A megascale view of reservoir quality in producing sandstones from the offshore Gulf of Mexico
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A publicly available data set has been examined for relationships between average values of porosity, permeability, depth, temperature, pressure, thickness, age, and play type for 11,833 sandstone reservoirs, mostly of Miocene age and younger, from the United States offshore Gulf of Mexico (GOM). Porosity shows wide scatter as a function of burial depth, but the median (P50) porosity trend decreases smoothly with depth. The GOM trend has much higher porosity for the given depth than the P50 trend of sandstone reservoirs worldwide, reflecting rapid sedimentation rates and young ages of GOM reservoirs, most of which have spent relatively little time at temperatures more than 80C, where quartz cementation becomes active. Multivariate regression analysis shows that porosity is best predicted by temperature (r2 = 0.40), with the fit improved slightly by adding age and then depth (r2 = 0.44). Arithmetic average permeability (represented by its logarithm) shows a correlation of maximum and P50 trends with porosity. GOM P50 permeability lies 0.2–0.4 log units below the P50 trend for sandstone reservoirs worldwide, probably reflecting very fine grain size of most GOM sands. Water saturation can be used to calculate the effective (petroleum-filled) porosity of each reservoir, which shows strong correlation with permeability. Grouping the reservoirs by chronozone reveals regular trends of decreasing average porosity and permeability with increasing age, reflecting increasing average depth and temperature with age. Porosity and permeability functions representing depositional sand quality show only subtle differences between different age groupings and play types. The results presented here can be useful for specifying realistic distributions of parameters for both exploration risk evaluation and reservoir modeling.SUMMARY During the Cambrian, two types of continental margins occurred around Gondwana. The eastern margin (Antarctica, Australia and southern South America) was characterized by a narrow continental shelf with a steep slope separating the shallow water environment from a deep‐oceanic one accompanied by mafidultramafic volcanics. The western margin was characterized by a wider continental shelf, probably passing gradually to an unknown outer basin. This comprised three main domains: the Asiatic shelf, composed of distinct cratonic blocks, presumably separated from each other by deeper‐water/ volcanic intracontinental basins; the European shelf, characterized by the development of shallow intracontinental siliciclastic basins; and the Americanc‐African shelf, morphologically and depositionally uniform. The distinction of these two Gondwana continental margins expresses their different geodynamic behaviour during Cambrian extensional tectonics. In fact, the sedimentary/palaeogeographic evolution, suggests the establishment of an active Pacific‐like margin in the eastern domain, and the tentative establishment of a divergent Atlantic‐like margin, in the westem one.
Continental Margin
Siliciclastic
Passive margin
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Margin (machine learning)
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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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The Jurassic-lowermost Cretaceous succession of East Greenland was deposited in a seaway formed over a series of old extensional basins between Greenland and Norway. The succession is interpreted within a sequence stratigraphic framework with the main emphasis on the Middle and Upper Jurassic. The vertical and lateral dimensions of the stratigraphic units are measured in kilometers and hundreds of kilometers, respectively. The interpretation is comparable to seismic-scale sequence stratigraphy, and the results can be compared directly to those derived from conventional reflection-seismic studies of subsurface successions. Sequence boundaries, and thus sequences, are defined differently by various research groups. In contrast, systems tracts representing linkages of deposi ional systems are considered the basic building blocks in both genetic and sequence stratigraphy. In the present study, systems tracts are recognized as the unit of highest rank within the concept of sequence stratigraphy. Ten major systems tracts are recognized. A Pliensbachian-Toarcian transgressive systems tract consisting of a slightly retrogradational parasequence set is overlain by a thin uppermost Toarcian-Aalenian(?) highstand systems tract (290-420 m in total thickness). The 140-700 m thick upper Bajocian-middle Callovian interval is represented by a basal aggradational to slightly retrogradational parasequence set interpreted as a shelf margin systems tract transitional to a transgressive systems tract. This interval is overlain by a strongly onlapping retrogradational set form d under rapid sea level rise and high sediment input, and interpreted as a transgressive systems tract. The upper Callovian-middle Oxfordian forms a composite progradational parasequence set that downlaps onto the top of the transgressive systems tract and represents a highstand systems tract. The transitional strata between the two systems tracts are highly condensed distally and contain the maximum flooding surface. Major regional deepening began in the late Oxfordian, and a thick succession of shales and turbiditic gully sandstones was deposited across the whole region. This succession represents another transgressive systems tract formed during a rapid sea level rise reaching highstand in the early Volgian when a sandy highstand systems tract prograded into the basin. The middle Volg an-Valanginian interval was characterized by rotational block faulting in northern East Greenland, in contrast to southern East Greenland, which continued its regular subsidence. A succession of a lowstand or shelf margin systems tract, a transgressive systems tract, and a highstand systems tract is recognized in both regions. This similarity may allow separation of sea level and tectonic signals in the two contemporaneous successions. The sequence stratigraphic analysis forms the basis for a coherent genetic model for the Jurassic-lowermost Cretaceous succession. The model may prove to be of value in interpreting deeply buried correlative hydrocarbon reservoirs in the northern North Sea and the Norwegian shelf.
Sequence Stratigraphy
Sequence (biology)
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Continental Margin
Red beds
Diachronous
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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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Structurally, the Hugoton embayment is a large, southward-plunging syncline that represents a northerly extension of the Anadarko basin. It is bounded on the east by the Pratt anticline, on the northeast by the Central Kansas uplift, on the northwest by the Las Animas arch, on the west by the Sierra Grande uplift, and on the southwest by the Amarillo uplift. The embayment is approximately 150 mi wide and 250 mi long. Subsidence began during the Early Ordovician and reached a maximum from the middle Mississippi through the early middle Permian. Rocks of Paleozoic, Mesozoic, and Cenozoic ages are present in the embayment. The section thickens toward the axis of the embayment where it is about 9500 ft. The Ordovician through Cambrian section attains a thickness of about 650 ft. The Devonian and Silurian are largely absent from the area. The Mississippian and Pennsylvanian sections are about 3000 ft thick. Excluding the Permian, the Mississippian and Pennsylvanian contain the highest exploration potential. An evaluation of the deeper zones in the underexplored areas of the embayment identified several structural and stratigraphic trends that are presently untested or remain underexplored. The trends can be separated into those controlled by early structural developments whichmore » persisted through the section and later structural stratigraphic events. The probability of finding new fields in the 500,000 to 5,000,000-bbl range is good.« less
Devonian
Anticline
Syncline
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