Organic geochemistry of core samples from an ultradeep hot well (300°C, 7 km)

Abstract

South Texas cores of Lower Cretaceous rocks from a depth of 6400.8 to 7544.6 m at present-day temperatures of 262–296°C have high concentrations of C 15+ hydrocarbons. Bitumen coefficients range from 105 to 367 mg/g and C 15+ extractable bitumen ranges from 500 to 2200 ppm. Some generation potential remains associated with the kerogen of these rocks. In addition to exhibiting the above organic-geochemical properties, characteristic of the zone of intense hydrocarbon generation, these rocks also have organic-geochemical properties, attributed to the zone of hydrocarbon extinction or greenschist metamorphism. These characteristics are: high vitrinite reflectance ( R 0) values, 4.4–4.8; low H/C ratios, 0.30–0.58; high saturate/aromatic hydrocarbon ratios, 7.05–20.6; high hydrocarbon/NSO ratios, 2.65–4.66; and high transformation index ratios [ S 1 (S 1 + S 2) ], 0.61–0.87. The data from this (and other wells we have studied) show that high concentrations of C 15+ hydrocarbons are thermally stable to high temperatures (at least 300°C) in abnormally-pressured semi-closed systems over geologic time. Concepts prevelant among petroleum organic geochemists concerning the thermal fate of hydrocarbons, with subsequent graphite formation, and greenschist metamorphism, are in sharp contradiction to these data. Conventional concepts of the distribution of heavy hydrocarbons with increasing temperature and depth apparently require further review and revision.