Occurrence of Indigenous Biogenic Gas in Organic-Rich, Immature Chalks of Late Cretaceous Age, Eastern Denver Basin
Dudley D. Rice, 1984. "Occurrence of Indigenous Biogenic Gas in Organic-Rich, Immature Chalks of Late Cretaceous Age, Eastern Denver Basin", Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks, James G. Palacas
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Natural gas is produced at shallow depths from chalk beds of the Upper Cretaceous Niobrara Formation in northeastern Colorado and northwestern Kansas. The depth of the gas-productive trend increases to the northwest from 250 to 850 m (820–2,790 ft). The chalks are fine-grained limestone, consisting mainly of calcareous nannofossils and other microfossils, that are characterized by high porosity (30–45%) and low permeability (about 1 md). Core samples from the productive area average 30% acid-insoluble residue. Most of the insoluble residue, which consists of clay minerals and organic matter, is concentrated in alternating laminations.
The gases are methane-rich (C1/C1–5 > 0.98), are enriched in the light isotope 12C (δ13C1 values range from –65 to –55 ppt), and become isotopically heavier with increasing depth across the trend. The shallow gas in the Niobrara is interpreted to be of biogenic rather than thermogenic origin because of its chemical and isotopic composition and of source-rock studies indicating that Upper Cretaceous rocks in this region are immature with respect to thermogenic hydrocarbon generation. To the northwest, however, these rocks are mature and were capable of generating oil at the time of maximum burial and/or heat flow.
The biogenic gas was generated early in the burial history of the chalks by microbial degradation of organic matter in an anaerobic, presumably sulfate-free environment. In-situ gas generation is indicated, because low permeability inhibited long-range migration and because organic-rich laminae provided an adequate source for the gas. Organic-carbon values average 3.2%, and the organic matter consists primarily of hydrogen-rich sapropelic kerogen (type II), typical of an open-marine environment. The chalks are overlain by a thick section of shale containing many bentonite beds in the lower part that served as a seal for gas after the reservoirs were naturally fractured later in the burial history.
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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.