Oil was first discovered in
the Jurassic Smackover when a "deep" test was drilled in the shallow
Smackover Field in Union County, Arkansas in 1937. With that notable
discovery, the Smackover became one of the most sought-after Jurassic
objectives in the Gulf Coast. The Smackover Formation does not outcrop and is only encountered in subsurface
penetrations located in the U.S. Gulf Coast area. Within that
region, the Smackover is a carbonate facies that grades laterally
into an evaporitic facies in certain areas of the Mississippi Interior
Salt Basin.
For example, in Clarke County, Alabama, the upper 150 to 200
feet of the Smackover grades laterally (and abruptly) into a series of
anhydrite and halite beds - the result of a localized shallow-water
restrictive lagoonal area bounded on the west by a subtle paleoridge that
stimulated the growth and persistence of a linear algal barrier reef.
Further west, on the other side of this intriguing barrier, localized
sabkha facies grade into more open-water, oolitic grainstones in central
Choctaw County, Alabama. Proximity of these upper Smackover grainstones to
magnesium-rich brines associated with lateral and overlying Buckner
anhydrites led to extensive dolomitization of the oolitic, peloidal, and
oncolitic grainstones. The resulting oomoldic and pelmoldic dolomite is
exceptionally porous but uniformly low in permeability. The low
permeability results from the following diagenetic mechanism, focusing on
oomoldic reservoirs only in this simplified example: (1) the aragonitic
ooids are deposited; (2) calcite cement precipitates within, and
effectively occludes, the pore space between the ooids; (3) the aragonitic
ooids are then "preferentially" leached out, leaving void spaces ("molds")
where the spherical ooids once were, encapsulated in the very calcite
cement that had earlier occupied all interstitial pore space - effectively
leaving only the calcite cement matrix; finally, (4) the remaining calcite
cement is extensively dolomitized, creating an incredibly fine matrix of
oomoldic porosity. The presence of the large ooid molds creates
significantly high porosity (depending of course upon the size of the
original ooids), however, the dolomitization of the once impermeable
calcite cement creates what this geologist refers to as "fractal"
permeability - micropermeability so uniformly fine as to create a
latticework that, while "poor" by normal standards, is well-suited to
the production of gas
and high-gravity (i.e., volatile) oil . However, such micropermeability
is not conducive for the commercial production of highly viscous (lower
gravity) crudes, because the increased capillary pressure and small pore
size severely limits the producibility of such liquids. (Please
see our log example at the end of this discussion.)
Significant opportunities
still exist for the discovery and development of oomoldic gas-condensate
reservoirs within the Alabama/Mississippi Smackover Trend because many
explorers are unfamiliar with such reservoirs, their characteristics, and
their hidden potential.
Algal patch reefs are also
observed within the Upper Smackover, especially in southwest Alabama. The algal facies
within both the Upper and Lower Smackover is especially noted for good
porosity and very good permeability when dolomitized, because
dolomitization of the algal facies results in complete destruction of the
original algal fabric, accentuation of the fairly common vugular porosity, and
the formation of exceptionally large saddle dolomite rhombs that are
striking in appearance and accompanied by good inter-rhombic permeability.
A dolomitized algal facies is also common within the Lower Smackover Brown
Dense Limestone in Mobile and Escambia Counties, Alabama, and Escambia
County, Florida, just across the state line; this facies is a prolific
gas-condensate producer in several large Smackover fields in that area.
Because of their position at
the top of the unit, and the enhanced dolomitization that accompanies
proximity to the overlying Buckner Anhydrite (an obviously excellent top
seal), the Upper Smackover grainstones represent the most prolific
and widely distributed reservoirs within the Smackover of the
Gulf Coast. For example, it appears that the Upper Smackover Oil Trend of
central Choctaw County, Alabama, ranks at or near the top of Gulf Coast
oil trends with regard to
per-well production statistics, with the average oil well having produced over
900,000 barrels from a depth interval of between 11,000 and 12,000
feet. An excellent example of the prolific nature of the Choctaw County
Trend is the 400-acre North Choctaw Ridge Field, which has produced
over 8.7 million barrels of oil - 6 million barrels having been produced
from two 3 million-barrel wells atop this diminutive structure.
Elsewhere in the Interior
Salt Basins of Mississippi and North Louisiana, it is clear that several Jurassic rivers - most notably the
ancestral Mississippi River - contributed large volumes of quartz sand to
the Smackover during deposition. A series of fan deltas have been identified within the
Smackover, of varying geometries and thicknesses. The prolific but highly
overpressured Deep Smackover Gas Trend of west-central Mississippi has
produced almost a trillion cubic feet of gas from one such delta complex;
this trend includes that state's most prolific gas well, which has
produced over 139 billion cubic feet of gas
over two decades - and is still producing at significant
commercial rates. The Smackover C Sand reservoir of north Louisiana is
another prolific sandstone "play". Vision believes that considerable
potential still exists for new, sizable discoveries within the overall
Smackover Sand Trend, and is currently pursuing a large prospect within
its perimeter.
As with all carbonate trends,
reservoir trapping mechanisms within the Smackover vary greatly and range
from the simplest four-way (anticlinal) closure to the subtlest
stratigraphic entrapment of hydrocarbons on a monoclinal ramp with no
apparent structural component. Where present, the overlying Buckner
Anhydrite or Haynesville shales and evaporites provide excellent barriers
to vertical hydrocarbon migration. Fault juxtaposition of the Smackover
against salt - either LouAnn or Haynesville/Buckner - is a proven trapping
mechanism and a popular wildcat target for explorers. The juxtaposition
against the older Early Jurassic LouAnn Salt occurs when the Smackover
slides down a fault/salt interface on the steep flank of a salt wall or
anticline, creating a three-way downthrown fault closure; an example of
this trapping style would be the Chaparral Field in Wayne County,
Mississippi. Conversely, the juxtaposition of younger Haynesville/Buckner
evaporites occurs when those beds are dropped down against the Smackover
reservoir, creating a three-way upthrown fault closure; Buccatunna
Creek Field in Choctaw County, Alabama is representative of this
trapping style. Lateral facies changes combined with a structural
component often create enormous Smackover traps, including the giant
Jay - Big Escambia Creek Field Complex that straddles the Alabama -
Florida state lines. Patch reef Smackover traps represent another example
of a combination structural / stratigraphic trapping mechanism, with
development and encasement of the reefal porosity having occurred atop
topographically high paleo-structures in an otherwise non-porous (deeper
water) area.
A considerable portion of the
deeper, gas-prone trend in southeast Mississippi has recently been imaged
with 3D seismic data, resulting in a succession of several new
discoveries. This part of the trend (also known as the Wayne County
Trend), typified by high-relief salt structures, had escaped earlier
development because of the associated steep dips, deep depths, and
structural complexity. Of note is the recent North Clara Field
discovery well, the Twister #11-A Clay, which is currently producing 700
to 800 barrels of oil per day plus 3 to 4 million cubic feet of gas per
day with high flowing tubing pressure after a year of production.
The lower unit of the
Smackover is aptly named the Brown Dense Limestone. It represents
the most important source rock within the Mississippi Interior Salt Basin.
This limestone is an organic-rich mudstone that was deposited as the
ancestral Smackover sea transgressed across and inundated the
post-Norphlet Gulf of Mexico
geosyncline. Typically 200 - 300 feet thick, this brittle styolitic limestone
is remarkably uniform, with only rare developments of porosity such as
that noted above in extreme south Alabama. Fracturing of the Brown Dense
Limestone is also common, manifested by oil and gas shows and
enhanced by structural deformation and/or proximity to faulting. The Brown
Dense Limestone has
been proven to be a prolific generator of oil and gas, contributing large
volumes to both the Upper Smackover porosity and the underlying Norphlet
Sandstone (once the Smackover porosity has been totally filled). The
remaining oil and gas expelled from the Brown Dense Limestone found its
way into the majority of Jurassic and Cretaceous reservoirs distributed throughout the
Gulf Coast.
The Smackover thickens into
the center of the Mississippi Interior Salt Basin, but many geologists
mistake the thick Jurassic carbonates of that area to consist entirely of
"Smackover" Limestone when the carbonate section actually consists of a
fairly uniform Smackover equivalent section that rarely exceeds 500 feet,
topped by an additional Haynesville/Buckner carbonate section that can
exceed 1000 feet in total thickness. It is interesting to note that in
western Alabama, the total isopachous thickness of the combined Top
Haynesville - Top Norphlet section is approximately 2,000 feet, consisting
of approximately 500 feet of Smackover carbonate and 1500 feet of
Haynesville/Buckner evaporites; but, moving west into the western Jasper
County area of Mississippi, the same combined Top Haynesville - Top
Norphlet section is still approximately 2,000 feet, but now appears
to consist of
approximately 1500 feet of "Smackover" carbonate and 500 feet of
Haynesville/Buckner evaporites. In actuality, lateral facies changes have
simply caused the lower 1000 feet of Haynesville/Buckner evaporites to
transition into
1000 feet of limestone and dolomite that is indistinguishable (to most
geologists) from the underlying, "true" Smackover carbonate. Where this becomes
important is the observation that the critical structural movement (timing)
"window" for Smackover trapping has been well documented in Alabama to have
occurred during early Haynesville / late Buckner time - i.e., relatively
soon after Upper Smackover
deposition. In other words, critical structural timing and hydrocarbon
entrapment is closely associated with an isopachous interval approximately
600 - 800 feet above the base of the Smackover (i.e., top of the
Norphlet). Conversely, later structural growth, located some "distance"
(stratigraphic time) above that critical early Haynesville / late Buckner
migration "window", has been proven in most circumstances to have occurred
too late to have trapped
hydrocarbons in the underlying Smackover reservoir. It is clear,
therefore, that this observation becomes critical as one moves west from
Alabama into
Mississippi. Recall that the apparent top of the "Smackover" there appears
to be 1,500 feet above the base of the Smackover (when actually, this
is the top of a Haynesville carbonate that is laterally an evaporite
facies in
Alabama). Structural growth at that isopachous level - 1,500 feet
above the base of the Smackover - might be observed and appear to
be favorable for hydrocarbon entrapment in the Smackover, but - as
demonstrated definitively across the state line in Alabama - the growth
history actually occurred too late for favorable entrapment of hydrocarbons in the real
Smackover "equivalent", fully 1,000 feet below the so-called "top Smackover". Fields in Jasper County such as
Vossburg and
Lake Utopia bear testimony to this observation: most of the oil and gas
produced from these fields comes from the "Lower Smackover" (sic), while
relatively little production has come from the "Upper Smackover", much
higher in the section - and which actually represents the local
(Mississippi) equivalent of its mid-Haynesville evaporitic facies in
Alabama. Since the Haynesville carbonates rarely produce commercial
quantities of oil or gas, it is important to recognize just what the local
carbonate facies really represents - and to risk one's exploration targets
accordingly.
Vision Exploration cites this
observation as an example of how experience and familiarity with a given
trend - in this case, Vision's 20-year record of experience in Smackover
exploration - can aid newcomers to the area and help one to avoid the
geologic pitfalls that are not readily apparent upon first inspection of a
prospect or proposed drilling venture.
Considerable potential
remains for the discovery and development of significant Smackover oil and
gas fields across the Mississippi / Alabama / Louisiana area. For example, the
Grayson Field,
discovered in the early 1990's near Magnolia, Arkansas, is a rather small
Smackover structure that is projected to produce in excess of 20 million
barrels of oil; however, the Grayson Field structure - a simple anticlinal
closure, with a stratigraphic porosity pinchout on its west flank -
escaped detection for decades after the surrounding area had been heavily
explored, developed, and written off as having no future for any
additional significant oil or gas discoveries.
Many other Grayson Field-type
accumulations remain to be discovered and developed, throughout the entire
onshore Smackover Trend. Vision Exploration
has targeted the prolific Smackover carbonate with several high-potential
prospects that comprise an important part of its Program inventory.
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