West-east Cross-section Research Articles

Noble gas geochemistry and chronology of groundwater in an active rift basin in central China

Stable noble gas isotopes are excellent groundwater tracers. Radioactive noble gases are emerging new tools in the study of groundwater circulation dynamics. Among these, the 85Kr and 81Kr, and 39Ar have advanced very fast in recent years and exhibit strong potential in the reconstruction of the history of groundwater recharge and evolution in sedimentary basins at different scales. Here, we report the findings in groundwater circulation dynamics as relative to intensive water-rock interactions, heat transfer and He gas flux in Guanzhong Basin located in Xi’an, the geographical centre of China, which is a rift basin created by collision between the Eurasia and Indian plates, with active neotectonic activities. The recent technological breakthrough in noble gas isotope measurements, i.e. the atomic trap trace analysis (ATTA) techniques on Kr and Ar gas radionuclei, has revolutionized groundwater dating. Noble gas samples from shallow and deep wells to 3000 m depth have been collected to study isotope variations to reconstruct the history of groundwater recharge and understand the water-rock interaction processes. Stable isotopes of water show strong water-rock interaction in the formation, creating a strong positive O-isotope shift up to 10 ‰, a phenomenon that is rarely seen in a fairly low temperature environment. Analysis of 85Kr and 81Kr show groundwater ages up to 1.3 million years old along both North-South and a West-East cross sections, which offers strong evidence about the slow moving flow, strong water-rock interaction, rich geothermal resources as well as He gas resources.

Constraining a mafic thick crust model in the Andean Precordillera of the Pampean flat slab subduction region

Elastic properties in twelve representative rock samples from Central and Western Precordillera in the Andean backarc region of Argentina between 30 and 31°S were estimated from detailed petrological analysis. Thus, P and S seismic-wave velocities (Vp, Vs) as well as Poisson's ratio (σ) among other parameters were derived for gabbros, leuco-gabbros and wehrlites, in greenschist and amphibolite metamorphic conditions using a framework of a wide variety of empirical observations from active continental margins. In addition, Vs lithosphere models along two west–east cross sections were obtained using a joint inversion of teleseismic Rayleigh waves and receiver functions. These models clearly delineate the upper-plate crustal structure and the flat-slab subduction of the Nazca plate at about 100 km depth in this region. The suggested seismic velocity structure shows a relatively low (<3.3 km/s) Vs layer located in the first 15–18 km depth, then an increase of it from 3.3 to 4 km/s between 20 and 55 km depth with a mayor change at 40 km depth beneath the Precordillera showing an increase in Vs from 3.3 to 3.8 km/s. The Moho discontinuity was identified at around 65 km depth beneath the Precordillera (Vs = 4.3 km/s) and shows a low shear-wave velocity contrast in comparison with the upper continental mantle's parameters. Using this seismological model, Vs estimations derived from the petrological analyses for the 12 collected samples can be projected at depths greater than 30 km. These geophysical and petrological results agree with the hypothesis of a mafic thickened and partially eclogitized lower crust beneath the Precordillera, which has been predicted previously on a base of seismological studies only. Our petrological and seismological results collectively support a thick crustal model of a mafic–ultramafic composition extending to middle-to-lower crustal levels beneath Central and Western Precordillera; this region correlates with a suture zone between the eastern Cuyania terrane and the western Chilenia terrane.

Application of the authigenic 10Be/ 9Be dating method to continental sediments: Reconstruction of the Mio-Pleistocene sedimentary sequence in the early hominid fossiliferous areas of the northern Chad Basin

The concentrations of atmospheric cosmogenic 10Be normalized to the solubilized fraction of its stable isotope 9Be have been measured in the authigenic phase leached from silicated continental sediments deposited since the upper Miocene in the northern Chad Basin. This method is validated by the systematic congruence with the biochronological estimations based on the fossil mammal evolutive degree of faunal assemblages. The fifty-five authigenic 10Be/ 9Be ages obtained along 12 logs distributed along two West–East cross sections that encompass best representative Mio-Pliocene outcrops including paleontological sites show a systematic stratigraphic decrease when considering all studied sedimentary facies extending from the Pleistocene up to 8 Ma and allow performing geologic correlations otherwise impossible in the studied area. The resulting global sequence evidences and temporally specifies the succession of the main paleoenvironments that have developed in this region since the Miocene. Under the special conditions encountered in the northern Chad Basin, this study demonstrates that the authigenic 10Be/ 9Be ratio may be used as a dating tool of continental sedimentary deposits from 1 to 8 Ma. The half-life of 10Be theoretically allowing dating up to 14 Ma, it may have fundamental implications on important field research such as paleoclimatology and, through the dating of fossiliferous deposits in paleontology and paleoanthropology.

Multi-Well Subsurface Study of the Stratigraphic Sequence of the Potter Formation in the Midway-Sunset Field, Kern County, California: ABSTRACT

To understand the stratigraphic nature of the Potter Formation reservoirs, a multi-well study along a west-east cross-section was done using dipmeter, Formation MicroScanner, and FMI[trademark] (Fullbore Formation Microlmager) data. Thermal EOR wells are drilled only into the heavy oil-bearing portion of the reservoir, thus each well does not penetrate the complete Potter sequence, but penetrates successively younger sandstone and conglomerate members from west to east. Wireline logs and continuous core data were used for correlation and calibration of the dip and image data. The image data revealed detailed stratification, bedding, and sedimentary structures, identifying conglomeratic, bioturbated and stratified sequences. Correlation with continuous core clarified which features were more interpretive. The image data, available on three of the eleven wells, was then correlated to dipmeter data, both in the form of computed dip data and interpolated electrical images. Each of the eleven wells penetrated only one or two members of the Potter Formation. Using the dip data, it was possible to reconstruct a true stratigraphic thickness type log of the Potter Formation. The interpreted stratigraphic data, revealing bedded, bioturbated, and conglomeratic depositional systems was combined into a single type log of the Potter Formation. The resultant type log can be usedmore » for various sequence stratigraphic, engineering, and thermal EOR reservoir characterization applications in the Midway-Sunset Field.« less

Summer Circulation Patterns, Northern Smith Sound

The Naval Ocean Research and Development Activity conducted an oceanographic research program in northern Smith Sound during middle July 1977 using USCGC Westwind (W-AGB 281) as the data collection platform. One aspect of the program was to investigate the water mass interactions at the narrowest portion of Smith Sound which: (1) represents the boundary between this sound and Kane Basin to the north; and (2) is the area of the most well-known semipermanent open water area - the North Water Polynya. From an analysis of 2 West-East cross sections and a comparison of these data with data collected by the Coast Guard in 1963, it is suggested that the northern Smith Sound area is a meeting area of two different water masses: (1) a warm (&gt;-1.0&deg;C), dense (&gt;26.8 sigma tau) mass on the eastern side of Smith Sound which enters the area from the south; and (2) a cold (&lt;-1.0 deg. C), low density (&lt;26.7 sigma tau) mass on the western side of Smith Sound which originates in the Arctic. It is further suggested that since the warm, dense water mass was also observed in 1963, this feature in eastern Smith Sound may be a permanent or, more likely, a semipermanent (seasonal) phenomenon and could be highly modified Atlantic Water. This latter conclusion is in contrast to the general belief that Atlantic water does not enter Smith Sound.

Depositional Topography and Cyclic Deposition in West-Central Texas

The terminology introduced by Rich (1951) can be applied to the subsurface, post-Canyon rocks of west-central Texas. Rich's term denotes an environment within the zone of wave action; clino denotes a slope environment, and denotes an environment in the deepest part of a sea. West-east cross sections in the subject area show a cyclic development of the unda, clino, and fondo environments advancing generally westward as the Midland basin was filled. The filling of the Midland basin was accomplished by repeated, alternating phases of deposition; a clastic or sand-shale phase and a carbonate-black shale phase. The term is used to describe such deposition and includes rocks formed in all three environments: unda, clino, and fondo. Each phase of d position was probably related to sea-level. Turbidity currents were possibly responsible for transporting material that was formerly of unda origin to a topographically lower environment. Several oil fields of west-central Texas produce from cyclo-phase sandstones. These sandstones are locally called Canyon sands but it can be shown that they are actually Cisco or younger in age. Stratigraphic, structural, and depositional conditions are variously responsible for hydrocarbon accumulation in certain cyclo-phase fields of west-central Texas. Anticlinal conditions are responsible for trapping in the undaform sandstone of the Group 4,000^prime pool. An updip extension of the sand body is responsible for accumulation in the clino-fondoform sandstones of the Lake Trammel and Sweetwater Canyon Sand fields. The configuration of the undaform edge encouraged migration of oil into the clinofondoform sandstone of the Flowers field. Local thickening of an undaform sandstone forms a trap in the North Hamlin field. Because of the cyclic nature of the upper Pennsylvanian-lower Permian deposition, many more fields of the types discussed may be anticipated in the cyclo-phase sandstones of west and west-central Texas.

Report on the New West-East Cross Section Prepared by the Panhandle Geological Society: ABSTRACT

This paper consists of a discussion of the stratigraphy of the Texas Panhandle district as revealed by a west-east cross section prepared by the Stratigraphic Committee of the Panhandle Geological Society. The cross section includes seventeen wells and extends from Union County, New Mexico, to Ellis County, Oklahoma, crossing their previously constructed north-south cross section at the Sinclair No. 1 Lips in Roberts County, Texas. End_of_Article - Last_Page 2609------------

Stratigraphy and Structural History of East-Central United States

This article contains a discussion of the regional stratigraphic changes as revealed from studies of outcrops and cuttings from approximately 525 wells in the following states: Michigan, Ohio, Indiana, Illinois, Kentucky, and Tennessee. Three west-east cross sections have been prepared in order to show graphically these structural and stratigraphic changes. A tentative correlation table and seven logs made from studies of well cuttings are included as an aid to future students in the area. The structural history which began in pre-Cambrian time, continued with minor pulsations in post-Canadian, post-St. Peter, post-Brassfield, post-Chattanooga, post-St. Genevieve, and post-Pennsylvanian time. One major period of folding occurred in post-Mississippian pre-Pennsylvanian time.

Buckeye Field, Matagorda County, Texas

The Buckeye field is on a very deep-seated domal uplift; it covers about 800 acres, according to the surface indications and a magnetometer survey. The first well drilled came in as a producer, August 31, 1932. Seven wells have been completed, and of these, two were commercial producers. The wells on a west-east cross section indicate that the uplift may cover less than 800 acres. A study of the subsurface reveals that the Upper Catahoula is undoubtedly of Miocene age. The oil horizon occurs in the upper part of the Lower Catahoula (Frio), at the depth of approximately 7,800 feet. This was the deepest commercial oil production in the Texas Gulf Coast at the time of its discovery. The field produced 362,397 barrels before the wells stopped flowing, and is still producing s all quantities of oil on the pump. The field has been subdivided into 20-acre units.