Geology topics
Publications and source records attributed to J. W. Schmoker.
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Describes dolomite, gypsum, and sandstone units deposited in transgressive-regressive cycles. Three depositional cycles are partly exposed. The cycles observed in outcrop were informally labelled cycle 1, cycle 2, and cycle 3 in ascending stratigraphic order. West Mellott field represents a subsurface example of the facies and facies relationships observed in outcrop. The eolian-dune sandstone of the C cycle, which was partially reworked by the transgression of the B cycle, produces oil at West Mellott. The draping of dolomite and anhydrite of the B cycle on the eolian-dune sandstone of the C cycle is analogous to the draping gypsum on dune sand in cycle 2 in outcrop. -from Authors
This study examines relationships between porosity and time-temperature history, and the influence of rock properties upon porosity, for the Lower Cretaceous J Sandstone in the Colorado portion of the Denver basin. The J Sandstone is classified as a quartzarenite to litharenite and was deposited in nearshore-marine, deltaic, and fluvial-estuarine (valley-fill) settings. Principal elements of its paragenetic sequence include quartz cementation and pressure solution, carbonate cementation and dissolution, dissolution of feldspar and rock fragments, and formation of authigenic clays. Porosity versus vitrinite reflectance (R 0 ) regression lines of the form Phi = A(R 0 )Beta (where B is a negative number) depicting the 10th, 25th, 50th, 75th, and 90th porosity percentiles of the J Sandstone were derived from 963 core-plug measurements representing 31 wells. The data span a thermal maturity range of R 0 = 0.41%-1.14%. Porosity distributions at different locations within the basin can be estimated as a function of thermal maturity on the basis of these regression lines. Porosity trends of the J Sandstone, if considered as a function of R 0 , are similar to those of broad, composite data sets representing sandstones in general. The petrographic factors that most affect J Sandstone porosity variability at a given level of thermal maturity are carbonate cementation and clay content. Carbonate cement, where present, reduces porosity. If previously more widespread, carbonate cement could also introduce porosity heterogeneity by temporarily preserving the pore network relative to uncemented intervals. Abundant detrital and authigenic clay reduces porosity by occupying pores. Low clay content indirectly reduces porosity because the inhibiting effects of clay upon quartz cementation and pressure solution are largely absent.
Sandstones and dolomites of the Permian upper part of the Minnelusa Formation are treated here as four-component systems consisting of fluid-filled pore space, quartz, dolomite, and anhydrite. Response equations of sonic, neutron, and density logs form a system of four simultaneous equations. With four equations and four unknowns, the composition of upper Minnelusa facies is defined by the three-log suite and can be calculated by solving a 4 ?? 4 matrix. Such calculations of facies composition help in establishing subsurface correlations and yield information on the diagenesis and physical character of upper Minnelusa sandstones and dolomites. Applications of composition calculations are illustrated by examples drawn from the area of the West Mellott field (T52N, R68W), where the upper Minnelusa is at depths of about 7000 ft (2100m). -from Authors
Upper Minnelusa sandstones form a complex group of reservoirs because of variations in regional setting, sedimentology, and diagenetic alteration. Structural lineaments separate the reservoirs into northern and southern zones. Production in the north is from a single pay sand, and in the south from multi-pay sands due to differential erosion on top of the Upper Minnelusa. The intercalation of eolian dune, interdune, and sabkha sandstones with marine sandstones, carbonates, and anhydrites results in significant reservoir heterogeneity. Diagenetic alterations further enhance heterogeneity, because the degree of cementation and dissolution is partly facies-related.
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During field studies conducted in October 1977 and March 1978, borehole gravity surveys were completed by the U.S. Geological Survey at the Nevada Test Site in test well Ue19z, exploratory drill hole PH-1, and water well 5a. The U.S. Geological Survey-LaCoste and Romberg-1/ borehole gravity meter (McCulloh and others, 1967a; McCulloh and others, 1967b) was used in the logging program. The primary objective of this work was to obtain data for the determination of accurate in situ formation densities utilizing an instrument not significantly affected by casing, borehole rugosity, or other near-borehole conditions. This report provides a brief summary of the geology of the rock formations penetrated by drill holes Ue 19z, PH-1, and water well 5a, and tabulates the subsurface gravity data obtained in these, wells. Reduced sections of the gamma-ray logs run in PM-1 and 5a are also included.