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Origin of Stuart City (Edwards) Gases from Early Cretaceous Shelf-Margin Reservoirs of South Texas

Illich, Harold 1; Waite, Lowell 2; Ferworn, Kevin 1; Zumberge, John 1; Brown, Stephen 1
1 GeoMark Research, Dallas, TX.
2 Pioneer Natural Resources, Dallas, TX.

Gases and condensates from shelf-margin Stuart City (Edwards) reservoirs were analyzed geochemically for the purpose of describing their origin. Differences in the gases were suggested by the substantial reservoir heterogeneity characteristic of the Edwards and currently being exploited with horizontal well technologies. Fifty-six (56) gas samples were acquired from the shelf-margin.

Systematic differences occur in heating values of the gases. Gases from the southwestern and the northeastern parts of the shelf-margin producing trend typically contain 2-5% wet gas (C2+). Gases sampled from the central part of the trend (Pawnee and Three Rivers field areas) usually contain less than 0.5% wet gas. Gas wetness is not a simple function of depth. Gas wetness relationships suggest processes in addition to source maturity contributed to the evolution of the fluids.

Bulk chemical data and stable carbon and hydrogen isotope compositions of the gases are interpreted to indicate several gas “families” are represented in the shelf-margin trend. Two of the families appear to be maturity variants of a common source. The source is probably a basinal facies of the Stuart City Formation, or possibly the Eagle Ford Formation located on the expanded (seaward) side of the reef trend. An additional gas family is interpreted to originate from a separate but closely related facies of the source for the Edwards gases. The most compositionally distinct gas family is from a combination of Edwards and younger sources. The younger gases were probably derived from a Paleogene source also located on the expanded side of the reef play.

Source differences, maturity differences, gas-gas mixing (Cretaceous and Paleogene), and cross-stratigraphic migration are processes interpreted to be involved in the evolution of the Edwards shelf-margin producing trend. Mixing is particularly important because it demonstrates geographic overlap of two or more petroleum systems. The presence of multiple petroleum systems within the play has economic significance via the lowering of exploration and production risk.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009