at Nexen who contributed to
different parts of the project during my internship, including Tom Walker, Nils Andresen,
Jim Colliton, John Davis. I also want to acknowledge Reed Glasmann at Willamette
Geological Service.
Of course, this thesis project would not be possible without the many people at
Baylor University who helped me write and organize my thesis properly and provided
insight to the content as well. Most notably, I want to thank Dr. Steve Dworkin, my
advisor at Baylor, who took me on as a student and worked tirelessly with me on editing
and organizing my thesis, and who was always open to discuss ideas for its direction.
Thanks to Dr. Stacy Atchley, who helped with the organization and flow of my thesis.
Last but not least, I want to thank Dr. Steven Driese for being supportive throughout the
writing process and who provided geologic insight on my project.
viii
CHAPTER ONE
Introduction
Overview and Objectives
Ancient aeolian depositional systems like the Norphlet Formation of the Gulf
Coast United States have been studied extensively because of their thick accumulations in
the rock record, large aerial extent, and potential to store large volumes of hydrocarbons.
Aeolian deposits have proven to be effective reservoirs due to preservation of high
porosity with interconnected pore throats and high net-to-gross sand proportions.
However, even though aeolian systems are primarily sand, they are characterized by an
internal heterogeneity that is challenging to understand due to rapid changes in both
vertical and horizontal stratification. These rapid changes occur at various scales,
ranging from small incremental changes (laminations) between grainflow and windripple deposits to changes between dune and interdune facies.
The Jurassic Norphlet Formation is a widespread buried aeolian sandstone first
discovered in Norphlet, Arkansas in 1922 with an aerial extent stretching thousands of
square kilometers across the states of Louisiana, Mississippi, Alabama, and Florida, and
into the waters of the Gulf of Mexico (Mancini et al., 1985). The full extent of the
Jurassic aeolian system is not known, although the boundaries onshore are fairly well
constrained. Hydrocarbon accumulation is attributed to the Jurassic Smackover
Formation, which acts as both the source and seal for underlying Norphlet aeolian
reservoirs (Tew et al., 1991). Drilling and exploration in the deep-waters of the Eastern
Gulf of Mexico utilizing high-resolution 3D seismic data has led to further seaward
1
extension of the prolific aeolian system into water depths greater than 5000 feet (Godo et
al., 2006). This study summarizes the published information of the Norphlet Formation
located in the subsurface of the deep-water Eastern Gulf of Mexico (Figure 1).
Figure 1. Norphlet Formation study area in the deep-water Eastern Gulf of Mexico. Depths in the study
area are around 2000 meters, and about 150 km from the Mississippi River Delta (modified from Salvador,
1991)
Although well studied onshore and offshore (e.g., McBride, 1981; Mancini et al.
1985, 1986, 1990; Tew et al., 1991; Dixon et al., 1989; Story, 1998; Kugler and Mink,
1999; Ajdukiewicz et al., 2010), little is known about the Norphlet beneath the deepwaters of the Eastern Gulf of Mexico. Determining the relationships between facies and
reservoir properties such as porosity, permeability, texture, and petrophysical response
will aid in future production of these aeolian reservoirs. Additionally, the ability to
predict lateral facies extent will assist in further exploration of the deep-water Norphlet.
This thesis will investigate the following general objectives about the Norphlet in
the deep-water Eastern Gulf of Mexico: 1) Characterize the aeolian depositional facies
within the Norphlet; 2) Establish relationships between facies and reservoir
2
characteristics; 3) Understand effect of diagenesis on facies porosity and permeability; 4)
Determine the depositional setting of the Norphlet and reconstruct the paleo-dune field
(erg) based on existing data.
Norphlet Geologic Setting
During the late Triassic, rifting of the supercontinent Pangea was initiated and the
Gulf of Mexico basin began to form (Jacques and Clegg, 2002). Rifting continued
through the middle Jurassic, allowing seawater to enter the restricted ancestral Gulf of
Mexico. These waters evaporated leaving behind thick evaporite deposits known today
as the Louann Salt (Figure 2; Salvador, 1991; Bird et al., 2005).
Figure 2. Stratigraphic column of geologic units found in the eastern Gulf of Mexico region with study
interval highlighted. The thick Louann Salt evaporites are in red. The Norphlet aeolian sandstone is in
yellow. The overlying Smackover carbonates are in blue (modified from Mancini et al., 1986).
3
A regional paleogeographic reconstruction (Blakey, 2010) during late Jurassic
time (170 Ma) shows the ancestral Gulf of Mexico restricted from the open ocean (Figure
3). A eustatic regression following the deposition of the Louann evaporites during the
late Jurassic resulted in an ideal arid environment for the Norphlet Formation (Oxfordian
stage, 160 Ma) aeolian deposits to develop along the exposed continental shelf (Figures 2
and 3; Mancini et al., 1990).
Figure 3. Regional Paleogeographic map during the Mid-Late Jurassic. Pangea is breaking apart allowing
waters to enter the ancestral Gulf of Mexico and evaporate forming the Louann Salt. The Norphlet
Formation is not developing at this time. Close proximity to paleo-equator indicates a warm climate during
this time (170 ma; modified from Blakey, 2010).
The Norphlet Formation shows the marked change in sand thickness from a few
feet to around 1000 feet, depending on the proximity to the Norphlet erg margin. Thicker
deposits are located within the erg axis. Mancini et al. (1985) reconstructed a
depositional model of the Norphlet at Mobile Bay (Figure 4). The southern Appalachian
4
Mountains were paleo-topographic highs during this time that provided sediment to the
Norphlet system (Figure 4; Tew et al., 1991). Norphlet alluvial and fluvial sediments
deposited in close proximity to these mountains. Wind processes transported and
deposited aeolian sediment across the arid landscape towards the southwest (Figure 4B;
Mancini et al., 1985, 1986).
Figure 4. Depositional reconstruction of Norphlet from Appalachian highlands to Mobile Bay during the
Jurassic showing structural highlands shedding alluvial sediments to the southwest, which are reworked by
aeolian processes (Mancini et al., 1985, 1986).
Seismic mapping of the Norphlet aeolian deposits at Mobile Bay (Story, 1998;
Ajdukiewicz et al., 2010) shows linear dune morphologies preserved within the
subsurface oriented north-south indicating a northward paleo-wind-direction (Figure 5).
5
Trends within this linear dune system have permeability values that are higher when
parallel to the linear dune crests (Figure 5; Story, 1998).
5 km
Figure 5. Seismic isopach map showing linear dune bedform pattern at Mobile Bay. Anisotropy is
demonstrated by these morphologies, with higher permeability parallel to the dune crests (Story, 1998).
The depositional model of the Norphlet and suggested aeolian dune extent shown
in Figure 4 will be extended seaward due to the recent Norphlet discoveries within the
deep-waters of the Eastern Gulf of Mexico. A new paleogeographic reconstruction is
proposed based on the interpretation of known Norphlet maps From Mancini et al.,
(1985, 1990) and a recent publication by Lovell, 2010 that extends the Norphlet further
south (Figure 6). Within the study area, the nearby basement structure, known as the
Middle Ground Arch, was previously documented and mapped by Dobson and Buffler
(1991). The Middle Ground Arch was a Jurassic highland area, which is interpreted to
have provided a sediment source for this southern Norphlet system. Alluvial, fluvial, and
aeolian sediments are interpreted to be deposited adjacent to the highland areas associated
with the arch (Figure 6). Much is still unknown about the boundaries of the system,
6
particularly where the coastline between the Norphlet aeolian deposits and the Jurassic
Gulf of Mexico Sea is, or if it is present at this time.
Figure 6. Paleogeographic map of the Norphlet about 160 Ma (Lovell, 2010). The Norphlet includes
alluvial, fluvial, and aeolian deposits shed off of Jurassic mountain areas. Connectivity between each
proven Norphlet aeolian system is uncertain. In addition, the location of the Norphlet coastline is also
unknown. The map also shows source terrains providing sediment to the system.
During the late Oxfordian Stage (158 Ma; Figure 2), the Norphlet Formation was
buried and eroded by a transgression associated with the Smackover Formation (Mancini
7
et al., 1990). The Smackover is characterized by shallow water carbonates, whose sharp
basal contact suggests reworking and removal of an unknown amount of the Norphlet
deposits during this transgression. The breakup of Pangea around this time increased
accommodation space within the Gulf of Mexico basin, allowing for thick accumulations
of sediment to fill the basin throughout the Mesozoic and Cenozoic. The weight of these
sediments caused structural deformation of the ductile Louann Salt throughout much of
the Gulf of Mexico (Salvador, 1991). Salt remobilization created many structural
adjustments within and fragmentation of the deep-water Norphlet deposits, which are
located overtop thick Louann Salt deposits (Figures 2 and 7). The extent of the Louann
Salt is large, as the Gulf of Mexico is actively spreading apart during the Oxfordian
(Figure 7).
Figure 7. Tectonic Map of Gulf of Mexico during the Late Jurassic Oxfordian Stage. Pangea was splitting
apart along the active ridges (red) allowing for marine waters to enter into a newly formed Gulf of Mexico
and to evaporate, forming the Louann Salt (Modified from Pindell and Kennan, 2000).
8
Dataset and Approach
In addition to the published data available on the Norphlet, this project included
observations from whole core and petrophysical data. Petrophysical data included: 1)
gamma-ray logs that measure gamma radiation; 2) neutron porosity and bulk density
logs, which measure pore space; 3) formation resistivity logs, which measures resistance
to flow; and 4) Oil-Based MicroImage (OBMI) logs, which are used to generate dip
direction and azimuth diagrams. This project also utilized data gathered from highresolution three-dimensional seismic surveys of the area.
The project approach included of the following steps:
1) Compile literature from historical aeolian deposits, with emphasis on previously
studied Norphlet successions, in order to develop a clear understanding of aeolian
systems, particularly within the Norphlet.
2) Describe the facies observed in core using conventional core description
techniques. This involves determining the sedimentary features that separate each
facies from one another.
3) Understand the diagenesis of the Norphlet with respect to reservoir properties.
4) Tie petrophysical data into core-calibrated facies to determine relationships
between facies and well log responses.
5) Use the high-resolution three-dimensional seismic data to correlate and map out
the shape and size of the Norphlet subsurface extent.
6) Identify both modern (Namib Desert, southwestern Africa) and ancient (Permian
Cedar Mesa Sandstone, southeastern Utah) aeolian analogs in order to better
understand the lateral and vertical changes we may expect within the Norphlet
9
system. These analogs will aid in predicting dune morphology, determining the
lateral extent, and better understanding the overall sedimentary architecture.
7) Reconstruct a depositional model of the migrating dune field, including the
original dune height from grainflow thickness, paleo-directional depositional
trend, and overall lateral distribution of the erg axis vs. erg margins within the
aeolian environment.
10
CHAPTER TWO
Depositional Facies
Depositional facies present are all aeolian and included avalanche-dominated
grainflow, wind-ripple influenced grainflow, cross-laminated wind-ripple, planarlaminated wind-ripple (dry interdune), damp interdune, wet interdune, and reworked
aeolian (Table 1). A wide variety of aeolian dune, interdune, and associated extradunal
depositional facies similar to those in the Norphlet have been observed by Glennie
(1970), McKee (1979), Ahlbrandt and Fryberger (1981a), Ahlbrandt and Fryberger
(1981b), Fryberger et al. (1983), Driese (1985), Rubin (1987), Fryberger et al. (1990a),
Fryberger et al. (1990b), Kocurek (1991), and Mountney (2006a), (complete list in Figure
A1.1).
Depositional facies were combined into four facies associations based on
similarities between sedimentary stratification and environmental interpretation. These
include the Grainflow, Wind-Ripple, Wet Interdune, and Reworked Aeolian Facies
Associations (Table 1). A schematic diagram representing these facies associations’
relative location to each other in aeolian systems is shown in Figure 8.
In addition to the depositional facies characterized of dunes (grainflow, wind
ripple, wet interdune), extradunal facies (e.g., wadi, sabkha, sand sheet, fluvial, alluvial
fan, etc.) are also believed to be associated with the Norphlet Formation and are likely
laterally adjacent to the aeolian facies.
11
Table 1. Facies Summary Table for the Norphlet Formation. Facies names and detailed descriptions are given in text. Seven facies identified in core were
combined into four facies association based on similar stratification patterns.
Facies
Number
1
2
3
12
4
5
Depositional Facies
Fine- to medium-grained, cm-scale,
planar-tabular cross-bedded sandstone
(20-35o dip)
Fine- to medium-grained, mm- to cmscale, cross-bedded sandstone (10-25o
dip)
Fine- to medium-grained, mm-scale,
cross-laminated, inversely graded
sandstone (pinstripe, 5-10o dip)
Fine- to medium-grained, mm-scale,
planar-laminated, inversely graded
sandstone (pinstripe, 0-4o dip)
Very fine- to coarse-grained, poorly
sorted, mm- to cm-scale, crinklylaminated argillaceous sandstone
6
Fine- to coarse-grained, mm-scale,
rippleform cross-laminated sandstone
7
Fine- to medium-grained, mm- to cmscale, irregularly deformed, crossbedded sandstone
Facies Stratification
(Interpretation)
Avalanche-Dominated Grainflow
(High-Angle Dune)
Abbreviation
Facies Associations
GA
1- Grainflow (G)
Grainflow with interbedded WindRipple
(Low-Angle Dune)
GW
Wind-Ripple cross-laminated
(Dune Plinth)
WR c
2- Wind Ripple (WR)
Wind-Ripple planar-laminated
(Dry Interdune)
Wavy or crinkly laminated watertable influenced deposits
(Damp Interdune)
Current ripple ephemeral flood or
fluvial deposits
(Wet Interdune)
Marine flooded and reworked
aeolian deposits
(Reworked Aeolian)
WR p
DI
3- Wet Interdune (WI)
WI
RW
4- Reworked (RW)
Figure 8. Grainflow, wind-ripple, and interdune deposits represented within an aeolian system. Windripple deposits occur along any surface, but are preserved at the toe-sets of the grainflow deposits.
Grainfall is reworked into grainfall deposits as the avalanche down a steep dune slope (Modified from
Wagner and Moiola, 2010)
Aeolian Facies Descriptions
Facies descriptions illustrate changes in facies, facies associations, grain size,
sedimentary structures, lithology, and bed dip angles. Higher angle dune sets are
sometimes punctuated with slump features, which may be due to over-steepening,
wetting events, or even earthquakes (Doe and Dott, 1980). Each facies will be discussed
in detail in this chapter. Drying-upward cycles of sediment accumulation are represented,
which will be discussed further in Chapter Four.
Facies 1: Avalanche-Dominated Grainflow
Facies 1 is a fine- to medium-grained, cm-scale, planar-tabular cross-bedded
sandstone (Table 1). Facies 1 is characterized by strata that dip at high angles (>20o)
with fine- to medium-grained, cm-scale, sometimes faintly inversely graded grainflow
beds commonly separated by a single thin, fine-grained lamina of grainfall deposits.
Regular alternations of grainflow and grainfall layers in Facies 1 are observed in outcrop
(Figure 9). Facies 1 is interpreted as representing the high-angle slipface of a migrating
13
dune (Figure 10). Interpretations of this facies are based on similar observations by
Hunter (1977), Mckee (1979), Ahlbrandt and Fryberger (1981a), and Mountney (2006a).
Facies 2: Wind-Ripple-Interbedded Grainflow
Facies 2 is a fine- to medium-grained, mm- to cm-scale, cross-bedded sandstone
(Table 1). Facies 2 is characterized by strata that dip at moderate to high angles (10-25o),
with fine- to medium-grained, cm-scale grainflow beds interbedded with inversely graded
wind-ripple laminations. Irregular alternations of grainflow and wind ripple beds in
Facies 2 are observed in outcrop (Figure 9). Facies 2 is interpreted to be lower slipface
deposits along the lee slope of dune sets where grainflow tongues are interfingering with
the wind-ripple strata of the plinth of the dune (observed by Hunter, 1977; Mountney,
2006a).
Facies 3: Cross-Laminated Wind Ripple
Facies 3 is a fine- to medium-grained, mm-scale, cross-laminated, inversely
graded sandstone (Table 1). Facies 3 is characterized by low to moderate angle (5-10o)
inversely graded wind-ripple laminations. Inclined wind-ripple strata of Facies 3 are
observed in outcrop (Figure 9). Facies 3 is interpreted to be plinth deposits (Mountney,
2006a) dominated by wind ripple strata formed along the lower part of the migrating
dune. This facies would be found below where grainflows are pinching out into the
wind-ripple strata, but above flat lying interdune strata (Figure 9; Ahlbrandt and
Fryberger, 1981a; Fryberger et al., 1990a; Kocurek, 1991).
14
Figure 9. Ancient outcrop of Navajo Sandstone (Jurassic) with Facies 1-4 represented. Facies 1,
Avalanche-Dominated Grainflow, represents high-angle dune deposits. Facies 2, Grainflow with
Interbedded Wind-Ripple represents the moderate angle dune deposits. Facies 3, Cross-Laminated WindRipple, represents moderate angle wind-ripple deposits. Planar Laminated Wind-Ripple represents low
angle wind-ripple deposits.
Figure 10. Modern example of aeolian facies. Dune is migrating over dry-interdune (wind-ripple).
Grainfall deposits are blown over dune crest, deposit on the leeward side of the dune, and avalanching
(grainflow) carries the grainfall deposits downslope (Namibia; modified from Mountney, 2006a). For a
detailed diagram of modern aeolian processes, see Figure A1.2.
15
Facies 4: Planar-Laminated Wind Ripple
Facies 4 is a fine- to medium-grained, mm-scale, planar-laminated, inversely
graded sandstone (Table 1). Facies 4 is characterized by flat lying (0-4o), inversely
graded wind-ripple laminations. Flat-laying wind-ripple strata in Facies 4 are observed in
outcrop (Figure 9). Facies 4 is interpreted to be a dry interdune deposit where windripples dominate, and there is little to no water table influence (Figure 10). Some thicker
deposits of Facies 4 may be interpreted as a sandsheet (Fryberger et al, 1990a; Ahlbrandt
and Fryberger, 1981a; Mountney, 2006a; Hunter, 1977).
Facies 5: Damp Interdune (Crinkly-Laminated Sandstone)
Facies 5 is a very fine- to coarse-grained, poorly sorted, mm- to cm-scale, crinklylaminated argillaceous sandstone (Table 1). Facies 5 is characterized by wavy or crinkly
laminated, low angle beds representing water-influenced deposits. Facies 5 is interpreted
to have formed in a damp interdune where capillary forces of the water table caused the
surface to be damp, trapping windblown suspended silt and clay (Ahlbrandt and
Fryberger, 1981a, b; Fryberger et al., 1990a). Evaporation of water on the interdune
surface during deposition likely caused the beds to become crinkled and deformed
(Figure 11; Ahlbrandt and Fryberger, 1981b). This facies also includes damp adhesion
ripples, similar to the deposits documented in Namibia by Kocurek and Fielder (1982).
Facies 6: Wet Interdune (Rippleform-Laminated Sandstone)
Facies 6 is a fine- to coarse-grained, mm-scale, rippleform-laminated sandstone
(Table 1). Facies 6 is characterized by low angle, cross-laminated, current-ripple
bedforms, with a wide range in grain size, from fine to coarse sand. Facies 6 is
16
Figure 11. Modern example of a damp interdune (Facies 5). The crinkly bedded nature of the interdune is
due to capillary action of the water table deforming the depositional surface (Namibia;
http://www.imaginature.nl/pages/Southern%20Africa%203.html)
Figure 12. Modern example of a wet interdune (Facies 6). An ephemeral flood is depositing sediments
over the interdune surface. These flood deposits are short lived and are quickly buried by aeolian deposits
overtop (Great Sand Dunes, CO; http://mansurovs.com/category/digital-photography/wallpapers).
17
interpreted to represent a wet interdune environment, where ephemeral fluvial (wadi)
processes deposited sediment during a flash flood event (Figure 12), based on similar
interpretations by Mountney (2006b).
Facies 7: Marine Reworked Aeolian (Irregular Cross-Bedded Sandstone)
Facies 7 is a fine- to medium-grained, mm- to cm-scale, irregularly deformed,
rippleform cross-bedded sandstone (Table 1). It is characteristically irregularly
deposited, cross-bedded sandstone that appears to have aeolian and marine (subaqueous)
elements. Identified only in the uppermost part of the Norphlet, Facies 7 is interpreted to
be aeolian dune sand that has been reworked by marine processes (Figure 13; Fryberger
et al., 1990b) during the flooding of the Smackover Formation sea-level rise. This upper
reworked zone is also identified in the Norphlet in updip onshore and offshore
environments (Tew et al., 1991).
Figure 13. Modern example of reworked aeolian (Facies 7). Seawater is reworking aeolian sands of a dune
field along the coast (Namibia; http://www.ecdc.co.za/fromabove/photographers-of-the-eastern-cape.asp)
18
Norphlet Facies Associations
The seven identified depositional facies were grouped into four facies associations
in order to combine small-scale variants of similar stratification types (sedimentary
fabrics) and processes of formation. This facilitates the correlation of facies to reservoir
characteristics (next section) and permits assessment of large-scale distribution of the
facies (Chapter Four). Facies Association 1 is Grainflow, combining high angle dune
deposits of Facies 1 and 2. Facies Association 2 is Wind-Ripple, combining all windripple deposits of Facies 3 and 4. Facies Association 3 is Wet Interdune, combining
water-influenced interdune deposits of Facies 5 and 6. Facies Association 4 is the same
as Facies 7, Reworked Aeolian. The percentage of each aeolian facies association in the
Norphlet suggests much thicker accumulations of the Grainflow Facies Association than
Wind-Ripple Facies Association, and subsequently even less Wet Interdune Facies
Association.
The Grainflow Facies Association is more abundant which indicates potentially
more dune development (dryer system). Conversely, the presence of wet interdune facies
indicates that environmental conditions were wetter due perhaps to a lower sediment
supply (Fryberger et al., 1983). The percentage distribution, however, may not
accurately represent percentages for these facies associations due to the low preservation
potential of aeolian deposits. Truncation from dune migration and deflation during
periods of non-deposition has likely skewed the percentage of preserved facies within the
Norphlet (Loope, 1985).
19
Depositional Facies Summary
Seven depositional facies were identified within the Norphlet, which were
grouped into four facies associations. The Grainflow Facies Association (1) is a fine
grained, high-angle (with a consistent dip direction on dip log), cross-stratified sandstone.
The Wind-Ripple Facies Association (2) is a medium grained, low to moderate angle (on
dip log), laminated sandstone. The Wet Interdune Facies Association (3) is a medium to
course grained, low-angle (on dip log), crinkly laminated, argillaceous (often slightly
higher gamma-ray response) sandstone. The Reworked Facies Association (4) is a fine
grained, random low to moderate angle (on dip log), irregularly bedded sandstone.
Because of similar sandstone lithology, differences in neutron-density, bulk-density and
gamma-ray response between facies were small. No relationship was found between
facies and resistivity logs because its resolution is too course to depict sedimentary
differences in the sandstone. Other non-aeolian facies are interpreted to represent an
extradunal fluvial system that is either interfingering with, or is a precursor to, the
Norphlet Formation.
20
CHAPTER THREE
Analysis of Reservoir Characteristics
In this section, the relationship between depositional facies and the reservoir
characteristics 1) porosity, 2) permeability, 3) texture, and 4) diagenesis are analyzed. If
a correlation between key reservoir characteristics and sedimentary facies can be
established, it will aid in understanding reservoir quality. Facies that display higher
porosity and permeability and more mature textural attributes can be used to predict
where best to target future hydrocarbon recovery in the Norphlet. Porosity, permeability,
and texture commonly display a strong relationship to facies types at other localities (e.g.
Beard and Weyl, 1973; Dixon et al., 1989; Mancini et al., 1990).
Facies-Controlled Porosity and Permeability
The total range and average porosity and permeability values for each facies
within the Norphlet was calculated, but focus will be placed on the four grouped facies
associations. The relationship between each facies association and porosity and
permeability in the Norphlet is illustrated in the scatter plot in Figure 14. When these
facies are combined into facies associations, a clear distinction between porosity and
permeability values can be observed. The highest porosity and permeability are found
within the Grainflow Facies Association, followed by the Wind-Ripple Facies
Association (Figure 14), and the lowest reservoir quality is found within the Wet
Interdune Facies Association. These results match facies porosity and permeability
defined in the Norphlet at Mobile Bay, as grainflow cross-lamination facies display the
21
highest porosity and permeability, wind-ripple cross-lamination facies display moderate
values, and interdune facies displaying the lowest values (Figure 15).
Overlap of core facies porosity and permeability is attributed primarily to aeolian
facies heterogeneity, which was shown by Weber (1979) to have large ranges in
permeability over short (bed scale) vertical and horizontal distances (Figure 16).
Figure 14. Abundance of measured porosity and permeability with respect to facies. The yellow
confidence ellipses represents Grainflow Facies Association. The red confidence ellipses represents WindRipple Facies Association. There is a clear distinction between Grainflow and Wind-Ripple Facies
Associations.
22
Figure 15. Facies controlled porosity and permeability defined in the Norphlet fields at Mobile Bay.
Grainflow cross-lamination facies display the highest porosity and permeability, wind-ripple crosslamination facies display moderate values, and interdune facies displaying the lowest values (Dixon et al.,
1989)
115 md
7 md
Figure16. Permeability measurements ranging from 2 to 115 md over 6 inches of the Rotliegendes
Sandstone core. This figure illustrates depositional heterogeneity in short segments (Weber, 1979).
23
Norphlet Detrital Texture Analysis
Depositional facies all display relationships to the textural characteristics of grain
size, grain sorting, and grain rounding within the Norphlet indicating the processes of
deposition control the grain populations within the sandstone. The sandstones have grain
sizes that range from very fine- to coarse-sand, based on the Udden-Wentworth scale
(Wentworth, 1922). The Norphlet Sandstone displays an upper-fine to lower-medium
grain size average, moderately well sorting, and is typically subrounded to sububangular.
Grain Size
Grain-size distributions in the Norphlet display unique relationships with respect
to facies. The Grainflow Facies Association is concentrated in the upper fine- to lower
medium-grained size category, and have small amounts of very fine or very coarse sand.
The Wind-Ripple Facies Association spans the size categories of very fine to coarse sand,
and has a distinct bimodal grain size distribution in the fine and medium sand sizes. This
is attributed to the alternating finer and coarser, mm-scale, stratified nature of wind-ripple
deposits. The poorly sorted nature of the Wet Interdune Facies Association shows a wide
range of grain sizes. This range is attributed to finer material being trapped by water
along the interdune surface and courser material being deposited by ephemeral streams
across the interdune surface (Albrandt and Fryberger, 1981b).
Grain Sorting
Grain sorting ranges from well-sorted to very poorly sorted, with moderately
well-sorted sand most common. The Grainflow Facies Association displays the best
sorting, the Wind-Ripple Facies Association is more moderately sorted, and the Wet
24
Interdune Facies Association has the poorest sorting. The Reworked Facies Association,
although there are few samples, has a similar well-sorted nature as the grainflow facies.
Grain Rounding
Grain rounding in the Norphlet sands ranges from well-rounded to angular, with
the average being subrounded to subangular. The relationship between facies and grain
rounding indicates that the Grainflow Facies Association is mostly well-rounded,
whereas Wind-Ripple Facies Association is mostly subrounded to subangular, and the
Wet Interdune Facies Association is subangular to angular.
Textural Summary
The textural attributes of the Norphlet Sandstone observed in this study are within
the typical range of aeolian sand (Bagnold, 1941), although the sand grains are a little
coarser grained than is typical. The well-sorted and well-rounded nature of the sand is
due to the aeolian processes of saltation (Schenk, 1983; Bagnold, 1941). The reason so
few grains have coarse or very fine sizes has been explained by Bagnold (1941), who
stated that it is difficult for wind to carry coarse, heavy sediment in saltation across an
erg; and conversely, the wind holds the finer grain sizes suspended in the air over long
distances.
The Grainflow Facies Association has a consistent upper fine to lower medium
grain-size, is well-sorted, and is well-rounded, which identifies it as being the most
texturally mature sandstones (Schenk, 1983). The Wind-Ripple Facies Association has a
bimodal grain-size distribution, with moderate sorting and rounding indicating less
texturally maturity. The Wet Interdune Facies Association is the least mature and
25
commonly displays a very poorly sorted nature. Fine-grained sediment more readily
becomes incorporated into the Wet Interdune Facies Association because the water traps
fine sediment. The Wet Interdune Facies Association will trap coarse sediments from
wind as well. The maturity of sandstone has been shown experimentally to be directly
related to porosity and permeability. Sandstones with better sorting and coarser grain
sizes typically display higher porosity and permeability values (Figure 17; Beard and
Weyl, 1973; Nagtegaal, 1978). This parallels facies-controlled porosity and permeability
data of the Norphlet sandstones.
Figure 17. Experiments on different grain sizes and sorting characteristics in sandstones indicate higher
values of porosity and permeability for coarser, more well-sorted sands (Beard and Weyl, 1973)
26
Norphlet Diagenesis
Norphlet sandstones have similar diagenetic constituents, which include the
following cement phases:
1) Chlorite: Chlorite cement is present throughout the entire interval of study, and is the
most abundant cement. Chlorite occurs as books and rosettes that nucleate on sand
grains and grow tangentially into intergranular areas. The amount of chlorite
precipitated is somewhat random throughout the core (Figure 18A).
2) Illite: Illite has a mostly mesh-like (tangential) and platy occurrence. Illite occurs
randomly throughout the reservoir in relatively low abundances and is always found
associated with chlorite cement (Figure 18B), indicating that if formed after chlorite.
Sources of chlorite and illite authigenic clays are likely from deep, high-temperature
brines dissolving local volcanic rocks (Taylor et al., 2010).
3) Quartz: Quartz cement occurs as small euhedral crystals that nucleate on detrital
quartz grains. The amount of quartz cement in the Norphlet is low, and its abundance
is inversely proportion to the abundance of chlorite cement. Quartz cement
precipitated on detrital quartz grains only in locations where chlorite coats are
missing. This suggests that quartz cement formed after chlorite (Taylor et al., 2010;
Figure 18C and D).
4) Calcite and anhydrite: Calcite and anhydrite occur in low abundance throughout the
Norphlet reservoir. These cements are most common in patches near damp and wet
interdune facies, and are interpreted to have formed as early evaporate minerals as the
interdune waters dried up (McBride, 1981; Driese, 1985).
27
Figure 18. Diagenetic phases within the Norphlet. A) Thick chlorite filling pores in some areas, open porespace in blue (Taylor et al., 2010). B) Tangential illite found in Norphlet (Ajdukiewicz et al., 2010). C)
Authigenic quartz common in areas lacking chlorite (Taylor et al., 2010). D) SEM photograph showing
authigenic quartz crystals growing out of windows without chlorite coating (Taylor et al., 2010). E)
Dissolution of feldspar and VRF common (Ajdukiewicz et al., 2010). F) Stylolitization from pressure
solution occurring within the Norphlet (Ajdukiewicz et al., 2010).
5) Hematite: Detrital sand grains are coated with a thin layer of iron-oxide in the lower
part of the Norphlet.
28
6) Pyrite: Pyrite occurs as opaque, gold-colored patchy clusters. It occurs in very small
amounts throughout the reservoir.
Secondary porosity formed from the dissolution of detrital grains and occurs
sporadically throughout the Norphlet (Figures 18A and E). Secondary porosity can be
identified by the oversized pores that contain the rimming cements. Secondary porosity
is very effective at increasing porosity. Compaction is evident throughout the Norphlet
interval as well. Grains have become tightly packed in some areas, particularly in the
lower part of the Norphlet (Figure 18F). Fused grain boundaries are common (Figure
18B) and result from pressure solution during deep burial. In addition, stylolites have
formed in some intervals due to extreme pressure solution (Figure 18F).
The petrographic analysis of cements, dissolution, and compaction in the Norphlet
has revealed many diagenetic phases, which have formed since initial sand deposition.
Refer to the paragenetic sequence presented in Figure 19 for the relative timing of the
major diagenetic phases. The diagenetic events in the Norphlet are explained below:
1) During early burial, calcite, anhydrite, and possibly halite filled the pore space to an
unknown amount in the sandstone. It is also probable these cements were at least
partially syndepositional. During later stages of diagenesis, these cements were
mostly dissolved away (McBride, 1981; Driese, 1985).
2) Chlorite cement occurred pervasively throughout the Norphlet Formation, and covers
most detrital grains with thick, fibrous coatings (Figure 18A).
3) Illite cement occurred in close association with chlorite precipitation, and covers the
chlorite with a mesh-like appearance (Figure 18B).
29
Figure 19. Paragenetic sequence of the diagenetic phases present in the Norphlet in Mobile Bay and
onshore wells. Black bars represent confident stages of paragenesis. A diagenetic phases occur within the
deep water Norphlet extension (Ajdukiewicz et al., 2010).
4) As burial continued, dissolution of detrital grains, particularly Volcanic Rock
Fragments (VRFs) resulted in the formation of secondary porosity (Figures 18A and
E).
5) Late-stage, silica-rich pore fluids permeated the Norphlet, resulting in the
precipitation of small amounts of authigenic quartz where chlorite cement was thin
(Figures 18C and D).
30
6) Although compaction was occurring throughout the burial history, major pressure
solution and stylolitization began to occur at greater depths during late stages of
diagenesis (Figure 18F).
Diagenetic phases within the Norphlet Sandstone have altered the porosity and
permeability in the many ways throughout the burial history. For example, porosity is
unusually well preserved for a sandstone that has been buried over four miles deep. The
reason for the preservation of the pore space is most likely due to the early precipitation
of cements that kept intergranular areas open (McBride, 1981). Evaporite minerals, such
as calcite, anhydrite, and halite, were the likely culprits that may have prevented
destruction of InterGranular Volume (IGV) in some areas of the reservoir (Paxton et al.,
2002). These cements were probably dissolved away after the sandstone had been deeply
buried.
Because chlorite is the most abundant cement, it is likely the reason for much of
the porosity reduction in the Norphlet (Dixon et al., 1989). However, it has prevented
quartz cement from forming and occluding the porosity. This remarkable relationship is
thought to be one of the primary reasons for the high effective porosity within the
Norphlet Formation (McBride, 1981). In general, diagenetic phases did not preferentially
affect specific facies within the Norphlet.
Reservoir Characteristics Summary
Analysis of the Norphlet sandstone revealed that depositional facies are the
primary control of porosity and permeability throughout the system. The Grainflow
Facies Association has the highest porosity and permeability, followed by the WindRipple and Wet Interdune Facies Associations. The Grainflow Facies Association is also
31
the most texturally mature, which parallels the fact that it has the highest porosity and
permeability of the Facies Associations. Diagenesis showed little preference towards
facies, and has caused decreased porosity and permeability in the entire Norphlet. Calcite
has prevented early compaction in this system during burial and was later dissolved
away. Chlorite has significantly preserved porosity occlusion from quartz overgrowth.
32
CHAPTER FOUR
Norphlet Depositional History
Developing an accurate composite depositional model for the Norphlet in the
deep-water Eastern Gulf of Mexico is difficult because very little is known about the
lateral extent of the Norphlet Formation in the deep-waters of the Gulf of Mexico.
Regional mapping of the lateral extent of the Norphlet Formation utilizing threedimensional seismic data will aid in the interpretation.
Additionally, grain composition within the Norphlet sandstone can be used to aid
in determination of provenance (Lovell, 2010). This provides information on the location
of the source(s) of the sediment and the distances that it traveled. Comparative analogs,
both modern and ancient, are also discussed to help understand the depositional setting of
the Norphlet.
Seismic Analysis
The seismic line displayed in Figure 20 includes two well locations for the study
area. The Norphlet interval identified in seismic covers roughly one seismic wave
making depositional facies identification at a seismic scale difficult due to low seismic
resolution.
The Norphlet has been fragmented into geographically isolated “pods” in this
region due to Louann Salt movement and formation of salt diapir structures (Figure 20).
This salt remobilization occurred post depositionally as a result of the accumulation of
thick packages of sediment in the Gulf of Mexico since the Late Jurassic, causing the
present day Louann salt to vary in thickness drastically (Figure 20). Variations in salt
33
Figure 20. A) Seismic profile through the Norphlet trend in the Eastern Gulf of Mexico. The top of the
Norphlet is shown in green. The line is in time-depth, and represents roughly 22,000 feet from the seafloor
to the base of the salt. The Louann salt (orange) forms a domed diapir beneath the Norphlet and Smackover
Formations. The distance between these two wells is about 15 miles (Chowdhury, 2009).
thickness are also syndepositional as rifting caused transitional continental crust to thin
and sink, allowing more accommodation space in some areas (Salvador, 1991).
Difference in accommodation space is at least part of the reason for variations in deepwater Norphlet locality thickness as the thicker deposits on the western side of the field
would have deposited in a topographic low (Kugler and Mink, 1999).
34
Additionally, differences in sediment preservation also contributed to the
thickness variations. The consistent thickness of the Smackover Formation and the sharp,
erosive nature of the contact between the underlying Norphlet Formation give evidence
for removal of Norphlet sediment during sea-level transgression, with erosion depending
on the basin configuration at the time of transgression. Dune morphology observed in
Mobile Bay (Figure 5; Story, 1998; Ajdukiewicz et al., 2010) is not visible in the
Norphlet of the deep-water Eastern Gulf of Mexico, which is due to the Smackover
transgression flattening field topography. Finally, thickness changes of the Norphlet are
can be tied to the erg-margin proximity, as aeolian deposits often thicken towards the ergaxis (Mountney, 2006b).
Core-Log Correlation
A number of bounding or truncation surfaces can be seen in the Norphlet cores,
which mark sharp changes in aeolian facies. The significance of aeolian bounding
surfaces can be determined from their lateral continuity (Brookfield, 1977). Determining
the stratigraphic significance and the spatial extent of these surfaces is important for
understanding reservoir continuity. Because dune sediment preservation is so low in
aeolian systems (usually 20-30%; Bagnold, 1941; Loope, 1985), the origin of aeolian
bounding surfaces has been a subject of considerable debate over the years. These
surfaces are best interpreted by examining modern dune fields and large, well exposed
ancient outcrops (McKee and Moiola, 1975) and are controlled by factors such as wind
regime, sediment supply, water-table influence, and climatic changes (Rubin, 1987;
Kocurek and Havholm, 1993). Early workers identified and established many different
types of aeolian bounding surfaces (Stokes, 1968; Brooksfield, 1977; Kocurek and Dott,
35
1981; Kocurek, 1988) including a set of hierarchical bounding surfaces ranging from
first- to fourth-order, each representing different amounts of geologic time. More current
aeolian bounding surface terminology, which describes the processes of surface
formation, is described by Fryberger et al. (1990, 1993), and Mountney (2006a). These
surfaces include reactivation surfaces formed from brief periods of wind direction
change (seasonality), stacking surfaces formed from dune sets migrating overtop one
another, stabilization surfaces formed from periods of aeolian deflation to the water table,
and supersurfaces, which represent a drastic change to non-aeolian deposits during wetter
conditions controlled by allocyclic changes (Fryberger, 1990, Mountney, 2006a; Figure
A1.3). Although all surfaces are likely to exist in the Norphlet, only stabilization
surfaces and supersurfaces are laterally continuous enough to correlate stratigraphically
across a regional or sub-regional area. Even these surfaces, however, are difficult to
interpret from core.
The Norphlet-Smackover transition in all wells marks a major change in climate
and associated deposition. This results in the development of a supersurface, which was
used as a datum for stratigraphic analysis. This surface is sharp, indicating significant
erosion of the Norphlet erg during Smackover sea-level transgression. Stratigraphically
equivalent cycles, bounded at the top and base by regionally significant stabilization
surfaces can be interpreted within the Norphlet. These surfaces were defined by a course
grained basal lag commonly followed by thick (10 ft) low-angle deposits (Wet Interdune
or Wind-Ripple Facies Associations) and succeeded by an upward-“drying” trend which
commonly fined upward. Drying upwards cycles were observed progressive changes in
accumulation each defined by successively thicker dune sets, increased Grainflow Facies
36
Association abundance, and decreased Wet Interdune and Wind Ripple Facies
Associations abundance. Cycles exhibited similar thicknesses across the field.
Stabilization surfaces mark periods of erg deflation down to the water table.
Surfaces with extensive, thick deposits of wind-ripple or wet interdune elements may
denote more prolonged periods of aeolian dampening caused by a high water table. It is
possible that these surfaces are not correlative or significant, considering sparse data
density and the inability to trace these surfaces out laterally. In addition, these surfaces
lack features such as soil development, plant roots and animal burrows that would
suggest prolonged deflation (Mountney, 2006b). Nonetheless, the repeating cyclic drying
nature of these sediments cannot be ignored. If, in fact, these surfaces are significant,
they will certainly act as reservoir baffles closer to the erg margins.
Sediment Provenance
As shown by the recent work of Taylor et al. (2010) and Ajdukiewicz et al. (2010)
Petrographic studies of the Norphlet sandstones reveal important characteristics about
detrital components. These detrital components define the source(s) of the Norphlet
sediments and provide information on the distance that the sediment may have traveled
before deposition. Quartz grains are mostly monocrystalline and nonundulose, Kfeldspar (orthoclase) is the dominant feldspar, and lithic fragments are dominantly
volcanic in origin. These detrital components are common in thin section throughout the
Norphlet interval (Figure 21). The Norphlet Sandstone from Mobile Bay and onshore
wells plot as arkosic sandstones (Figure 22).
37
Figure 21. Detrital constituents present in the Norphlet include Quartz (Qtz), K-feldspar (Kspar) and
Volcanic Rock Fragments (VRF). Constituents in this thin section are typical throughout the Norphlet
(Taylor et al., 2010)
Figure 22. Sandstone classification of all point-count data for Mobile Bay and Onshore Wells. Rock is
much more arkosic than the Norphlet in the Deep Water Eastern Gulf of Mexico (Ajdukiewicz et al., 2010).
Volcanic rock fragments (VRFs) are more abundant within the Norphlet
Formation than in most aeolian sandstones (Pye and Tsoar, 2009), which typically
contain much higher percentages of quartz. Lithic fragments are the best indicator of
sediment provenance. The abundance of these fragments indicates close proximity to the
38
source area, because lithic fragments readily breakdown during prolonged transport.
Well and seismic data to the east provided by Salvador (1991) and Dobson and Buffler
(1991) indicate that beneath Florida’s present day western gulf shelf there is a Jurassic
highland area, termed the Middle Ground Arch, that is composed of extrusive volcanic
basalts and andesites, which likely provided sediment during Norphlet deposition.
Depositional History
Work done by Mancini et al. (1985, 1990) at the Mobile Bay fields indicates the
lower part of the Norphlet represents a fluvial or distal alluvial fan environment, with a
number of ephemeral wadi elements. Although these accumulations are roughly 100
miles away, a similar facies assemblage containing wadi elements and lacking aeolian
facies may exist throughout most of the lower part of the Norphlet Erg System. Soil
development, muddy laminations and sandstones with mud intraclasts would all be
indications of a lower alluvial fan or fluvial overbank environment (Blakey et al., 1996;
Kocurek, 1981). The natural progression of aeolian deposition is interrupted and buried
by a sea-level transgression represented by the deposition of the shallow water
Smackover carbonates, likely scouring the upper Norphlet interval.
The Norphlet varies in thickness throughout the erg system and ranges in
thickness from a few feet onshore to 1200 feet in Mobile Bay. The reasons for the drastic
thickness difference are due to both the erosive Smackover transition, which would have
scoured the Norphlet differentially, and differences in accommodation space during time
of deposition. Aeolian systems are commonly deposited within a basin or topographic
low (Ahlbrandt and Fryberger, 1981a). Tectonic deformation from salt loading onto the
Louann probably had some syndepositional effects; however, structural influence from
39
salt tectonics largely did not affect the system until later Mesozoic and Cenozoic
sediment accumulation, adding an enormous load on top of the thick, ductile salt deposits
(Salvador, 1991).
Dune Morphology and Wind Regime
Many researchers have described modern dune morphologies (e.g., McKee, 1979;
Fryberger and Goudie, 1981; Lancaster, 1989; Mountney, 2006a). A complied list of
these morphologies can be found in Figure A1.4. Work by Albrandt and Fryberger
(1981a) and Fryberger and Dean (1979) studied aeolian dip-angle relationships to dune
morphologies and concluded that unidirectional dip azimuths signify a unidirectional
paleo-wind direction. A consistent dip direction in the dune facies of the Norphlet
indicates that the Norphlet was accumulating from dominantly southeasterly winds.
Unidirectional winds commonly form barchan, barchanoid, and transverse dune types,
depending on sediment supply, wind velocity, and seasonal variability (Albrandt and
Fryberger, 1981a; Rubin, 1987; Mountney, 2006a). This hierarchical progression of dune
forms with increasing sediment supply is interpreted to be the general setting of the
Norphlet system (Figure 23). Generally, the lower-angle grainflow sets represent isolated
barchan dunes, whereas the higher-angle dune sets represent migrating transverse or
barchanoid dune complexes (Fryberger and Dean, 1979).
Kocurek and Dott (1981) constructed a plot that relates stacked sets and grainflow
bed thickness to the original dune height (Figure 24). Although their results were
somewhat inconclusive and variable, the concept appears to be correct. This plot (Figure
24) was applied to Norphlet sandstones and indicates that the original dune height ranges
between 5 to10 meters for most of the dunes. Thicknesses vary quite a bit from set to set
40
C
B
2000 m
A
1000 m
50 m
Figure 23. Hypothetical interpretation of hierarchical progression in the Norphlet dune field from erg
margin to erg axis. Dune forms increase in size, amalgamate together, and interdunes decrease in
abundance. A) Barchan dunes (gomyclass.com/wind.html); B) Barchanoid dune ridges (Google Earth,
2010); C) Transverse dune ridges (Google Earth, 2010)
throughout the core, indicating a range in original dune heights. This was likely caused
by changes in sediment supply and seasonality (Rubin, 1987).
Norphlet Analogs
The Norphlet in Mobile Bay and onshore areas displays a northwest-southeast
trending “band” of aeolian deposits. This sand sea was bounded by highlands to the east
(Appalachian Mountains) and a restricted Jurassic sea to the west as the Gulf of Mexico
41
Figure 24. Maximum cross-strata thickness plotted against height of the dune slipface in the Little Sahara
Dune Field indicates that thicker grainflows match larger dune slipfaces (Kocurek and Dott, 1981).
was opening up (Mancini, 1985, 1990). One of the more important questions about the
Norphlet Formation in the deep-water Eastern Gulf of Mexico is whether it was
physically connected to the Norphlet onshore and in Mobile Bay. The scale of an aeolian
system is difficult to determine with limited well/core data and a lack of outcrops. The
Namib Desert along the southwestern coast of Africa provides a potential analog of a
modern aeolian system with similar geographic constraints. Given that this system has
continuous dunes for around 500 miles of north-south African coastline, the system is
identical in geographic length to the total known distribution of the Norphlet Formation.
Because this elongate band of Norphlet deposits is bounded to east by highlands and to
the west by the ancestral Gulf of Mexico, the depositional setting is also similar to the
Namib Desert. The extent of the Namib Desert and a detailed overlay of the
environments of deposition are illustrated in Figure 25.
42
Another way of gaining an understanding the Norphlet is by comparison with well
exposed ancient aeolian deposits. For example, the Cedar Mesa Formation in
southeastern Utah is a lower Permian succession of aeolian sediments that has very
similar stratification patterns and cross-bed thicknesses to the Norphlet Formation. The
dune bedforms in the Cedar Mesa have mostly unidirectional dips (Mountney, 2006b),
identical to those in the Norphlet. An outcrop view of the Cedar Mesa Sandstone depicts
these similarities in stratification, with stacking dune set thicknesses also comparable to
those observed in the Norphlet cores (Figure 26). The central erg sediments in the Cedar
Mesa Sandstone are mostly continuous dune deposits, with a few interdune environments
represented (Figure 27A). Dune sets are thinner at the erg margins and interfinger with
extradunal fluvial facies (Figure 27B; Mountney, 2006b). This proximal-to-distal faciestract relationship is probable within the Norphlet System. Finally, deposition of the
Cedar Mesa was punctuated by deflationary surfaces throughout its depositional history
(Loope 1985, Mountney, 2006b). These surfaces may be analogous to the stabilization
surfaces identified in the Norphlet, and tracing them may aid in understanding the lateral
scale of the system.
Depositional Model
The depositional model illustrated in Figure 28 depicts the Norphlet during later
stages of widespread aeolian deposition. The model represents a time of maximum
aeolian growth just before the Smackover seas transgressed over the erg and buried the
entire system.
43
Figure 25. The Namib Desert located along the coast of Namibia is comparable in scale to the Norphlet
aeolian system. Overlaying environments of deposition and the three wells give an idea for scale of these
aeolian systems (images modified from Google Earth, 2010).
44
Figure 26. Comparison of the Norphlet core with an outcrop of the Cedar Mesa. Bounding surfaces
(yellow lines), grainflow facies (GF), and wind-ripple facies (WR) are shown. A hypothetical core taken
from the Cedar Mesa between two bounding surfaces shows similarities can be seen between the Norphlet
and Cedar Mesa Sandstones in facies types, stratification patterns, cross-bed thicknesses, bedding
thicknesses, and dune set thicknesses. The difficulty of understanding these systems distributions without
outcrop exposure to trace out lateral and vertical changes in the system is apparent.
A
B
10 m
20 m
Figure 27. Cedar Mesa outcrops at two different localities A) Cedar Mesa erg axis (central erg) displaying
thick stacked dune sets with high net to gross sand proportions. B) Cedar Mesa erg margin displaying thin
dune sets with many interdune and interfingering fluvial intervals. The distance between these two Cedar
mesa outcrops is roughly 20 km, which is similar to the distance separating the Norphlet wells.
45
46
Figure 28. Schematic depositional model of the Norphlet in the deep water Eastern Gulf of Mexico in the Late Jurassic Oxfordian Stage. Highlands are in close
proximity with alluvial deposits shedding from them. Distal alluvial deposits interfinger with the aeolian system. The aeolian system is more widespread,
maturing upward and laterally towards the central erg.
CHAPTER FIVE
Conclusions
1) Seven aeolian depositional facies were identified within the Norphlet Formation,
which display a range of dune and interdune elements. These facies are combined
into four facies associations defined as Grainflow, Wind-Ripple, Wet Interdune, and
Reworked Aeolian. A variety of non-aeolian facies (fluvial/wadi) recognized by
previous workers (Mancini et al., 1985, 1986) occur in the lower Norphlet and may
reflect regional changes in the system to extradunal environments of deposition.
2) Grainflow facies displayed the highest porosity and permeability, wind-ripple facies
displayed moderate values, and wet interdune facies displayed the lowest values.
Porosity and permeability values show heterogeneity between and within each facies.
3) Textural changes in the Norphlet are facies-controlled. The Grainflow Facies
Association is texturally the most mature, the Wind-Ripple Facies Association has
bimodal grain-size distribution with moderate maturity, and the Wet Interdune Facies
Association is poorly sorted and relatively immature. The Reworked Aeolian Facies
Association is similar to grainflow in textural attributes.
4) Diagenetic constituents include calcite, anhydrite, quartz, chlorite clay, and illite clay,
all of which affect reservoir quality differently. Cementation showed no relationship
to facies. Early evaporates may have preserved intergranular volume (IGV). Chlorite
cement reduced porosity and permeability, but prevented quartz precipitation from
occluding pore space (Taylor et al., 2010; Ajdukiewicz et al., 2010).
47
5) Seismic analysis defined the full extent of the Norphlet Formation to deeper waters of
Eastern Gulf of Mexico. It is structurally fragmented into separate pods, primarily
due to salt tectonics, which resulted from sediment loading on the widespread and
thick, subjacent Louann Salt.
6) Detrital grains in the Norphlet contain a high abundance of VRFs, indicating
relatively close proximity to a volcanic source rock.
7) Aeolian deposits along the erg margin may interfinger with the fluvial extradunal
facies (and/or bury a Norphlet precursor) and eventually dominate the system during
later stages of Norphlet deposition. Aeolian deposition is eventually interrupted by
the Smackover sea-level rise, which buried and eroded the system severely.
8) Depositional reconstruction of the Norphlet in the Gulf of Mexico suggests that wind
direction was dominantly from the southeast throughout deposition. Based on
modern and ancient analogues, the system is forming mostly barchanoid and
transverse dune forms on the order of around 10 meters high. Determining the
Norphlet connectivity from onshore to Mobile Bay to deepwater would stretch the
dune field out nearly 500 miles, which is a similar scale to Namibia.
Future work on the Norphlet Formation in the deep-water Eastern Gulf of Mexico
should involve utilizing higher-resolution seismic data to better delineate the system’s
morphology on a regional scale. Petrophysical, core, and thin section analysis from
current and future wells will provide a better understanding of the architectural elements
within the dune environments and better constrain facies reservoir characteristics.
Finally, understanding the geographic extent of the Norphlet by determining 1) the
connectivity of the deep-water Norphlet to the deposits in the north, and 2) the Upper
48
Jurassic coastline to the Norphlet deposits (if one exists) would be invaluable to future
exploration on a more regional scale.
49
APPENDIX
50
APPENDIX
Supplemental Figures
Arid Environments Dune High‐
angle Aeolian Stratification Facies and Sedimentary Structures Grainfall Fine grained, thin, mm‐scale laminated Sandstone Fine‐medium grained, massive, cm‐scale planar tablular to trough cross‐strata, inverse and normal graded Sandstone Grainflow (Avalanche) Other Low‐
angle Interdune Dry Damp Adhesion Ripples Dessication Polygons/Mudcracks Wet ephemeral flood/wadi Fine‐medium grained, thin, mm‐scale pin stripe laminated Sandstone Fine‐medium grained, thin, mm‐ to cm‐scale rippleform laminated Sandstone poorly sorted, wavy‐laminated Argillaceous Sandstone mud‐silty laminations with infilled sand in cracks rippleform laminated, course grained, poorly sorted sandstone paleosol poorly to non‐stratified argillaceous siltstone and mudstone common rooting, burrowing, walking traces; Also Extradune lacustrine/pond dark laminated shales Also Extradune Carbonate carbonate cemented bedding Evap‐
oritic Sabkha minor rooting, burrowing, walking traces; Also Extradune Evaporites (playa) Extradune Sandsheet (zibar) Marine Reworked fluvial channel Defromation structures Wind‐Ripple Translatent Rippleform crinky to wavy laminated poorly sorted sandstone and evaporites bedded halite or anhydrite to interlaminated with argillaceous sandstone minor rooting, burrows, salt crusts Also Extradune deformed aeolian bedding overbank cravasse splay Horizontally bedded sandstone rippleform laminated, irrgularly deformed cross‐stratified sandstone normally graded ripple and/or cross‐bedded sandstone interbedded fine silts and muds with thin, sandy sheets/lobes sandy red beds hematite stained arkosic sandstones (oxidized) Upper (proximal) alluvial fan course conglomerates and debris flows Lower (distal) alluvial fan (plain) fine interbedded muds and silts common rooting, burrowing, walking traces soft sediment deformation Convolute, wavy beds or bedsets syndepositional slump features Figure A1.1. Facies summary table displaying a compiled list of facies found within or associated with
aeolian systems. Facies in table supported by McKee (1979), Ahlbrandt and Fryberger (1981a), Ahlbrandt
and Fryberger (1981b), Driese (1985), Rubin (1987), Fryberger et al. (1990), Kocurek (1991), and
Mountney (2006a).
51
Wind Direction
Figure A1.2. Diagram of modern processes aeolian dune migration. Wind direction forms wind ripples on
windward slope picking up sediment and depositing it as grainfall on the leeward slope. Over-steepening
of the leeward slope leads to Avalanching, or grainflow.
Figure A1.3. Various types of bounding surfaces defined by Mountney (2006a), including reactivation
surfaces, superimposition surfaces, interdune migration surfaces, and supersurfaces.
52
Figure A1.4. Illustrations of various modern dune types. Arrows indicate wind direction (Mountney,
2006a). Dune forms A, B, and C are common for unidirectional winds, such as those interpreted in the
Norphlet.
53
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