Shale Interbeds Research Articles

Petroleum potential of the interpreted Paleozoic geoseismic sequences in the South Diyur Block, Western Desert of Egypt

ABSTRACT The South Diyur exploration block of nearly 38,000 sq km is located in the Farafra Oasis region in the Western Desert of Egypt. It is a frontier exploration area, the nearest well being Ammonite-1, a dry hole drilled by Conoco in 1979 immediately outside the southwestern corner of the block. The South Diyur Block is located on the probable northeast extension of the Kufra Basin in southeast Libya. Although prolific reserves of oil and gas occur in Paleozoic basins in North Africa and throughout the Middle East, to date, the targets for petroleum exploration in the northern Western Desert have been in Jurassic and Cretaceous rocks. The regional structural surface features in the South Diyur Block are the NE-trending Bahariya and Farafra anticlines interpreted as a deeply eroded and inverted Late Cretaceous structure on the southern extension of the Syrian Arc system. The oldest exposed rocks are a Cretaceous sequence of sublittoral sediments (the Campanian Wadi Hennis Formation) in the core of the anticline. The interpretation of the subsurface is based on 1,175 line-km of reprocessed 1970s-vintage 2-D seismic. Four sequence boundaries have been identified from the seismic data. SB-1 correlates with the Jurassic/Cretaceous boundary in Ammonite-1. SB-2 is regionally correlated with the Late Devonian to Early Carboniferous Hercynian unconformity that overlies deeply eroded and truncated Paleozoic sequences and possibly marks the regionally extensive Late Paleozoic basin inversion. SB-3 near the base of the interpreted Silurian sequence coincides with the ‘hot shale’ petroleum source rock that is present throughout North Africa and the Middle East. SB-4 is interpreted as a major unconformity at the top of an Upper Proterozoic sedimentary section that was misinterpreted as the Precambrian acoustic basement in Ammonite-1. Five seismic sequences relate to the seismic boundaries. SS-1, from the surface to SB-1 is characterized by subparallel seismic stratification and is composed mainly of sandstone with shale interbeds in Ammonite-1. SS-2, bounded by SB-1 and SB-2, is distinguished by parallel to subparallel seismic stratification. In Ammonite-1, the sequence of interbedded sandstone and shale is fresh-water bearing and lacking in top seals, thus reducing its prospectivity. The underlying SS-3 (SB-2 to SB-3) directly underlies the Hercynian unconformity and is characterized by semi-transparent seismic facies that may correspond to a thick Silurian shale sequence. SS-4 (SB-3 to SB-4) of probable Cambrian–Ordovician age has parallel seismic stratification. Deep channels are interpreted as evidence of a Late Ordovician–Early Silurian glacial phase that is present throughout North Africa and the Middle East. SS-5 (below SB-4) is marked by partial subparallel seismic stratification and block faulting. It probably belongs to the Late Proterozoic (Pan-African) phase of block faulting and pull-apart basins. Similar seismic geometries and facies occur in the Kufra Basin in southeast Libya and in many parts of the Arabian Plate, including the prolific petroleum systems of Oman. Exploration plays in the South Diyur Block are a combination of Paleozoic structural and stratigraphic traps associated with prospective fairways, and possible stratigraphic traps in the Late Ordovician–Early Silurian glacial channels. In addition, the interpreted Late Proterozoic sequences (SS-5) warrant further evaluation. In order to identify future exploration plays and drill targets, additional 2-D seismic (4,490 line-km), aeromagnetic and airborne gravity surveys will be integrated with the present seismic data and drilling results from Ammonite-1. This will allow a proper assessment of the magnetic basement, basin configuration and prospective fairways.

The analysis of frequency-dependent characteristics for fluid detection: a physical model experiment

According to the Chapman multi-scale rock physical model, the seismic response characteristics vary for different fluid-saturated reservoirs. For class I AVO reservoirs and gas-saturation, the seismic response is a high-frequency bright spot as the amplitude energy shifts. However, it is a low-frequency shadow for the Class III AVO reservoirs saturated with hydrocarbons. In this paper, we verified the high-frequency bright spot results of Chapman for the Class I AVO response using the frequency-dependent analysis of a physical model dataset. The physical model is designed as inter-bedded thin sand and shale based on real field geology parameters. We observed two datasets using fixed offset and 2D geometry with different fluidsaturated conditions. Spectral and time-frequency analyses methods are applied to the seismic datasets to describe the response characteristics for gas-, water-, and oil-saturation. The results of physical model dataset processing and analysis indicate that reflection wave tuning and fluid-related dispersion are the main seismic response characteristic mechanisms. Additionally, the gas saturation model can be distinguished from water and oil saturation for Class I AVO utilizing the frequency-dependent abnormal characteristic. The frequency-dependent characteristic analysis of the physical model dataset verified the different spectral response characteristics corresponding to the different fluid-saturated models. Therefore, by careful analysis of real field seismic data, we can obtain the abnormal spectral characteristics induced by the fluid variation and implement fluid detection using seismic data directly.

Spatial Distribution of Thin Interbeds in Point Bars of Meadering Channels: Case Study of Block X in Daqing Oilfield

Reservoirs in point bars of meandering channels account for a high proportion in oil and gas reservoirs, with much impact of complicated architecture on remaining oil accumulation and distribution. Thus study on inner architecture of point bars and its impact on remaining oil distribution are important. This paper selects Unit III in Block A in Daqing Oilfield with data in filled well patterns and cores to study inner architecture of subsurface single sand bodies, and analyzes 3D structural features of meandering point bars in Xingbei deltaic plain in Daqing Placanticline. Architecture parameters of lateral accretion shale interbeds are: eastward dip averagely 7°, the upper, middle and lower dip in same accretion face are respectively 10°, 7° and 3°, the width is 15-42m. They are vertically imbricated series and areally corner points of abandoned channels on both sides of meandering rings as track collection intersections, and progressive and curvature reducing arc sets with abandoned channels of meandering rings as outer boundary. This pattern provides geologic basis for successive study on remaining oil.

Mixed Siliciclastic-Carbonate Upward-Deepening Cycles of the Upper Cambrian Inner Detrital Belt of Laurentia

Abstract High-resolution sedimentological and biostratigraphic data recently recovered from Upper Cambrian strata in the northern Rocky Mountain and central Appalachian regions reveal that meter-scale cycles of very different character developed synchronously during deposition of the basal subzones of the Ibexian Series in both the carbonate facies belt and the distal part of the inner detrital, mixed siliciclastic–carbonate facies belt of the Laurentian paleocontinent. A typical mixed siliciclastic–carbonate cycle in the Snowy Range Formation of Wyoming and Montana consists of the following lithofacies in ascending order: shale, shale with very thin grainstone interbeds, grainstone with subordinate thin shale interbeds, flat-pebble conglomerate, and (in some cycles) thrombolitic bioherms. Internal transitions between the constituent lithofacies are mostly gradational, but cycle boundaries are sharp with shale directly overlying flat-pebble conglomerate and/or thrombolitic boundstone. We postulate that these mixed siliciclastic–carbonate cycles developed in response to different depositional dynamics than meter-scale cycles developed in carbonate-belt facies. Specifically, cycles in the carbonate belt were produced largely by temporal variation in accommodation space, and associated filling of that space through progradation of the carbonate system and aggradation to sea level. In contrast, cyclic deposition within the inner detrital belt was controlled by variations in terrigenous sediment input and resulting effects on carbonate productivity. In this case, enhanced delivery of siliciclastic mud by rivers was triggered by regression and/or a shift to more humid climatic conditions. An upward increase of carbonate in the main part of the cycles reflects enhanced carbonate production in response to the reduction of terrigenous clay supply during shoreline retreat. Widespread cementation of fine grainstone substrates led to deposition of extensive beds of flat-pebble conglomerate, possibly in association with elevated sea-surface temperatures and enhanced storm intensity. Thrombolites formed at times of maximum transgression as microbial communities thrived in the absence of turbidity, colonizing the irregular surfaces of flat-pebble conglomerate beds. We thus interpret these wholly subtidal cycles to record full changes in paleobathymetry with the bulk of the cycles recording upward deepening, rather than shoaling associated with regression. Comparison with age-equivalent shoaling cycles of the Conococheague Formation of the carbonate belt of the Appalachian Mountains highlights the different depositional dynamics in the mixed siliciclastic–carbonate systems, and underscores the peril in extrapolation of depositional models derived from carbonate systems to mixed systems, regardless of age or paleogeographic setting.

Differentiation of the significant Late Valanginian (Early Cretaceous) transgressive event in the Spiti Himalaya, India

Nearly continuous sedimentary succession of Early Cretaceous age has been long known in the Tethyan Himalayan belt without differentiation of the individual stages. We discuss here our just realised differentiation of a relatively thin well bedded sandstone interval with significant presence of the characteristic Late Valanginian–Early Hauterivian ammonoid genus Olcostephanus. It lies within a dominantly sandy succession with shale interbeds of Giumal Sandstone Formation, near Gate in Spiti Himalaya. This is the first stratigraphically precise record of Olcostephanus from the Cretaceous of India. It is here circumstantially age constrained to the Standard Tethyan Verrucosum Subzone Olcostephanus expansion event to the High Himalayan belt.

Hybrid sedimentation in Late Permian-Early Triassic in western Sichuan basin, China

Mixed carbonate-siliciclastic sedimentation is common in the Upper Permian-Lower Triassic in the western Sichuan (四川) basin. The extensional movement was strong in the Late Permian, resulting in differential uplifting of fault blocks. During this period, there was an extensive retrogression. The Kangdian (康滇) ancient land progressive uplifted and enlarged, forming the major provenance of terrigenous clastics. In Changxingian age, mixed terrigenous-carbonate sedimentation was distributed in the area between Dafeishui (大飞水)-well Dashen (大深)-1-well Shoubao (寿保)-1 and Beichuan (北川)-well Guanji (关基). The terrigenous clastics sourced from Kangdian ancient land input into the relatively shallow water, resulting in an extensive limited platform-mixing tidal flat where moderate-thick laminated grey limestone, thin muddy limestone and purplish red sandy shale were deposited. The tectonic framework in Feixianguanian (飞仙关) stage was inherited from the Changxingian stage. The hybrid sedimentation was limited to the south of Dujiangyan (都江堰)-Xindu (新都). In Jialingjiangian (嘉陵江) stage, the palaeogeographic features were also unchanged and two complete transgression-retrogression cycles also occurred. The mixing tidal flat was distributed to the west of Baoxing (保兴)-Ya’an (雅安)-Hongya (洪雅)-Leshan (乐山), where purplish red sandstone with thin limestone and shale interbeds and grey-dark grey silty mudstone with limestone were deposited. The carbonates in the mixed facies tract have few primary pores preserved and poorly-developed secondary pores due to the relatively high content of terrigenous clastics, strong compaction and weak dissolution. Therefore, they are unfavorable for the development of effective reservoirs.

Hydrocarbon-Generative Potential of Coal and Interbedded Shale of Mamu Formation, Benue Trough, Nigeria

Coal and its interbedded shale samples were collected from Okaba, Onyeama, and Okpara seams in the Mamu Formation, Lower Benue Trough, Nigeria. In the present study, source rock evaluation and biomarker distribution in the samples were investigated by Rock-Eval pyrolysis, vitrinite reflectance measurement, and gas chromatography–mass spectrometry (GC-MS) analysis. All of the samples analyzed contained the minimum of 0.5 wt.% and 2 mg/g of total organic carbon (TOC) and genetic potential (GP) respectively of organic matter required to serve as good source rock for oil and gas. Several plots from the Rock-Eval pyrolysis classified the organic matter in the samples as type II/III kerogen. The abundance of hopanes, homohopanes (C31–C35), benzohopanes, and C29 steranes in most of the samples indicates terrigenous materials, phytoplankton, and cyanobacteria contributions to the organic matter that formed the coal. High Pr/Ph ratio (1.73–12.47) and n-alkane distribution in the samples showed that Mamu samples consisted of terrestrial organic matter with marine incursion deposited under oxic/suboxic-oxic in lacustrine-fluvial/deltaic environments. The distribution patterns of C32–C35 benzohopanes in the samples confirmed the redox condition of organic matter deposition within the formation. The occurrence of olean-18-ene, olean-13 (18)-ene, and olean-12-ene in Okaba samples favors terrestrial organic matter deposited in lacustrine-fluvial/deltaic environment. The vitrinite reflectance values (0.48–0.60% Ro) and all the maturity parameters derived from the Rock-Eval analysis and biomarker distributions showed that Okaba samples are immature, whereas Okpara and Onyeama are at the beginning of the oil window.

High-resolution isotope stratigraphy of the Lower Ordovician St. George Group of western Newfoundland, Canada: implications for global correlation

The Lower Ordovician St. George Group of western Newfoundland consists mainly of shallow-marine-platform carbonates (∼500 m thick). It is formed, from bottom to top, of the Watts Bight, Boat Harbour, Catoche, and Aguathuna formations. The top boundary of the group is marked by the regional St. George Unconformity. Outcrops and a few cores from western Newfoundland were sampled at high resolution and the extracted micritic materials were investigated for their petrographic and geochemical criteria to evaluate their degree of preservation. The δ13C and δ18O values of well-preserved micrite microsamples range from –4.2‰ to 0‰ (VPDB) and from –11.3‰ to –2.9‰ (VPDB), respectively. The δ13Ccarb profile of the St. George Group carbonates reveals several negative shifts, which vary between ∼2‰ and 3‰ and are generally associated with unconformities–disconformities or thin shale interbeds, thus reflecting the effect of or link with significant sea-level changes. The St. George Unconformity is associated with a negative δ13Ccarb shift (∼2‰) on the profile and correlated with major lowstand (around the end of Arenig) on the local sea-level reconstruction and also on those from the Baltic region and central Australia, thus suggesting that the St. George Group Unconformity might have likely had an eustatic component that contributed to the development–enhancement of the paleomargin. Other similar δ13Ccarb shifts have been recorded on the St. George profile, but it is hard to evaluate their global extension due to the low resolution of the documented global Lower Ordovician (Tremadoc – middle Arenig) δ13Ccarb profile.

Distribution of Uranium Contamination in Weathered Fractured Saprolite/Shale and Ground Water

The objective of this study was to determine how structure, stratigraphy, and weathering influence fate and transport of contaminants (particularly U) in the ground water and geologic material at the Department of Energy (DOE) Environmental Remediation Sciences Department (ERSD) Field Research Center (FRC). Several cores were collected near four former unlined adjoining waste disposal ponds. The cores were collected, described, analyzed for U, and compared with ground water geochemistry from surrounding multilevel wells. At some locations, acidic U-contaminated ground water was found to preferentially flow in small remnant fractures weathering the surrounding shale (nitric acid extractable U [U(NA)] usually < 50 mg kg(-1)) into thin (<25 cm) Fe oxide-rich clayey seams that retain U (U(NA) 239 to 375 mg kg(-1)). However, greatest contaminant transport occurs in a 2 to 3 m thick more permeable stratigraphic transition zone located between two less permeable, and generally less contaminated zones consisting of (i) overlying unconsolidated saprolite (U(NA) < 0.01 to 200 mg kg(-1)) and (ii) underlying less-weathered bedrock (U(NA) generally < 0.01 to 7 mg kg(-1)). In this transition zone, acidic (pH < 4) U-enriched ground water (U of 38 mg L(-1)) has weathered away calcite veins resulting in greater porosity, higher hydraulic conductivity, and higher U contamination (U(NA) 106 to 745 mg kg(-1)) of the weathered interbedded shale and sandstone. These characteristics of the transition zone produce an interval with a high flux of contaminants that could be targeted for remediation.

Early Silurian glaciation along the western margin of Gondwana (Peru, Bolivia and northern Argentina): Palaeogeographic and geodynamic setting

The record of the Ordovician–Silurian transition within the southern Peru–Bolivia Palaeozoic basin (extending across Peru, Bolivia and northern Argentina) is characterized by a diamictite-bearing unit which overlies several different Ordovician units, and underlies mid Silurian shales. We review the lithostratigraphy, sedimentology and biostratigraphy of this unit, revising stratigraphic relationships and several ideas and interpretations previously proposed. Analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition. Turbidite and shale interbeds within the diamictites indicate a deep marine environment, with interbedded mud flows, debris flows, slumps and large displaced slabs providing evidence for sediment instability and resedimentation. Chitinozoan biostratigraphy indicates a Llandovery age, in contrast with previous proposals of a Hirnantian age, and suggests the need for more detailed sedimentologic and biostratigraphic studies, and a reassessment of this diamictite unit in relation to adjacent regions in South America. The resedimented character of the deposit explains some of the Ordovician fauna previously described, which must be considered as recycled from underlying units. In situ fauna needs to be reassessed, as it may be endemic, diachronic and/or indicative of migrations within Gondwana. Glacially-faceted and striated clasts, as well as large granitoid boulders within the resedimented materials, provide evidence for glaciation of the source area, and are interpreted as recycled from former glacigenic deposits. The evidence found in the Central Andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late Llandovery (Telychian). The precise age of glaciation in this part of Gondwana cannot be confirmed do to a lack of true tillites. Tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western Gondwana during the Late Ordovician and Early Silurian. The new data imply that the Ordovician–Silurian boundary was not preserved in some areas of the basin, and that an erosional hiatus (disconformity with variable regional significance) is present between different Ordovician units and the diamictite unit. Late Ordovician (?late Caradoc–?early Ashgill) euxinic black shales locally present in the central Altiplano and Eastern Cordillera of Bolivia may represent the youngest Ordovician unit in the region, and more research should be focused on them. Paleogeographic reconstruction of the Early Silurian Peru–Bolivia Basin depicts the extension of the diamictite unit southwards into northern Argentina and western Paraguay, connecting with the Paraná basin through the Asunción Arch. To the north it continued into northern Peru and Ecuador, connecting with the Silurian record in Colombia and Venezuela, and to the northeast into Brazil (Solimões and Amazonas basins). The sedimentary record of glaciation in the Peru–Bolivia basin was due to the development of local ice fields to the west of the basin, related with tectonism and resulting reliefs along the active margin of Gondwana, and probably did not coincide with the development of the North African late Ashgillian ice cap.

Grain-scale deformation in the Palaeoproterozoic Dongargarh Supergroup, central India: implications for shallow crustal deformation mechanisms from microstructural analysis

Analysis of the grain-scale deformation mechanisms in folded rocks of the Dongargarh Supergroup, central India, reveals that deformation was accomplished by a combination of pressure solution, microfracturing and dislocation creep processes. The finite strain was assessed using the Rf/ϕ method (X/Z ≈ 2). Partitioning of strain into various deformation mechanisms revealed that dislocation creep and pressure solution were the major contributors to the finite strain, followed by microfracturing. Analyses of microstructures suggest a sequence of dislocation creep followed by pressure solution and microfracturing, that ultimately gave way to microfracturing and limited crystallization or recrystallization. Overall constancy in volume during deformation is suggested from the balance between fracture-related grain-scale dilatancy and solution-related volume loss. Observations on cleavage spacing within various lithologies in a specific structural setting suggest that lithology played a vital role in cleavage development. Cleavage development in sandstones of the Dongargarh Supergroup required thin shale interbeds (for competency contrast) and grain size &lt;0.036 mm (4.75 ϕ).

Geochemical characteristics of the Zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the Jixian Sag of Bohai Bay Basin

Natural gas from the Zhaolanzhuang field of the Jizhong Superdepression, Bohai Bay Basin contains the highest proportions of H 2S (40–92%) among the sour gases encountered in China. The gas payzones include the Eocene–Oligocene Kongdian Formation (Ek) and the Es4 member of the Shahejie Formation. The sedimentary sequence consists of halite, anhydrite, carbonate, sandstone and shale interbeds deposited in the evaporative brackish water lacustrine – salt lake setting. In the deepest part of the Jinxian sag, the total thickness of evaporites is more than 1000 m, of which halite accounts for over 40%. Various organic-rich mudstones intercalated with the evaporites are currently within the conventional hydrocarbon window (with a depth of 2500–3500 m), and likely the source for the oil and sour gas in the Zhaolanzhuang field. The temperatures of the gas reservoirs range from 75 to 100 °C, too low for significant thermochemical sulfate reduction. The co-occurrence of abundant elemental sulfur with the sour gas and the δ 34S values of the various sulfur-containing compounds indicate that the H 2S gases were most likely derived from much deeper source kitchens where significant thermochemical sulfate reduction has occurred.

Occurrences and displacements of landslides by an earthquake with a subsequent rain: the 1999 Chi-chi earthquake in central Taiwan

Many landslides occurred during the Chi-chi earthquake (ML=7.3) on 21 September 1999 in western central Taiwan. These land-slides have been discerned from either aerial photographs or SPOT images, digitized, and coregistered with topographic maps at 1: 25, 000 scale. The data of the landslides were manipulated using ArcView GIS. Results indicate that the rock formation consisting of sandstone with shale interbeds in Miocene age was most susceptible to landslides, whether by the earthquake or by the rain. Most earthquake-induced landslides occurred on the upper part of the slopes and many transported displace material to river channel. The average travel ratio of the earthquake-induced landslide was 1.2.Study results indicate that the Toraji typhoon-related landslide in 2001 was a secondary disaster of the Chi-chi earthquake. Due to relatively high water content in the displaced material, the average travel ratio of the typhoon-related landslide reached 1.5, greater than the earthquake-induced one.

Eocene Clinoform Growth in Front of a Storm-Wave-Dominated Shelf, Central Basin, Spitsbergen: No Significant Sand Delivery to Deepwater Areas

ABSTRACT Eocene shelf-slope clinoforms on Spitsbergen reflect asymmetric basin infill across the coastal plain, shelf, slope, and deepwater segments of a foreland basin. Although four main types of clinoforms are recognized in the basin, this study focuses on clinoforms generated by a storm-wave regime on the shelf margin. The key aspect of this type of clinoform (Type 3) is that only modest volumes of sand were delivered across the shelf edge onto the slope, and little or no sand onto the basin floor, despite the sand-prone nature of the sediment delivery system, right to the shelf edge. The sedimentology and geometry of a series of such clinoforms are documented along a continuously exposed outcrop transect, some 800 m high and 4 km long. Sandstone units of individual clinoforms are 2-12 m thick, and are separated by thicker shale. Clinothems have subhorizontal topset and bottomset segments and steeper mid-portions, and are up to 150 m high, nearly an order of magnitude higher than the deltas and shorefaces that constructed them. This clinoformal configuration is thus interpreted to reflect shelf, shelf-edge, slope, and base-of-slope depositional environments, and water depths at least as great as the decompacted clinoform amplitudes. Exposed shelf segments dip up to 0.4 degrees whereas slope segments (some 2150 m wide) dip up to 4 degrees. The shelf and upper-slope reaches of clinothems consist of hummocky and swaly cross-stratified sandstone units without any significant interbedded shale. On the middle slope and below, these sandstone units become finer grained, are ripple- and plane-parallel laminated, and separated with interbedded shale. Individual topsets of clinothems are interpreted to have formed by strandplain or wave-dominated delta progradation across the morphological shelf platform during short periods of sea-level stillstand or slight fall. The shale units immediately above sandy clinothems developed during longer intervals of relative sea-level rise, when the wave-dominated shoreline retreated back across the shelf platform. The long-term trajectory of shelf-margin growth shows a significant aggradational component during overall progradation. Compared to other clinoforms in this same basin where the sediment-delivery system was fluvially driven and brought great volumes of sand onto the basin floor and slope, respectively, storm-wave-generated clinoforms brought only limited sand to the upper slope and little or no sand to the basin floor.

A geological comparison of the Sishen and Sishen South (Welgevonden) iron ore deposits, Northern Cape Province, South Africa

The Palaeo-Proterozoic Transvaal Supergroup in the Northern Cape Province of South Africa is host to the largest known resources of high-grade hematite ore on the Southern African continent. These ores are currently being exploited at Sishen and Beeshoek. Hematite occurrences on the Sishen South exploration project (some 65 km south of Sishen mine) are, at present, the focus of an intensive investigation. At both the Sishen and Sishen South deposits, mineralisation is found within lithologies belonging to the Transvaal and Oliphantshoek Supergroups. The Transvaal Supergroup comprises a basal, carbonate platform sequence (the Campbell Rand Subgroup) overlain by thick units of banded iron formation belonging to the Asbestos Hills Subgroup. The Asbestos Hills Subgroup is unconformably overlain by sedimentary rocks of the Gamagara Subgroup. The latter Subgroup has been correlated with the lowermost unit of the Oliphantshoek Supergroup. The Sishen and Sishen South deposits are located close to the western margin of the Kaapvaal craton and have been subjected to intensive, structural deformation. Both localities have undergone folding, faulting and thrusting. Early regional tectonism (2400-1700 Ma) played a critical role in the genesis and preservation of the ores from younger erosive events. At both deposits, four similar iron ore occurrences have been recognised. The ores have developed in specific depositional environments and hence display unique physical, chemical and metallurgical properties. First, the bulk of the high-grade hematite comprises laminated and massive ores, which are found in the upper parts of the Asbestos Hills Subgroup. The laminated and massive ores are best developed in basinal and pseudo-graben structures. Second, Collapse breccia ores of medium- and lower-grade are preserved within deep palaeosinkholes. The sinkholes have developed within the dolomites of the Campbellrand Subgroup. Finally, Lower-grade conglomeratic and gritty ores constitute the remainder of the deposits. These ores are found in the overlying clastic succession of conglomerates, shales and quartzites belonging to the Gamagara Subgroup. Although there are many stratigraphic and lithological similarities between the Sishen and Sishen South deposits, there are also important differences,which will impact on the profitable exploitation of the ores. The most striking contrast between the deposits is their size and geometry. Sishen mine is a single pit operation with an opencast mineable reserve of 895 Mt. The Sishen orebody is thicker, more continuous and significantly larger than the Sishen South deposit. The Sishen South deposit has a potential opencast reserve of 228 Mt. It comprises a number of small and narrow, isolated orebodies, spread over an area of ~64 km2. There is a significant difference in grade between the deposits. On average, the orebodies of the Sishen South deposit have a higher iron content (+0·25%Fe), less potassium (-0·090%K2O) and less phosphorus (-0·019 %P). The Sishen South deposit has less interbedded shale in the laminated and massive ores. The footwall of these high-grade ores is a light-density,intrusive sill, compared to banded iron-formation at Sishen. With a proposed heavy-medium separation plant at Sishen South, the impact of dilution on product grades is expected to be less than that currently experienced at Sishen. There is evidence at both localities to supportdifferent ore-forming processes.Most researchers are of the opinion that the laminated and massive ores are the product of supergene enrichment of primary banded iron-formations belonging to the Asbestos Hills Subgroup. Processes of laterisation and even basinal dewatering have been proposed to explain this supergene enrichment process. The exact mechanisms and events which led to the upgrading of the main ore horizon are, however,yet to be clarified. After the enrichment of the primary iron-formations (~2200 Ma), the massive and laminated ores were folded into basinal and dome structures. Karstification took place in the vicinity of the Maramane anticline, which led to the development of sinkholes within which subsidence and collapse took place. These structures facilitated the later upgrading and preservation of the breccia ores. Some 400 Ma later,the laminated and massive ores were subjected to rapid mechanical erosion. Conglomeratic and gritty ores were deposited in extensive, alluvial fan systems at the base of the Gamagara Subgroup. These lithologies heralded the change to an oxidising palaeo-environment and the appearance of the first red-bed sequence in the region. The Sishen and Sishen South deposits developed in the same palaeo-environment and were subjected to similar tectonic deformation. There are, however, a number of differences between the deposits, which will impact on the proposed exploitation of the Sishen South orebodies.

Distribution of Upper Cenozoic Carbonate-Cemented Sandstones, Kuche Depression, Northwest China: Insights from High-Resolution Borehole Micro-Resistivity Imaging

ABSTRACT High-resolution micro-resistivity images are used to study the distribution of neoformed carbonate concretions in Upper Cenozoic shallowly buried fluvial sandstones in the Kuche Depression, northwest China. The carbonate concretions are high-resistivity white facies on borehole images. Most of them occur as patches 1-40 cm thick and in shapes ranging from equant, oblate, to elongate. These concretions contrast markedly with the poorly cemented or uncemented host sandstones, which appear as dark to immediate grayscale facies on the images. Three patterns of occurrence of carbonate concretions are identified: isolated concretions, discontinuously cemented layers, and continuously cemented layers. Most concretions occur in fine-grained, low-permeability laminae, suggesting that their growth was controlled more probably by diffusion transport rather than advective transport. Grain size, lamination, and distribution of intergranular clay all potentially influenced the selective distribution of carbonate cement in the sandstones. Shale interbeds and terrigenous carbonate rock fragments in the sandstones are potential sources of carbonate cement.

Physical modeling study of the influence of shale interbeds and perforation sequence on sand production

Centrifuge tests were performed to study sand production problem that is a major issue in many oil and gas recovery projects. The tests simulated seepage-induced failure around a multi-perforated vertical well. Two different scenarios were considered: (1) perfs evenly spaced along the casing within a homogeneous sand layer, and (2) ditto but a shaley seam was introduced in the middle of the layer. Data collected during the experiments included time-dependent variation of pore pressure within the pay zone, flowrate, sanding rate, and tracking of the enlargement of the cavity around the well. Numerical modeling was used to better interpret and support the experimental results. Overall, the study performed shows that (1) the location of perfs affects the mode and magnitude of sand production and the concomitant productivity, and (2) long-term productivity can be improved through managed sand production. These findings are consistent with some field cases where extraordinary increases in production were noted due to sanding.

Rock deterioration in the Royal Tomb of Seti I, Valley of the Kings, Luxor, Egypt

A comparison of present tomb conditions of the Royal Tomb of Seti I in the Valley of the Kings with Burton's photographs (1921–1928) reveals that the rock structure and tomb decoration have undergone severe deterioration. In fact, several other Royal Tombs in the Valley of the Kings have shown similar decay features. Most of the Royal Tombs in the Valley of the Kings and the West Valley were excavated into the marls of the middle and lower part of Member I, the lowermost unit of the Thebes Formation. However, several tombs penetrate into the underlying interbedded shales and marls of the Esna Formation. All of them show severe, irreversible rock structure deterioration originating from swelling and shrinkage. Since the burial of Seti I, 3300 years ago, water and debris from flash floods had minor impacts on wall decoration of the uppermost chambers and on wall structure in the lower chambers. Historic flooding since the discovery of the tomb has caused major destruction of walls and pillars by repeated swelling and shrinkage of the shale. Moreover, accelerated humidity changes over the past 70 years have contributed to increasing deterioration of the rock structure. Rock mechanic analyses were done on samples from the tomb of Seti I in order to understand the propelling mechanism of deterioration. Marls and limestones of the Thebes Formation are predominantly composed of calcite, with 10–40% clay minerals and minor amounts of quartz and anhydrite. Sepiolite and palygorskite are the dominant clay minerals in the Thebes Formation, with subordinate amounts of kaolinite and illite/smectite mixed layers. The shales of the Esna Formation contain more than 70% clay minerals, which are composed of illite/smectite mixed layers, with high swelling capacity, kaolinite and subordinate palygorskite and sepiolite. The marls of the lower Thebes Formation showed dry compressive strength values between 48 MPa (7000 psi) and 78 MPa (11 300 psi) before violent failure, with a maximum elastic deformation of 0.4–0.5%. The Esna Shale samples had maximum stress loads of 60 MPa (8500 psi) and 74 MPa (10 800 psi) with a maximum deformation of more than 2%. The deformation character changed with increasing pressure from an elastic to an inelastic mode. Long-term swell tests on Esna Shale samples revealed an enormous water absorption and swelling capacity. Swell heaving is between 50 and 80%, most of which takes place within the first hour. The swell pressure exceeds 5 MPa (700 psi) and 13 MPa (1900 psi) with a deformation of 1.5 and 3.3%, respectively. The swell pressure increases to its maximum over a period of 7–12 days. The rock tests showed that the swell pressure capacity of the Esna shales might exceed the strength of the brittle and highly jointed marls of Member I, into which most of the tombs are hewn. The swell and water intake tests demonstrated that water absorption of the Esna shales is immediate. Mineralogical analyses showed that mixed-layer clay minerals are able to release enormous pressure, which may be ‘compensated’ through ductile deformation by the shales or may result in brittle failure of the marls.

Carboniferous Mixed Terrigenous Clastic, Carbonate and Sulphate Sediments in the Bachu Area, Xinjiang

AbstractThe Carboniferous system in the Xiaohaizi area, Bachu County, Xinjiang Uygur Autonomous Region, composed of typical mixed terrigenous clastic, carbonate and sulphate sediments, was mainly deposited in the tidal flat and lagoon environments. The mixed sediments occur as the following eleven types: 1. limestone intercalated with siltstone; 2. interbeds of shale and limestone; 3. gypsolyte interbedded with limestone; 4. gypsolyte intercalated with siltstone; 5. gypsolyte interbedded with shale; 6. gypsolyte intercalated with siltstone, limestone and dolomite; 7. siltstone interbedded with gypsolyte and limestone; 8. terrigenous detritus scattered in carbonate matrix; 9. carbonate as cement in clastic rocks; 10. mixed sediments of carbonate and terrigenous mud; 11. mixed sediments of carbonate grains with terrigenous sand. Based on the analysis of the dynamic mechanism of mixed sediments, it is believed that these types of mixed sediments in the study area were controlled by climate, sea level change and sources of sediments.

Abstract: Characterization and 3D Modeling of Turbidite Reservoir: A Case Study in Miocene Slope Deposits, Main Pass Area, Offshore Gulf Coast of Mexico&amp;nbsp;

ABSTRACT A case study of a detailed characterization and 3-D modeling of a turbidite reservoir has been completed. Well correlation and seismic attribute analysis revealed a clear three dimensional facies architecture of a turbidite deposit reservoir. The study shows that a turbidite lobe is developed along with multiple and relatively high density gravity flow events. Reservoir property and heterogeneity are controlled by facies architecture in a predictable pattern. The turbidite lobe has a typical mounded shape with a fairly straight feeder channel. The main body of the lobe is composed of stacked lenticular sand bodies, each representing a single gravity flow event. Reservoir sand is cleaner and thicker at the center lobe, rapidly becoming finer grained, with increasing shale interbeds, and finally pinches out towards the edge. Inside the lenticular sandbodies, thin shale patches can serve as vertical barriers to fluid flow during production. Facies distributions together with the structural pattern control the hydrocarbon resource potential and production performance in this area. They also provide important information for further exploration and development. A 3-D reservoir model is built using stochastic object based techniques. To preserve the observed facies architecture in the model, the General Marked Point Process algorithm is employed. Specific object shapes are designed in the computer program for different facies. Multiple stage facies modeling enables the preservation of different levels of reservoir heterogeneity. Major geological features are preserved in the model and can be incorporated in future flow simulations.