Downdip Yegua Trend--Overview

Thomas A. Ewing, W. Grant Fergeson

The Downdip Yegua Trend of overpressured gas-condensate reservoirs has produced over 400 bcf of gas and 10 million bbl of oil and condensate since 1979. The trend has indicated reserves in the range of 1.5-2.0 tcf.

The trend was opened only in the late 1970s and 1980s because its sandstone fairways are mostly separated from the updip, sandstone-rich Yegua section (which has produced since the 1930s) by a "mid-dip" region where sandstones are scarce. The trend is also separated from overlying Frio and Vicksburg targets by over 1,000 ft of highly overpressured Jackson shale.

Five fairways can be defined: (1) the Duval-Jim Wells fairway, with thin but continuous sandstones and little expansion of section, (2) the Wharton-Jackson fairway, with thick but discontinuous sandstones of high quality and very active faulting and expansion, (3) the Fort Bend-Brazoria frontier, with deeply-buried sandstones and growth faulting affected by salt movement, (4) the Harris-Jefferson fairway, with abundant sandstones, but absence of a "mid-dip" region, salt structures and growth faulting, and (5) the Orange-Calcasieu fairway, with widespread sandstones but a narrower belt of growth faults and traps.

The Harrison-Jefferson fairway has the most production to date (250 bcf), but it was also one of the earliest to be developed (beginning in 1953). The Wharton-Jackson fairway has produced 122 bcf since its discovery in 1982, and is the most prolific fairway to date. The Duval-Jim Wells fairway has produced 30 bcf since 1979, and the Orange-Calcasieu fairway has produced in the vicinity of 8 bcf. The other two fairways are in the least mature stage of development.

Sandstones in the Wharton-Jackson fairway (and in some other areas) were fed by narrow, incised channel systems that extend through the "mid-dip" region; these channels are presently a focus of exploration for normally pressured gas. Downdip, the channels feed a variety of deltaic and bar and/or strandplain environments. The episodic deposition downdip from the main Yegua (Katy) delta system is due either to episodic lowering of eustatic sea level, or to episodic progradation of narrow bird-foot deltas into the middle shelf.

At present, activity is most concentrated on detecting "sweet spots" of a few hundred acres or less, evaluating downdip and lateral extensions of known trends, and in defining large, potentially prolific structures in frontier areas using integrated geologic and geophysical analyses.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.