ABSTRACT

In studying the stratigraphic and geographic distributions of oils initially produced from seven East Texas Cretaceous formations, the effects of depth, temperature and rate of burial on increasing hydrocarbon maturation, expressed as increasing API gravity, can be determined statistically. Analysis of both linear-regression and data-density trends indicates that API gravity increases as the oil matures during burial. Linear-regression analyses yield positive API-gravity gradients and low correlation coefficents for the data populations. Data-density trends show well-delineated and differing API-gravity gradients with ordinal and abscissal limits.

From younger to older formations, there are two main trends of increasing API gravity. The oil from Sub-Clarksville reservoirs, showing the best-delineated high API-gravity gradient, represents one main trend. The oils from Woodbine and Paluxy reservoirs show both main trends; API gravity increases rapidly, then slowly, as burial continues. The oils from Glen Rose, Rodessa, Pettet and Travis Peak reservoirs show the second main trend; that is, a low API-gravity gradient. A composite plot for the seven formations, showing one curvilinear trend, with both high and low API-gravity gradients implies different hydrocarbon maturation rates for the East Texas oils. Almost all maturation trends are within a temperature range of 110° to 250°F (43° to 350°C), which falls below the theoretical temperature window of 250° to 350°F (121° to 177°C) for maximum hydrocarbon generation.

For each formation, the geographic distributions of API gravity, depth of burial and formation temperature establish a stratigraphic, geographic and tectonic framework for studying the statistical distributions. Generally, high-API-gravity oils have been produced from the deep southern and shallow eastern parts of the basin. Lower-API-gravity oils are produced from the northern and western shallow parts of the basin.