ABSTRACT: Facies and Reservoir Characterization of an Upper Smackover Interval, East Barnett Field, Conecuh County, Alabama
Gail R. Bergan, James H. Hearne
Excellent production from an upper Smackover (Jurassic) ooid grainstone was established in April 1988 by Coastal Oil and Gas Corporation with the discovery of the East Barnett field in Conecuh County, Alabama. The discovery well, the Grisset 36-16 No. 1 in Section 36, of Township 4 North, Range 7 East, was completed flowing at a rate of 1020 BOPD and 1762 MCFGPD on a 15/64 in. choke with a flowing tubing pressure of 2200 psi. A structure map on the top of the Smackover Formation and net porosity isopach map of the producing intervals show that the trapping mechanism at the field has both structural and stratigraphic components.
Two diamond cores were cut from 13,580 to 13,701 ft, beginning approximately 20 ft below the top of the Smackover. This interval has been extensively dolomitized and contains several of the better reservoir quality facies common to the Smackover.
Two shallowing-upward sequences are identified in the cores. The first sequence starts at the base of the cored interval and is characterized by thick, subtidal algal boundstones capped by a collapse breccia facies. This entire sequence was deposited in the shallow subtidal to lower intertidal zone. Subsequent lowering of sea level exposed the top portion of the boundstones to meteoric or mixing zone waters, creating the diagenetic, collapse breccia facies. The anhydrite associated with the breccia also indicates surface exposure. The second sequence begins with algal boundstones that sharply overlie the collapse breccia facies of the previous sequence. These boundstones grade upward into high-energy, cross-bedded ooid beach (?) and oncoidal, peloidal beach shoreface deposits. Proximi y of the overlying Buckner anhydrite, representing a probable sabkha system, favors a beach or a very nearshore shoal interpretation for the ooid grainstones.
The ooid grainstone facies, which is the primary producing interval, has measured porosity values ranging from 5.3% to 17.8% and averaging 11.0%. Measured permeability values range from 0.04 md to 701 md
and average 161.63 md. These high porosity and permeability values result from abundant primary intergranular pore space, as well as secondary pore space created by dolomitization and dissolution of framework grains.
The algal boundstone facies has sporadic reservoir quality, with porosity values ranging from 1.0% to 21.9% and averaging 6.0%. Permeability ranges from <0.01 md to 3731 md and averages 64.6 md. The highest porosity and permeability values occur in zones where localized dissolution of trapped allochemical grains or solution-enhancement of preexisting fenestral pores has created additional porosity. Most of the primary pore space is tightly cemented by baroque dolomite or anhydrite. The remaining facies in the core have local porous and permeable streaks created by secondary dissolution, but are volumetrically insignificant in the total rock volume.
Depositional and diagenetic studies such as these add to our regional knowledge of the Smackover Formation and help us better understand (and hopefully predict!) the rapid facies changes that occur in the upper Smackover in southern Alabama.
AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990