Occurrence of Indigenous Biogenic Gas in Organic-Rich, Immature Chalks of Late Cretaceous Age, Eastern Denver Basin

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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 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 (13C1 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.