O&G is not deposited uniformly in the subsurface. Various types of "traps" exist to capture O&G.
The older field areas you mention have been depleted and paid out. There has been a continuing stream of wells drilled since that time period trying to find "new" O&G traps. Lots of dry holes along with a low percentage of successful wells that have found some "new" O&G traps.
Deeper drilling and new technology has pushed operators to try new things - but no guarantee that they will work economically.
The unconventional shale play that is rampant today (Eagle Ford, Permian Basin, Barnett, Bossier, Haynesville, etc.) does not extend into the area you mention - although some operators have tried to chase some "shales" in such a way.
Venado put together a large acreage block in Duval and Jim Hogg Counties chasing a "shale" play. After 3-4 wells (vertical and horizontal) and tons of capital spent, play was deemed uneconomic (produced some O&G but not enough to come close to paying out drilling and acreage costs).
The subsurface and geology is a complicated and sometimes fickle mistress - it is not easy to find new O&G traps / but operators are always trying.
That is the "game" that O&G operators play.
Hi Mark: I haven't been on this forum for awhile. Glad to see you are still here and helping. Any chance there will be renewed interest in dry gas drilling in these counters with dry to liquid gas conversion plants going up along the coast?