Abstract

Geochemical characteristics of hydrocarbons from the South Liberty field, Liberty County, Texas, were integrated with local stratigraphy, pressure, temperature, seismic data, and formation water chemistry to determine the source, maturity, and migration pathways of hydrocarbons associated with the salt dome in the southeast Texas Gulf Coast. Fourteen crude oil samples from the soft-geopressured Cook Mountain and Yegua (both Eocene) and Frio (Oligocene) reservoirs (989–2886 m [3245–9469 ft]) were analyzed by whole-oil high-resolution gas chromatography (GC) and GC–mass spectrometry. Pressure and temperature data from 36 wells were used to model the thermal maturation history in the vicinity of the dome.

South Liberty oils were found to belong to a single genetic family, sourced from rocks with a similar level of maturity. Biomarkers indicate they were formed in a marginal marine environment with notable terrestrial input from a likely lower Tertiary source rock (probably the downdip lower Claiborne and/or Wilcox Group). Oils were generated within the peak oil window (vitrinite reflectance, ~0.6–0.9%) at expulsion temperatures of 125 to 130°C. Thermal modeling indicates the lower Claiborne and/or upper Wilcox beds attained optimum maturity in and around the dome between 8 to 20 Ma and 18 to 31 Ma, respectively. Oil migration toward the structure probably occurred through faults breaching the deep-seated lower Claiborne/upper Wilcox source. Two severely biodegraded samples were found, and these were associated with cooler formation water (50 to 55°C) with a greater meteoric water component (48–60%). Comparison with Brazoria County oils (100 km [62 mi] to the southwest) indicates that the South Liberty oils belong to the same genetic family, with minor differences resulting from a greater input from terrestrial kerogen.

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