Gas Field Redevelopment
A NEW TREND DIES ON THE VINE AFTER ONLY ONE COMMERCIAL DISCOVERY
The existence of shallow gas reservoirs in the Miocene and Oligocene sands of southwest Mississippi and the Florida Parishes of Louisiana was first revealed on modern seismic data in the mid-1980's, when large grids of new 2D seismic data acquired for the Lower Tuscaloosa Mid-Dip Stratigraphic Play - in southwest Mississippi and the Florida Parishes of Louisiana - illuminated dozens of probable "bright spots" and velocity anomalies in the shallow Tertiary section. At first, the exploitation of these shallow gas sands was limited to those areas located within a mile or two of existing gas pipelines, for economic reasons. As pipeline infrastructure slowly expanded throughout the region in the next 20 years, more and more of the shallow seismic anomalies were targeted - especially in Amite and Wilkinson Counties in Mississippi and West Feliciana Parish in Louisiana. These shallow gas sands were predominantly Frio in age, but some Miocene reservoirs were also discovered and developed. Results were mixed, with many gas accumulations being rather small in volume and deliverability. Because only two significantly large (>5 BCF) fields were discovered in the first 15 years of exploration - with the largest field establishing only 15 BCF of recoverable reserves - interest was lost in the Wilkinson / Amite / West Feliciana Parish Trend, and that play has been fairly dormant in the last five years.
However, in 2000 an intriguing discovery occurred far from the established trend as described above. A Mississippi geologist, intrigued with the potential for shallow gas production in localized patch reef build-ups of the Heterostegina ("Het") Limestone, utilized existing 2D seismic data to identify what appeared to be a patch reef in the Het Lime on the west flank of Waveland Field, a Lower Cretaceous (predominantly Mooringsport) gas field in Hancock County, Mississippi. A subtle velocity "sag" that developed beneath the anomaly was suggestive of shallow gas, because of the presumed time delay attributed to the displacement of water in the reservoir by gas - through which acoustic waves travel at much slower velocities. A Het Lime test was drilled to investigate the anomaly, but the Het was apparently devoid of hydrocarbons; however, a 28 foot-thick Miocene sand located not far above the Het Lime was found to be gas-bearing and (as one would expect, at 4,000 feet) very porous and permeable.
Casing was run and the well was subsequently perforated across the Miocene Sand, later named the "Amphistegina Sand" in recognition of the foraminifera associated with that stratigraphic interval. Unlike the Tertiary gas completions further to the west, this reservoir flowed at sustained rates in excess of 2,500,000 cubic feet of gas per day (> 2.5 MMCFGPD) with over 1,500 psi flowing tubing pressure. In fact, aside from some downtime associated with mechanical issues, this initial discovery - the #1 International Paper 7-5 - continued to produce 2 MMCFGPD with only a minor loss of reservoir pressure until February of 2004. The operator successfully offset this well with a second gas producer (the #1 International Paper 7-10) that also encountered 29 net feet of gas pay and also potentialled at an approximate rate of 2.6 MMCFGPD. Logs for the #1 International Paper 7-5 & 7-10 wells are shown immediately below. Unfortunately, as of March 2004, two developmental offsets for the field area (subsequently named "Mariner Field") have been plugged, and the #1 International Paper 7-10 and the discovery well, the 7-5, have watered out. It appears that Mariner Field represents a 4 BCF combination structural/stratigraphic trap, formed by the drape of the Amphistegina reservoir sand over a thick underlying channel; and that the gas production has been laterally swept by active water encroachment. Numerous other wildcats drilled in the immediate area have been plugged. At this point in time (fall 2009), it would seem that this young Trend has "died on the vine". Check back with us in the near future for further updates.