Abstract Moderate volumes of dry gas are produced in the South Padre Island Area of offshore south Texas from a 120-m-thick aggregation of mixed shoreface and shelf sandstones that are interbedded with thin neritic shales. These shallow marine sediments aggraded on a broad stable shelf platform during the middle Miocene depositional episode when block faulting and basin filling within the Rio Grande Rift disrupted regional drainage patterns and caused an abrupt decrease in sediment supply to the Rio Grande Embayment. Because sediment influx to south Texas was significantly reduced, middle Miocene sandstones were deposited less than 10 km landward of the previous shelf edge, which was constructed during the early Miocene (pre-Amphistegina B) regressive episode. Thick, widespread marine shales, containing the Cibicides opima and Textularia stapperi faunal assemblages, occur beneath and above the sequence of nearshore sediments. The Textularia stapperi transgressive shale serves as the primary seal for hydrocarbon accumulation that is preferentially trapped on the upthrown side of minor down-to-the-west faults. These reactivated early Miocene faults did not significantly influence middle Miocene sediment distribution or interval thickness except near the contemporaneous slope. Individual sandstone reservoirs of the delta fringe and mid-shelf facies are 3 to 12 m thick and exhibit thin interbedded to upward-coarsening log responses. They form laterally-persistent but irregular sand sheets that pass landward into either thick coastal-plain sandstones or embayment mudstones, they merge updrift (northeastward) into fluvial-deltaic sandstones, and they grade seaward into slope mudstones. Progressive basinward and southwestward decreases in net sandstone thickness and sandstone percent indicate predominant sediment transport by shelf currents in the same directions. Repetitive cyclic depositional events are also indicated by the nearly uniform vertical spacing of sandstone beds near their southwestern pinchout. Shallow reservoir depths (~ 1500 m, 5000 ft) and relatively young sediment age suggest that the hydrocarbons trapped in middle Miocene sediments originated in older Tertiary shales and migrated into the shelf sandstone reservoirs when early Miocene faults were reactivated. The presence of humic organic matter in Miocene sediments and high thermal gradients within the Rio Grande Embayment probably are responsible for the predominance of methane which results from cracking of previously generated hydrocarbons.