HYDROCARBON SYSTEMS IN THE SENO MEXICANO (SOUTH TEXAS AND BURGOS BASIN): AN INITIAL EXAMINATION

The Seno Mexicano is a prolific hydrocarbon province, particularly rich in natural gas. The Texas part of the basin (RRC district 4) has produced over 3200 million barrels of oil (Mbo) and 93 trillion cubic feet of gas (Tcfg) since the 1920s; 68% of the gas is from fields greater than 0.1 Tcfg, and 45% of the oil is from fields greater than 10 Mbo. The Mexican part (Burgos basin) has produced 15 Tcfg from Cenozoic reservoirs, 86% from major fields. Casinghead gas and condensate are not included here.

Gas production occurs in six major plays; the Cretaceous rim on northwest and west (2.6 Tcfg), the Lobo slide complex (12.5 Tcfg, 6.4% in Mexico), the Wilcox and Queen City shelf-margin trends (15.5 Tcfg, 34% in Mexico), the Frio-Vicksburg trend (30 Tcfg, 15% in Mexico), the expanded shelf-margin Frio trend (21.7 Tcfg, 9.3% in Mexico), and the expanded Miocene (only 1.0 Tcfg, offshore Texas). Oil production occurs from a belt of shallow Eocene reservoirs (471 Mbo), and in the northeastern corner of the basin (780 Mbo), as well as from thin oil rims on many Frio-Vicksburg gas reservoirs.

In the Seno Mexicano, the Mesozoic contains two world-class oil and gas source rocks, the Upper Jurassic (Pimienta) and the mid-Cretaceous (Eagle Ford). The Mesozoic section was buried deeply in the Cenozoic and is gas-generating to overmature at present. The large amounts of dry gas (no condensate) in the Lobo and Wilcox may come from mature Mesozoic sources, but may also come from the thick, terrestrial-derived Paleocene-Eocene fill of the Rosita Trough. Gas-condensate charging of the Vicksburg and Frio reservoirs may come from the deep trough beneath the present coast, filled with Eocene and Oligocene material. Oil reservoirs in the shallow Eocene are anomalously gas-poor, suggesting a marine middle Eocene source. The dearth of significant Miocene production suggests a lack of source rock where Eocene source rocks were not present and Mesozoic sources were overmature. Abyssal oil production from Eocene reservoirs in the Perdido fold trend is sourced from Jurassic source rocks where they remain in the oil window.