About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 931

Last Page: 932

Title: Alteration of Organic Matter and Generation of Hydrocarbons in Austin Chalk, Southeast Texas: ABSTRACT

Author(s): George J. Grabowski, Jr.

Article Type: Meeting abstract


Kerogen in the Upper Cretaceous Austin Chalk from the subsurface of southeastern Texas is predominantly amorphous and sapropelic, with minor amounts of spores, pollen, and woody fragments. Over a range of 7,000 ft (2,134 m), the amorphous kerogen increases in maturity from a T.A.I. of between 1+ and 2- (medium yellow) at 2,300 ft (701 m; present depth below surface), to a maximum of between 2+ and 3- (orange-brown to light brown) at 9,100 ft (2,744 m). This

End_Page 931------------------------------

interval spans most of the zone of oil generation, as supported by a continuous decrease in H/C atomic ratios of the kerogen and hydrogen index from pyrolysis of the whole rock. The ratio of extractable organic matter (bitumen) to total organic carbon increases with depth, recording the formation and reservoiring of hydrocarbons in the chalk. The extractable organic matter becomes enriched in saturated hydrocarbons at the expense of non-hydrocarbons with increasing depth of burial, while the saturated hydrocarbons themselves become more like crude-oil hydrocarbons and less like immature, bitumen hydrocarbons in the deepest samples. The Austin Chalk appears to have acted as a source rock for at least part of the crude oil reservoired in and produced from the formation.

End_of_Article - Last_Page 932------------

Copyright 1997 American Association of Petroleum Geologists