The environments of deposition of the San Miguel lignite, a commercial quality deposit in a 4.5 by 0.3-km area in Atascosa and McMullen counties, Texas, was determined through analysis of nearly 122 m of continuous core and over 600 electric and radiation logs. The lignite is in the Jackson Group and is part of the south Texas Eocene lagoonal-coastal plain system.

The lignite is overlain and underlain by a unit of gray-green bioturbated siltstone and claystone 33.9 m thick. The lack of body fossils and abundance of root structures indicates this unit was deposited in a coastal grass-flats environment.

Below the bioturbated unit is a unit of massive green claystone 3.4 m thick, which contains abundant macro-invertebrate fossils. The fossils indicate this unit was formed in an open bay or lagoon.

Below the green claystone is a coarse, carbonaceous sandstone 3.6 m thick. Sedimentary structures and petrographic trends of this unit are analogous to those of modern back-barrier flats deposits.

The lignite interval is composed of lignite and carbonaceous clay partings of 4.2 m average thickness. The lignite interval has an overall strike-trend with local dip-trending segments. The lignite represents the accumulation of plant matter in a coastal swamp behind a lagoon. The clay partings formed during occasional flooding of the swamp by coastal streams.

Analysis of sedimentary structures, petrography, and paleontology from continuous cores is considered essential to oil and gas exploration. This study demonstrates these techniques are also important in lignite exploration.

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