Hydrocarbon Resources in Lower Miocene Reservoirs--Texas State Submerged Lands

Steven J. Seni

The lower Miocene depositional episode is the major gas- and gas-condensate-producing trend in state submerged lands along the Texas coast and offshore, producing in 1985 about 11 billion ft³ onshore and 100 billion ft³ offshore. Eight reservoir plays are recognized in a diverse suite of depositional environments but in relatively similar, structurally controlled traps. Production is from earliest progradational deltaic and submarine fan facies and retrogradational barrier-bar/strand-plain and shelf facies. Fault-controlled traps include growth-fault rollover anticlines, dip-reversed fault blocks, and antithetic faults. Combination stratigraphic-structural traps in barrier-bar sandstones are locally significant in the Matagorda Bay area and offshore. Significant exploration potential remains in underexplored, deep (10,000-16,000 ft), progradational, sand-rich continental margin facies, including submarine fan systems along the upper coast and in smaller gas fields in progradational and aggradational delta front, delta flank, and shoreface environments at more modest depths (6,000-10,000 ft) along the central and upper coast.

Production and pressure-decline data indicate that most reservoirs produce by depletion drive. Water-drive reservoirs are concentrated in the south Texas area (Holly Beach) in regionally extensive, transgressive sandstones on the upthrown blocks of counter-regional faults and in the Coastal Bend region (Six-sixty, Sherman Offshore) in combination stratigraphic-structural traps in transgressive barrier/lagoonal environments. Lower Miocene reservoirs having water drives are commonly less compartmentalized by faulting than are depletion-drive reservoirs and thus often have greater reservoir volumes.

The size of fault blocks and of closure in rollover anticlines primarily affects reservoir size. Sand-body geometry also influences reservoir size and is significant where growth faulting influences sand distribution (El Gordo). Most additional reserves will be supplied within existing fields by newly discovered reservoirs, reservoir extensions, and deeper pool discoveries. Experience with onshore Texas reserves suggests that extended conventional recovery through geologically directed infill drilling and enhanced gas recovery through coproduction are further techniques for increasing reserves.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.