Facies architecture and production characteristics of strandplain reservoirs in the Frio Formation, Texas

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Tyler, Noel

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University of Texas at Austin. Bureau of Economic Geology
University of Texas at Austin. Bureau of Economic Geology


Many modern shore zones comprise a continuum of depositional environments that encompass both strandplain and barrier-island systems. Strandplains are further subdivided into two classes: sand-rich beach-ridge plains and mud-rich chenier plains. Tertiary shorezone systems of the Texas Gulf Coast Basin contain a significant proportion of the Texas oil resource in clastic reservoirs. These reservoirs display better-than-average oil recovery efficiencies. This report describes the production attributes of three Frio strandplain reservoirs--the Cayce, the Cornelius, and the Carlson--in the North Markham-North Bay City field, Matagorda County, Texas.Hydrocarbons in the North Markham-North Bay City field were trapped in a simple rollover anticline. Oil is produced from stacked strandplain sandstones in this multiple reservoir field. Composite sandstones of beach-ridge plain/distributary/delta-front origin constitute the Cayce oil reservoir. Internal heterogeneity results in considerable fluid-flow anisotropy, as displayed by sequential water-cut maps and oil production maps. Water influx in the strandplain deposits follows broad fronts, whereas water invasion in channel deposits is more restricted and erratic. The Cornelius reservoir was deposited in a system intermediate between sand-rich beach plains and mud-rich chenier plains. Sandy beach ridges, separated by muddy swales, compose the productive framework of this class of strandplain reservoir and furthermore 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 strandplain deposits. Oil is contained in upward-fining transgressive sandstones that rest on thicker but oil-barren progradational facies. Facies analysis indicates that the Carlson had a complex and episodic depositional history, yet water-influx maps 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 from the North Markham-North Bay City reservoirs follows a predictable trend. Recovery efficiency is highest from the transgressive sheet sands of the Carlson, which is the shallowest of the major oil reservoirs; intermediate from the composite Cayce; and lowest from the depositionally complex and mud-rich Cornelius, which overlies the Cayce. Reservoir efficiency of the strandplain sandstones in the North Markham-North Bay City field exceeds that of barrier and back-barrier deposits productive elsewhere in the Frio Formation of the central Texas Gulf Coast.


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