Page 252 - Geology of Carbonate Reservoirs

DIAGENETIC RESERVOIRS 233

an average depth of 4900 feet and is interpreted to have been deposited originally
as a localized buildup that formed on a structural terrace along the eastern margin
of the Midland Basin. Work underway at this writing may change the initial inter-
pretation because evidence has been found that the entire carbonate section in
Happy Field may have been displaced downslope after having broken away from a
shallow, shelf - edge terrace.
Originally known locally as Happy Spraberry Field, it was discovered in 1988 by
Bennett Petroleum during recompletion of an existing well. It is referred to here as
Happy Field because it produces from shallow - water carbonates interpreted to be
in the Permian, lower Clear Fork Formation, not from deep - water Spraberry sand-
stones that occur in the center of the Midland Basin. After the initial discovery,

Bennett Petroleum ran 3D seismic surveys to further define the Happy Field struc-
ture. Encouraging results led Bennett and their partners, Torch Energy, to drill six
new wells. Torch Operating Company acquired production interests and nine more

wells were drilled by 1992 as part of a waterflood that achieved moderate success

in the northern part of the field. The small increase in oil and water production after
waterflooding was less than the volume of water injected and the reservoir pressures

remained virtually unchanged, suggesting that fl ow units were not in good commu-
nication and that potentially significant amounts of oil may have been bypassed.

Original oil in place was estimated to be 17.2 MMBO and cumulative production

through 1996 was 2.4 MMBO and 4.5 MMCFG.

Structural Setting Happy Field is located on the Eastern Shelf of the Midland
Basin. Recent seismic studies by independent operators indicate that the field is a

stratigraphic trap located on a paleotopographic terrace that, during lower stands
of sea level, was the site of shallow subtidal carbonate sedimentation. However,
ongoing work suggests that slope failure may have allowed the entire carbonate
section at Happy Field to slump basinward, moving it to a deeper water, downslope
environment. The shapes of present structural contours and interval isopach con-
tours of reservoir thickness are strongly similar, indicating that the present - day

structure in the field represents depositional relief on the top of the Happy carbon-
ate buildup, not the result of postdepositional tectonic deformation.
Borehole cores reveal that the reservoir consists of two low relief, NE – SW trend-
ing pod - like accumulations of oolitic and peloidal grainstones and packstones with
lesser amounts of bindstones, rudstones, and floatstones. Minor tilting of the basin

margin has not added to the amount of structural closure on the reservoir. The
northern pod is the larger of the two; it has structural closure of about 70 feet on
its south side. The southern pod has about 50 feet of structural closure to the west.
There is no evidence of faulting on wireline log correlation sections. The reservoir
pods are separated by an interval isopach thin that may be a submarine channel.

Depositional and Diagenetic Characteristics Happy Field carbonates consist of
oolitic and peloidal grainstones to packstones, reef rudstones and fl oatstones, and,
rarely, Tubiphytes bindstones, some of which are in growth position. Bindstones and
other reef - like facies were originally deposited near the base of a shallowing - upward
succession that ended with deposition of crossbedded, oolite grainstones and sub-
ordinate packstones. The reservoir is developed mainly in the packstone – rudstone
trend where grain - moldic, solution - enhanced intergranular and vuggy porosities