ABSTRACT

Analyses of gas from the Edwards limestone in South Texas show that the percentage of methane decreases systematically up-dip, to the northwest, while the heavier hydrocarbons (ethane through hexane +) increase correspondingly. The isocomposition lines trend in a more easterly direction than the structural contours and the contact of reef and back-reef facies, so methane also decreases, and the heavier hydrocarbon gases increase, northeastward along structural and depositional strike. The non-hydrocarbon gases -- hydrogen sulfide, nitrogen, and carbon dioxide -- tend to follow the heavier hydrocarbons, except around the three salt dome fields in McMullen County where exceptionally high percentages are found.

The source of the gases was fine-grained sediments deposited during Edwards time, principally down-dip on the clinoform (continental slope) and secondarily to the southwest in the Rio Grande embayment. The hydrocarbons were fractionated first in the source beds as compaction reduced the size openings between the mineral grains, and secondly in the reservoir rock in the course of bulk flow as a separate phase by density stratification within traps, small as well as large. Migration of the gases in solution in moving water or by diffusion, and differential adsorption on mineral surfaces, would have produced areal distributions the reverse of the one actually observed.

Meteoric water entering the Edwards Formation in northern Mexico and southwestern Texas added a northeasterly component to the up-dip movement of expelled formation fluids, causing the decrease in methane and increase in "heavy" hydrocarbons northeastward along structural and depositional strike.

The non-hydrocarbon gases originated and migrated with the hydrocarbons, except in the three salt-dome fields where enrichment in these gases is attributed to sulphate-reducing bacteria acting on hydrocarbons and anhydrite associated with the salt, and the resulting gases migrating upward along faults.