Thin sandstones and shales of the Muddy Formation produce large amounts of oil at Hilight field. The Muddy section is generally tight, and the best reservoir sandstones have effective porosity of 17% and average permeability of only 115 md. High production rates are caused primarily by extensive fractures, and the reservoir will yield an ultimate recovery of more than 80 million bbl of oil.

Sedimentary structures and petrographic analyses show that Muddy sands were deposited in littoral marine, lagoonal, and fluvial environments. Porous sandstones average 10 ft and rarely attain 20 ft in thickness. Lower Muddy sandstones are fluvial, whereas upper Muddy sandstones are mostly littoral or lagoonal in origin. Fluvial sandstones are associated with shales and siltstones that are highly carbonaceous and were deposited in poorly drained marshes. Lagoonal sandstones

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and marsh deposits also occur lateral to the littoral sandstones.

The presence of thin, littoral marine sandstones with widespread marsh sediments suggests a depositional environment similar to the modern chenier plain of South Louisiana. The entire Muddy sequence is transgressive toward the east and southeast over a low-relief topography developed on the underlying Skull Creek Shale.

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