This document summarizes the depositional and diagenetic factors controlling reservoir quality in the Little Creek Field in Mississippi. The field contains point bar sandstone reservoirs in the Upper Cretaceous Tuscaloosa Formation deposited by meandering rivers. The primary reservoirs are the Q and Q2 sandstones which have average porosities of 24% and permeabilities of 100 md. Diagenetic chlorite coatings on grains impact petrophysical properties by reducing effective porosity but allowing production of oil at high water saturations. Depositional features like shale drapes and cemented zones create some permeability variability but overall the reservoirs have good quality and continuity controlled by depositional and diagenetic factors.

1. Name: Md. Shamsul Arefin
Designation: MS Student, Petroleum Geology (2017-18)
Institute: Department of Geology, University of Dhaka, Bangladesh
Depositional and Diagenetic Factors
Controlling Reservoir Quality of
Little Creek Field, Mississippi

2. LITTLE CREEK FIELD
Little Creek Field is located in Lincoln and Pike Counties in southwest Mississippi.
It is within the Upper Cretaceous Mid-Dip Tuscaloosa trend of southwestern
Mississippi and northeastern Louisiana.
Little Creek was one of the larger fields discovered in the trend with more
than 102 MMBO (1.6 X 107 m3) in place.
The depositional system consists of composite point bars and oil production
from a structurally and stratigraphically trapped Q and Q2 sandstone within the
Upper Cretaceous lower Tuscaloosa Formation.

3. Figure: Three producing trends in the lower tuscaloosa and location map of little
creek field within the mid- dip tuscaloosa trend.

4. FIELD AND RESERVOIR DEVELOPMENT
 Little Creek field was discovered in 1958 by Shell Oil Company.
 The producing horizon in the field is Lower Tuscaloosa Q-Q2 sandstones and
productive extent is about 6,200-acre.
 The average pay zone is at a depth of approximately 10,750 ft (10,350 ft subsea).
 The discovery well, Lemann No. 1 was drilled based on based on the
seismic interpretation of simple closure at the lower Tuscaloosa horizon at a
depth of 10,750 ft.
 Following discovery, development drilling on 40-acre spacing was rapid.
 A total of 208 wells were drilled with 162 being successful.

5. STRUCTURE
The structure is an elongate, unfaulted, north-south low-relief nose with
maximum dips on the flanks of 1 to 2°. The feature trends gently south
from Mallalieu Field and is about 14 miles long and 4 to 6 miles wide.
Little Creek and Sweetwater fields are located near the southern end of this
anticlinal nose.
The approximately 100-foot oil column at Little Creek is controlled by both
structural and stratigraphic closure.
The stratigraphic aspects of the trap result from the configuration of a
sinuous belt of point-bar sandstones.
The structural nose that formed later served to localize and increase closure.

6. Figure: Structure of the Lower Tuscaloosa Q sand stratigraphic marker and the outline of the
sandstone distribution. Little Creek and Sweetwater fields are structurally (gentle anticlinal nose) and
stratigraphically trapped.

7. STRATIGRAPHY
The Upper Cretaceous Tuscaloosa Formation unconformably overlies the Lower Cretaceous
Dantlzer Formation and the Washita/Fredricksburg Group. It is bounded above
unconformably by the Eutaw Formation.
The Tuscaloosa Formation is divided into the basal Lower Tuscaloosa Formation, the middle
Tuscaloosa Marine Shale, and the Upper Tuscaloosa Formation.
The focus of this study is the Lower Tuscaloosa Formation, which can be divided into the
Massive Sand and Stringer Sand.
During Moore’s (from John et al., 1997) analyses of approximately 50 wells drilled across the
region, he was able to determine that the Tuscaloosa Marine Shale was highly fractured and
that the fractures were interconnected leading him to conclude that the shale unit was the
source of the hydrocarbons migrating into the Lower Tuscaloosa Formation.

8. Figure: Regional stratigraphic column of southwest Mississippi (Rutherford, 1988)

9. RESERVOIR CHARACTERIZATION
 Depositional Setting
 Petrography and Diagenetic History
 Reservoir Quality and Petrophysics

10. DEPOSITIONAL SETTING
The depositional sequence in Little Creek Field consists of a
transgressive sequence of sediments grading from braided and
meandering fluvial deposits at the base, through the meandering Q-Q2
sandstones, to overlying littoral sediments that are primarily fossiliferous,
burrowed shales.
The Lower Tuscaloosa Q-Q2 sandstones are a series of point bars
deposited by highly sinuous meandering streams on an upper deltaic
plain.
Mudstones and claystones underlying and lateral to the Q-Q2 sandstones
are mottled red and green and are typical of floodbasin deposits; they
serve as excellent lateral seals.

11. Figure: Type log illustrating the lithology, depositional environment,
and log response of the transgressive Lower to Middle Tuscaloosa
interval. The point bars are the objectives at Little Creek Field.

12. LOWER TUSCALOOSA Q-Q2 SANDSTONES
 Q2 Sandstone: The Q2 sandstone represents the oldest meander-belt
system in Little Creek Field. The Q2 sandstone grades from a
narrow belt 1,300 feet wide in the north to a wider (approximately
2,500 feet) belt in the south. The Q2 sandstone limits are not as
well defined as those of the overlying Q sandstone.
 Q Sandstone: The Q sandstone is thick (40 feet), laterally
continuous (2,600 to 8,000 feet), and widespread. Its limits are well
defined and the top of the Q sandstone shales out as the channel
edge is approached.
The Q sandstone contained an estimated 87% of the in-place
hydrocarbons.
The maximum net thickness of the Q sandstone is 55 feet while it is 30 feet
for the Q2 sandstone.
The average net thickness of the Lower Tuscaloosa Q and Q2 sandstones is
40 feet.

13. Figure: The objective sandstones in Little Creek Field consist of the Q2
and Q. These type logs illustrate SP and resistivity responses of the
three characteristic log types penetrated.

14. Figure: Q sandstone net pay isopach map contoured in feet. The Q sandstone may be
grossly divided into three main pay areas (three point bars) connected by narrower
zones of channel sandstones.

15. MAJOR SEDIMENTARY FEATURES
 Point-bar deposits: erosional bases with channel lags, large scale
cross-beds grading up through horizontal and small-scale ripple cross-
laminae, clay drapes, local mud balls and intraclasts, micaceous and
carbonaceous streaks, and some calcareous patches and discontinuous
claystone layers.
 Abandoned channel fill: includes interbedded very fine-grained
sandstone, siltstone, and mudstone, small-scale cross-stratification, flow
structures, microfaulting, clay intraclasts, and carbonaceous plant
remains.
 Floodbasin deposits: primarily contains mottled red, green, and
brown floodbasin mudstone.

16. Figure: The Q sandstone has characteristic point-bar features induding large-scale cross-stratification, isolated shale
laminae, small-scale ripple cross-stratification, and abandonment facies.

17. Figure: Intraformational shale rip-up clasts above an erosional surface in the Q sandstone. The sequence grades
through parallel-laminated and large- to small-scale cross stratified sandstone. The Soloman Atkinson No. 1 well ,
10,783.5-10,799.5 feet. The bottom of the cored interval is at the lower right. The scale bar is 1 foot.

18. Figure: Large-scale cross-stratified and parallel laminated sandstone passing upward to ripple cross laminated
sandstone in the Q sandstone. The Soloman E.G. Werren et al. Atkinson No. 1 well, 10,770.5-10,783.5 feet. The bottom
of the cored interval is at the lower right. The scale bar is 1 foot.

19. Figure: Idealized depositional model of the large meander system that deposited the Q point-bar
sands.
 The Q sandstone in Little Creek Field is made up of three large point-bar accretion
loops formed by a river flowing generally from the northwest to the south.
 Sweetwater Field is another possible upstream, abandoned meander loop of this river
system and consists of point-bar and oxbow-lake deposits.

20. PETROGRAPHY AND DIAGENETIC
HISTORY
The sandstones are very fine- to medium-grained, moderately to well-sorted,
quartz rich sublitharenites.
 Quartz and igneous rock fragments are the most abundant framework
minerals, but chert, metamorphic rock fragments, lignite, mica, and pyrite
are also present.
 Clays are present as detrital and authigenic types. Detrital clays occur as
clay drapes and as matrix( in the upper parts of upward fining channel
abandonment sequences). The authigenic clays include pore-lining chlorite
and minor kaolinite.
 Minor cements, which are locally important to reservoir quality, include
ferroan dolomite, quartz overgrowths, and siderite.

21. Figure: Most framework grains in this photo are quartz (q),
but partially dissolved and relatively fresh volcanic rock
fragments (rf) are present. Also shown is a small patch of
pore-filling ferroan dolomite (fd), and most of the grains have
a 5 to 10 micron coating of chlorite. This sample has good
macroporosity (p) as well as abundant microporosity within
the clays. Scale bar is 0.50 mm.
Figure: A generalized diagenetic sequence
for the Lower Tuscaloosa Formation in
Little Creek Field.

22. CHLORITE
Chlorite occurs throughout the reservoir sandstones and is considered most important in determining the
reservoir quality and petrophysical properties.
The chlorite occurs as a uniform "druse" of grain-coating hexagonal plates except in areas where abundant
quartz overgrowths occur.
Chlorite may be important in inhibiting quartz cementation and in partially preventing compaction.
The chlorite is also the most important compositional component in terms of its effects on the petrophysical
properties. Extremely low resistivities (<1 ohm-m) and average high water saturations (> 55%) are related to
the bound water within the microporous chlorite. It is therefore possible to produce clean oil even with these
extremely high water saturations.
Figure: Scanning electron photomicrograph of graincoating chlorite.

23. RESERVOIR QUALITY AND PETROPHYSICS
Reservoir quality is generally good. The
Q and Q2 sandstones have average
porosities and permeabilities of 24% and
100 md, respectively.
Most of the porosity is primary
intergranular but minor secondary
dissolution porosity is also present.
However, as much as 50% of the
measured porosity may be microporosity
associated with the pervasive grain-
coating chlorite, thus reducing the
effective porosity.
Local barriers to vertical flow (clay
drapes and cemented zones) as well as
the grain size changes lead to a slightly
higher horizontal versus vertical
permeability. This occures in a very
limited extent.
Figure: Plot of porosity versus permeability for
Lower Tuscaloosa sandstones. The large range
of values (unstressed) is primarily related to the
abundance of clays and to grain-size variations.

24.  The distribution of typical SP shapes
for Q-Q2 sandstones
Blocky toward the center of the channel
Bell-shaped toward the edge of the
channel
More serrate near the channel edge
Permeability profiles vary from location
to location
Fairly uniform permeabilities in blocky
curves
Decreased permeability near the top in
bell-shaped curves
Figure: Representative SP curve shapes are
helpful but not unique in trying to delineate
the sandstone boundary.

25. CONCLUSION
The Lower Tuscaloosa Q-Q2 sandstones are primarily point bars deposited in sinuous fluvial
meanders on a deltaic plain.
Reservoir quality and continuity is generally high and is controlled by both depositional and
diagenetic processes. Recovery efficiencies are quite high in this reservoir.
 Depositional controls include slightly reduced vertical to horizontal permeability, local shale drapes,
and channel abandonment facies.
 Diagenetic controls include compaction, chlorite grain coatings, and quartz and ankerite cements.
The chlorite is most important in controlling the observed petrophysical properties of the sandstone
including the water saturation, porosity, and bimodal pore system.
Clean-oil production from sandstones with high water saturations is possible.
Meander-belt systems, like Little Creek Field, provide reservoirs that are inherently difficult to
delineate but are generally high-quality sandstones with "built-in" top and side seals. Production
from these reservoirs requires that close attention be paid to permeability stratification and
potential shale baffles and barriers within the point-bar sands.

26. REFERENCES
 Werren E.G., Shew R.D., Adams E.R., Stancliffe R.J. (1990) Meander-Belt Reservoir
Geology, Mid-Dip Tuscaloosa, Little Creek Field, Mississippi. In: Barwis J.H., McPherson
J.G., Studlick J.R.J. (eds) Sandstone Petroleum Reservoirs. Casebooks in Earth Sciences.
Springer, New York, NY
 Shew, R. D., Werren, E. G., Adams, E. R., & Stancliffe, R. J. (1989, January 1). Depositional,
Diagenetic, Production, and Seismic Characteristics of a Mid-Dip Tuscaloosa Point Bar
Complex, Little Creek Field, Mississippi. Society of Petroleum Engineers.
 Warren, Alexandra (2018) Correlation of Sand Reservoirs of the Lower Tuscaloosa
Formation in the Smithdale and East Fork Fields in Amite County, Mississippi.
Undergraduate thesis, under the direction of Greg Easson from Geology and Geological
Engineering, University of Mississippi.