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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Late Cretaceous Volcanism in South and
Central Texas - Stratigraphic, Structural,
and Seismic
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Since their discovery in 1915, hydrocarbon traps in and
around "serpentine plugs" have produced about 47 million
barrels of oil, and have significant potential for additional small
discoveries. Production is from isolated reservoirs within
mounds of altered volcanic tuff and associated shoal-water
carbonates. A review of the more than 200 volcanic centers
and intrusive bodies of South and Central Texas has led to
development of stratigraphic and seismic models
useful in
exploration and production.
The so-called serpentine plugs are largely tuff mounds formed by accumulation of volcanic ash (altered to palagonite) on the seafloor around a submarine volcanic vent. Volcanic activity waked during deposition of the chalk and marl of the upper Austin and lower Taylor Groups (about 80 million years ago). After their eruption, the tuff mounds localized the deposition of shoal-water carbonates with-good porosity and permeability. Low-permeability, organic-rich marine shale and marl d the Taylor Group capped the carbonates, serving as both a hydrocarbon source and a stratigraphic seal. Compactional draping of overlying San Miguel and Olmos sands, with minor offset faulting, created important additional traps in South Texas.
Central Texas volcanic centers are highly aligned along strike-oriented regional faults and fractures of the Balcones and Luling systems. The magmas in both Central and South Texas were ultramafic and alkaline, suggesting that partial melting occurred at depths of about 40 miles (60 kilometers). The magma row rapidly to the surface, probably in an extensional mesa regime controlled by pre-Tertiary Balcones- Luling faults.
The palagonite tuff of a typical productive volcanic center
has low seismic velocity and is encased in high-velocity
carbonates. The strong velocity contrast, coupled with the
distinctive shape of the tuff mass, yields a characteristic
seismic pattern. Modern seismic techniques, together with
stratigraphic data, allow accurate delineation of buried tuff
mounds and prediction of productive carbonate facies.
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