Organic-Rich Chalks and Calcareous Mudstones of the Upper Cretaceous Austin Chalk and Eagleford Formation, South-Central Texas, USA

Abstract

The Austin Chalk and Eagleford Formation are widespread Upper Cretaceous deposits of the Gulf Coast region of south-central Texas. The upper part of the Austin is formed by prograding high-stand sequences of light-colored bioturbated chalks. The lower part of the Austin and the Eagleford are composed of organic-rich chalks and calcareous mudstones that form the transgressive systems tracts of three sequences. These organic-rich rocks are laminated or sparsely burrowed and contain few benthic fossils, suggesting deposition in a basin with oxygen-deficient sediments. The organic-rich rocks of the lower Austin are basinally restricted deposits that reach 200 ft (61 m) in thickness. They contain an average of 3.7% TOC composed of a mixed type II/III kerogen (HIo = 481 mg/g), dominated by amorphous organic matter but with 5–40% vitrinite, inertinite, and spore-pollen. Pristane/phytane ratios of 0.6–1.3, minor amounts of oleanane, a predominance of moretane and hopane over normoretane and norhopane, and predominance of hopanes over steranes indicate deposition of mixed marine and terrestrially derived organic matter under dysoxic conditions. The Eagleford is generally thinner (40–150 ft or 12–46 m) but more widespread than the lower Austin. TOC values average 3.7% in shallow cores and 4.5% in one deep core. The organic matter in the shallow cores is a mixed type II/III (HIo = 414 mg/g). However, predominance of normoretane and norhopane over moretane and hopane, increased amounts of tricyclic terpanes, absence of oleanane, and predominance of steranes over hopanes indicate deposition of mainly marine-derived organic matter under anoxic conditions. Generation of liquid hydrocarbons has occurred below 6000–7000 ft (1830–2130 m) present-day burial for both formations. Rocks at depths below 9000–10 000 ft (2740–3050 m) are now overmature for generation of liquid hydrocarbons. Calculated specific oil yields are 0.3–0.4 bbls/acre-ft (0.3 bbls/km2/m) for the lower Austin and the shallow Eagleford cores and 1.2 bbls/acre-ft (1.0 bbls/km2/m) for the deep Eagleford core. Where thick lower Austin is present, it probably has generated and expelled hydrocarbons. However, the more widespread Eagleford has most likely generated the majority of the oil found in the Austin Chalk. Oil has migrated into more permeable intervals of the upper Austin and can be discriminated by composition from in situ EOM. Pristane/phytane ratios and stable-carbon isotopes do not differentiate between the lower Austin and the Eagleford as the source rocks for these oils.