Lower Sandstone Research Articles

Process Mineralogy Study on a Sandstone Copper Ore

In this thesis the object of study is a low grade sandstone copper with complex composition. Through the process mineralogy research on a sandstone copper ore,the result showed that the primary sulfide copper minerals are tetrahedrite and chalcopyrite; secondary sulfide copper minerals are bornite, chalcocite and azurite; the main oxide copper mineral is malachite; the main gangue mineral is quartz. The grade of copper is 0.77% and the oxidation rate is 45.45% in the run of mine, it is an oxide copper ore with not thoroughly oxidation. There is a part of disseminated particle size of copper minerals is relatively fine. Mainly sulfide copper minerals are paragenesis with dolomite and pyrite; oxide copper minerals are often blended with limonite. So the key of obtaining the good flotation index is selecting reasonably grinding fineness and making the part of fine-grained disseminated copper full liberation.

Late Paleoproterozoic geodynamics of the North China Craton: Geochemical and zircon U–Pb–Hf records from a volcanic suite in the Yanliao rift

The Yanliao rift zone along the northern margin of the North China Craton exposes a suite of volcanic rocks comprising trachybasalts and trachytes interlayered with dolostones in the Tuanshanzi Formation of the Changcheng Group. The overlying Dahongyu Formation is composed of olivine basalts, trachybasalts, trachyandesites, trachytes, and minor sub-alkaline basalts, intercalated with a lower sandstone and an upper dolostone sequence. LA-ICP-MS zircon U–Pb age data from the volcanics reveal that the Tuanshanzi Formation was deposited at ca. 1670Ma, and the Dahongyu Formation at ca. 1625–1664Ma. Combined with previous studies, these new data indicate that the lowest boundary age of the Changcheng Group, marking the timing of initiation of the Yanliao rift, is ca. 1680Ma. Except for the sub-alkaline basalts, the other alkaline volcanic rocks of the two formations are characterized by high Nb/Y ratios (1–3), and possess SiO2 contents of 43.3–60.5wt.%, K2O of 0.72–15.69wt.%, MgO of 0.10–8.56wt.%, and with Mg# of 2–64. These rocks also display strongly fractionated REE patterns and generally positive Nb and Ta anomalies, with high (Nb/La)PM and (Nb/Th)PM values of 0.85–1.39 and 0.96–2.05, respectively. They show positive zircon εHf(t) values (+0.1 to +1.8 for the trachybasalt sample 11PG31-2) and whole-rock εNd(t) values (+0.83 to +2.25). These geochemical features, together with trace element modeling, suggest that the alkaline volcanic rocks were generated by fractional crystallization of magma derived from low-degree partial melting of an OIB-like depleted asthenospheric mantle source, with only minor involvement of melts generated from the overlying enriched lithospheric mantle source. In contrast, the sub-alkaline basalts are characterized by low Nb/Y ratios of 0.09–0.19, and show fractionated REE patterns and pronounced negative Nb–Ta anomalies with low (Nb/La)PM and (Nb/Th)PM values of 0.07–0.21 and 0.10–0.33, respectively. In combination with the positive zircon εHf(t) values (+0.8–+4.3), we infer that these rocks were derived from the partial melting of a depleted lithospheric mantle source previously metasomatized by fluids from subducted slab. Combined with previous studies on the 1680–1780Ma magmatic rocks along the northern margin of the NCC, our study identifies a late Paleoproterozoic asthenospheric upwelling in this region. The generation of the Yanliao rift and the eruption of these volcanic rocks were possibly triggered by the delamination of the continental lithosphere in a post-orogenic setting following the final amalgamation of the North China Craton within the Columbia supercontinent.

Research and Application of a Novel Deep Profile Modification Agent for Complex Sandstone Reservoirs

According to low temperature, high salinity, heterogeneous and complex sandstone oil reservoirs in the Qaidam Basin. A novel profile modification agent (PMA) was developed by the author. The microscopic structure of the agent was analyzed by SEM. The PMA of 0.3% has stronger plugging capacity to cores. Its blocking coefficient was larger than 91%, and it was not less than 90% after being flushed with 20PV injected water. In parallel core test the agent could be used to adjust the reservoir permeability and enhance oil recovery substantially. The field test achieved good results.

Geoelectric Structure of the Crust along the Mbere Trough (South Adamawa) from Audio-Magnetotelluric Data

The South Adamawa trough represents a major structure associated to the tectonic activity in the Adamawa region. The Mbere and the Djerem troughs represent the eastern and the western parts of the South Adamawa trough respectively. The audio-magnetotelluric (AMT) method is used to study the Mbere trough and to bring out tectonic features that affect the basement of this region during the Cretaceous. From analysis and interpretation of AMT data, based on pseudo-section and geoelectric section, two geologic models are constructed across the trough at Belel and Nyambaka. Those models bring more information on the sediments thickness and the borders faults of the trough compared to previous studies. The results shows that the Mbere trough is dissymmetric, and bounded on its northern side by an intra-gneissic fault and on its southern side by two faults with at least one considered as a faulted contact between the granitic and gneissic rocks. The sediments filling of the trough are consist mainly of conglomerates on which lay sandstones. The thickness of these sediments is about 2060 m at Belel and decreases to about 1800 m at Nyambaka, with a low sandstones thickness, which does not exceed 360 m. The basement consists of granite in the South of the trough and gneiss in the North, with resistivity more than 10,000 Ω.m. The Moho is located between 25 and 35 km in the South Adamawa region.

Production Dynamic Method to Determine the Critical Production of Bottom Water Reservoir

Tahe oilfield of Triassic oil under low amplitude anticline group monolithic sandstone reservoirs with bottom water features, having a uniform oil-water interface, water energy sufficient. The current production wells generally see water, water cut rising too fast. Horizontal well bottom water coning phenomenon is outstanding, later period of high water cut stage manages difficulty to increase, water blocking measures have a low efficiency. Determine critical output, to improve production well life, efficient development reservoir has important significance .Based on the study of dynamic characteristics of single well and geological analysis of factors determining the critical output, and the application of numerical simulation method to verify the reasonable threshold quantity production. This method is simple and practical, can accurately make the critical output of single well.

Integrated outcrop reservoir characterization, modeling, and simulation of the Jackfork Group at the Baumgartner Quarry area, western Arkansas: Implications to Gulf of Mexico deep-water exploration and production

The lower Pennsylvanian Jackfork Group in Arkansas has been the subject of studies, field trips, and publications for many years because of excellent outcrop exposures of different deep-water architectural elements. This latest study is focused on the Baumgartner Quarry located near Kirby, Arkansas, which exposes a series of vertical walls in three dimensions. This quarry has not been as well documented as other popular exposures, although three-dimensional (3-D) quarry faces exist, and the quarry strata comprise part of a complete 600-m (1970-ft)-thick near-continuous Jackfork stratigraphic sequence not unlike younger deep-water stratigraphic exploration targets in the Gulf of Mexico and elsewhere. Subsurface problems including reservoir uncertainties and reservoir performance of lobe versus channel-fill deposits are addressed based on our work in the quarry. A 3-D sequence-stratigraphic model was developed using a correlation of seven measured stratigraphic sections in the quarry. The 180-m (590-ft)-thick quarry strata consist of a lower lowstand systems tract (LST) (lower sandstones) dominated by channel-fill sandstones, overlain by a shaly transgressive systems tract (condensed section), and then by an upper LST (upper sandstones) dominated by sheet or lobe sandstones. This model was translated into an updip against salt field, which is analogous to some deep-water Gulf of Mexico reservoirs. Performance simulation was conducted on the model using a one-injector water well and two vertical producing wells, one of which was connected to the injector via a channel sandstone and the other of which was offset from the channel sandstone. Results yielded 60% more production from the connected injector-producer pair than from the nonconnected pair. Comparison between the lower (channel-prone) sandstones and the upper (sheet-prone) sandstones revealed that the sheet-prone sandstone is more sustainable, whereas the channel-prone sandstone exhibits a larger drop in production rate during a 10-yr production period. These results illustrate the value of 3-D outcrop models for reservoir performance simulation for development planning of deep-water fields with limited data control, such as in the deep-water Gulf of Mexico.

The Effects of Structural Characteristics of Low Seepage Sandstone Uranium Ore Rock on Permeability

It is great important for the development of China’s uranium mining to efficiently extract low seepage sandstone uranium ore rock. It is the objective of this paper to present the effects of the structural characteristics of the low seepage sandstone uranium ore rock on permeability. Samples of sandstone from a uranium mine in XinJiang province were used to carry out experimental research on permeability. These tests were carried out on ore rock with particle size and distribution characteristics determined by a homemade permeability test device. The research indicates that the sandstone uranium deposits may be classified as low seepage because the particle size falls within the range 0.1~1.6mm, that the distribution of particle sizes observed conforms to low seepage fractal characteristics, the permeability decreases when the value of the ore particle size distribution of fractal dimensions increases, and the permeability decreases with particle size decreasing, and that the decrease of the permeability is not a linear change, but is an obviously exponentially decreasing functional relationship.

Long-Term Reliability of Historical Monuments Built with Low Quality Sandstone

The presented research concerns the long-term stability of a series of historical monuments, which were constructed with low-strength, ferrous sandstone. The main issues are the overall low compressive strength of the sandstone, the large scatter on these strength values, the sensitivity of its characteristics to water absorption and the lack of new original sandstone to replace the damaged zones. The sandstone reacts poorly under sustained high load levels, a situation which typically occurs at the base of bell towers and medieval city towers, as the dead load is considerably high compared to the compressive strength of the sandstone material. To assess the long-term behaviour of the sandstone, a test program has been set up to obtain information on its strength characteristics under monotonic and sustained loading. Therefore, test specimens were taken from the original material of a collapsed church tower. The results of these laboratory tests were used to adapt the parameters of an existing creep model to simulate the long-term behaviour of the sandstone under specific stress levels. Additionally, a number of strengthening solutions are discussed.

Tracing past migrations of uranium in Paleoproterozoic basins: New insights from radiation-induced defects in clay minerals

Radiation-induced defects were identified in kaolinite, illite, and sudoite by electron paramagnetic resonance spectroscopy of the alteration halo surrounding the uranium orebodies in the Athabasca Basin (Saskatchewan, Canada). Clay minerals are assumed to behave similarly under irradiation. In all samples, defects are similar in nature, but their concentrations can vary widely over several orders of magnitude. The maximum fluctuations in defect concentrations are observed along the regional Paleoproterozoic unconformity between the lower sandstones and the metamorphic basement rocks and close to crosscutting brittle structures, both of which appear to be the main vectors of uranium-bearing fluid transfer in the basin. In the basement, some Hudsonian faults connected to this unconformity also show high defect concentrations, attesting that uranium-bearing fluids may have circulated in the fracture network. The proximity of mineralization can be revealed through defects that record the past presence of uranium in altered rocks at significant distances from the mineralized bodies. The absence of correlation between defect concentrations and present dose rates indicates that migrations of uranium-bearing fluids took place after the formation of clay minerals.

Overpressure development and oil charging in the central Junggar Basin, Northwest China: Implication for petroleum exploration

The Junggar Basin is one of the largest and most petroliferous superimposed petroleum basins in China. The central depression area has become the frontier field for petroleum exploration. The characteristics of potential source rocks and reservoir sandstones, and the pressure regime in the central Junggar Basin were studied. Permian shales are dominated by hydrogen-rich, oil-prone algal organic matter, and Jurassic mudstones are dominated by hydrogen-poor, higher-plant derived organic matter. These source rocks are widespread and have been mature for hydrocarbon generation, suggesting good to excellent exploration potential, both for crude oils and for natural gases. The deeply buried Jurassic sandstones usually have low porosity and permeability. However, sandstones beneath the Jurassic/Cretaceous unconformity display relatively high porosity and permeability, suggesting that meteoric water leaching had improved the quality of the sandstones. Overpressure developed over much of the central Junggar Basin. The overpressured rocks are characterized by slightly increased interval transit time, low sandstone permeability, increased organic matter maturity, and high relative hydrocarbon-gas contents. Mudstones in the overpressured system have quite the same clay mineral compositions as mudstones in the lower part of the normally pressured system. Overpressure generation in the central Junggar Basin is best to be explained as the result of hydrocarbon generation and fluid retention in low-permeability rocks. Petroleum generated from Permian and Jurassic source rocks could migrate laterally through preferential petroleum migration pathways and accumulated in structural traps or lithological traps in the overpressured system, or migrate vertically through faults/hydraulic fractures into the overlying, normally pressured system and accumulate in structural or lithological traps. Therefore, commercial petroleum reservoirs could be potentially found in both the overpressured system, and in the normally pressured system.

The engineering geology of the Nottingham area, UK

Abstract Nottingham was built near a crossing point on the River Trent in the East Midlands of England. Initially, the city developed on a low sandstone hill close to the north bank of the river, which provided a secure, well-drained location above the marshes that bordered the river. Geologically, Nottingham stands at the boundary between Palaeozoic rocks to the north and west, and Mesozoic and Cenozoic strata to the south and east. The area is underlain by coal-bearing Carboniferous Coal Measures, Permian dolomitic limestones, Permo-Triassic mudstones and weak sandstones, Jurassic clays and Quaternary glacial and alluvial deposits. Artificial deposits, resulting from the social, industrial and mineral extraction activities of the past, cover the natural deposits over much of the area. This geological environment has underpinned the economic development of the area through the mining of coal (now largely ceased), oil extraction that was important during World War II, brickmaking from clays, alluvial sand and gravel extraction from the Trent Valley, and gypsum extraction from the Permo-Triassic mudstones. The Permo-Triassic sandstone is a nationally important aquifer, and has also been exploited at the surface and from shallow mines for sand. However, this history of the use and exploitation of mineral deposits has created a number of environmental problems, including rising groundwater levels, abandoned mine shafts and mining subsidence, and, within the city itself, the occasional collapse of artificial cavities in the sandstone and contaminated land left by industrial activities. Natural constraints on development include gypsum dissolution, landslides, rockfalls, swell–shrink problems in Jurassic clays and flooding. Occasional minor earthquakes are attributed to movements related to coal mining or natural, deep geological structures. Thus, Nottingham's geological context remains an important consideration when planning its future regeneration and development.

Casing-Collapse Strength Reduction Under Lateral Loads From Yielding Shales in the Daqing Oilfield

Summary Casing failure has been found in nearly 20% of production wells in the Daqing oil field after 45 years of oil-production history. Plastic failure of casing strings has been recognized in the zones of yielding shales overlying reservoir pay zones in the field. This type of casing failure is one of the most costly problems in the field. No research has been conducted to study the resistance of the casing string to the lateral loads from the yielding shales. This paper offers results from such research. Assuming a lateral load uniformly distributed along a casing section from a clay zone clamped between an upper and a lower sandstone zone, a mechanistic model was developed in this study. Results of the model indicate that the resistance of casing string to the lateral loads drops sharply in the early stage of development of radial deformation. The resistance of the casing string to the lateral loads is not very sensitive to the longitudinal deformation. The remaining strength of the casing string also depends on properties of the casing string. A calculation example is illustrated in this paper. This work provides an analytical tool that can be used for optimizing casing programs against lateral loads from yielding-clay zones to minimize casing failure and field-operation cost.

Nutrient retention within the hyporheic zone of a low order sandstone stream (Weidlingbach, Lower Austria)

In this study at the Weidlingbach, a fourth order sandstone stream, the hydrological exchange between surface and interstitial water as well as the retention potential of the hyporheic zone for inorganic nutrients were studied. For this purpose, we performed shortterm nutrient additions (ammonium, nitrate, phosphate) at a 75 m long, third order study reach between May and August 2004. Additionally, hydrological and chemical parameters to estimate water exchange (VHG, hydraulic conductivity, conductivity, oxygen concentration) were measured in the stream and interstitial water. Sediments at the study reach could be divided into sites with either upwelling or restricted water exchange (low permeability, positive VHGs, low oxygen content) and sites which were characterized by a significant downwelling of surface water. Hyporheic nutrient uptake was measured at these latter sites. The nutrient with the highest retention efficiency was ammonium, followed by phosphate and nitrate. Vertical uptake curves indicated that the retention efficiency was highest in the sediments at the surface. These results imply that, at the Weidlingbach, the hyporheic zone does not considerably contribute to stream retention and that the surface sediments and the surface water play a greater role in nutrient cycling.

Geological characterization and potential for carbon dioxide (CO2) enhanced oil recovery in the Cardium Formation, central Pembina Field, Alberta

Research Article| June 01, 2008 Geological characterization and potential for carbon dioxide (CO2) enhanced oil recovery in the Cardium Formation, central Pembina Field, Alberta Shahin E. Dashtgard; Shahin E. Dashtgard Department of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, BC V5A 1S6, Canada Search for other works by this author on: GSW Google Scholar Maja B.E. Buschkuehle; Maja B.E. Buschkuehle Strike Oil Lilmited, Perth, Australia Search for other works by this author on: GSW Google Scholar Brent Fairgrieve; Brent Fairgrieve Energy Resources Conservation Board, Calgary, AB, Canada Search for other works by this author on: GSW Google Scholar Habtemicael Berhane Habtemicael Berhane Energy Resources Conservation Board, Calgary, AB, Canada Search for other works by this author on: GSW Google Scholar Bulletin of Canadian Petroleum Geology (2008) 56 (2): 147–164. https://doi.org/10.2113/gscpgbull.56.2.147 Article history received: 16 Jan 2008 accepted: 06 May 2008 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Shahin E. Dashtgard, Maja B.E. Buschkuehle, Brent Fairgrieve, Habtemicael Berhane; Geological characterization and potential for carbon dioxide (CO2) enhanced oil recovery in the Cardium Formation, central Pembina Field, Alberta. Bulletin of Canadian Petroleum Geology 2008;; 56 (2): 147–164. doi: https://doi.org/10.2113/gscpgbull.56.2.147 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyBulletin of Canadian Petroleum Geology Search Advanced Search Abstract Injection of CO2 into the Cardium Formation, central Pembina Field, Alberta was initiated in 2004 in two inverted 5-spot waterflood patterns. The purpose of this flood was to assess the economic viability of enhanced oil recovery (EOR) using CO2. To study CO2 behaviour within the reservoir and assess long-term risks of CO2 leakage and storage potential, a geological and hydrogeological model was developed for the Cardium Formation, and for both the underlying and overlying strata. The injection zone, Cardium Formation, was sub-divided into four permeable and three impermeable reservoir units. The underlying and overlying strata were subdivided based on their geological potential to contain CO2 over the long-term (5,000 years).Permeable reservoir units in the Cardium Formation include the lower, middle and upper sandstones, and the conglomerate. Most of the storage capacity of the Cardium Formation is contained within the three sandstone beds, which exhibit average porosities between 14.8 percent and 16.4 percent. Permeability in the sandstones increases predictably with increasing porosity, but is variable from sandstone to sandstone. The lower sandstone has the lowest average permeability (9.5 md), which is about half the average permeability of the middle and upper sandstones (21.4 and 19.8 md, respectively). There is no relation between porosity and permeability in the conglomerate. The conglomerate on average is 50 percent more permeable (33 md) than the middle and upper sandstones; although, permeability may exceed one Darcy in individual beds. The higher permeability of the conglomerate results in it acting as a “thief” zone for injected CO2. This tendency is further compounded by both the endemic use of hydraulic fractures (which propagate vertically) to stimulate oil production, and the positive buoyancy of CO2 relative to water.The Cardium Formation at Pembina occurs within the middle of the 650 m thick Colorado Group shale. The effectiveness and integrity of the overlying shale succession, as a seal for the Cardium Formation, is vital to the long-term storage of CO2. Leakage through this shale is very unlikely as it is thicker than 300 m and separates two aquifers with significantly different regional pressure regimes (the Cardium Formation was originally over-pressured whereas the overlying Wapiti Formation is sub-hydrostatically pressured). You do not currently have access to this article.

RESERVOIR QUALITY, DIAGENESIS AND SEDIMENTOLOGY OF THE PALE AND SUBU SANDSTONES: RE-VISITING THE EASTERN PAPUAN BASIN, PAPUA NEW GUINEA

Two shallow diamond drill holes (Subu–1 and Subu–2) continuously cored in August and September 2001 by InterOil Australia represent the first sub-surface penetrations of reservoir quality sandstones in the Eastern Papuan Basin of Papua New Guinea. These wells intersected two sedimentologically distinct thick quartz sandstones (>100 m). The upper sandstone unit is Campanian in age and is correlated with the Pale Sandstone, whereas the lower sandstone is of Turonian age and has not been reported previously, and is tentatively named as the Subu Sandstone in this paper.The core has been the subject of detailed reservoir quality and diagenetic study as part of a multi-disciplinary study conducted by CSIRO Petroleum. The results of the reservoir quality portion of this study form the basis of this report and demonstrate the following:There are two distinct depositional systems present with a lower sandy slope apron and basin floor fan system (Subu Sandstone) and a younger upper shoreface-shallow marine depositional system (Pale Sandstone).While the porosity and permeability data for subsurface samples (5 to 16% and 0.1 to 1000mD) are lower than previously reported by Boult and Carman (1990) for surface samples both the sandstone units demonstrate thick, good reservoir quality reservoir capable of holding significant volumes of hydrocarbons.Bitumen is present in the pore space through out the sandstones in both wells. The presence of biodegraded hydrocarbons demonstrates that liquid hydrocarbons have been generated in the basin and have either migrated through the Subu and Pale sandstone or have been reservoired in them.Associated with the bitumen is pyrite precipitated as an in-situ by-product of shallow biodegradation of the parent liquid hydrocarbon as indicated by sulphur isotope analysis.Diagenetic effects include compaction (the dominant control on reservoir quality), minor quartz cementation, minor secondary porosity generation, and in thin zones localised carbonate cementation.Despite their very different depositional settings and age difference the thin section petrology of the Pale and Subu sandstones are very similar. The subtle difference between them is textural (grain size, sorting) and detrital clay content. The Subu Sandstone is typically finer grained, displays a higher degree of sorting and has a higher detrital clay content than the Pale Sandstone.The character of these sandstones may have as much to do with provenance as with depositional environment and may indicate a separate quartz-rich depositional system sourcing sediment from the Australian craton independent of the Fly Platform Toro/Imburu systems.

Radarsat investigation of a circular feature in east central Jordan

A typical ring-like feature, located in east central Jordan about 150km south-east of Amman, is very obvious on Radarsat images. Four concentric circles, but only two perfect ones, were discriminated, ranging in size from about 6km to about 1km in diameter. At first glance, this feature greatly resembles an ideal impact structure. Previous geological studies, geophysical investigations and recent field observation proved the presence of a domal structure that incorporates lower Cretaceous sandstone at the core; middle of the central ring, and successively surrounded by a highly fractured and steeply dipping sequence of Cretaceous and lower Tertiary carbonates and chert. The principal faults and lineaments in the region follow N-W, N-E and E-W trends, the intersection of which is believed to have acted as a preferable place for the emplacement of an igneous body, most probably in the post-Eocene, which causes the domal uplift and the associated ring faults at Jabal Waqf as Suwwan. Interpretation of 1:60 00...

Lower Miocene Nukhul Formation, Gebel el Zeit, Egypt: Model for structural control on early synrift strata and reservoirs, Gulf of Suez

The Aquitanian-early Burdigalian (lower Miocene) Nukhul Formation at Gebel el Zeit, Egypt, was deposited during early stages of Gulf of Suez rifting. The unit dips 8-15° less than underlying prerift strata, indicating that significant rotation and extension preceded subsidence of the Gebel el Zeit fault block. The Nukhul Formation at Gebel el Zeit is up to 75 m thick in outcrop and consists of a lower sandstone and an upper carbonate unit. The formation varies considerably along strike because of syndepositional differential movement of small fault-bounded blocks. The lower clastic unit at South Gebel el Zeit contains poorly sorted, conglomeratic, marly sandstone that commonly displays grading and Bouma sequences. Beds were deposited below storm base by sediment gravity flows. Thicker intervals are inferred to fill small, structurally controlled, submarine gullies that funneled sand and gravel southwestward to a half-graben basin. In contrast, an inferred correlative, thin, basal conglomeratic unit in North Gebel el Zeit was deposited in a shallow-marine setting. The presence of basement clasts in Nukhul strata indicates early syndepositional uplift due to structural tilting. The upper carbonate unit consists of bioclast, peloid, and intraclast packstone, wackestone, and grainstone with minor floatstone, rudstone, and coral-algal boundstone. Carbonate strata were deposited variously in deep-marine, low-energy peritidal and subtidal, and reefal environments. Deeper submerged blocks were the site of carbonate resedimentation or deeper shelf deposition. Reefs and shallow-marine bioclast shoals formed on higher submerged blocks. Nukhul strata show that synrift reservoir prediction in the Gulf of Suez, the Red Sea, and presumably in other rifts requires mapping of synrift cross faults and fault block by fault block facies analysis. Robert Winn is a consulting geologist. He received a Ph.D. from the University of Wisconsin-Madison in 1975 and joined Marathon Oil Company as a research geologist in 1977. He left Marathon in 1994 for the University of Papua New Guinea, where he was senior lecturer and then associate professor. He was head of the Geology Department from 1996 to 2000. His primary interests are in clastic sedimentology and sequence stratigraphy.Paul Crevello is a consulting geologist and technical director of PetrexAsia, which he formed in 1997. He received an M.S. degree from the University of Miami (1978) and a Ph.D. from Colorado School of Mines (1989). He was employed by Marathon Oil as a research geologist from 1978 to 1994 and by the University of Brunei as senior lecturer from 1994 to 1997. His specialties are in sequence stratigraphy and sedimentology of carbonate and turbidite systems and the integrated calibration of 3-D reservoir models. William Bosworth is employed by Marathon Oil Company. He joined Marathon in 1984 and has worked principally on international exploration and development projects. From 1980 to 1984 he taught structural geology and tectonics at Colgate University in Hamilton, New York. His research interests are principally in extensional tectonics, continental stress field evolution, and the paleogeodynamics of the African plate.

Teacher's Aide: Geologic Characteristics of Hole-Effect Variograms Calculated from Lithology-Indicator Variables

Variograms calculated from binary variables, such as from two lithologies, tend to show sinusoidal forms with decreasing amplitudes for increasing lag distances. This cyclicity is observed often when analyzing drill-hole data for rock sequences with alternating lithologies, and the variograms are thus labeled “hole-effect variograms.” Such variograms show a variety of forms: (1) Low to moderate variation in lithologic-body dimensions causes variograms to have strong cyclicity with decaying amplitude. (2) Variograms with one or more peaks and troughs usually result from a binary variable for which lithologies are about equally abundant but possibly large variations exist in the size of lithologic bodies. (3) Variograms show poor cyclicity if one lithology has highly variable body sizes and the other has moderately variable body dimensions. (4) Variograms that attain a plateau at short lag distances represent extremely high or low sandstone fraction, high variability in size of the most abundant lithology, and low variability in the other. Information about the dimensions of lithologic bodies makes it possible to approximate characteristics of the variogram of the lithology variable without numerous wells. Conversely, a hole-effect variogram of lithology may be used to estimate lithologic dimensions.

Abstract: Does Climate Affect Amount of Bioturbation? Comparison of Ichnofabric in Low vs. High Latitude Triassic Fluvial Sandstones

Abstract: Does Climate Affect Amount of Bioturbation? Comparison of Ichnofabric in Low vs. High Latitude Triassic Fluvial Sandstones

Hog's Back Formation: a new (Middle Ordovician) stratigraphic unit, Ottawa Embayment, eastern Ontario, Canada

The Hog's Back Formation, exposed in Ottawa, Ontario, is a new stratigraphic unit that disconformably overlies the Rockcliffe Formation and underlies, with apparent conformity, the Pamelia Formation of the Ottawa Group. The Hog's Back Formation is 14.3 m thick at its type section (Prince of Wales Falls, Ottawa) and thickens eastward to 27 m in Russell County, about 40 km southeast of Ottawa. It consists of lower, dark green shales with thin lime-mudstone and sandstone interbeds, and upper thin to thick beds of fine crystalline greenish grey sandy and calcareous dolostones with thin to medium (10–30 cm) pinkish grey, bioclastic packstone and grainstone interbeds. The latter thicken eastward and correlate with the "St. Martin calcarenites" of the Beaconsfield Member, Laval Formation, in the Montréal area. Together, these beds correlate with the Valcour Formation, Chazy Group, in the Lake Champlain area of New York. Conodonts and macrofossils indicate a Chazyan (Llandeilian) age for the Hog's Back Formation. The lower sandstone–shale–carbonate assemblage of the formation indicates sedimentation in peritidal environments, and documents initial regional flooding of nearshore sandy facies of the Rockcliffe Formation. The overlying carbonate facies represent sabkha environments punctuated by storm deposition. The gross shallowing-upward succession of the formation was terminated by renewed flooding associated with another regional transgressive event heralding deposition of the Ottawa Group.