About This Item
- Full text of this item is not available.
- Abstract PDFAbstract PDF(no subscription required)
Share This Item
The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: A Microbial Smackover Formation and the
Dual Reservoir-Seal System at the Little Cedar Creek
Field in Conecuh County, Alabama
By
1 Department of Physics, Jackson State University, Jackson, MS
2 Jura-Search Inc., Jackson, MS
Little Cedar Creek Field appears to be the largest Smackover field discovered in the Northern Gulf Coast Province in the last three decades. At this time, the field demonstrates an oil column of at least 700 feet and extends along strike over 7 miles. Development drilling is ongoing.
From nearly 20 conventional cores taken throughout the field, it is apparent that two separate reservoirs exist in the Smackover: an upper ooid/peloid grainstone shoal facies and a lower microbiolite/thrombolite bindstone reef facies. These two reservoirs are underlain, separated and overlain by tight mid-ramp, lagoonal and tidal flat limestones, respectively.
The lower, reefal reservoir is only partially dolomitized and characterized by porosities of 6% to 25% with permeabilities in excess of 1 Darcy. The thickness of this lower reservoir varies from 2 to 50 feet. The upper ooid shoal reservoir varies from 0 to 30 feet in thickness and is somewhat tighter, less permeable and only partially dolomitized. Each reservoir possesses its own distinct oil/water level and each reservoir pinches out in an updip direction.
Unlike virtually all other Smackover fields in the Eastern Gulf, Little Cedar Creek Field does not possess a Buckner anhydrite top seal immediately overlying the Smackover reservoir. Furthermore, Little Cedar Creek Field is also unique because both of its reservoirs are composed predominately of limestone, not dolomite, as is the case in most Smackover fields in the region.
The Smackover Formation is only 80 to 100 feet (24 to 30 m)
thick and consists of seven distinct lithofacies at Little Cedar
Creek Field. From the base to the top, the following lithofacies are
recognized: (1) a laminated peloid wackestone (mid-ramp)
which overlies the red conglomerates (alluvial fan) of the
Norphlet Formation with a sharp contact; (2) a bioturbated,
peloid packstone (mid-ramp); (3) a microbial bindstone (inner
ramp); (4) a laminated peloid wackestone-packstone (inner
ramp); (5) a bioturbated peloid packstone (lagoonal); and
(6) a peloid-ooid grainstone (beach). These
nearly pure carbonate lithofacies are overlain
by a mixed regime of lime mudstones,
red and green shales, sandstones and
conglomerates (lithofacies 7) interpreted as
Smackover mixed carbonate and siliciclastic
tidal flat deposits. The sequence of lithofacies
and their respective depositional environments
indicate a shoaling-upward cycle
that formed by southward progradation
following the rapid transgression of the
Smackover sea. Virtually every lithofacies of the Smackover
Formation exhibits microbial features, making the entire
thickness of the formation microbial in origin at this location.
Such a situation was probably caused by harsh environmental
conditions imposed by the geometry
of the embayment and the
ramp, low-energy conditions and poor seawater circulation.
The microbial bindstone and ooid grainstone lithofacies are highly porous and permeable, forming two distinct reservoirs at the Little Cedar Creek Field. The microbial bindstone reservoir consists primarily of pellets and peloids bound by microbially and abiotically precipitated cements. Framework and intergranular pores generate porosities of 6% to 25% and permeabilities as high as 1.5 Darcies. The microbial reef reservoir is overlain by the nonporous and nonpermeable bioturbated peloid packstone lithofacies (5 to 20 feet) forming the seal over this reservoir. The cause of the preservation of porosity in the microbial bindstone was marine cementation that prevented extensive burial compaction.
End_Page 45---------------
The ooid grainstone reservoir is cross-laminated and has intergranular, moldic, vuggy and intercrystalline porosity types. The abundance of microbially coated grains and composite particles suggests a low-energy beach where microbial activities were an integral part of the environment. The ooid grainstone reservoir grades upward into nonporous and nonpermeable wackestone and packstone facies, and eventually to green and red shale and sandstone layers. The reason for the preservation of porosity in the ooid grainstone reservoir was early meteoric diagenesis, which produced moldic and intercrystalline pore spaces.
The Little Cedar Creek Field was discovered in 1994 when Hunt
Oil Company drilled the #1 Cedar Creek Land
& Timber 30-1.
The Smackover Formation was perforated at a depth of
11,870–11,883 feet and tested at the rate of 108 barrels of oil per
day. The original bottom hole pressure was 4300 psi, producing
46 degree API gravity oil. The Hunt well produced for several
years, flowing at an average rate of 43 BOPD.
Midroc Operating Company offset the Hunt discovery in 2001
with the drilling of the #1 Cedar Creek Land
& Timber 19-15.
The Midroc well was completed at a rate of 250 BOPD from the
same stratigraphic interval in the upper Smackover Formation.
Since that time Midroc Operating Company has drilled 22
additional successful wells in an east-northeast direction from the
original discovery. The average completion on the last 22 wells is
270 BOPD and roughly 250 MCFPD.
Such an important and unique discovery prompted us to conduct a comprehensive study of the Smackover at Little Cedar Creek Field to evaluate the conditions that led to the formation of such a major dual-reservoir system. The purposes of this investigation are the following: (1) to provide a detailed description of lithofacies of the Smackover Formation in the field, with particular attention given to the reservoir and seal lithofacies characteristics; (2) to interpret the depositional environments of the Smackover Formation in order to decipher conditions that led to deposition of this dual reservoir setting; and (3) to speculate on future exploration strategies for similar Smackover reservoirs.
End_of_Record - Last_Page 47---------------