REASONS FOR AND SIGNIFICANCE OF DEEP, HIGH-RANK HYDROCARBON GENERATION IN THE SOUTH TEXAS GULF COAST

Abstract

ABSTRACT Organic geochemical analyses of Tertiary to Jurassic rocks from a 6,704 m wellbore in LaSalle County, Texas (interior South Texas Gulf Coast) revealed two hydrocarbon (HC) generation zones: a shallow (2,255–2,682 m) zone with a HC concentration maximum at a vitrinite reflectance (Ro) of 0.9, and a deep zone (4,000–5,791 m) with a HC concentration maximum at Ro = 3.4. The shallow zone was due to generation from terrestrial-based (Type III) organic matter (OM), and organic-geochemical data of this zone agreed with results of other studies of HC generation from Type III OM and closely followed the generally accepted model of organic metamorphism. In contrast, the deeper, carbonate-rich, Lower Cretaceous to Jurassic rocks contained a marine-derived OM, and significant organic-geochemical changes accompanied this change in OM type at a lithologic break at 3,300 m in the well bore: 1) Hydrogen indices sharply increased. 2) ROCK EVAL Tmax and Transformation Ratio values sharply decreased. 3) Gas chromatograms of C15+ saturated and aromatic HCs had mature characteristics above the break and immature characteristics below the break. 4) In both the saturated and aromatic HCs, molecular compositions and compound ratios, which had been continually increasing to mature values with depth, strongly reversed themselves to very immature values. The immature characteristics of bitumen from the carbonate-rich rocks below the lithologic break are believed to be largely original characteristics which had not yet been diluted by significant HC generation. Below the lithologic break, the maturity of both the saturated and aromatic HCs increased up to a deep generation zone defined by a sharp increase in the concentration of C15+ bitumen at 4,116 m (Ro = 1.9), followed by increases in the concentrations of C15+ saturated and aromatic HCs at 4690 m (Ro = 2.5), and 4,910 m (Ro = 2.8), respectively. A maximum in C15+ HC and bitumen concentrations at 5,242 m (Ro = 3.4) was followed by a decrease to background values by 5,821 m (Ro = 4.5) from primary migration. Thus, a generation sequence was encountered of resins and asphaltenes being generated first, followed by saturated and then aromatic HCs. A thermal deadline for C15+ HCs was apparently being approached at well bottom at Ro = 7.0. The moderate to high amounts of C15+ bitumen and HCs, and the measurable to moderate amounts of generation potential in the deep, high-rank (Ro = 1.3–7.0) rocks of this well bore do not agree with the hypothesis of a thermal deadline for C15+ HCs in fine-grained rocks by Ro = 1.35. The organic maturation path of the marine-derived (presumed hydrogen-rich OM) from the deep rocks of this well supports the conclusions of other studies, that hydrogen-rich OM may have higher activation energies than Types II-S and III OM and therefore requires higher burial temperatures and maturation ranks to initiate and sustain HC generation and primary migration.