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The North Markham-North Bay City field, Matagorda County, Texas, is one of the major multiple-reservoir oil fields of the central Texas Coastal Plain that produce from stacked Frio barrier/strand-plain sandstones. The three principal oil reservoirs in the field are interpreted to be transgressed strand-plain (Carlson), progradational strand-plain (Cornelius), and composite progradational strand-plain/wave-dominated delta (Cayce) systems. Production characteristics of strand-plain facies are modeled using these reservoirs as examples.
Reservoir continuity is greatest in transgressed and progradational strand-plain sandstones where crosscutting channel facies are of minor importance. Hydrocarbon distribution is laterally continuous in both reservoir types. Broad edgewater incursion indicates an absence of internal facies barriers. Progradation of the Cornelius strand plain resulted in a composite reservoir in which the older sands and contained hydrocarbons pinch out against the overlying offlapping sequence.
In contrast, reservoir continuity in the Cayce is poor. Crosscutting fluvial sands produce oil at lower rates, act as conduits for early water influx, and provide facies boundaries against which hydrocarbons in adjacent progradational facies are pooled. Facies changes and pinchouts in heterogeneous reservoirs such as the Cayce are in part responsible for limiting ultimate recovery from major clastic reservoirs along the Texas Gulf Coast to approximately 50% of the original oil in place.
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