Observations on the Geochemistry and Origin of Gases Occurring along Lower Cretaceous Shelf Margins, South Texas

Illich, Harold (Pioneer Natural Resources USA, Inc.) | Waite, Lowell (Pioneer Natural Resources USA, Inc.) | Tinnin, Beau (Pioneer Natural Resources USA, Inc.) | Covarrubias, Edgardo (Pioneer Natural Resources USA, Inc.)



Gas and liquids produced from the Eagle Ford Formation (Cenomanian-Turonian), Edwards Formation (Middle Albian), and Wilcox Group (Late Paleocene-Early Eocene) represent resource and conventional petroleum plays that occur along the Lower Cretaceous shelf margins of South Texas. The interrelationship or independence of these petroleum systems is of considerable interest to workers seeking to expand production in the region. Gases and co-produced liquids were collected for the intervals and used as a basis of this work.

The Eagle Ford is an authentic resource play (source and reservoir for hydrocarbons). Wilcox gases and oils have been interpreted to have a source younger than the Eagle Ford, probably Paleogene Wilcox shales and/or the Midway Shale, but little data have been presented in support of this conclusion for the South Texas onshore area. The origin of gases in the Edwards reservoirs is less apparent but several possibilities can be suggested: Eagle Ford on the expanded side of the margins (southeast of the margins); Edwards basinal facies also occurring southeast of the margins; and/or older intervals such as the Pearsall shale (Aptian). Onshore, southeast of the Lower Cretaceous shelf margins, only the Eagle Ford and Tertiary intervals have been demonstrated to have significant source potential.

Gas chromatographic and isotope data were collected, analyzed, and interpreted for fifty-five Edwards, forty-nine Eagle Ford, and six Wilcox gases distributed along the Lower Cretaceous shelf margins from Lavaca County (northeast) to La Salle County (southwest). The gas data are used to conclude that the Eagle Ford is its own source and a Tertiary source provides gases and oils to the Wilcox. The main source for the Edwards is probably the Eagle Ford occurring on the expanded southeast flank of the Lower Cretaceous Shelf margins. Gases and oils in the Edwards occurring northeast of the shelf margin “cross-over” (gases from Karnes, DeWitt, and Lavaca counties) are interpreted to be mixtures of Eagle Ford and Tertiary gases (gas-gas mixing). This conclusion is supported by the occurrence of oleanane-bearing biomarkers in the geochemistry of condensates from Edwards reservoirs.

Introductions and Objectives

In the last decade, exploration and development of “resource” plays has provided an opportunity to study the geochemistry of oils and gases that are native to the formations from which they are produced. It is the goal of this presentation to acquire an increasingly robust knowledge of South Texas shelf margin petroleum systems and develop additional interpretive schemes useful in the study of such systems. Specifically, the probable sources for hydrocarbons in the Edwards, Eagle Ford, and Wilcox will be studied and processes that have acted to modify the hydrocarbon geochemistry identified. Emphasis in this study is placed mainly on the gases from these systems. The distribution of gases in South Texas used in this project is shown in Figure 1.