A Breccia Pipe in the Deseret Limestone, South Flank of the Uinta Mountains, Northern Utah

This article provides information on the main most common and most promising sediment complexes: Riphean, Vendian-Lower Paleozoic and Upper Paleozoic-Mesozoic.
 Riphean deposits within the area under consideration have now been uncovered on the northwestern side of the Vilyui syneclise. Within the southern slope of the Anabar anteclise, the Tankhai and Sokhsolokha structural-formation zones are distinguished.
 Permian deposits are most widely distributed within the Predverkhoyanskiy trough. They were opened by deep drilling wells in the northern part of the Anabar-Lena branch, in the central region (the Kyutingdinsky graben) and in the Aldan branch of the trough.
 The Permian and Lower Triassic deposits adjacent to large gas generation zones of deflections and depressions are characterized by the most favorable conditions for the placement of large oil and gas accumulation zones. In the regional assessment of the conditions for the placement and preservation of large oil and gas accumulation zones, the most favorable conditions are associated with the second hydrogeological floor.
 Seven oil and gas accumulation zones have been identified within the Predverkhoyanskiy pericratonic trough: the Hapchagai-Tomporuk and Loglor-Linden oil and gas accumulation zones are zones with proven industrial gas content. All deposits of the Khapchagai megaval fall into the Vilyuisk zone, due to the presence of horizons with relatively high filtration and capacitance properties in the near-axial part of the hemisyneclise.
 The Predverkhoyanskаyа oil and gas accumulation area combines linear pre-thrust structural traps formed parallel to the face of the structures of the Verkhoyansk folded region advancing on the Mesozoic cover of the platform. Typical traps are pre-thrust and especially sub-thrust anticlines, to a lesser extent closed kinks of layers in the hanging wings of shaft-like up-thrust structures.
 The Kyutingda possible oil and gas accumulation zone is one of the most promising sites for setting up regional exploration. The capacity of sedimentary discharge in the left-bank near-platform part of the Kyutingdinsky graben is estimated at about 5 km. This significantly increases the prospects of oil and gas potential in the north of the Predverkhoyanskiy trough.
 The Prialdanskaya and Prilenskaya possible zones of oil and gas accumulation are located along the platform side of the latitudinal and meridional branches of the Predverkhoyanskiy pericratonic trough, respectively. The prospects for the oil and gas potential of these zones are due, on the one hand, to the development of basal sandstones of the Upper Paleozoic, in which lithologically and stratigraphically shielded traps may be present. On the other hand, numerous bitumen manifestations indicate the intermediate, transit nature of this territory with respect to the migration of hydrocarbons. The terrigenous section of the Upper Paleozoic-Mesozoic deposits of the Prialdanskaya and Prilenskaya is favorable for the accumulation and formation of hydrocarbon traps.

The entrapment of oil and gas at an unconformity at Prudhoe Bay, North America's largest oil field, is well known. It now appears that other large oil and gas accumulations such as those in the Kuparuk River sandstones west of Prudhoe Bay are trapped at an unconformity. Prudhoe Bay field is on a large anticline on which the reservoir rocks have been truncated by an unconformity. The unconformity places organically rich Cretaceous shales in juxtaposition with Sadlerochit reservoir rocks. Cretaceous shales above the unconformity also provide the updip seal for the oil accumulation. Oil in the Jurassic Kuparuk River sandstones is trapped on a large structural nose by truncation of the reservoir rocks. The Cretaceous shales above the unconformity provide the seal and possibly the source for the oil. Uplift of the Barrow arch in Late Jurassic and Early Cretaceous time resulted in a regional unconformity. The truncation of older rocks by this unconformity, and the seal provided by the overlying shales are key factors to exploration of this arch which extends for more than 300 mi (483 km) along the Beaufort Sea coast. The delicate relations of source, reservoir, and trap, which are commonly controlled by one or more unconformities, are important to exploring this rich petroleum province. End_of_Article - Last_Page 684------------

Over the last years major petroleum fields in different regions of the world have been rapidly running out of their resources due to intensive production. Therefore, it is now essential to explore new territories and aquatories where large oil and gas accumulations could be revealed. One of the most effective methods of analyzing petroleum potential of prospective regions is basin modeling which consists in reconstruction of sediment deposition and formation of oil and gas accumulations by using special computer software. The objective of this research was studying petroleum potential of the South Kara Basin of the Russian Arctic Shelf. The study included 1D modeling for the wells of the Leningrad Petroleum Field in the Kara Sea, the Bovanenkovo Field on the Yamal Peninsula and 2D modeling for the seismic lines crossing the region in different directions. The 1D models were calibrated with the present day parameters of vitrinite reflectance measured in the studied wells. Thus, appropriate heat flow values for the region were obtained and used for further 2D modeling. As a result of 2D modeling, hydrocarbon accumulations were obtained for several known structures of the region, including the discovered petroleum fields of the Kara Sea and the Yamal Peninsula. This fact could approve the validity of the created models. Therefore, the results of conducted modeling could be used for the forecast of petroleum potential of the region. As a consequence, presence of petroleum accumulations was confirmed for some local structures that had been revealed earlier by seismics. Moreover, the zones of distribution of accumulations with different phase composition were determined within the South Kara Basin. Local structures defined by the modeling were considered to be the most prospective objects for further petroleum exploration in the region.

Cenomanian rocks in western Abu Dhabi comprise a sequence of shallow and deep marine carbonates with terrigenous shales and sandstones. Four rock units, in ascending order; the Shilaif, the Tuwayil, the Ruwaydha and the Mishrif are recognized within the sequence. The Shilaif Formation is represented by pelagic carbonates, deposited in anoxic intrashelf basin, developed in central Abu Dhabi during the Cenomanian. It proved to be the source rock of the oil reservoired in the overlying Mishrif. The Tuwayil, consists of shales and sandstones, and represents a period of terrigenous clastic influx, which has formed a large tongue protruded into the Shilaif basin. It includes four sandstone units, which are found oil bearing in some localities. The Ruwaydha represents the last basinal depositional phase of the Cenomanian in the area. The Mishrif Formation is a thick shallow marine carbonate sequence, exhibiting minor cyclic deposition reflecting periodic oscillation of sea level. It represents deposition on epicontinental shelf, with its margin prograding eastward into the Shilaif basin. It is a good reservoir, known to be oil productive in east Abu Dhabi and offshore Dubai from rudist reefal build-ups and/or accumulations. Large oil and gas accumulations have been found in the area, mainly in structural traps of the Middle and Lower Jurassic, with moderate oil accumulations in Middle and Lower Cretaceous rocks. A transitional zone between the Mishrif and Ruwaydha/Tuwayil exists in the area. The width of this transitional zone appears to extend along few kilometres. However, interfingering of the Tuwayil, the Ruwaydha and possibly the Shilaif appear to extend into the Mishrif reflecting possible occurrence of localised paleo-embayments at the Mishrif shelf margin. Seismic-stratigraphic investigation, helped to define facies limits, patterns and occurrence of prospective stratigraphic traps in both the Mishrif and the Tuwayil Formations. The Tuwayil sandstone and Mishrif carbonate clastics pinching out against structural highs, are prospective localities for stratigraphic hydrocarbon entrapment. In addition, areas where the Mishrif develops possible carbonate build ups and/or rudist reefal accumulations are also of interest and could have hydrocarbon potential. The sequence of seismo-stratigraphic/facies definition procedure, presented in this paper, were applied to define stratigraphic traps in the Mishrif and Tuwayil Formations.

The participation of water-dissolved gases in the formation of large oil and gas accumulation zones has been discussed many times, but despite this, the role of underground hydrosphere degassing in the processes of generation and accumulation of hydrocarbons is not completely clear. The purpose of this paper is to detail the ideas about the mechanisms of water- dissolved gas participation in the localization of industrially significant gas accumulations. According to the author, the role of water-dissolved gas in the formation of oil and gas accumulation large zones and giant gas fields is to increase the degree of isolation of phase-separated hydrocarbons due to the specific cap formation. A concomitant effect is the release of a colossal volume of fresh condensation water with increased geochemical aggressiveness to rock-forming minerals; this contributes to the sanding of production wells.

On the continental margin of Tanzania two big sedimentary basins - Tanzanian and Rovuma are distinguishing. Within the Tanzanian basin the following six subbasins from north to south along the littoral are distinguishing: Tanga, Pemba-Zanzibar, Latam, Rufidji, Mafia and Mandava. The big part of Tanzania is situated on the East African plateau. Sedimentary cover of the basin is presented by Phanerozoic deposits of 7-10 km thickness on the Archean basement. The Rovuma basin consists by two blocks Mrvara and Linde. The basin is characterized as a passive continental margin with a complex tectonic history. Rovuma basin is the principal gas bearing basin in the country. The formation of gas accumulations on the southeastern continental margin of Africa is associated with the spread of the Karoo complex, dated to the Permian-Triassic age. Analysis of more than 25 discoveries gas fields on the Tanzanian continental margin results to conclusion that all discovered gas fields gravitate mainly to the western side of the blocks. Large gas accumulation is marked in the deepwater areas, on the slope. Moreover, large gas discoveries have been made in the south of Tanzania. As far as advancement forward to the north the quantity and reserves of gas fields reduced. At the north of Tanzania gas accumulations have not been found, but oil accumulations have been noted.

Quasi-continuous hydrocarbon accumulation: An alternative model for the formation of large tight oil and gas accumulations

Oil Prospects of Israel

Accumulation of water-soluble gas by degasification: One of important mechanisms of large gas accumulations in the Xujiahe Formation, Sichuan Basin

Little investigation of the continental slope and rise with respect to their oil potential has been made, mainly because their depths are much greater than presently is practicable for oil and gas exploitation, and partly because national jurisdictions have not been established. The continental slope overlies the general seaward limit of continental-crustal rocks, but these rocks are covered with sedimentary strata that have prograded beyond the original position of the continental slope. The sedimentary strata are chiefly Cenozoic and Mesozoic in age; they have a seaward dip, and they include few sandstone layers suitable for accumulation of oil and gas. Sediments of the continental rise lap against the continental slope and contain many good acoustic reflecting horizons that are believed to be the surfaces of sand bodies deposited by turbidity currents. The strata between the reflectors are silt and clay that were deposited slowly from suspension; many of them have slumped from the continental slope where they were deposited originally beneath oxygen-poor water that permitted the accumulation of relatively high concentrations of organic matter. The interbedded position of organic-rich silt and clay (source beds) and of turbidite sand bodies (reservoir beds), which pinchout and are structurally deformed near the heads of the continental rises of the world, may constitute a geologic environment in which large oil and gas accumulations ave developed. Exploratory drilling is needed before possible exploitation can be considered. End_of_Article - Last_Page 717------------

Practically all oil and gas accumulations throughout the world are associated with some extensive productive series which consist of highly permeable rocks containing oil and gas pools, interbedded with shales and marls containing dispersed organic matter. All of the important recent discoveries between the Volga and the Urals were based on the observed association of oil and gas pools with certain lithologic-stratigraphic complexes of the Devonian, Carboniferous, and part of the Permian; although accumulation is related to many diverse Structural zones. Here, the productive series is the source rock as well, for the pelites contain dispersed organic matter including bitumen whose composition is related to the oils. Where accumulations occur in sediments deposited under oxidizing conditions, factors are sought which favor migration from possible source rocks along faults, unconformities, or surfaces of facies change. For example, in Azerbaidjan and Turkmenistan large oil and gas accumulations in Pliocene continental beds occur along structural zones that allowed migration from underlying, principally Paleogene and Miocene, deposits. As shales compact, their organic constituents gradually change; the coaly part from a ligniferous to a carboniferous stage and, ultimately, to graphite. The mobile bituminous substances formed by dissociation of organic matter and disproportioning of hydrogen, change from asphaltines and resins to oils and then to methane in the subcapillary pores of pelites, where they are in a dispersed, loosely bound state. Compaction causes slow removal of the loosely bound water and molecular migration with differentiation of the bituminous substances. After differentiation, the more mobile hydrocarbons are carried by the water into reservoir rocks. The bituminous substances move from small to large pores also by capillary forces. Migration in subcapillary pores apparently ceases when rocks become lithified and are no longer plastic. In the super-capillary pores and fissures of reservoir rock, free migration takes place as both molecular migration and movement in larger masses. Water circulates in reservoir beds from elevated areas of the outcrop toward lower discharge areas. When the hydrocarbon-bearing water moves into different environments of temperature and pressure, the hydrocarbons may come out of solution, rise to the top of the carrier-beds, and unite into oil and gas pools if traps are available. The location of oil and gas reflects the present equilibrium of fluids with the structural pattern and hydrogeological environment. --D. C. Van Siclen.

In the Province of Fars and in its offshore, gas reserves in excess of 600Tcf, discovered in Permo-Triassic carbonates well sealed by the anhydrite of the Dashtak Fm, were originated from Early Llandoverian highly organic and radioactive shales. The current distribution of this source rock was influenced by the pre-Permian erosion on a system of N-S tilted blocks. Extensive geochemical modeling has shown that oil generation began in the Middle Jurassic in some areas, while the onset of gas window was reached locally as early as Middle Cretaceous. A suite of cumulative isopachs shows the evolution of the source rocks maturity through time. It also provides a reliable image of the geometry of the reservoirs, the long range migrations and the location of large size oil and gas accumulations on a few continuous regional highs, prior to the Zagros orogeny. Huge amounts of gas and limited amounts of light oil later re-accumulated in the Zagros folds, formed during the Late Miocene and Pliocene. The characteristics of the Dalan and Kangan reservoirs, and the areal extent of the Dashtak Fm, which seals the system, could be deduced from paleogeographic reconstruction, distribution of facies, and isopachs. The distribution of the reservoirs, seals, surface oil and gas seepages and hydrocarbon-related indications, together with the location of the gas fields, makes possible to select, among the numerous PermoTriassic prospects, those which are the most attractive. Some prospects may also contain light oil rings below gas. THE SILURIAN SOURCE ROCKS : In the Zagros Foothills of Iran, a variety of Proterozoic rocks were uplifted by Proterozoic salt diapirs (Hormuz Fm), but no source rocks were found in the few Hormuz samples we analyzed. No source rocks were found either in the Cambro

Large-scale accumulation and distribution of medium-low abundance hydrocarbon resources in China

The early-to-middle Miocene Nurbani Reef is one of several seismically defined Batu Raja carbonate build-ups located on the western flank of the Sunda Basin, offshore southeast Sumatra. It contains a large sub-commercial oil and gas accumulation, discovered by HAPCO in 1983 with the Nurbani-I well. Seven delineation wells were drilled, five tested oil at rates ranging from 86 to 1747 BOPD. A basal transgressive platform limestone makes the substratum on which several successive carbonate build-ups, which make up the Nurbani Reef complex, grew. Three lithofacies have been recognized and mapped in the carbonate build-ups. The first facies comprises skeletal packstones and wackestones, developed along the eastern flank of the complex in a reef front environment. The second facies corresponds to a lagoonal back-reef environment developed on the western flank of the reefal complex and comprises mudstones and marly limestones, with occasional beds of reef-derived packstones. A narrow band of reef core, made up of in situ coral and algal bound-stone, forms the third facies. This facies is interpreted to constitute the framework of the Nurbani Reef Complex. The best reservoir development is associated with the skeletal packstones and wackestones facies. These facies exhibit extensive mouldic and vuggy porosity, together with some fracturing, and contains produceable hydrocarbons. The other facies contain large volumes of hydrocarbons which are nonproduceable because of their poor reservoir characteristics. Detailed petrological studies combined with wireline log analyses, test results, and seismic interpretations, have demonstrated facies control of reservoir distribution and performance. Therefore, a thorough understanding of the facies distribution has been found to be essential in establishing the extent of produceable hydrocarbons in the Nurbani Reef complex.

Beier sag is one of the most oil-rich sags in the south of Hailaer Basin in China. The purpose of the study is to clarify the identification characteristics and geological origin of calcareous mudstone strata in Nantun Formation of Beier sag, so as to provide new ideas for the identification and evaluation of high-quality source rocks, and to provide effective support for efficient exploration of oil and gas in the basin. The comparative analysis of logging curves and seismic reflection characteristics shows that the calcareous mudstone layer has the “three high and one low” logging response with high GR (Natural Gamma Ray), high LLD (Deep Investigate Induction Log), high AC (Acoustic) and low DEN (Rock Density), showing continuous and strong seismic reflection characteristics. Through the comparative analysis of experimental data of rock thin slice and microscopic image and organic geochemistry, the layer lithology is mainly gray-black mudstone, locally mixed with oil shale and marl, rich in calcium and algae. The layer has the high abundance of organic matter, good organic matter type ( II1-I ), high hydrocarbon-generating potential and high hydrocarbon-exhausting efficiency. It is the highest quality source rock found in Beier sag, and is the material base to form the large scale oil and gas accumulation. The formation of calcareous mudstone formed in the brackish water and wide lake basin by a wide range, represents the anoxic event early in Cretaceous period and is the significant stratigraphic correlation marker bed of the sag. In addition, this layer and the fan-delta front sand widely distributed in the lower part of Nantun Formation form the most favorable reservoir combination, which controls the plane distribution of oil and gas reservoirs. Above and below the calcareous mudstone strata, oil and gas are very rich.