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D. L. Gautier

Publications and source records attributed to D. L. Gautier.

At least 37 records · Page 2Linked to original sources
USGS1988

Sandstone porosity as a function of thermal maturity

Sandstone porosity decreases in the subsurface as a power function of thermal maturity: ϕ = A( M ) B , where ϕ is porosity and M is a measure of thermal maturity representing integrated time-temperature history; A and B are constants for a given sandstone of homogeneous properties but vary between data sets. The commonly observed exponential dependence of sandstone porosity upon depth follows as a special case from this power-function relation when temperature increases linearly with depth. The consideration of sandstone porosity in terms of time-temperature exposure offers advantages in the comparison of porosity data from diverse geologic settings, the recognition of unusual porosity within a sandstone sequence, and the prediction of porosity ahead of the drill and at times in the geologic past.

Geology↗
USGS1987

Isotopic composition of pyrite: Relationship to organic matter type and iron availability in some North American cretaceous shales

The S isotope composition of pyrite in Cretaceous shales from the Western Interior of North America is related to organic C abundance, kerogen type and Fe availability. Both calcareous and noncalcareous rocks show a correlation between S and C, but noncalcareous rocks are relatively enriched in S with a higher S/C ratio. This higher ratio probably shows that pyrite formation was Fe limited in the calcareous rocks. Organic-carbon-rich noncalcareous shales accumulated slowly beneath anoxic bottom waters. The anoxic bottom waters allowed hydrogen-rich organic matter to be preserved. Such shales have a narrow range of 34 S-depleted sulfide and have Fe/S ratios like stoichiometric pyrite, suggesting that pyrite formation in organic-rich shales was also limited by Fe availability. Conversely, organic-poor shales commonly accumulated at comparatively high rates, contain hydrogen-poor and refractory organic matter, and have a wide range of pyrite-S isotopic compositions. These organic-poor shales contain post-sulfidic authigenic minerals such as siderite and have excess reactive Fe rather than pyrite stoichiometry. Evidently Fe played a large role in early diagenesis and determined the course of post-sulfidic diagenesis. Fe availability was, however, mainly controlled by provenance, by the rates of sediment accumulation, and by the oxygen content of the depositional environment.

Chemical Geology: Isotope Geoscience Section↗
USGS1987

Median-porosity contour maps of the J Sandstone, Dakota Group, in the Denver Basin, Colorado, Nebraska, and Wyoming

The Lower Cretaceous J sandstone of the Dakota Group is present in the Denver basin in eastern Colorado, southeastern Wyoming, and southwestern Nebraska. Deposited during a regression of the Cretaceous epicontinental sea, this informally named unit is composed primarily of sandstone and shale of deltaic and near shore-marine origin. The J sandstone can be divided into an upper transgressive sand, a middle marginal-marine and deltaic facies, and a lower prodelta sequence (Clark, 1978). The depth from the surface to the top of the J sandstone increases from about 4,000 ft on the gently-dipping eastern flank of the basin to more than 8,000 ft at the basin ax is near the steeply-dipping western flank. Porosity data compiled in this study were determined from J sandstone cores from 134 widely spaced boreholes. Porosity in areas of poor core coverage was determined from neutron density logs from an additional 20 boreholes ( corrected to core average grain density). Median, rather than average, porosity was used in order to minimize the statistical effect of anomalously high and low porosity values. Thirty-five oil companies and independent operators supplied core porosity data. Core porosities were determined by means of helium porosimetry, primarily by Core Laboratories of Denver, Colo.

Colorado, Nebraska, Wyoming↗
USGS1986

Cretaceous shales from the western interior of North America: Sulfur/carbon ratios and sulfur-isotope composition

Carbon and sulfur abundance and δ 34 S of pyrite sulfur were studied in cores of selected Cretaceous marine shales from the western interior of North America. Sulfur/carbon ratios average 0.67, a value greater than that observed in recent marine sediments and much higher than global values calculated for the Cretaceous. Increased S/C ratios probably result from generally low levels of bioturbation and enhanced efficiency of sulfate reduction due to low oxygen levels in the Cretaceous seaway. Isotopic compositions of pyrite sulfur vary systematically with the level of oxygenation of the depositional environment and therefore with organic carbon abundance and type of organic matter. Samples with organic carbon in excess of 4 wt% contain disseminated pyrite that is extremely depleted in 34 S (mean δ 34 S = −31‰); these samples are laminated clay shales that contain hydrogen-rich (type II) organic matter. Samples containing less than 1.5% organic carbon display relatively “heavy” but wide ranging δ 34 S values (δ 34 S = −34.6‰ to +16.8‰; mean δ 34 S = −12.4‰); these samples are highly bioturbated and contain only type III, hydrogen-poor organic matter. Samples containing intermediate amounts of organic carbon contain pyrite with δ 34 S values averaging −25.9‰ and contain mixed type II and type III organic matter. The higher organic carbon content and the preservation of hydrogen-rich organic matter generally correlate with slow sedimentation. Samples rich in organic carbon and containing isotopically “light” sulfide sulfur accumulated beneath anoxic and perhaps sulfidic bottom waters. Samples with intermediate organic matter content and intermediate sulfur isotopic compositions accumulated under mainly dysaerobic bottom waters. Samples with relatively low amounts of organic carbon and wide-ranging but less negative sulfur isotopic values were deposited beneath oxygenated bottom waters. Sulfur-isotope data are apparently a sensitive indicator of diagenetic or depositional facies of fine-grained Cretaceous rocks in the western interior.

Geology↗
USGS1986

Median-permeability contour maps of the J sandstone, Dakota Group, in the Denver Basin, Colorado, Nebraska, and Wyoming

The Lower Cretaceous J sandstone of the Dakota Group (MacKenzie, 1965) is present in the Denver basin in eastern Colorado, southeastern Wyoming, and southwestern Nebraska. This informally named unit deposited during a regression of the Cretaceous epi continental sea and is composed primarily of sandstone and shale of deItaic and near-shore marine origin. The J sandstone can be divided into an upper transgressive sand, a middle marginal-marine and deltaic facies, and a lower prodelta sequence (Clark, 1978). The depth from the surface to the top of the J sandstone increases from about 4,000 ft on the gently-dipping eastern flank of the bass n to more than 8,000 ft near the steeply-dipping flank at the western boundary. Permeability values compiled in this J sandstone study were determined from cores from 134 widely scattered boreholes. Median permeabilities, rather than average permeabilities, were used in order to minimize the effect of anomalous samples. Thirty-five oil companies and independent operators supplied core data. Core Laboratories of Denver, Colorado analyzed the core, which was submitted over a period of 25 years.

Colorado, Nebraska, Wyoming↗