Lawrence J. Drew

The Bayan Obo iron-rare-earth-niobium deposits, Inner Mongolia, China

Abstract The plate tectonic setting, regional geology and certain aspects of the economic geology of the iron-rare-earth-niobium ore bodies at Bayan Obo, Inner Mongolia, China, were studied by a team of geologists from the Tianjin Geologic Research Academy and the U.S. Geological Survey between 1987 and 1989. These ore bodies were formed by hydrothermal replacement of Middle Proterozoic dolomite in an intra-continental rift setting. A variety of veins and/or dikes that have a carbonatitelike mineralogy cut the footwall clastic rocks and migmatites. A stockwork of veins occurs at several locations in the footwall. The hanging wall is a shale that has been converted to a K-metasomatite and has microcrystalline potassium feldspar as its principal constituent. This shale served as a sealing caprock that contained the chemical solutions that reacted with the dolomite and created the enormous concentration of mineralized rock in an 18-kilometer-long syncline. The rocks that host these ore bodies and the associated mineralized areas occur today as roof pendants in granitoid rocks of Permian age that were emplaced during a continent-to-continent collision during that period.

A procedure to estimate the parent population of the size of oil and gas fields as revealed by a study of economic truncation

An estimation technique has been derived to predict the number of small fields in a geologic play or basin. Historically, many small oil and gas fields went unreported because they were not economical. This led to an underestimation of the number of undiscovered small fields. A study of the distributions of reported oil and gas fields in well-explored areas suggests that the large fields when grouped into log base 2 size classes are geometrically distributed. Further, the number of small fields reported is a function of the cost of exploration and development. Thus, the population field-size distribution is conjectured to be log geometric in form.

Geology and structural evolution of the Muruntau gold deposit, Kyzylkum desert, Uzbekistan

Abstract The Muruntau gold deposit in the Kyzylkum desert of Uzbekistan is the largest single deposit (⪢ 1100 tonnes of gold) of the class of low-sulfide syndeformation/synigenous gold deposits formed in the brittle/ductile transition zone of the crust within transpressional shear zones. Hosted by the Cambrian to Ordovician Besopan Suite, the ores were deposited in pre-existing thrust-fault- and metamorphism-related permeabilities and in synmineralization dilational zones created in a large fault-related fold. The Besopan Suite is a 5,000-m-thick sequence of turbiditic siltstones, shales and sandstones. The ore is primarily localized at the base of the Besopan-3 unit, which is a 2,000-m-thick series of carbonaceous shales, siltstones, sandstones and cherts. Initial gold deposition took place within the Sangruntau-Tamdytau shear zone, which was developed along the stratigraphic contact between the Besopan-3 and Besopan-4 units. During the mineralization process, folding of the Besopan Suite and a left-step adjustment in the Sangruntau-Tamdytau shear zone were caused by two concurrent events: (1) the activation of the left-lateral Muruntau-Daugyztau shear zone that developed at nearly a 90° angle to the preceding shear zone and (2) the intrusion of granitoid plutons. These structural events also resulted in the refocusing of hydrothermal fluid flow into new zones of permeability.

Environmentalism and natural aggregate mining

Sustaining a developed economy and expanding a developing one require the use of large volumes of natural aggregate. Almost all human activity (commercial, recreational, or leisure) is transacted in or on facilities constructed from natural aggregate. In our urban and suburban worlds, we are almost totally dependent on supplies of water collected behind dams and transported through aqueducts made from concrete. Natural aggregate is essential to the facilities that produce energy—hydroelectric dams and coal-fired powerplants. Ironically, the utility created for mankind by the use of natural aggregate is rarely compared favorably with the environmental impacts of mining it. Instead, the empty quarries and pits are seen as large negative environmental consequences. At the root of this disassociation is the philosophy of environmentalism, which flavors our perceptions of the excavation, processing, and distribution of natural aggregate. The two end-member ideas in this philosophy are ecocentrism and anthropocentrism. Ecocentrism takes the position that the natural world is a organism whose arteries are the rivers—their flow must not be altered. The soil is another vital organ and must not be covered with concrete and asphalt. The motto of the ecocentrist is “man must live more lightly on the land.” The anthropocentrist wants clean water and air and an uncluttered landscape for human use. Mining is allowed and even encouraged, but dust and noise from quarry and pit operations must be minimized. The large volume of truck traffic is viewed as a real menace to human life and should be regulated and isolated. The environmental problems that the producers of natural aggregate (crushed stone and sand and gravel) face today are mostly difficult social and political concerns associated with the large holes dug in the ground and the large volume of heavy truck traffic associated with quarry and pit operations. These concerns have increased in recent years as societys demand for living space has encroached on the sites of production; in other words, the act of production has engendered condemnation. Many other environmental problems that are associated with dust and noise and blasting from quarry and pit operations have been reduced through the efficient use of technology. Recycling concrete in buildings, bridges, and roads and asphaltic pavements will ultimately reduce the demand for virgin natural aggregate. The impact created by the large holes in the ground required for the mining of natural aggregate can be greatly reduced by planning their reclamation before mining begins.

Observed oil and gas field size distributions: A consequence of the discovery process and prices of oil and gas

If observed oil and gas field size distributions are obtained by random samplings, the fitted distributions should approximate that of the parent population of oil and gas fields. However, empirical evidence strongly suggests that larger fields tend to be discovered earlier in the discovery process than they would be by random sampling. Economic factors also can limit the number of small fields that are developed and reported. This paper examines observed size distributions in state and federal waters of offshore Texas. Results of the analysis demonstrate how the shape of the observable size distributions change with significant hydrocarbon price changes. Comparison of state and federal observed size distributions in the offshore area shows how production cost differences also affect the shape of the observed size distribution. Methods for modifying the discovery rate estimation procedures when economic factors significantly affect the discovery sequence are presented. A primary conclusion of the analysis is that, because hydrocarbon price changes can significantly affect the observed discovery size distribution, one should not be confident about inferring the form and specific parameters of the parent field size distribution from the observed distributions.

Statistical variability of the geochemistry and mineralogy of soils in the Maritime Provinces of Canada and part of the Northeast United States

A soil geochemical survey in the Maritime Provinces of Canada and part of the Northeast United States was completed for the North American Soil Geochemistry Landscapes Project. Soil samples, derived largely from unsorted glacial till, were collected over 349 sites, from 0 to 5 cm depth (regardless of horizon), A-, and C-horizons. The 0 to 5 cm depth interval represents the soil of interest in health risk assessments and is termed the Public Health (PH-) layer. The <2 mm fraction of each sample was analysed for a broad suite of major and trace elements using a near-total four-acid digestion, and major mineralogical components were determined by quantitative X-ray diffraction. Multivariate statistical analyses of the logcentred soil geochemistry from the PH-layer and the two soil horizons, and of the soil mineralogy from the A- and C-horizons, reveal distinctive inter-element relationships from deeper soil (represented by the C-horizon) upwards into topsoil (represented by the A-horizon and PH-layer). Statistical dispersion of several elements increases upwards in the soil profile. Maximum data dispersion occurs in the PH-layer and A-horizon soils. Elements including S, P, Pb, Hg, Cd, Se, Mo, Sb, Bi and Sn are relatively enriched in the PH-layer and A-horizon, and are positively correlated with increasing organic carbon contents. The relative enrichment of groups of elements in the C-horizon, in contrast to those elements in the A-horizon and PH-layer, suggests a composition that reflects the geochemistry of the glacial till that is derived from the local bedrock. Elements such as Ni, Mg, Cr, V, Co, Fe and Sc, represent a mafic component of the parent material, and relative enrichments of K, Rb, Zr, rare-earth elements, Li and Al indicate a more felsic component. The patterns revealed by the application of multivariate methods to the soil chemistry and mineralogy are attributed to underlying geology, soil-forming processes, and anthropogenic activity, or combinations of all three factors. Both the soil geochemistry and mineralogy were tested in their ability to predict soil horizon and underlying bedrock lithology or time-stratigraphic assemblages. The geochemistry and mineralogy of the soils are both good for predicting soil horizon; however, the soil geochemistry is better for predicting the underlying lithologies/assemblages than the soil mineralogy.

The Evolution and Use of Discovery Process Models at the U.S. Geological Survey

The development and the application of discovery process models have evolved to the point that these models now can be used to estimate the number of oil and gas fields remaining to be discovered in partially explored regions, and the rates at which fields will be discovered in the future. The fundamental data used to calibrate these models are taken from the systematic order that exists in the historical pattern of discovery in such regions (exploration plays, basins, provinces). The characteristic patterns of discovery vary from region to region, and ordinarily exhibit diminishing rates of return to wildcat drilling as a function of field size. The number of fields remaining to be discovered in each field-size class is estimated by a two stage procedure: direct nonlinea estimation of fields in the portion of the size distribution not truncated by economic factors (larger fields) and indirect estimating by using a relative frequency factor for the field-size classes in the economically truncated portion of the distribution (smaller fields). The forecast of the future rates of discovery then considers the total range of field sizes, on the basis of wildcat wells drilled, by applying the discovery process model to both segments of the estimated size distribution. The application of this procedure is illustrated for a variety of onshore and offshore exploration plays from different geologic environments that have different economic conditions (size truncation points).

A petroleum discovery-rate forecast revisited—The problem of field growth

A forecast of the future rates of discovery of crude oil and natural gas for the 123,027-km2 Miocene/Pliocene trend in the Gulf of Mexico was made in 1980. This forecast was evaluated in 1988 by comparing two sets of data: (1) the actual versus the forecasted number of fields discovered, and (2) the actual versus the forecasted volumes of crude oil and natural gas discovered with the drilling of 1,820 wildcat wells along the trend between January 1, 1977, and December 31, 1985. The forecast specified that this level of drilling would result in the discovery of 217 fields containing 1.78 billion barrels of oil equivalent; however, 238 fields containing 3.57 billion barrels of oil equivalent were actually discovered. This underestimation is attributed to biases introduced by field growth and, to a lesser degree, the artificially low, pre-1970s price of natural gas that prevented many smaller gas fields from being brought into production at the time of their discovery; most of these fields contained less than 50 billion cubic feet of producible natural gas.

Aggregation of carbon dioxide sequestration storage assessment units

The U.S. Geological Survey is currently conducting a national assessment of carbon dioxide (CO2) storage resources, mandated by the Energy Independence and Security Act of 2007. Pre-emission capture and storage of CO2 in subsurface saline formations is one potential method to reduce greenhouse gas emissions and the negative impact of global climate change. Like many large-scale resource assessments, the area under investigation is split into smaller, more manageable storage assessment units (SAUs), which must be aggregated with correctly propagated uncertainty to the basin, regional, and national scales. The aggregation methodology requires two types of data: marginal probability distributions of storage resource for each SAU, and a correlation matrix obtained by expert elicitation describing interdependencies between pairs of SAUs. Dependencies arise because geologic analogs, assessment methods, and assessors often overlap. The correlation matrix is used to induce rank correlation, using a Cholesky decomposition, among the empirical marginal distributions representing individually assessed SAUs. This manuscript presents a probabilistic aggregation method tailored to the correlations and dependencies inherent to a CO2 storage assessment. Aggregation results must be presented at the basin, regional, and national scales. A single stage approach, in which one large correlation matrix is defined and subsets are used for different scales, is compared to a multiple stage approach, in which new correlation matrices are created to aggregate intermediate results. Although the single-stage approach requires determination of significantly more correlation coefficients, it captures geologic dependencies among similar units in different basins and it is less sensitive to fluctuations in low correlation coefficients than the multiple stage approach. Thus, subsets of one single-stage correlation matrix are used to aggregate to basin, regional, and national scales.

Is there a metric for mineral deposit occurrence probabilities

Traditionally, mineral resource assessments have been used to estimate the physical inventory of critical and strategic mineral commodities that occur in pieces of land and to assess the consequences of supply disruptions of these commodities. More recently, these assessments have been used to estimate the undiscovered mineral wealth in such pieces of land to assess the opportunity cost of using the land for purposes other than mineral production. The field of mineral resource assessment is an interdisciplinary field that draws elements from the disciplines of geology, economic geology (descriptive models), statistics and management science (grade and tonnage models), mineral economics, and operations research (computer simulation models). The purpose of this study is to assert that an occurrenceprobability metric exists that is useful in “filling out” an assessment both for areas in which only a trivial probability exists that a new mining district could be present and for areas where nontrivial probabilities exist for such districts.