Generation and Migration of Hydrocarbons in Upper Cretaceous Austin Chalk, South-Central Texas
George J. Grabowski, Jr., 1984. "Generation and Migration of Hydrocarbons in Upper Cretaceous Austin Chalk, South-Central Texas", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
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The Austin Chalk of south-central Texas was deposited on a carbonate ramp marginal to the Gulf of Mexico during the Late Cretaceous Epoch. Dark-colored, laminated, and sparsely burrowed chalks containing > 1.5% total organic carbon (TOC) were deposited under disaerobic conditions and predominate in deep, basinal cores. Light-colored, thoroughly bioturbated chalks containing < 1.5% TOC formed in oxygenated shelfal environments and predominate in shallow cores.
The kerogen in the Austin Chalk is uniformly composed primarily of amorphous, type I or II material irrespective of carbonate content or lithology. The kerogen occurs disseminated in the dark-colored chalks and concentrated in microstylolites and stylolites. Extractable organic matter (EOM) is most abundant in porous chalk between zones of pressure solution.
EOM in shallow cores (< 5,000 ft or 1,525 m) contains 70% NSO (nitrogen, sulfur, and oxygen) compounds and 30% hydrocarbons. The saturated hydrocarbons are dominated by geochemical fossils in these shallow cores. With increasing depth of burial, the EOM becomes enriched in hydrocarbons, up to 60–80% in the deepest core (9,100 ft or 2,780 m). These hydrocarbons formed from alteration of kerogen and NSO compounds: the kerogen becomes progressively condensed and less aliphatic with increasing depth of burial. This alteration is independent of the carbonate content of the rocks. Peak oil generation occurs between 5,000 and 8,000 ft (1,525 and 2,440 m), with gaseous hydrocarbons becoming abundant below 8,000 ft (2,440 m).
EOM that was formed during catagenesis migrated in the Austin Chalk through micropores to zones of higher porosity. This migration caused enrichment of the EOM in both absolute amounts and relative proportions of hydrocarbons and resins in porous chalks. Larger molecules, such as asphaltenes, did not readily migrate in the chalk. Migration of EOM occurred at all depths in the Austin Chalk but was most prevalent in mature chalks below 5,000 ft (1,525 m). Changes in the composition of EOM owing to migration are independent of carbonate content or lithology and are dependent primarily on the maturity of the rock. The light (> 39° API) crude oils produced from deep wells in the Austin Chalk are similar to and may be formed from migrated EOM.
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Carbonate rocks have diverse characteristics. They can be excellent reservoirs as well as prolific source rocks for oil. Oils from carbonate rocks commonly have distinctive bulk chemical and molecular characteristics that reveal their origin. The lack of widespread appreciation for these facts in the geological community was one reason that a symposium entitled “Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks” was organized and held at the Geological Society of America annual meeting in Atlanta, Georgia, in October 1980. The symposium was sponsored by the Organic Geochemistry Division of the Geochemical Society during my term as chairman of the division. Of the 18 papers given in the symposium, 12 papers and four abstracts are included herein. Also included in this volume are two papers that were prepared later.
I hope that this collection of original papers, which synthesize data from about 20 different sedimentary basins, will help to correct any lingering misconceptions concerning the effectiveness of carbonate rocks as major sources of petroleum. I also believe that the information presented herein, including the references, will serve as a valuable resource for evaluating petroleum occurrence in other carbonate sequences and for locating petroleum reserves in unexplored, partially explored, and even maturely explored basins where possible carbonate-generated oil and gas may have been overlooked.
The first 11 papers, arranged in geo-chronological order, are descriptions and interpretations (that is, case histories) of specific carbonate source rocks that range in age from Precambrian to Miocene. Some of the highlights of these papers are summarized below.
The paper by Fu Jia Mo, Dai Yong Ding, Liu De Han, and Jia Rong Fen, in addition to describing the geochemistry of petroleum accumulations and source rocks ranging in age from Precambrian to Triassic, points out some interesting differences in thermal histories of Precambrian carbonate-rock sequences in eastern China. In one basin, Precambrian carbonate rocks are surprisingly thermally immature and have yielded heavy oils and asphalts. In another basin, on the other hand, Precambrian carbonate rocks are definitely overmature and have generated methane-rich gas.
The paper by McKirdy, Kantsler, Emmett, and Aldridge on the Eastern Officer basin, South Australia, includes the first reported examples of nonmarine carbonate rocks and oils of Cambrian age that are similar to those of the Eocene Green River Formation, Utah.
In their study of crude oils in the Michigan basin, Gardner and Bray indicate that the interreef, laminated carbonate rocks of Silurian age are the primary source of commercial oil accumulations in the Silurian pinnacle reefs.