Source and commingling features of light oils from the Chepaizi uplift in the Junggar Basin, Northwest China
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Light crude oil
Stratigraphic relations between the La Luna and Cogollo limestones of Cretaceous age in northwestern Venezuela are discussed. Both formations are somewhat petroliferous. The lithology, microscopic character, and occurrence of oil in the two formations are compared in detail. The conclusion is that oil in the La Luna limestone is indigenous, whereas that in the Cogollo has migrated into the rock from some other source. Analyses and simple tests of the La Luna rock indicate further the probability that it is, or has been, a source rock of petroleum. The specific source of the oil-generating material in the La Luna sediment is discussed from the viewpoint of what is known of the present composition of the rock and what may be inferred as to its original environment of deposi ion.
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Recent studies of the Redding Formation of California provided new information concerning the extent of Cretaceous deposits underlying the southern Modoc Plateau. Understanding the stratigraphy and depositional environments of the Cretaceous rocks in this region is important for both paleogeographic reconstruction and hydrocarbon exploration. The Redding Formation is an approximately 1600-m thick clastic sequence that can be divided into five lithologic members. Biostratigraphic data indicate that the members of the Redding Formation are time-transgressive. The lowest three members were deposited during the middle to late Turonian, whereas the upper two accumulated during the Coniacian to santonian. The disconformity in the section developed during the latest Turonian to early Coniacian. Deposition in the Redding region was restricted to shelf environments and may have been controlled partly by euastatic sea level rise and fall. Initial transgression was directed northward and eastward with turonian strata accumulating across the basin. After the early coniacian hiatus, maximum marine inundation occurred briefly during the Santonian. Then late Santonian conglomerates and sandstones of the highest member prograded rapidly across the basin from the north, and shoaling apparently followed shortly thereafter. The southern limit of these late Santonian conglomerates appears to be the Tuscan Springs region where theymore » interfinger with deep shelf mudstones of the Chico Formation. These mudstones are considered to reflect an eastward swing of the Santonian shoreline around the northern Sierra Nevada. Thus, by the late Santonian, deposition had ceased in the Redding region but continued in a narrow trough to the south and southeast. The observed stratigraphy suggests that a thick sequence of Upper Cretaceous clastics beneath the southern Modoc Plateau is unlikely.« less
Marine transgression
Lithology
Cenomanian
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To date, exploration in the Pannonian basin of Hungary has concentrated on oil and gas believed to be derived from source rocks of the Cenozoic. In this sense, the basin is a mature hydrocarbon province. However, exploration of the Mesozoic and Paleozoic sections has just begun. These section may contain significant quantities of hydrocarbons derived from source rocks of the Mesozoic. Much of the buried basement consists of a complex system of stacked nappes composed of Mesozoic and older rocks. Basement structures from three areas are shown: (1) southwestern Hungary, where oil and gas produced from fracture zones in the crest of nappes; (2) southeastern Hungary, where thrusting occurred subsequent to Upper Cretaceous deposition; and (3) western Hungary, where thrusting occurred prior to Upper Cretaceous deposition. In general, Paleozoic-Middle Triassic rocks are overmature; however, Upper Triassic-Cretaceous rocks entered the oil-generation window during the Neogene. The heavy oils of the Zala basin were generated from organic-rich marls of the Late Triassic. In the Mecsek area, Toarcian shales are likely a good source for oil. Upper Cretaceous rocks, because of their terrestrial character, are inferred to be gas prone. Reservoir properties of Triassic and older rocks are expected to be poor, exceptmore » where fracture porosity occurs. In the Nagylengyel field, rudist limestones of Late Cretaceous contain prolific reservoirs with primary solution and fracture porosity. Although the average porosities of these reservoir rocks are relatively low (2-4%), permeabilities are >1-2 d as a result of paleokarst development.« less
Basement
Neogene
Petroleum reservoir
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Two silicoflagellate species, Dictyocha speculum and D. crux are described from the Upper Cretaceous of Washakie Basin, Wyoming and Colorado, and Piceance Creek Basin, Colorado. This is, to the knowledge of the author, the first clearly established record of an occurrence of Dictyocha (previously known from Tertiary to Recent) in Cretaceous sediments.
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Several paleomagnetic studies have been made in Arctic Alaska, by industry, by the U.S. Geological Survey, and by the University of Alaska. In general, the results available to the public have been disappointing--most samples of pre-Cretaceous rocks give very steep magnetic inclinations with respect to present horizontal. This has been generally interpreted in terms of a Cretaceous overprinting event. A study of the paleomagnetism of Cretaceous rocks from the North Slope shows that although the Cretaceous field was steeply inclined, it was not as steep as conventional paleogeographic reconstructions would indicate, and not as steep as the bulk of the apparently remagnetized older rocks. This finding leaves open the possibility that the steeper directions recorded by the older rocks are the result of regional tilt, or the result of a paleogeography that allowed an earlier, steeper remagnetizing field. The shallower inclinations seen in the Cretaceous sediments of the Nanushuk Group (Albian-Cenomanian based on the fossil record with one K-Ar age of 100 Ma from an ash parting) give paleolatitudes of about 75°N. The predicted paleolatitude based on North American paleogeographic reconstructions is 80-85°N. Circumstantial evidence that the paleolatitude was shallower than 80-85°N comes from the enormous biogenic productivity needed to form the extensive coal deposits of the Nanushuk Group. Lower paleolatitudes also may be needed to explain the apparent existence of broad-leaved evergreens and the recently reported dinosaur tracks and skin imprints in the Nanushuk Group. End_of_Article - Last_Page 680------------
Overprinting
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