Pre-Cretaceous rocks in the northern Sierra Nevada are subdivided from west to east into the Smartville, central, Feather River peridotite, and eastern belts. Cretaceous and younger sedimentary rocks form the western boundary of the Smartville belt, but various reverse-fault segments of the Foothills fault system separate the other belts. The Foothills fault system and associated structures involve rocks as young as Kimmeridgian (Late Jurassic) and are truncated by Early Cretaceous plutons. This relationship is often cited as evidence for the Nevadan orogeny which is commonly viewed as a temporally restricted event involving deformation and metamorphism during the Late Jurassic. Recent work, however, suggests that some of the Mesozoic structural fabric in the northern Sierra Nevada may not have been produced during the Late Jurassic, but instead may have formed between Early and Middle Jurassic time. Thus, distinguishing Nevadan-age deformation from older Mesozoic deformation is now one of the more important problems facing geologists working in the northern Sierra Nevada. The Haypress Creek pluton crops out in the eastern belt and historically has been cited as a post-Nevadan pluton. It intrudes the Early to Middle Jurassic SailorCanyon Formation that, together with the overlying Middle Jurassic TYittle Lake Formation, contains a domainally developed, locally penetrative, northwest-striking cleavage (S 2 ). S 2 can be traced into the contact metamorphic aureole of the Emigrant Gap composite pluton, where structural and microtextural evidence indicates that it predates pluton intrusion. New U-Pb zircon data for the Haypress Creek pluton suggest an age of 166 ± 3 Ma and previously published U-Pb zircon data for the oldest phase of the Emigrant Gap composite pluton suggest an age of 168 ± 2 Ma. The fossiliferous Sailor Canyon Formation ranges in age from Early Jurassic (Sinemurian) in its lower parts to Middle Jurassic (Bathonian or Bajocian) in its upper parts. The overlying Tuttle LakeFormation contains S 2 , which formed prior to emplacement of the Emigrant Gap and Haypress Creek plutons at ca. 168-166 Ma. This relationship suggests that the TXittle Lake Formation must have been deposited and deformed entirely within the Middle Jurassic. Thus, S 2 and associated structures within the eastern belt formed prior to Late Jurassic Nevadan deformation associated with the Foothills fault system.There are two end-member models used to explain the plate tectonic evolution of pre-Cretaceous rocks in the northern Sierra Nevada. These are referred to as the arccontinent collision and single, wide-arc models. Data discussed herein do not preclude either of these models for Early to Middle Jurassic time. However, regardless of which of these models is favored, both scenarios place the approximately 168 Ma and younger Jurassic volcanic and plutonic rocks of the Smartville, central, and eastern belts in a distinctly intra-arc setting and further imply that the Foothills fault system and related Late Jurassic structures are also...
The hydrocarbons discovered to date on the southern margin of Australia have been assigned to the Austral Petroleum Supersystem based on the age of their source rocks and common tectonic history. Modelling of the source facies distribution within this supersystem using tectonic, climatic and geographic history of the southern margin basins, suggests the presence of a variety of source rocks deposited in saline playa lakes, fluvial, lacustrine, deltaic and anoxic marine environments.Testing of the palaeogeographic model using geochemical characteristics of liquid hydrocarbons confirms the three-fold subdivision (Al, A2 and A3) of the Austral Petroleum Supersystem.Bass Basin oils are assigned to the Austral 3, Eastern View Petroleum System. The presence of oleanane in the biomarker assemblages of these oils, together with their negatively sloping, heavy, isotopic profiles, indicate derivation from Upper Cretaceous-Tertiary fluvio–deltaic source facies.In the eastern Otway Basin, oils of the Austral 2, Eumeralla Petroleum System are sourced by Lower Cretaceous (Aptian–Albian) coaly facies. Oil shows reservoired in the Wigunda Formation at Greenly-1 in the Duntroon Basin are possibly sourced from the Borda Formation and are assigned to the Austral 2, Borda Petroleum System.In the western Otway, Duntroon and Bight basins, a lack of definitive oil-source rock correlations precludes the identification of individual Austral 1 petroleum systems.
The Penola Trough is an elongate, Late Jurassic to Early Cretaceous, NW-SE trending half graben filled mainly with synrift sediments of the Crayfish Group. Katnook-1 discovered gas in the basal Eumeralla Formation, but all commercial discoveries have been within the Crayfish Group, particularly the Pretty Hill Formation. Recent improvements in seismic data quality, in conjunction with additional well control, have greatly improved the understanding of the stratigraphy, structure and hydrocarbon prospectivity of the trough. Strati-graphic units within the Pretty Hill Formation are now mappable seismically. The maturity of potential source rocks within these deeper units has been modelled, and the distribution and quality of potential reservoir sands at several levels within the Crayfish Group have been studied using both well and seismic data. Evaluation of the structural history of the trough, the risk of a late carbon dioxide charge to traps, the direct detection of gas using seismic AVO analysis, and the petrophysical ambiguities recorded in wells has resulted in new insights. An important new play has been recognised on the northern flank of the Penola Trough: a gas and oil charge from mature source rocks directly overlying basement into a quartzose sand sequence referred to informally as the Sawpit Sandstone. This play was successfully tested in early 1994 by Wynn-1 which flowed both oil and gas during testing from the Sawpit Sandstone. In mid 1994, Haselgrove-1 discovered commercial quantities of gas in a tilted Pretty Hill Formation fault block adjacent to the Katnook Field. These recent discoveries enhance the prospectivity of the Penola Trough and of the Early Cretaceous sequence in the wider Otway Basin where these sediments are within reach of the drill.
The Kenmore oil field in the Eromanga Basin of southwest Queensland was discovered in 1985. Since then, a further 32 wells have been drilled and more than 12.5 MMSTB of oil has been produced from the Birkhead Formation/Hutton Sandstone. Oil production over the last year has averaged 1,220 barrels per day totalling some 0.45 million stock tank barrels (MMSTB)Oil reserves in Kenmore were originally estimated at 2.2 MMSTB following the Kenmore–1 discovery well drilled in 1985. In the following 20 years, infill drilling, a 3D seismic survey, various reservoir studies and better -than-expected recovery efficiency, have steadily increased the ultimate recoverable reserves to the current estimate of 14.3 MMSTB.The growth of reserves at Kenmore is primarily attributed to better drainage of the complex reservoir framework within the lower Birkhead Formation resulting from recognition of the variable lateral connectivity of the reservoir. Due to the initial estimate of the ultimate field reserves being significantly smaller than now recognised and the resultant conservative drilling program, the economic value of the field was not maximised. This experience has implications for the ongoing development of the Kenmore field and suggests that other Birkhead/Hutton oil fields should be developed more aggressively to prevent history repeating itself.
Recent hydrocarbon discoveries in Early Cretaceous (pre-Aptian) reservoirs of the western Otway Basin offer encouragement to future petroleum exploration and also contain clues to the palaeoenvironments and early evolution of this rift basin. The Late Jurassic/Early Cretaceous Casterton Formation and the Early Cretaceous Crayfish Group have traditionally been regarded as fluvial and lacustrine deposits. Indeed, the source rock characteristics inferred from the geochemistry of the Katnook Field condensate and the oils from Wynn-1 and Sawpit-1 are those of siliciclastic freshwater facies. However, the biomarker assemblage of the Troas-1 condensate implies that its source beds were deposited in a marginal marine setting. Even more unexpected are the biomarker compositions of reservoir bitumens from Crayfish-Al and Zema-1 which provide evidence for the existence of saline to hypersaline palya lakes during the early rift phase of the Otway Basin.
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