Abstract

The Downdip Yegua Trend of overpressured gas-condensate reservoirs has produced over 600 BCF and 15 MMBBL of oil and condensate, mostly since 1979: The trend has indicated reserves in the range of 1.6-2.0 TCF, and substantially higher potential.

The trend was opened only In the late 1970s and 1980s because its sandstone fairways are for the most part separated from 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 1000 ft of highly overpressured Jackson shale.

Five fairways can be defined:

--I. The Duval-Jim Wells Fairway, with thin but continuous sandstones and little expansion of section;

--II. The Wharton-Jackson Fairway, with thick but discontinuous sandstones of high quality and very active faulting and expansion;

--III. The Fort Bend-Brazoria Frontier, with deeply buried sandstones affected by growth faulting and salt movement;

--IV. The Harris-Jefferson Fairway, with abundant sandstones, no mid-dip region separating it from updip deltaic sediments, abundant salt structures, and growth faulting;

--V. The Orange-Calcasieu Fairway, with widespread sandstones but a narrower belt of growth faults and traps.

Fairway IV has the most production to date (372 BCF), but it was also one of the earliest to be developed (beginning In 1953). Fairway II has produced 164 BCF since Its discovery in 1982, and is the most active fairway to date. Fairway I has produced 70 BCF since 1979, and Fairway V has produced In the vicinity of 11 BCF. Fairways III and V are in the least mature stage of development.

Sandstones In the Wharton-Jackson Fairway are fed by dip-elongate channel systems which 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, bar/strandplain and shelf environments. The episodic deposition downdip from the main Yegua (Katy) delta system is due either to episodic lowering of eustatic sea level, or to infrequent progradation of narrow blrdsfoot deltas Into the middle and outer shelf.

At present, activity is mainly 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 analysis.