The North Markham-North Bay City field, Matagorda County, Texas, produces oil and gas from multiple barrier and strand-plain sandstones of Oligocene age stacked over a rollover anticline in the Frio fault zone. Three principal oil reservoirs--the Cayce, Cornelius, and Carlson--account for 83% of the 49 million bbl produced from the field. All three were deposited in strand-plain systems and display considerable heterogeneity. The Cayce reservoir is composed of beach-ridge plain, distributary, and deltaic facies. Reservoir heterogeneity results in anisotropic fluid behavior. Water influx in the beach-plain deposits follows broad fronts, whereas water invasion in channel deposits is more restricted and erratic. The Cornelius reservoir was deposited in a system intermediate etween sand-rich beach plains and mud-rich chenier plains. Sandy beach ridges, separated by muddy swales, compose the productive framework of this class of strand-plain reservoir and act as conduits for early water influx. Sandstones, possibly of washover origin, in the intervening swales produce oil but are more rapidly drained than are beach-ridge sandstones. The Carlson reservoir produces from transgressed strand-plain deposits. The Carlson had a complex and episodic depositional history, yet water-influx and oil-production maps suggest isotropic fluid behavior. Modern sand-rich transgressive shore-zone deposits are characteristically sheetlike, as is the transgressive component of the Carlson reservoir. This distinctive morphology appears to have fostered reservoir productivity.

Oil recovery follows predictable trends. Recovery efficiency is highest from the transgressive sheet sands of the Carlson, intermediate from the composite Cayce, and lowest from the depositionally complex and mud-rich Cornelius. Reservoir efficiency of strand-plain sandstones exceeds that of barrier and back-barrier deposits productive elsewhere in the Frio Formation of the central Texas Gulf Coast.