Geology
The primary productive geologic unit is the Cretaceous-age Woodbine Formation, a regional petroleum-bearing unit which had been known since the early part of the 20th century. This sandstone unit was deposited during a period when East Texas was a shallow sea, approximately 100 million years ago. During a subsequent period it was uplifted, eroded, and then covered again by the sea, which this time deposited a layer of impermeable chalk, creating a stratigraphic trap – a situation where oil, which is lighter than water and migrates upwards, reaches a point where it can move no farther, and pools. The source rock for the oil in East Texas is the lower-lying Eagle Ford Shale.
Water intrusion from deeper in Woodbine Formation is the mechanism pushing oil through the reservoir toward the producing wells. A 1932 study showed that oil wells stopped flowing when water pressure dropped below 800 pounds per square inch.
Read more about this topic: East Texas Oil Field