Texas shallow water hunt simmers

G. Alan Petzet
Exploration Editor

Brazos-Matagorda Shallow Water 3D Data Sample (177383 bytes)

An exploration resurgence may be near in Matagorda Bay and shallow water areas along southern portion of the Upper Texas Gulf Coast.

An indicated gas discovery by Shell Western E&P Inc. in Matagorda Bay is said to have fueled nominations of bay and gulf near shore tracts at the Oct. 4 Texas state lease sale.

Shells 1 East Matagorda Bay Unit, 9 miles southwest of Matagorda, has 512 in. casing set at 15,596 ft in a sidetrack hole, Petroleum Information reported. The well, about 7 miles southeast of the onshore Collegeport gas field complex, was unlikely to be flow tested until after the lease sale.

Industry sources said Shells new field wildcat, permitted to 20,000 ft and drilled to just below 17,000 ft before drillpipe became stuck, is thought to be an Oligocene Frio discovery.

Companies have drilled several deep Frio wildcats along the coast. Frio has produced gas and condensate from below 16,000 ft onshore, but most of the production offshore has come from 8,000 ft or less below the sea bed.

Meanwhile, the first large scale 3D seismic data ac- quired just off the coast are becoming available. About 20 fields in the Matagorda and Brazos areas offshore have produced more than 1.2 tcf of gas, mainly from Lower and Middle Miocene.

Nearshore 3D seismic

Two nonexclusive surveys in the Brazos and Matagorda areas are providing some of the clearest data yet in this mature province.

One survey, by Jebco Seismic Inc. and PGS Exploration (U.S.) Inc., covered more than 450 sq miles off Brazos and Matagorda counties. Data were acquired into less than 15 ft of water. The ocean bottom cable survey at its nearest point came within four miles of Shells Matagorda Bay wellsite.

The survey area was 80% unleased when shooting began in March. Operators have since nominated for inclusion in the lease sale all of the bay and nearly all the adjacent offshore area.

A 3D dip line from the survey provides fine detail in shallow zones and a new look at heretofore unimaged geology below the regional basal Miocene fault plane to a depth of 7 sec two-way travel time, perhaps in Cretaceous.

Deep Frio sediments that might be analogous to Matagorda Bay Frio sands appear at about 5 sec TWTT.

Examples of prospect types Jebco lists include:

• Low-relief, subtle closures associated with large and small faults in Middle Miocene.
• Reinterpretation of complexly faulted producing structures for new fault block and deeper pool tests in Lower and Middle Miocene.
• Extension of the Lower Miocene Marginulina A trend toward the northeast, where sands are known to exist.
• Extension of the deep Lower Miocene Siphonina davisi sands from Matagorda 519 field and in adjacent blocks.

Operators have asked the geophysical contractors to expand acquisition next year along the coast in the Matagorda and Galveston areas. Gulfward from the Jebco/PGS survey, Diamond Geophysical Service Co., Houston, has acquired 3D data on the upthrown side of the Corsair fault in the Brazos and Brazos South areas (OGJ, Sept. 18, p. 45). Copyright 1995 Oil & Gas Journal. All Rights Reserved.